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KD8OW
01-03-2012, 04:02 PM
I'm replacing the speaker in a radio, Drake SW-8, read limited room. Ordered a replacement, same part number, SAMMI cw77b1o a 10w 8 ohm speaker. When the speaker arrived it is marked cw77b10 10w 4 ohm.
Installed the new speaker, it works, but then started to worry about drawing to much current through the amplifier circuit. Is there some simple way to match the 4 ohm speaker to the 8 ohm circuit requirment.

Bill

W0LPQ
01-03-2012, 04:59 PM
Don't sweat it .... people have been doing this for decades..

K8ERV
01-03-2012, 06:15 PM
If it sounds good, do it.

TOM K8ERV Montrose Colo

KB0MNM
01-05-2012, 01:53 AM
Bill, If you are worried about using a 4 ohm speaker on an 8 ohm amplifier, consider adding a resistor in series. The most common value would be 4.7 ohms if you are looking for safety, though 3.9 should be close enough. I would resist ( pun intended ) any urge to save money by using a 5 watt version, unless it turns out that you put two eight ohm ( or 10 ohm ) resistors in parallel to make what you need. The reason: at low audio frequencies, use the formula P=IxIxR rather than P=ExI. You are dealing with alternating current. Feel free to use an older 10W wirewound type- the coil in the speaker is wound. 73-

KD8OW
01-05-2012, 02:21 AM
Thanks Jon

Who'd a thunk....Ohms Law, Duh

Bill

KB0MNM
01-05-2012, 02:43 AM
No problem, Bill. Just avoid a 'bill' by making sure that the solution is either a series addition to make 8 ohms, or a matching transformer that shows the same 8 ohm impedance. If you use any resistance in parallel with the new speaker, you subtract load and add to over-current problems. But if you put two 5 watt 8 ohm resistors in parallel, then put that combination in series with the new speaker- you add 4 ohms. Z squared equals the sum ( Xl squared ) less (Xc squared) plus R squared, where Z is impedance, Xl is inductive reactance, Xc is capacitive reactance. Or you could limit amplifier gain... Have fun- 73 - Jon

WA6TKD
01-05-2012, 02:48 AM
This kind of question came/comes up a lot with solid state hi-fi stereos. There is no problem running with 4 ohm speakers as long as you don't crank the volume up so high that the higher current draw with a 4 ohm speaker Vs a 8 ohm speaker would exceed the current capacity of the audio amp output stage.

-73

KB0MNM
01-05-2012, 02:59 AM
Yup, posts #2, #3, and #7 are all the usual fix. Then along comes a two-year-old who wants to play while the cat has jumped into your lap ( presumably to avoid the child ). This is what I call "provoking Murphy's law". For about $5.00 worth of parts ( terminal strip or other mounting may cost ), the issue is not possible. The extra heat might be a new concern...- Jon

KF5LJW
01-05-2012, 04:30 AM
Resistor will work, but you give up 3 db or half your audio power. But honestly feeding a 4 ohm speaker with a amp designed for 8 is no real problem. Only time it could be a problem is if you listen at full volume with a continuous tone.

KM3F
01-06-2012, 07:38 PM
If you want to try something to demonstrate what happens with impedence missmatch at it's worse, connect in a #47 pilot lamp in series with the speaker.
Then as you increase volume listen to what happens to the audio for distortion.
At low levels it makes little difference but at higher drive levels the lamp will light up.
When this happens the lamp resistance increases on audio peaks increasing the missmatch and causing distortion.
I have use this trick long ago with stations close in to prevent blowing my brains out while listening to weak signals with the volume up on a radio that had poor AGC action.

WA7PRC
01-06-2012, 09:45 PM
The thing to remember is that a speaker is an awful load... the impedance is all over the map. Its nominal impedance (http://en.wikipedia.org/wiki/Nominal_impedance#Loudspeakers) is the impedance at ONE frequency. If you look at it over the range of frequencies driving it, it's anything but a straight line, like this:

http://upload.wikimedia.org/wikipedia/en/thumb/7/72/Speaker_impedance.svg/800px-Speaker_impedance.svg.png

The speaker will work fine without anything added.

KB0MNM
01-07-2012, 01:03 AM
Bryan-
Did you look at post #1? He said that the speaker works fine, but that he does not want to turn the amp up for fear that the replacement part at 4 ohms will draw more than an 8 ohm speaker. Also, where did you find the graph and/or info on his Drake equipment? -Jon

K1ZJH
01-07-2012, 01:36 AM
I agree with Bryan. It should work fine, and speaker impedances are indeed all over the map.

Besides, adding the resistor is a 3dB power loss, so the volume control will still be cranked up to make up for
the missing audio energy, which is wasted as heat in the resistor. Not a very efficient method of impedance matching. And he
will still be running the amp harder for a given amount of audio regardless.

If you are really anal, the proper way to do it is with an autotransformer--the taps on a 70.7 volt PA universal audio
transformer would be ideal. Use the 4 and 8 ohm taps.

Pete

WA7PRC
01-07-2012, 01:49 AM
Did you look at post #1?
Yes. I read all the posts before replying.


He said that the speaker works fine, but that he does not want to turn the amp up for fear that the replacement part at 4 ohms will draw more than an 8 ohm speaker.

The advice is still to not turn the level up more than is needed, or past the point of distortion (whichever is less). Since squarewaves have more total energy (fundamental + harmonics) than any other waveform, that is when the power delivered to the load is greatest.


Also, where did you find the graph and/or info on his Drake equipment?

I didn't; I doubt such a graph exists. And, the load impedance for the driver (the enclosure and room) has an effect. The graph can be found at the link I provided. I didn't state that the graph was for a specific speaker.

The point is, in this kind of application, there's no point in getting overly excited about the NOMINAL impedance of a speaker driver since it's all over the place anyway.


Besides, adding the resistor is a 3dB power loss
If the resistor equals the speaker impedance, the loss is 6dB; -3dB = 0.707 EIN.

K1ZJH
01-07-2012, 01:59 AM
If the resistor equals the speaker impedance, the loss is 6dB; -3dB = 0.707 EIN.

If nothing else changes, I agree. But he would end up having to run the AF gain up to have the same
amount of audio by ear. That is, for the same average power delivered to the speaker.

My point is for the same amount of audio energy (mechanical, moving air)
1/2 of the power is wasted as heat in the resistor. Assuming the speaker impedance always matched the
resistor.

K7JEM
01-07-2012, 04:32 AM
If you think about the speaker, changing from 8 ohms to 4 is a 2 to 1 "SWR". Most of us have run transmitters into antennas of 2 to 1 SWR with little affect on our radios. I'm sure this mismatch won't bother anything.

And the loss would be 3 dB when we compare a 4 ohm speaker with a 4 ohm resistor to the original 8 ohm speaker, assuming speaker efficiencies are the same. It could be that a modern replacement speaker is more efficient than the original, too. The actual difference may be minimal.

Since most people listen to their speaker at an average level of 1 watt or less, the resistor will not develop a lot of heat anyway. 1 watt into a speaker is very loud, when in the same room. Not so much on a PA horn in a ballpark, though.

Joe

WA6TKD
01-07-2012, 04:47 AM
And the loss would be 3 dB when we compare a 4 ohm speaker with a 4 ohm resistor to the original 8 ohm speaker, assuming speaker efficiencies are the same. It could be that a modern replacement speaker is more efficient than the original, too. The actual difference may be minimal.


Correct, without knowing the specific SPL/W ratings of the two speakers in question, there is simply not enough information to correctly determine which speaker would sound louder at any given power level. Without that spec all is just an assumption or WAG. I would never bother to add a series resistor to a 4 ohm speaker, I would just make sure I don't turn the volume up where distortion is evident, but that is the same way I would operate even if I was using a 8 ohm speaker.

KF5LJW
01-07-2012, 05:00 AM
This thread is getting kind of funny. A whole bunch of talk about a non-existent make believe problem. There is no real problem driving a 4-ohm speaker with an amp made for 8-ohms. It is no different using your 50 ohm transmit amp into a 25 ohm antenna. It works, every day, all day.

K1ZJH
01-07-2012, 05:04 AM
Most IC audio amps have extremely low output impedances. In theory, the load impedance sets the maximum
power to the load for a given Vcc supply; within the device's maximum ratings.

SWR has nothing to do with source impedance.

Pete

K7JEM
01-07-2012, 06:30 AM
I downloaded the manual and the specs say 2 watts into a 4 ohm load, so I don't think a 4 ohm speaker is going to bother anything.

http://www.dproducts.be/drake_museum/SW8.pdf

K1DNR
01-07-2012, 07:30 AM
It will work fine.

Stereos, SWR, guitar amps, etc don't qualify as being analogous in any way. You might only drive a stereo speaker with an average of 1 loud RMS watt. But if you are listening to a modern recording, the dynamic range is over 90db. You can blow the smack out of an amplifier and the speaker by putting a 4 ohm speaker in an 8 ohm cabinet.

K7JEM
01-07-2012, 07:38 AM
It will work fine.

...... You can blow the smack out of an amplifier and the speaker by putting a 4 ohm speaker in an 8 ohm cabinet.

??

So will it work fine, or blow the smack out of the amplifier? The post is confusing.

Joe

K1DNR
01-07-2012, 08:05 AM
??

So will it work fine, or blow the smack out of the amplifier? The post is confusing.

Joe

Read it again. Have you ever seen a ham radio receiver with a dynamic range of 90db?
I said in the case of stereos.

Your SWR analogy was especially poor.

K7JEM
01-07-2012, 08:17 AM
Read it again. Have you ever seen a ham radio receiver with a dynamic range of 90db?
I said in the case of stereos.

Your SWR analogy was especially poor.

The analogy was to get people thinking. A ham TX is quite restrictive as to load impedance, yet a 2 to 1 SWR is generally not an issue. An audio amp is even more forgiving than an RF amp, so the same impedance mismatch would have far less effect. As some have pointed out, the output impedance of a modern IC audio amp is probably less than an ohm anyway, so with a speaker the actual impedance is of little importance, at least within reason.

And I'm not sure the dynamic range is the reason an amp or speaker would blow. It would tend to be more of a peak or average power situation to cause that, nothing to do with dynamic range.

Joe

K1DNR
01-07-2012, 08:41 AM
The analogy was to get people thinking. A ham TX is quite restrictive as to load impedance, yet a 2 to 1 SWR is generally not an issue. An audio amp is even more forgiving than an RF amp, so the same impedance mismatch would have far less effect. As some have pointed out, the output impedance of a modern IC audio amp is probably less than an ohm anyway, so with a speaker the actual impedance is of little importance, at least within reason.

And I'm not sure the dynamic range is the reason an amp or speaker would blow. It would tend to be more of a peak or average power situation to cause that, nothing to do with dynamic range.

Joe

An RF vs audio analogy breaks down pretty quickly I think - If we are talking about RF, we have a carrier on one frequency, with a relatively small bandwidth. If we are talking about an audio signal we are talking about a frequency of 30 or so Hz to well over 15,000Hz - so an impedance matching perspective doesn't work the same way - so I think that argument is a bit of a false start - at least from the perspective of a signal into a load.

We are talking about the range of amplitude of the signal or the voltage at the speaker. If I measure the range or amplitude, or voltage, the dynamic range or loud to soft, of the audio on a ham radio receiver - it is relatively small.

If you measure the same range of amplitude, or volume, on a stereo reproducing a digital recording of the 1812 Overture, the range is over 90db from soft to loud.

That means when those canons go off in the 1812, it is taxing the amp far more heavily than the audio from the ham radio. The peaks are high enough to damage the speaker quite easily - particularly one rated at half the design impedance.

Put another way - plug an electric guitar into an amp and turn the volume to 10 - wonderful tube effects and hearing damage not withstanding - no problems.

Replace that speaker with one rated at half the impedance and try the same thing. You'll be calling your technician in no time to replace the speaker and possibly repair the amp.

Its far less critical with a low power/low dynamic range ham radio audio stage. The range just isn't there. The overtones and harmonics aren't there. Those play a big part as well. The attack of a piano hammer, guitar pick, or cannon is huge - a huge spike, rich in overtones that also tax the amp. This is why a ham radio audio and a hi fi audio comparison differ.

K7JEM
01-07-2012, 03:23 PM
OK, but it has nothing to do with dynamic range, it has to do with output power, peak and average. If you are listening to the 1812 overture, your audio output would be adjusted such that the loudest sounds are set to deliver peak rated power, or less. If you set your output to deliver peak output at a soft point in the program material, then yes you will get overload on the peaks. But the same would occur with a 2M radio, or SWL radio.

I've never seen a speaker blown just because the impedance was changed, although it might happen if the person controlling the volume couldn't hear distortion, or was trying to get a distorted sound.

In any event, relative the particular case in the OP, there would be no problem anyway, since the radio is specifically rated at 2W output into 4 ohms, and the replacement speaker is rated at 10w, 4 ohms. The rest of the discussion here has been enlightening, but the truth is that the speaker matches the specified load anyway, so no "problem" ever existed.

Joe

KB0MNM
01-09-2012, 02:35 AM
Guys- The link to Wikipedia was not exactly obvious, and did not cover every situation. The link to the manufacturer's book shows that it is possible to use a 4 ohm speaker rated at 2 Watts for EXTERNAL use, and does not state whether or not that external speaker jack includes a switch ( as many do ) to bypass the internal speaker. I stand by what I said about provoking Murphy's law. If you look at the possible current drain for 3 volts peak-to-peak into either a 4 ohm or 8 ohm load, you might think again. No, this has nothing to do with 300mV at the line-out jack ( 4.7K impedance ). This is just a reasonable approximation of maximum volume. A transformer probably would not fit (& may be too costly). Reminder- we are dealing with AC current. Think current squared times impedance equals power. Try 35-707 ma. - 73

W0LPQ
01-09-2012, 04:10 PM
Almost 3 pages ... just hook it up and use it. That simple.

KB0MNM
01-09-2012, 04:21 PM
I'm replacing the speaker in a radio, Drake SW-8, read limited room. Ordered a replacement, same part number, SAMMI cw77b1o a 10w 8 ohm speaker. When the speaker arrived it is marked cw77b10 10w 4 ohm.
Installed the new speaker, it works, but then started to worry about drawing to much current through the amplifier circuit. Is there some simple way to match the 4 ohm speaker to the 8 ohm circuit requirment.

Bill
W0LPQ- It is already hooked up and works. He is trying to avoid future damage caused by excessive current. There are two ways at the speaker to do that: current limiting or a transformer. Considering the power supply, the space inside- the resistor solution would probably be the best one, shy of getting the correct ( original ) speaker impedance. Just because you can use a speaker and it works, does not mean that damage may not occur at higher volume levels, hence 'Murphy's Law'. Those who fail to plan, plan to fail.-Jon KB0MNM (/5)

KB0MNM
01-09-2012, 04:25 PM
& by the way- one could always decrease the gain of the last audio stage. There is often a capacitor that affects this. A smaller capacitor would have the correct effect, if it is indeed a 'gain' cap.

K7JEM
01-09-2012, 04:29 PM
W0LPQ- It is already hooked up and works. He is trying to avoid future damage caused by excessive current. There are two ways at the speaker to do that: current limiting or a transformer. Considering the power supply, the space inside- the resistor solution would probably be the best one, shy of getting the correct ( original ) speaker impedance. Just because you can use a speaker and it works, does not mean that damage may not occur at higher volume levels, hence 'Murphy's Law'. Those who fail to plan, plan to fail.-Jon KB0MNM (/5)

The manual already indicates that it is specified for a 4 ohm load. He installed a 4 ohm speaker, so what it the deal?

NO damage will occur, unless he just turns the thing up to full blast, and lets the distortion kill the speaker or amp. But that could occur with any speaker and amp setup. At normal levels, there is no problem.

Joe

KB0MNM
01-09-2012, 09:51 PM
Joe, If you see it as no problem, that is fine. If you want to be contrary because I answered the original question, that is fine. I seriously doubt that what is said here is of national importance. I can imagine that if excessive current flows through the 4 ohm INTERNAL speaker that is inside the Drake, it might fry either the speaker- the audio amp - or the power supply- because fuses work on excessive current and you seldom see them at the output of audio amplifiers of this sort. Since he has already done the work with the speaker, he may or may not decide to follow his original plan to see if he could adjust the circuit. The deal is in answering the question, that is the deal. See Kelley's corollary ( Murphy was an optimist... ).:)

K7JEM
01-09-2012, 10:22 PM
The radio is designed for a 4 ohm load. The radio is rated at 2 watts output, and he has installed a 10 watt speaker. I really don't know where this "excess current" thing is coming from. By the time he gets to that point, the audio would be very loud and distorted, hopefully that is an indication to turn down the volume to a decent level. Two watts is very loud on a small radio. Average power might be closer to .2 watts, or less.

The radio uses the same amp for the internal and external speaker. There is no additional amplifier in that radio to power the external speaker. So the single amplifier is actually rated at 4 ohms and two watts.

It shouldn't be that big of a deal to understand that his replacement speaker is fine for the application, and this has been proven by the fact that he replaced it with no mods, and is now enjoying his SWL activities again.

Joe

K9STH
01-10-2012, 12:28 AM
MNM:

How is more current going to flow through the internal speaker?

If the speakers are in series, then the effective resistance is going to be 8-ohms (ignoring all of the things that happen due to frequency, etc.), which, with the same power from the amplifier, will be 1/4th of what the current would be in each speaker if independently connected. If the speakers are in parallel, then the current through the internal speaker is independent of the current flowing through the external speaker. Theoretically, with the same audio power applied, since each speaker has the same impedance, then equal currents would flow through each speaker.

Glen, K9STH

AC0FP
01-10-2012, 12:41 AM
The radio is designed for a 4 ohm load. The radio is rated at 2 watts output, and he has installed a 10 watt speaker. I really don't know where this "excess current" thing is coming from. By the time he gets to that point, the audio would be very loud and distorted, hopefully that is an indication to turn down the volume to a decent level. Two watts is very loud on a small radio. Average power might be closer to .2 watts, or less.

The radio uses the same amp for the internal and external speaker. There is no additional amplifier in that radio to power the external speaker. So the single amplifier is actually rated at 4 ohms and two watts.

It shouldn't be that big of a deal to understand that his replacement speaker is fine for the application, and this has been proven by the fact that he replaced it with no mods, and is now enjoying his SWL activities again.

JoeDon't argue with MNM he is obviously an expert and needs no advice!

fp

KB0MNM
01-10-2012, 02:41 AM
MNM:

How is more current going to flow through the internal speaker?

If the speakers are in series, then the effective resistance is going to be 8-ohms (ignoring all of the things that happen due to frequency, etc.), which, with the same power from the amplifier, will be 1/4th of what the current would be in each speaker if independently connected. If the speakers are in parallel, then the current through the internal speaker is independent of the current flowing through the external speaker. Theoretically, with the same audio power applied, since each speaker has the same impedance, then equal currents would flow through each speaker.

Glen, K9STH
Glen- You must not have read all of the pertainent material, because I was carefull to go ALL CAPITAL letters on the business of internal and external speakers. The original factory speaker was 8 ohms. Now if you replace that one with a 4 ohm speaker, you will draw more current ( should junior turn the volume way up ). Enter the post #1, it is installed. Then he asks how to ensure that this does not happen. How about a 4 ohm, 10 watt resistor in series ( or two 8 ohm 5 watt in parallel, then in series )?? Now you get someone who says '... but the rated impedance is 4 ohms...' Look carefully- that is for an EXTERNAL add on speaker- this is what provokes Murphy's law- everybody's happy- huh? Now we have 2 ohms impedance.... As to being an expert, I never made that claim. I just read enough to know when someone asks for help, don't assume that 'working' is the same as 'right'.
BTW- I used to be expected to teach basic electronics... to guys that wanted to learn. I was not born with any 'silver spoon'.

K9STH
01-10-2012, 05:58 PM
MNM:

Again, how is more current going to be drawn through the INTERNAL speaker when an external speaker is attached?

You are forgetting basic Ohm's law. When the speakers are in parallel, twice the current is going to flow through the 4-ohm speaker than through the 8 ohm speaker.

When in series, the same current is going to flow through both speakers. However, due to the increased resistance that current is going to be less than what the current would be through the single 8-ohm speaker.

Using the formula P = (I^2)R, with 2-watts power, when in parallel, the effective resistance will be 2.666 ohms. R = (R1 x R2)/(R1+R2) = 32/12 = 2.666. The total current provided by the 2-watts will be 2 = (I^2)(2.666), 2/(2.666) = I^2, 0.750 = I^2, 0.866 = I. This current will be drawn on a 2:1 ratio with the 4-ohm speaker being the "2". This means that 0.577 amps will be drawn by the 4-ohm speaker and 0.289 amps will be drawn by the 8-ohm speaker.

When only the 8-ohm speaker is in place, then 2/8 = I^2, I = 0.354 amps.

When the speakers are in series, the same current is going to be drawn through both speakers. In series, the resistance adds being 8-ohms + 4-ohms = 12 ohms. Therefore, 2 = (I^2)(12), 2/12 = I^2, I = 0.167.

In both cases, the current drawn through the 8-ohm internal speaker is less than when the 8-ohm speaker is alone.

Now, since we are dealing with AC and not DC, the calculations are more complex. However, the same basic logic remains and that is when the second speaker is added the current through the 8-ohm internal speaker is decreased.

Glen, K9STH

K1ZJH
01-10-2012, 06:11 PM
In both cases, the current drawn through the 8-ohm internal speaker is less than when the 8-ohm speaker is alone.


Why is that? Most solid state amps have essentially 0-ohm output impedances. For two speakers in parallel, the AF voltage
remains the same regardless of the load, within reason of course...

Pete

K9STH
01-10-2012, 08:38 PM
ZJH:

Yes, the voltage applied is constant. However, the current through the resistance is not constant. Again, E=IR or E/R=I. Assume an applied voltage of 1 volt. For a resistance of 4-ohms then the current equals 1/4 or 0.25 amps. For a resistance of 8-ohms the current equals 1/8 or 0.125 amps.

Again, the voltage is constant but the current, through the load, is not.

Using the formula for 2 resistors in parallel, (R1R2)/(R1+R2), you get 32/12 which equals 2.6667-ohms. Then 1/2.6667 = .375 amps supplied by the voltage source to the resistive load. 0.250 amps + 0.125 amps = 0.375 amps. The total current drawn by the effective load is equal to the current drawn by each section of the load.

Glen, K9STH

K1ZJH
01-10-2012, 08:54 PM
In both cases, the current drawn through the 8-ohm internal speaker is less than when the 8-ohm speaker is alone.


The above statement is not correct for a parallel circuit.

The AC voltage remains the same, the 8-ohm speaker's impedance remains the same,
the power delivered to the 8-ohm speaker, and the current flowing through that speaker, remains the same.

The total load current supplied by the amp increases when the second speaker is added, but it doesn't
affect the current flowing through the original 8-ohm device. P = E squared / I.

Does turning on a second lamp on a AC branch circuit somehow affect the wattage drawn by other
lamps on the same branch?

Pete

K9STH
01-10-2012, 10:40 PM
Theoretically, the power drawn would be less. However, the difference is usually not even measurable because of the reserve power from the AC lines. But, like during hot weather, when the power grid is taxed, there can be "brown outs" caused by exceeding the capabilities of the supply.

In the case of the speakers, there is a limit to the power which can be supplied by the amplifier. This is specified at 2-watts. Therefore, there will be a division of the power in the ratio of the 2 impedances to each other. If there was not a limit on the power, then the power dissipated in each speaker would be the same if operated individually or in parallel. However, there is a limit specified and therefore the power through each speaker will be measurably different when in parallel than when operated as a single unit.

With an unlimited power source the voltage will remain constant and therefore the power dissipated through each speaker (load) will remain constant. However, in the "real world", the power source has limitations and therefore the voltage and current will be different. This is why there are "brown outs". When the load is within the capabilities of the source then each load will dissipate the same amount because the voltage remains constant. But, when the source is limited, then the voltage drops and the power dissipated in each portion of the load will be less.

In the situation described, there is a definite fixed capability of the source, 2-watts. Then, there are 2 loads, of unequal resistance, presented to that source. The effective resistance of the 2 loads reduces the voltage that can be supplied by the source. The power is still constant, but, because the power has to be distributed between 2 loads, to achieve the same power load the current through each segment of the load has to be reduced from what either segment would draw if the power source was only supplying power to that load.

The key to this is the fact that the power capabilities of the source is limited. If the power source was unlimited, then the power dissipated through each device, and therefore the current through each device, would remain constant. But, since the power source capabilities are limited, then the power dissipated in each load will decrease. If the power source was capable of supplying more power than the total dissipation of the loads combined, then the power dissipated by each load would be constant. However, in this example, the source is limited and therefore so is the current through each device.

Glen, K9STH

K1ZJH
01-10-2012, 11:29 PM
Hi Glen

It is still has a near zero output impedance, unlike a tube amplifier. The voltage is a constant, and indeed the maximum
power handling ability of the amp is finite. But, the output devices would likely fail before the voltage is reduced
an additional load. You can only have so much voltage drop across a semiconductor junction :)

A blanket statement that adding a second speaker in parallel reduces the power of the first speaker isn't accurate for this
type of amplifier.

Pete

KB0MNM
01-11-2012, 05:40 PM
Glen- It appears that you agree that *if* a second 4 ohm speaker were added to the system in parallel, the true impedance of that system now changes to around 2 ohms. Not knowing if the speaker jack is wired for cut-out to put them in series or parallel- I have applied Murphy's law in advance. I am sure that everyone knows the original internal speaker was intended to be an 8 ohm device. I will agree that the impedance is non-linear, but for purposes of preserving the equipment * I answered the original question with regard to cost*.-73

KB0MNM
01-11-2012, 05:49 PM
ZJH:

Yes, the voltage applied is constant. However, the current through the resistance is not constant. Again, E=IR or E/R=I. Assume an applied voltage of 1 volt. For a resistance of 4-ohms then the current equals 1/4 or 0.25 amps. For a resistance of 8-ohms the current equals 1/8 or 0.125 amps.

Again, the voltage is constant but the current, through the load, is not.

Using the formula for 2 resistors in parallel, (R1R2)/(R1+R2), you get 32/12 which equals 2.6667-ohms. Then 1/2.6667 = .375 amps supplied by the voltage source to the resistive load. 0.250 amps + 0.125 amps = 0.375 amps. The total current drawn by the effective load is equal to the current drawn by each section of the load.

Glen, K9STH

Glen- Your math looks good, but try it with two 4 ohms speakers in parallel, then (an 8 and a 4). When done, you would also need to look at the power supply of the entire radio as a system. The original speaker was supposed to be 8 ohms, it is now 4 ohms. The external speaker jack wiring is not specified- could be series or parallel. Drake owners could pipe-up here. Series would be safer, but repairfolk sometimes shortcut that.-73

K1ZJH
01-11-2012, 07:13 PM
Should be easy to jack in a speaker and see if the internal one disconnects, or not. I suspect that radio is using an dual audio chip
in a BTL configuration given the fact that neither speaker lead is tied to the chassis.

Pete

K9STH
01-12-2012, 01:13 AM
With a power limit of 2 watts, 2 each 4-ohm speakers will present a load of 2-ohms. Each speaker will draw 0.5 amps. Using P = (I^2)R = (0.5^2)(4) = (0.25)(4) = 1. Therefore, as to be expected, each speaker is dissipating 1 watt while drawing a current of 1 amp.

Going back to the calculations that I presented yesterday, when in parallel, with a power limit of 2 watts, the current through the 4-ohm speaker drawn through the 4-ohm speaker will be 0.577 amps and the current drawn through the 8-ohm speaker will be 0.289 amps. In that configuration, the 4-ohm speaker will be dissipating 1.333 watts and the 8-ohm speaker will be dissipating 0.667 watts.

If I remember correctly, the 4-ohm speaker in question is rated at 10-watts. Therefore, the capabilities of the speaker will not be exceeded.

Of course, if the source of power is not limited to 2-watts, then the calculations change. How the power is limited is open for discussion. Is the current to the output amplifier limited? It could be. Are there other limiting circuits? Could be. Or, is the power expected to be kept within specifications by the operator? Again, could be.

In a practical sense, unless someone is virtually deaf, 2-watts into a speaker will produce a painful and/or harmful sound level in most applications. Then, there are those persons who insist on "feeling" the sound rather than just "hearing" it. Unfortunately, a goodly number of those persons have damaged their hearing beyond the point of no return.

A number of years ago, there was a "blurb" in some newspapers about the fact that the Swedish Navy could not find enough young men who's hearing was not damaged to operate SONAR on their warships. I suspect that the same thing is true in many countries. That is, finding people who have not damaged their hearing from listening to LOUD music.

Glen, K9STH

KB0MNM
01-12-2012, 01:44 AM
Glen- See post #1. The original internal speaker was intended to be 8 ohm, 10 watt. The one which arrived is rated the same as the EXTERNAL speaker for impedance, 4 ohms ( 10 Watt shipped ). So the question was one of making sure that the Drake radio system (any part ) would not be damaged by excess current. If there is any question about the power supply, it is 'What happens when the speaker load is low enough to draw up to 2 amps from a supply rated for 1 amp? ' The answer is ( I hope ) a fuse blows. 2<2.66 for impedance, but one should also consider that there would be some small impedance to the audio driver which works the speaker or speakers. The answer is about making the 4 ohm speaker which is already installed a safe repair. The added resistor may warm things up, but should not make the audio much worse, if the gain is adjusted to compensate ( volume control ). A transformer would work, except that space is at a premium. -73

KB0MNM
01-12-2012, 02:07 AM
The radio is designed for a 4 ohm load. The radio is rated at 2 watts output, and he has installed a 10 watt speaker. I really don't know where this "excess current" thing is coming from. By the time he gets to that point, the audio would be very loud and distorted, hopefully that is an indication to turn down the volume to a decent level. Two watts is very loud on a small radio. Average power might be closer to .2 watts, or less.

The radio uses the same amp for the internal and external speaker. There is no additional amplifier in that radio to power the external speaker. So the single amplifier is actually rated at 4 ohms and two watts.

It shouldn't be that big of a deal to understand that his replacement speaker is fine for the application, and this has been proven by the fact that he replaced it with no mods, and is now enjoying his SWL activities again.

Joe
Joe- Look again at post #1. If no external speaker may be added to the radio, the 4 ohm impedance may be fine. But someone provided the specs. The EXTERNAL add on speaker may be 4 ohms. This assumes that the radio is in the 'as manufactured' state, which it is not. Glen is showing some of the math as to why this may work fine - for now-. As long as no one turns up the volume, or adds another 4 ohm speaker, it may be safe to assume that no further work is needed. But that is not what he asked about. He asked about a way to protect the system from excess current. BTW- The radio external speaker is rated 2W, but that is not the
same as the radio being set up for 2W, or why is the normal internal speaker 10W?

K7JEM
01-12-2012, 02:12 AM
I'm almost absolutely sure that the internal speaker will cut out when an external one is plugged in. That is the way 99+% of all newer radios are wired. People don't want the internal speaker running if they have attached a better external unit. I cannot think of a single radio made in the last 30 years that does not do this. So the discussion about parallel speakers is moot in this case.

The speaker is 10W because that is the rating for a speaker of that size. You can use a 10W speaker on a 1, 2 or .3 watt audio amplifier, hams do it all the time. I used to use a 12w external Motorola speaker on my handheld radio, which could probably put out .3 watts, Sounded very, very good.

Joe

K9STH
01-12-2012, 02:48 AM
By "power supply", I must assume that you mean the audio output stage and NOT the actual source of voltage for the radio.

I just went over to BAMA to look at the SW-8 manual. Unfortunately, the manual does not have a schematic. However, looking at the specifications it looks like the SW-8 is just an updated SW-1 with the addition of a product detector. Now none of the SW- series receiver manuals on BAMA have a schematic. However, I not only own a Drake SW-1 but I also have the full factory documentation on that receiver including several very large schematics.

The audio output IC in the SW-1 is 1-watt compared to 2-watts in the SW-8. Looking at the manufacturer's data sheet on the IC used in the SW-1, a TDA7052A, that particular "chip" definitely has current limiting circuitry. I strongly believe that Drake would have continued to use an IC with current limiting although with a 2-watt rather than a 1-watt capability. They probably just used the next "higher" capability chip.

Therefore, the current that can be provided by the audio output stage is limited and therefore, it doesn't matter if the impedance of the speaker is 4-ohm or 8-ohm. Also, it doesn't really matter if a second speaker is put in parallel with the first speaker because the current output from the amplifier is restricted to a specific level. As such, the replacement speaker provided is fine and even if when an outboard speaker is added, it really doesn't matter if that speaker is in parallel or series with the internal speaker because even with the additional load, the audio circuitry will not be damaged.

Glen, K9STH

K1ZJH
01-12-2012, 02:57 AM
Glen

I believe it used a LA-4555 audio IC configured as a BTL amplifier.

Pete

K7JEM
01-12-2012, 02:57 AM
As such, the replacement speaker provided is fine and even if when an outboard speaker is added, it really doesn't matter if that speaker is in parallel or series with the internal speaker because even with the additional load, the audio circuitry will not be damaged.

Glen, K9STH

External speakers never go in series. But the question would be, does the internal speaker cut out when the external speaker is plugged in?

KB0MNM
01-12-2012, 03:12 AM
I'm replacing the speaker in a radio, Drake SW-8, read limited room. Ordered a replacement, same part number, SAMMI cw77b1o a 10w 8 ohm speaker. When the speaker arrived it is marked cw77b10 10w 4 ohm.
Installed the new speaker, it works, but then started to worry about drawing to much current through the amplifier circuit. Is there some simple way to match the 4 ohm speaker to the 8 ohm circuit requirment.

Bill
Bill- The simple way is to add 4 ohms in series, a 10W resistor. It seems as if the amp may be fine without it, provided that the
amplifier output is from an LA-4555 chip ( and that chip has overcurrent protection ). BTW- How did the last speaker fail?
Kudos to K9STH and others who have provided some time and research in recent posts.

K9STH
01-12-2012, 03:52 AM
JEM:

There are both amateur radio and commercial two-way radios that do put the external speaker in series with the internal speaker. No where near as many as those that disconnect the internal speaker, but there are still a number that don't disconnect the internal speaker. The Uniden dash mount radios are one example of the commercial two-way radios that put the external speaker in series. Those radios have a "Molex" type jack on the back of the unit which, when an external speaker is not used, there is just a jumper that connects the low side of the internal speaker to ground. The external speaker is connected instead of the jumper which puts it in series with the internal speaker. Of course, the Uniden HR-2510 and HR-2600 also use the same arrangement with the speakers. Standard X66 radios used a similar arrangement. Now I haven't looked at the newer Vertex/Standard units, so I can't comment on those.

Other radios which have something like a Molex connector instead of an "earphone" type jack, usually put the external speaker in series. Those with the jack usually disconnect the internal speaker.

Glen, K9STH

K7JEM
01-12-2012, 04:18 AM
Glen, it makes no sense that the speaker would be in series. The jumper is usually on the high side. One side goes to amp, the other side to the speaker. To install an external speaker, you remove the jumper and add the external speaker to the high side, removing the internal speaker from operation.

The reason is the internal speaker usually is not as efficient as a much larger external unit. The bigger external speaker always sounds better, but some applications have little room for an external unit. People want the sound to go to the best sounding speaker. Once it is installed, there is no need for the inferior internal speaker.

I'll see if I can find some schematics on the radios you mentioned.

Joe

K7JEM
01-12-2012, 04:25 AM
Here is a copy of the HR2510 schematic. It does have the Molex connector you talk about, but it goes to the high side of the audio amp. The low side of the internal speaker is always grounded, the high side flows through a jumper.

http://www.cbtricks.com/radios/uniden/hr_2510/graphics/hr2510_sch_sheet1.gif

KD8OW
01-12-2012, 01:20 PM
Thank You Gentlemen,

I have certainly learned alot about audio circuitry and impedance matching. The reason that I asked the original question was that after I replaced the speaker I barely had to open the volume control to get room level sound and was worried that I was drawing too much current through the audio amp. I have Installed some additional resistance in series with the speaker and it is functioning closer to original.
Once again THANK YOU
Bill

W8JI
01-12-2012, 01:53 PM
I'm a little late, but assuming a 2:1 mismatch, the extra voltage from a high impedance for the same power is only about 40%. The extra current is again an additional 40% for the same power.

Assume a 4 ohm load, and 1 watt average power. Voltage and current would be 2 volts and 1/2 ampere.

Change that to 8 ohms, and it is 2.828 volts and 0.3535 amps.

This is hardly worth the worry, unless you have the audio stage running at maximum and it has a marginal design.

There is FAR more variation in performance and stress caused by speaker efficiency for a given sound level than a 2:1 load mismatch.

Extra voltage from a high load generally just causes saturation and clipping distortion at or near full output power, extra current from a low load generally just causes distortion from current limiting at or near full output power. The ONLY time mismatch is a problem is when the stage is operating right near limits, and generally only then when the mismatch is severe.

Load the 8 ohm line with 600 ohms, and it is problematic. Load it with 16 or 32 ohms, and unless you crank the gain way up it is unlikely you will notice anything other than a slight volume drop. If my radio's audio output stage went critical over a 2:1 mismatch, I get another radio or work around the exceptionally poor design. With heavy internal feedback in solid state, it is possible to have stability issues, but again that usually takes a pretty gross mismatch.

73 Tom

KB0MNM
01-13-2012, 01:16 AM
Thank You Gentlemen,

I have certainly learned alot about audio circuitry and impedance matching. The reason that I asked the original question was that after I replaced the speaker I barely had to open the volume control to get room level sound and was worried that I was drawing too much current through the audio amp. I have Installed some additional resistance in series with the speaker and it is functioning closer to original.
Once again THANK YOU
Bill
Bill- I think that there has been a lot of effort brought forth to illuminate the common issue, notably by: WA7PRC ( 8 ohm graph from Wikipedia ), W7JEM ( link to specs ), W1ZJH ( notes regarding output I.C. LA-4555? ), and especially K9STH ( observations regarding wiring and math ). While nobody was 'perfect' ( IMHO self included )- I think everyone tried to advise you as best we could. I am very pleased that you did install the extra resistance- ( I have had the displeasure of finding equipment modified before I started work in such a way as to cause small disasters and much embarrasment )- thus I think that "Murphy's law" and our imaginary 2 year-old operator will not be visiting your Drake radio someday on account of the work that you did. Thank you for asking!!! 73

KB0MNM
01-13-2012, 07:54 PM
BTW- That callsign should be K7JEM, not W7JEM- Lots of other W calls- sincerely sorry, Joe. Thanks Bill.

BRAINS
01-25-2012, 04:35 AM
I apologize in advance for barging into a thread, because I'm not going to be very on topic AND I'm not even a HAM. Was studying as a younger gent, picked up a Yaesu FT-101E way back when, built a simple dipole antenna and did a lot of listening - just never finished up and took the tests. I really wanted to work the 20, 40 and 80m bands.

Anyway, the reason for my intrusion - To the original poster (KD8OW, Bill). Could you point me in the right direction on where you found the Sammi cw77b10's for sale? Those would fit perfectly as replacements in some small speakers I have, and I can't seem to find them.

KD8OW
01-26-2012, 02:35 AM
I found it at the David Levy Company (www.dlcparts.com)
the internal part no. is oem-3096

contact info:
sales@dlcparts.com
Sherri Williams
David Levy Company
Direct Line
562-926-5758

Nice people to do business with
Bill

BRAINS
01-26-2012, 02:09 PM
Thank you very much!

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