K7DYY Mk II audio processor board question...

Discussion in 'Amplitude Modulation' started by W1BR, Jul 25, 2020.

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  1. W1BR

    W1BR Premium Subscriber QRZ Page

    I have one of these installed on a D=104 that has been retrofitted to an electret cartridge (never found a decent original element that survived!) I installed a K7DYY Mk II processor board to take advantage of the all phase filter and compressor. The noise gate feature is disabled. I noted on the Wireless Girl's webpage she suggests reducing the value of R-17 to limit the compression control range. I can easily do that. I also noticed the board's response curves offer a good peak at about 3-5 kHz, with a sharper roll-off than "stock" above 4.5 kHz.

    I've read some negative comments about this board on this forum, and I'm curious as to why this board is supposedly only usable with the K7DYY transmitter? The design seems reasonable??
     
  2. N2DTS

    N2DTS Ham Member QRZ Page

    You can use it with any rig, but it may not sound good into a vintage transmittter looking for a 1 to 5 meg ohm microphone as the output of the dyy compressor is 600 ohms and line level.

    I did not like it because it does not have a threshold and brings all the shack noise up.
    I was modulating 30% in a mostly quiet shack, just the dyy transmitter fans which are not loud...
     
  3. K5UJ

    K5UJ Ham Member QRZ Page

    Perhaps that's due to the response curve of the D104's xtal element.
     
  4. W1BR

    W1BR Premium Subscriber QRZ Page

    Well, none of that makes much sense... a higher impedance input than the source impedance shouldn't make a bit of difference so long as the proper level can be achieved Conversely, a low Z audio stage can load down a high Z source, but the opposite is true here. My board has an output level pot and the P-P audio voltage is easily reduced to a level compatible with either the Ranger I or B&W 5100. I still don't understand the previous concerns regarding using this board with other than the DYY transmitter. Regarding the response curve, my comments were related to the curves shown by the Wireless Girl, in which the D-104 response is pretty faithful, except for the pronounce roll off above 4.5 kHz which is steeper than the D-104 curve. Her webpage seems to suggest that the value for R17 will correct some of the issues, as well as turning off the noise gate feature. I haven't tested on the air as of yet, since the vintage bench needs a lot interface and control wiring to completed.
     
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    It does make sense with the D-104 because of its Impedance vs. capacitive nature.

    Here is the discussion of an experiment done by Stu, AB2EZ

    There has been some recent discussion of D-104 microphone heads (elements)... and various approaches to designing a preamplifier to work with a D-104.

    Although there seems to be some disagreement regarding the electrical equivalent circuit of a D-104... it is my opinion... based on both the physics of how a crystal microphone operates, and measurements I have recently made... that the electrical equivalent circuit of a D-104 is a voltage source in series with a capacitor. There is no series resistance.

    Recently, I measured the series capacitance of two (2) different D-104 microphone heads (elements) that I have: by placing a .01 uF capacitance across the microphone head, and using the resulting output as the input to a 10-megohm input-impedance instrumentation amplfier.

    Note: I am NOT using an "amplified" D-104.

    The results were as follows:

    When I measured a recent-production D-104 microphone head (element), that I have been using on the air for the last 6 months, I found that placing a .01 uF capacitor across the output resulted in a 10 dB (approximately a factor of 3 in voltage) drop in the output of the microphone... with no significant effect on its frequency response or the way it sounds. This is exactly what one would expect if the series capacitance of that microphone (element) is approximately 5000 pF. The .01 uF capacitor in parallel with the microphone (element) forms a 1:3 capacitive voltage divider.

    When I measured another D-104 microphone head (element), that I happen to have as a spare, I found that placing the .01 uF capacitor across the output resulted in a 20 dB (approximately a factor of 10 in voltage) drop in the output of the mcirophone... with no significant effect on its frequency response or the way it sounds. This is exactly what one would expect if the series capacitance of that microphone (element) is approximately 1100 pF. The .01 uF capacitor in parallel with the microphone (element) forms a 1:10 voltage divider. I believe that this 2nd microphone head (element) is an older one.

    What are the implications of this with respect to driving a preamplifier with a D-104 microphone head (element)?

    Well, for the case of a D-104 microphone head (element) that has a capacitance of around 1100 pF, i.e., the older-production units: if you drive a preamplifier whose input resistance is 1 megohm, then the input to the preamplifier will drop off by 3dB at a frequency of 145 Hz. If you want the response to extend down to 50 Hz, then you need to use a preamplfier with an input resistance of 2.9 Mohms. This is the advice that most people give with respect to interfacing a D-104 to something like a Ranger... i.e., "increase the input grid leak resistor value to at least 3 megohms".

    But, for the case of my more recent-production D-104 microphone head (element), since it has a series capacitance of approximately 5000 pF... if you use it in conjunction with a preamplifier whose input resistance is 1 megohm... then the low frequency 3 dB cutoff will be 32 Hz. Thus, for that "flavor" of D-104 microphone head (element), a 1 megohm input resistance is fine.

    Best regards
    Stu


    Pheel
     
    Last edited: Jul 26, 2020
  6. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    In the data sheet that came from the factory with the D-104, Astatic recommends at least 5 (or 4.7) megohms. I haven't run that test with the capacitor, but I recall reading somewhere, either in a sheet from Astatic or one of my engineering handbooks, that a generic crystal microphone can be considered equivalent to an ideal a.c. generator in series with a ~600 pf capacitor. With the relatively large Rochelle salt crystal in the D-104, it may well be closer to 1100 pf.

    I have several D-104 crystal elements. No two sound exactly alike. One in particular seems to have a better low frequency response than the others. My most "recent" element is one I bought new from Astatic in the mid 1970s.

    I use considerable overkill in the loading resistance in my push-pull pre-amp, with a 10 megohm resistor from each grid to ground, or 20 megohms total load across the microphone. I can see on my monitor scope modulation from the low-frequency rumble of distant aircraft well before I can hear it audibly. This is far better low frequency response than what is shown on the Astatic data sheet.

    One of the beauties of the stock D-104 xtal element is that it is non-polarised. Either terminal can be used as hot or ground, or both can be used for balanced output with the shell of the mic head serving as ground reference. Astatic recommended the balanced connection / push-pull pre-amp configuration if the mic cord is extremely long (~20 ft or more) to keep a.c. hum pick-up to a minimum. Mine is only 6' long, but the push-pull preamp works with a.c. on the tube filaments and minimal hum; my other conventional unbalanced preamp using a 12AX7, requires DC on the filament to get rid of the hum. With a.c. on the filament, the hum modulates the transmitter about 20%. With the push-pull configuration there is no RF pickup. I don't have any RF suppressor capacitors or resistors, nor any blocking capacitor at the input. The only thing connected to the grids are the mic cord conductors that go directly to the xtal element, and the 10 megohm grid resistors. I had to use a resistor and small capacitor at the input to the unbalanced pre-amp to keep RF out.
     
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    But, I stated this mike has an electret element. I could never find a decent working crystal cartridge... :)
     
  8. N2DTS

    N2DTS Ham Member QRZ Page

    The DYY board has an input for an electret element I think, so that part is ok, if the 600 ohm output is ok and you don't overdrive something it should be fine, but I don't think it works very well really.

    It DOES seem to hard limit though.
     
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    Looking at the schematic I don't see how the output impedance can be any higher than 500 ohms and that is with the pot set at midrange. The 2167 action is very aggressive and I did see some overshoot on the start of the first syllable when I was testing the newly installed board in the mike base. I'll try some of the WirelessGirl's suggested changes and if I get bad reports I'll just bypass the 2167 and just use the all phase filter section of the board. There were some initial problems with slow audio recovery when the PTT was hit, and I ended up sending the board back to K7DYY to see if he could work on the issue. There was some problem concerning the 5 VDC supply on the board; as I recall he reduced the electret bias voltage down to 2.5 volts to minimize the AGC recovery on the PTT spike.

    If I ever get time to finish the AC wiring and control wiring for the vintage gear I may get on the air by fall, if I am lucky!! I'd love to play around in some of the heavy metal and AM events.
    Pete
     
  10. N2DTS

    N2DTS Ham Member QRZ Page

    What are you going to run it into?
    I have three D104 microphones and they all work well.
    One I bought new back in the day, the other two I got off ebay cheap.
    One sounds very close to my original, the other sounds more thin, its a newer mic I suspect.

    With a change of the grid resistor on the preamp, they work well into most rigs.

    With most vintage rigs all you need is a mic transformer, 600 ohms to high impedance output, or a circuit change inside the radio.
     

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