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Broadcast Audio limiter/compressor in audio chain for newbie

Discussion in 'Amplitude Modulation' started by KD6CXW, Jul 28, 2019.

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  1. N1BCG

    N1BCG Ham Member QRZ Page

    A Dorrough 310 is way more than enough for processing just your voice for hamateur use, so nothing else except a mic pre-amp (to bring your mic up to line level) would be needed. The unit was designed to drive broadcast transmitters back in the days when they took a lot of level to reach full modulation and had headroom to spare.

    The isolation, typically an "H" pad, is used when feeding an inductive load such as a transmitter with a transformer input. Depending on your transmitter, you might need to attenuate the output considerably, particularly for microphone level inputs.

    Having a manual for the 310 is a must because there are several critical settings such as the FET bias for each band. There are also low-noise substitutes for the original 741 op-amps that would be worth installing.
  2. K5UJ

    K5UJ Ham Member QRZ Page

    He has a Bauer broadcast rig, not a D104 ham rig.

    That depends on the mic.

    For a typical modern studio or broadcast mic., you'll need a preamp and equalizer ahead of the 310. Before you go connecting the 310 to the rig, you should make up an audio dummy load -- 600 ohm resistors or a combination that = 600 ohm from the + and - audio to ground to load the 310, then put something on it to see or hear what's happening like an oscilloscope or use the 310 to drive an audio amp that has an 8 ohm speaker or headphone output -- just rig up some way to make sure the box is working okay and isn't putting out a lot of noise pops and spikes and other high peak transients or weirdness that might crap something out in the audio input stage of the Bauer. I never take an old audio box and blindly put it on my transmitter without first making sure it's clean.
    N1BCG likes this.
  3. N1BCG

    N1BCG Ham Member QRZ Page

    Right, but this thread "Broad Cast Audio limiter/compressor in audio chain for newbie" has gotten over 2100 views. A lot of other ops, even newbies, with other rigs are reading this. For those with such rigs, it's important not to drive a mic input with a +8dBm balanced output, although the 310 is transformer isolated.
  4. W2WDX

    W2WDX Premium Subscriber QRZ Page

    You know ... that is a good point to make; however, I would hope that even a newbie knows the difference between a line input and a mic input. If they don't than this whole thread and the topic it covers is way beyond their scope in the first place.

    I mean using any of this gear and not having or using the line input (or making a mod to the transmitter to have one) is a matter of fundamentals. If you do not have that knowledge in place, your chances of success is limited. Those of you who understand gain structure know what I mean. Sure, you can take a microphone, gain it up to line level, process the signal ... but to then externally reduce the signal via a pad back to mic level is an assbackwards approach with many downsides. I always modify my transmitters to have a properly designed balanced line level input for this reason, or purchase transmitters that have one.

    And again ... broadcast is one word! :D
    Last edited: Sep 3, 2019
  5. N1BCG

    N1BCG Ham Member QRZ Page

    I mention things like this based on what I've heard. There seems to be a surge of interest in AM and optimizing audio. I've lost track of how many newcomers to the mode that I've met on 75M ranging from millennials refurbishing a DX-100 and homebrewing an 810 rig to others trying out their Flexes and Anans for the first time. All good...

    But the learning curve is unavoidable. One op was transmitting squarewaves with his Flex because he fed the mic input with the line output of a mixer. He turned the mic input gain down to achieve the proper modulation, but this caused the input pre-amp to clip (it was before the attenuator in the circuit). Another tried to feed a hamfest purchase Inovonics 222 directly into a mic jack. Same result.

    I give these guys a lot of credit for at least trying, and even more credit for taking constructive advice as it was meant. Both ops now sound great as a result and visit 75M often.
  6. W2WDX

    W2WDX Premium Subscriber QRZ Page

    That's why I said it was a good idea to mention it. This is one of the reasons I bought a Flex 5000; it has a balanced line input with an XLR connector, unlike most of more recent Flex's.

    (The following I write for the aforementioned newbies reading this).

    With the single caveat regarding the specific instructions to use a pad between the 310 and a broadcast transmitter, I have never liked the idea of resistor based padding, especially when going from balanced to unbalanced, line to mic. It is more of function of impedance than simple signal reduction when going from line to mic. Even using a transformer to drive an unbalanced mic input is problematic. In the case of boatanchors and ham transmitters, exactly where the audio is inserted in the audio stage of the transmitter is the crucial factor. Does the mic input use a lowpass filter directly after the connector and how does its parameters change with an external pad or transformer? Is there also a high-pass filter in the first audio stage (mic amp) of the transmitter and how does it effect the processed signal? If using a external transformer, is its secondary properly loaded by the transmitters mic input? These factors, along with a host of others, are basically unknown or at least difficult to calculate, especially for newbies. So the resulting audio quality through the transmitter itself is less predictable. In many cases, the performance value expected of the hi-end audio gear is negated due to this unpredictable performance of the audio once inside the transmitter.

    I have a standardized method for adding a line level input to older hollow state ham transmitters. Simply put, it involves using a small (1"x 1" can) high-quality Jensen line-to-grid transformer added to the transmitters second audio stage grid input, using a proper value grid resistor to load the transformer secondary. Switching can be added to keep the mic input usable; for that ubiquitous D-104. This method seems to work quite well with low susceptibility to RFI, maintains the audio signal quality, has low distortion and noise, and keeps the gain structure in a proper sequence. I usually add an XLR connector on the back of the transmitter wired to the primary of the Jensen, sometimes adding a few matched low-value (67µH?) miniductors in series on the leads for RFI filtering.
    Last edited: Sep 3, 2019
  7. W2VW

    W2VW XML Subscriber QRZ Page

    IIRC some Junkston transmitters used a connection to the cathode of the second speech amplifier stage for phone patch duty.

    I tried this years ago and it worked FB for 0dBm .

    Modernizing old studios comes with the task of disposing of lots of those pesky UTC audio transformers which were everywhere.

    I like to stack them up in a pyramid and see how many can be pulled out before the structure crumbles.

    WA3VJB likes this.
  8. W2WDX

    W2WDX Premium Subscriber QRZ Page

    There ya go! The crucial thing IMO with most hollow state transmitters is getting past the first audio stage and injecting the signal into the second, regardless of the method or topology.

    I like the newer Jensen transformers. They are in current manufacture and can be ordered online direct. I prefer them since they are tiny, have better specs and can fit inside most ham transmitters close to the audio stages. Most times underneath where the tube wiring is. I usually shrink wrap them (to insulate) and use capacitor clamps to secure them. Not cheap though, but not as expensive (and more reliable) as the vintage stuff like UTC; the price of which has skyrocketed from the audiophools market.
    Last edited: Sep 3, 2019
    W2VW likes this.
  9. K5UJ

    K5UJ Ham Member QRZ Page

    I'll gladly take any UTC interstage transformers, or actually any UTC audio transformers you want to throw away.

    LS47 LS18 would be wonderful.
    Do they make anything that can go between a pair of 845s and 833As? High power class B audio is getting harder to do well (hi-fi).
  10. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    AFIK, no one is making production driver iron anymore. I know of a boutique outfit or two that will wind them for ouch money.

    The usual source is hoarders, I mean, collectors that seem to amass such treasures.

    Doing so in the face of many that use Hi-Fi amps, cathode followers, and other "workarounds" gets only two results:

    They secretly coerce one away from you, or just declare you a dinosaur with no credibility.

    But, the fact that a pair of big triode modulators properly transformer driven by a smaller set of them in lo-mu topology gets the ears happy in every case.


    ps...Broadcast---one word---in title now---sleep easy, boys.
    K5UJ, N6YW and WZ5Q like this.

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