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Pentron Microphone Mixer / Preamp

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  • Pentron Microphone Mixer / Preamp

    I found this Pentron MM4 microphone mixer and would like to try using it for recording. It has 4 channels each going through one section of a 12ax7. I thought I would try the circuit as is and then simply wire 2 channels with cascading gain each through 2 sections of 12ax7 with 2 outputs instead of 1.

    I dont have much experience with this but understand some basics. I have not tested it yet. I will definitely replace the power cord with a 3 prong and most likely the filter and cathode bias caps. should the selenium rectifier be replaced? How do you do that without changing the B+ voltage?

    Any ideas or suggestions are welcome thanks for your help

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  • #2
    should the selenium rectifier be replaced?
    You should be able to use the existing rectifier,check the voltage after replacing the filter caps. If it's 160V you're good. If it were mine I would use a full wave bridge. The Voltage would be almost the same, with less ripple. With a bridge you could use smaller filter caps.

    The inputs are high Z. If you are using low Z mics you can get a cheep transformer like this one.
    Shure A85F - Microphone Matching Transformer A85F B&H Photo


    • #3
      What mics are you using it with? If hi-z unbalanced mics this circuit should work ok but not terrible quiet. There seems to be a mistake on the print, the summing resistors all need to be the same, the first one is 330 ohms instead of 330k.
      If you want to built a good preamp for balanced pro mics, the main thing missing is a high quality mic to grid transformer. Bad ones are cheap, good transformers are expensive. More than another use of a transformer in audio(interstage, output, etc) the mic transformer determines the quality of the rest of the circuit to a greater extent. Double shielding for example is omitted from consumer quality input transformers but gives up quite a bit of signal to noise ratio.
      For most semipro recording the solid state balanced line ICs are far cheaper and have better performance and CMR than all but the very best input transformers. Analog Devices SSM series of chips are orders of magnitude lower distortion and noise with higher CMR and PSRR than probably anything you can build for less than $300 a channel. Here is a low cost example about $2: 130db CMR and 124 PSRR!! and single resistor gain setting.
      If you are using a balanced line output, their transformerless diff line driver is $3.
      Your summing amp can be another SSS2019.
      So, what mics and what level quality are you planning on?


      • #4
        Yeah thats a mistake on the schematic, its 330k

        I have a very basic home recording setup basically a couple of SM57's and a Studio Projects C1 condenser (which obviously needs separate phantom power). So ideally some combination of those for acoustic guitar, & vocals. I have a two channel Symetrix solid state preamp that suits my needs for electric guitar.

        Balanced ins would be a huge plus though because I could run longer cables and have more flexibility with mic placement, right?

        I'm tempted to modify this as a 2 channel preľ channel one for balanced mic in, channel two for recording bass direct in.

        Any suggestions? thanks for all of your useful info...


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