Adding an output gain adjustment to Troodent Preamp.

OK Im super fresh, FYI.

I recently bought a set of Troodent PCB's and Im starting to get everything figured out before I go to my local (Amazing) electronics supply store. I would really like to add an output adjustment of some sort so I could drive this preamp before going into my converters. This seems like it would be super simple, I just am lost as to the best place to put it. Would I just put a pot in across one of the XLR pins? That seems too easy/ would color the sound, right? Again I'm super fresh, so I apologize.

The only other thing Im up in the air on this thing is whether I should add a PPM meter instead of just the peak meter. I realize this would be adding a circuit, and I haven't researched that too well yet.

I attached the schematic for reference to what Im working on.

withering_drums said:

I would really like to add an output adjustment of some sort so I could drive this preamp before going into my converters. This seems like it would be super simple, I just am lost as to the best place to put it. Would I just put a pot in across one of the XLR pins?

Click to expand...

You need an "attenuator". But a pot would not work well because it would make the output impedance inconsistent and potentially too high. As it is the circuit output impedance is very low (maybe a few ohms although it depends on the gain position). So if you just put a 10K pot across the output, then when it's in the middle position the output impedance will be 5K in parallel with 5K which is 2.5K which is too high. You want the output impedance to stay constant and not be more than ~600 ohms. You could probably use a 1K pot in which case the output impedance will be at most 500 in parallel with 500 which is 250 which is ok. But since you have a nice gain control you could just use a simple attenuator created from 2 resistors. For example, that circuit looks like the output can swing maybe 30Vpp (so +15 to -15) and your converter probably takes at most 3Vpp (1.5 to -15). So you want to attenuate by a factor of maybe 10 (20 dB). So if you want the output impedance to be low but not load the amp too much you could do something like 2K and 220R. That would make the load on the amp 2.22K and the output impedance 220 in parallel with 2K which is ~200 ohms and attenuate 20dB.

You have a gain switch that controls both the first stage and second stage gain, so do you really need more adjustment than this? Most converters have huge dynamic range compared to a mike amp like this, so perhaps just drive your converters a little more gently than you'd prefer and keep things simple? Your noise floor won't suffer much by running the preamp at a slightly lower gain than the absolute maximum you can use before clipping.

If you insist on adding a pot to this, the best place to put this would be to replace R26 with a pot, probably with a value of 10kΩ, audio taper. It already looks like R26 was a pot in the original circuit, which was turned into a simple 1k5Ω resistor in this variation, since it has a needless pair of DC blocking caps in series with it - only one blocking cap is needed for the resistor as drawn, but for a pot, you would want both send and return coupling caps to keep DC out of the pot.

The output of C89 (the negative side of the 22µF Tantalum to the left of R26) will go to the top of the pot's track, the bottom of the pot's track would go to ground, and the wiper of the pot goes  to the 10µF C89 Tantalum cap to the right of R26. (Not sure why there are two C89s, but what the heck - that's how the print was drawn).

I'm not certain that 10kΩ is the best value, but it'll be close. Again though, you really do not need a level control here - switchable gain is fine unless you deliberately want to clip the preamp and then pad it down for your converters.

Best of luck!

Ahhh OK Im seeing whats going on a little better now. So normally do Preamps with an input and output gain adjustment, its controlling the Mic Amp gain and the Line  Amp gain separately? Would it make more sense for me to seperate the 2 decks into 2 switches or am I really doing anything positive with this, or is replacing R26 basically doing that?

The PCB I have sends both ends of R26 to connector for future use to connect an eq circuit.  The original Tridents had an output gain adjustment, Im guessing this would drive the EQ circuit then.

withering_drums said:

Ahhh OK Im seeing whats going on a little better now. So normally do Preamps with an input and output gain adjustment, its controlling the Mic Amp gain and the Line  Amp gain separately? Would it make more sense for me to seperate the 2 decks into 2 switches or am I really doing anything positive with this, or is replacing R26 basically doing that?

Click to expand...

No. The two decks work together. You could make them separate controls but it wouldn't adjust the gain that much (not at all in the upper 9 of 12 positions). Using a pot in place of R26 would either load the preamp too much or add series resistance which would equate to more noise in the output stage. Also, noise performance is usually best at maximum gain so I would set it to maximum gain and then craft a simple attenuator to match the maximum level that your converter will accept.

Setting the gain in a mic pre is a balancing act between headroom, noise and distortion. In classic Class A Neve mic pres, one whole stage gets switched in only for the highest gains. Notice how in your schematic the output amp gain is one value for most of the gain switch range, only decreasing for the lowest gains.

Cheers

Ian