Happy New Year!
I have a pair of these Gates "The Yard" Console Mic Preamp Cards that I would like to rack up. These were originally fixed gain with a trim following the output. I would like to change that obviously and make it into a standard, standalone circuit. I already have a few things planned out but I had a few ideas to throw around so I welcome all input. My planned mods are:1. -20db pad switch
2. Phase switch
3. Soft-start phantom power
4. MILA-style solid state power supply
5. XLR I/O
6. 1/4" Hi-Z Direct Input
7. Rewire output transformer for 600 ohm
Now for gain control I had a few ideas and I'd like to see what would be the best way to go about it. First off I'd like to replace R5 (220K fixed resistor) with a 250KA Pot.
Then I'd like to make the feedback adjustable somehow. I was thinking of either a single switch, multi-position switch, or potentiometer in the feedback loop replacing R1 (56K) and possibly changing C4 (.47uf) with a larger value.
What is a "MILA-style solid state power supply"? I've searched and googled it, to no avail...
For the volume control, I would be asking whether I wished to try and maintain factory specs with regards to fidelity, distortion etc. If so, using a 3 position 10, 15, 20db or finer resolution pad on front would be the cleanest way. If you wanted more of a nuke n smash amp it may not matter.
I did that interstage volume control mod once, before I knew better. You can't, since it's inside the feedback loop. Feedback variation is your only option. FB is high here, like 25 dB, and turning an interstage down actually decreases input headroom while decreasing output volume and increasing distortion.
Grid pot in the first stage possible too, leads need to be short as possible. Few inches total.
Seems to be almost mandatory, although I would suggest you add a 1/4" TRS to the output.
"What is the as-is circuit not doing that I wish it to do?"
You are thinking way too much. You need to wire this up on the bench, and get proof of practical performance.
Will: 2x 470R with 220R across them, then add 2x 1k in series with 470R, and then 2x 1k5 in series with 470R and 1k work as roughly -10db, -20db, and -30db pads respectively?
If you look in the pad thread from the above link you will see a formula that will give you the exact db attenuation value of these or any resistor values you choose.
If you use that PSU circuit, take care not to forget that the filament supply is floated at HT potential.
QuoteYou are thinking way too much. You need to wire this up on the bench, and get proof of practical performance. Of course, I always overthink! I was just looking for a good place to start and looking for parts I need to order. Of course I'm going to check it out before I wire it up as a finished piece. Now I'm thinking I only need the -20db pad on a regular DPDT switch, but I still think the 6 position gain control switch could be cool. It gives me variable gain and the ability to remove the feedback completely while cathode bypassing the first stage. Are my guessed values no good?
> a pentode based preampLook where C2 is connected. V1 is NOT working as a Pentode. If we assume V2 has gain of 0.5 from grid to cathode, then V1 works as a triode with Mu of 21/0.5 or about 40. This gives V1 gain near 30. If V2 is loaded in 20K(?), it has gain near 12. Open-loop gain 30*12= 360. Closed-loop target (56K/1.2K)+1= 47. Feedback margin 360/47= 7.5 or 17dB. This will take-out 10+ dB sag, or take the 5%THD point down below 1%THD.