GHR Audio

Here’s an alternative for the wall wart
https://www.tme.eu/fi/details/ge40i36-p ... mean-well/

You can use different wall wart voltages if you change some other components like I did here.
viewtopic.php?t=74

Main board BOM

Here's a general replacement for input transformer in vari mu compressors. varimuinput.pdf This won't work for Fairchild 660 because power supply max is +/- 45V. For Fairchild you would need opamp with higher power supply voltage. Here are the KiCAD design files varimuinput.zip - Picture of the boar...

U3 pin 3 is the "center tap" where the control voltage goes. Since U3 is a buffer you wouldn't need power amplifier for the sidechain. Yes U2 pin 13 & 9 go to the grids. Fairchild input transformer has large step up ratio, U1 could be replaced with differential amplifier with adjustabl...

Heikki. could your transformer less input circuit work in the 670 Fairchild comp. Short answer is no. Slightly longer answer is maybe. Assuming you would keep rest of the 670 without any changes you could use something like in the picture below. Except Fairchild has very large control voltage which...

It's very difficult to make the chips clip. Input should take more than 26 dBu before clipping. Before you can get the output stage to clip you're most likely clipping what ever it is connected to.

Scott I accidentally deleted your last post and I don't know how to return it.

INA input chips are the limiting factor for max input level. If your gain is higher than unity, then output chips will clip before the input. Since there are no volume pots after the tubes it's hard to get any significant distortion from the tubes. It was designed so that it's difficult for the user...

Even when threshold is at max, large enough input signal will start gain reduction.

For GR indication, I ran a cable on underside of board from sidechain CV1 out to onboard J13 (CV1 VU meter driver input). Stuffed R93 and R94 with resistors equivalent to VU meter resistance. Looking good for now. Value of R93 and R94 will depend on what meters are used but probably resistor equali...

In the schematic they are just blank resistors so I was looking for confirmation that I have these shown correctly. Those values look good to me. If your timing cap is 1uF your attack is approximately variable between 1 ms and 50 ms and release between 50ms and 550ms + attack time. What vu meter sp...