GHR Audio

The LEDs are not for just indication. You can put one of the LEDs on the front panel and wire it to the board if you want to use it for indication. That three LED string needs to have 2 red LEDs and one green to get the right voltage across them.

The LEDs are there to help set the proper bias voltage for the gain reduction diode strings. When adjusting RV1 at some point the LEDs should start lighting up and that's a good starting spot where to set it.

For quick test so far:

Didn't have 4004, so I installed UF4007 for D73 and D74 sidechain bias -- should be OK, no?

I trimmed the SC until LEDs are barely visible lit up in the dark.
Then measured after R33, yielding 4V62.
Then measured directly at the diode junction, yielding 2V2x.
I finally trimmed to 2V2 sharp at the junction.

For the other channel,
I trimmed so that directly at junction it reads 2V2 too and then checked after R34 for 4V62.
It was 4V63 so I adjusted it to 4V62.
Both measuring points now read equal across channels.

It would be nice to have junctions there for easier measuring


As for RV3 and RV4 (500R trimmer) I installed B1K with 1K across to wiper. This yields 500R when fully turned and also bends the pot law. Just a 1K might be fine here too, I assume. This is the Threshold sitting before the rectifier, but it's a trim on the PCB for fixed installation. That's a nice option but it was slightly confusing at first. Having parallel pads here, maybe, for connecting a pot or rotary and/or for precision-bending might be nice, I think.

For first listening test:
ATT at 1K, fastest, effectively bridging a face plate control.
REL at 47k, also at fastest, effectively bridging a face plate control.

For unity gain (J9 to J10), it sits somewhere around 5K.

For RATIO I installed B10k for now.

But a bit weird maybe, cos with ratio dialled to zero, when THR is dialled in, unit starts limiting ever so slightly...

Got it:
I was driving the input rather hot. Cool, useful function ! For the 1k on the ratio string to ground.
Something like LED starts blinking when feedthrough is (too) hot -- depends, of course, on what gear is hooked down the chain.

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.

Flicker LED indicator is onboard.
VU meter driver is onboard.
LED meter would require external board.

For diode string, I used 1N4448.

Matching of diodes is not required, they should average out. Still, from a batch of 100 diodes, I sorted them using a simple DM diode tester. Ended up using the 64 ones that measured closest to one another.

Also all resistors in parallel positions across channels and in differential path positions are hand-matched.

Just as long D73 and D74 are the same type of silicon diode the exact type shouldn't matter too much.

If RV3 and RV4 are 1k instead of 500 ohm it will change the threshold range.

Exact type of diode shouldn't matter in the diode string and any silicon diode should work. Again just as long as they are the same type.

I suggest measuring distortion on both channels to make sure it's about the same on both channels when gain reduction is happening. I had one diode the wrong way on one of the diode strings and it caused large distortion.

On right side of the schematic there are resistor values for 1 dB steps for the gain.

Value of R93 and R94 will depend on what meters are used but probably resistor equaling the meters resistance is good starting point. With some meters D83 and D84 might have to be removed.

You don't necessarily have to measure anything when adjusting RV1. Just set it where the LEDs are starting to light up.

The more accurate way to set it is to feed the channel input 1kHz signal and adjust threshold to make sure there is NOT any gain reduction happening. Monitor the output and adjust RV1 and you should see the gain dropping when the LEDs start lightning up. If you're feeding +4 dBu signal to the input and have the gain set at unity, then output should be +4 dBu too. Adjust RV1 just to the point where the output drops to +3.9 dBu. Then slowly turn RV1 back just to the point where the output is back at +4 dBu. Then do the same thing to the other channel while adjusting RV2.

Heikki wrote: ↑Thu Jan 18, 2024 7:21 pm If RV3 and RV4 are 1k instead of 500 ohm it will change the threshold range.

Yes, 1k gives deeper threshold. Biur 500R is enough, I think, at least for now.

GAIN rotary
I looked at the gain rotary today. Could you please share ideal resistor values, meaning not rounded to E standard? I checked by typing the values into excel. Looks like there are two 'kinks' between steps. That's fine as long as it's on both L and R. Yet, getting unusual resistor values is not too difficult when putting two resistors in parallel.

I'll need to check if I still have the ideal values somewhere. The E standard values should be pretty close to 1 dB per step.