GHR Audio

Turn trimmer RV1 from both sidechain boards to the most clockwise position and measure the voltages

Voltages for the rotary sidechain version here!
* Measure the voltage and then turn the trimmer RV1 on sidechain board counter clockwise until the voltage is approximately 0.1V greater than the original value. If the voltage originally measures 3.9V then turn the trimmer RV1 on sidechain board counter clockwise until it is 4.0V. Do this measurement and adjustment for both channels!

0dB led should be always lit when the power is on. Rest of the leds should only light up when compressing except there might be light glow on 1dB led when at idle.

The latest revision of the led board has a trimmer to adjust the 1dB led. Adjust the trimmer until there is only light glow on the 1dB when not compressing.


Tube Balance

For checking the tube balance audio interface and software like ARTA or Room Eq Wizard are needed. Or use your DAW if you have a signal generator plugin.


Channel 1

Switch link on.

Turn threshold to max on both channels

Turn attack and release on both channels to fast position.

Turn ratio on both channels to max.

Feed channel 2 input with 50Hz signal. Do NOT feed any signal to channel 1 input!

Turn channel 2 threshold down until 8dB led lights up on both channels.

Measure output voltage from channel 1. Adjust balance trimmer RV1 on main board until lowest output voltage is found. Output level should -40 dBu or even much less if the tubes are in good balance.


Channel 2

Switch link on.

Turn threshold to max on both channels

Turn attack and release on both channels to fast position.

Turn ratio on both channels to max.

Feed channel 1 input with 50Hz signal. Do NOT feed any signal to channel 2 input!

Turn channel 1 threshold down until 8dB led lights up on both channels.

Measure output voltage from channel 2. Adjust balance trimmer RV2 on main board until lowest output voltage is found. Output level should be -40 dBu or even much less if the tubes are in good balance.


Another method of testin tube balance

20 kHz - 1s on 1s off wav file
https://ghr.fi/proaudio/varimut/20k.wav

Turn attack and release to fast position. Turn ratio to max. Play the audio file and and adjust the volume and threshold so that 3 dB led lights up and listen for audible "thumps". Adjust balance trimmer for minimal "thump".

It could be cool to add some other tests to verify things are working as they should. I can't think off of the top of my head what was useful but can look through my notes/correspondence.

Here's one for testing compression and as it relates to the leds on the metering. Feel free to correct this if wrong.

Feed 1 kHz signal to the compressor and monitor the output. Turn threshold to max/off position. Take note what the output is, let’s say for example it is +4 dBu. Then start to turn threshold towards min until the output is + 3 dBu, now the 1 dB led should be lit. Then turn the threshold towards min until the output is +1 dBu, now the 3 dB led should be lit. Then turn the threshold toward min until the output is -1 dBu, now the 5 dB led should be lit. And so on

Testing // matching / calibrating tube gain

This should help find tubes for better matching.
.
1st step is to trim SC CV so that it rests at (i.e. slightly above) required SC out junction, effectively calibrating the sidechain CVs. Then:


"If you need some (minor) unity gain calibration -- and instead of putting trimmers on the outputs (which also requires additional changes) , and also instead of offsetting the CV voltages for this -- you could install four 100k trimmers into the 'empty' parallel resistor slots as in the picture (looks a bit ugly in my case but works).

This allows matching the gain of the four individual tubes before the signals go thru the INA stages.

I matched (up to 0.4dB of difference between tubes before) with mono test signal into L and R channels and no compression dialled in. The trade-off is minor.

Mind you though, once compression is happening, it will all be off again due to non-perfect tube behaviour -- but hopefully better than without."

"

I think it was brought up to me in the past but, what are acceptable input levels for this unit . I'm guessing it's determined by what the input INA chip can handle?
The gain pot's are after and would affect the tubes so this would be the best place to try and push level for effect? But this would be limited to some degree by the output chips?

Hope that makes sense.
Thanks!

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 to make it distort audibly.

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.

Thanks. No worries on the post.
You answered the question I had.