You can bring signal from any source you want right to the sidechain transformer. You could add a switch to select the signal source to the sidechain. For feedforward I would take the signal after U1/U2, right from the junctions of 22u caps and 2.2k resistors.
Transformerless Vari Mu Compressor build thread
- Thread starter Heikki
- Start date
Help Support GroupDIY Audio Forum:
manulaudic
Well-known member
I finished the rotary version build and I want to say that everything works perfectly ! waiting for a set of tubes now to do final adjustments.
Regarding the added trimmer on the output, would you have a recommendation on how to adjust it ?
Also regarding this operation, turning Threshold on both to max is OFF on the rotary version, is that the right procedure?
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.
Manu
Regarding the added trimmer on the output, would you have a recommendation on how to adjust it ?
Also regarding this operation, turning Threshold on both to max is OFF on the rotary version, is that the right procedure?
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.
Manu
Before adjusting the output trimmers adjust RV1 on both sidechain boards according to the instructions here. Variable Mutual Contuctance Compressor
"* 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!"
If you used the coarse gain rotary https://ghr.fi/proaudio/varimut/stepped.jpg turn the gain rotaries to position 7. Feed some signal to CH1 input, monitor the output and adjust RT1 until gain is unity. Then feed signal to CH2 and adjust RT2 until gain is at unity.
Threshold at fully clockwise position is OFF = MAX = or whatever marked on the front panel.
"* 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!"
If you used the coarse gain rotary https://ghr.fi/proaudio/varimut/stepped.jpg turn the gain rotaries to position 7. Feed some signal to CH1 input, monitor the output and adjust RT1 until gain is unity. Then feed signal to CH2 and adjust RT2 until gain is at unity.
Threshold at fully clockwise position is OFF = MAX = or whatever marked on the front panel.
manulaudic
Well-known member
Perfect, i did adjust rv1. so to optai. U
gain unity, should i have the threshold off?
manu
living sounds
Well-known member
Just realized I ordered the 200V zener twice but no 100V... However, I've got a 1.3W 100V zener here. Can I get away with using this one in the build?
manulaudic
Well-known member
I received my matched tubes. When setting the unity gain, I'm feeding a 1khz signal and measuring the voltage coming in the compressor in AC (0.394v) and matching the output is not a problem with the trims with threshold off, stack and release at fastest and ratio max with Stepped gain on position 7 (out of 12)
Now when I have the link on and lower threshold I get a difference in output from one channel to another. Is that the part I am supposed to do the 8db signal in gain reduction and adjust rv1 and rv2 trimmers on main board?
Manu
Now when I have the link on and lower threshold I get a difference in output from one channel to another. Is that the part I am supposed to do the 8db signal in gain reduction and adjust rv1 and rv2 trimmers on main board?
Manu
manulaudic
Well-known member
after more testing it's the link switch that does that. If I don't use the link and set both channels identical it's a perfect match !
any ideas?
any ideas?
Link not giving exactly same amount of gain reduction on both channels is a feature not a fault. If you don’t like it you can wire the link like described in this post.
manulaudic
Well-known member
Sweet, once again super clear build and help from Heikki and everyone else on this forum. It's done and perfect ! I'll put a few pics here tomorrow
living sounds
Well-known member
I've got the finish line in sight now, too.
So far I think this was the most fun build I ever had, thanks to the extremely well-thought-out design and the near-perfect documentation.
manulaudic
Well-known member
Here are some pics of my build !
Thanks Heikki for such a great design, the great documentation and answering every question I've had on this build
I'd like to say this is a clean build but there so many more connections from the previous pot version that I didn't really wired it in the proper order. I think it's fairly clean considering it really goes all around the boards, I definitely know it can be improved but I guess it takes one to know how to do it faster and better next time
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manulaudic
Well-known member
I have a question on the ratio values. Have I missed it somewhere? I can see the attack and release ones but not the ratio values it corresponds.
Manu
Manu
Because of the soft knee ratio increases with compression.
Here are the compression curves with ratio in min and max position.
https://ghr.fi/proaudio/varimut/6sk7compchart.png
Here are the compression curves with ratio in min and max position.
https://ghr.fi/proaudio/varimut/6sk7compchart.png
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Is fully counterclockwise on the ratio control truly 0 (1:1, I guess), or is there always some compression potential depending on how much you drive the input?
paolomunarini
Well-known member
As you can see from last Heikki's post the lowest setting is something less than a 2:1 at +20dB input.
The highest setting is something more like a 5:1...
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Oh, you are right. I read the graph wrong the first time. Thanks for the correction.
paolomunarini
Well-known member
Actually I have a general question about ratio calculation.
I did approx calculations based on dB to dB ratios.
But dB is a logarithmic scale, so wouldn't it be more correct to calculate ratio based on intensities? The scale is very similar for low ratios but a 1:4 ratio in dB is a 1:10 ratio in intensities...
Sorry about the aftermath (literally) but I'm used to rethink on my calculations.
EDIT: looks like everyone does dB/dB ratios. So don't consider my thoughts.
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