Transformerless Vari Mu Compressor build thread

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
402
Location
Finland
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.
 

manulaudic

Well-known member
Joined
Nov 7, 2011
Messages
358
Location
Finland
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
 

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
402
Location
Finland
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.
 

manulaudic

Well-known member
Joined
Nov 7, 2011
Messages
358
Location
Finland
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.
Perfect, i did adjust rv1. so to optai. U
gain unity, should i have the threshold off?

manu
 

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
402
Location
Finland
1.3W 100V zener will work just fine. 1W zener would be a little too close but I wouldn’t worry with 1.3W zener there.
 
Last edited:

manulaudic

Well-known member
Joined
Nov 7, 2011
Messages
358
Location
Finland
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
 

manulaudic

Well-known member
Joined
Nov 7, 2011
Messages
358
Location
Finland
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?
 

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
402
Location
Finland
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.
 

living sounds

Well-known member
Joined
Jul 26, 2006
Messages
3,446
Location
Cologne, Germany
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
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
Joined
Nov 7, 2011
Messages
358
Location
Finland
DSCF2146.JPG DSCF2147.JPG DSCF2148.JPG

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
 
Last edited:

manulaudic

Well-known member
Joined
Nov 7, 2011
Messages
358
Location
Finland
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
 

Script

Well-known member
Joined
Nov 27, 2008
Messages
1,889
Location
Japan
With soft knee, numerical ratio 'values' really do not mean much. I tend to label from '0' to '+'. Also, the harder you drive with Input Gain, the more 'value' you get.
 

JMan

Well-known member
GDIY Supporter
Joined
Oct 26, 2019
Messages
573
Location
Midwest, US
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

Active member
Joined
Aug 8, 2021
Messages
34
Location
Italy
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?
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...
 
Last edited:

paolomunarini

Active member
Joined
Aug 8, 2021
Messages
34
Location
Italy
As you can see from last Heikki's post the lowest setting is something less than a 1:2 at +20dB input.

The highest setting is something more like a 1:5...
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.
 
Last edited:

Latest posts

Top