Transformerless Vari Mu Compressor build thread

[paolomunarini]

If I can, I open a quick discussion.
I have two spare 2x23 rotaries. I was wondering... What if I create two resistor ladders on each wafer with different gain steps and with a DPDT toggle I switch between the two ladders?

In other words, I wire one of the wafers to be a 0.5dB step atten (for mastering) and the other one to be a 1db or more (for tracking).

What do you thing about this idea?

 

[paolomunarini]

I post this here because it could be of interest for the life of the project. Texas Instruments is discontinuing INA2134 and 2137 line receivers in the DIP14 package.
They will only be available in SOIC14 package in the future. If you are planning on one unit go for it right now Or be prepared to adapt SOIC to DIP...

Dates from the discontinuance notification:

• Last date to order: March 7, 2024
• Last date to ship: Sept 7, 2024

 

[Making_Waves]

Anyone have any boards to sell for this and can ship to the US?

 

[scott2000]

Anyone have any boards to sell for this and can ship to the US?

Yes. I have some extras.
You can pm me

 

[Making_Waves]

Yes. I have some extras.
You can pm me

Awesome, will do!
Edit: I might still have an account restriction that prevents me from sending a PM. Maybe it'll get resolved soon? Would you be able to PM me?

 

[scott2000]

Sent PM..

BTW for everyone else, there are some transformer bypass boards available that use the UTM 2571 and will be included with a set of boards. They are relay bypass that allow the option of switching/using a transformer at the input or not.
Been meaning to make a listing for the few sets I have extras of in the bm but am lazy. Transformer boards are black in color.

Should mention that I'd like to give anyone in the US who already has the vari mu boards the option to get these if they want. ...
(2) boards shipped to continental US for $7. They have to go parcel because they're in rigid mailers.

PM if interested...

 

[flyingeyepro]

This build looks very interesting. Might try to do it with analog meters for my own tastes.

 

[thelivingroom]

I post this here because it could be of interest for the life of the project. Texas Instruments is discontinuing INA2134 and 2137 line receivers in the DIP14 package.
They will only be available in SOIC14 package in the future. If you are planning on one unit go for it right now Or be prepared to adapt SOIC to DIP...

Dates from the discontinuance notification:

• Last date to order: March 7, 2024
• Last date to ship: Sept 7, 2024

Thanks for the heads up! Just received the boards and won’t realistically start this until next year sometime (gotta finish my Orange 86 and D-LA2A first - both are getting close to the finish line), but I’m going to order these and some of the other parts, like the DC-DC converters, while they’re in stock now.

 

[duende]

Hi,

I have these boars since a while and i will starting to build this compressor (Mastering version), but i have some questions :

- Where can i find a matched set of tubes for this project ? Or a substitute ?

- Regarding the psu, what's the best option/performances ?

Thanks.

 

[Script]

You might try to ask Heikki himself through his website. Not sure though he ever got around to sorting, matching and offering tubes.

Other than that it's best to buy a couple extra tubes (via an evolbay etc of your choice) and test in circuit. 6SK7 are comparatively 'cheap'.

Maybe also search for 100k trimmer in this thread -- additional trimmers for adjusting gain right at the tubes. Worked for me.

As for psu, Heikki's suggested solution is quite nifty, except I don't like external power bricks much. The 15V rails only give 100mA if I remember correctly. So as long as you do not plan on adding extra stuff to the circuit (like me), you'll be fine. Alternatively could hook an extra single rail off from after the first power transistor (is what I did).

 

[Script]

Oh, and if you use shorter standoffs for the PCB, it might fit into a 2U case. Very much depends on the case though (need to test before committing) and will likely need extra ventilation slots/holes. Over here it fit by 3mm into a repurposed case. Front assembly was a pita though.

 

[duende]

Thanks,

I contacted Heikki here but no response. What's his website ? Are you talking about the backup files website ?

I'm really a newbie concerning tubes, it's my first tube building, so sorry for my questions... Is unmatched tubes a problem just about gain differences or also in frequencies linearity or others domains ?

External PSU is not a problem for me, i want the less noisy solution. I think i remember a pic of this unit with a toroidal transformer ?

3U is ok for me, i already have a 3U face plate, the only problem for me on this project is the space between knobs, i would have liked more space to put bigger knobs and silkscreen, and vu meters...

 

[Script]

Yes, contact thru 'backup file' website.

Unmatched tubes can give differences in gain both (1) when no compression is happening ('idle') or/plus (2) when attenuating ('curve'). You need to go thru unit calibration, before you will notice.. Just follow the calibration instructions.

I added the 100k trimmers to adjust for differences at 'idle', so that I only had to find tubes with acceptable variance in 'curve'. (The trimmer on the CV board thus serves mainly to adjust CV on the sidechain board itself, not the tube CV feed).

I finally added a diode link at the sidechain out for stereo operation, but with above trimmers and tubes, the difference is almost negligible -- the unit gives soft compression anyway.

If you are new to tubes, maybe just build as per build instructions first. Adding the 100k trimmers later is easy enough. I started with four NOS tubes, unused, and only felt the need to buy another two cos one was off by too much (so four tubes out of six).

Could wire the rotaries. Would allow you to place wherever you want -- but many wires = a lot of time.

 

[duende]

Yes, contact thru 'backup file' website.

Unmatched tubes can give differences in gain both (1) when no compression is happening ('idle') or/plus (2) when attenuating ('curve'). You need to go thru unit calibration, before you will notice.. Just follow the calibration instructions.

I added the 100k trimmers to adjust for differences at 'idle', so that I only had to find tubes with acceptable variance in 'curve'. (The trimmer on the CV board thus serves mainly to adjust CV on the sidechain board itself, not the tube CV feed).

I finally added a diode link at the sidechain out for stereo operation, but with above trimmers and tubes, the difference is almost negligible -- the unit gives soft compression anyway.

If you are new to tubes, maybe just build as per build instructions first. Adding the 100k trimmers later is easy enough. I started with four NOS tubes, unused, and only felt the need to buy another two cos one was off by too much (so four tubes out of six).

Could wire the rotaries. Would allow you to place wherever you want -- but many wires = a lot of time.

Thanks,

What to the added diode at the side chain ?

Yes i could wire, not really a problem but i prefer avoid wires when i can solder roataries directly on board.

I hope to start this compressor, very interesting and reliable project.

 

[Script]

Please post some pictures when ready.