Transformerless Vari Mu Compressor build thread

[paolomunarini]

Very interesting project. Do you have PCBs available in EU?

Edit: approaching the build.

I found this source for the 6sk7. Does anyone have experience with reichelt?

https://www.reichelt.com/it/it/elektronenroehre-pentode-octal-8-pol-6-3-v-tube-6sk7-p258579.html

 

[Heikki]

Here are some suggested modifications for mastering use.

 

[steves.mastering]

That’s fantastic, I was going to start working on adding stepped controls
Looking forward to getting started on this.

 

[jarvis]

Very interesting project. Do you have PCBs available in EU?

Edit: approaching the build.

I found this source for the 6sk7. Does anyone have experience with reichelt?

https://www.reichelt.com/it/it/elektronenroehre-pentode-octal-8-pol-6-3-v-tube-6sk7-p258579.html

I've only had good experiences with them. I actually lived not too far away from their shop, so apart from ordering online I've also picked up parts in person. They were nice, easy to work with and have been delivering fast and reliably.

But I have no idea about their quality of service outside of Germany.

Best
Jannis

 

[paolomunarini]

Thanks for the feedback man.

I ordered 4 tubes for 15€ shipping included. I'd say it was a bargain!

I'll post here if they are worth it!

 

[manulaudic]

I've only had good experiences with them. I actually lived not too far away from their shop, so apart from ordering online I've also picked up parts in person. They were nice, easy to work with and have been delivering fast and reliably.

But I have no idea about their quality of service outside of Germany.

Best
Jannis

I have had also only good experience with them and just ordered 8 tubes from them last week to have for this compressor and future build.

 

[alphasnk]

Yes Reichelt is great, if there is a problem they'll take care of you!

 

[manulaudic]

For those in Finland, Uraltone has the 6sk7gt on discount for 3eu a piece.

 

[manulaudic]

My next question to Heikki and everyone else here. can the stepped version be considered a Mastering grade vari mu? I'm considering building one for a friend, matching resistors across the side chain boards etc... Would that be something he could consider using with reliability on his mastering ?

tHanks

 

[steves.mastering]

For those in Finland, Uraltone has the 6sk7gt on discount for 3eu a piece.

good spot, and only 5 left in stock. Someone jump on them quick.
I was gonna go for them myself, but I really want to get a set of 4 matched tubes and can’t seem to find matched sets that’ll send to the UK.
Anyone know of a good replacement that is still in production?

 

[steves.mastering]

My next question to Heikki and everyone else here. can the stepped version be considered a Mastering grade vari mu? I'm considering building one for a friend, matching resistors across the side chain boards etc... Would that be something he could consider using with reliability on his mastering ?

tHanks

I’m planning on building one of these with stepped switches and like you said matched high quality components throughout for mastering purposes. I see in Heikki’s notes that being a tube style compressor, variances can happen due to tubes all being different. That’s why I’m hoping to find a matched set of 4 tubes in hope to get it tightly calibrated on both channels

 

[Heikki]

My next question to Heikki and everyone else here. can the stepped version be considered a Mastering grade vari mu? I'm considering building one for a friend, matching resistors across the side chain boards etc... Would that be something he could consider using with reliability on his mastering ?

tHanks

With the modifications suggested here it should be as reliable as any mastering gear with tubes.

good spot, and only 5 left in stock. Someone jump on them quick.
I was gonna go for them myself, but I really want to get a set of 4 matched tubes and can’t seem to find matched sets that’ll send to the UK.
Anyone know of a good replacement that is still in production?

There are no substitues for 6SK7 that are still in production but there should be plenty of 6SK7's left in the world. I doubt anybody sells matched 6SK7 tubes anywhere or not at least at reasonable prices. I suggest buying few extra tubes so you can find good enough matches yourself.

I do have two or three hundred old soviet 6SK7 tubes in boxes sitting on a shelf. I do plan at some point to get around matching them for compressor use and selling matched pairs if anybody wants to buy them. It would be a time consuming effort and it might be the price I would end up selling them might not be reasonable.

 

[steves.mastering]

I’m down for 4

 

[scott2000]

Would that be something he could consider using with reliability on his mastering ?

tHanks

Yes. The additional suggested mastering modifications were actually born out of a visit with Bob Katz here in FL who spent some time with it but these were in some ways specific to his workflow . That's it open on the table in the one pic before he ran it through a series of tests.

 

[steves.mastering]

Yes. The additional suggested mastering modifications were actually born out of a visit with Bob Katz here in FL who spent some time with it but these were in some ways specific to his workflow . That's it open on the table in the one pic before he ran it through a series of tests.

Wow Bob Katz the legend. I’m just re-reading “Mastering Audio, the art and the science”.

well done to everyone who’s managed to make such an incredible project that’s even been used by legends

 

[manulaudic]

That's just fantastic ! Thank you guys

 

[duende]

Is it possible to have boards or gerber files ?

Thanks.

 

[Heikki]

Everybody please read the first post.

Anybody who wants to buy boards please send me a PM or email. If you are in America and want to buy boards send PM to Scott.

 

[Heikki]

Here are some suggested modifications for mastering use.

It's probaply best to explain the modifications here too. The idea is that you have unity gain when the gain rotary is at middle position. From the middle position you can adjust the gain between -5.5 dB and +5.5 dB in 0.5 dB steps. The sidechain steps are changed to 1 dB steps and a switch is added to get to get 10 dB more range from the 12 step switch. Step 12 (fully clockwise) is always off position.

U9 and U10 are changed to INA2137. This is related getting the gain to unity when gain is at middle position. R43, R44, R45, R46 are changed to 330 ohm. RT1 and RT2 with the 1k resistors are added to make make and adjustable U pad for the outputs. These trimmers are used to adjust the gain to unity when the gain rotary is at middle position. This also makes it much easier to balance the gain between channels because there's no need to mess with the bias trims to balance the gains.

R39, R40, R41, R42 are changed to 1 ohm to make sure the +15V and -15V voltage doesn't sag to U7, U8, U9, U10, now under heavier load.

I do not recommend doing these changes for any planning to use compressor in tracking or perhaps in a mix bus. Potentiometer sidechain is more than adequate for plenty of uses. I do personally prefer this coarse stepped attenuator to a gain pot.

Edit: Updated version of the mastering modifications.
https://ghr.fi/proaudio/varimut/layout/stepped.pdf

 

[Heikki]

I recently got an email from somebody wanting my KiCAD design files for this project. I responded in a manner that I thought was appropriate for somebody wanting something for free on which I have spent a lot of time and money on. I really like the part where he threatens to shoot me with a gun.

Here is his second email.

[The design files will cost 15000 (fifteen thousand) €] -- I do not know if you are just "kidding around", being sarcastic or if you are just inherently an ******* *******!!!

[Will you be paying by paypall or do you want an bank account number to pay to?] -- The attitude of yours that I detected in the beginning of your response makes me feel more like paying you by means of a "Smith & Wesson"!!! If you don't know what that is, look it up.

[you can also design your own boards from scratch] -- Which is what I was planning on doing.....anyway!!! However, I was planning on designing either a 4-layer or a 6-layer PCB and placing the vacuum-tube B+ and heater voltages on the inner-layers. Doing something like that is well beyond the "usual DIY'er" on the GroupDIY forum, but I have designed up to 16-layer PCB's for NASA that were used within their space signal-processing instrumentation and defense contractors for circuit boards used within U.S. fighter jets.

In case I didn't make myself clear, I was only requesting to receive your KiCAD schematics files so I could start from that point and then update the schematic with PCB-mounted XLR connector footprint data and rearrange the schematic layout in such a manner as to either eliminate or significantly reduce the amount of physical wiring that I saw within your photos. Whenever I am designing a piece of electronic equipment, the approach that I take is to try and have as little wiring as possible. I was only wanting to obtain your KiCAD schematics files to use as a "starting point" instead of me having to re-create what you already have in KiCAD. But, apparently.....that ain't gonna happen!!! THANKS!!!

I am so sorry that I even contacted you. SHEESH!!!

Anyway since the schematics are free and available for all the world to see anyone with the necessary skills can design their own boards from them and do what ever they please with them. Just don't expect me to give my KiCAD design files away.