Mix Bus Compressor from scratch

[DaveP]

I didn’t think I would be around to start another “from scratch” project, but it seems I am in remission, so life goes on as normal. My last from scratch project was a departure for me to make a basic transistor amp, but it was a total failure! Since then I have rebuilt a Marantz Hi-Fi amp with a new circuit and it works beautifully so confidence is restored! I will post that separately.

My friend asked me for a simple mix-bus compressor using some tube circuits that have given him some good results in the past. It won’t need much more than about 3dB compression, so this opens up other options that we couldn’t use for deep compression. The main effect will be to warm up the sound with transformers and tubes and to glue the mix together.

I have chosen to use the special EF83 tube for gain reduction, it maintains a near constant anode current flow down to -10V, so it should not “thump”. This tube was used in Grundig tape recorders for many years and it does not need to be push-pull or work into an interstage TX. I have attached the spec sheet and what I'm referring to is on page 10.

I start these projects with a scale drawing of the front panel, sorry the contrast is not very strong.



This helps me to calculate the width and height of the box behind the front panel, then I can start cutting metal to size.



As you can see, it will be a 3u box, I found a couple of nice vintage german VU meters which will be fitted where X marks the spot.

best
DaveP

 

[Spiritworks]

Glad to hear you are doing well.

 

[rock soderstrom]

I have chosen to use the special EF83 tube for gain reduction, it maintains a near constant anode current flow down to -10V, so it should not “thump”. This tube was used in Grundig tape recorders for many years and it does not need to be push-pull or work into an interstage TX. I have attached the spec sheet and what I'm referring to is on page 10.

Very interesting project, as always. I'm pleased that the EF83 is being used, I have a lot of them lying around, mostly Valvo.

Yes, they were used as AGCs in slightly better equipped Grundig tape recorders in the past (from which I harvested mine). Nice curves in the data sheet, similar to the EF86, built for audio.

 

[zamproject]

@DaveP nice to see you'r back !
As always I will follow this one

Cheers
Zam

 

[ruffrecords]

Interesting tube. Is it used as a pentode or as a triode in gain reduction applications?

Cheers

Ian

 

[MidnightArrakis]

Anxiously "Standing By....." for a completed schematic!!!

Gonna -- LOVE -- seeing the VU-meters!!!

Will the power-supply also be tube-based? Or, a more modern design?

/

 

[DaveP]

Interesting tube. Is it used as a pentode or as a triode in gain reduction applications?

Hi Ian, it's always used as a pentode, I'm not sure the constant current effect would work as a triode, although it does do more limited gain reduction in that mode. I will post some test results later

best
DaveP

 

[DaveP]

Glad to hear you are doing well.

Thank you guys, it was a surprise to me and the doc, he said that sometimes cancer goes dormant, but he does not know why! The chemo was pretty rough but it did the job.
best
DaveP

 

[DaveP]

Midnight, don't get too excited, they are only vintage 1960's, not 1930's!



best
DaveP

 

[DaveP]

These are the results of my tests on an EF83:



100k resistor on both anodes, 390k for g2 in pentode mode

I would have needed more negative voltage for the triode option and there was the uncertainty about losing the constant current feature of this tube. This is why I stuck with the pentode mode.

best
DaveP

 

[DaveP]

This is the side chain circuit:



best
DaveP

 

[DaveP]

This is the provisional circuit, it may need adjustment later on.



best
DaveP

 

[zamproject]

single or dual side-chain ? what do you sum ? audio or CV ?

Little suggestion, for a -mix bus- comp I think an -auto- position with 2 or 3 RC network is useful.

 

[DaveP]

single or dual side-chain ? what do you sum ? audio or CV ?

It's a single side chain that sums audio.
All my gear has those timings so my friend is used to them, but its a good suggestion though
best
DaveP

 

[zamproject]

It's a single side chain that sums audio.

So some issue may occur with out of phase L/R material, but usually at master bus / final mix this is properly handled.
I'll be more concerned with mid emphasis compared to side, maybe you should implement a high-pass filter to dial with this ?
Like a switchable C6 ?

Best
Zam

 

[DaveP]

The question is "where does it stop"? My brief was transformers and three or four tubes, now we have six. my friend often puts audio through boxes just to get some tube sound so this may well do the job as it is. Another switch might fit in the front panel, we shall see!

best
DaveP

 

[rock soderstrom]

Why the rare 6DT8? You probably have some of them in stock. They are easier to wire for 6V heaters, but can surely be replaced by 12AT7/ECC81?

 

[ruffrecords]

It is not clear to me how the side chain works, especially the threshold control. Can you elaborate?

Cheers

ian

 

[DaveP]

Rock, you are right, the 6DT8 is easier to wire and it also has a higher max heater /cathode voltage than the ECC81, which makes it ideal for these circuits.
best
DaveP

 

[DaveP]

It is not clear to me how the side chain works, especially the threshold control. Can you elaborate?

There is a voltage divider from the HT and the threshold control sets the voltage where the tube starts to conduct. If a peak overcomes the threshold, it starts to generate negative DC via the cathode follower. The stage is fed from the output tube which has higher peak to peak voltages than the rms voltages at the output. I hope all that makes sense.
best
DaveP