My First Compressor/Limiter

[chrispbass]

Great work Ian, really interesting reading your analysis
Best
Chris

 

[ruffrecords]

Prototype tube FET PCbs arrived in the post today. I will build one up and check I can get dc out of the side chain first.

Cheers

Ian

 

[chrispbass]

Prototype tube FET PCbs arrived in the post today. I will build one up and check I can get dc out of the side chain first.

Cheers

Ian

That's quick Ian. Who do you use for the PCB's?
Best
Chris

 

[ruffrecords]

Seeedstudio

Cheers

Ian

 

[chrispbass]

Thanks Ian

 

[ruffrecords]

Thanks Ian

Seedstudio used to be quite slow. Production was quick but the low cost Singapore post option I chose used to take several weeks. Not a problem because I am usually not in a hurry. However, in the last month or so the same option costs a little more but is now delivered as a tracked parcel and they seem to get here much quicker.

Cheers

Ian

 

[chrispbass]

Thanks for the extra info Ian

 

[MountCyanide]

Greetings Ian-
Did you ever get this compressor up and running?
Definitely sounds like the type of unit I'd be interested in getting into.

 

[ruffrecords]

I had some prototype PCBs made but I still have not got round to  building one. Last year was I was so busy building lunchboxes, Classic Solos and Tube Tone boxes for other people I had no time for new development. The only thing I am building this year for customers is the mastering EQ so I can catch up on the MKIII mixer design.

Cheers

Ian

 

[MountCyanide]

Ok, gotcha - pretty much figured that was the case.
I'm a bit of a novice with the valve circuits so I'd be better off waiting until you got one up and cracking first.

 

[pvision]

Just re-read this thread, Ian. There's something in the back of mind about FET compressors applying signal as well as the control voltage to the FET to reduce distortion

When I used 1176s every day on vocals & bass I tended to use 4:1 virtually all the time. Higher ratios were strictly for effect. So you could omit some complexity here and have 4:1 and ∞:1 (like the MXR PWM limiter) or not have a ratio control at all (like the Orban 418A)

The Orban 418A might be a good source of ideas. Bob Orban seems to have been very particular in his designs and the PCB is a work of art. There's a complete schematic, layout, block diagram & functional description in the manual

Nick Froome

 

[ruffrecords]

Just re-read this thread, Ian. There's something in the back of mind about FET compressors applying signal as well as the control voltage to the FET to reduce distortion

This is a very well known technique. In the schematics I have posted this is achieved by the pair of 1M resistors, one feeding the control voltage and the other the input signal to the FET gate.

When I used 1176s every day on vocals & bass I tended to use 4:1 virtually all the time. Higher ratios were strictly for effect. So you could omit some complexity here and have 4:1 and ∞:1 (like the MXR PWM limiter) or not have a ratio control at all (like the Orban 418A)

The Orban 418A might be a good source of ideas. Bob Orban seems to have been very particular in his designs and the PCB is a work of art. There's a complete schematic, layout, block diagram & functional description in the manual

Nick Froome

Thank for the tip I will look up the Orban.

Cheers

Ian

 

[pucho812]

Why not go the other direction and have a variable ratio?

 

[ruffrecords]

Why not go the other direction and have a variable ratio?

No real reason at all except is it very difficult to identify a single parameter that controls ratio and even harder to make it track in a stereo compressor.

The big problem with (simple) compressors is the variations in the gain control element both from unit to unit and in a stereo pair. The ratio is largely set by the transfer function of the gain control element and/or which part of its characteristic you are working on.  Now if you could come up with a simple tube VCA with 10dB/volt gain I think you would really be on to something.

I knew about these problems back in the early 1970s when I built a compressor as my degree dissertation project. To overcome them I came up with a scheme that generated the desired output level from a piecewise linear expression of the input signal and compared it with the actual output signal to generate and error voltage that fed the gain control element. The beauty of it was that it was almost entirely independent of the characteristics of the gain control element. It used loads of 741 op amps (that was all that was available at the time) so I would not fancy building that using tubes.

Cheers

Ian