Mix Bus Compressor from scratch

[Johnny1234]

Hi Dave, the 1n4148 diode feed from c6 on the side chain output is reversed in the left or right channel.

 

[DaveP]

Well spotted, it's an error on the right channel drawing, the finished circuit has them both positive going.
best
DaveP

 

[DaveP]

This project is not finished by the way, I just had a break to catch up on my "Honey do list".

Work on the timing dials is continuing and will be posted soon.
best
DaveP

 

[slipperyflax]

Looking forward to updates!

 

[DaveP]

I am trying to match the "Ohmite" dials of the input and threshold controls, for the attack and Release switches.





I have produced the dials in MS Paint and will use some cellophane circles to give a durable finish.



best
DaveP

 

[Johnny1234]

Hi Dave, Been waiting on your results before ordering iron and other goodies. Does the unit work as intended? How does it track compared to chart in post #10.
Many Thanks Again

 

[DaveP]

I have some time to work on it tonight, so I may have the answer later.
best
DaveP

 

[DaveP]

OK, I have finished the dials and installed the attack and release switches.



The threshold control was very free-running so I fitted a circle of antistatic foam to give it a nice resistance.



I will try to pass audio tomorrow
best
DaveP

 

[thomasdf]

Another success in the making ! Bravo Dave !

 

[Johnny1234]

Wow, I'm amazed. Beautiful work. Please let us know your results.

 

[DaveP]

Thanks for your support everyone.
It passes audio but I need to make a few adjustments and learn how to operate it!
More testing to do tomorrow
best
DaveP

 

[DaveP]

There appears to be a problem with the side-chain, all I am getting is a square wave with a very short space between squares which does not give any negative DC. The threshold works OK. At first I thought it was an overload on the tube but reducing the input voltage just reduces the square wave, so back to the drawing board. Anyone planning to make this project should wait until I have sorted out the problem.
best
DaveP

 

[DaveP]

I have modified the side-chain as the following schematic.


The original side-chain needed to be fed by two diodes, ( I guess to stop cross-talk between channels) I have changed this to two cathode followers to achieve the same effect. Also note the second diode to earth feeding the attack circuit, this is absolutely necessary to get a negative voltage.

So I now have a controlable negative voltage feeding the EF83's but it is not reducing the gain at present, so I suspect that I have to make the circuit more like the original tape recorder schematic for that to happen, we shall see. At least this shows that real life designs take a while to get right.
best
DaveP

 

[Johnny1234]

Hi Dave, Looking forward to any updates. I'm sure you will figure it out.
Again many thanks to you.

 

[DaveP]

I have made some progress today. The negative voltage is not getting through the 1 M resistor feeding the grid of the EF83. When I short this out, the negative DC reduces the audio input voltage so the output voltage is reduced, but the gain remains roughly the same at about 20x. weird! The EF83 tube spec sheet mentions the source resistance should be less than or equal to 220k, but it is often more than this on many circuits as there is usually a resistor from the preceeding EF86, I am not sure how much this affects things.
best
DaveP

 

[DaveP]

OK, I have added a 220k resistor in series followed by a 100nF cap and the negative voltage is no longer lost down the 1M resistor.


I think the negative voltage was shorted out through the IPT and now the input cap is stopping that happening. This makes the circuit more like the taperecorder circuit and it is possible to have up to -9V on the grid now although it should not need that much in normal use.
A good days work!
best
DaveP