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

 

[DaveP]

These are the results of a full test on the RH channel.

This chart shows that indeed the anode current stays level as the negative voltage increases, but the Vg2 current drops considerably.
The gain versus negative voltage shows almost a 6dB reduction which is as much as we will ever need. After -11V the signal starts to become unstable but that may be because the cover is off during testing. Bear in mind that there is no bypass cap on the cathode resistor.

The threshold control dial runs from 0 to 100 and this table shows the negative voltage corresponding to the dial numbers.

best
DaveP

 

[gyraf]

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

In this circuit, the speed of your attack and release times will probably be dominated by the 100nF/1M/220K time constants

 

[DaveP]

Yes, the same thought had occurred to me, which was why I didn't fit them from the start. I think that I should try to find the minimum value that would work without it affecting the attack times too much.
best
DaveP

 

[gyraf]

..or input your DC control voltage through the input transformer secondary, connect transformer directly to tube grid, no capacitor or noisy resistors (and no current will flow through transformer, it's only a grid out there)

i.e. instead of referring input transformer secondary to ground, refer it to your sidechain DC voltage. Then you'd only have to fight the inter-winding capacitance, and you can do this by applying control signal with low impedance (this low-z drive of trafo intercapacitance is most probably the reason for the 20W-amp in fairchild sidechain)

/Jakob E.