Here is the schematic of the 'modernized' version I am using.
If you would like to use side chain filtering, this can be done at the point C8/P3.
(Even inserting a lower value capacitor would work here.)
I don't have problems with serious LF distortion, as long as I don't use extreme fast release times.
The standard release can give 'agressive' limiting (when using a fast release time), but sounds very pleasant at medium or longer release times. The 'composite' release acts more as a leveller, without audible pumping.
I have added an extra resistor in series with the input, because otherwise the circuit was too sensitive with +4dB signals.
I used a quad OpAmp for the circuit. (TL074)
Signal to noise ratio is 81 dB ('A' weigthed, re. +4dBv), good enough for me.
Maximum distortion is 0.4% THD @ 400 Hz (10 dB limiting)
For stereo, it is important to match the FETs. (I found two pairs from 20 pcs.)
I used a multiturn trimpot for the FET bias, because this setting is pretty sensitive.
And for those who can use Sprint6 files, here is the project file:
You can download a free Sprint viewer here: http://www.abacom-online.de/updates/Sprint-Layout60_Viewer.exe
Note: on the PCB design I have omitted C8, because there is no significant DC offset at the output of the OpAmp.
(And if there is an offset, this can be compensated with the 'Detector balance' trimpot.)
Oh thank you very much! Can you pleas also post how did you solve the VU meter, bypassing and input output balancig/debalancing? Do you designed your own circuit for VU meter? Which VU meter did you use?
The VU meter I used is a pretty standard one (I think it was a Monacor), for the measurement of the output level it is simply connected through a 3.6K resistor to the output. (Driving impedance is low here, so no extra distortion from the rectifier in the VU meter.)
For the gain reduction measurement I am using an experimental circuit.
Without signal, the VU meter indicates 0 VU and moves back when gain reduction occurs.
At the moment I am still optimizing this circuit, because there are a few (small) problems:
- The '0 VU' drifts a little with temperature (You can see the small holes between the meters for '0 VU' calibration.)
- Indication isn't really exact. (At 5 dB a bit too much, at 10 dB a bitt too little.) Not a real problem, error is in the 1 dB range, but I would like to see the exact amount of gain reduction.
Inputs and outputs are unbalanced at the moment, because the inserts on my mixer are unbalanced.
Will be continued...
This is the circuit I use at the moment for the gain reduction meter:
The negative voltage on the inverting input of the OpAmp is amplified and inverted.
P1 and P2 are adjusted in such a way that the VU meter indicates 0 VU without any gain reduction.
If gain reduction occurs, the FET will conduct more and the voltage on the non-inverting input of the OpAmp will get lower.
This will result in a lower voltage for the VU meter.
The trick is to adjust P1 and P2 in such a way (they interact!) that the indication of the gainreduction is about right, and the meter is still on 0 VU without any gain reduction.
If someone gets crazy and uses more than 20 dB gain reduction, the polarity at the output of the OpAmp would change, making the needle rise again! To prevent this, a diode is placed between the output of the OpAmp and the meter.
Values are still experimental.
My version is unbalanced (as was the original RM58), because I use it on the inserts of my mixer and those inserts are unbalanced.
But I suppose the best way to add balanced inputs and outputs, is to use the well knows THAT line receiver/transmitter chips.
Or real transformers, if you like...