Poor Man's Tube Compressor

squib said:

what is the effect of the inherent turn on and turn off response of the vactrol? With a 35mS release time, wouldn't it be generating significant distortion at low frequencies? Do you really want such a fast release? The opto cells in a LA2A had a much slower release or turn off time and indeed were hand graded to pick the few from any one batch that met the characteristcs required.

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If I remember correctly, the turn on and turn off times are quoted at 40mA or 10mA currents and they lengthen considerably as the current lowers and the tests are also based on the opto being adapted at 40mA for 24hrs - hardly a realistic condition for a compressor. My design at present leads to a maximum current of about 2mA and in the listening tests I have done so far there seems to be no obvious distortion at low frequencies.

I used a VTL5C3  for this current project for a client simply because it had a high output impedance preamp driving the gain reduction element so the VTL5C3 seemed the better choice and so far seems to work. My earlier experiments were based on the VTL5C4 which has a much longer decay time of 1.5 secs although the specs themselves are a bit confusing as they insist on defining decay as the time to 100K despite the widely different nominal ranges of the opto resistance of the 5c3 and 5C4. I notice that at the end of the 5C3 and 5C4 data sheet there is a note that the 5C4 can be substituted by the Silonex NSL-32SR2. This device though seems to have a resistance at 0.1mA about one tenth of that of the 5C4. So the specs are confusing to say the least but that might be simply because the original application was intended to be remote antenna termination.

The bottom line is nothing is yet cast in concrete but you are right, the compression characteristics do depend very much on those of the opto cell.

Cheers

Ian

Hi Ian,


  watching this with great interest. I ordered a couple of VTL5C3 today, just in case. They are getting a tad scarce, methinks . . .

  KIndest regards,


    ANdyP

strangeandbouncy said:

Hi Ian,


  watching this with great interest. I ordered a couple of VTL5C3 today, just in case. They are getting a tad scarce, methinks . . .

  KIndest regards,


    ANdyP

Click to expand...

Yes, I found a bloke in the USA selling 34 of them on eBay. I was thinking of buying them as a sort of pre-emptive group buy thing.

Cheers

Ian

Probably a good idea, 'cos there are precious little of the blighters out there!

strangeandbouncy said:

Probably a good idea, 'cos there are precious little of the blighters out there!

Click to expand...

That's good 'cause I just went and bought them. When they get here I'll make them available at cost in the Black Market.

Cheers

Ian

Wow, 2 years sine the last post on this topic. How time flies. I finally go around to knocking up a simple Vactrol compressor using a pair if 5C3 types connected with the LDR parts in parallel and the LEDs in antiphase so it works on both half cycles of the audio. I used a twin line amp with one amp for the main audio patha nd the other for the side chain. The LEDs are just fed from the side chain amp via a 2K reistor. The gain reduction part uses a 47K resistor and the Vactrols in a pot divider between the input pot and the audio amp. I'll post a schematic later. What matters about compressors is what they sound like. Here are links to four files of a drum track with 0, 5, 10 and 20dB of gain reduction. Each sample is untreated for the first half and via the comp for the second half.

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/audiotests/EZcomp/compdrum0dB.wav
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/audiotests/EZcomp/compdrum5dB.wav
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/audiotests/EZcomp/compdrum10dB.wav
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/audiotests/EZcomp/compdrum20dB.wav

Cheers

Ian

Man I just got a free box of 6922 tubes that all 200 are working... Might be right up my ally to play around with this circuit.  Has there been any development since?

pucho812 said:

Man I just got a free box of 6922 tubes that all 200 are working... Might be right up my ally to play around with this circuit.  Has there been any development since?

Click to expand...

None at all - too busy with other things - you know how it is!!

I am going off optos because they are getting scarce because of their cadmium content. They are also quite hard to drive with tubes. lately I have been thinking about using a FET instead.

Cheers

ian

understood.  might just stick these with mic pre's  and eq's for now....    Maybe a mic or two 8)

back in late 80's i bought this pocket size electronic handbook ( one of those 555 circuits type book), it had many basic but useful circuits...
one was an "opto comp". it was extremely simple, led, LDR and couple more components... no ratio, no any other adjustments... plain as hell !
direct mic through it on voice  sounded so good and thick on this comp...
it was on breadboard and i didnt even light_sealed it.. we were using it in dark  8)
none of the compressors i have used over the years gave the same result.. i lost the book and the design many years ago :'(
looking forward to get the same taste again...

I have recently had a couple of other enquiries about this circuit. It is so long since I did anything on it I will need to open it up and trace the schematic! I'll post it here.

Cheers

Ian

As promised, I have attached the schematic of the prototype poor man's tube compressor I built a while back. There is no PCB for the; I just wired the additional components onto the back of the input and threshold pots. I do not remember the exact gains I set the two halves of the TLA to. The audio path should be set so that with no compression, the gain at 100% and an input level of -20dBu, you get 0dBu at the output. This basically gives you 20dB of gain make up. The side chain should similarly be set for about 20dB of gain. This means the lowest threshold will be -20dB [ir]elative to the output[/i]. Since there is another 20dB gain available in the audio pst, this means the threshold relative to the input can be as low as -40dBu

Cheers

Ian

Thanks Ian....

Ill try this, and info here...

Since those opto cells seem to get rare: can't we rectify the control voltage and just feed it to a single cell?

Michael

Michael Tibes said:

Since those opto cells seem to get rare: can't we rectify the control voltage and just feed it to a single cell?

Michael

Click to expand...

You have two options:

1. Just use one cell and live with the fact you are only using half of the waveform. In many situations this will not be a problem. It s only if you have a highly asymmetric waveform will there be any error and in an ac coupled system they tend to be rare.

2. Full wave rectify the waveform before applying it to the the opto as you suggested. The only downside of this is it raises the threshold by two diode voltage drops - definitely worth a try.

Cheers

Ian

Looks good! The TLA is your Twin line amp, right?

Deepdark said:

Looks good! The TLA is your Twin line amp, right?

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Correct. The prototype was actually built on an EZMixer Eurochannel PCB but that is just a TLA with some extra bits that I did not need to fit. The TLA just a Eurochall mic pre which has the bits for a mic pre input (switches and gain) removed and replaced with provision for an extra transformer which you don't need for this design anyway. Here is a pic of the guts. You can just see the two optos soldered directly onto the back of the gain pot. Two pots, two optos, two resistors  and a TLA is all you need.

Cheers

Ian

I had some vactrols i ordered a few years back, when the poor mans compressor was discussed. I built it this afternoon. I used a mic preamp card and took the pultec out so i could use the panel.
It works but it is very subtle. I will need to test some more but i really needed to feed the preamp with a very loud signal before i realized it was actually compressing. Could i lower the threshold by playing with the 47k resistor? I might be doing something wrong because the grahill stepped gain switch is not working anymore...
How much voltage are you feeding the led's when compressing?
Where is it best to take the 0v for the led's? i took it from the foot of the ht resistor next to output 2.
For a bypass switch could we just bypass the threshold pot?

Anyway thanks Ian for doing this.

Regards,

Pierre

Hi Pierre,

That was quick!! Let me guess what you did. Your signal comes is as normal via the mic or line input via the push buttons to the mic transformer. You took out the link the connects the transformer secondary to the input of the first amp and connected the transformer secondary to the 10K pot. That could be your first problem. The mic transformer really needs to be loaded with no more than 150K. The 10K pot is reflected to the primary as 100 ohms  so it is not surprising you need a lot of signal to get anything out. I cannot think of an easy fix for this. One possibility is to remove the 10K pot entirely and rely on the mic pre gain switch to set the gain. Then the worst case load is the 47K resistor in series with the opto. If you made this 100K it would not change the threshold but it would increase the available gain reduction and perhaps alter the ratio a little. The load on the transformer would be 100K worst case which does not affect the reflected primary impedance anywhere near as much as the 10K.

You then connect OUT1 to IN2 via the 10K threshold pot and connect the opto LEDs to OUT2 via a 2K resistor. The audio output comes from OUT1.  You probably already have about 20dB gain in the second amp if it was set up to be gain make up for the Pultec so that should be fine.

To test it, turn the threshold fully off and feed 0dBu into the line input. Adjust the gain switch position for approximately 0dBu out. You should then be able to turn up the threshold pot and see the output level reduce. If you turn it up so it is 6dB lower you have approximately 6dB of gin reduction. You can then adjust the gain switch to get the level back to about 0dBu.

Cheers

Ian

An alternative, possibly more accurate way of setting a specific amount of gain reduction is feed in 0dBu with the threshold off and adjust the gain control for +6dBu out. The raise the threshold control until the level drops to 0dBu. For 10dB of gain reduction set the output to +10dBu instead.

Cheers

Ian