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PostPosted: 06 Dec 2010, 06:20 
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Matt: I guy at work who is sound man for a number of commercial groups in Melbourne has asked me to make a tube compressor for him. This is the only schematic I (and others) have been able to come up with.

At a guess what would you expect the HT to be. I think quite low volatge e.g. 12V to 90V. Would you please make an educated guess for me. I have started building the cct. brd. while I wait parts for the 300B amp.

I may have the cct. brd. complete tomorrow. Also can you suggest a method by which I can simply test that the HT is "about" the correct value e.g. current through the 12AX7??
Attachment:
TubeCompressor.jpg

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PostPosted: 06 Dec 2010, 22:42 
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It's surely an interesting approach to building a compressor.

The normal approach to volume compression (or expansion) is to run the signal into a detector and use the output of the detector to modulate an amplifier. It looks like whoever designed this circuit was trying to make do with what they had. So, I'll explain how I interpret the circuit and see if anyone has any additions or corrections to what I have to say.

The first tube looks to be doing two functions. The left hand triode is a cathode follower with a very steep dc load line (10kΩ). The AC input is divided by two with the two 470kΩ resistors and feeds the grid of the cathode follower. The cathode is then AC coupled to the input of a diode detector formed by shorting the right hand triode grid and plate together. The output of the detector is loaded with the 1kΩ-0.01µf RC pair. The attack rate of the detector will be driven by the RC time constant of the 10kΩ-10µf RC pair on it's input. The detector decay will be tuned with the adjustable RC pair on the output. I believe that the first triode is running in a very nonlinear fashion just to drive the detector.

The second tube is more confusing. The input is clearly another cathode follower with an adjustable load. The adjustment seems to be used to set the bias point of the other triode which appears to be a simple common cathode amplifier. Now comes the confusing part. Based on everything up to this point I would say that the circuit is designed for a B+ in the range of 40v to 50v. However, the output stage has a load resistor of 470kΩ! :shock: I would think that using this high a load at such a low B+ would produce a fair amount of distortion. But maybe the 12AX7 is cleaner in the lower corner then I thought. I just don't know. The overall output level appears to be set based on the value of the forward feed resistor (470kΩ at the top) and the divider ladder driving the input cathode follower.

So overall, I guess the circuit kind of makes sense. I stand by my estimate of B+ being about 40v. But I would have to do some more rigorous circuit analysis to be certain. You should be able to power it up with just the left hand tube in the circuit to verify the detector operation. Then power it up with just the right hand tube to verify the amplifier stage (at least that's how I would approach it).

The use of the 12AX7 is interesting here. This is the exact function for which the 6AT6 and 6AV6 were designed. Each is a single triode (1/2 a 12AX7 in the 6AV6) with dual detector plates off the cathode. I'm kind of surprised that no one could find a circuit using one of these tubes. They were widely used in vacuum tube car radios in the 1950s and 1960s.

Does anyone else have anything to add? (Or correct. I made a lot of assumptions in my analysis and I have to admit that vacuum tube detectors are not exactly my strong suit. ;) )

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PostPosted: 07 Dec 2010, 01:22 
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Matt: I guess I was sort of in the ball park with 12 to 90V. You know what I would dearly like? Is another schematic. Something I would feel more confident in. Thanks for your input so far. I'm part way through the cct.brd. Should have it done tomorrow.

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PostPosted: 07 Dec 2010, 15:49 
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mwhouston wrote:
Matt: I guess I was sort of in the ball park with 12 to 90V. You know what I would dearly like? Is another schematic. Something I would feel more confident in. Thanks for your input so far. I'm part way through the cct.brd. Should have it done tomorrow.

If you ever find a good schematic of a tube compressor please let me know. I've looked, and looked. The most unusual design I've come across used the screen voltage on a pentode to control the gain, turning the tube into a VCA. It didn't look like it would give that much gain reduction so I abandoned it. Perhaps two or more in series would provide enough gain reduction???

If someone could point me to a good quality voltage controlled amplifier using tubes I could design a decent compressor. I've built analog compressors with independent attack, release, threshold, and ratio controls, and with both peak and pseudo RMS detection to control the VCA. The only part I don't know how to design using tubes is the VCA (other than changing the screen voltage as mentioned above). The detector circuitry isn't that difficult.

Look at the cost of some of the tube compressors at the online music stores. $2000-$3000 is not unusual, although there are much less expensive ones.

I also draw a distinction between a compressor and a volume leveler. I don't want an AGC circuit, I want a fully adjustable compressor, and the attack time must be no longer than 1ms MAX at the shortest time setting. If it takes longer than about a millisecond you can't compress transients. At 1ms attack time, the transients that make it through wil be of such short duration that they will generally be inaudible (at least when clipped by a tube amp). 0.1ms or less would be preferrable.

Anyway, this is a big interest of mine (having used compressors for recordng instruments for over 25 years now) and I've been looking for a good tube compressor circuit for some time. The only one I found that looks really good was so complicated I said the heck with it. I've got too many digital compressors that have a zero attack time and sound great on instruments. If your friend performs live and needs super high quality compression, my band uses a computer and an ASIO interface to process all of our instrument sounds live. It's a little work to set up but it will out perform any compressor you can buy (to my ears - and I'm one PICKY guy when it comes to compressors). I would recommend Native Waves C1 compressor, a VST plugin. The round trip time for processing (latency) from input to output is about 10ms on my computer, but again that's with a zero attack time (anticipatory compression). Then there are several good multi-band compressors available, but I digress...

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Last edited by sampleaccurate on 07 Dec 2010, 16:13, edited 1 time in total.

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PostPosted: 07 Dec 2010, 16:01 
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Suncalc wrote:
The output of the detector is loaded with the 1kΩ-0.01µf RC pair. The attack rate of the detector will be driven by the RC time constant of the 10kΩ-10µf RC pair on it's input. The detector decay will be tuned with the adjustable RC pair on the output.


Wouldn't a 1k and a 0.01uf give a decay (release) of 0.01ms? That seems way too fast to me. Or did I misinterpret?

The attack time looks to be 0.1ms, which is great IMHO. But the release time would cause serious perceptable distortion (especially in the bass region) if it's really that fast.

Don't know, just asking.

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PostPosted: 07 Dec 2010, 16:16 
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Here are a bunch of schematics. Guess which one is first?

http://imagesfrom.us/keyword/tube%20com ... schematic/

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PostPosted: 07 Dec 2010, 16:47 
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Look familiar? Anybody know what book this is? It says B+ should be 150 volts!

They even mention the need for a fast rate of compression and a slower rate of release!

My interpretation is that the 1Meg pot controls the attack time (compression speed) and the 25K pot is either a ratio or threshold control or a combination of both.

Image

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PostPosted: 07 Dec 2010, 16:56 
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I have searched and the guy I am building the compressor for has search the 'net and the one I have posted is the only semi-sensible one we can find. I would have thought it a lot easier.

Hey Matt, bored? How about a nice and easy tube compressor? Should only take you a couple of beers to knock one up.

By the way Matt I think my power tranni for the 300B is ready for pick-up. I get the feeling this thing is going to be big, heavy and ugly.

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PostPosted: 07 Dec 2010, 17:02 
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Good work mate finding the origianl cct. Well 150V is a far cry from what Matt and I thought. I would have said 12 to 90V. Matt thought 50V. 150V is cool. I just built a 180V supply for a preamp and can make it small and dead quiet.

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PostPosted: 07 Dec 2010, 20:09 
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I just wish I understood how the circuit works. I've looked it over for 30 minutes and there are so many connections between points that you don't normally see connected that it's tough to figure out. You can't break it down into pieces - it appears to be very interactive. There are just lots of connections! Some of what Suncalc wrote sounds right, some I'm not so sure, I just plain don't know.

But this is one of those circuits that just won't leave me alone, churning in the back of my mind, a cheap, easy, MUSICAL tube compressor. Too good to be true. And honestly, I think this thing probably won't sound too good. But a suspicion isn't a valid reason to discount something that MIGHT be good.

This would be interesting to simulate to possibly help me figure out what's going on and why. If I can model it and understand it I can improve upon it. Two knobs, one of which is attack, means that there is only one other control. It should be possible to add additional controls to complete the normal attack, release, ratio and threshold controls on a typical compressor. Some form of make up gain would also be desireable IMHO. Threshold could be omitted and the input level changed to change the threshold, but that's not very convenient or accurate.

If it does work well it would probably be cool to incorporate some kind of gan reduction indicators to visually indicate the signal attenuation. 4 LEDs 6dB apart could show 18dB of range assuming the first lit up at the onset of compression (resistive divider, LEDs and some comparitors required). A point in the circuit where the DC voltage is related to the attenuation would be required to provide the gain reduction meter input signal. It need not be a linear relationship.

Who knows, this thing may rock. I'm definitley eager to hear (about) how it sounds.

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