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Old 6th April 2015, 11:27 PM   #1
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Default Tube Mic preamp with compressor - any ideas?

Hi there, I'm interested in making a microphone tube preamp that also has a built in limiter/compressor so there's little difference between loud and quiet sounds. Designing and making a tube preamp that can drive a relatively low-impedance input such as a computer's line-in is easy enough, I can pretty much do that in my sleep.

Trouble is I want to add a compressor to the preamp, but the only schematics I can find either require a LOT of tubes or have a very high output impedance meaning that I cannot connect it to most of the stuff I have.

I tried one schematic for a compressor (the one that's on the very left), but again this is one of the ones that has a ridiculously high output impedance and I couldn't get it to work anyway, I thought I could connect it to the output of my preamp but like I said it was a no-go. Also I've attached a schematic of my preamp.

Another compressor I found uses a pentode with what appears to have the third grid "modulated" to adjust the gain. What about that? Would that work?

So can anyone direct me to a schematic that can amplify a microphone, uses standard tubes that most tube nuts like my self would have (such as the ECC83/12AX7), has a built in limiter and has a reasonably low-impedance output (100k or less)?

any help on that would be appreciated.
Attached Images
File Type: jpg OneTubeComp just the circuit.jpg (474.7 KB, 232 views)
File Type: png mic preamp.png (17.4 KB, 223 views)
File Type: gif Compressor.gif (3.7 KB, 228 views)
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Old 7th April 2015, 01:42 AM   #2
kevinkr is offline kevinkr  United States
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Hint: The solution is just a cathode follower away...
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Old 7th April 2015, 09:18 AM   #3
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For the lowest tube count I would recommend using an opto resistor as the gain controlling element. There are plenty available with built in LEDs.

Here's a link to a two tube design:

http://www.ianbell.ukfsn.org/EzTubeM...L/PMTCLsc.jpeg

Cheers

Ian
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Old 7th April 2015, 12:30 PM   #4
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Quote:
Originally Posted by ruffrecords View Post
For the lowest tube count I would recommend using an opto resistor as the gain controlling element. There are plenty available with built in LEDs.

Here's a link to a two tube design:

http://www.ianbell.ukfsn.org/EzTubeM...L/PMTCLsc.jpeg
Nice diagram. Ambiguous penmanship! The cathode-bypass capacitor on the rightmost tube… is that 220 nF or 220 μF? could be that old 'μ' symbol, you know. … … granted which 95% of everyone have long given up using, but still.

GoatGuy
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Old 7th April 2015, 02:21 PM   #5
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Quote:
Originally Posted by GoatGuy View Post
Nice diagram. Ambiguous penmanship! The cathode-bypass capacitor on the rightmost tube… is that 220 nF or 220 μF? could be that old 'μ' symbol, you know. … … granted which 95% of everyone have long given up using, but still.

GoatGuy
Ah, I see what you mean. You think my μ looks like a capital N - which would of course be quite wrong for nano-Farads.

Can't understand why engineers and electronics hobbyists would have given up using the μ for micro. Probably the advent of keyboards and business oriented software that made access to technical symbols such a chore.

Cheers

Ian
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Old 1st April 2016, 11:22 AM   #6
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Hello,

Sorry for dig out this thread from ages ago .

Did someone build the very nice looking compressor schematic from Ian?? I´m really interested in a good working tubae based schematic without "magic" parts or very expensive tons of transformers.

Best regards
Karsten
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Old 1st April 2016, 09:52 PM   #7
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Quote:
Originally Posted by ruffrecords View Post
For the lowest tube count I would recommend using an opto resistor as the gain controlling element. There are plenty available with built in LEDs.

Here's a link to a two tube design:

http://www.ianbell.ukfsn.org/EzTubeM...L/PMTCLsc.jpeg

Cheers

Ian
I second that. All vari-mu approaches add extra distortions.
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Old 2nd April 2016, 11:00 AM   #8
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Hello Wavebourn,

Thanks for the comment. As lon as the extra distortion is something which sound nice... It´s what I expected from a tube designe for audio..

You know the value for the HT+ ??

Regards
Karsten
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Old 2nd April 2016, 11:41 AM   #9
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Before even worrying about the HT+ (I think anything 200V~300V should work), you probably want to check with Ian on the availability of the Vactrol used in the design.
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Old 2nd April 2016, 12:01 PM   #10
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Hey Jazbo8,

You´r right. The VTL5C3 seemed to be discontinued. Well, I will try to buy one if it will not cost to much first before go forward with this project.
I will start a recherce if there a similar device available ( should be ) or if it possible to build a similar Vactrol using a discrete LED/LDR combination. I think I´m not the first one who try to get a replacement or build a discrete version.
So there seemed to be hope, to find some information in the net...

Best regards
Karsten
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