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Has anyone used an opamp to split phase for a tube amp?
Has anyone used an opamp to split phase for a tube amp?
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Old 5th April 2016, 09:22 PM   #21
stocktrader200 is offline stocktrader200  Canada
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a npn transistor or n channel fet running a few milliamps works great
20k on collector , 20k on emitter 150 volts @ 2 ma
can provide at least 60v p-p at low distortion

Last edited by stocktrader200; 5th April 2016 at 09:25 PM.
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Old 6th April 2016, 02:43 AM   #22
repair guy is offline repair guy
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I tried this circuit and it sounds incredible, far better than the one I tried to cook up. This is very similar to every one I've seen here. They use a type of paraphase with opamps. It works well because the opamp is so fast there is almost no delay. Right now I'm getting a sound that's somewhere between tube LTP and a split load. It's warmer but cleaner than a tube split load phase inverter. Sounds alot like a tube LTP but with less distortion. It's really weird how well this works, doesn't sound like there's an IC stuck in the circuit. I guess it makes sense, professional audio has been splitting phase like this for a long time. All those XLR ins and outs, they don't use signal transformers, they use opamps, maybe hundreds of them.


The schematic I used, disregard the output tubes in the schematic here.
TL082, 12AU7, 2x EL84
Click the image to open in full size.
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Old 6th April 2016, 02:55 AM   #23
rayma is offline rayma  United States
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Quote:
Originally Posted by repair guy View Post
I tried this circuit and it sounds incredible, far better than the one I tried to cook up.
That's an instrumentation amplifier input stage, not a paraphase.

An oldie but a goodie from Radford, 1979.
http://www.radfordrevival.co.uk/literature/tt100/
http://www.radfordrevival.co.uk/word...uctions-P2.jpg

Last edited by rayma; 6th April 2016 at 03:00 AM.
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Old 6th April 2016, 03:15 AM   #24
repair guy is offline repair guy
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Quote:
Originally Posted by rayma View Post
That's an instrumentation amplifier input stage, not a paraphase.

An oldie but a goodie from Radford, 1979.
TT100 | Radford Revival
http://www.radfordrevival.co.uk/word...uctions-P2.jpg
You're right, it is indeed an instrumentation amp. Can't believe I missed that.


Interesting, 2 opamp input stages with a transistor LTP. Not sure why they went with transistor drivers. Large voltage swings are not a transistor's strong suit. They certainly have more than enough current to overcome the tiny capacitance of the KT88 grids though. Must be a fast, clean sounding amp with very high damping factor, for tubes.

Last edited by repair guy; 6th April 2016 at 03:25 AM.
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Old 6th April 2016, 03:24 AM   #25
BinaryMike is offline BinaryMike  United States
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Has anyone used an opamp to split phase for a tube amp?
If you're running EL84 finals in pentode mode with about 300V from cathode to anode and cathode bias around 11~12V, then it seems a bit silly to include the tube driver stage. You could drive the EL84 grids directly from the op-amps, which kicks multiple problems in the butt simultaneously. Op-amp gain should be increased to about 12~15 if you do this --- more if you add feedback. Op-amp power rails should be 15V or a bit more if possible. Some op-amps can handle up to 40V total.
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Old 6th April 2016, 03:48 AM   #26
repair guy is offline repair guy
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Originally Posted by BinaryMike View Post
If you're running EL84 finals in pentode mode with about 300V from cathode to anode and cathode bias around 11~12V, then it seems a bit silly to include the tube driver stage. You could drive the EL84 grids directly from the op-amps, which kicks multiple problems in the butt simultaneously. Op-amp gain should be increased to about 12~15 if you do this --- more if you add feedback. Op-amp power rails should be 15V or a bit more if possible. Some op-amps can handle up to 40V total.
I would have to build another stage for gain. Would this circuit stay linear outputting over 10V? I don't know, but I have to try it, it's too tempting. I bet it would have excellent S/N ratio. I'll try it tomorrow.
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Old 6th April 2016, 03:55 AM   #27
rayma is offline rayma  United States
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Originally Posted by repair guy View Post
I would have to build another stage for gain. Would this circuit stay linear outputting over 10V?
If you need more output swing: http://joebrown.org.uk/images/DualPS...pingOpAmps.pdf
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Old 6th April 2016, 04:36 AM   #28
repair guy is offline repair guy
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Originally Posted by rayma View Post
This is a fine line. I don't want to build a solid state amp with a tube output, but it is tempting to do. Opamps are best with small voltages, tubes do large voltage swings with ease, but have capacitance issues. I think the setup I have now is using each device's strength, but I will try it out. After all, that's what diy is all about.

Also the EL84 might be the easiest to drive power tube in the world.
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Old 6th April 2016, 04:59 AM   #29
ericj is offline ericj  United States
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Quote:
Originally Posted by repair guy View Post
This is a fine line. I don't want to build a solid state amp with a tube output, but it is tempting to do. Opamps are best with small voltages, tubes do large voltage swings with ease, but have capacitance issues. I think the setup I have now is using each device's strength, but I will try it out. After all, that's what diy is all about.

Also the EL84 might be the easiest to drive power tube in the world.
You can go further than that. See page six of this application note showing how to build an amplifier for electrostatic headphones with an lm4562:

http://www.ti.com/lit/an/snaa046a/snaa046a.pdf

It's not the best stat amp for sure, but it's more ambitious than the Stax SRM-001mk2 I listen to all day at work. Which is based on a TL062.
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Old 6th April 2016, 08:03 PM   #30
BinaryMike is offline BinaryMike  United States
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Has anyone used an opamp to split phase for a tube amp?
Quote:
Originally Posted by repair guy View Post
I would have to build another stage for gain. Would this circuit stay linear outputting over 10V?
No need for another stage. Just rejigger the resistors around the op-amps. Most op-amps stay linear up to about 1~2V from each supply rail. That's why I suggested 15V or higher supply rails -- to be sure that your power tubes run out of gas before the driver does. There's no real harm if the grids go slightly positive on signal peaks in a direct-coupled circuit. Output power simply increases a bit. Cathode bias creeps upward any time the tubes venture out of class A operation and class AB2 operation adds to that problem. Some folks tackle this issue with a huge cathode bypass cap, e.g. 10,000uF. At some point, you might want to rework the bias arrangement for better stability.
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