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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 4th April 2016, 09:37 PM   #1
repair guy is offline repair guy
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Default Has anyone used an opamp to split phase for a tube amp?

I just threw together one and it looks great on the scope. Almost no phase shift and pretty linear, provided you keep it at 2:1 gain. Seems like you could put a tube driver stage after it and drive the outputs. Then use global feedback. I know it would be a hybrid, but would that be a bad thing.

It has far less phase shift than a 12AX7 stage.

Has anyone tried this?
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Old 4th April 2016, 09:52 PM   #2
BinaryMike is offline BinaryMike  United States
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Has anyone used an opamp to split phase for a tube amp?
Here's an example. I built one amplifier channel on a breadboard four years ago and haven't done any more, but it performs surprisingly well. Output power is about 30W with a regulated PSU.
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Old 4th April 2016, 10:03 PM   #3
SpreadSpectrum is offline SpreadSpectrum  United States
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I saw this done in the McIntosh MA2275 schematic. I have never tried it myself.
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Old 4th April 2016, 10:18 PM   #4
Enzo is offline Enzo  United States
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From guitar amps, but the concept is valid. These are driving the output tubes via the cathodes.

http://bmamps.com/Schematics/Peavey/..._schematic.pdf

and similar:

http://bmamps.com/Schematics/Musicma..._&_2100-rp.pdf
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Old 4th April 2016, 10:24 PM   #5
repair guy is offline repair guy
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Quote:
Originally Posted by BinaryMike View Post
Here's an example. I built one amplifier channel on a breadboard four years ago and haven't done any more, but it performs surprisingly well. Output power is about 30W with a regulated PSU.

That's really interesting, solid state amp with tube outputs. How does it sound on speakers?

Mine would be simpler than that. Just a tube stage between the opamp and outputs.

You should see the atrocious performance of a tube paraphase splitter on my scope. Anything past 5Khz and it just falls apart. At 20Khz the phase shift is so bad it looks like some kind of joke circuit. It's not the loading of the scope leads, it's actually that bad.
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Old 4th April 2016, 10:54 PM   #6
Eli Duttman is offline Eli Duttman  United States
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Quote:
I know it would be a hybrid, but would that be a bad thing.
IMO, absolutely not! Performance is the key, not any specific technology. IME, tubes do several things better than SS. However, carefully crafted combinations of "hollow state" and SS frequently do a better job than either technology does alone.

FWIW, I believe the 12AX7 triode should be restricted to voltage amplification duties, as its low gm and high RP make it a very poor driver of downstream circuitry.
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Old 4th April 2016, 11:13 PM   #7
tomchr is offline tomchr  Canada
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I've often thought the LME49811 would make a great output stage driver for a tube amp. Sadly, it's going out of production. One could play with a high-voltage opamp, such as the LTC6090, though.

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Old 4th April 2016, 11:45 PM   #8
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
That's really interesting, solid state amp with tube outputs. How does it sound on speakers?

Mine would be simpler than that. Just a tube stage between the opamp and outputs.
That design didn't have a notable sonic footprint that I can recall, despite running in class AB2 and using a $12 speaker line matching transformer as OPT. The apparent complexity stems from the fact that voltage gain in 6080 tubes is only about 1.0 in this application. All of the transistors in the middle of the schematic are just implementing a pair of high-voltage op-amps in inverting configuration. I'll be interested to follow your amplifier development story.
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Old 5th April 2016, 12:40 AM   #9
smoking-amp is offline smoking-amp  United States
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"It has far less phase shift than a 12AX7 stage."

I think 12AX7 is the key there. So much old stuff being built to rubbish standards. Use a suitable tube!
It can be done. A video tube for example. A Mosfet Concertina can do it well too.

Last edited by smoking-amp; 5th April 2016 at 12:52 AM.
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Old 5th April 2016, 02:48 AM   #10
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I built it today on breadboard and it really sounds good. Sounds way cleaner than the original paraphase type splitter. The new circuit sounds very clean in the highs. I've yet to figure out a way to get the global feedback into the ic's input, but I think this is a good start. I used a stereo magnavox EL84 amp as the test bed. I'll get a schematic up soon. Each scope channel is connected to the EL84 grids with global feedback disconnected.



Using a TL082 and 12AX7. Right at output tube grids. 20Khz.
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Using it's original paraphase at output tube grids. 20Khz
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The circuit on breadboard
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