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?
It has far less phase shift than a 12AX7 stage.
Has anyone tried this?
From guitar amps, but the concept is valid. These are driving the output tubes via the cathodes.
http://bmamps.com/Schematics/Peavey/Peavey_Heritage-vtx_schematic.pdf
and similar:
http://bmamps.com/Schematics/Musicman/musicman_2165-rp_&_2100-rp.pdf
http://bmamps.com/Schematics/Peavey/Peavey_Heritage-vtx_schematic.pdf
and similar:
http://bmamps.com/Schematics/Musicman/musicman_2165-rp_&_2100-rp.pdf
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.
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.
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.
Tom
Tom
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.
"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.
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.
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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.
Using it's original paraphase at output tube grids. 20Khz
The circuit on breadboard
Using a TL082 and 12AX7. Right at output tube grids. 20Khz.

Using it's original paraphase at output tube grids. 20Khz

The circuit on breadboard

repair guy,
You have shown why tubed paraphase splitters are out of favor. The current consensus is that split load, AKA "cathodyne", AKA "concertina", and differential (LTP) splitters are best.
I definitely want to see your schematic. FWIW, I'm thinking that it may be possible to drive the non-inverting opamp I/P with the signal from the 'X7 and apply the NFB to the chip's inverting I/P or vice versa.
You have shown why tubed paraphase splitters are out of favor. The current consensus is that split load, AKA "cathodyne", AKA "concertina", and differential (LTP) splitters are best.
I definitely want to see your schematic. FWIW, I'm thinking that it may be possible to drive the non-inverting opamp I/P with the signal from the 'X7 and apply the NFB to the chip's inverting I/P or vice versa.
Here's the schematic so far. I didn't draw the opamp power supply, it's regulated +15, -15. I know it needs some optimizing, but it works really good. I might change out the 12AX7 for a 12AU7 for less gain. If anyone has any better design for the resistors, and how to add the global feedback I'd appreciate it.

Most of today's hi-end dac have XLR output, should be possible to use that as well.
MC275 does it by the way
MC275 does it by the way
Has anyone tried using the DRV134/DRV135?
http://www.ti.com/lit/ds/sbos094b/sbos094b.pdf
http://www.ti.com/lit/ds/sbos094b/sbos094b.pdf
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Unless you're driving long cables with unknown terminations, there is no point using DRV. Opamps will do.Has anyone tried using the DRV134/DRV135?
http://www.ti.com/lit/ds/sbos094b/sbos094b.pdf
Got any measurements you could post us of the amps performance? I'd be curious to see how this compares to other el84 amplifiers.
If anyone has any better design for the resistors, and how to add the global feedback I'd appreciate it.
The McIntosh MA2275 has an implementation of an opamp splitter with global feedback returned to the splitter. Here is a site with the schematic. The splitter is on page 16: here
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