I am going to test a Texas OPA454 high voltage opamp to drive an EL34 in my SE amp. It's not a particularly quiet opamp but probably no worse than 6SN7 or 12AX7.
(what is the noise figures for these?)
I'll be initially usiing + - 36v switch mode psu with common mode choke on the output and gain will be 20x.
Comments please?
(what is the noise figures for these?)
I'll be initially usiing + - 36v switch mode psu with common mode choke on the output and gain will be 20x.
Comments please?
Hi batteryman, this is something I thought about for some time. I am just too lazy to do it.
I would say it worth to try. My preference would be NO NFB, keeping the EL34 working in most linear region.
Cheers, Johnny
I would say it worth to try. My preference would be NO NFB, keeping the EL34 working in most linear region.
Cheers, Johnny
The OPA462 has higher rail voltage (up to +/-90V) and much lower noise than the OPA454.
My amp has been modified to remove the 6SN7s and the VAS mounted in a separate chassis at present (using an Akido) so there is no global NFB.
I will leave it like this for now and maybe then install the opamp in the EL34 chassis supplied from the HT via a Zener stabilised supply. I can then experiment with NFB to see which my ears prefer.
It's unusual to have an opamp driver for a valve power amp, when all the voltages are already conveniently available for a valve driver. Is this an experimental setup where you need to isolate the valves from all other factors?
Yes, just experimenting with alternative VAS schemes. To be honest, I'm struggling to make a valve design work with low distortion with 10v RMS to the EL34. (It's my lack of valve knowledge) .
There were op-amps built with vacuum valves, including DC coupling, many years ago. See G. A. Philbrick. You many even find some premium brand 9A base dual triodes marked "GAP", sometimes with color coded tip paint colors like a medical select but usually not. I'll not print the type for fear of bots, but it's the common mu of 100 button base dual triode. Snag one when you see it it affordably.
All good fortune,
Chris
All good fortune,
Chris
Should not be a problem at all.
Distortion in SE output stage is much higher that any sort of voltage amplifying stage, especially with 6SN7.
It's a problem for an old man whose brain feezes at the sight of formulae! Even Merlinb's webpages can't help me.
The Chinese SE amp was advertised as being 10w. They didn't quote the distortion which I measured at over 10% at about 4watts. I found that replacing one of the 6SN7s anode resistor with a 10M45 constant current source helped.
The 10 Watt spec is obviously bogus - no surprise. You might find member stephe's YouTube channel "Skunkie Designs" helpful. She's tackled several of these Chinese wonders with useful and practical advice.
All good fortune,
Chris
All good fortune,
Chris
Thank you, I’m looking for a device to drive output tube grids into A2/AB2 applying differential local feeeback, and this seems the right opamp. Is it needed to sink some heat when running at full throttle?
My approach was use an inverter to step up the power supply just for the valves, and a DC-DC converter generating +/-15V for the opamps, with high gain tetrode (actually an RF dual tetrode, QQV3-10) as the power valve.
Power section, with differential input, this feeds a small OT:
The input section, input network with RFI filter, gain stage, feeding an adjustable offset buffer and an inverter to generate something like 16V rms differential output to drive the QQV3-10.
The whole unit driven from 12V battery or mains PSU - its s toy guitar-amp style amp (but too low input impedance for a real guitar, just a vanity piece really).
The lack of mains transformer solves the hum issue, only 10nF caps needed to decouple the HT...
The two LEDs in the anode circuits allow visual indication of offset + bias adjustment, tune for same brightness for offset, just starting to illuminate for bias!
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Did a quick test of the OPA454. Input is 1v RMS and output is 15v RMS: (Just the opamp)
Tabulated results are at 1khz. As expected, far superior to a valve VAS. Now to build another and listen to it.
Tabulated results are at 1khz. As expected, far superior to a valve VAS. Now to build another and listen to it.
I've used OPA453's to drive KT88's using DC coupling to the grid. I ran them off +5V and -75V supplies.
Chips like the LM4702, LME49810 and LME49830 can still be found among the usual suspects. While discontinued by TI (after they bought National Semi) they would fit the bill. I may have some, PM if interested.
Scott wrote an article describing tube noise relative to 390R (2.5nV/RtHz): https://tavishdesign.com/pages/audio-tube-noise-measurments