• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Anyone else tried an opamp as a driver?

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#42
Ian444 said:
Yes it is, the first stage gives some gain, the 2nd stage is the inverter for one tube of the PP tube pair. If anyone can come up with a better scheme that would be fantastic, I'm all ears.
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Ok, I would aim for one op-amp per tube, with one of the op-amps inverted. You'll have to look after the input with the -ve used for both input and nfb however so it may get messy.

Maybe there isn't a really clean way of doing this - I would look though 😉
 
#44
The Tubecad Op amp splitter may look more symmetrical, but the signal still goes thru one Op amp for the top path and two Op amps for the lower path. Ian's good results just go to show that this "signal delay" paranoid fear mythology over nanosecond delays is just that.
 
#46
I was referring to the often hysterical avoidance of non-symmetry in splitters I see on the forum. Admittedly, an assymetry can get one into problems if the output Zs are not identical or the inverter stage non-ideal (linearity), but two Op Amps should be pretty darn close.
 
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#47
smoking-amp said:
I was referring to the often hysterical avoidance of non-symmetry in splitters I see on the forum. Admittedly, an assymetry can get one into problems if the output Zs are not identical or the inverter stage non-ideal (linearity), but two Op Amps should be pretty darn close.
Click to expand...

Oh. Well I think if you can eliminate a surplus op-amp in a signal path it's always a good idea - regardless of any religion surrounding it!
 
#48
teemuk, thanks again for the link (to tubecad). I tried that cct today, the upper one that is, but I prefered the sound of the modified P87B cct (post #13 has a link to the P87B cct). I changed it some more after reading up on opamp theory, now I know enough to be dangerous, which isn't very much. This is the schematic that seems to work best so far. It has some similarity to a paraphase inverter due to the two 10K resistors forming a voltage divider across the two outputs. Then I tried the DRV134 IC, with an OP275 with 4 times gain before it, the DRV134 has an internal gain of 2. This IC also sounds very good (thanks for the suggestion Cookj and panomaniac), and according to the datasheet it should have plenty of voltage swing for full power into EL84. Since I have been testing with +/-10V supply, I need to build a +/-18V supply before going any further.
 
#49
I bought another Mengyue for AUD$180 shipped as a test bed. It has two independent heater windings, I put them in series into a voltage doubler to come up with close to +/-18V dual supply for the opamps (OP275). Sounds great, strong bass, sharp detail, I'm very happy with it. I suspect the low output impedance of the opamps DC-coupled to the output tube grids has a lot to do with the sound characteristics. Just what I needed, another Chinese tube amp 😉 Well, I was going to use it at work, but now the missus likes this one...

Some pics...pic1... pic2... pic3...
 
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