Simple hybrid amplifier (35W/8Ohm)

[12E1]

Nice concept IMO, using a power op-amp in this way for the buffer function. If I had the time (I don't) I'd be tempted to build one myself just for fun. What's the purpose of the LM317 configured as a current sink on the output? I note that you added it in the second version, but I could not find a reason for it when I scanned the OPA458 data sheet.

[kacernator]

LM317 constant current source makes OPA548 to stay in sinlge ended class A for first cca 0.5W. Simply it sounds better. It moves crossover to higher power levels.

Look at the bottom of the page at last schematics:
"http://www.tubecad.com/2004/blog0013.htm"

[12E1]

Cool. Thanks for that. It makes sense. I see that JB's schematic shows the old LM12 - I remember those from many years back - not the first high power op-amp, but one of the first that was really usable for audio.

[kacernator]

Some tweaks there and there...

[FOXYE]

Very interesting amplifier, just need to find a way to get more power.

[kacernator]

I am thinking two options:

1) make LM3886 or TDA7294 unity-gain stable
2) OPA549 unity-gain in bridge mode configuration

Second should be doable, hopefully I can use CCS on output too woth bridge mode. BUT OPA549 is quite expensive, cca 25$. It is a lot money just for output stage. It should do about 150W into 8 Ohms. Nice thing about OPA549 is that you can current limit them with resistor, which should be usefull with bridge mode especially.

[Junm]

I wish you can do Lm3886 since I also have 6 pcs of it to use..also LM3875 and Lm1876. If these chips can be stable as unity gain buffers..My plan before is to have a gain of 5 and the rest is from the tube pre...but not done yet hope you will...

[kacernator]

Originally Posted by Junm

I wish you can do Lm3886 since I also have 6 pcs of it to use..also LM3875 and Lm1876. If these chips can be stable as unity gain buffers..My plan before is to have a gain of 5 and the rest is from the tube pre...but not done yet hope you will...

I am trying LM3886 non-inverting mode at unity at this thread, but I need help with it:

LM3886 unity gain

[jcx]

I suspect that with the shown Hybrid Amp schematic's series isolating 1 k and 220 pf to gnd the source impdance may be controlled enough to try the noise gain hack

the "sportiest" I would want to try with LM3886 would be to aim for noise gain of ~20 with asymptote/pole at 200kHz, which puts the low frequency zero at 10 kHz, I'd try noise gain R ~ 1kOhm

[heyyou]

I like your idea of making cooling holes using a shotgun.