Hi all. I have designed a small amplifier for use in my small listening room. I am posting it looking for opinions and suggestions. I realize that some of you will not agree with the tubes that I have chosen and that is OK with me. I think it will work and sound fine but I wanted to make sure that it is as polished as it can get in spite of the crappy tubes I chose.😉
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470k gridleak is kinda high, and a .68µF / 470k is almost going to let DC through, hehe... (rolloff is like 0.5Hz... go to 100k or something.)
That and I hate direct coupled stuff, I'd cap couple it and put in a divider to bias the CF... or drop it altogether, an AU7 shouldn't have a problem driving a 12B4, I wouldn't think... but whatever.
You could parallel the AU7 and have plenty of drive, if you want to use the whole tube. or if it's stereo, use 1/2 a channel.. or something like that.
have fun 😀
That and I hate direct coupled stuff, I'd cap couple it and put in a divider to bias the CF... or drop it altogether, an AU7 shouldn't have a problem driving a 12B4, I wouldn't think... but whatever.
You could parallel the AU7 and have plenty of drive, if you want to use the whole tube. or if it's stereo, use 1/2 a channel.. or something like that.
have fun 😀
Colt45 said:
The reason for the CF is to drive a subwoofer amp in parallel with the output stage. Also I would think that a .68uF cap into a appoximately 50K load(when driving two loads in parallel) would be a big enough coupling/output cap. I can make the cap as big as needed. I left the 470K grid leak in to keep the input impedance of the total load I will be driving as high as possible.
...kinda looks like the original Bottlehead Foreplay circuit driving the 12b4, which benefited from ccs(source) loading of the VA section, and ccs(sink) on the CF.
If I were building a little 12b4 amp, I would be tempted to go push-pull. 🙂
If I were building a little 12b4 amp, I would be tempted to go push-pull. 🙂
pmillett said:
Do you see any problem with it (the modified Foreplay part) drivng a subwoofer amp and the output section in Parallel Pete? Do I have the coupling cap sized too small for a 50K load?
pedroskova said:
It is basically the original Foreplay circuit with a few changes of the operating points. I plan on building the SE version on a rather large chassis (17x10) and if I don't get enough volume out of my full ranger speakers then I can modify it as a PP amp. I could use the CCS boards on it but I really don't need all of the gain. The output stage will only take 48v PP before clipping. This is designed with efficient speakers and a small room in mind. 95 dB from a full ranger in a small room is loud.
Not as long as the sub amp is well-behaved. I'd be a little worried that it might inject some DC back into the grid of the 12B4.
A safer bet would be to use two coupling caps - maybe a 0.1 or 0.22 to the 12B4, and a seperate .68uF or 1uF to the sub amp.
You could even try taking the 12B4 cap to the plate of the first 12AU7 (bypassing the CF) - I don't think it would have too much trouble driving the 12B4 directly.
I didn't calculate what gain you'll wind up with, but I expect it will be a little low. Might be fine for your system. If you need more gain you could swap in a 12AT7 or something else (5687/7119 come to mind).
Pete
A safer bet would be to use two coupling caps - maybe a 0.1 or 0.22 to the 12B4, and a seperate .68uF or 1uF to the sub amp.
You could even try taking the 12B4 cap to the plate of the first 12AU7 (bypassing the CF) - I don't think it would have too much trouble driving the 12B4 directly.
I didn't calculate what gain you'll wind up with, but I expect it will be a little low. Might be fine for your system. If you need more gain you could swap in a 12AT7 or something else (5687/7119 come to mind).
Pete
pmillett said:
This is the point where my ignorance starts to really embarrass me. THe datasheet for the 12AU7A shows a "amplification factor" of 20. Will I get a voltage gain of 20 out of this circuit? If not then you are correct in that I will have to choose another tube. I would like to get as close as I can to swinging 48v PP into the 12B4A without driving it into clipping. I bought four "Vitamin Q" .68uF caps for this project so I can use one to the sub amp and one to the output stage without a problem. I can also resize the grid leak resistor on the 12B4A to something a little smaller if you feel that it is needed with the .68uF cap but I would like to use the caps I bought.
pmillett said:
That was one of my concerns as well, however I see it has been addressed.
How would the input of the sub amplifier behave with a 40Vpp waveform at its input if you drove the 12B4 to approximately full output?
I would consider buffering the subwoofer output separately and directly from the volume control. Perhaps you could dispense with the existing CF stage and drive the 12B4 directly with 1/2 of the 12AU7, use the other half for a unity gain buffer (CF) off of the volume control to drive your sub amplifier.
Also the 12AU7 has better linearity and sound at somewhat higher currents - I'd recommend something > 5mA..
kevinkr said:
My subwoofer amp has a built in attenuator so I had planned on just dialing it way back and hoping for the best. I'm driving it now with basically the same circuit so I don't anticipate any problems.
As far as the 12AU7 goes I agree that a little more current would increase the linearity but it would cut back on the headroom at the input of the amp. I have sacrificed linearity for headroom. Hopefully the tradeoff is not too drastic.
G said:
Nope. You can approach getting the "amplification factor" with a CCS on the plate - with a resistive load you'll get quite a bit less. I'm too lazy to work it out but I would guess you'll get a gain of closer to 12.
Pete
pmillett said:
Well that sucks. So that would mean that I won't get all of the potential output out of the 12B4A that I would if I used a higher gain tube right? I could use a 12AT7 but then I would risk my source clipping the input. Interestin' conundrum. I suppose I could just use CC sources but even then I wouldn't get enough gain to push the 12B4A to full potential output.
Just put in a 12AT7 in the front gain stage and you will be fine with the gain your looking to get.....the operating points are close enough....or just tweak them....
Chris
Chris
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