Hi folks,
A while ago i build a Alembic F2b copy, and hooked it up at a Hypex UCD180 power amplifier. It sounded amazing, but since the output impedence of the circuit is fairly high, at high power, the poweramp goes to safe mode for a few seconds.
To solve this problem i wanted to build a buffer stage after the preamp to decrease the output impedence. Since i don't have much experience i hope you guys can take a look at it.
The alembic f2b design. I removed the decoupling capacitor since it's allready in the buffer stage.
The buffer circuit. A cathode follower circuit with one half of a 12AU7 tube, so there not much voltage loss, and enough current gain. The C1 is 470nF
Do you guys think this could work out?
Thanks for the help.
A while ago i build a Alembic F2b copy, and hooked it up at a Hypex UCD180 power amplifier. It sounded amazing, but since the output impedence of the circuit is fairly high, at high power, the poweramp goes to safe mode for a few seconds.
To solve this problem i wanted to build a buffer stage after the preamp to decrease the output impedence. Since i don't have much experience i hope you guys can take a look at it.
The alembic f2b design. I removed the decoupling capacitor since it's allready in the buffer stage.
An externally hosted image should be here but it was not working when we last tested it.
The buffer circuit. A cathode follower circuit with one half of a 12AU7 tube, so there not much voltage loss, and enough current gain. The C1 is 470nF
An externally hosted image should be here but it was not working when we last tested it.
Do you guys think this could work out?
Thanks for the help.
Last edited:
Adding an output buffer is fine, in fact is needed, if you want to use that fine preamp more than a few meters away from the power amp.
It also makes it an easier driver for any amp; otherwise it would be quite limited, so do it.
That said, I very much doubt that preamp impedance mismatch by itself makes any amp go into protect mode; there must be something else going on.
Load, temperature, etc.
Only problem is that its output might be too high for that power amp input, and making it low impedance will make the problem worse, not better.
Personally, I would :
a) add the buffer.
b) feed , say, 100mV @ 1KHz (or 440Hz) at the preamp input to simulate an average instrument
c) set all tone and volume controls to "7" (don't ask why, it works 😉 )
d) measure audio voltage at the preamp output.
If value measured there is roughly what the power amp needs (say power amp needs 1V RMS for full power and you get something between 1 and 2 V RMS) , fine, your preamp is matched to your power amp.
But I fear you will have much more, say 8V RMS or some other too high value ... which will slam your power amp input and may drive it into protect.
In that case, add some attenuation (say, 220K/47K or whatever's needed) between original preamp output and buffer stage.
Which by the way now can be an unity gain Op Amp or a transistor or Fet emitter follower.
Much easier 🙂
Edit : what does slam mean?
Something like this:
That preamp can easily put out some 60V RMS ... your power amp does not need much more than 1V RMS.
Hope the graphic example becomes clear now 🙁
It also makes it an easier driver for any amp; otherwise it would be quite limited, so do it.
That said, I very much doubt that preamp impedance mismatch by itself makes any amp go into protect mode; there must be something else going on.
Load, temperature, etc.
Only problem is that its output might be too high for that power amp input, and making it low impedance will make the problem worse, not better.
Personally, I would :
a) add the buffer.
b) feed , say, 100mV @ 1KHz (or 440Hz) at the preamp input to simulate an average instrument
c) set all tone and volume controls to "7" (don't ask why, it works 😉 )
d) measure audio voltage at the preamp output.
If value measured there is roughly what the power amp needs (say power amp needs 1V RMS for full power and you get something between 1 and 2 V RMS) , fine, your preamp is matched to your power amp.
But I fear you will have much more, say 8V RMS or some other too high value ... which will slam your power amp input and may drive it into protect.
In that case, add some attenuation (say, 220K/47K or whatever's needed) between original preamp output and buffer stage.
Which by the way now can be an unity gain Op Amp or a transistor or Fet emitter follower.
Much easier 🙂
Edit : what does slam mean?
Something like this:
An externally hosted image should be here but it was not working when we last tested it.
That preamp can easily put out some 60V RMS ... your power amp does not need much more than 1V RMS.
Hope the graphic example becomes clear now 🙁
Last edited:
I agree with JM somewhat. I think the power amp needs a limiter to protect it, not a buffer.
When I last played with a Class-D power amp, I used the diode bridge clipper circuit from the Marshall JCM900 ahead of it.
When I last played with a Class-D power amp, I used the diode bridge clipper circuit from the Marshall JCM900 ahead of it.
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