Can Gain Structure be affected by increased current introduced into the system by magnetic conduction power chords?
Beside invasive alterations to power amps to adjust Gain structure, what other methods can be recommended? In line attenuators, TVC placement between preamp/output and power amp/ input...
Beside invasive alterations to power amps to adjust Gain structure, what other methods can be recommended? In line attenuators, TVC placement between preamp/output and power amp/ input...
Always fun when you discover an article addressing the very things you've been thinking about for some time but didn't know where or how to find answers!
I'm building a Pass Labs F4 power amp. It is rated at 25 watts per channel into 8 ohms, with a max unclipped output of +/-20 volts, and max output current of 5 amps.
The F4 has no gain, potentially requiring a pre-amp with gain, so I'm considering - but not decided on - Nelson Pass' BA-3 (link).
Based on the diagrams in Pano's article, can someone help me figure out the math to calculate how much/little gain I need at the pre-amp stage?
Collective wisdom on the F4 threads points out that under certain circumstances gain might not be required at all. Nonetheless, I'd still like to know how to calculate for this.
Good questions, I'm glad the article was of some use to you.
If the F4 is rated at 25 watts into 8 ohms, we can suppose that it can supply about 14 volts RMS max. If the F4 has no voltage gain of its own, then you'd need 14 volts into it to get full power out of it.
How much gain do you need in the preamp? If your source is a typical CD player with a 2 volt output, then you'd need a voltage gain of at least 7X to fully drive the F4. 7X voltage gain is close to 17dB. However if your source is at 1 volt max (many of mine are) then you'd want 23dB of gain. Not too hard for most preamps, but some might not give you a clean 14 volt RMS output. That's something to consider. 14 volts is 25dBu, which is a healthy level, even in pro equipment.
If the F4 is rated at 25 watts into 8 ohms, we can suppose that it can supply about 14 volts RMS max. If the F4 has no voltage gain of its own, then you'd need 14 volts into it to get full power out of it.
How much gain do you need in the preamp? If your source is a typical CD player with a 2 volt output, then you'd need a voltage gain of at least 7X to fully drive the F4. 7X voltage gain is close to 17dB. However if your source is at 1 volt max (many of mine are) then you'd want 23dB of gain. Not too hard for most preamps, but some might not give you a clean 14 volt RMS output. That's something to consider. 14 volts is 25dBu, which is a healthy level, even in pro equipment.
Thanks for explaining that Pano, although I'm still grappling with understanding it all.
So if 7x gain is needed to drive the F4 to full output, then I imagine less being needed to drive large speakers of 90-ish dB sensitivity at modest listening levels?
My sources are a turntable via phono preamp, a CD player, and a raspberry pi based streamer + DAC.
I see merit in not having abundant gain, only to have to attenuate it right after (or before?) the preamp; but also don't want a situation where some is needed but none is available.
What say you?
So if 7x gain is needed to drive the F4 to full output, then I imagine less being needed to drive large speakers of 90-ish dB sensitivity at modest listening levels?
My sources are a turntable via phono preamp, a CD player, and a raspberry pi based streamer + DAC.
I see merit in not having abundant gain, only to have to attenuate it right after (or before?) the preamp; but also don't want a situation where some is needed but none is available.
What say you?
I agree, you don't need more gain than you need. 
Most systems have far more gain than they need because gain is cheap and it helps to make the system usable across a wide range of situations. You can always turn it down if there is too much gain, but it's hard to turn it up if there isn't enough.
Most people with efficient speakers in a medium size rooms won't be using much, if any, voltage gain from source to speaker - which is surprising. I've run systems (or parts of systems) where the voltage at the speaker was noticeably less then the source. Bigger room and inefficient speakers do need gain, and sometimes a lot of it!
If you can do the test linked in my signature line, you'll have a good idea of how much voltage you need at the speakers. Given that the F4 has no voltage gain, you'd want at least that much voltage out of your preamp. And it doesn't hurt to have a few dB extra. I would think that you'd want somewhere around 12dB of gain for your use, but there are ways to actually determine that thru measurement.
Most systems have far more gain than they need because gain is cheap and it helps to make the system usable across a wide range of situations. You can always turn it down if there is too much gain, but it's hard to turn it up if there isn't enough.Most people with efficient speakers in a medium size rooms won't be using much, if any, voltage gain from source to speaker - which is surprising. I've run systems (or parts of systems) where the voltage at the speaker was noticeably less then the source. Bigger room and inefficient speakers do need gain, and sometimes a lot of it!
If you can do the test linked in my signature line, you'll have a good idea of how much voltage you need at the speakers. Given that the F4 has no voltage gain, you'd want at least that much voltage out of your preamp. And it doesn't hurt to have a few dB extra. I would think that you'd want somewhere around 12dB of gain for your use, but there are ways to actually determine that thru measurement.
I agree, you don't need more gain than you need.
Thanks for that bit of guidance Pano. I will run the speaker voltage test (and read the thread) over the weekend and see what I come up with.
Cheers!
