Pre-amp gain and the power amp?

Mulburg

Paid Member
I don‘t understand how the gain from a pre-amp affects the power amp. Will someone give me a beginner’s explanation? My pre-amp is a bottlehead moreplay, the spec states “Gain: (with balance control centered) 9dB”. My amp is an aleph j, my source is an old iPad.

Gruesome

9 dB gain means the voltage coming into the preamp is multiplied by 2.828.

6 dB = factor 2 in voltage (and factor 4 in power, since power = voltage squared over impedance), 3 dB = factor square root of two = 1.41421.... in voltage.

Maybe your source voltage is a bit low for your power amp, or maybe it isn't. At least one of the three (iPad, pre amp or power amp) should have a knob to control its gain. You could look up what your different gizmos (iPad, preamp) put out in terms of voltage, and compare that to what your power amp expects or needs.

Not sure what an F4 thread is.

6L6

Paid Member
Source - in comparison to everything else, a small signal. Small voltage, very small current. Some are smaller than others (think phono)

Preamp - Most have some gain, and also they will make the signal low-impedance, meaning they will add some current, just in case there's long cables to run or a particularly grumpy (low) amplifier input impedance. Your preamp has a gain of 9db (2.8x) and a small current gain.

Power amp - now almost all power amps have quite a bit of voltage and current gain, because the signal is still too small to power speakers. Speakers need a lot of current and more voltage. Every power amp you've ever used has some voltage gain (as well as current). The Aleph J has a voltage gain of about 20db (10x), and a current gain of a heck of a lot, because it drives speakers.

Ok...

Let's say your source is outputting 1v.

When it goes to your preamp it will increase the signal by 2.8x. The signal is now 2.8v

The 2.8v signal now enters the power amp. it has a voltage gain of 10x, making the signal 28v (and adding a whole lot of current)

28v peak to peak is 9.8vRMS.

Amplifier power is Vrms^2/load impedance (let's say 8ohms)

9.8^2 = 96

96/8 = 12W

So with a 1V signal your system can run 12W into the speakers. 1V then 2.8V then 28V.

If you want full power out of your Aleph J, you'll need a stronger source, or a preamp with more gain.

NOW, since you mentioned F4, you have to understand that the voltage gain of that amplifier is zero. The current gain is still lots and lots.

But the voltage at the input of the F4 will always be the same as the output. (in reality it's a teeny bit smaller, but close enough to zero it doesn't matter)

So in your case, you will have the 1v from your source, then the 9db (2.8x) of your preamp, giving you a total of... yep... 2.8v.

2.8pk-pk is 0.98vrms

.98^2 = .96

.96/8 = 0.12W

Yes, really. (this will be, however, louder than you'd expect coming out of the speakers...)

Anyway, the point of the F4 is you need a preamp with about 30-35db, and the ability to swing 40v pk-pk to drive it to clipping. (But the vast majority of people don't listen anywhere near that loud, so something less may be just fine)

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Mulburg

Paid Member
9 dB gain means the voltage coming into the preamp is multiplied by 2.828.

6 dB = factor 2 in voltage (and factor 4 in power, since power = voltage squared over impedance), 3 dB = factor square root of two = 1.41421.... in voltage.

Maybe your source voltage is a bit low for your power amp, or maybe it isn't. At least one of the three (iPad, pre amp or power amp) should have a knob to control its gain. You could look up what your different gizmos (iPad, preamp) put out in terms of voltage, and compare that to what your power amp expects or needs.

Not sure what an F4 thread is.
Thank you. It’s a thread on the forum, someone asking about what preamp to use for the F4 amp.

1 user

Paid Member
Thank you 6L6.