Which F5 to build?

Apologies if I missed the thread(s) that cover this question.

I’m trying to decide on which version of the F5 to build.
My speakers are 91.5dB SPL
Nominal impedance is 4Ω, but they do drop to 3Ω4 with a 25º phase shift.

My first thought was the original F5 as Nelson says “It will drive a 2 ohm load without burping and 1 ohm without misbehaving.”

But the thought of a little more Class A bias appeals to my desire to reduce the Class “A” to Class “B” hump with higher bias.

So my second thought is the F5 Turbo V2, to accommodate a higher bias. But will two O/P devices biased at 1 amp each, produce the same curve as one device biased at 2 amps?

BTW, what’s meant by the statement “This amplifier will poop out slightly above 1 ohm.” (P.S. Voltage limitation?)?

Thanks in advance for your input,

Mark
 

ra7

Member
2009-02-07 6:47 am
Davis, CA
I keep coming back to Papa's original remarks, which were that this amp (and other Pass amps) usually have less gain than is the norm, and so, depending upon your room and your speakers, you may need a preamp to drive the amp to full output.

For example, if you use the B1 buffer for a preamp, and let it run full out, the output of a CD player, which is normally 2V, will result in a output at the speaker of 11V given 15db of gain in a standard F5, which at 8 ohms is about 15 watts.
 
Last edited:

ra7

Member
2009-02-07 6:47 am
Davis, CA
Right. The F5 is unfairly criticized for not having enough power. 25W is a lot of power, at least for me, as long as you drive it properly or make other changes, such as increasing the gain of the circuit. In fact, its preferable to have low gain in the power amplifer. It will not amplify the noise in the system as much as a high-gain amp would.