Which F5 to build?

[Mark Bartley]

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

 

[Zen Mod]

you need Amps , not Volts

dunno for nomenclature ( neither I can remember it ) but I'll make regular F5 , with two pairs of outputs , biased to as much your sinks can sink , up to 35W dissipation per device ( meaning 140W per channel max.)

 

[AudioSan]

agree With mighty Zen here🙂 go for "standar" F5. dual output pairs and higher bias. but stay at +/-23V supply. With dual pairs and 2.5A bias, it will stay in class A all the way at 4ohm.

 

[6L6]

The gentlemen are saying to build an F5C, with two outputs and 24v rails.

Hopefully the pcb will be restocked in the store soon.

 

[Mark Bartley]

Thanks

Thanks for comfirming what I was leaning toward.

Now for those darn boards to come in stock.......

Thanks again, Mark

 

[ra7]

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.

 

[Nelson Pass]

That said, you can easily increase the gain by increasing the value of the 100
ohm feedback resistors. There's lots of open loop gain to spare in the design.

😎

 

[6L6]

The answer from the Master himself. Neat!

220ohm 3W resistors in the feedback circuit (R9, 10, 11, 12 in the original F5) will bring it to 'normal' gain.

 

[ra7]

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.

 

[Bksabath]

I have 4 homs Polks LS15i 87dB
running 26V rails twins F5
do not solder on the board the current limiters circuit get speaker protection
I run 50W dissipation on TO220

Question you shuld ask yourself is how much power do I need
25 F5 W may be plenty

 

[Mark Bartley]

Power

Yes, I'm figuring that a F5T-V2 should hit 100W into 4 ohms.
With 91.5 SPL sensitivity speakers, that should be plenty.

As far as gain, I estimated that ~2.5V from a pre-amp will to drive the F5 to full O/P. I have no problem reaching that level. Comments?

Thanks, Mark

 

[6L6]

Mark- your assumptions are correct. It's a great choice.

 

[Mark Bartley]

Thanks again

Your input and assurance is appreciated.