An illustrated guide to building an F5

[LucasAdamson]

I am about to start my build. Can I get some advice please.

Heatsinks are the Conrad MF35-151.5 (2 of them, 1 per channel), 0.02mm mica washers and silicon grease.

Antek AS-4218 400VA 2x18v, 8x 50v 18,000uF caps, CRC standard Pass power supply.

PCBs are DIYAudio, PRP 1/4 watt resistors, Panasonic 3w/5w resistors, hand-matched JFets (K170, J74), not matched mosfets (IRFP9140 & IRFP240)

No thermal protection or current limiting portions of the standard circuit. No DC speaker protection either. Just a speaker delay at power-on to prevent surges.

What special precautions do I need to take to make this honed down version of the circuit? I know biasing is likely to take longer, and that drift may occur in extremes of temperature, so any experience or advice is appreciated.

[6L6]

You may remove the limiter (be careful), but leave the thermistor in the bias circuit. It's not worth the brain-damage with keeping the bias stable if it's not there.

The F5 is one big teeter-totter, thermally tracking the bias should be considered mandatory.

Nelson removed the limiter in the F5Turbo, but not the bias thermistor. That's a very strong hint.

[AudioSan]

agree With 6L6. and you can drop the speaker deley. there is no need for it on a F5.

[LucasAdamson]

OK, so you are right. I was inviting somebody to tell me how stupid a plan that was. I was just taken in by what Peter Daniel said about his F5, that it was fairly average until he removed those parts of the design. I guess he is expert enough to deal with the consequences of an amp that loses bias relentlessly.

So, what really constitutes the thermal tracking part? Is it just R15, R16 and the thermistors?

[jacco vermeulen]

Assume nothing

[LucasAdamson]

OK, so obviously R11 & R12 are also for thermal stability.

I will try and assume nothing.

[6L6]

Originally Posted by LucasAdamson

I was just taken in by what Peter Daniel said about his F5, that it was fairly average until he removed those parts of the design.

As much as I like Peter and his genuinely fantastic products, he is the type that talks about the different sounds of heatsinks...


Anyway, the easiest way to disable the limiter is to omit the ZTX transistors.

[RCruz]

This one is for me also

Thank you for sharing 6L6

[6L6]

I'm glad you found it useful.

Please take a look at the F4 guide, there is much in common to both amps. Obviously the amp circuit is different, but the PSU, chassis, and many connections are similar.

A guide to building the Pass F4 amplifier.