F5 Turbo Builders Thread

It’s been 6 years or so since I built my F5V3. IIRC the stock gain is 10. When I built my F5V3 I ended up with a gain of 13 (22db). I believe I increased the series feedback resistor to a value I had on hand. Distortion is still low at that gain. You could also try boosting gain by changing feedback resistors. The results will need to be looked at before you connect to a speaker. As well as an increase in distortion, which may not be of consequence to you, there will be less feedback at dc, so your offset voltage at the speaker terminals could have larger variations with temperature and be more difficult to adjust initially. Your best approach may be to increase preamp gain if possible. As an alternative you could use a step up transformer between the preamp and power amp. This would avoid placing any active circuitry into the signal chain but comes with it’s own set of problems.
 
It’s a good thing you had DC offset protection! Was the offset stable when you ran the amp without a signal? Seems like it could be a thermal issue to me. What could be happening is that the P and N mosfets are running at different temperatures. Just how hot are you running the output stage? Over 30w/device is pushing into thermal considerations to where you need to take great care of all the details to maintain reliability. Check to be sure all thermistors are making good thermal contact with the mosfet. Use thermal compound to help with that. If not, readjust them and re bias the output stages. I would check the mounting bolts for the mosfets to make sure they are all at roughly the same torque. You are also running 10ma Jfets at 32 volts so they may be running hot as well, which could cause an offset issue.
Hi
Now I changed schematic to F5T V3 with +/-32V with only 2 pairs output mosfets. Still used 10mA IDSS JFETs but changed R25,R26 to 4K7 for divided voltage over JFETs +/-16V instaed of original value (10K = 11VDS). After I finished all parts and made them work I can measured 250mV across all SOURCE resistors for total biased 1A and there's no DC FAULT not over +/-5mV .

SAME PROBLEM AGAIN >>> HIGH DC VALUE (2Vdc) at the output still remain with 30Wrms @8 ohm !
 
I assume you are driving the amplifier with a sine wave source to get the 30w output. How long does it take for the DC bias to show up when you are testing in this manner? Do you have an input cap for the F5 or are you DC coupled? If you are DC coupled check your signal generator to make sure there is not a DC offset on your input signal.
 
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I assume you are driving the amplifier with a sine wave source to get the 30w output. How long does it take for the DC bias to show up when you are testing in this manner? Do you have an input cap for the F5 or are you DC coupled? If you are DC coupled check your signal generator to make sure there is not a DC offset on your input signal.
Oh my godddd...
I used FE2022 for input buffer with +/-24V supply and this's a major problem ! Output couple cap was 3.3uF from Mundorf EVO it contains a huge DC voltage (300mVdc) when played a drum tracks then my F5T V3 amplified this voltage to 2Vdc. Now I shorted this output cap 3.3uF and my problem is gone.

Very happy with full power played now.

Thank you very much.
 
So I'm sure someone is running there F5T or other variants with switching supplies. Also sure of posts discussing it, but as always searching out these things can take forever. Anyone know where to direct me on this. I'm considering buying two meanwell 36vdc 160W power supplies to connect as bipolar config for my rails. My F5 is built as a V1 with the extra output transistors. Also the front end is cascoded too. The power supplies can be adjusted from 32-38 volts each. As far as biasing I'll be shooting for 1 amp each transistor. Question do you all think the power supplies are big enough to support 50W per channel and will it be quiet??
 
Hi guys.

I'm not sure of the Pass Labs licensing arrangements for past projects (or whether Nelson is bothered!) but there's a commercial guy selling a bunch of other people's designs on Gumtree in my state. Included in his stock are the F5 and F5 Turbo, with the latter offered for AU$1,650 for example (using Jim's Audio PCB's).

Is this crossing some line or other, or should I just ignore it?

Stu