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.
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.
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
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
I have a question regarding this section in the article:
Is there a "rule of thumb" when parallel J-Fets are recommended? Currently running the amp with 4 output pairs per side (Toshiba 2SK1530 / 2SJ201 which seem to have higher capacitance than the Fairchild parts).
Many Thanks
Sven
Is there a "rule of thumb" when parallel J-Fets are recommended? Currently running the amp with 4 output pairs per side (Toshiba 2SK1530 / 2SJ201 which seem to have higher capacitance than the Fairchild parts).
Many Thanks
Sven
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If you have the Fairchild output devices and Vishay MUR3020 diodes, go for it! However, if your like me, using IRFP9240 & 240 output devices and OnSemi diodes, be prepared to deal with a difficult amp to stabilize. At first I thought my skills adjusting bias and offset were seriously lacking, especially when sparks, flames and smoke filled the workshop the first time. Hint: When that happens to you, check your JFET's any time you fry an output device. When I set bias to 300mV across the test points, offset would wander off, I would adjust offset again and then bias would take off.
Very frustrating, I stated this project in 2018, only today do I finally have both of the monoblock amps stable. Stable where the bias and offset values when I turn it off, after 2.5 hours being on, the next day when I turn it on I have the same bias and offset values after 30 minutes.
Besides replacing multiple output devices and several K170 JFET's, and various resistors, removing the diodes and changing the gate stopper resistors from 47 ohms to 470 ohms on one amp and 560 ohms on the other.
I'm about to hook up the amp to my workshop speakers, they have endured 7 different PASSDIY amps so far, I hope they survive the F5T. I built the v3 version, 48vdc rails, Balanced with an N & P channel boards for each positive and negative phase. So 4 output boards for each channel, instead of in parallel for a stereo amp. I have the bias set to 370mV on the N channel and 350mV on the P channel, I have plenty of heatsink, that has never been an issue.
I encourage you to build this amp, the last week or so adjusting the bias and offset has been very satisfying. I'll be spending tomorrow removing the 470 ohm gate stopper resistors and installing 560 ohm resistors so both amps are the same. I've learned so much building these amps, I'm not sure how much power (watts) they will produce, I have a lot of respect for them.
After 6 years, my Balanced F5Turbo Mono blocks are playing music! No hum or any noise with either the inputs shorted or the preamp at full volume. I pulled my recent F6 amp from the work shop system and the F5t's will play for the couple of weeks.
Only tow more chores:
1 - Adjust P3 on both amps, right now they are set in the middle.
2- Figure out how to take +48vdc and change it to 12vdc for the VU meter driver board.
I looked in the forum on how to calculate amplifier power, I have + - 48vdc rails, biased to 0.370mV across the test points. All my speakers are 8 ohms, with two N channel and 2 P channel boards per channel, I came up with 120W per channel.
I'm using 600 VA torroid transformers for each channel, How do I calculate how much class A power output?
Only tow more chores:
1 - Adjust P3 on both amps, right now they are set in the middle.
2- Figure out how to take +48vdc and change it to 12vdc for the VU meter driver board.
I looked in the forum on how to calculate amplifier power, I have + - 48vdc rails, biased to 0.370mV across the test points. All my speakers are 8 ohms, with two N channel and 2 P channel boards per channel, I came up with 120W per channel.
I'm using 600 VA torroid transformers for each channel, How do I calculate how much class A power output?