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Old 6th November 2013, 12:39 PM   #13441
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Sorry for th last message,
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Old 11th November 2013, 12:03 AM   #13442
crcook is offline crcook  United States
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Join Date: Mar 2009
Location: Silicon Valley, California, USA
I recently built an F5 from diyAudio boards. All parts from Mouser, except input JFETs, bought, as suggested from Spencer. In diyAudio 4U case.

Amp powers up fine, bias is set to .59v with virtually no offset at the output. Music comes though, sounds nice. One problem.... output on right channel is low. Put in 2 volts 1K hz signal from old HP generator. Measured output on a scope. Left channel is ~12vAC out, right channel is about ~6.5 vAC out. So, the gain in the right channel is roughly half. Output FETs seem to have correct voltages, bias set correctly. Only change from original plans is that I am using a 16v transformer vs. 18v as specified. 18v was out of stock, and I had access to a 16v 300v a unit. Output of power supply is roughly +/- 20vDC, using diyAudio PSU.

I am fairly new at FET troubleshooting, pretty good with tubes. Ideas? I am thinking the input FETs are bad/weak. I did order an extra set from Spencer, but want to check other things before replacing.

Thanks for any advice.
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Old 11th November 2013, 12:06 AM   #13443
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Most likely a resistor off value or not connected properly.

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Old 11th November 2013, 04:57 AM   #13444
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On a tube amp you would start doing a voltage table, measuring the voltages on all the active devices, without any signal fed into the amp.

I suppose the same would help here. If you compare the good side with the bad side, you will probably quickly be able to dial in to the area were the problem is.

I havn´t fixed mine yet. Just got the power supplies yesterday.

Best regards
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Old 11th November 2013, 05:32 PM   #13445
crcook is offline crcook  United States
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Location: Silicon Valley, California, USA
Something simple. Put in 4.7K resistor where 47K was needed. Gee, the 'red' band looked 'orange' for some reason. Must have been the Zinfandel....

Thanks to all.
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Old 11th November 2013, 10:04 PM   #13446
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Nelson nailed it!
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Old 12th November 2013, 04:45 AM   #13447
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Sorry stupid question but I would like to build the F5 amp but I was wondering that I do not have grounded electrical outlet in the apartment. There are only two holes, brown and blue. Not yellow-green at all. What harm or danger it could cause?
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Old 12th November 2013, 06:27 AM   #13448
Corpius is offline Corpius  Netherlands
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Lot's of commercial gear is not earthed nowadays, but in the EU there are some rules that apply to non-earthed gear.
You could use double isolation in the internal wires, make sure the circuit boards and other live parts have a distance of at least 10mm (preferably more) from the chassis, use extra isolation between the transformer and chassis, do use heat shrinks on all wires/connections, use heatsinks on the rectifier diodes instead of bolting them to the chassis and omit the ground-to-chassis connection. All at your own risk of course, but that also accounts for building an amplifier with a connection to earth.

A better (read safer) option would be to replace your mains wiring and add a earthed line to it. That's what I did a few years ago.
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Old 24th November 2013, 12:28 PM   #13449
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I have read the F5 manual a few times more and think I understand the circuit better now: R9 is the gate osc prevention for resistor Q1, R10 sets the input impedance, Q1 is the voltage amplifier with a gain of 10+ (probably more, not sure how much). R3 and R1 sets the gain together with Q1´s ´internal resistance´. TH1 and R15 prevents thermal runaway. Q5/R17/R19/R21 limits the current by overbias or output short. R13 is the gate osc prevention resistor for Q3, and Q3 is the current amplifier. R11 stabilizes the circuit, and R5/R7 are FB resistors together with R3 (which I dont understand).

Understanding the circuit is part of the way. So does Q5 also securely limit that the output FETs does not get more than 20V input, which I have read in Bob Cordels book will kill it ?

In all the many posts that I have read, debugging has been a very minor subject. It is almost like: Build the thing and Hope and Pray :-)

My idea was to test the circuit backwards: Desolder the Drain and Gate on the N-channel Power MOS FET and test if it works according to Nelsons document ´How to - Matching Devices, with an external PS. Then desolder the Source and Gate on the P-channel and test that too. Test the voltage across R11/R12 and test if it possible to get around 4V across R3 and R4. I dont know how to test the Q5 and Q6 current limiting circuits neither the thermistor circuits.

You could also use a signal from a signalgenerator or if such is not present, send an audio signal to the input to see if it gets amplified by Q1 / Q2. A standard DMM will probably not measure higher than between 100 - 1000 hz on AC mV, but that doesnt matter. You will still get a good indication. A scope would be better.

I have always been quite uncomfortable with solid state power amps, because I have had my accicents with some, and therefore I would prefer to test the board without the Power MOS FETs connected. Is that possible, or does the circuit need the Power devices to work correctly ? That would have been a good extension to the illustrated build manual.

As stated earlier I am mostly used to tube amps, and I would never install any tubes, before I have checked the voltages on all pins. And I allways test the PSU unloaded. That has saved me a lot of trouble over time.

... But I disconnected the PSU´s and it showed that the fancy pancy double diodes had died on me. I changed the contruction to Vishay 35A bridges - the PSU worked again, unloaded.

I then used the recommandation to use som smaller external PSU´s to test the circuits. It seems that both side work. No problem getting voltage across R3/4 and R11/12. I went up to drawing 0.5A in each side.

Quite happy I connected the newly working PSU in one channel - blowing the 1A fuse I had put in for safety. Ok I thought I will put in a lager one - 1,6A - also blown.

My F5 is allmost standard, but I have used the Toshiba MOS FETs and 2,2 mH coils in the power supply. Do the coils draw so much current that they will smelt the fuse? I did check the wiring, there does not seem to be anything wrong with that.

I am not that happy with just putting larger fuses in, without knowing what I am doing.
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Old 24th November 2013, 12:35 PM   #13450
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I should add that I use 500VA transformer and that I have VDR inrush protection.
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