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Prince3121 11th June 2012 08:40 AM

F5 output voltage problem
Hello All

I'm a new member here, but have built an F5 with the AVForums guys in South Africa.
Anybody here got an itch to build a FirstWatt F5 amp?

I simply love the sound, as I had it running sweetly in my test phase, but wanted to house it in a nice solid case. So, upon rebuilding it, I had some setbacks, with biasing issues.

I have now got the F5 to a point where the one channel is biasing nicely with an output voltage of 0.44Mv, but the second channel is giving me a -0.44Mv, after rebuilding the second board.

I'm using the IRFP240 and IRFP9240 Mosfets, ZTX450/550 transistors and 2SJ74 and 2SK170 gain transistors. My current bias voltages are 4.2v and 4.3v. Using 24V input power rails - 300VA toroidal transformer.

My question is, what can I check that affects the polarity of the amp circuit in this way?

Any help would be appreciated.


KatieandDad 11th June 2012 09:36 AM

Anything less than 10mV is a bonus. 0.44mV + or - is perfectly OK.

Prince3121 11th June 2012 12:03 PM


Originally Posted by KatieandDad (
Anything less than 10mV is a bonus. 0.44mV + or - is perfectly OK.

-Thanx for that info. I was just not sure. Will hook up the inputs tonight and do some test listening.


Prince3121 11th June 2012 08:01 PM

Tested the amp tonight, but found that the -ve output channel was playing way softer than the +ve output one. So, went back to the drawing board and discovered that one of the tracks had lifted under the MOSFET resulting in shorting out the IRFP240. Repaired the broken track, replaced the MOSFET and 2SJ74 JFET and all is well with the output voltage.

Readings are: 4.20v/4.27v and output voltage: 0.44mV


soonsak 23rd June 2012 03:30 PM

Hi I'm from Thailand I've got as same problem. My F5 got about 0.8 mV out put voltage both. And decided to connect to my speakers it haven't any problem with my speakers and normal sound good. Please don't worry let it go. Cheers

h_a 26th June 2012 07:21 AM


My F5 got about 0.8 mV out put voltage both.

So, even if you're speakers are extremely low impedance ones (R=1 Ohms) the voice coil would get heated by 0.001^2/1=0.000001 W.

Even for 100mV offset, that number rises only to 0.01 W.

You're perfectly right - just enjoy your amps, no need to worry about Milliwatts and Microwatts ;)

AndrewT 26th June 2012 02:15 PM

Just check the output offset on a regular basis while the amp is still fairly new.
Once you have checked it, say daily, for a month you can be reasonably sure it won't go wonky on you, until you abuse it.

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