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Old 9th November 2012, 11:53 AM   #12661
Itsmee is offline Itsmee  England
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Quote:
Originally Posted by valvesound View Post
I biased both channels nicely as described 0.6V over the .47R and output dc offset to less than 10mV on both channels. I then went and connected source and speakers and the one channel is dead
Check your input wiring. If you biased the output ok, then you have some resistance in series with the drains of Q1 & Q2 - do you have a signal at the drain of Q1 & Q2? Ok, it will be small and not necessaraly equal.
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Old 9th November 2012, 12:16 PM   #12662
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There are only 4 active devices.
All 4 work in controlling the output bias current and in turn controlling the output offset.
As far as I know all 4 must be working to at least some performance to achieve both output bias and output offset.

As far as I can see/understand the no output signal has to be before the gates to the jFETs.
i.e. an input error.
Swap channels at the outputs terminals,
Swap channels at the input terminals.
Swap channels at the Source.
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Old 10th November 2012, 11:51 AM   #12663
Itsmee is offline Itsmee  England
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Quote:
Originally Posted by AndrewT View Post
As far as I know all 4 must be working to at least some performance to achieve both output bias and output offset.
Output bias can still be achieved with short circuit JFET's.

Valvesound,

Hopefully you have found the problem, if you haven't, check the voltage at the drains of Q1 & Q2 with a meter, with a fully biased output and working JFET's, it should be between 3 & 4.5 volts lower than the supply rail(s).

Note, N & P FET devices require different bias voltages so don't expect them to be the same.
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Old 10th November 2012, 12:17 PM   #12664
AndrewT is offline AndrewT  Scotland
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Output bias can still be achieved with short circuit JFET's...........
I'm not sure about this. Are you sure you are right?
Short out one or both jFETs and the currents in the resistors will allow output bias and output offset to be set up properly?
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Old 10th November 2012, 12:22 PM   #12665
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It will be very sensitive, but yes it can be done.
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Old 10th November 2012, 12:30 PM   #12666
AndrewT is offline AndrewT  Scotland
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I can't see it.
That shorting of both jFETs to mainatin symetry will pull both output gates to the same voltage.
That will put supply rail voltage across the two sets of GS resistors.
If both VR are @ zero ohms then the input burns up the traces due to tens/hundreds of amperes flowing.

If one jFET is shorted then the other will have to take double the Vds and I'm not sure what happens when one tries to set up the bias and the offset by adjusting the VRs from their zero r point.
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Old 10th November 2012, 09:20 PM   #12667
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If both VR are @ zero ohms then the input burns up the traces due to tens/hundreds of amperes flowing.
The 10 ohm source resistors would limit the maximum current to 2.4A, if there is one turn on the ten turn pot from short circuit, the current would be aprox 100mA

24v/(5K/10)//(2k2+~4k7)=~0.1A
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Old 11th November 2012, 10:19 AM   #12668
AndrewT is offline AndrewT  Scotland
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2.4A^2 * 10r = 57.6W for the Source resistor.
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Old 12th November 2012, 05:13 AM   #12669
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it is all sorted! The mosfets were replaced and there was sound! I will place some pics and a review compared to the JLH 25W they replaced.
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Old 12th November 2012, 09:42 AM   #12670
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The amp is still burning-in but overall I am very happy with the F5.

A quick comparison F5 25W class A to JLH 25W class A is:
There is an overall improvement with the F5 but subtle, not night and day.... yet, there has been improvement from Friday evening to last night.
Bass is less than JLH but not lean in any way.
Female vocals are superb with more texture.
The music sounds higher in definition (like comparing full 1080p Blue ray to upscaled DVD)

The JLH is a great amp but took a lot of tweaking with boutique components to sound as it did, where the F5 was great as is "out the box".
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