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Old 6th February 2009, 07:33 PM   #2731
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Ok... an update.

1. I connected up a different CDP and set it playing. Almost immediately the bias jumped all over the place and the resistors started to cook big time.

2. That was with no speakers connected but using the full line output of the CDP. Last night it would have being playing with a much lower input level.

3. So definitely somehow this thing is unstable, despite having 2 gate stoppers and also it wasn't even at full bias voltage, or warmed up fully. However the level of the signal input would have been much higher tonight (direct from CDP) compared to last night (rather low level through preamp).

4. Temp of the mosfets was about 70 degC just before this (taken on the metal tab). When fully warmed up last night this was 80deg C.

5. So something here is sending bias wild (ran up and down erratically between 0.5-1.5V when resting bias was 0.35V), causing huge current draw and burning out the resistors.

So to answer more Qs:

AndrewT: only took music playing for a few seconds to see this... last night with speakers connected it took much longer.

Zenmod: I can't really hook up the CRO when this is happening, i wouldn't be fast enough.

What you guys think??!? Other channel gets the same signal and is absolutely stable.

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Old 6th February 2009, 09:27 PM   #2732
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i would try to replace q1 and q2 and if no change q3 and q4. it seems to me that one transistor is the culprit ( a bad soldering excepted). If it solves your problem you can always check the semiconductors one by one.
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Old 6th February 2009, 11:00 PM   #2733
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Yeah, I think so.

Why doyou think to change Q1 or Q2 first rather than Q3/4? Just wondering in an attempt to learn more is all.....

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Old 7th February 2009, 02:20 AM   #2734
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Default There's something puzzling me --

Chart 7 and Chart 11 in the audioXpress article seem to be a little anamolous -- if we look at Chart 7, the THD% @ 2W into 8 ohms is 0.0027% -- but in Chart 11, the same data point has a THD% of 0.006% Expliquez cela a mois, s'il vous plais.

FWIW, in an F5 I just put together this afternoon, the 1kHz, 2W, THD% was 0.006%. I know we're counting the number of angels which can fit on the head of a pin, Aquinas, but.....
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Old 7th February 2009, 06:36 AM   #2735
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Woodturner- fran
No spécial reason. It is up to you.
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Old 7th February 2009, 06:57 AM   #2736
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wich caps you guys recommend me for Power Supply?
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Old 7th February 2009, 07:35 AM   #2737
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Hi, Fran

It seems to be beyond my ability.

I understand from you that the amp is ok with dc bias, but it is problem with music signal at the input. Is it parasitic oscillation? If so, I would try 220 ohms of the mosfet gate resistors or greater values, replacing the original 47 ohms.

Is there any possibility of positive feedback?

If I am sure of all others, I would try to replace jfets and mosfets as bobodioulasso says.

Good luck!

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Old 7th February 2009, 08:26 AM   #2738
AndrewT is offline AndrewT  Scotland
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does the input have an RF filter fitted?
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 7th February 2009, 08:39 AM   #2739
rhysh is offline rhysh  United Kingdom
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I have just had a thought, i am using floorstanders with two drivers a piece. Im sure i read somewhere that the F5 was designed for single driver loudspeakers?
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Old 7th February 2009, 09:06 AM   #2740
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but F5 has voltage gain, and so it shouldnt be a problem with crossovers. I also use a two way system.
Or are there any other criteria than current/voltage gain for "preferred single driver"-mode?
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