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aleopeldd 23rd November 2004 03:23 AM

The AV400......
I have realized the AV400 from Mr.Holton. I have designed my own pcb with 2 channel on a 10x18 board.
First impression: never listened to a so clean and naturaly power amp!
But there is a problem: the biasing!!
I'm running it with +-35v and with the trimmer i can set the bias more high than the 18mv read on the Source resistor of the mosfet... The amp became hot in fiew minutes...
What's the problem? +-35v is right? I have only this PSU.

Kilentra 23rd November 2004 01:14 PM

Yeah, that's one of the problems with this kind of output stage... the higher you set the bias, the more unstable it gets. The best you can do is to mount the Vbe multiplier directly on top of the hottest Mosfet.

BobEllis 23rd November 2004 01:50 PM

Basic question - How much heat sink are you using? When you crunch the numbers, how much temerature rise do you predict? How does this compare with what you see?

You must mount the Vbe multiplier transistor on top of one of the outputs or you'll get thermal runaway. This is the method Holton recommended to compensate for the tendency of the bias to rise as the output stage temperature rises. (not an issue with lateral FETs, but it is with bipolars and vertical FETs). See the Leach amp for another technique. If you've already done this, then your heat sinks are too small. You can reduce the bias to get normal temperature down, but you still should have bigger sinks for high power operation.

I used that technique on my Pass/Thagard A-75 when I reduced the bias and it seems to be a bit overcompensated - the bias takes a couple of minutes to come up to useable levels and probably 5-10 to reach full bias. I'd rather be overcompensated than undercompensated and blow outputs, though.

aleopeldd 25th November 2004 06:35 AM

I use a quite big heat sink, the problem is that i can set a bias current of 250ma x device as minimum... with the trimmer at 0ohm! Probably there is to modify some resistor value for run @ +-35v.....

The Vbe multiplier transistor is mounted on the heat sink at 0,5cm from the OP devices....

After 5 minutes of a sine wave @ 2 watt on a 8 ohm load the heat sink reach 40'!!!
I have not done power test for the moment because i don't feel confident with the amp.
The sound quality is excellent!

BobEllis 25th November 2004 12:46 PM

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Check your vbe multiplier circuitry- something is seriously wrong if you can't lower the bias current. maybe a bad solder joint or missing connection.

How big is quite big? You might be surprised at how big you really need, idling at 9 watts per device. For example, I have a protptype mini aleph that gets a 30 degree rise on the sinks at 35 watts on two devices. The heat sink is 4"x2"x12" long - one of these from Apex Jr.

Kilentra 25th November 2004 05:48 PM

Maybe you could check the values of the resistors on either side of the potentiometer?

When I used a Mosfet for the VBE multiplier I had a similar effect, but the correct bias actually came near the top of the potentiometer range when using a BD139 there and IRFP240/9240 output devices.

tiroth 25th November 2004 08:15 PM


Originally posted by BobEllis
For example, I have a protptype mini aleph that gets a 30 degree rise on the sinks at 35 watts on two devices. The heat sink is 4"x2"x12" long - one of these from Apex Jr.
Great sinks those are. :)

aleopeldd 26th November 2004 08:36 AM

2 Attachment(s)
This a pic of the amp

aleopeldd 26th November 2004 09:02 PM

Every thing resolved!! Was a pin of the Vbe multiplier transistor (collector) that has broken in a not visible point!! Now the amp run cool and sound better!!!
It reach only 35 degrees at full output power on 4 ohm! Now the power is around 150w on 4 ohm (+-35v 600VA supply).
There is a little problem more... the amp is not very sensible, i have to put in about 4v rms to reach full power!!!
A great thing: when it is on seems to be off without input signal and without potentiometer on the RCA!!
Thank you to all for help!!!!

ciko2k 16th September 2008 07:46 PM

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ciao, volevo chiederti se hai dovuto apportare qualche modifica allo schema originale poichè lo alimenti a +- 35v e non a +- 70v.
anch'io ho costruito un av400, ma ho alcuni problemi..posso chiederti una mano per risolverli?

io lo alimento a +-56v, seguendo la modifica apportata da un utente. lo schema modificato è quello allegato.

riferendomi allo schema, i transistor q6 q7 q8 q9 e q5 scaldano un po.. ma il problema principale è che ai capi delle resistenze di source qualle da 0.22 nn ho i 18mv..
ho misurato le tensioni ai capi di R2 R3 R7 R25 R29 e sono vicine a quelle riportate nel PRE-FLIGHT TEST del documento di holton.
sapresti aiutarmi?
grazie mille e spero a presto..

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