Help me fix the AV800 amp

[radu8982]

I managed to build an AV800 and I have some problems. It sounds very weak.
Here are the measurements
R1 1,46v
R2 1,45v
R3 0,98v
R55 450mV
R56 14V

[Lee Knatta]

Your voltage values look reasonable to me.

Here you find a schematic with voltage values for the very similar AV1000.

http://www.diyaudio.com/forums/attac...-av1000_op.pdf

Do you know how to use LTspice?

http://www.diyaudio.com/forums/attac...av1000.asc.txt

[radu8982]

What do you think the problem is that the sound is so poor about 40w, i measured some other components R28 R29 69mV ,Vbe Q24 Q25 540 mV,volts from base to base Q6 Q7 4mV and they seem good from what somebody tell me

[ostripper]

sound is so poor = C2/9 too low. Try 68p -100p. Possible oscillation.

Original mje340/50 for VAS can use 22-47p with no problems.

OS

[radu8982]

I don't think this capacitors are the problem in the amp manual it says that r56 in my case shood have~500mV not 14v and I think this influence the input transistors thus the power output but i don't now ho to fix the problem

[Lee Knatta]

To which manual do you refer?
Strange schematic from Epsicom Ltd. Doesn't look like the knew what they were doing when blindly copying and modifying the schematic from Holton's AV amp.

I think OS is right with the capacitors. The amp has a lot of other compensation schemes as well but increasing C2 and C9 isn't a bad point to start with.

According to simulation 14V across R56 is okay. There are some deviations from the Epsicom and AV schematic but at this point they are near identical.

[radu8982]

I am going to relplace the capacitors tomorow,and I have an idea my toroidal has two windings of 10v I am going to redres that take of R4 R57 and ad 15v to each piont were this resistor are.I heard that mosfets aren't completly open unles the driver stage has voltage increase of +/-15V like in the schematic below.

[radu8982]

My mistake i am gona put +/- 15V from the point wich the prefinal transistors are

[Lee Knatta]

Yes, split supplies are good for vertical FET output stages. But first better get the thing stable.

Do yourself a favor and add gate protection to the output stage.

http://www.diyaudio.com/forums/attac...amp_v04r00.pdf

In this schematic the protection diodes are D4, D5, D7, D8.

Also consider lowering R22 and R44 a bit to maybe 47R or 22R. Someone pointed out a high value could cause instability but I can't confirm this.

[radu8982]

I replaced 10p c2 c9 with 100p and the amp stop working,now i hear hum like ther is a short ,i knew it that something is gona hapend.From what i understand i can replace the snoby 2sa1837 4793that i have for vas with mje340 350 if they popt and i did not tel you that i replace 2sc2240 2sc1845 but i think that s not a problem