Help stabilizing this amplifier...

[mchambin]

The two zenerdiodes, D1 and D2, are 10 Volts.
It is way to high..

Indeed, I had not looked at D1 D2 Q5 Q6.
With R14 =R15 = 47 Ohm: Current thru D1 D2 is 200 mA, so, 2 Watts in D1, 2 Watts in D2.
Q5 and Q6 saturated.

 

[Boscoe]

I have corrected the diodes, i tested the buffer without the op amp and it doesn't work. The waveform is incredibly distorted going above 2V.

 

[sajti]

Hi,

there are many problems with this amplifier.
I would set the DC parameters first. Without any driving signal, and without feedback.
There are too high gain of the two amplifier stages which drives the outputs. This means, that the opamp have to give the gain less than 1. MOst of the opamp will oscillate with this settings.

My only question is: Why You want to use VAS between the opamp, and the otput, if the power supply is only +/-20V? Connect the EF directly to the output of the opamp.

Sajti

 

[Boscoe]

Yes I have sorted the dc conditions of this amp now with the zeners. The second stage I only want gain of about 3-4 so I can use it on higher supply rails.

 

[mchambin]

I wish you have recovered from your flu.

So far we do not know exactly the changes you have made.
I suggest you post an update of your amp..

 

[Boscoe]

I wish you have recovered from your flu.

So far we do not know exactly the changes you have made.
I suggest you post an update of your amp..

Thanks!

That's a good idea i will do it tonight.

 

[Boscoe]

I've managed to loose the design.

Does anyone have any ideas for a reasonably low distortion buffer with a voltage gain for around 3-4?

 

[mchambin]

I am sorry to hear about your lost design.
To be honest I found no solution a liked about a x3 , x4 gain amp for this kind of an op amp based amplifier.

Here is a solution I found on DIYaudio for a high voltage amp output:
+ 54V- 54V rails ( 150Watts output in 8 Ohm load ). Pretty complex, very interesting..
I dowloaded the files and tested with LT Spice.
http://www.diyaudio.com/forums/solid-state/212875-yapa150-yet-another-power-amplifier.html

 

[ChristianThomas]

I've managed to loose the design.

Does anyone have any ideas for a reasonably low distortion buffer with a voltage gain for around 3-4?

You've got the original design on page one of this thread, if you need it.

I suspect that the problem you are having boils down to not defining the gain of the discrete amplifier at your desired 3 to 4 times. So you need a bit of feedback there to set the gain, which could be done any number of ways.

Incidentally, the Musical Fidelity A1, or A100, did something similar with an op amp and a common base stage. There is a long thread on here from about 18 months ago and Arkless (who used to work there) posted a circuit diag of it. I expect the link is still up. Also there is a website by someone whose name escapes me, who does both rather nice builds of chip amps and started on, I think it's a B100, and he has a page on it too.