LA-2A strange asymmetrical waveform

[beatnik]

Hi guys

I have just finished building an LA-2a point to point

The compressor works fine and sounds good

However I've noticed that the output waveform is clipped in a strange way. The top half is clipped, whereas transient can still be seen on the bottom half. I have attached an image of a drum track unprocessed (top) and then through the compressor.

I can't figure out if there is anything wrong because the compressor seems working fine actually

I hope someone can give advice if something wrong is happening

Thanks

 

[weiss]

i think this is no error. try to lower the gain or use a preamp..

http://groupdiy.com/index.php?topic=19054.0
http://groupdiy.com/index.php?topic=11517.0
http://groupdiy.com/index.php?topic=48490.0

Stock LA2 has asymetrical clipping.
It's just that hot line amp.
Everybody overdrives it.
When you use the La2 with a mic pre, you can set up the gain correctly between devices.
Otherwise you need to pad a hot cd input, or turn the gain way down.

 

[beatnik]

I did read those post while searching for possible problems

However, it seems a lot of difference between the two halves of the waveform

The top is chopped while the bottom still has a lot of transient movement

 

[weiss]

did you try to lower the gain?

 

[beatnik]

with lower gain is less evident, but still there. i am taking a screen picture

 

[beatnik]

here is with the input signal turned down 10dB and the gain on the la2a at unity

 

[beatnik]

here i turned up the gain from 50 (unity) to 60

 

[beatnik]

i've also done some tests with sine waves at 100 an 1k Hz and confirmed that at unity the output has a symmetric waveform, if i go past unity gain it starts to become asymmetric.

 

[gyraf]

We'll probably need to know absolute analogue output level (mV or dBu) in order to tell anything real from waveforms...

Jakob E.

 

[beatnik]

Something was wrong actually.

I discovered very high voltages on V1 and V3 anodes. Around 200V for both.

I have replaced the tubes and now the compressor works better and the waveforms look fine

But I still have higher voltages compared to the schematic.

On V1 anode 160V instead of 130V and on V3 anode 130V instead of 100V

B+ voltage is 265V

Do you think they are fine ?