GSSL VCA input offset voltage?

I have just finished adjustments to my 2180LA gssl .Sounding nice! :grin:

But I just noticed something which may be wrong:

I can see a small DC voltage at pin 1 of the vcas .. left channel = 4mV
right channel = 0.5mV


I tried replacing the right channel vca with another 2180LA , and this time the dc voltage on pin 1 is 1.6 mV .

Is this anything to be worrying about? Or is it the input offset voltage I'm seeing?

The same discrepancy is also visible at the two summing resistors.

I did the above test without an input signal present.

When a input is applied , the (ac) signal levels do seem to be fine throughout..

Do I have a problem , houston ??

Don't worry - that seems to be within specs. And we have dc-decoupling.

Jakob E.

Ok! Thanks again Jakob..more piece of mind!

Well with all these adjustments and the mods , this gssl is sounding FAB!!

I'm just about to put it to bed back in the rack,but I thought I should check one final thing...

Just to check noise levels , I stopped the cd I was playin thru it , and cranked the channel and desk level to the max, and the make up gain..

The hissy type of noise , which I'd call actual circuit noise is almost none existant , but there is a mains hum there ...actually must stronger on the left than the right .

In practice I don't think it's a prob (if i pressed play now , it would bust my speakers..and ears!) , but in the course of further education , I am interested to learn where that hum is being induced.

1. Should I be checking any decoupling caps , or some of my electrolytics (pana fcs) ??

2. I am using a small 6va encapsulated tx , mounted to the right of the pcb ...could the field from this be hitting the left vca more than the right one?

3. I (stupidly) used a small neon mains switch (I now understand these aren't a great idea!)..could this be it?

4. Could it have anything to do with the earth strap being connected to pin 1 of the left channel input?


5. Or could it be something I haven't thought of , with my limited knowledge!


If it should be possible to reduce this hum , I'd love to have a go . It's all great fun , trying to perfect this stuff!


Cheers all

nEon

Hi.

I am also seeing in speclab a little mains hum on the left cannel of my GSSL. Built with dbx 2150 vca's and with an external toroid mounted in a separate case. It's definately not the field from your on pcb trafo I would think as I've tried both with the same result.
What I'm seeing is a 5 db louder 50/60 hz peak in the left channel. It's still pretty quiet though and in normal operation it's no problem but still I would like to see both channels equal.

I have measured the E series console we have in our studio and the real bus comp is pretty close thd wise when offsets trimmed properly, but a bit quieter compared to my GSSL.

Any ideas on what to check for?

Hi.

I am also seeing in speclab a little mains hum on the left cannel of my GSSL. Built with dbx 2150 vca's and with an external toroid mounted in a separate case. It's definately not the field from your on pcb trafo I would think as I've tried both with the same result.
What I'm seeing is a 5 db louder 50/60 hz peak in the left channel. It's still pretty quiet though and in normal operation it's no problem but still I would like to see both channels equal.

I have measured the E series console we have in our studio and the real bus comp is pretty close thd wise when offsets trimmed properly, but a bit quieter compared to my GSSL.

Any ideas on what to check for?

It's better to look in - and post to - the main error-fixing thread for GSSL-questions:

http://www.groupdiy.com/index.php?topic=110

..nearly every problem that you may encounter has been had and fixed before..

Jakob E.