overvoltage on audio circuits

Hi,

I have here a batch of SSL channel strips rack mounted.
The regulated PSU that came with them is marked +-18v but actually produces +- 21v with a 1A quick test load, with the adustment pots at minimum as they arrived, .
(I suspect these are 24v based supplies - they are modular linear ones made in USA i think, 2 x "18v" and a 48v phantom)

It seems the channel strips have been running with this supply at 21v for a long time now.

What could be the possible detrimental effects?


I have only had a quick peek inside the first one of eight, and noticed a bulging capacitor on the SSL dynamics card.

I would not power any of them up with this supply at present.


Cheers,


Gareth.

gareth magennis wrote:

----------------------

>
> I have here a batch of SSL channel strips rack mounted.
> The regulated PSU that came with them is marked +-18v but actually produces +- 21v with a 1A quick test load, with the adustment pots at minimum as they arrived, .
> (I suspect these are 24v based supplies - they are modular linear ones made in USA i think, 2 x "18v" and a 48v phantom)
>
> It seems the channel strips have been running with this supply at 21v for a long time now.
>
> What could be the possible detrimental effects?

** Very few, likely there is PCB level filtering of the DC supply via resistor and electros.

Many Soundcraft analogue desks had 68ohm resistors and 100uF caps fitted in each rail feed - so dropped a volt or two in order to remove residual noise from the external PSU.

Op-amps like the NE5532 are happier with 16 volt rails.



...... Phil



>
> I have only had a quick peek inside the first one of eight, and noticed a bulging capacitor on the SSL dynamics card.
>
> I would not power any of them up with this supply at present.
>
>
> Cheers,
>
>
> Gareth.

gareth magennis > wrote:
>
>I have here a batch of SSL channel strips rack mounted.
>The regulated PSU that came with them is marked +-18v but actually produces +- 21v with a 1A quick test load, with the adustment pots at minimum as they arrived, .
>(I suspect these are 24v based supplies - they are modular linear ones made in USA i think, 2 x "18v" and a 48v phantom)

Who made themm? If they are at 21V with a 1A load, they are likely failed and
might even be running full open.

>It seems the channel strips have been running with this supply at 21v for a long time now.
>
>What could be the possible detrimental effects?

Electrolytics will go bad. Can cause failures of semiconductors too, but if
they were going to fail they would have failed by now. If the equipment was
well-designed, they used 25V or 35V electrolytics and the lifetime will be
shortened a little bit. If it was poorly-designed they used 16V electrolytics
and the lifetime will be shortened a whole lot.

>I have only had a quick peek inside the first one of eight, and noticed a bulging capacitor on the SSL dynamics card.

They do that naturally, though. They are wear items. It's probably time
to replace them.

>I would not power any of them up with this supply at present.

21V isn't enough to worry a lot about but if the regulation in the supply is
not working properly, the noise floor will be impaired too.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

OK, thanks Phil and Scott, I somehow thought 21v would be a bigger problem that it apparently is.

Gareth.

gareth magennis > wrote:
>OK, thanks Phil and Scott, I somehow thought 21v would be a bigger problem that it apparently is.

Maybe, but if the supply is producing 21V, it might be producing noise too.

Incidentally, did you see SSL has introduced a low cost console at the AES
show? Well, it's $40k so it's not that low cost, but by SSL standards it's
low.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Well I haven't scoped the supply as yet but will certainly do so.

Just stuck 20 ohms on each leg to find out what it might do, before connecting any module, and didn't like the overvoltage.


Gareth.

Scott Dorsey wrote:

--------------------

>
> Maybe, but if the supply is producing 21V, it might be producing noise too.
>
>


** FYI:

Just about the dumbest thing any console designer can do is fill up the modules with rail bypassing electros ( tants or otherwise) without series resistors.

Doing that injects wide band noise from the regulated PSU directly into the ground pattern and ribbon cabling - millivolts of the evil stuff.

Seen it done.


..... Phil

On 26/10/2019 1:18 pm, gareth magennis wrote:
> Well I haven't scoped the supply as yet but will certainly do so.
>
> Just stuck 20 ohms on each leg to find out what it might do, before connecting any module, and didn't like the overvoltage.
>
>
> Gareth.
>


Sounds like a fairly random shonky empirical tack-on to me, rather than
a solid scientific 'good' firm supply. If you want +/-15V JUST DO IT.
iF YOU WANT +/-18v JUST DO IT. dON'Y DO SOMETHING ELSE AND STICK A
SERIES RESISTOR IN TO DO MAYBE WHATEVER DEPENDING ON WHAT HAPPENS. And
+/-24V- what's that all about ?

Ooops capslock. Too late to give a ****.

geoff

geoff > wrote:
>On 26/10/2019 1:18 pm, gareth magennis wrote:
>> Well I haven't scoped the supply as yet but will certainly do so.
>>
>> Just stuck 20 ohms on each leg to find out what it might do, before connecting any module, and didn't like the overvoltage.
>>
>Sounds like a fairly random shonky empirical tack-on to me, rather than
>a solid scientific 'good' firm supply. If you want +/-15V JUST DO IT.
>iF YOU WANT +/-18v JUST DO IT. dON'Y DO SOMETHING ELSE AND STICK A
>SERIES RESISTOR IN TO DO MAYBE WHATEVER DEPENDING ON WHAT HAPPENS. And
>+/-24V- what's that all about ?

I think he means he used a 20 ohm shunt resistor as a test load.
Not a series resistor.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 27/10/2019 2:59 am, Scott Dorsey wrote:
> geoff > wrote:
>> On 26/10/2019 1:18 pm, gareth magennis wrote:
>>> Well I haven't scoped the supply as yet but will certainly do so.
>>>
>>> Just stuck 20 ohms on each leg to find out what it might do, before connecting any module, and didn't like the overvoltage.
>>>
>> Sounds like a fairly random shonky empirical tack-on to me, rather than
>> a solid scientific 'good' firm supply. If you want +/-15V JUST DO IT.
>> iF YOU WANT +/-18v JUST DO IT. dON'Y DO SOMETHING ELSE AND STICK A
>> SERIES RESISTOR IN TO DO MAYBE WHATEVER DEPENDING ON WHAT HAPPENS. And
>> +/-24V- what's that all about ?
>
> I think he means he used a 20 ohm shunt resistor as a test load.
> Not a series resistor.
> --scott
>


I was more referring to the claimed practice of SSL to stick a series
resistor in the power supply legs after regulation.

geoff