AP System One - Input DC limit?

hey there - does anyone happen to know the if the analog inputs of the Audio Precision System One can handle DC offset - and if so - what is the actual limit?

Im wondering if I can use it to take some noise measurements on a tube power supply I'm working on...

I have a few other meters I can read noise with - a Sound Tech 1710a, HP400GL & 400FL - all of these can handle the +275VDC offset I'm dealing with, but the AP has a few bells and whistles the others don't.

I have scoured all the manuals and data sheets and can't find a definitive answer - even have the service manual with a schematic where I see 1uf 250V film caps on the +/- legs - thing is - there is some detection circuitry that happens before this - in looking at it I don't think it would be an issue - but again - an actual DC input spec would be nice.

anyway - any info at all would be much appreciate. its an amazing tool.

thanks guys,
Timothy

(attached is the input schematic I'm referencing)



The System One will take DC inputs It should not exceed 200 volts. I would not do this myself as it could cause the CMRR to change by pulsing one of the LAMPS (this is part of the over voltage input limiting). The lamps are in almost all of the systems. If your s/n is >45xxx it may have a different input limiter. This is a  change as the lamps were discontinued.

I would use an  external atten /cap/ probe that will make sure that the AP or other instrument won't be damaged.

I service the older AP Systems.  You may want to look on LinkedIn " Classic AP Audio Systems Group LA"
[email protected]
Duke

Hey Duke - thanks so much - thats super helpful.

I have heard of the CMRR/Lamps issue, thanks for explaining that.

I just made a test lead with two 1uF 200V film caps in parallel and it seems to be working great.

and Im really glad to meet someone who can work on these things - since getting it I have a hard time imagining my life without it. I know someday its going to need some love - I'm thankful to have someone to turn to.

I just asked to join that Linked in group...

so one other question - the generator output is transformer coupled - what are the DC bias considerations there, if any. The primary is isolated - looks like just the attenuator/source impedance circuitry on the secondary...

thanks again Duke.
T.

About the question you asked about DC bias? I don't really under stand it. DO NOT PLACE DC VOLTS on the generator outputs as this can burn up or change the output resistors.  The system will not work within the specs.
Please be more specific about the question.
Duke

Audio1Man said:

DO NOT PLACE DC VOLTS on the generator outputs

Click to expand...

that answers my question.

which was - is it safe to hook the generator output across a circuit that has a DC bias such as the input of the impedance converter/amp stage of an active microphone, and if so, what is the safe operating range.

thank you.