Preamp power supply

[YISH313z]

Hey guys got a question. I bought a used gap 73 mic preamp, it came with no power supply so I ordered a new one. When I got the PSU unit I tested the preamp, it sounded different then the sample sounds I heard on YouTube, I'm talking about everything below 200hz was missing. While sitting in disgust, I noticed the PSU was buzzing like hell, hair clipper buzzing. Hmm. I banged on the supply a few times and it resolved to a mild hum heard up close(the PSU unit not the preamp). I returned that one in exchange for a new but it has yet to arrive.

My question is whether or not the low freq loss could be due to the bad power supply or is the preamp simply bad?

[jksuperstar]

Is there a roll off switch for low end? Still, I don't think it would be as high as 200Hz.

I wouldn't expect specific frequency loss due to the power supply, but that probably depends on the preamp topology.

Hope it works out well for you.

[YISH313z]

No LF roll off switch. I really hope its the power supply.

[JoPo]

I don't want to be unpleasant but IMHO, I don't see how a preamp power supply would affect mic signal frequency.

A power supply delivers a voltage with a current max and that's it. It mays be full of filters to avoid 50 or 60Hz noise but not 200Hz. But it must be much better to have the right power supply for any preamp.

What is this preamp for ? Is it this one ? :
http://fr.audiofanzine.com/preampli-tra ... re-73.html

It looks very good ! There must be something you're missing, how such a piece of gear could not record below 200Hz ?
In the french issue, they say it's a very good preamp, they only beware about the switch quality.

[astroman]

the low freq 'error' my well interact with the regular bass region (phase out)
but the description reads like a switching PSU ran amok - that can totally mess up the amp
it doesn't deliver plain DC but pulses

cheers, Tom

[YISH313z]

yes, that is the preamp. Forgive me Astroman i only have a high school level background in EE that is fuzzy at best, the power supply for the preamp outputs 24V AC.

There was nothing on the Preamp its self through switching that modified the sound that drastic.

here is a sample of what it sounds like compared to my alesis mini mixer preamp both mic with an AT2020 and no pop filter...

https://www.facebook.com/video.php?v=10152744823045959

[astroman]

if an AC PSU (just a transformer) makes such noise, it may be short circuited some way
the isolation may be broken and leak high voltages into the electronics
capacitors are quite sensitive to this and a broken capacitor can in fact act as a high pass
but it could as well be just a broken solder joint of the main supply
(I was mislead by the sound description)

cheers, Tom