Hey all. I'm currently using a pair of API 525's and 560's both of which are noisy in the upper frequencies (above 4k). They are mounted in a 6B lunchbox. Any advice on troubleshooting to reduce or eliminate the hiss?

Thanks in advance,

Mizzle

I'd check your cabling first.

What's after the 525's?

I use exactly that combination, and it's pretty damn quiet.

I had to send back a 525 for a failed opamp, but that made it crunchy, not noisy.

Are the APIs being used in a +4 dBu level environment?

Bri

SO.....

To answer a couple questions:

Yes, it has been operating in a +4 dBu environment.

The 560's immediately follow the 525's (there is no patch bay or other device in between).

-- Upon further inspection it seems that the 560's only contribute a neglidgeable amount to the noise. I have checked the cabling and this has no effect on the level of noise. It seems that the source must be post input-gain as the only pots that control the hiss level are the ceiling and output controls. These are original 525's and their condition and noise level are similar.

Should I simply be sending the 525's the hottest signal I can to improve the S/N ratio?

And by the way... does anyone know what some of the button combinations will do (example: "all buttons out" or "1 and 2 in" etc.)?

Thanks,
Mizzle

All buttons out: 0.1 sec release
button 1 in: 0.5 sec release
both in: 2 sec release
button 2 in: 2.5 sec release

By pressing the compress and limit buttons in at the same time you can get an "all button in" mode on the 525 which gives it the 'swimming' sound of a very high compression ratio. I believe the attack time is freq. dependent, but don't quote me on that.

The ceiling control is like a linked threshold and make-up gain control. As you turn it up, the threshold goes down and the makeup gain goes up.

I have a pair of newer ones (late nineties) and they're really quiet since they've been back from the factory.

I have a PDF of an old 525 manual if you want it.

Thanks for the info, Max! That .pdf would be great; the more information the better.

Mizzle