Mystery issue with MacIntosh C47 preamp

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avahifi

Mystery issue with MacIntosh C47 preamp
« on: 18 Sep 2019, 04:53 pm »
We have a mystery problem with a client trying to interface a new Vision SET 400 power amplifier with his MacIntosh C47 preamplifier.

When the SET 400 is plugged into the unbalanced RCA output 1 set on the Mac preamp, the system produces audible white noise.

His other power amp, a Prima Luma is quiet with the same connection.

However, when the SET 400 is plugged into the unbalanced RCA output 2 set on the Mac, it is quiet.

The amp also is quiet on my test bench and in my reference system with any preamp we use here.

It is also quiet with open inputs.

Although it appears that the issue is some incompatibily with the Mac audio output unbalanced set 1, I have no clue what that could be.

The customer is returning the amp.

Any thoughts there as to what the issue could be?  Anyone know of anything unusual about the C47 RCA output set 1?

This is a one off issue that we cannot identify so far.  The SET 400 has a 65K ohm pure resistive input impedance so should not be loading down the preamp or producing an unusual load for it.

Thanks for your help.

Frank

raindance

Re: Mystery issue with MacIntosh C47 preamp
« Reply #1 on: 18 Sep 2019, 07:47 pm »
Did you try a resistance box set to 65K on the output of the C47 and see if the preamp oscillates?

avahifi

Re: Mystery issue with MacIntosh C47 preamp
« Reply #2 on: 18 Sep 2019, 08:39 pm »
We do not have access to the Mac preamp as it belongs to the client far across the country from us.

Frank

raindance

Re: Mystery issue with MacIntosh C47 preamp
« Reply #3 on: 18 Sep 2019, 09:06 pm »
A little bit of research turns up other users with hiss issues on the C47. Seems that loosening & tightening some silver screws on the back panel resolves some sort of grounding issue... Some say that only certain magical cables work with it (yeah right) and some say that a firmware update fixes the issue. It occurs with volume at 0. So I doubt returning the amp to you will resolve the actual problem.