2. amps-preamps
  3. 163294-7172

Hiss from speakers that increases as loudness control is decreased

I know there are a ton of articles on the web and our forum on hiss and hum but I didn't find anything that could be of help. My gear comprises of a Audiolab cd transport, Mccormack DAC1, Mcintosh C27 and a Mccormack DNA .5 and Vandy 2CE Signatures. They are all connected through Kimber PBJs. There's a noticeable hiss and a faint hum that becomes apparent when the C27 loudness control is at 12'o clock. As the loudness is turned down (towards flat) the hiss gets significantly louder and the hum also becomes extremely apparent. When I disconnect the preamp, the speakers are quiet as a tomb. I have swapped the cd transport with a streamer and it hasn't made any difference. 

I don't know if this is a ground loop issue or an issue with the C27. The last time it was serviced was about 8 years ago.

Any pointers/tricks are appreciated before I haul the preamp over to the technician for diagnosis..

Thanks
uditnaik
7 responses Add your response
atmasphere's avatar
atmasphere
11,841 posts
 
The preamp needs service. The first thing if servicing this that I would check would be the power supplies- making sure the filter caps are fresh, and that the regulator isn't oscillating.


If that did not sort it out I would be looking at the circuitry around the volume control itself- it sounds like the active device after the volume is lacking a stopping resistor, or has developed a noise problem.
uditnaik OP
20 posts
 
@atmasphere thanks a ton.. I thought as much.. I am going to get the preamp in capable hands...
elliottbnewcombjr
5,471 posts
 
While it is being serviced, have them thoroughly clean those old rca jacks:

Those old McIntosh jacks are close together, hard to clean, and not gold plated, subject to corrosion.

My vintage McIntosh Preamp mx110z (from the 1960’s). I was in a hurry to hear it, gave them a simple cleaning, hooked it up.

Last week, disconnected, took down to the shop, used very fine Emory cloth, super fine steel wool to remove corrosion, used tight rca jack pushed repeatedly in/out, alcohol, compressor. IOW, a real cleaning inside and out of the jacks.

I figured out a trick that led to greater success: I got a length of 1/2" wide leather (could use strong cloth) to make a loop around the jack, and put a thin layer of steel wool inside the loop, pulled the loop up/down without needing room to jamb my fingers between the tightly spaced jacks.

Made a noticeable difference
uditnaik OP
20 posts
 
Can’t the jacks be replaced with newer ones?
elliottbnewcombjr
5,471 posts
 
not easy, they will stay clean a long time now

your C27 jacks are further apart, much easier to clean. one of the changes from prior C26 which had tightly spaced jacks like mine

http://www.roger-russell.com/c26pg.htm#c27

http://www.berners.ch/McIntosh/de/Frame_McIntosh.htm

mine are tight together

http://www.berners.ch/McIntosh/en/MX110.htm
imhififan's avatar
imhififan
2,205 posts
 
Have you try change those Kimber PBJ to some shielded cable?
uditnaik OP
20 posts
 
@imhififan I have not.. Honestly, I didn’t even think that the cables would be a reason for the hiss. In any case, the preamp is with the technician. If he can’t reproduce it then I guess I will be sorting through my wires for some shielded ones.
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