Yamaha C85 Line Noise Versus C2a

[sonavor]

Hi,

I have a restored Yamaha C85 preamp as well as a restored C2a preamp. The restorations were mainly replacing the electrolytic capacitors, cleaning switches and resoldering connectors.

Both preamps have been used in my home systems for over a year. Early this year I noticed that when no music was playing there was a very faint but audible hum coming from my speakers. The room had to be dead quiet otherwise I had to put my ear right on the speaker grill to hear the hum...but it was present. I was surprised as I hadn't noticed this before. I rechecked all of the wires that were connect (even changed some). I also played around with grounding the preamp to the power amp (a Yamaha M85). I even swapped in a Yamaha MX1000 as a test. The hum remained.

To further test the problem I listed to another system I had set up which has a Yamaha C2a preamp and a Yamaha MX1000 power amp. When no music is playing I can set the C2a volume wherever I want and there is no sound from the speakers.

So I took the preamplifiers to my bench.

Using a QuantAsylum QA400 Analyzer I could see the difference.
Here is the L & R C85 outputs to the power amplifier when there is no music playing and the volume is turned all the way down. You can see there is the 60Hz line noise at a pretty low level (-95dBV).


-sonavor

 

[sonavor]

Next...
Here is the C85 with no music input and the volume set to normal listening level. Normal being the level I set the C85 volume to when listening to the music source for this test. It was half way in this case.


You can see a significant increase in the 60Hz noise level. This is audible when I put my ear to the speaker.

-sonavor

 

[sonavor]

Here is the same C85 at the normal listening level when some music is playing (iPod source into the C85 CD input).
You can see the level is high enough where the hum is drowned out.


-sonavor

 

[sonavor]

Here is the C2a preamplifier swapped in on my test bench for this test.
This is with the volume at normal listening level and no music playing. The same iPod music source is connected to the C2a Aux input. The L & R preamp line outputs are what are being monitored (just like the C85).


You can see how low the 60Hz line noise level is even when the C2a volume is turned up.

-sonavor

 

[sonavor]

Last, here is the C85 again. This is a repeat of the test where no music source is playing and the preamplifier volume is at normal playing level (like the previous test with the C2a).
The difference in this test is I moved the L & R monitoring from the preamplifier line out to the Tape 1 Record outputs. I wanted to see if the faint hum would appear on a recording if I used the C85. As you can see, the L & R outputs to the Tape record outputs have a much better noise level. It still isn't as good as the C2a but very good.


-sonavor

 

[sonavor]

A question I have is whether this is normal for the Yamaha C85 or is there something wrong with mine? I would think the C85 line outputs should be completely quiet when no music is playing. If that is the case, does anyone have any ideas of what to check? I have a C80 and C70 I can put to the test but I need to restore them first as they are not currently working.

Thanks,
-sonavor

 

[savatage1973]

Have you tried repeating the tests with shorting plugs inserted into the inputs that you use while performing the tests?

Perhaps, thoroughly go through and test each component in the PS of the C85?

 

[whoaru99]

Can't speak to the C-85, but when I was using my C-80 on a regular basis I don't recall hearing any hum at all. The only noise I could hear was an extremely faint hiss when my ear was right against the tweeter and the volume was high.

I think there is something wrong, as in still broken, with the C-85. I don't believe Yamaha would permit audible hum in the design intent.

 

[sonavor]

That's what I feel as well. When I had a new C70 back in 1983 it was completely quiet when nothing was playing. The C85 is an excellent preamp so I don't think that 60Hz noise level should be that high. Yamaha wouldn't have put it out there if that problem was common. Thanks for the replies. I will have to start going through the preamp output to try and find the culprit.

 

[avionic]

I'll have to check it out when I get off work.

 

[amr2]

I have C-80 and C-70 preamps. They are completely quiet.

 

[avionic]

I use a C-80 and a C85. Both are dead quiet.

 

[avionic]

I'd be looking for cracked solder ground connections.

 

[avionic]

Try rotating the AC plug 180 degrees.

 

[sonavor]

I'll go through the solder joints again. I've been using the C85 with an MX2 in my workshop the last few days. It works great other than that extra 20 to 30 DB on the 60Hz component of the idle voltage.

 

[avionic]

I'll go through the solder joints again. I've been using the C85 with an MX2 in my workshop the last few days. It works great other than that extra 20 to 30 DB on the 60Hz component of the idle voltage.

Have the power supplys been recapped lately ?

 

[sonavor]

I checked my old pictures and it was June 2015 when I did the recap on this C85. I did all of the electrolytic capacitors and the rear audio jacks. The audio jacks were done because the preamp kept losing a channel on various input and changing cables didn't resolve the problem.

Here are some pictures of this C85 from today.

 

[sonavor]

With that test setup here is what the analyzer shows

 

[sonavor]

Here are some pictures of the inside.

 

[sonavor]

A little closer for more detail -