Sonographe SC25 preamp hum/buzz LOUD!

[TheFinisher]

Hi guys.
Picked up the Sonographe pre and power yesterday and despite being assured that it was sounding good when I got home there was a loud buzz which after a few seconds became a low level hum , I tried it again this morning ( again with cheap speakers) and the sound is now distorted too.
I have swapped the pre with my alchemist kraken which shows that the power amp is not the issue ( I even tried the SC25 with my alchemist Kraken power amp and the hum/buzz was there .
Could it simply be a few of the small capacitors?

[admin]

Sorry to hear that you are having issues with the SC25.

There can be many causes of hum or buzz. In the best-case scenario, a low level 60 hz hum may be a ground loop issue. This can show up with preamp and not another which could still fit the picture. Any chance you have a ground loop isolator? Those can be helpful in quickly ruling out ground loop issues.

Of course, there could be a faulty internal component as well which would take more effort to identify. Does the hum increase/decrease with changing in the volume control?

[TheFinisher]

Hi, initially it was a very very loud screeching sound which decreased to a low level hum ( regardless of volume levels )
At normal listening levels the hum wasn’t audible, but then when I tried again this morning i got the same symptoms as before with the addition of the sound being distorted. I tried a bit of bell wire connected from the preamp ground to my mains electricity ground ( we have ground or earth over here ) this made no difference at all . I have also tried plugging the preamp into a different circuit in the house ( whilst having the Hifi connected to another circuit ) but that made no difference either.

[admin]

Since there is a change in sound with the warmup, I would start by removing the covers and doing a thorough inspection of the internals looking for broken or loose connections. At the very least, you can give the unit a couple of firm but gentle hits on the housing and see if that elicits and change in the sound. We just had a member this past week with a hum that changed as the unit warmed up and it was a transformer issue, as the transformer warmed up the metal inherently expands and can dampen itself, etc. However, this could happen with any component. You can even try running the unit open and WITH A NON-CONDUCTIVE "poker", just push on each component to see if manual manipulation can cause a change in sound. This could identify something that has come loose and making poor contact. It's a place to start at least.

The only other thing I would mention is that if you just bought the unit, I would bring this up with the seller as you were obviously sold something not as described.

[TheFinisher]

I have just given it a thorough examination and found nothing obvious though interestingly the hum now goes quieter when I place my hand near the preamp, that surely is related to a ground loop ?

[admin]

If you think it's a ground loop issue, I highly recommend having one of these on hand to test it. They're cheap and a nice "tool" to have in your diagnostic toolbox.

https://www.amazon.com/InstallGear-Grou ... 134&sr=8-2

If it was a malfunctioning internal component I don't know why moving your hand close to the preamp would make any difference in the sound. It could also be electromagnetic interference of some kind. That can potentially have an effect of body position close to the unit.

[TheFinisher]

I have now ordered a very similar ground loop isolator, cheers

[TheFinisher]

A recap has done the trick

[admin]

Glad you figured out the cause of the issue. Technically, you can keep it in the system as it will not do any harm to any of the components. However, I would not recommend it. I think it sucks the pleasure out of the sound. I find the music loses its liveliness, dimension, and soul. Again, I own one because I have used it as a diagnostic tool,... much like my voltmeter or screwdriver.

I would recommend trying to actually get rid of the ground loop problem and not masking it with the ground loop isolator.

[TheFinisher]

Unfortunately I rushed home today in order to have a good listen and the fault is back ! With more jiggery pockery I got rid of the buzz/hum but now I have really bad distortion which wouldn’t go , I’ve unplugged it for now but it looks like I’ll have to get a professional to look at it .

[admin]

Sorry to hear that it wasn't just a "simple" ground loop issue. Let us know how it goes.

[TheFinisher]

Problems solved
I did a complete recap on the preamp ( like for like ) and now I’m a happy camper.
£46 was my total cost , result.

[velfunaud]

Hi,
I have the schematic of the sc25 with the exact components. I can send for mail . I have set iq to 0.9V (9ma) final stage ) is this correct? I am looking for the diagram of the sc25P phono card or a sc25P card to buy? greetings velfunaud

[admin]

Hi,
I have the schematic of the sc25 with the exact components. I can send for mail . I have set iq to 0.9V (9ma) final stage ) is this correct? I am looking for the diagram of the sc25P phono card or a sc25P card to buy? greetings velfunaud

First, welcome to CJO, great to have you with us.

If you could email the schematic to me, I can add that to our online database for people to utilize in the future.

[mastri81]

Hi to everyone, i looking for sc25 service manual or schematic.

[AnotherJohnson]

The sort of problem posed here is nearly always bad filter caps that are passing 60 Hz.

Caps block DC as though they were an open circuit, but they allow alternating currents to pass to varying degrees, depending on the frequency. The complex impedance for L or C components is related to the frequency trying to pass through. Inductors (coils) easily pass DC, but block alternating currents to varying degrees, again with complex impedance related to the frequency.

[Hutsut5]

I'm new to this forum would love to have a copy of the sc25 Schematic also I have an sa power 400 amp if any one has a copy of that Schematic that would be greatly appreciated thanks so much