• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Shanling SP-80C Problem - DC Spikes?!

Dear all,

I have a problem with my Shanling monoblocks where sparadically whilst playing music at moderate volume i hear a loud instantaneous sound like a dog yelping!

The noise occurs at irregular times and is independent of the music volume. The most likely cause would seem to be the amplifier going open circuit very briefly and dumping a large DC through the speaker outputs.

I have replaced all valves, but this has had only limited effect, and have examined the internal PCB's, with nothing looking untoward. Unfortunately the amps are China imports so no warranty can be claimed.

Has anyone experienced this, or can suggest possible cause or remedy? If anyone has a circuit diagram that would be of great help?

Thanks in advance
 

kevinkr

Administrator
Paid Member
It seems unlikely that both would exhibit the exact same behavior at the same moment if it were a fault internal to each unit - you haven't indicated whether they do it independently of each other or at the exact same moment.

Independently then your hypothesis would be valid that the amplifiers are at the root of this..

If simultaneously this indicates some other component in your system such as the pre-amplifier is the culprit. It is even more likely that the problem is a severe disturbance on your AC mains or a strong magnetic source like a fridge or AC unit nearby turning on and off.
 
Clarification

Hello,

Thank you for replying to my message. Yes, the amps are monoblocks, and the problem appears to be from one of the amps. I have moved the Shanlings to a new system and the problem persisted, which would suggest that one of the monoblocks is the cause of the problem.

The noise is really rather pronounced, but being intermittent is very difficult to isolate. I wonder if any other Shanling owners have had this problem, or for that matter any tube amp owners?

I have opened the amps up and there is no obvious component damage. A brief scan around with a multimeter has not identified any failed components either. If it is DC spiking as i suspect has anyone an educated guess as to the cause of the problem? Perhaps supply caps? faulty transformer?

Thanks again
 

dhaen

diyAudio Moderator Emeritus
2002-06-10 12:01 pm
U.K.
www.keystrobe.co.uk
Ok, thanks for the clarification. As Adam points out, it is unlikely to be DC, but more likely that it is breaking into oscillation. You will need an oscilloscope to verify this.

Another speculative thought is arc-over. This might occur at higher volume levels, if the case.
Arcing is best detected in a dark room with the covers off. (Beware of safety considerations).

I notice that the amp's advertised valve lineup is 4x EL34 output valves and 1x 6N8P & 1x 6N9P valves, which may be fairly conventional, circuitry wise. What isn't, is the "tracking volume control" system they employ, and of course the problem might be in there.