I've had these amps for a few years now and they've always been somewhat noisy, but it was bearable and I've always blamed the Broskie's Aikido 6SN7 preamp I was using (had some issues with 100hz hum from it). How does this noise sound? 50Hz + 5khz, based on rough measurements done with my iPhone.
However, now I've switched to a passive preamp (which is basically a switch and a 25k shunt attenuator), and the noise is significantly worse. I'm trying to troubleshoot the problem and fix it (hopefully).
What I've noticed:
Based on the above, here's my suspicion: noise is coupled via the input cable, based on the terminating resistance. When I was running with the 6SN7 preamp the output resistance was constant, so noise was constant, but with the passive preamp the output resistance varies with volume.
I'm going to try to move those input cables (get them away from power cables) and also replace them with some different cables, to see if it makes a difference, but I'd appreciate any other ideas/suggestions.
However, now I've switched to a passive preamp (which is basically a switch and a 25k shunt attenuator), and the noise is significantly worse. I'm trying to troubleshoot the problem and fix it (hopefully).
What I've noticed:
- poweramp without input cable connected, no significant noise
- poweramp with input cable connected only on its side - high noise (which increased while the amplifier turned on, but I never left it on enough to see how high does it go, for fear of damaging the speakers)
- poweramp with input cable connected to 6SN7 preamp, but preamp off - same as above
- poweramp with input cable connected to 6SN7 preamp (on) - constant noise, but bearable
- poweramp with input cable connected to passive preamp, volume down - no noise, more or less like the poweramp without a cable connected
- poweramp with input cable connected to passive preamp, volume past 50% - noise is increasing with volume
Based on the above, here's my suspicion: noise is coupled via the input cable, based on the terminating resistance. When I was running with the 6SN7 preamp the output resistance was constant, so noise was constant, but with the passive preamp the output resistance varies with volume.
I'm going to try to move those input cables (get them away from power cables) and also replace them with some different cables, to see if it makes a difference, but I'd appreciate any other ideas/suggestions.
Well, for the sake of posterity, gotta report I've fixed it. It was the cables indeed - moving them around made quite a difference, so I then tried replacing the Chord Crimson I had in there with some DH Labs Silver Sonic BL-1 I had around and poof, dead silent.
Had a look at the Chord pair and it looks like they're not screened at all. Moved them to another system and they work fine, so it's probably dependant on environment conditions.
Still, I'd love to understand more about this interaction between cable and amp. Why did the terminating resistance on the source end of the cable affect noise coupling?
Had a look at the Chord pair and it looks like they're not screened at all. Moved them to another system and they work fine, so it's probably dependant on environment conditions.
Still, I'd love to understand more about this interaction between cable and amp. Why did the terminating resistance on the source end of the cable affect noise coupling?