Valve Amp Throbbing Hum

[lilmountain]

Hi everyone,

Not sure this is the best forum for Valve amps so happy to be directed elsewhere. I'm based in the UK, South East. Recently bought a second hand Cayin CS-150A integrated stereo amp, KT150 x 4, 6SN7 x 4 and 22DE4 x 1. Most expensive amp I've bought and was a real step up in sound. Absolutely no hum through the speakers, lovely sound and all was well for the first month or so. I've upgraded bits around it, there was a varying level of hum from zero to audible when standing next to the amp but not through the speakers, the speakers were always deathly quiet. I played and tinkered by providing a separate mains supply (but still from the main breaker board), hooked everything up to an Audioquest Niagara power conditioner. Running an old 90's upgraded Regar Planar turntable, Black Cube phono pre-amp and SMSL M500 MIII and AE509 speakers.

I recently swapped the Ortofon 2m Blue cartridge for a second hand Rega Exact, wired the latter up wrong assuming it had the same wiring as the Ortofon. There was no bang or pop, just really muddy sound before I read the instructions and corrected it, this was all done within an hour. I don't know if they're connected but ever since then there has been a low, rhythmic throbbing hum from the speakers that initially was audible from my listening position a good few feet away. This noise is exactly the same level whether the volume is at zero or turned up. Never had it before. I messed about changing power outlet, process of elimination taking out the Niagara etc. but no difference. Also checking with a different amp, looking at electrical interference, pulling all the inputs and so on. Then I tried tapping the valves to spot a bad one. I bought it with a full set of spare 6SN7's but completely swapping them out made no difference.

One of the KT150's sounded microphonic (based on a bit research on YouTube) so I bought a new match pair in the knowledge that this is a long term investment and I would need them at some point anyway. This reduced the volume of the hum but it's still there. I can't tell if the overall quality once playing is affected but basically I want to get it back to where I know it was.

So firstly any thoughts for what it could be?

Secondly, I really want this to work and plan to keep the amp for years so I'm prepared to invest in a proper expert to look at it and whilst it's open maybe suggest some improvements/upgrades. Does anyone know someone who could do this in the London/Kent area. It weighs a ton so posting it isn't realistic and would cost a bomb so ideally I'd keep it within 50 miles of either West London or Gillingham Kent.

I've attached audio of the buzz if it works on this forum.

Sorry for the long post, any help is welcome!

Thanks,

Sunny

 

[lostwin]

I have used both Juancho (South London) or toprepairman (East Sussex) for work on valve kit. Both recommend. Those are their WAM forum names.

 

[pmcuk]

Have you located where the hum is coming from?
- Disconnect the preamp from the amp. Does the amp still hum?
- Reconnect the preamp but without the phono connected - is the hum still there
- Reconnect the phono - is that where the hum is coming from?

 

[rabski]

I would suggest as above to start with.

Disconnect everything except the speakers and if there is still noise, then it's an amp issue. Assuming it is, then the cartridge bit is just coincidence. There's nothing you could do with cartridge wiring that would damage an amplifier.

 

[Disarmamant]

Is the turntable ground wire from the tonearm connected to the phono preamp?

 

[wizmax]

@toprepairman is in Fetcham near Leatherhead Surrey not East Sussex.

 

[lilmountain]

I have used both Juancho (South London) or toprepairman (East Sussex) for work on valve kit. Both recommend. Those are their WAM forum names.

That’s brilliant thanks, I’ll reach out.

 

[lilmountain]

Is the turntable ground wire from the tonearm connected to the phono preamp?

The Rega has never had a visible ground cable and has never been a problem before plus I’ve tried the amp with this disconnected and the him is still there.

 

[lilmountain]

@toprepairman is in Fetcham near Leatherhead Surrey not East Sussex.

Good to know if I can’t get through too there’s. Thanks!

 

[lilmountain]

I would suggest as above to start with.

Disconnect everything except the speakers and if there is still noise, then it's an amp issue. Assuming it is, then the cartridge bit is just coincidence. There's nothing you could do with cartridge wiring that would damage an amplifier.

Yes I’ve disconnected everything bar the speakers and still the hum. Also tried another amp and the speakers were fine so yes it looks like an amp issue.

 

[lilmountain]

Have you located where the hum is coming from?
- Disconnect the preamp from the amp. Does the amp still hum?
- Reconnect the preamp but without the phono connected - is the hum still there
- Reconnect the phono - is that where the hum is coming from?

I’ve tried disconnecting everything but the hum is still there, also it’s an integrated amp so no separate pre.

 

[lilmountain]

I could be very unlucky with the valves but I don’t have the budget to go out and get a full set speculatively. Is there anyone who can test my valves as a relatively easy first exercise?

Other than that I’ll try the contacts shared. I tried loading the recorded hum as a file and it hasn’t made it. Does anyone know if it’s possible to upload it to the thread? Thanks again everyone!

 

[lilmountain]

I would suggest as above to start with.

Disconnect everything except the speakers and if there is still noise, then it's an amp issue. Assuming it is, then the cartridge bit is just coincidence. There's nothing you could do with cartridge wiring that would damage an amplifier.

Phew! I’d have been gutted if that had been the cause!

 

[rabski]

I’ve tried disconnecting everything but the hum is still there, also it’s an integrated amp so no separate pre.

Assuming it is 'throbbing' at a relatively low frequency, that's classic 'motorboating', which is frequently down to failing capacitors in the power supply. There are other potential causes relating to feedback circuits, but none of this ought to be any problem to diagnose or fix.

Juancho and toprepairman are both well-known on the forum (in good ways) and both highly capable and knowledgeable with valve amps.

 

[DomT]

I see that Rabski used the word motorboat sound and I was going to ask if it was this. I have had this problem a few times with valve amps. Once it was due to internet mains extenders and another time cables being in proximity to things that they shouldn’t be. When you disconnect everything it’s probably also an idea to unplug all interconnect cables and then reconnect making sure that all cables are tidy and separated as much as possible. It may sound silly but it cured a problem that I had once and it’s free. If it doesn’t work then maybe it’s something else. Maybe there is a Wammer in your area who could pop round?

 

[lilmountain]

I see that Rabski used the word motorboat sound and I was going to ask if it was this. I have had this problem a few times with valve amps. Once it was due to internet mains extenders and another time cables being in proximity to things that they shouldn’t be. When you disconnect everything it’s probably also an idea to unplug all interconnect cables and then reconnect making sure that all cables are tidy and separated as much as possible. It may sound silly but it cured a problem that I had once and it’s free. If it doesn’t work then maybe it’s something else. Maybe there is a Wammer in your area who could pop round?

I did dig around the back and nothing had really changed but I'll have another go, given it happens even when everything is disconnected I guess I need to check the amp power supply cable and speaker cables in terms of touching something they shouldn't. Any thinking about it maybe changing the power cable. I'll look into it and whatever I find I'll report back for anyone following this. I'll try and attach sound file again but motorboating definitely seems to capture the sound, rarely it also sounds like vinyl surface noise but rarely, hissy crackle.

 

[Jazid]

A cursory glance at the online information is all I can rely on about your amp since I've no experience of one, but if what I've read is accurate then I'd be looking at the rectifier and first filter capacitor.

The sales blurb states that there is one 22DE4 for rectification duty. If the amp is using one single damper diode to rectify the total B+ supply then that valve will be operating way beyond it's design spec. The 22DE4 datasheet states an output of 175mA steady state DC current, though the plate dissipation curve imply it can actually deliver about 240mA DC. It also states 1.1A peak pulse at max 15% duty, and TV damper diodes are famously resilient.

I reckon the B+ for four KT150s and four 6SN7 will draw around 640mA DC in triode mode. With my rather crude maths this requires the peak current through one diode to be around 1.8A, or 1.28A averaged over a 50% duty cycle. That is a big ask. So is there a hidden solid state rectifier under the hood? If there is not then I am impressed that the tube has held up for as long as it has! The first filter capacitor after the damper diode will be dealing with significant ripple current from a half wave rectified supply, and it's perhaps unsurprising that the transformer makes some noise because it is being used only 50% of the time in each cycle.

Anybody know the PSU schematic for this amp?

 

[lilmountain]

A cursory glance at the online information is all I can rely on about your amp since I've no experience of one, but if what I've read is accurate then I'd be looking at the rectifier and first filter capacitor.

The sales blurb states that there is one 22DE4 for rectification duty. If the amp is using one single damper diode to rectify the total B+ supply then that valve will be operating way beyond it's design spec. The 22DE4 datasheet states an output of 175mA steady state DC current, though the plate dissipation curve imply it can actually deliver about 240mA DC. It also states 1.1A peak pulse at max 15% duty, and TV damper diodes are famously resilient.

I reckon the B+ for four KT150s and four 6SN7 will draw around 640mA DC in triode mode. With my rather crude maths this requires the peak current through one diode to be around 1.8A, or 1.28A averaged over a 50% duty cycle. That is a big ask. So is there a hidden solid state rectifier under the hood? If there is not then I am impressed that the tube has held up for as long as it has! The first filter capacitor after the damper diode will be dealing with significant ripple current from a half wave rectified supply, and it's perhaps unsurprising that the transformer makes some noise because it is being used only 50% of the time in each cycle.

Anybody know the PSU schematic for this amp?

Okay well if that’s the information a cursory look uncovers I’d love to see what a detailed look looks like! Thanks for looking into this, I have to confess most of that is way beyond me. I can only say it’s top of the range (or was at the time a year or so ago), fairly well regarded by some YouTubers. Yes it’s a Chinese amp but Cayin have a relatively good reputation for quality as I understand it but I don’t want to start a hot debate on whether Chinese amps are ever any good or not. I’m no expert! What’s clear to me is that apart from checking the mains and external interference I probably need to get it to someone who knows what they’re doing. If seeing the schematics reveals the weakness your suggesting then it would be interesting to know what the solution is and whether I can get someone to mod it accordingly. It’s a lovely sounding amp and I really want it back to the way it was. If it means replacing one valve every couple of years then it’s still worth it as long as it is t dangerous.

 

[Jazid]

Dont worry, I do not doubt that its an excellent amp and, more importantly, an easy fix. Theres some great Chinese amps available at all price ranges, top designers working and good parts being made. Always I have some caution about 220 vs 240V mains, and 'shortcuts' we wouldn't take in Europe, but these are at the value engineered end of things.
If there is an apparently underspec'd rectifier used there will be a designer happy to defend that decision. And more likely a solid state full wave rectifier doing the heavy lifting behing the scenes!

 

[simon g]

You definitely need to have this looked at by a pro. When I was in Bexhill, I used the below on a couple of occasions. He did a great job. That was many years ago; I've long left valve amps behind. OK, he's a bit further away from you, but one to consider.


https://www.servicesound.com/