Tracking Down Hum and Buzz

[Paully]

So I rebuilt a Dynaco ST-35 and today it has hum and buzz equally in both channels that is really bad in a normal configuration with interconnects connecting it to a Bottlehead Bee Pre preamp and hooked up to Altec 846A speakers. Last night it just had a little hum and buzz in one channel. This is what I have done to trouble shoot just not sure what it means (and I will list in what I think is order of importance):

• Shorting plugs in the RCA jacks: Hum and buzz pretty much gone. Maybe a tiny bit of buzz.
• Same set-up upstream, same interconnects running to different amp: no hum or buzz.
• Take out interconnects and run Dynaco with RCA jacks open: slight hum and buzz both channels, hum a little worse than buzz.
• Different set of interconnects to preamp (which is off) with amp plugged into various outlets: very bad hum which overwhelms any buzz that might be there plus some clicking noise.
• Running a jumper from amp to preamp makes no difference.
• Lifted ground from power cord using a cheater plug into a power strip, bad hum and buzz, no clicking.
• Radio Shack ground tester shows good ground in the three outlets used.

I'm going to keep reading, but any insight would be very appreciated.

 

[Paully]

Bump for help!

 

[trobbins]

Can you confirm that amp shows negligible hum when its inputs are shorted, and only minor hum when inputs are open.

Can you link to amp and preamp circuits so people don't have to chase them up.

 

[Paully]

Can you confirm that amp shows negligible hum when its inputs are shorted, and only minor hum when inputs are open.

Can you link to amp and preamp circuits so people don't have to chase them up.

Yes, absolutely, when the inputs are shorted there is negligible hum. Nothing that would be a problem, almost completely quiet. Maybe the tiniest bit of buzz but unimportant it is so little. Hum is minor when open but nothing I would want to listen to but certainly minor compared to sound with interconnects in to preamp. Hum is still very bad even with the preamp off.

Dynaco ST-35 Schematic:
http://www.thehistoryofrecording.com/Manuals/DynaCo/Dynakit_ST35.pdf

EFB Power Supply Schematic:
http://tronola.com/ST35-EFB_Assembly.pdf

Preamp Schematic is Bottlehead Bee Pre, can't post that.

 

[s-petersen]

Does it hum with only 1 RCA connected? it sounds like some kind of ground loop.

 

[Paully]

Does it hum with only 1 RCA connected? it sounds like some kind of ground loop.

Leaving the preamp off and turning the Dynaco on, it hums very badly in the channel with the RCA interconnect on it. The other channel is greatly reduced, but still would be a bit annoying when listening to music.

 

[rogerh113]

It certainly is an option to just make sure that the amp is terminated - that is how it will be used, with input that will properly terminate it. I have 2 amps that do the same as your amp. I've been running one of the amps for years, and it sounds great and there have been no problems. I guess you could spend a pile of time trying to 'fix' it, but it may just be the way it is designed, or a marginal component or wiring issue that will be complex to find (and even if you find it, the problem may not be resolved).

If you terminate the amp inputs with shorting plugs, and there is no problem, then it sounds like your problem might be further upstream, or a ground loop as mentioned before. You could try floating the rest of the setup with a 3 to 2 plug just to see if the ground loop noise stops. Best to know where the problem is before you start digging in....

 

[maxhifi]

How's the bottle head preamp work into a different amp?

 

[Paully]

How's the bottle head preamp work into a different amp?

Perfectly, no hum or buzz.

 

[maxhifi]

Perfectly, no hum or buzz.

How did you lift the ground, when you used the cheater plug?

 

[Paully]

How did you lift the ground, when you used the cheater plug?

I just used a plug with two prongs and the ground missing. A three prong to two prong adapter.

https://en.wikipedia.org/wiki/Cheater_plug

 

[maxhifi]

I just used a plug with two prongs and the ground missing. A three prong to two prong adapter.

https://en.wikipedia.org/wiki/Cheater_plug

What I mean to say is, which amp did you do this with? Amp, pre, or both?

 

[Paully]

What I mean to say is, which amp did you do this with? Amp, pre, or both?

Ah, sorry, amp only. Should I go try to do it on all of the amplification at the same time?

 

[maxhifi]

Ah, sorry, amp only. Should I go try to do it on all of the amplification at the same time?

Try it on the preamp instead of the amp. I'm not sure how the bottle head stuff is grounded. Also, try a different set of interconnects.

 

[Paully]

Try it on the preamp instead of the amp. I'm not sure how the bottle head stuff is grounded. Also, try a different set of interconnects.

Will do. I did try a different set of interconnects and there was no difference but I will definitely go put the cheater plug on the Bottlehead pre shortly. Thanks!

 

[maxhifi]

Will do. I did try a different set of interconnects and there was no difference but I will definitely go put the cheater plug on the Bottlehead pre shortly. Thanks!

Does it buzz with the interconnects plugged into the Dynaco on one end, and left unplugged on the other end?

 

[Paully]

Does it buzz with the interconnects plugged into the Dynaco on one end, and left unplugged on the other end?

Ok, ground lifted on the preamp (and the phono preamp just unplugged) still have high levels of hum and buzz with the amp and pre connected but I left the preamp off. Disconnected from the preamp but still with interconnects in the amp have significantly lower levels of hum and buzz but audible from the listening position. Seems to be the same as when I had the amp running with the RCA jacks just open. Low but noticeable.

Would a cold solder joint to ground in the amplifier explain the behavior? When the RCA jacks are shorted it is completely quiet, what does that mean?

 

[maxhifi]

Ok, ground lifted on the preamp (and the phono preamp just unplugged) still have high levels of hum and buzz with the amp and pre connected but I left the preamp off. Disconnected from the preamp but still with interconnects in the amp have significantly lower levels of hum and buzz but audible from the listening position. Seems to be the same as when I had the amp running with the RCA jacks just open. Low but noticeable.

Would a cold solder joint to ground in the amplifier explain the behavior? When the RCA jacks are shorted it is completely quiet, what does that mean?

What if you hook up just one channel to the preamp, and leave a shorting plug on the other.

 

[Paully]

What if you hook up just one channel to the preamp, and leave a shorting plug on the other.

Channel with the shorting plug is quiet, channel with the interconnect has noticeable hum and buzz but I think (think) that it was somewhat reduced from the noise with both interconnects.

 

[maxhifi]

Channel with the shorting plug is quiet, channel with the interconnect has noticeable hum and buzz but I think (think) that it was somewhat reduced from the noise with both interconnects.

OK one more question for you. What if you have both interconnects plugged into the dynaco but not the preamp, and then touch their shields together (not the middle conductor, just the outer shell)

Does this increase hum?

The Dynaco is set up with an amp board at either end. As such, it is kind of set up for a ground loop. The ST70 is better in this regard since the amp boards are right beside each other. I would be interested to see photos of the top and bottom of the amp, with the covers removed.