Tracking Down Hum and Buzz

What is so baffling to me is that shorting out the inputs and having no noise should prove it isn't the amp.

The testing you have done thus far gives me the impression that each of the two inputs has a different level of noise at its ground, so when you connect them together at the pre
I appreciate everyone's input. I have used the Dynaco with the Bottlehead preamp for a good while prior without incident and they sounded wonderful together. So I wouldn't have thought it would be a design problem inherent in the Bee Pree. And the preamp doesn't buzz or hum with other another amp. Doesn't mean something hasn't gone wrong but I just don't understand how it could be the preamp by design.

I am using Cardas cables and Blue Jean that again don't have a problem in any other context, and I thought they were decent quality. I will of course go do the test touching the ends of the cables. What is so baffling to me is that shorting out the inputs and having no noise should prove it isn't the amp. The preamp has no problems in any other context and never had a problem with the Dynaco which should prove it isn't the culprit. I am using two sets of pretty good cables. Shouldn't be them. Nothing is left! I am going to follow everyone's advice and go searching for a cold ground solder joint and do that test you suggest with the interconnects. I hope to get to it tomorrow but it may have to be Monday. Thanks to both of you for trying to help me, it is appreciated.

Good luck!
 
The testing you have done thus far gives me the impression that each of the two inputs has a different level of noise at its ground, so when you connect them together at the pre


Good luck!

I am also writing a paper for a master's class so I can't do all of the testing I really want to and certainly can't get into the amplifier just yet even though it is killing me not to have a shot at it. I did take a passive preamp I have (just a volume pot hooked up to RCA jacks) and ran the CD player straight into it and using the Cardas cables that I have been using connected it to the Dynaco. Almost no hum and buzz, at least not audible at the listening position. The slight amount of noise was equal in both channels as far as I can tell. I am going to have to make a chart of what I have done and the results once I have searched for a bad ground. So obviously still need to do the cable thing and look for a bad ground connection and will just as soon as I can.
 
I am also writing a paper for a master's class so I can't do all of the testing I really want to and certainly can't get into the amplifier just yet even though it is killing me not to have a shot at it. I did take a passive preamp I have (just a volume pot hooked up to RCA jacks) and ran the CD player straight into it and using the Cardas cables that I have been using connected it to the Dynaco. Almost no hum and buzz, at least not audible at the listening position. The slight amount of noise was equal in both channels as far as I can tell. I am going to have to make a chart of what I have done and the results once I have searched for a bad ground. So obviously still need to do the cable thing and look for a bad ground connection and will just as soon as I can.

Yeah, better focus on studies! There will always be broken amplifiers to figure out
 
Max is going exactly where I am thinking as well. I am completely unfamiliar with the Bottlehead, so I'm at a disadvantage there. But I would suggest two things:

1. Make sure you are using quality interconnects. In a pinch, I recently went to Home Depot and purchased a 6 foot set of interconnects for use between my Fisher preamp and power amp -- only to have it hum notably. Swapped the interconnects out with a different set -- no hum. What I found was that cheap cables today do not even remotely wrap or shield the inner conductor but rather, simply run a bare ground lead next to the inner conductor, and count on the very low output impedance of today's modern SS devices to reduce any hum pickup to minimal levels. So, make sure you're using good, well shielded interconnects.

2. Taking Max's suggestion even further, using a known good set of interconnects, plug one end of the interconnects into the Dynaco, and at the other end, bring them together face to face, so that the outer ground shield of one plug is grounding out the top of the other plug, and visa versa. In this way, the top of both inputs will be grounded out by the ground lead of the other channel.

If the Dynaco is quiet in this test, then the problem must lie within the Bottlehead, and particularly with the grounding circuit between the output jacks of the two channels. On the other hand, if the buzz and hum are present, then the problem may be the inherent issue with the physical lay out of the Dynaco due to the power transformer being mounted in between the two amplifier input jacks. This allows the input and ground circuits for each channel to be on opposite sides of the magnetic field generated by the power transformer, which then generates a ground current through the interconnects when their ground leads are connected together at the source end. This is often a problematic way to do things, but normally, the ST-35 is not cited for this kind of problem -- but that is also based on it being used in conjunction with a Dynaco PAS preamp. Most particularly however, note that to minimize such concerns, Dynaco did not connect the ground terminals of the output jacks in the PAS directly to ground, but to ground through 10Ω .5 watt resistors. These resistors act to allow an appropriate ground reference to be made, but also minimize any ground current that might flow from the ground loop conditions described. You might look at using these resistors in your bottle head if the suggested test indicates that the noise is originating from the Dynaco.

As Max said, this problem is all but eliminated in the ST-70, since the input jacks are spaced right next to each other in the middle of the power transformer's center line. This kind of configuration allows for little if any ground currents to be developed between the channels, all but eliminating the problem. However, just to be sure, Dynaco included the 10Ω resistors inside the ST-70 as I recall as well.

I hope this helps -- let us know!

Dave

With the interconnects just running into the Dynaco the hum and buzz is less than connected to the Bee Pree but still there.

Taking the interconnects and touching various parts together:
Shield to shield: no change
Pin to pin: no change
Shield to pin: seems to eliminate the buzz and either just reveal or increase the hum, I think increase but honestly it is hard to tell.

Put in a passive preamp and the buzz and hum is very low, almost inaudible. Swapped in another Bottlehead preamp I have, a Foreplay II, and the hum and buzz is just as bad as with the Bee Pre. I have used three sets of quality interconnects, those make no difference. I plugged the shorting plugs back in and the hum and buzz is extremely low, again almost inaudible.

I didn't have this problem with the Bee Pre before and the Bee Pree isn't humming with my Paramount amps. Seems like there has to be something in the Dynaco but I just can't imagine what. I resoldered some grounded points on the power supply board including the Ground Pad, no effect.
 
I went ahead and opened up the amplifier and resoldered every ground connection I could find. No dice. I am putting the amp up for now because I don't have time to mess with it and I don't even know what to mess with anyway. I am going to try to figure out what I might do to trace the actual circuit and figure out exactly where the hum enters the system and go from there.
 
It is possible that the dynaco amp is just more sensitive than the Bottlehead amp, and therefore amplifies hum at the output of the Bottlehead preamp more.
 
It is possible that the dynaco amp is just more sensitive than the Bottlehead amp, and therefore amplifies hum at the output of the Bottlehead preamp more.

That is possible, for sure. But I would go back to the fact that it didn't use to do this and it is humming badly with two different preamps. It can't be part of Bottlehead design if the problem was nonexistent prior, could it?
 
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