Mitch2, as much as I think you should be buying my preamp :) instead of the Dude, I am sure the Dude can work with your amplifier. The problem is that the adapters you were using were not wired right.
The problem is that pin 3 of the XLR was left floating, allowing the amp to buzz of hum. Cardas adapters are often wired this way. Its easy to fix- you just connect pin 3 to pin 1 and you're all set. |
We had a Dude in here for repair a while back and found that the ground setup was peculiar. In a nutshell is would not pass UL/CE certification. I understand later units are grounded differently.
I think there are several things going on with the various responses we have seen on this thread. I am certain that in all cases a transformer is not needed, and I suspect that the reason that it eliminated hum has to do with ground isolation that transformers can provide. But we must keep in mind that there are variances in the production of the Dude, so what may be causing hum in one system may not be what is causing it in another.
So let's start with the AC ground. It is possible to have the Dude chassis tied to the ground connection of the AC power cord, but in some cases this will result in a buzz if the amps are also grounded. This will vary with amplifier, for example some amps (like Classe) are grounded correctly while others (older Cary) often have the chassis and circuit ground tied to the AC ground (which is incorrect).
As you can see, its not just the Dude that has variance, so this can be a tricky matter! It is possible to test the amps and preamp without plugging them in to see if they are grounded correctly. If you measure from the AC ground pin of the power cord, it should have a direct connection to the chassis. However the audio input should not- you should be able to measure a resistance (under 200 ohms but more than zero!) between the audio input and the chassis. This is also true of the preamp, however since the preamp is the heart of the system, it can be alright if its audio circuit is the same as chassis ground, so long as the amplifiers are either grounded into the AC as above **or** not grounded at all (in the case of improperly grounded amps like the older Carys mentioned).
Now let's examine cables. If you are running from the Dude, which is single-ended, it is a good idea to check the resistance from the shield connection of the RCA to the same at the other end of the cable. It should measure nearly zero ohms. If you are using an adapter to convert to XLR, there should be zero ohms between the RCA shield connection and pin 1 **and** pin 3 of the XLR. The center pin of the RCA is thus pin 2 of the XLR.
Now I have also seen amplifiers that purport to have balanced inputs that have some really strange stuff going on- a year and a half ago we discovered a well-known solid state amp that used an input transformer (which allowed it to theoretically operated balanced or single ended with ease) was set up so that the input transformer ignored pin 3! On another occasion we found an Audio Note DAC that used pin 2 for ground instead of pin 1 of the XLR!
So if you want to be scientific about it the connections at the input of the amp should be tested to see that they are correctly wired. Many high end audio manufacturers are not aware that there are standards for the balanced operation...
If the above issues are satisfied, there will be no hum/buzz. This is of course assuming that the preamp is working correctly in the first place (hum and buzz was a problem with the sample we saw). So its a good idea to test the preamp with a known-good alternative amplifier, or to replace it temporarily with a known-good preamp (quite often one of your sources like a CD player can be sufficient for this).
Good Luck- keep posting. |
Hi Grannyring, if the the chassis is grounded to the middle pin of the IEC, but there is no resistance (open) between the RCA ground and the chassis, then the chassis can act as a sort of antenna- and thus can can buzz.
A resistor of about 100 ohms (1/2 watt) between the RCA shield and chassis should take care of it if this is the problem. If it were me this would be the next thing I would try, based on the measurements you have reported.
Keep posting... |
Yes- if pin 3 is not tied to pin 1, while the RCA is in use, the amplifier will buzz whether the preamp is on or not, and whether its plugged into the wall or not.
Also, FWIW it is not uncommon for a preamp to buzz if it is not powered up. The reason is that the output impedance is usually much higher and can act like an antenna.
If the Aesthetix is really balanced, you should not have any troubles making this work. |
This is *starting* to sound like the preamp is off the hook. Did you make sure a jumper is there between pin 1 and 3 of the XLRs? If that is missing it may not matter what the source is.
A balanced amplifier has 2 inputs: inverting and non-inverting. When you are using the RCA input you are usually using the non-inverting input, which is often the same as pin 2 of the XLR. If you don't install the jumper the unused input can pick up noise. |
It is possible that the power supply ground of the speaker is somehow also connected to the speaker terminals. That could cause some amplifiers to be ground-looped. A continuity test between the ground pin of the AC cord and the speaker terminals would sort that out pretty quick. |
From the descriptions above, IMO the preamp is off the hook- this is a problem with the power amp.
Also based on these descriptions, its my theory that the amplifier has a problem with noisy rectifiers, which is getting picked up at the input if the source impedance is too high. This is why it is quiet with a shorted input and a 1K resistor, but not with the Dude (which has a relatively high output impedance as the volume control is at the output of the preamp), and likely a few other tube preamps as well.
Noisy rectifiers can be tricky as the commutation noise they produce acts very much like RF (which *is* part of their noise component), in that it can easily get to other parts of the amplifier, and use input connections as an antenna.
However this is only a theory, another possibility is a layout problem where the AC power is somehow too close to the audio input. Either way though- things are pointing at the amp. |
Grannyring, that info weakens my theory about the specific malfunction, but strengthens the idea that the amp is at fault. If the amp is in warranty I would return it for repair. |
The 1000 ohm value on pin 1 (grid) represents the grid stop resistance at the input. That is not the problem. |
So simple impedance of the source is partially ruled out- we are OK with a relatively high impedance at the input, so long as it is balanced. What happens if we use the same source to drive the RCA single-ended?
Grannyring, are you able to use the Jensen running single ended? How you would do that is to use the RCA input of the amplifier by using an adapter with the Jensens, one that ties pin 3 and pin 1 of its XLR output together.
This is still pointing to an issue with the input wiring where it is able to pick up noise from inside the amp. I suspect that it is common mode to the inverting input and so is not picked up when the amp is driven balanced. |
Hi Al, in practice 100 ohms can be quite effective. We use that value a lot. It does not seem like much in the face of the higher impedances that tubes employ, but it can make the difference between RF oscillation (resulting in hum) and absolute stability.
So right now I am thinking that grid stops are not the issue.
Its interesting that using the Jensens sorted it out. This would be a way of reducing the output impedance of the source. If the amplifier is having troubles with AC line noise at its inputs, this would be the sort of thing to shut it up. I suggested that this might be one of the explanations back on the 13th; that theory has now gained some juice. |
