Thought on quieting an amp

[Kingrex]

It seems some people here have a pretty good grasp on amp engineering. I have a KT88 pentode monoblocks that hum 60 cycle. When the input is shorted and the 12ax7 pulled, its totally quiet. My local tech surmises the noise therefore originates before that tube or in the tube.

Amp is a typical wood bread box built on the over the shelf DIY Tube CB. He is mocking up a tube socket he can stick in the existing socket, then power the anode with 5 volts DC, instead of AC.

He is also wondering about the case. Do I need a shielded case. Any thoughts on what to try?

There is a Lundahl input transformer from the single ended rca to the CB.

I have already swapped tubes, shielded tubes, cheated ground, different power cables, different locations in home, isolation transformers, shunyata, isotek, turned off house, ground is 6.7 ohm. I'm pretty sure its an inherent internal noise and I would love to isolate it and get rid of it.

 

[Folsom]

Shielded transformer?

Honestly my guess is a bad solder/wire. One of the grounds may not be connected.

 

[213Cobra]

Shielding the case couldn't hurt. You can try the DC feed to the 12ax7. But my guess is the prime suspect is the Lundahl input xformer being improperly grounded relative to the input and the circuit. Also, do you know the input impedance for the amp with that xformer as the 1st stage? If you find the 12ax7 is exceptionally noisy, you can also try a the SPAX7 or other exceptionally low-noise 12ax variant.

Phil

 

[Kingrex]

 

[Kingrex]

Input is 10k. This is the guts of my amp. Supposedly grounded as well as can be. No shielding to speak of.

 

[Kingrex]

I am told grounding the case of the input transformer makes more noise. Right now, i get an open circuit at the input transformer case and the aluminum top plate.

 

[Folsom]

So you can try a nylon screw or something like that?

 

[Kingrex]

Cobra said he thought the input transformer was not grounded proper. My Fluke read the case as open ground. It is installed proper if indeed it needs to float to be as quiet as can be.

Is that Cinemag a good transformer. Or are they noisy.

What about the twisted wire. I'm back to shielding. Are the wires an antenna. Is there an easy way to make a cage to test RF interference.

 

[Folsom]

They should only be an antenna if they aren't connected.

What power transformer? Imbalances in those cause hum. Does it hum?

 

[Solypsa]

I'd stay focused on 60hz hum and potential ground loops and set rfi/emi aside for the moment.

1) physical hum from ps transformer- you have dealt with this before iirc (Rega)

2) hum on signal part of circuit when amp is powered on. Is the hum present on the either side of the input transformer? Or appears after?

 

[Kingrex]

The hum is from the speakers. Loud enough to be heard at the listening chair between songs. At night when the power gets worse, loud enough you hear the stereo when muted and the tv is on. Or just in quiet passages in songs.

Please dont start with ground loops in my system. I have tested this with all my stereo gear, accept for the amps turned off, unplugged and the amp inputs shorted. I am focusing my attention on the amps themselves. They need to be not heard at the listening chair when installed as such.

 

[213Cobra]

Cobra said he thought the input transformer was not grounded proper. My Fluke read the case as open ground. It is installed proper if indeed it needs to float to be as quiet as can be.

Is that Cinemag a good transformer. Or are they noisy.

What about the twisted wire. I'm back to shielding. Are the wires an antenna. Is there an easy way to make a cage to test RF interference.

Cinemag transformers are normally quiet and are quite good. Different Cinemags are used in a variety of moving-coil phono cartridge SUTs, including all of the Bob's Devices units. I don't think you have an rf/emi problem. I note that the input xformer is installed to float, but I also have to say that in many SUTs and in many other amps with similar input transformers, a switch is installed to allow running grounded or floated, depending on the rest of the system connected. I doubt you are getting intolerable 60Hz from twisted pairs.

You may find that in your system, that input transformer cannot float and be quiet. I emphasize may. It's going to take some trial and error to isolate your problem. But I'd try a temporary ground for the xformer and if that works, then install a ground lift switch. Another thing to try is an Ebtech Hum X. This is a ground voltage filter that maintains ground. It just plugs into your AC outlet and then plug the amp, or the whole system feed into it. These are often used with guitar amp and PA amp setups and can solve some otherwise vexing 60Hz hum problems.

PHil

 

[Folsom]

The Ebtech Hum X isn't going to do anything since the ground loop is broken by the Cinemag. The hum either starts somewhere up stream to be modulated into the signal path or it's generated after Cinemag. (maybe in it) It just isn't generated through it.

 

[213Cobra]

The Ebtech Hum X isn't going to do anything since the ground loop is broken by the Cinemag. The hum either starts somewhere up stream to be modulated into the signal path or it's generated after Cinemag. (maybe in it) It just isn't generated through it.

I am not convinced we can conclude that. We don't know there isn't some voltage on the ground line in the first place. Second, I've had instances where with the presence of a ground lift allowing test, hum that starts upstream has been mitigated or eliminated by one or more Hum X and same if it's generated after. We don't know what's going on with the ground relationship between the preamp and the amps, or the total combination of the sources. Even if other amps don't hum in the system, I've been surprised to find Hum X sometimes effective in situations where it seems it should not be. I'm not saying it will work. But first I'd try not floating the Cinemag.

Phil

 

[Kingrex]

I will clamp a 20 awg silver wire to the input transformer foot and take it to my main bond point in the amp. Its about 5" away.

I'm going to put my 2.4kva topaz back in series with my amps only. I hate two feeds to my gear because you can get ground loops easy that way, but it does stop the night noise.

 

[Kingrex]

 

[Kingrex]

That is a general plug next to my rack. My rack reads about 39 volts with my server, dac and Add Power operating. The preamp and phono pre as well as amps are off with 39 volts induced.

This is a topic for another thread. What is the impact of induced voltage between the ground and neutral. If I spread the wires I would drop the voltage but the inductance would increase and choke the electrons.

 

[Folsom]

Phil, read up on galvanic isolation. I can conclude.

Rex don’t attach safety (earth) ground to the Cinemag. Just attach a wire from the ground on RCA to it.

 

[213Cobra]

Phil, read up on galvanic isolation. I can conclude.

Rex don’t attach safety (earth) ground to the Cinemag. Just attach a wire from the ground on RCA to it.

I understand galvanic isolation. I'm not convinced the 60Hz hum is exclusively related to the existence of the Cinemag and whether it's grounded or not, but it is the first question I'd want to settle.

Attached is the reference sheet on your Cinemag xformer. There are both single and dual shield versions of that transformer. The dual shield version is about -7db quieter @ 60Hz than the single shield, FYI. The design was optimized for 600 ohm feeds, interestingly. The circuit samples show both floating and grounded implementations, so still think that's an experiment worth doing.

Phil

 

[Folsom]

They are showing the Cinemag enclosure tied to safety ground. Is your enclosure safety grounded? The screw would do that if it were making proper contact. But still I wouldn't run a real long wire to ground it, I'd try the RCA first.

Is there an online schematic of the amplifier?