Line pre-amp, benefit of collective wisdom needed!

[Deco-Doctor]

I have been spending the last few weekends on my latest project, a valve line level pre-amp with output transformers.





The valve is a russian 6N6P double triode, one half for each channel. Both triodes are biased using a 3volt lithium cells in the grids, the cathodes are grounded. Each triode is running at 138volts on the anode at 22mA standing current

The output transformers are Sowter 9560s. http://www.sowter.co.uk/specs/9560.htm



The regulated power supply is a solid state Maida regulator. http://www.diyaudio.com/forums/tubes-va ... lator.html

OK so far, but this is the bit where I need the benefit of the collective wisdom on this forum


Without any power or other connections to the preamp just plugging the outputs into my power amps results in hum. moving the whole preamp out of the rack and into the middle of the lounge floor reduces said hum. Turning the beast on it's side also reduced the hum so I rotated the transformers to the current position in the last picture.

On power up as the current rises in the transformer primary the hum reduces greatly, but does not go away. Aside from shielding both transformers in a mumetal can what else can the team suggest?

Thanks

Martin

[Andrew]

How are the output sides of the transformers wired?

[Deco-Doctor]

How are the output sides of the transformers wired?

Pink and Green wires. Pink to the ground side of the RCA socket, Green to the centre pin. The output is loaded with a 680ohm resistor. Ground side of the RCA to common signal ground. I tried lifting the ground but the hum is worse. Not tried it without the load resistor yet.

[Andrew]

Pink and Green wires. Pink to the ground side of the RCA socket, Green to the centre pin. The output is loaded with a 680ohm resistor. Ground side of the RCA to common signal ground. I tried lifting the ground but the hum is worse. Not tried it without the load resistor yet.

Well, that's pretty much what I would have done. Is that a choke in there, on the middle left?

And, have you tried lifting the heaters?

Andrew

[Deco-Doctor]

Is that a choke in there, on the middle left?

And, have you tried lifting the heaters?

Andrew

Yes that is a 10H 80mA choke. PSU is 250-0-250 TX then 2 xUF5408 full wave rectifier to a choke input filter then 150uf>220ohms>150uf filter to Maida regulator board.

The heaters are DC, LM317 regulator. I have tried floating the heater, grounding one side and now the latest is lifting it to 10volts above ground, actually no difference in all cases.

Apart from the hum it sounds superb.

[Deco-Doctor]

The output stage is virtually the same as the last schematic on this page.

http://www.troelsgravesen.dk/TRAM.htm

Mine is biased with a 3volt battery though.

[Andrew]

I would try several things..

Put a scope the output, its it mostly 50Hz or 100Hz?

If its 100Hz, its mostly coming from the power supplies or after a set of diodes. If its mostly 50Hz, it is likely getting coupled in from a source that's not being rectified, could be the mains TX or environmental noise.

Also put a large cap on the B+ to ground and perhaps a small-ish resistor in series to make sure any 50Hz/100Hz is not on the B+. Basically, add a temporary RC filter and see if it helps.

I also would try and get the outputs away from the choke and the mains TX, the choke is probably the worst culprit it will be radiating 100Hz out of its air gap.

I notice the chap in the website using separate boxes, I wonder if he really sites them as close together as they are in the pic when he has them in his system.

Having something that can do an FFT on the signal is dead handy when chasing these problems down, do you have access to anything?

How much above the cathode do the heaters need to be to make sure there's no unwanted noise getting coupled in? I usually shove them up a bit further than 3 times cathode voltage, but I have no idea if more is better....or your 3x is enough....

Andrew

[Nick]

My suspicion would be the choke. choke input will put a big demand on the choke, and is likely to emit a lot of field. Was the choke designed for choke input, if not it will be having a hard life. Maybe try moving the choke away as a experiment.

[Deco-Doctor]

I would try several things..

Put a scope the output, its it mostly 50Hz or 100Hz?

If its 100Hz, its mostly coming from the power supplies or after a set of diodes. If its mostly 50Hz, it is likely getting coupled in from a source that's not being rectified, could be the mains TX or environmental noise.

the choke is probably the worst culprit it will be radiating 100Hz out of its air gap.

Having something that can do an FFT on the signal is dead handy when chasing these problems down, do you have access to anything?

How much above the cathode do the heaters. I have no idea if more is better....or your 3x is enough....

Andrew

Andrew - Thanks for your thoughts. I have not had chance to put a scope on the output yet, but before any power is applied there is a lot of 50Hz hum, which I guess must be induced hum in the output transformers. On power up the hum reduces, but I can still hear 50Hz, of course it's difficult to tell if that's induced hum from the power transformer, or if it's coming from elsewhere.

I don't have an FFT analyser, but I know a man who has! So I could get it on an FFT at the weekend.

I have the heater at 10 volts above the cathode which is grounded as I am using battery bias in the grid circuit. I have no idea how much is enough, but I can easily move it to 30 or volts or so by changing a resistor.

[Deco-Doctor]

My suspicion would be the choke. choke input will put a big demand on the choke, and is likely to emit a lot of field. Was the choke designed for choke input, if not it will be having a hard life. Maybe try moving the choke away as a experiment.

Nick, thank you. I'm going to remove the choke completely, actually I think it's overkill anyway. I don't know if the choke is rated for choke input service, but there is no mechanical noise from it.

The Maida regulator is good enough to kill any ripple, the designer claims no measurable ripple on the output with 50vpp ripple on the input.

I need to find another transformer before I can ditch the choke though. I have just ordered a toroid with an electrostatic screen that should be a lot better than what I have at the moment in terms of radiated field.

[Nick]

Just make sure the madia is ok with the input voltage changing from choke to cap input will mean.

If the output picks up hum on it sown it sounds like the transformers are sensitive to hum fields, maybe some screening tape around the core. The hum will reduce on power up because the Ra of the driving valve will shunt some of the hum to ground.

[Deco-Doctor]

Just make sure the madia is ok with the input voltage changing from choke to cap input will mean.

If the output picks up hum on it sown it sounds like the transformers are sensitive to hum fields, maybe some screening tape around the core.

I'm astonished at how sensitive the transformers are to hum fields, even inside a steel case and several feet away from any electrical devices. Screening the core might be the only answer, but it won't be easy. It usually requires the transformer to be inside a cylindrical mumetal can to be effective. I think I will ask Brian Sowter what he can suggest.