Faint Hiss/Hum.

sonata149 · 24th November 2010, 07:11 PM

I've built Eli Duttman's El Cheapo PP power amp, with 6BW6 outputs, most components coming from Jim McShane. The amp sounds nice except for a faint noise (between hiss and a hum). It's not that "efffff-ing" valve sound either and it doesn't interfere during play but can be slightly annoying without a signal coming through. It's not the preamp, because it's still there without it.

I've just made a "major surgery" to isolate each section's grounding (including power supply grounds) and brought them to a star-earth connection. The faint noise remained. BTW the heaters are nowhere connected to earth.

Is this normal for a valve amp? Is there anything to do to eliminate such a noise?

Thanks for any comments/suggestions.
Regards,
Joe A

DF96 · 24th November 2010, 07:29 PM

What are the heaters connected to? They need a reference voltage from somewhere. Heater supplies should not be left floating.

sonata149 · 24th November 2010, 07:33 PM

Thanks DF96, I'm not sure I understand your question properly, but the heaters are daisy-chain connected in parallel to a 6.3V secondary winding.

SY · 24th November 2010, 07:53 PM

Is that 6.3V winding referenced to ground? If you leave it floating, annoying sounds can happen.

Soonerorlater · 24th November 2010, 08:10 PM

This is a good read

The Valve Wizard

DF96 · 24th November 2010, 08:15 PM

If the 6.3V secondary has a centre tap, ground that to somewhere near the HT supply ground. If not, try grounding one side or the other side. One may give less noise so experiment. An ungrounded heater supply can pick up noise via capacitive coupling in the mains transformer.

Miles Prower · 24th November 2010, 08:32 PM

Quote:

Originally Posted by SY

Is that 6.3V winding referenced to ground? If you leave it floating, annoying sounds can happen.

Not necessarily. I left the heater winding floating for one project that uses a power xfmr with multiple secondaries (5.0V / 3.0A for 5U4GB; 6.3V / 6A; 650Vct / 150mA). The positive rail is supplied with a 5U4GB, and has a solid state negative rail. The negative rail hits 450Vdc within a couple of seconds, and would really bust the spec for Vhk for the 6FQ7s it feeds.

I tried it with floating heaters before resorting to Zener protect diodes, and all I get is a 6.0mVp-p, 60Hz, residual at the output. You can barely hear it, and if not for the glow, you'd never know it was on.

SemperFi · 24th November 2010, 08:52 PM

Either ground one side of the heater or connect it to a positive voltage of up to +80volts. If output tubes are cathode biased, simply connect the heater to the cathode.

sonata149 · 25th November 2010, 08:02 AM

Thanks a lot for all that useful information. In my case the filament winding (one of multiple secondaries) is not centre-tapped. It is connected as Miles Prower describes (before the Zener protection) and although I don't have the necessary test equipment to measure it, the noise I get is something similar.

I also read the Valve Wizard post (thnx Soonerorlater!) and I will experiment with the various methods described by contributors.

sonata149 · 25th November 2010, 02:36 PM

I connected a potential divider network (220R each leg) across the heater winding and connected the junction point to ground as proposed in the Valve Wizard link. While the noise has imperceptibly been reduced there is still some residual noise. Nothing serious but it's still there. To my ears it sounds like it is transformer noise, which is a toroidal affair carrying all secondary voltages.

OTOH, other members may have a different view.

24th November 2010, 07:11 PM	#1
sonata149 diyAudio Member Join Date: Sep 2006 Location: Malta, Europe	Faint Hiss/Hum. I've built Eli Duttman's El Cheapo PP power amp, with 6BW6 outputs, most components coming from Jim McShane. The amp sounds nice except for a faint noise (between hiss and a hum). It's not that "efffff-ing" valve sound either and it doesn't interfere during play but can be slightly annoying without a signal coming through. It's not the preamp, because it's still there without it. I've just made a "major surgery" to isolate each section's grounding (including power supply grounds) and brought them to a star-earth connection. The faint noise remained. BTW the heaters are nowhere connected to earth. Is this normal for a valve amp? Is there anything to do to eliminate such a noise? Thanks for any comments/suggestions. Regards, Joe A

24th November 2010, 07:53 PM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Is that 6.3V winding referenced to ground? If you leave it floating, annoying sounds can happen. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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24th November 2010, 07:29 PM	#2
DF96 diyAudio Member Join Date: May 2007	What are the heaters connected to? They need a reference voltage from somewhere. Heater supplies should not be left floating.

24th November 2010, 07:33 PM	#3
sonata149 diyAudio Member Join Date: Sep 2006 Location: Malta, Europe	Thanks DF96, I'm not sure I understand your question properly, but the heaters are daisy-chain connected in parallel to a 6.3V secondary winding.

24th November 2010, 08:10 PM	#5
Soonerorlater diyAudio Member Join Date: Jul 2008 Location: Belfast	This is a good read The Valve Wizard

24th November 2010, 08:15 PM	#6
DF96 diyAudio Member Join Date: May 2007	If the 6.3V secondary has a centre tap, ground that to somewhere near the HT supply ground. If not, try grounding one side or the other side. One may give less noise so experiment. An ungrounded heater supply can pick up noise via capacitive coupling in the mains transformer.

24th November 2010, 08:52 PM	#8
SemperFi diyAudio Member Join Date: Jan 2002 Location: nowhere	Either ground one side of the heater or connect it to a positive voltage of up to +80volts. If output tubes are cathode biased, simply connect the heater to the cathode.

25th November 2010, 08:02 AM	#9
sonata149 diyAudio Member Join Date: Sep 2006 Location: Malta, Europe	Thanks a lot for all that useful information. In my case the filament winding (one of multiple secondaries) is not centre-tapped. It is connected as Miles Prower describes (before the Zener protection) and although I don't have the necessary test equipment to measure it, the noise I get is something similar. I also read the Valve Wizard post (thnx Soonerorlater!) and I will experiment with the various methods described by contributors.

25th November 2010, 02:36 PM	#10
sonata149 diyAudio Member Join Date: Sep 2006 Location: Malta, Europe	I connected a potential divider network (220R each leg) across the heater winding and connected the junction point to ground as proposed in the Valve Wizard link. While the noise has imperceptibly been reduced there is still some residual noise. Nothing serious but it's still there. To my ears it sounds like it is transformer noise, which is a toroidal affair carrying all secondary voltages. OTOH, other members may have a different view.

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