s-5 tube amp hum troubles

ambience exists · 21st July 2008, 06:36 PM

I just got around to fitting my S-5 electronics stereo tube amp in a case with a wood bottom and a metal top with vent holes. The binding posts are attached to the metal top but are isolated from it. The top is something I found at a kitchen store and is teflon non-stick coated. I got a volume knob from radio shack for $3 that has ALPS stamped on it. I soldered it off the board with wires and bolted it to a hole in the metal top. With the source paused, the amp is quiet until the pot is around 1 oclock, until about 5 oclock, when it hums kind of like a guitar amp. The hum doesn't get louder, it just exists. There is no hum with the pot all the way up. Does anyone have any idea what is going on?

Jeb-D. · 21st July 2008, 06:51 PM

Is the pot's shaft connected to the amplifiers ground? It should be, unless you have the amplifiers ground and the pot shaft both referenced to earth ground.

Also, I found on my kit (it's an old revision S5 kit) the hole where the provided on the PCB to ground the pot shaft, was actually connected to the input of one of the channels instead (this causes the buzz characteristic to change when you touch the pot shaft at certain volume settings). If your board has the same error, do not use the provided hole.

ambience exists · 21st July 2008, 06:53 PM

Do you mean the pots ground lug or the actual shaft itself?

When I touch the shaft the buzz is reduced.

ambience exists · 21st July 2008, 08:16 PM

thanks, I was not aware of that error.

I now have that hole in the PCB connected via copper wire to the pot shaft with a star nut, and scraped off the chassis paint to get the connection.

The hum reduced, but it is still present and changes its character at differrent volume settings.

gmilitano · 21st July 2008, 11:22 PM

Did you twist the wires as noted in the build instructions? Is your input close to the AC?

When you touch the pot not that it is grounded does the hum still change?

ambience exists · 21st July 2008, 11:30 PM

I actually purchased this as a completed amp, and just fit it to a chassis.

Which wires should be twisted? I twisted the wires that lead to and from the power switch I supplied.

I used the advice Jeb-D gave me and changed the grounding wire from the hole it was in to a spare hole on the ground trace in the rear of the board. It worked very well, reducing the hum to little, but I still get some buzzing when the pot is almost full bore. There is also some very quiet what sounds like power AC hum that doesnt change with pot position. Is this the "AC under the caps" hum I've been reading about?

gmilitano, I actually got this amp after being inspired by your build on DIY Audio Projects. Youve got some interesting builds.

Thanks

Jeb-D. · 21st July 2008, 11:55 PM

Quote:

. There is also some very quiet what sounds like power AC hum that doesn't change with pot position.

This is normal for these kit's. Especially if your speakers are rated above 90dB 1W/1M. If S5 were to put in more into the power supply it would jack up the cost of the amp kit. There are ways to improve the power supply which will reduce this, if it is bothering you and you have have some cash to spend.

If the amps buzz characteristic changes with knob position, this usually indicates grounding scheme. If you can post pictures of the amp guts, it would be easier to tell what is going on. Is this happening with a source connected or no source?

ambience exists · 21st July 2008, 11:58 PM

Source connected, CD paused so I can hear whats going on.

By the way, is there a way to rig this amp as a power amp only, with no volume control? I have a preamp I could use.

Also, If this helps I also took out the input capacitors since I measured no DC from my source.

Jeb-D. · 22nd July 2008, 12:29 AM

Quote:

By the way, is there a way to rig this amp as a power amp only, with no volume control? I have a preamp I could use.

Of course. Remove the pot. On the PCB holes for the pot, put jumpers from the pots input terminals to the pots output terminals. Then add a pull down resistor from the input tubes grid to ground. The value will depend on your pre-amps capabilities. 100k is the safe side, but if your pre-amp can handle a 10k load, I'd go with that.

Quote:

Also, If this helps I also took out the input capacitors since I measured no DC from my source.

I removed mine too. but check for grid leak voltage when a source isn't attached. Check it when the pot is at maximum setting.

ambience exists · 22nd July 2008, 01:07 AM

Sorry for being a bit innexperienced, but where exactly do I rig the resistor?

21st July 2008, 06:36 PM	#1
ambience exists diyAudio Member Join Date: Jun 2008	s-5 tube amp hum troubles I just got around to fitting my S-5 electronics stereo tube amp in a case with a wood bottom and a metal top with vent holes. The binding posts are attached to the metal top but are isolated from it. The top is something I found at a kitchen store and is teflon non-stick coated. I got a volume knob from radio shack for $3 that has ALPS stamped on it. I soldered it off the board with wires and bolted it to a hole in the metal top. With the source paused, the amp is quiet until the pot is around 1 oclock, until about 5 oclock, when it hums kind of like a guitar amp. The hum doesn't get louder, it just exists. There is no hum with the pot all the way up. Does anyone have any idea what is going on? __________________ ((AE))

21st July 2008, 06:53 PM	#3
ambience exists diyAudio Member Join Date: Jun 2008	Do you mean the pots ground lug or the actual shaft itself? When I touch the shaft the buzz is reduced. __________________ ((AE))

21st July 2008, 08:16 PM	#4
ambience exists diyAudio Member Join Date: Jun 2008	thanks, I was not aware of that error. I now have that hole in the PCB connected via copper wire to the pot shaft with a star nut, and scraped off the chassis paint to get the connection. The hum reduced, but it is still present and changes its character at differrent volume settings. __________________ ((AE))

21st July 2008, 11:22 PM	#5
gmilitano diyAudio Member Join Date: Apr 2004 Location: Winnipeg, Manitoba	Did you twist the wires as noted in the build instructions? Is your input close to the AC? When you touch the pot not that it is grounded does the hum still change? __________________ DIY Audio Projects [ 10" DIY Subwoofer \| FE127E Sealed Bipole \| [ LM3886 Chip Amp / Gainclone \| K-12M Tube Amp Kit ]

21st July 2008, 11:30 PM	#6
ambience exists diyAudio Member Join Date: Jun 2008	I actually purchased this as a completed amp, and just fit it to a chassis. Which wires should be twisted? I twisted the wires that lead to and from the power switch I supplied. I used the advice Jeb-D gave me and changed the grounding wire from the hole it was in to a spare hole on the ground trace in the rear of the board. It worked very well, reducing the hum to little, but I still get some buzzing when the pot is almost full bore. There is also some very quiet what sounds like power AC hum that doesnt change with pot position. Is this the "AC under the caps" hum I've been reading about? gmilitano, I actually got this amp after being inspired by your build on DIY Audio Projects. Youve got some interesting builds. Thanks __________________ ((AE))

21st July 2008, 06:51 PM	#2
Jeb-D. diyAudio Member Join Date: Nov 2005 Location: SoCal	Is the pot's shaft connected to the amplifiers ground? It should be, unless you have the amplifiers ground and the pot shaft both referenced to earth ground. Also, I found on my kit (it's an old revision S5 kit) the hole where the provided on the PCB to ground the pot shaft, was actually connected to the input of one of the channels instead (this causes the buzz characteristic to change when you touch the pot shaft at certain volume settings). If your board has the same error, do not use the provided hole.

21st July 2008, 11:58 PM	#8
ambience exists diyAudio Member Join Date: Jun 2008	Source connected, CD paused so I can hear whats going on. By the way, is there a way to rig this amp as a power amp only, with no volume control? I have a preamp I could use. Also, If this helps I also took out the input capacitors since I measured no DC from my source. __________________ ((AE))

22nd July 2008, 01:07 AM	#10
ambience exists diyAudio Member Join Date: Jun 2008	Sorry for being a bit innexperienced, but where exactly do I rig the resistor? __________________ ((AE))

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