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Old 20th August 2015, 01:50 PM   #1
thehoj is online now thehoj  Canada
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Default 845 amp.. Now I've got a faint hum..

I just got my 845 amp put together, and I'm dealing with a very faint hum. A while back I started a thread about dealing with some buzz above 3Khz. With the assistance of forum members on here I resolved that issue by improving my layout.
Here's that thread: Faint buzz through power stage (845)

Now that the amp is put together in the chassis, I noticed a very faint hum almost inaudible coming through the woofer of my test speakers. It's so faint I wasn't concerned with it at all, with my 92db @ 1w/1m test speakers you can only hear the hum at maybe 2 inches from the woofer.. BUT, with my 96db @ 1w/1m TL speakers with fostex full range drivers it's a bit more noticeable. Maybe up to a few feet in front of the drivers. Still not bad, but I kind of wanted to get rid of that.

First of all, I'll post some images of the schematic, and the build.
This amp is JUST the power amp, no driver in this chassis. The entire driver for this amp is in a different chassis. And this hum is coming just from the power amp, no driver connected.

Here's the actual power amp schematic (including HT power supply, as well as filament power supply):
Click the image to open in full size.

Here's how my power supply is wired. Power transformer CT connected to first filter cap (C1), and then negative of that connected to negative of C2, negative of C2 connected to C3. All of my signal ground is then connected to C3, and my earth ground is connected to C3:
Click the image to open in full size.



Here's a few pics of the amp:

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

It's definitely a low frequency hum. It sounds like 60Hz to me. But when measuring over an 8 ohm resistor load with my oscilloscope I actually measure that it's a frequency of 30Hz at about 6mV peak to peak. It's messy looking sinewave. In talking to others about this, they seemed to think maybe my oscilloscope was out, and so the 30Hz reading might not be accurate.

Here's a picture of the waveform though (amplitude set to 2mv/Div, t/div set to 10mS):
Click the image to open in full size.

Now, I've painstakingly analyzed my grounding, and believe it is sound. As I mentioned, earth is connected to the chassis near where earth ground enters the amp, and this is only connected to C3. All of my signal ground is also only connected to C3.

There's no driver in this amp. Also, the hum is present when the grid of the 845 is grounded.

One other test I just did yesterday, was to eliminate the filament regulator as a potential cause of this. So I disconnected the filament circuitry I built into the amp from the 845 socket, and hooked up a car battery with a dropping resistor to the filament connections for the 845. With the dropping resistor I was reading 10.25V over the filament while using this car battery. When doing this I still heard the faint hum, and measured that exact same waveform over an 8 ohm resistor. I feel like that should eliminate my filament rectifier / regulator as a cause of this.

I'm starting to wonder if I should just leave this alone. It's not very noticeable.. Does anyone have any thoughts on what could be causing this?
Someone on a different forum suggested that it looks like it could be plasma hum, and wondered if I had any florescent lighting in the house. But I don't.

Last edited by thehoj; 20th August 2015 at 02:06 PM.
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Old 20th August 2015, 03:35 PM   #2
DF96 is offline DF96  England
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Check your 'scope to see whether you have 30Hz or 60Hz hum.

Calculate what hum you should expect from that amp fed by that PSU. Compare this with the hum you see. Recalculate on the assumption that one of the rectifiers is not conducting - with a two stage LC smoother any 60Hz ripple could be attenuated 30dB less than 120Hz ripple.
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Old 20th August 2015, 04:01 PM   #3
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Default 845 hum

Try fitting a hum bucking pot across the heaters say 22R 3-4 watt with the wiper connected to the bias components. Oe even just a couple of 10R 3 Watt resistors in series with the junction of the two Rs connected to the bias components.

John
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Old 20th August 2015, 04:03 PM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by thehoj View Post
I just got my 845 amp put together, and I'm dealing with a very faint hum. earth is connected to the chassis near where earth ground enters the amp, and this is only connected to C3. All of my signal ground is also only connected to C3.
Try moving the earth chassis connection to C1 instead of C3.
Also, all those pairs of wires (like between C1/C2 and C2/C3) should be twisted together.
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Old 20th August 2015, 04:04 PM   #5
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Magnetic coupling straight to the output transformers is never zero, and a real bear to resolve with all the chassis holes cut. If you can do it safely (safety first!) you might try unbolting one channel's transformer and moving it around, listening to only that channel. A long shot, but might help.

All good fortune,
Chris
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Old 20th August 2015, 04:06 PM   #6
kevinkr is offline kevinkr  United States
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I assume DF meant 60Hz or 120Hz, if the coupling is magnetic you might expect 3rd harmonic of the 60Hz line frequency to predominate. (180Hz)

Determine the period of that waveform and from there it should not be too difficult to determine the cause.

Your supply seems much more lightly filtered than the supply in my GM70 amps. I am not sure how much additional input capacitance your rectifiers can handle, but some additional capacitance here pays pretty big dividends.

You may still be fine ripple wise on the HV supply. I do not know what your level of experience is with measuring high voltages, but a supply ripple measurement would be useful if you can do that SAFELY. (Fixed resistor divider string across the supply of a known ratio would work) Ideally ripple pk-pk should be under 50mV on this supply if you want a quiet output with no negative feedback. An alternative would be to simulate the filter performance using LTSpice.

I have managed to get my GM70 amps quiet enough that with 100dB + efficient bass bins (onkens with Iconic 165-8G 16" woofers) to be near dead silent - it took LF output noise of <250uVrms to get this far, so it is quite possible.
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Old 20th August 2015, 04:07 PM   #7
thehoj is online now thehoj  Canada
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Quote:
Originally Posted by DF96 View Post
Check your 'scope to see whether you have 30Hz or 60Hz hum.

Calculate what hum you should expect from that amp fed by that PSU. Compare this with the hum you see. Recalculate on the assumption that one of the rectifiers is not conducting - with a two stage LC smoother any 60Hz ripple could be attenuated 30dB less than 120Hz ripple.
As originally designed I should have about 1mV of ripple
Click the image to open in full size.

But I also tried adding a 3rd LC leg to the power supply to reduce ripple to an insanely small amount like this (waaaay under 1mV of ripple) which didn't affect the hum at all.
Click the image to open in full size.

Are you saying one of the tubes in the rectifier could be bad? Or rectifier could be wired incorrectly?
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Old 20th August 2015, 04:13 PM   #8
kevinkr is offline kevinkr  United States
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Are you sure you are actually achieving output ripple levels anywhere close to this magnitude under load? If so the HV PSU is not likely to be the culprit, and magnetic coupling probably becomes the primary suspect..

Your wiring approach looks very similar to what I do and it works for me...
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Old 20th August 2015, 04:17 PM   #9
thehoj is online now thehoj  Canada
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Quote:
Originally Posted by John Caswell View Post
Try fitting a hum bucking pot across the heaters say 22R 3-4 watt with the wiper connected to the bias components. Oe even just a couple of 10R 3 Watt resistors in series with the junction of the two Rs connected to the bias components.

John
Even with a regulated heater supply?


Quote:
Originally Posted by rayma View Post
Try moving the earth chassis connection to C1 instead of C3.
Also, all those pairs of wires (like between C1/C2 and C2/C3) should be twisted together.
I was under the impression that I would then hear pulses from that first filter cap / PT center tap if I connected earth to C1.


Quote:
Originally Posted by Chris Hornbeck View Post
Magnetic coupling straight to the output transformers is never zero, and a real bear to resolve with all the chassis holes cut. If you can do it safely (safety first!) you might try unbolting one channel's transformer and moving it around, listening to only that channel. A long shot, but might help.

All good fortune,
Chris
I did think about that.. It would be difficult.. But I could maybe give it a try..



Quote:
Originally Posted by kevinkr View Post
I assume DF meant 60Hz or 120Hz, if the coupling is magnetic you might expect 3rd harmonic of the 60Hz line frequency to predominate. (180Hz)

Determine the period of that waveform and from there it should not be too difficult to determine the cause.

Your supply seems much more lightly filtered than the supply in my GM70 amps. I am not sure how much additional input capacitance your rectifiers can handle, but some additional capacitance here pays pretty big dividends.

You may still be fine ripple wise on the HV supply. I do not know what your level of experience is with measuring high voltages, but a supply ripple measurement would be useful if you can do that SAFELY. (Fixed resistor divider string across the supply of a known ratio would work) Ideally ripple pk-pk should be under 50mV on this supply if you want a quiet output with no negative feedback. An alternative would be to simulate the filter performance using LTSpice.

I have managed to get my GM70 amps quiet enough that with 100dB + efficient bass bins (onkens with Iconic 165-8G 16" woofers) to be near dead silent - it took LF output noise of <250uVrms to get this far, so it is quite possible.
Well as measured in that screenshot above, my t/div is set to 10mS.. The waveform measures 3 divisions, so that converts to 30Hz.. Is that what you meant?

I've been measuring my HV with a HV probe, basically just to check what my actual B+ is.. As for attempting to measure ripple, I have a string of resistors in place to discharge the caps, so I could do a direct reading over one of those resistors in the string. The discharge string is made up of 4 x 150Kohm resistors, So I should measure about 230V over each resistor.

I'm calculating 1mV of ripple though with PSUD2.. Why would my ripple potentially be so much higher than that?
And with that third LC leg bringing the simulated ripple down even more, it should be way under 1mV.

Last edited by thehoj; 20th August 2015 at 04:26 PM.
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Old 20th August 2015, 09:55 PM   #10
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Default *$% hum

Yes even with a regulated heater supply. Every little bit of hum rejection helps!

John
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