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Old 4th June 2015, 04:12 PM   #1
thehoj is offline thehoj  Canada
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Default Faint buzz through power stage (845)

Yes, another noise related thread.. But mine's different! I'm not dealing with the driver stage at all here.. lol.. Just the power stage.

I've been prototyping an 845 based SET amp.. Got everything put together and it sounds great, like ALMOST there.. But there is a very very faint buzz coming through the speakers.

In trying to narrow things down, I've disconnected the driver stage completely from the amp, so I'm just dealing with the power stage with no signal connected to the grid. I still get a very faint buzz coming through the speakers.

Here's my power supply
Click the image to open in full size.

And here's the power stage itself
Click the image to open in full size.

I believe my grounding scheme is sound, as I've used it in the past to great effect.. The CT of the power transformer is connected to the ground of the first filter cap, this first cap is then connected to the ground of the second cap, and then the second cap ground is connected to the ground of the third cap, my ground connections for the power tubes are then connected to the ground directly at this third cap.
I have the ground junction at that first filter cap then grounded to earth ground at that single point.. I've attempted using a 0.1uF X2 cap / 100ohm resistor between that ground point and earth ground, to try to reduce noise in that way as well, but it doesn't seem to make a difference either way.

If I ground the grid input then the buzz disappears, but otherwise there is this frustrating buzz sound.

I'm using Rod coleman regulators implemented in the suggested way, and when I measure AC on the filament with my TrueRMS Fluke meter, I read 0.000V, so I don't think I'm dealing with ripple on the filament..

If I attempt to use my meter to measure AC ripple on the B+ rail, I get crazy readings that jump around from 50mV to 500mV.. I feel like maybe my meter isn't the best way to attempt to measure ripple on a high voltage line.. Based on my PSUD2 simulations, with the power supply configured as I've shown it, the ripple should be in the 1mV range. Could I actually be dealing with ripple even though I'm using a fairly well filtered CLCLC design (as shown in the screenshot above)? I've read elsewhere that I could be dealing with Poor PS filtering at frequencies higher than the fundamental of the ripple.. I do have an oscilloscope, but I'm not very familiar with using it, and so have been unable to attempt measuring the B+ ripple with it..

I'm using an Arcotronics in that first position http://capacitoredge.kemet.com/caped...44ASGP5100ZA0J, and Cornell Dubilier MKP film caps in the power supply http://www.cde.com/resources/catalogs/944U.pdf, so ESR should be low, although I have tried adding a 0.22uF snubbing cap on the final cap in the rail which didn't make any difference.

I've even attempted to use a makeshift faraday cage (grounded to PS ground) covering one of the power tubes, to see if there was some sort of strange interference coming through. I have no fluorescent lights in my house, no dimmer switches, nothing that I can think of that might cause interference.

Because I've eliminated the driver portion for this testing, I'm not really sure what else I should be looking at.. Maybe I need to figure out how to use my 'scope a bit better..

One thing I'm thinking about is that the entire power stage is just laid out on a slab of wood.. if it were properly encased in an aluminum chassis, could that could alleviate things..? I'm hesitant to build this into a chassis until I have all the kinks worked out though.

Or what about the mains.. Would it be worth looking into sort of input mains filter..?

One last thought for now, could long wires in my power supply be picking up some sort of emi or other interference?.. I didn't cut any of the wires on the chokes, so there are some pretty decent lengths of wire between each node in the power supply.. And those wires are all very close to the power transformer.

Does anyone have any thoughts or ideas on this?

Last edited by thehoj; 4th June 2015 at 04:29 PM.
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Old 4th June 2015, 05:00 PM   #2
Keit is offline Keit  Australia
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I suggest you take some good photos of the wiring and layout - we may be able to spot something then.

Since the hum dissappears when the grid is earthed, it follows that a stray field eg from the power transformer or AC wiring is coupling into the grid. Ensure the spacing is adequate. A metallic chassis, will help. Equally clearly, power supply ripple is not an issue - if it was you'd still get the hum with the grid earthed.

Is the heater wiring close to the grid?
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Old 4th June 2015, 05:48 PM   #3
thehoj is offline thehoj  Canada
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Quote:
Originally Posted by Keit View Post
I suggest you take some good photos of the wiring and layout - we may be able to spot something then.

Since the hum dissappears when the grid is earthed, it follows that a stray field eg from the power transformer or AC wiring is coupling into the grid. Ensure the spacing is adequate. A metallic chassis, will help. Equally clearly, power supply ripple is not an issue - if it was you'd still get the hum with the grid earthed.

Is the heater wiring close to the grid?
The AC heater wiring isn't very close to the grid, it feeds into the rectifier, which feeds the regulator, and then that DC heater wiring is close to the grid.. Could that still be an issue?

I don't have any good pictures of it in its current state. I'll get some this evening.
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Old 4th June 2015, 08:36 PM   #4
hooman is offline hooman  Iran
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turn on your amp whiteout 845!
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Old 5th June 2015, 12:54 AM   #5
Keit is offline Keit  Australia
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Quote:
Originally Posted by thehoj View Post
The AC heater wiring isn't very close to the grid, it feeds into the rectifier, which feeds the regulator, and then that DC heater wiring is close to the grid.. Could that still be an issue?
It's possible -depends on the circuit. Post the FULL circuit.

If the heater supply filter capacitance is too low, and/or the power transformer heater winding voltage is too low, the regulator will be unable to regulate fully, and will pass through to the tube ripple and noise.

Directly heated tubes like the 845 are designed to be fed with balanced AC heater supplies, and when this is done correctly, there won't be a trace of hum audible. In theory there may be intermodulation distortion arising from the mixing of AC heater voltage and signal - though this is a non-issue in practice if hum is not audible without signal - and isn't your problem in this case. Once you go to DC heating, there are multiple circuit-dependent traps to fall into.
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Old 5th June 2015, 03:39 AM   #6
thehoj is offline thehoj  Canada
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Quote:
Originally Posted by Keit View Post
It's possible -depends on the circuit. Post the FULL circuit.

If the heater supply filter capacitance is too low, and/or the power transformer heater winding voltage is too low, the regulator will be unable to regulate fully, and will pass through to the tube ripple and noise.

Directly heated tubes like the 845 are designed to be fed with balanced AC heater supplies, and when this is done correctly, there won't be a trace of hum audible. In theory there may be intermodulation distortion arising from the mixing of AC heater voltage and signal - though this is a non-issue in practice if hum is not audible without signal - and isn't your problem in this case. Once you go to DC heating, there are multiple circuit-dependent traps to fall into.
Here's the full circuit.

Click the image to open in full size.

I won't be able to get a pic of my wiring until tomorrow or Saturday.
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Old 5th June 2015, 12:07 PM   #7
Keit is offline Keit  Australia
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As far as your buzz is concerned, nothing stands out as an obvious cause from your circuit. My rough calculation shows that the DC voltage into the regulator should be close to 15V (I note you have marked it 15.5V) and teh ripple will be about 50 mV RMS. With any reaonable sort of electronic regulator that should be fine.

I have no knowlege of the "Rod Coleman Regulator". I was unable to find proper specifications or a circuit by a quick bit of Googling. I did find a couple of websites that imply it is a constant current regulator. Constant current regulation is a dumb fad way to feed a vacuum tube, but that has nothing to do with the buzz you are getting.

I notice that the 2nd filter cap is 33 mF (33 millifarad ie 33,000 uF) - more than adequate. But you have the first cap shown is only 68 uF. That is way to small to do anything usefull - your circuit will act as a choke input filter. That's why the filter output is only 15 V and not the 25 V approx you would get from a conventional pi filter. I guess you did it this way so as not to exceed the ratings of the regulator while using a 20 V transformer you have on hand.

If the 68 uF nichicon fails, the circuit will work just the same.

Why do you need an LC filter ahead of an electronic regulator? Usually electronic regulators regulate out the ripple, making the choke redundant.
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Old 5th June 2015, 12:50 PM   #8
M Gregg is offline M Gregg  England
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Quote:
Originally Posted by hooman View Post
turn on your amp whiteout 845!

Ah yes is it magnetic pick up from the power transformer or choke..
Magnetic induction to output transformers..
The problem with removing the output tubes is possibly over voltage on the PSU caps.
(Turn off B+) does it still hum?


Regards
M. Gregg
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Last edited by M Gregg; 5th June 2015 at 12:54 PM.
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Old 5th June 2015, 01:30 PM   #9
thehoj is offline thehoj  Canada
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Quote:
Originally Posted by Keit View Post
As far as your buzz is concerned, nothing stands out as an obvious cause from your circuit. My rough calculation shows that the DC voltage into the regulator should be close to 15V (I note you have marked it 15.5V) and teh ripple will be about 50 mV RMS. With any reaonable sort of electronic regulator that should be fine.

I have no knowlege of the "Rod Coleman Regulator". I was unable to find proper specifications or a circuit by a quick bit of Googling. I did find a couple of websites that imply it is a constant current regulator. Constant current regulation is a dumb fad way to feed a vacuum tube, but that has nothing to do with the buzz you are getting.

I notice that the 2nd filter cap is 33 mF (33 millifarad ie 33,000 uF) - more than adequate. But you have the first cap shown is only 68 uF. That is way to small to do anything usefull - your circuit will act as a choke input filter. That's why the filter output is only 15 V and not the 25 V approx you would get from a conventional pi filter. I guess you did it this way so as not to exceed the ratings of the regulator while using a 20 V transformer you have on hand.

If the 68 uF nichicon fails, the circuit will work just the same.

Why do you need an LC filter ahead of an electronic regulator? Usually electronic regulators regulate out the ripple, making the choke redundant.
It's somewhere between 15 and 15.5V.. I think it was actually closer to 15.5V though.

Here is the website for the regulators Rod Coleman DHT Filament Regulator

https://www.dropbox.com/s/lc1mcmccl8...DHT_TX_V43.pdf

It is a CCS regulator, and I've discovered that there seem to be two camps on this type of regulator, those who strongly stand by the concept, and those who feel it is faddish. Whether it is faddish or not, or whether there is a better way to regulate this voltage, I have the equipment to use them, so I plan to unless there is an actual issue with doing so..

The design is basically choke input.. maybe I should just remove the 68uF cap. I put it in there because the design notes show one in that position.. Although he uses a 10uF film cap.. To be totally honest I'm not sure why it's there. I need those small value resistors before the choke to bring the voltage down a bit, so this cap sits between the resistor and choke.. Maybe that's not necessary.

I used a choke input filter so that my ripple current would be a lot lower.. With capacitor input, the rms current in the transformer secondary is almost double the dc current.. The design notes indicate that the low peak current reduces EMI..

The build notes indicate that the voltage ripple should be no more than 150mVp-p going into the regulator, and I believe I'm just under this, like 149mVp-p voltage ripple, or yea right around 50mV RMS. So I'm using the choke input design with the 33,000uF cap after to get the voltage ripple down to this level.

Quote:
Originally Posted by M Gregg View Post
Ah yes is it magnetic pick up from the power transformer or choke..
Magnetic induction to output transformers..
The problem with removing the output tubes is possibly over voltage on the PSU caps.
(Turn off B+) does it still hum?


Regards
M. Gregg
My caps are rated for 1400V, so they should be fine if I pull the tubes out. I'll give that a try.

I'll also try powering up without B+.. That's a good idea.

Last edited by thehoj; 5th June 2015 at 01:32 PM.
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Old 5th June 2015, 02:40 PM   #10
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A faint buzz (not a hum) could also be coming from your mains power line (noisy appliance on the circuit) or a nearby dimmer or fluorescent light fixture. You could try turning off various appliances or adding a line filter to your power cord into the chassis.
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