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Old 30th June 2007, 05:42 PM   #1
pftrvlr is offline pftrvlr  United States
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Default Noisy PSU?

I am building a 6dj8-6as7 headphone amp, and it all working fine. This is my first scratch build project. So I have been doing a lot of SPICE sim and experimenting different things. Ragarding the PSU, I observed something interesting. The current draw is less than 60ma, power transformer is toroidal. AC heater supply comes from a separate toroidal heater transformer.

With
Rectifier->45uf ASC Cap->10H-125ma Hammond 159P Choke->45uf ASC Cap
I hear loud hum noise.

With
Rectifier->470uf electrolyptic Cap->10H-125ma Hammond 159P Choke->470uf electrolyptic Cap
I hear no hum at low volume.

The SPICE sim result shows that 10H/45uf PI filter should give low enough ripple.

Not having an o-scope to do more test, I can only suspect the Hammond choke might be the cause of hum.

Anyone has similar experience?
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Old 30th June 2007, 05:51 PM   #2
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When you exchange that last cap, I guess because of the different form factor, you also change the ground connection of that cap to a different one.
Very often these things are a result of ground loops.

If you indeed change the connection, try to connect the caps both (when changing) exactly to the same point, and see if it still makes a difference in hum. Possibly not, and then you know the correct connection.

Jan Didden
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Old 30th June 2007, 11:57 PM   #3
d2134 is offline d2134  Romania
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Quote:
Not having an o-scope to do more test, I can only suspect the Hammond choke might be the cause of hum.
Connect a 0.47 - 1 uF (non polarised with adeqate rated voltage ) in series with an AC Voltmeter (DMM set on AC Volt) and test the ripple.
Beware to hight voltage!
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Old 1st July 2007, 06:56 AM   #4
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Hi pftrvlr ,

First of all , your headphone amp is drawing less than
60 mA , from the power supply , your choke is rated for
10 H @ 125 mA , so it is not developing the total self-in-
ductance , then the SPICE sim , does not reflect the actual
situation .

Second , as a rule of thumb , the capacitor value have to be
aprox. 2200 uf / Ampere , to give you a low ripple . In your
case , 2200 uf x 0.060 A = 150 uf .

Third , to give you a “perfect” capacitor behavior , you need
to connect in parallel with each electrolytic cap , a mylar
or a polypropilene cap , with 1 / 100 of its value . Say , if
you are using a 150 uf elec. cap , you have to use a 1.5
uf in parallel with it , and the best solution , another plastic
capacitor of 1.5 uf / 100 = 15 nf , in parallel with the others .
( total 3 capacitors in parallel , 150 uf + 1.5 uf + 15 nf )

The new filter will be :
150 uf + 1.5 uf + 15 nf --> Choke --> 150 uf + 1.5 uf + 15 nf

Obviously I am assuming that you are using solid state
rectifiers , so the input capacitor value will not be a problem .

Try this solution , and give us a feedback ,

Regards ,

Carlos
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Old 1st July 2007, 08:48 AM   #5
EC8010 is offline EC8010  United Kingdom
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Carlos: The choke's inductance might be a little low from drawing 60mA rather than 125mA, but nothing much to worry about. I think you may be coming from a solid-state viewpoint with your capacitor rule of thumb - valve rectifiers won't tolerate much capacitance. Adding small values of capacitance in parallel with an electrolytic will tidy up HF behaviour, but not change hum.

Pftrvir: I think the problem is more likely to be that headphones are very sensitive transducers so the acceptable levels of hum are much lower. It could always be a wiring problem as Jan has suggested, in which case the hum is likely to have a buzzy sound, but it may simply be that you need to add another stage of filtering. If you're keen and competent with SPICE, then why not calculate the level of hum that would be 120dB down on the headphone's maximum output and see if your amplifier sims at that level?
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Old 1st July 2007, 03:32 PM   #6
pftrvlr is offline pftrvlr  United States
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Thanks everyone.

The PSU has both a solid FRED rectifier and a 5Y3 tube rectifier. I can swith between this two easily depending the output voltage I need. Immediately after the rectifier is a 50 ohm resistor. I'm too concern about the in-rush current of the 5Y3.

I do parrallel a 1.5uf MKP cap with the electrolyptic cap. Having too many of the 1.5uf MKP, I put them everywhere.

Wiil do more SPICE and DMM work and report what I find out. Also thinking to get a cheap o-scope from EBay.
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Old 1st July 2007, 08:03 PM   #7
Jeb-D. is offline Jeb-D.  United States
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If your using your PC as a source it is a grounded signal supply. If you have your preamp grounded as well the hum is probably caused by a system level ground-loop.

SoCal hu? Do you live near Santa Clarita by any chance?
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Old 1st July 2007, 08:10 PM   #8
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Is the filament supply floating? Connect a 1.uf cap from one side to ground.
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Old 1st July 2007, 08:43 PM   #9
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Default Re: Noisy PSU?

Quote:
Originally posted by pftrvlr
I am building a 6dj8-6as7 headphone amp, and it all working fine. This is my first scratch build project. So I have been doing a lot of SPICE sim and experimenting different things. Ragarding the PSU, I observed something interesting. The current draw is less than 60ma, power transformer is toroidal. AC heater supply comes from a separate toroidal heater transformer.

With
Rectifier->45uf ASC Cap->10H-125ma Hammond 159P Choke->45uf ASC Cap
I hear loud hum noise.

With
Rectifier->470uf electrolyptic Cap->10H-125ma Hammond 159P Choke->470uf electrolyptic Cap
I hear no hum at low volume.

The SPICE sim result shows that 10H/45uf PI filter should give low enough ripple.

Not having an o-scope to do more test, I can only suspect the Hammond choke might be the cause of hum.

Anyone has similar experience?
Hi

SPICE capacitor don't have tolerance and parasitic component.
The real inductor is a non linear component with a serie resistor
also the conductors have a small resistance.
In many cases these parts can be neglected in others they
become remarkable.
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Old 1st July 2007, 11:39 PM   #10
pftrvlr is offline pftrvlr  United States
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Quote:
Originally posted by Jeb-D.
If your using your PC as a source it is a grounded signal supply. If you have your preamp grounded as well the hum is probably caused by a system level ground-loop.

SoCal hu? Do you live near Santa Clarita by any chance?
I am in Thousand Oak 91362.

When I test the amp hum, the amp is either connected to an Oppo 970 player or nothing. I thought about the ground-loop/pin-1 issue, but it does not look like it is case.
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