Variations of DC Main Filter against buzzing Toroid Transformers - what is the right? - Page 6 - diyAudio
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Old 2nd October 2010, 12:26 PM   #51
AndrewT is offline AndrewT  Scotland
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Hi,
the capacitor passes the normal operating AC current.
These caps must be sized to drop as little voltage as is tolerable during normal working conditions.

The diodes pass overload current during short term operating conditions.
eg. start up current for the mains transformer, extreme overload current if the amplifier is put on full power test signals.

A single diode pair allows ~500mVpk to 550mVpk across the capacitor/s without significant diode current.
A double diode pair allows ~1000mVpk across the capacitor/s without passing significant diode current.

What must not happen is allowing the capacitors to develop sufficient voltage drop so that the diodes are passing during normal operating conditions.
The diodes will run hotter than necessary. The capacitors will suffer severe ripple current stress. The DC blocking action of the capacitors is defeated if the diodes pass.

Let's take a 120Vac example.
300VA transformer has a maximum operating current of 2.5Aac.
During charging of a capacitor input filter the crest:average ratio can be quite high. During normal music reproduction it is sensible to assume that the peak current (at the crest) is ~maximum AC current peak, i.e. ~3.5Apk.

Assume a 10mF pair of caps in series. The effective capacitance is 5mF, the frequency is 60Hz (USA).
The impedance of the caps is ~ 1/2/Pi/F/C ~0.53ohms.
3.5Apk will develop ~1.8Vpk across the pair of series caps.
A double diode will pass current when the peak primary current >~1.9Apk

For a 300VA transformer on 120Vac with 10mF+10mF and a 4diode bridge, the diodes will pass a little peak current when the SPL is turned up. But most domestic reproduction will probably just keep the diodes from passing.

I tend to adopt 10m+10mF for 240Vac with double diodes. I actually use 10off 3m3F 16V in series parallel for ~8mF effective
I think 110/120Vac needs at least double the capacitance for medium power draw transformers.
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Last edited by AndrewT; 2nd October 2010 at 12:31 PM.
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Old 22nd April 2011, 09:58 PM   #52
OllBoll is offline OllBoll  Sweden
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Here comes the questions...

I'm having mechanical transistor humm, none at audio output and the humm varies with time...

The conclusion I've come upon is that it could crap in the AC, which is why I'm considering filtering it. I've seen lots of circuts, but since it's in AC-land I would prefer a pre-built unit with a metal housing for safety reasons. So I wonder if this unit would do the magic needed?

https://www1.elfa.se/data1/wwwroot/a...FKH_data_e.pdf

The circut is:

Click the image to open in full size.

L = 2x4 mH
R = 1M ohm
Cx = 68 nF
Cy = 2.2 nF

I'm eying the 2 amp non-medical unit.

Thanks in advance,
Olle
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Old 22nd April 2011, 11:04 PM   #53
djk is offline djk
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I doubt it, there is nothing in that filter to deal with DC off-set.
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Old 23rd April 2011, 09:08 AM   #54
AndrewT is offline AndrewT  Scotland
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Do you mean mechanical transformer hum?

The filter you have shown is to attenuate RF and spike interference coming from the mains wiring. It does nothing for airborne interference and cannot solve a noisy transformer that may be suffering intermittant saturation due to unbalanced AC waveforms, i.e. DC on the mains.
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Old 23rd April 2011, 10:08 AM   #55
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by Eva View Post
First image is current (red) and voltage (blue) in a 750VA toroid with the usual DC offset at my place, .2A/div and 100V/div. Little audible noise.

Second image is current and voltage when I turn on the 1500W heater in half power mode, 2A/div and 100V/div. Note the huge current peaks due to saturation. Big audible noise.

Third image is current and voltage when a simple DC filter is added (1000uF and 2 back to back diodes), .02A/div and 100V/div. Transformer becomes completely quiet, even with the heater in half power mode.
EVA, I can completely back your findings with pretty much the identical measurements I made recently, in fact extending them with frequencies below 50Hz, not restricting the test to "pure DC".

One or two back-to-back diode sets shunted with not too big capacitors usually do the trick. Too large capacitors pass too much of sub-50Hz components (there are lots of subharmonics and beat frequencies on the mains).

Another important variable is the often too low primary inductance of cheaper toroids in an attempt to reduce copper cost. Quite often one can see a steep exponential rise of idle current starting at 220V and lower. At 240V/50Hz there often are already significant signs of saturation and, depending on the construction, corresponding mechnanical hum. Under such conditions even smallest amounts of DC and sub-50Hz stuff make things go rapidly worse.

Once I spec'd toroids to maintain low and stable idle current up to 250V@50Hz (5..10% more turns for all windings) the transformers became much quieter in general and also react much less severe to DC. Together with the diode blockers this issue now is completely settled, for me.

Last edited by KSTR; 23rd April 2011 at 10:11 AM.
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Old 24th April 2011, 10:37 AM   #56
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Quote:
Originally Posted by KSTR View Post
EVA, I can completely back your findings with pretty much the identical measurements I made recently, in fact extending them with frequencies below 50Hz, not restricting the test to "pure DC".

One or two back-to-back diode sets shunted with not too big capacitors usually do the trick. Too large capacitors pass too much of sub-50Hz components (there are lots of subharmonics and beat frequencies on the mains).

Another important variable is the often too low primary inductance of cheaper toroids in an attempt to reduce copper cost. Quite often one can see a steep exponential rise of idle current starting at 220V and lower. At 240V/50Hz there often are already significant signs of saturation and, depending on the construction, corresponding mechnanical hum. Under such conditions even smallest amounts of DC and sub-50Hz stuff make things go rapidly worse.

Once I spec'd toroids to maintain low and stable idle current up to 250V@50Hz (5..10% more turns for all windings) the transformers became much quieter in general and also react much less severe to DC. Together with the diode blockers this issue now is completely settled, for me.
because the costs the air-coils for crossover networks is increased significantly by Intertechnik (Kerpen, Germany) in the last years, I am assuming that the price of copper was also higher. I have the impression that this annoying transformer hum behaviour in the amp devices just for newer models therefore can be observed currently more often than before.
In order to effectively remove, one can also reduce the secondary voltage by introducing an additional transformer.
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Old 28th April 2011, 10:16 PM   #57
JKoch is offline JKoch  Poland
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Hi

Few years ago I built and still use DC Filter + RFI/EMI Filter found somewhere in internet. Here is the schematic:
- diodes STTA1206
- elcaps 22000μF/16V BC Components
- film caps X1, X2, Y2 Evox Rifa
- varistor S20K300
- chokes RN222-4/02 4A/250V, RN112-4/02 4A/250V Schaffner
- resistors 3W
Attached Images
File Type: gif 321227_2.gif (15.2 KB, 854 views)
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Old 28th April 2011, 10:22 PM   #58
JKoch is offline JKoch  Poland
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And here is pcb drawing:
Attached Files
File Type: pdf PSU.pdf (83.0 KB, 305 views)
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Old 28th April 2011, 10:26 PM   #59
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by JKoch View Post
And here is pcb drawing:
Oooo... you just don't *happen* to have pcb drawing as a file, or as black/white image such that one can print and etch some boards oneself?

The grateful (greedy) Olle

EDIT:

Or even better: it seems that pre-built EMI filters are abdunant, but does someone know a decent DC filter buyable through say mouser?

Last edited by OllBoll; 28th April 2011 at 10:43 PM.
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Old 29th April 2011, 03:32 AM   #60
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Quote:
Originally Posted by JKoch View Post
Hi

Few years ago I built and still use DC Filter + RFI/EMI Filter found somewhere in internet. Here is the schematic:
- diodes STTA1206
- elcaps 22000μF/16V BC Components
- film caps X1, X2, Y2 Evox Rifa
- varistor S20K300
- chokes RN222-4/02 4A/250V, RN112-4/02 4A/250V Schaffner
- resistors 3W
A real all-in-one filter!
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