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Old 13th June 2011, 05:32 PM   #11
DF96 is offline DF96  England
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Yes, caps across the diodes might increase the transfer of any RF in the supply, but there shouldn't be much RF there and a filter will stop it.

Modulation hum occurs when RF (probably from an oscillator in the equipment) is chopped at mains frequency by the rectifiers. This creates wide sidebands, which the RF equipment then picks up. It can be a particular problem in AC/DC radio receivers where the rectifier connects straight to the incoming mains. A mains transformer will not pass much RF, so AC-only sets are better in this respect. Putting caps across the diodes means that at RF the diodes no longer switch anything, so no modulation hum. This should not be a problem in audio equipment (apart from tuners and possibly some digital stuff) because there should be no RF oscillators inside. Note that a snubber will not stop modulation hum as the diode is still changing the RF impedance of the circuit.
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Old 24th December 2012, 03:30 PM   #12
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Default position of the snubber circuit

Hello,

first post in this great forum, by the way! I am planning on using such snubber circuits for the secondary of a toroidal transformer that is going to be used for powering a guitar amplifier. I am going to use full wave bridge rectification. And, having read the Hagerman article, I am a bit confused on where I should build the snubber circuits. Looking at the equivalent circuit, I firstly thought that I would need two such snubber circuits, one placed between the "+" of the secondary and audio ground, and the second symmetricallly placed between the "-" of the secondary and audio ground.
Is the above argument right, or is it adequate to put only one snubber circuit between the terminals of the transformer's secondary?
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Old 24th December 2012, 04:03 PM   #13
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After reading the Hageman article a single but with a damping closer to .707 rather than the under damped .5 will generate a smoother transfer function . Google Zobel circuit which is the name for this snubber circuit from the 1930 after the man who came up with it . Zobel did work on speaker response but that what this, a circuit for to damp resonance .
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Old 24th December 2012, 04:11 PM   #14
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Well, I am aware of the Zobel network, thank you! So your advice would be to place only one snubber across the secondary, targeting at a 0.707 damping?
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Old 24th December 2012, 06:10 PM   #15
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I found that the transformer secondary could be significantly detuned with a variable R and variable C, while watching the switching transient on an oscilloscope. Works best with small transformers. At the null the period and amplitude both went down, a lot, but the observed ringing was never very great and what I could see was mostly low RF, certainly not VHF. Maybe a couple of volts P-P?

I think the best I can do is use soft recovery rectifiers, a Zobel on the secondary, and some generic RC snubbers across the rectifiers to help mop up some of the remaining noise. I think a plain C just doesn't work well to kill the actual resonance that a RC can.

We need a RF spectrum analyzer to see what's really going on, and I know I won't be getting one for Christmas.

Last edited by Damon Hill; 24th December 2012 at 06:13 PM.
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Old 24th December 2012, 06:28 PM   #16
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Quote:
Originally Posted by Damon Hill View Post
I found that the transformer secondary could be significantly detuned with a variable R and variable C, while watching the switching transient on an oscilloscope. Works best with small transformers. At the null the period and amplitude both went down, a lot, but the observed ringing was never very great and what I could see was mostly low RF, certainly not VHF. Maybe a couple of volts P-P?

I think the best I can do is use soft recovery rectifiers, a Zobel on the secondary, and some generic RC snubbers across the rectifiers to help mop up some of the remaining noise. I think a plain C just doesn't work well to kill the actual resonance that a RC can.

We need a RF spectrum analyzer to see what's really going on, and I know I won't be getting one for Christmas.
Well, such a gift would be a whole life's investment!

This is what I was thinking. I was planning to use four UF5408 diodes, and implement a zobel network on the secondary, as described in Hagerman's article. But I did not plan to use anything in parallel with the diodes, out of simplicity. In the worst case, some 100nF capacitors, as suggested by Morgan Jones.
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Old 24th December 2012, 08:06 PM   #17
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For a guitar amp? Tube? I jump to thinking tube when you say guitar amp, even though you mention a torroidial transformer. Well, I can't add to the engineering theory or math, but looking at the tube guitar amps on my bench at the moment for a purely pragmatic baseline, LOL:

The Peavey Classic 50 and classic 100 are built to a price, and use...nothing before the main rail filter caps. But there is a small .01 ceramic cap across the primary (not a MOV).

The more powerful 180-watt Fender Super Twin with 6 6L6's uses some teensie .002 1KV ceramic disc caps, one across each of the 4 diodes in the bridge, then the main 220mfd filter caps are not directly across the 500 volt output, they are rated at 265v and go from each rail to the center tap of the secondary, and there's a 39K 2 watt resistor across each of those main filter caps. So that's like a 78K 4 watt resistor would be across the 500v rails. I always thought the resistors were to damp such resonances, but a note says they're mostly bleeders to drain the filters within 10 seconds as a safety precaution. That double-duty might be a practical consideration for you too.

I imagine the practical downside of a damping network across the secondary is of course the waste of a watt and generation of a little heat.

The .002mfd caps Fender uses across each diode are a lot bigger than the 100nF you propose.

If yours is solid-state or cost-no-object...then nevermind and Merry Christmas.
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Old 24th December 2012, 11:53 PM   #18
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The uf5408 generate a lot less switching noise than the old 1n5408 so 100nf is good rather than .002mfd (2nf) . so it 50 times bigger then what fender used and it corner is thus much lower. Here a reference chart West Florida Components Capacitor Conversion Chart
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Old 26th December 2012, 10:53 AM   #19
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Originally Posted by Damon Hill View Post
We need a RF spectrum analyzer to see what's really going on, and I know I won't be getting one for Christmas.
That's what work is for, BTW they are quite handy for testing power supplies for noise. Power a single ended RF amplifier from the power supply to test, inject a fairly pure RF signal into the amplifier, compare output to input, look for AM sidebands. FWIW getting rid of 500 kHz noise is fairly easy with small passive filters, 50Hz and 100Hz are much tougher
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Old 26th December 2012, 11:49 AM   #20
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Just to clarify things a bit, yes it will indeed be a tube amp. And saying "will", I am suggesting that this is a project that I am going to design and construct, for personal use. Thus, I want to do it the right way from all aspects, even though I may not be able to hear or feel any difference. I am going to use all this planning as a lesson for future builds, that could be hi-end - or simply demanding, in general - amplifiers.
Plus, this amp is going to be a Hiwatt replica, using slight amendments - which has already some hi-fi roots!
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