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Old 18th Sep 2019, 3:36 pm   #1
tritone
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Default Diode switching noise ?

Dear forum, what’s the easiest or best way to reduce or cancel out silicon diode rectifier spikes from a vintage audio guitar amplifier without going into too much overboard expense ?

The rectifier in question is a fullwave rectifier with a zero centre tapped transformer, the supply does not have a choke/inductor in the smoothing section, just those two 32+32Uf and 50+50uF capacitors RC filtered, please see the pic's below.

On the image from my scope you can see the 50Hz sinusoid and other hash which I am not too concerned about as of yet, but, I would like to reduce or eliminate that huge 100Hz diode switching noise as its coming out in the speaker.

The diodes in question are BYX 94’s, two in series for each phase obviously. I also have already tried out new glass passivated IN4007’s and also tried Vishay fast switching BA159 diodes, both of those sets of diodes didn’t make any difference and the noise was still there.

I have read that a resistor and capacitor, or snubber capacitors will do the job, but I need a little help here please to go the right way about it and to choose properly the proper components to do the trick ?

Lot’s of thank you’s in advance.

Tritone.
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Old 18th Sep 2019, 3:48 pm   #2
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Default Re: Diode switching noise ?

Use 'ultrafast' rectifier diodes of the appropriate ratings. It's counter intuitive but they really do make less rubbish, and you'll get away with milder snubbers.

Snubbers are normally decided upon empirically. They depend on the leakage inductance of your transformer and other stray effects.

Start with 10nF 1kV and maybe 50Ohms and see how that goes.

David
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Old 18th Sep 2019, 5:38 pm   #3
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Default Re: Diode switching noise ?

That spike seems to occur around the point where the diode switches on. Very unusual. Spikes usually come from diode switch off. I would not rush to blame the diodes until other issues like grounding and magnatic induction had been eliminated as possible culprits.
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Old 18th Sep 2019, 6:03 pm   #4
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Default Re: Diode switching noise ?

It's worth stocking up with a few dozen fast recovery diodes anyway as they only cost pennies - in fact, they cost little more than bog standard 1N4007s nowadays.
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Old 18th Sep 2019, 9:21 pm   #5
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Default Re: Diode switching noise ?

That's what makes me think of transformer leakage inductance.

THe fast diodes are so that when the turn on is fixed, you don't wind up with a turn off problem.

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Old 18th Sep 2019, 10:06 pm   #6
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Default Re: Diode switching noise ?

Thanks for your replies fellas,

@ G8HQP Dave & Radio Wrangler, what makes you’s think that it may be something else other than the diodes ? Interesting,

From what I can see on the capture viewing from the negative side of the wave is that there is a small spike at about 6 ms in on the negative side, moving in a positive direction, which suggests to me that that’s a switch off as current and voltage are inversely proportional, then at the large corresponding negative spike later 14 ms the voltage spike’s again away in the negative direction, same opposite thing goes in the + side also.

I have included a better pic of the PSU scem for a better look at whats there.
Also;
I can take some more focused captures on my scope to investigate further, I want to get to the bottom of this, What do you’s reckon ?

(edit)

Yea, I'm gonna post up more detailed captures. On that capture pic above I had my scope set to trigger on the positive rising leading edge and also had the trigger function set to AC coupling if thats of any useful info ?

Thank you.

Thanks guys.
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Old 18th Sep 2019, 10:46 pm   #7
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Default Re: Diode switching noise ?

Leakage inductance seems logical.

Also, I don't understand why the scope is triggering at t = 0 on the negative spike side when it is set for the positive up leading edge if you take a look again ?

What am I missing here guys ? I'll get better, more captures.

Thanks.
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Old 18th Sep 2019, 11:33 pm   #8
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Default Re: Diode switching noise ?

Have you checked the spikes are not on the incoming mains.
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Old 19th Sep 2019, 12:18 am   #9
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Default Re: Diode switching noise ?

Hello PJL, thank you,

I have checked that one out already and I appreciate your input on this matter.

Thats a very viable point to make and I have experienced that in the past at a different location to where I am now, switching noise coming in on the line from close by commercial machinery, mains voltage variants from 240v to 212v every day and lots of trasients and garbage.


It happens, but here where I am now its probably only hash garbage coming down the wires from my residential neighbours SMPS things, whereas before I lived next to an industrial estate. I do keep a close eye on that one.

Thanks for your reply.
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Old 19th Sep 2019, 6:21 pm   #10
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Default Re: Diode switching noise ?

I really don't like the wiring of diodes-in-series without suitable equalising R-C networks across them; it only takes one diode to be a bit faster/slower than the other, or to have higher leakage, and it can be 'Goodnight Vienna" for the transformer.

I'd suggest wiring suitably-rated 100KOhm resistors in parallel with each diode and 0.01uF capacitors in the same place too.

That's what I always did (following Pye's approach as in their big base-station transmitters....) in the days of BY100s and BY127s, and continued the practice with the coming of modern 'fast recovery' diodes in the 80s.
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Old 19th Sep 2019, 6:57 pm   #11
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Default Re: Diode switching noise ?

Hello G6Tanuki,

Thank you so much for your kind reply, all options are open still,, your advice is most welcome sir, thank you.

I may have come to the end of this project, my steam is runnng low at the moment as the owner of this equipment will not respond to many txt's and calls, But I still want to investigate the diode issue thing, It's important to me to learn, improve and give back to others.

Thank you sir, w'ell get this nailed for sure mister !
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Old 19th Sep 2019, 9:36 pm   #12
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Default Re: Diode switching noise ?

I'd suggest not loosing sight that this could be from wiring or layout or mechanisms not shown in a schematic, as per post #3.

Just relating to the diodes, I'd recommend just replacing them with 2x UF4007 in series, without any additional parts for equalising/etc. Buy the diodes as a batch, to be more confident that they are all fairly similar - as that will negate any need for equalising, especially in this situation where you would be using new diodes, and the secondary is only 370-0-370V.

Using ss diodes and not a valve rectifier can make the inherent power transformer secondary winding leakage inductance more apparent (even when the diode has no significant reverse recovery) due to the more rapid current steps as current rises or falls from zero. The energy in the leakage inductance may be alleviated by using a tuned CRC snubber across each secondary winding section. That may alleviate subtle forms of noise spikes transferring via eg. the heater winding into preamp circuitry.

Have you confirmed that the HV secondary CT is appropriately routed to the first filter cap negative terminal, and that that terminal is then the single point of connection to other power supply and load circuitry? The first 2 caps appear to be a can cap, which is problematic when trying to restrict exactly where charging pulses loop, and where other circuitry connects to, and forces perhaps multiple connections of 0V to chassis.

Have you confirmed that the rectified but unfiltered B+ lead, that passes through the standby switch and fuse, is kept well away from any preamp circuitry and preferably run next to chassis and with a minimum length and 'loop area', as it caries the pulsating high peak current charging pulses of the power supply. Some internal photos may help show the situation.

If the speaker observed noise is coming via the B+ to the output transformer, then there may be some benefit in separating the two 32uF caps using a small dropping resistor with a small series inductor, such that the second 32uF connects to the OT, and may have some attenuation of high frequency noise.

And is the speaker noise observed by the scope actually audibly noticeable, or in some other way a concern, or is this just an aesthetic 'fault', given that 60mVrms at 50Hz across an eight ohm speaker with not much low frequency response may not be noticeable compared to when a guitar is plugged in and ready to play.
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Old 20th Sep 2019, 12:03 am   #13
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Default Re: Diode switching noise ?

Your scope also shows transients from something doing mains zero switching, therefore fast mains spikes get through the transformer.

Check that the offending spikes aren't already on tour mains.

David
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Old 20th Sep 2019, 12:38 am   #14
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Default Re: Diode switching noise ?

Replacing each of the two series diodes sets with one modern high voltage diode in each leg would be first on my list.

I'm betting one of them either has high reverse leakage or is actually faulty.

Either 1N5408's or their fast equivalent, UF5408, would be suitable
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Old 20th Sep 2019, 7:32 am   #15
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Default Re: Diode switching noise ?

Whatever the cause they are very fast spikes, and so high in amplitude that they are above the screen on the scope.

Expand the sweep speed and scope sensitivity so we can see an individual spike. The structure of the spike might give a clue as to its root cause.

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Old 20th Sep 2019, 8:21 am   #16
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Default Re: Diode switching noise ?

Quote:
Originally Posted by Terry_VK5TM View Post
Replacing each of the two series diodes sets with one modern high voltage diode in each leg would be first on my list.

I'm betting one of them either has high reverse leakage or is actually faulty.

Either 1N5408's or their fast equivalent, UF5408, would be suitable
A single 1N5408 in each half of the secondary circuit would not be suitable here; a biphase rectifier applies double the peak of the rectified voltage in inverse when each diode is off i.e. the reservoir capacitor is charged to the positive peak voltage and the transformer applies the negative peak voltage at the trough of the mains waveform. In this case the diodes see an inverse voltage of 1046.5V which exceeds the 100V PIV of the 1N5408.

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Old 20th Sep 2019, 9:48 am   #17
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Default Re: Diode switching noise ?

Quote:
Originally Posted by G6Tanuki View Post
I really don't like the wiring of diodes-in-series without suitable equalising R-C networks across them; it only takes one diode to be a bit faster/slower than the other, or to have higher leakage, and it can be 'Goodnight Vienna" for the transformer.

I'd suggest wiring suitably-rated 100KOhm resistors in parallel with each diode and 0.01uF capacitors in the same place too.

That's what I always did (following Pye's approach as in their big base-station transmitters....) in the days of BY100s and BY127s, and continued the practice with the coming of modern 'fast recovery' diodes in the 80s.
I totally agree most silicon and indeed valve rectifiers in more vintage equipment used RF bypass capacitors in parallel across each diode.
The capacitors were always fitted physically very close to the diode.
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Old 20th Sep 2019, 1:00 pm   #18
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Default Re: Diode switching noise ?

Quote:
Originally Posted by jjl View Post
In this case the diodes see an inverse voltage of 1046.5V which exceeds the 100V PIV of the 1N5408.

John
Perhaps you meant 1000V PIV for the 1N5408? Which is what they are, so, on your calculations, just under the wire.

Looking up 2000v PIV diodes will give you heart failure at the prices, so looks like two diodes per side it is.
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Old 20th Sep 2019, 6:29 pm   #19
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Default Re: Diode switching noise ?

Terry

Yes, of course you're correct, thanks. I did the maths but messed up the typing

John
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Old 20th Sep 2019, 8:27 pm   #20
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Default Re: Diode switching noise ?

With multiple diodes in series you'll need resistors and capacitors across them to not just force even sharing of reverse voltage and also sharing of fast transients.

If the leading edge spike is locally generated, I'm beginning to suspect some sort of avalanche effect... they can be very fast and circuits designed to put transistors into avalanche were some of the early ways to generate the fastest pulses until the SRD came along.

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