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Old 7th October 2013, 04:55 PM   #1
rongon is offline rongon  United States
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Default Push Pull amp power supply question

I've noticed that solid state PP amps use a large value capacitor as the smoothing cap from the rectifier. Values of 6800uF at 63V or 100V are common.

I've noticed that tube PP amps, because of the higher voltages concerned, use a choke for smoothing, usually in a CLC arrangement, but sometimes an LC arrangement.

My question is, taking a relatively low voltage amplifier like PP EL84 or 2A3, what are the consequences of using something like two or three paralleled 800uF 330V capacitors for a +270V plate supply? They'd be paralleled with something like 2.2uF 400V metalized polypropylene for lower ESR at higher frequencies.

I've attached a PSUD2 sim. It looks like the ripple on the output voltage is OK for a PP amp supply. Am I correct?

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Old 7th October 2013, 05:15 PM   #2
TheGimp is offline TheGimp  United States
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High charging current spikes.

Depending on the rectifiers (1) you can get 120Hz noise coupled into your amplifier, (2) the high current charging spikes can damage the diodes if not properly sized for the peak current, (3 ) it generates a lot of power line noise.

1N4007s probably would not survive very long with that load.
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Old 7th October 2013, 05:40 PM   #3
rongon is offline rongon  United States
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Thanks.

(1) That's what worries me, but it seems OK. Did you mean the filtering of the raw DC would be insufficient? Or 120Hz noise would couple into the audio circuits a different way? If the latter, how would that work?

(2) I put two 1N4007's in series for each side of the rectifier in the PSUD2 sim. In real life, I used those big IXYS TO-220 cased hexfred diodes. I guess that's why the amp has survived nearly daily use for over 15 years!

(3) How is that different than (1)? I'm trying to understand...

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I have put some additional filtering in place for this amp. There's a 20A Corcom filter on the AC primary. All heater AC windings are bypassed to ground by 22nF film caps, and all tube plates have film cap bypass to ground (as per Valve Amplifiers 3rd Ed). I don't remember hearing any night-and-day improvements from before I had those in place, but they seemed like a good idea at the time.

Last edited by rongon; 7th October 2013 at 05:45 PM. Reason: follow up:
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Old 7th October 2013, 06:53 PM   #4
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IMO if your amp has a Proper grounding you don't need those huge caps.
100 + 100 uF is more than enough.
Take a look on old schematics, 47 uF is the largest value used and works without hum.
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Old 7th October 2013, 07:06 PM   #5
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Quote:
Originally Posted by rongon View Post
My question is, taking a relatively low voltage amplifier like PP EL84 or 2A3, what are the consequences of using something like two or three paralleled 800uF 330V capacitors for a +270V plate supply? They'd be paralleled with something like 2.2uF 400V metalized polypropylene for lower ESR at higher frequencies.

I've attached a PSUD2 sim. It looks like the ripple on the output voltage is OK for a PP amp supply. Am I correct?

--
Quite correct.

The reasons for using LC or RC LPFs in a hollow state power supply has to do with the Isurge limitations of hollow state power diodes. One of the beefier vacuum diodes is the 5U4GB with an Isurge= 1.0A / plate spec. That looks pretty impressive until you consider that a load current of 150mA will limit you to a reservoir capacitor of less than 40uF. I did such a design, and even something as small as 47uF busted that spec. With 34uF (two 68uF/350V in series with the balancing resistors) the Isurge= 800mA.

Even small Si power diodes can have Isurge>= 5.0A.

Of course, that's nowhere near enough to have a quiet PS, and so that needs to be followed up with an LC LPF if you're to avoid ripple hum.

The other reason to include such a filter, even when using Si diodes, is the current limitations of the PTX. Vintage PTXs were never designed with the Isurge a Si based power supply can pull in mind. Your diodes will be fine with it, but your PTX just might complain by making below design nominal DC, or possibly by burning out. It's sure to squeal in protest in any case.
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Old 7th October 2013, 11:12 PM   #6
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Quote:
It's sure to squeal in protest in any case.
It's more likely to hum the line frequency blues. The larger the input cap, the narrower the conduction angle. This means that the diodes, and the HV secondary winding are only conducting current for a mS or two in each cycle. So all of the energy supplied for the amp is delivered in brief pulses at very high currents. These brief current pulses create large magnetic pulses that find their way into low level circuits, often through the heater supply. You hear this as a raspy buzz in the audio, or a mechanical buzz from the power transformer or other metal surfaces.
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Old 8th October 2013, 12:10 AM   #7
rongon is offline rongon  United States
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It's the craziest thing. This amp is a PP 2A3 with no gNFB. With the amp on, I put my ear up to the woofer (90dB/1W/1m sensitivity) and can hear just the faintest bit of hum. Completely inaudible if you don't have your ear right in the woofer.

I used to run the 2A3 filaments with two 30R resistors going from each side to ground. It did buzz, but I figured it was the AC filaments in the DHT's. So I put a 25R wirewound hum balance pot in there, and that cured it as long as the 2A3's were well matched. The funny thing is that the rotation of the pot doesn't make hardly any difference in the hum level. But it hums if I replace the big pot with resistors.

It's a big dual-mono amp on an 11" x 17" aluminum chassis, with a copper bar down the middle which acts as the ground plane.

The power transformers are only used to supply the 2A3 plates, not the driver tubes (they have their own power xfmr). They were bought from Fair Radio, a long time ago. Rated 500VCT @ 150mA, run with 120mA load. They do run hot to the touch, but you can put your hand on them for a second or two before it gets painful. They don't shake or buzz.

This amp was an experiment that's just hung around these last 15 years. I've meant to build a better, prettier version as my 'keeper,' but never got around to it.

I'll probably get something like a 2H choke with 30R DCR from Edcor, and use those with more rational cap values in a CLC filter. Make it according to Hoyle and not worry about it any more.

The thing is, why has thing been working all these years? It sounds great, looks good on a scope. I'm sure it would be better with CLC plate supplies.

???
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Old 12th October 2013, 02:59 AM   #8
jerrys is offline jerrys  United States
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Default high voltage caps

Quote:
Originally Posted by marl0w3 View Post
IMO if your amp has a Proper grounding you don't need those huge caps.
100 + 100 uF is more than enough.
Take a look on old schematics, 47 uF is the largest value used and works without hum.
It is my understanding they used low cap values because of tube rectifiers. I noticed an obvious "trick" to help things along: a resistor between the first cap after the rectifier and ground. That would slow the charging and spare the (tube) rectifier some stress. Then the choke, and then the next filter cap. I was also told by several reputable sources to use a small value (say 1 uf) high voltage cap on the first electrolytic and it will prolong the life of that electrolytic. Cheap insurance. Can't see that it would hurt.
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Old 12th October 2013, 04:17 PM   #9
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I hope I don't bore some members, repeating what I've previously posted.

IMO, the point about tiny conduction angles bears expansion. Large value cap. I/P filters, with their small conduction angles, exhibit sharp, "triangular", ripple waveforms. Per Fourier's Theorem, such waveforms contain overtones of the ripple fundamental that extend well into RF. In a sense, large value cap. I/P filters are "hash" generators. "Typical" PSU filter chokes, due to the many turns of wire in them, exhibit a capacitance that shorts the inductance out a high frequencies. So, some "hash" makes its way into the reservoir capacitor. My answer to the conundrum is to insert a LC section made from a high current RF choke and a 1000 or so pF. mica or NPO ceramic cap. between the large I/P capacitance and the "normal" filter choke. The "RF" parts attenuate the crud, regardless of where it originates, that sneaks through the shunt capacitance.

FWIW, I suspect the unpleasant artifacts found in quite a few SS amps are, at least in part, due to the huge value I/P caps. in their PSUs. CLC PSU filters could help out.
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Old 12th October 2013, 06:36 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by jerrys
I noticed an obvious "trick" to help things along: a resistor between the first cap after the rectifier and ground. That would slow the charging and spare the (tube) rectifier some stress.
Why put a resistor there, in the ground connection? The main effect will be to increase ripple. Put the resistor directly in series with the rectifier and it will have the same effect on reducing ripple current but a beneficial effect on reducing ripple voltage.

Quote:
Originally Posted by jerrys
I was also told by several reputable sources to use a small value (say 1 uf) high voltage cap on the first electrolytic and it will prolong the life of that electrolytic. Cheap insurance. Can't see that it would hurt.
It would be nice if one of those sources could give a plausible explanation for this. I can't think of one. As you say it probably won't do any harm; I can't see that it would do any good.
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