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Swapping class AB amp linear supply with a SMPS - incl measurements

McFly

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So my hypothesis, based on measurements seen on this site and others, is that (audio specific) Switch Mode Power Supplies produce less noise (all 3 ways - directly into the amps DC rails, and via radiated magnetic fields, and mechanically) within the audible 20-20khz band when installed in an amplifier. I set out to draw my own conclusions. And share them here for anyone that's interested.

Disclaimer: Now, as usual, the results are my own, others may vary, my measurement set up did change between the amp having a 'big iron' transformer supply vs the new SMPS. What does this mean? The noise floor and subsequently the THD+N measurements are not really valid between the before and after. Shock horror. But take a look at the 50hz and 100hz noise + harmonics themselves - I think the data there is still useful hence why I'm sharing. Take these with a teaspoon of salt. I am also no electronics engineer - I understand only a little about extra low voltage electronics, I'm much more in tune with speakers. But I am, as I have explained before, a high-er voltage commercial electrical engineer, so I know how to be safe when playing around with appliances, read diagrams and use electrical testing equipment.

The amplifier chosen was a 1982 Vintage class AB Perreaux PMF2150B - made right here in NZ. I believe my serial number puts it around 85-86 year. It has a heavy, noisy EI type transformer inside it. I wanted to find a nice classic piece of HIFI gear, especially after I heard @restorer-john singing this units praises in an older thread. This thing is heavy as hell and hummed like a fridge both mechanically, and through the speakers when connected. Especially the right channel. It was almost unusable as I picked it up on the second hand market. (it was also heavily oscillating which was easily fixed before any of this went down) I tried replacement smoothing caps, bridge rectifier and even soaking the transformer in varnish with very little success. There are other benefits to fitting a SMPS like seriously reduced weight and no mechanical hum. I think some poor SMPS designs can emit very high pitched whines, and some even block out AM/FM radio tuners from working.

Testing conditions;

Class AB amplifier with linear transformer/rectifier/smoothing caps supply. I have attached the amplifier schematic and specs for reference.
1, the right, channel tested - The right channel's input stage is mounted closer to the original transformer and had more power supply noise.
Initial test: - Focusrite Solo input and output - Right channel into 8 ohms - Factory 1200?VA iron transformer, 35A bridge rectifier, 2x 18000uf smoothing caps
After: SMPS test - input - E1DA ADC - output topping E30. Right channel into 8 ohms - Hypex SMPS1200A700 2x85VDC. 1200W supply.

Original Linear Power supply; Focusrite Solo, 5W/6.3V into 8R

RIGHT CH INITIAL 6.3V 8R.jpg


Test with Hypex SMPS installed and E1DA ADC: 5W/6.3V 8R

PMF2150 REWIRED SMPS 5W R CHANNEL 8R DISTORTION ADJUST.jpg




Notable problems with my testing;
  • I definitely didn't get enough output levels/tones/sweeps logged when I did the original measurements to compare different power levels etc
  • Obviously, the aforementioned testing equipment
  • I could have tried replacing the transformer with a Toroid type - supposed to have less magnetic field spray - but the cost of this to NZ was more than the hypex SMPS, much more.
  • The (82V) rail voltages are still slightly high with the hypex SMPS (measured 84VDC) - yes but they were also higher with the original transformer, even with the 240V tap (our mains are 230V, but hover around 238V at my place) when I first got the amp I measured 92VDC on the rails with the 230V transformer tap.
  • There could be much more out of audio band noise that I didn't test for/look at

Results;
  • Notwithstanding the test equipment, the THD+N has gone from 0.3% or -50dB to 0.003% or -90DB, which is somewhat correct due to the 50hz noise being -50dB down in the initial testing. Thats a 40dB SINAD improvement just changing the power supply.
  • The amp is now DEAD SILENT, everywhere. I can now use it. Cant hear anything with my ear 2cm from the cones of speakers or the amplifier chassis.
  • The amp now measures well below factory specifications for THD, THD+N, hum and noise. I cant say for certain that it didn't before due to the test equipment limitations.
  • The amp is probably saved from the landfill, should someone less enthusiastic have purchased it
  • Subjectively (notwithstanding the original hum) the amp sounds exactly the same, if not better because there's no hum in silent passages. I actually enjoy using it now.
  • There are numerous references to SMPS sounding 'flat' and lacking the 'transient current availability' of linear supplies. I hear none of this, although it is hard if not impossible to do an AB comparison. Would need another factory PMF2150b (and use my auto amp switcher I made.) This could be the case if I drove the amp hard
  • What I will admit I have likely done, is limited the 'headroom' of the amplifier - the old linear supplies have been provably better in temporarily overloading which does translate to better headroom for dynamic peaks, however my peaks in my listening never go beyond the 200WPC output of this amplifier anyway. Sure, if you always run your amps hard you may want to stick with old iron, or, oversize the SMPS to something like a 2KW or even 3KW - then you'd be fine.
  • The rail voltages don't sag as much under full load with the SMPS compared with the linear supply - could this be going against the design topology and affecting the sound somehow?
  • I also admit, the amplifier will likely no longer meet the brochure specs with regards to the full FTC sections, as the power supply would overheat before getting there. I don't listen to full scale sine waves though so I'm happy to live with this.

Conclusion;

I can say for certain my testing method is flawed. I cant say for certain that SMPS installed into any class AB (or other class) will net an improvement as this is only one amplifier. There could be much more out of audio band noise that I didn't test for/look at. But the result here is a subjective and objective success, at least to me. First, it fixed the actual physical problems with the amp and second, appears to measure better at least in the areas the power supply affects the output. I didn't set out to say that old iron linear supplies are sh*t - not at all, there are amps with fantastic low noise iron supplies, but for sure the one in my amp here was either no good, or had deteriorated so much over time. Now I don't get a sore back lifting it.

Thoughts, was it worth it, should I stick the iron back in?

P.S. I got the matching SM2 preamp with it and am going to the E1DA to see if a good old fashion 'recap' does anything to the preamp.
 

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RayDunzl

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Why the difference in levels (top right):

-0.50dBfs and -22.72dBfs?

Oh. Different ADC.

Why different ADC?
 

Miercoles2022

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Nice work, McFly! I wholeheartedly agree that a high-quality SMPS can be an upgrade to some Class AB amps with linear supplies, especially older ones.

I did something very similar to one of my amps, which started out life long ago as a Hafler DH-220. I built and installed new front end and output circuits designed by Bob Cordell and the amp sounded great, though there was a slight hum from the big EI transformer. After several months I removed the old linear power supply and replaced it with a 600W unit from MicroAudio. The amp has a completely silent background with the SMPS and I’ve been using it like that for over a year and a half since.

Enjoy your rejuvenated Perreaux!
 

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OP
McFly

McFly

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Why the difference in levels (top right):

-0.50dBfs and -22.72dBfs?

Oh. Different ADC.

Why different ADC?
As I mentioned, multiple times, the test equipment changed. The works took place over the course of a year because life, and my measurement equipment and skills got better. If I find myself really bored these holidays, I could put the old iron back in and remeasure.
 

restorer-john

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I applaud your experiment with the old beast PMF-2150B!

The hum and noise levels on those old Perreauxes was high, so high in fact they issued a bunch of mods (which you may have seen) which consisted of shields around the low level front ends on the amp boards on later models and models with more power. Much of the noise was radiated directly from the big EI transformer, rectifier and caps.
 
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McFly

McFly

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Yes I have seen that. Its funny because I see in the brochure specs the hum and noise combined 100db down. I got 50db down. Perhaps the orignal traffos were cheap and the windings shifted after a few years or something causing unbalanced EMI from the EI. I would've liked to have tried a big Toroid - I would've been able to fit a 1K in there, but the cost just didn't make it viable.
 

restorer-john

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Its funny because I see in the brochure specs the hum and noise combined 100db down. I got 50db down.

Their spec is 100dB down from rated output. Rated output being 200wpc- 40V RMS.

1660616827747.png

100dB is a factor of 100,000 times, which equates to 0.4mV or 400uV.

1660616694266.png


The issue of course is the actual noise (ripple etc) will be much greater at elevated powers (as opposed to a shorted & undriven amplifier), where the supply is modulated by the output stage currents into a load.

I'd like to play with the front end supply decoupling as the cap values at 10uF/0.1uF are very much on the low side. Too low IMO.
 

DonH56

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Perreaux demo'd some amps at the store I worked back in the early 1980's. Nice stuff, solidly built, though we did notice some hum. I made a mu-metal shield around the transformer as a simple mod that seemed to help IIRC. The problem was it made it run hotter... I'd forgotten about the caps; 100 uF ~ 1000 uF was a popular mod IIRC. I was moving on about that time so lost track of the audio world.
 

restorer-john

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I'd forgotten about the caps; 100 uF ~ 1000 uF was a popular mod IIRC. I was moving on about that time so lost track of the audio world.

It was the 1980s, Don. Losing track was pretty normal.

You know what they say about the 60s, "if you can remember it, you weren't there". Not sure the 80s had a similar saying, but it was a rather heady period, with Miami Vice, big shoulder pads and bands you'd rather forget.
 

fpitas

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I'm glad to see some testing, instead of audiophile supposition that switching = bad.

If people try this, be cautious about adding huge capacitors at the switcher output. They really don't need it, and worse yet they can get unhappy and oscillate.
 
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McFly

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I no longer have the 18000uf smoothing caps in the amp. The transformer, rectifier and smoothing caps were removed. The hypex replaced the lot.

Thanks for the tip on the rail caps - this looks like an easy mod to do

Edit: Here's the right channel at spec'd full power 40Vrms into 8 ohms.

PMF2150 REWIRED SMPS 200W RIGHT CHANNEL 8R DISTORTION ADJUST PERCENTAGE.jpg
 
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sq225917

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Even with the test kit change that's an obvious win. I have a Muse model 200 power amp I might try it with, noisy as shit since day 1.
 

sq225917

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Does the smps noise leak through to any other components, pre amp, sources?
 
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McFly

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Not at all. Due to the amplifiers input filtering it doesn't make its way back along interconnects to preamps/sources, and also the radiated EMI is extremely low - that's one of the main reasons there is such an improvement here over the linear transformer/rec/cap supply.

Look at it physically, when the original linear supply was installed, I could not have a preamp sit on top of the power amp. The speaker hum would get WAY worse, and you could literally lift the preamp up of the top of the power amp while connected and running, and hear the hum drop away as the preamp was lifted higher. Now? now the preamp can sit on top of the power amp, and the whole setup is nice and quiet.
 

ta240

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I'm glad to see some testing, instead of audiophile supposition that switching = bad.

If people try this, be cautious about adding huge capacitors at the switcher output. They really don't need it, and worse yet they can get unhappy and oscillate.

Recently, I've seen a lot more positive comments on smsp power supplies on other forums and even Paul at psaudio danced around praise for them.
I know they aren't as bullet proof but I do like the lighter weight and not having to fight to keep noise out.
I'm curious at the difference in current draw. I run a diy amp with one and it doesn't pull enough to trigger my energy saving out strip to turn on the other components.
 

fpitas

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Recently, I've seen a lot more positive comments on smsp power supplies on other forums and even Paul at psaudio danced around praise for them.
I know they aren't as bullet proof but I do like the lighter weight and not having to fight to keep noise out.
I'm curious at the difference in current draw. I run a diy amp with one and it doesn't pull enough to trigger my energy saving out strip to turn on the other components.
Good ones are bullet proof. You can short them all day, they just shut down. No idea what the cheap ones do.
 

ta240

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Good ones are bullet proof. You can short them all day, they just shut down. No idea what the cheap ones do.
Well built, sure. But in terms of life spans reaching into multiple decades and beyond with just capacitor replacements, probably not. But then again it is just a power module that can be replaced.
 
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