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

[Piere]

You mean of the M-Audio Audiophile192? The breakout cable comes with the card and is an integral part. That part isn't sold separately.

 

[Chrise36]

You mean of the M-Audio Audiophile192? The breakout cable comes with the card and is an integral part. That part isn't sold separately.

Yes it seems i might have to make a new one.

 

[McFly]

Seeing as my measurement method was mediocre in the OP, I set out to do the experiment again. This time all tests done with Laptop on battery -> E30 dac fed from samsung wall wart -> DUT -> non inductive power resistors -> E1DA grade B -> Laptop.

This time, another class AB amp of identical typology and schematic, A later model of the one I originally tested which recieved much improved PCB layout. Still 200WPC 8 ohm, but powered by a Toroid transformer and not an EI type. More resistant to magnetic flux leakage I've heard, and this was true when I looked at the outputs. This amplifier exhibited a far lower 50hz (60 in 'murica) spill than my orignal one in the OP, but still large rectifier 100hz (be 120hz in the states) noise and harmonics of. So the toroid was indeed quieter I would say, but the power supply typology of dual winding TX to Full bridge rectifier to a pair of large reservior caps is still noisy.

With the Linear TX-Rectifier-Caps I observed a SINAD of around 83dB and removing the whole lot and connecting a hypex SMPS in place got me an 11dB improvement to 94dB.

Noise floors - The previous setup, a slight hum JUST audible at the listening position 3m away to completely inaudible even with my ear next to the speaker cone with the SMPS.

I say RIP linear power supplies, you served us well. Amp sounds exactly the same to me as it did before, amazing.

Heres the measurement loopback:

Heres the amp, 6.33V into 8R (5W). Both channels loaded at the same time (because why only load 1 channel in a stereo amp?)

And same again the left channel same power same loads but with the SMPS installed

I have yet to get to the bottom of the 16kz noise, I suspect the E30 as it is not there when I use another, lesser DAC. But for the purpose of this exercise to show power supply noise I think it can be ignored. Also the Harmonic distortion differs between the two, that is me adjusting the feedback mid measurement so Harmonic distortion did not improve due to the SMPS being installed. My measurement techniques and thoroughness wont get a me a job at NASA thats for sure. I have only shown the left channel but the right channel is the same, got the same improvement.

Any way this was a very quick and easy exercise that only took one rainy afternoon, and the SMPS now remains permanently installed in the amplifier and in daily use with no issues.

Bonus measurement both channels @ 20W

And a pic of the amp with the SMPS installed. (other mods - done well before I took the TX out as when I got the amp it didn't even work - include replaced burnt out transistors + little heatsinks installed and full electrolytic cap replacement)

 

[dlaloum]

Here's another question for you - the issue of continuous vs peak power capabilities have been discussed a few times recently.

Are you seeing any difference in peak output?

Typically SMPS & ClassD based tends towards higher continuous with reduced headroom beyond that...

In this hybrid with SMPS ClassAB - what is the impact (if any) on peak power?

(Probably academic given the starting point is 200W @ 8ohm... but still!)

Also of interest - what are the differences in terms of low impedance capability? - what are the differences (if any) between the transfo vs SMPS into 4 ohm and 2 ohm (and 1 ohm?) - those are typically power supply current limited, along with # of transistors and amount of heatsinking...

Given the right power supply - this beast should be able to handle low impedances well - but what happens to the PSU noise when high currents are demanded?

 

[Deleted member 48726]

Also there may be an issue regarding insurance with a mod like this. The listing of the product is void for sure.

I KNOW this is purely for testing. Thank you for doing it. @dlaloum raises some important questions that should be answered before we abandon our trusted supplies.

 

[egellings]

The Hypex does not. The DC rails fluctuate up and down with the incoming mains AC, exactly like a linear supply does.

That's why I said 'likely'. There will always be exceptions. It's also possible that the output may vary with input voltage, but does not as loading changes on the device, in spite of input voltage. So line regulation can be poor to nonexistent while load regulation can be good. As an example, an emitter follower buffered stage can show good load regulation while not caring one whit about its input voltage stability. Its output will vary with the input voltage, but not with the load current. Amplifying stages that care about voltage stiffness but not actual amount will benefit from that. (Edit: forgot 'not').

 

[somebodyelse]

If you have an older PC with an actual PCI slot (not PCIe), find the AP (Audiophile) 24/192 card and make sure you get the wire dongle with the 6 1/4" TRS leads.

It's perfectly valid as a measurement device now, but remember, it will need a Win7 install. The Vista drivers work perfectly with Win7. I've never tried to use those old cards in a win10 box, because none of mine have a legacy PCI slot.

It also works with linux. There are PCIe to PCI bridges available, some offering multiple PCI slots, but I haven't had to try one yet.

 

[McFly]

Here's another question for you - the issue of continuous vs peak power capabilities have been discussed a few times recently.

Are you seeing any difference in peak output?

Typically SMPS & ClassD based tends towards higher continuous with reduced headroom beyond that...

In this hybrid with SMPS ClassAB - what is the impact (if any) on peak power?

(Probably academic given the starting point is 200W @ 8ohm... but still!)

Also of interest - what are the differences in terms of low impedance capability? - what are the differences (if any) between the transfo vs SMPS into 4 ohm and 2 ohm (and 1 ohm?) - those are typically power supply current limited, along with # of transistors and amount of heatsinking...

Given the right power supply - this beast should be able to handle low impedances well - but what happens to the PSU noise when high currents are demanded?

All good questions - I dont have the capability to test for 'peak' or 'burst' power like Amir does, however the max power is still the same as it was with the LPS. 39.5V rms at the onset of clipping. I think this may be more the amps design, I'm not sure.

Keep in mind, the power supply I removed was a 500VA toroidal transformer. The hypex is WAY bigger at 1200W / 1500W peak so the VA rating is probably higher still. The rail voltages with the LPS were 83VDC and are 84-85 with the SMPS. Doesnt bother these MOSFETs at all.

Re low impedence loads, I'll have to check that - I did 8 and 4 ohm loads no problem but didnt do 2 ohm. I can do 2 ohm but my load will be noisy so harmonic distortion will be way higher.

I did some sweeps heres 1 channel at 100W 8 ohms:

Also there may be an issue regarding insurance with a mod like this. The listing of the product is void for sure.

I KNOW this is purely for testing. Thank you for doing it. @dlaloum raises some important questions that should be answered before we abandon our trusted supplies.

Hah - no way am I encouraging your average HiFi enthusiast to do such a thing. This is the DIY forum, aimed at anyone who might be building thier own amp and contemplating power supplys, or someone with the know how and an old noisy class AB amp.

Yes obviously a warranty would be void. No fn way do I suggest doing this to a modern amplifier. The first amp in this thread I did this with was ~40 years old. This amp here? 30 years old.

Insurance? Again, DIYers with the expertise only. I am an EE - so I'm competent in these modifications. I wouldn't dare go doing this if you had any doubt. Or if there's doubt, only do it on a test bench for your own learning with the appropriate protections in place and don't "daily" your mods you aren't sure of. Maybe practice on junk amps if you were going to. Someone doing this should have the means to safely measure and test such changes also.

 

[restorer-john]

All good questions - I dont have the capability to test for 'peak' or 'burst' power like Amir does, however the max power is still the same as it was with the LPS. 39.5V rms at the onset of clipping. I think this may be more the amps design, I'm not sure.

You can easily do peak/burst if you have a 'scope.

There is nothing special about the AP for dynamic/peak testing. The EIA (CEA equiv) toneburst is 1kHz, 20 cycles at 0dB, 480 cycles at -20dB (500mS total) repeated and the threshold being clipping or 1% THD.

If you want a copy of the EIA (CEA) burst or the EIAJ (Japanese) standard (they are different) wav files off one of my test discs, let me know. Or just make the file in whatever software you use.

 

[McFly]

Thanks John - will have a gander at that this weekend.

 

[Piere]

Hah - no way am I encouraging your average HiFi enthusiast to do such a thing. This is the DIY forum, aimed at anyone who might be building thier own amp and contemplating power supplys, or someone with the know how and an old noisy class AB amp.

You can diy a test burst at wish with the equation editor of Goldwave. It's share ware but its use is totally free.

Short bursts like the EIA, show peak power. Longer bust of at least 0.1s, show dynamic behaviour of the PSU also.

 

[McFly]

Ah looks like REW can possibly do CEA-2010 burst and custom bursts I didn't see these

 

[jamescarter1982]

could this be done to a valve amp ? be interesting

 

[egellings]

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

View attachment 224420

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

View attachment 224422



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.

It could be that except for 50/60Hz noise, the SMPS may well be voltage-regulated, while the big iron-diode bridge-caps supply bounces around more voltage wise, since it's likely not regulated for voltage, while the SMPS is.

 

[fpitas]

It could be that except for 50/60Hz noise, the SMPS may well be voltage-regulated, while the big iron-diode bridge-caps supply bounces around more voltage wise, since it's likely not regulated for voltage, while the SMPS is.

I think I read that the Hypex SMPS isn't regulated per se, the output follows the input. But it may indeed have a lower output impedance.

 

[McFly]

The hypex output voltage does drop with input, and also drops a bit with heavy load but doesn't drop as much as the big iron did.

 

[Piere]

I think I read that the Hypex SMPS isn't regulated per se, the output follows the input. But it may indeed have a lower output impedance.

True, the Hypex SMPS are intentionally not regulated. It is just a mains transformer as well, now at high switching frequency. What the amp sees is just the reflected input capacitors. And since the transformers contain a lot less copper than an EI transformer Ri is indeed a lot smaller. If that is an advantage is debatable --> Much higher charging current peaks for the bulk capacitors. The Hypex SMPS also contain several protection mechanisms in case of failure, for the SMPS as well as for the amp. Although most class AB amps are usually quite reliable and therefore of not much use imo.

 

[dlaloum]

True, the Hypex SMPS are intentionally not regulated. It is just a mains transformer as well, now at high switching frequency. What the amp sees is just the reflected input capacitors. And since the transformers contain a lot less copper than an EI transformer Ri is indeed a lot smaller. If that is an advantage is debatable --> Much higher charging current peaks for the bulk capacitors. The Hypex SMPS also contain several protection mechanisms in case of failure, for the SMPS as well as for the amp. Although most class AB amps are usually quite reliable and therefore of not much use imo.

?? Most class AB amps are quite reliable and therefore not much use???
Seems to me that is a bonus?

 

[Piere]

In what sense? If an amp fails, it fails. Most time by some sense of overload or in the long run by wear out of electrolytic capacitors. The Hypex SMPS are designed for their own Class-D amps, not for general use.

 

[HarmonicTHD]

Seeing as my measurement method was mediocre in the OP, I set out to do the experiment again. This time all tests done with Laptop on battery -> E30 dac fed from samsung wall wart -> DUT -> non inductive power resistors -> E1DA grade B -> Laptop.

This time, another class AB amp of identical typology and schematic, A later model of the one I originally tested which recieved much improved PCB layout. Still 200WPC 8 ohm, but powered by a Toroid transformer and not an EI type. More resistant to magnetic flux leakage I've heard, and this was true when I looked at the outputs. This amplifier exhibited a far lower 50hz (60 in 'murica) spill than my orignal one in the OP, but still large rectifier 100hz (be 120hz in the states) noise and harmonics of. So the toroid was indeed quieter I would say, but the power supply typology of dual winding TX to Full bridge rectifier to a pair of large reservior caps is still noisy.

With the Linear TX-Rectifier-Caps I observed a SINAD of around 83dB and removing the whole lot and connecting a hypex SMPS in place got me an 11dB improvement to 94dB.

Noise floors - The previous setup, a slight hum JUST audible at the listening position 3m away to completely inaudible even with my ear next to the speaker cone with the SMPS.

I say RIP linear power supplies, you served us well. Amp sounds exactly the same to me as it did before, amazing.

Heres the measurement loopback:

View attachment 293612



Heres the amp, 6.33V into 8R (5W). Both channels loaded at the same time (because why only load 1 channel in a stereo amp?)

View attachment 293613

And same again the left channel same power same loads but with the SMPS installed


View attachment 293614

I have yet to get to the bottom of the 16kz noise, I suspect the E30 as it is not there when I use another, lesser DAC. But for the purpose of this exercise to show power supply noise I think it can be ignored. Also the Harmonic distortion differs between the two, that is me adjusting the feedback mid measurement so Harmonic distortion did not improve due to the SMPS being installed. My measurement techniques and thoroughness wont get a me a job at NASA thats for sure. I have only shown the left channel but the right channel is the same, got the same improvement.

Any way this was a very quick and easy exercise that only took one rainy afternoon, and the SMPS now remains permanently installed in the amplifier and in daily use with no issues.

Bonus measurement both channels @ 20W


View attachment 293615


And a pic of the amp with the SMPS installed. (other mods - done well before I took the TX out as when I got the amp it didn't even work - include replaced burnt out transistors + little heatsinks installed and full electrolytic cap replacement)

View attachment 293623

Thanks for your work.

Check out Neurochrome. Tom designed his Modulus amps (A/B class) to be power supply agnostic ie the amps achieve a very high SINAD independently of the the PS (high CMRR).

Point is: One has to look at each AB amp individually if a change in PS benefits the amp performance.