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Class AB amplifier powered by non-polarized SMPS - cause for concern?

mike7877

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A Pioneer SX-950 born in 1976:

1696304035022.png

(tl;dr at the ------- before the pictures)


It was my dad's, I got it from him when I weighed just a bit more than it did and I used it as my main amplifier for the better part of 20 years.
It was rated 85WPC into 8 ohms, 110W into 4, but, like a lot of vintage gear, was much more powerful than that. But doubly so, because this thing was not just regular grade awesome, it was next level --- it could shove 43 volts from 20Hz to 200Hz into a subwoofer I made with a 6 ohm dual voice coil driver. Since 6 ohms means 4-5 ohms, let's be conservative and say 4.8 ohms - 43 volts makes just shy of four-hundred watts per channel - 400W x2!!!!
And we can't forget the transients! Easily 250W into 8 ohms. SX-950 has two 63V 22,000uf caps with extremely low ESR. How do I know extremely? I never measured, but size was the giveaway! (about as wide as an 8oz pop can, and an inch and a half taller). All power amp related components had wires wrapped around the leads about 6-10 times, extremely tightly! I love seeing such craftsmanship. Oh, last thing: stable down to below 2 ohms (probably 1.3-1.4ish), though not listed anywhere.

OK, now that you know how awesome the beautiful machine pictured above is (I mean, how could you not want to know? Wouldn't it be nice if these were the kinds of amps you could buy today walking into a major retailer?! I certainly think so!

Onto the issue: SX-950 had a 16 gauge aluminum power cord (lol mid 70s...), which was terminated into a non-polarized plug.
The entire time I used it, I never had a ground loop problem that could be solved by reversing the polarity of the plug.
...Not that there were many issues - problems were extremely rare. One thing that did happen, though, when the receiver wasn't plugged in right (I never paid attention and this might've happened all the time, but, knowing what I know, I doubt it), if you ran the back of your finger along the bezel around the glass covered AM/FM dial, you could feel the 60Hz power in the form of 120Hz buzzing in your finger, more intensely in your forearm, and about half way up your arm to a little past your elbow. It was great! Like being electrocuted without the pain.

So, like I said, never any issues with playback. Also never had anything break or malfunction from what I assume must have been some pretty serious voltage differentials between components at times. (Thankfully its input impedance (and the impedance of other components) were all something between 10 and 20k)

I also have a Pioneer RT-707, an excellent reel-to-reel which uses 1/4 inch tape at 3.75 or 7.5 IPS.
Oh, I forget the model number, but I also had their best (of two) equalizers from 1980 - the last silver-faced brushed aluminum model before all the manufacturers switched to the cheap black plastic crap that defined the 80s. Don't get me wrong, some stuff looked good, and some even still does. But you can't argue it wasn't a downgrade...
Anyway, both units had inputs and outputs (hooked up to "play" and "record" on the receiver) which I hooked up to Tape 1 and Tape 2.
Neither of these devices' power cables are polarized either. And the entire time, none had issues (with themselves or with other gear hooked up over the years, including computers and TVs). Unfortunately, pretty recently, my RT-707 finally begun exhibiting symptoms you'd expect it to have begun exhibiting a couple decades ago, not at nearly 48 years old. It's not horrible and I have a workaround to fix it, but a couple of its caps need replacing. Since the electrolytics are basically 50 years old now (they could be if they were ordered in '74, which is easily possible...), and the unit has been off for almost a year, it's probably best to do them all. Only a couple really really really need doing for performance issues, it's just going to be sooo much work when I have to...

So... all this stuff worked together and with other stuff that didn't have polarized plugs (and did..), including sensitive stuff, and only a couple times ever did I run into issues that needed fixing. And they could always be fixed (just never by reversing polarity of the power plug, like I said).


-------
In more recent news (and also related) I got myself a Topping LA90 Discrete - another class AB amplifier that's similar in some ways, but quite different in others. One of the biggest differences is probably the power supply, SX-950 being internal linear: unregulated, LA90 Discrete being external switch mode: regulated.

LA90 Discrete's external PSU also came with a non-polarized plug on its detachable wire. Before I remembered what I wrote above, I got pretty concerned about it, but now I'm not as concerned.
Not that I'm not conerned - I'm still a little concerned...
Does anyone know what potential issues could arise if the side of the SMPS that [I assume] was likely designed to be neutral, gets live (and v.v.)?
Reason being, I need to be careful with my LA90 Discrete - China is far away and I have a feeling that in the not-too-distant future, bad things are going to happen there, and when they do, the warranty I have could be pretty useless. Obviously yes I do care about the people living there, but music is my favourite and I care about it too. I'm not too concerned about it breaking my other stuff I connect to it, though it'd suck if one of my matched woofers got blown and I needed to replace both of them. Or tweeters. Or DAC or interface or etc.

Here's some pictures I took of my LA90 Discrete:
1696306963047.png

This is the part that goes into the wall. Both pegs are the same width

1696307225305.png

This hideous thing is on the SMPS itself - it's the female end of the other end of the power cable (the wall end of which is pictured above).


This is the top part of the SMPS sticker - the only informative part. As you can see, it's not too informative.
1696307099714.png

Technically these aren't of the LA90 Discrete, but its power supply / cord.


Thoughts on non-polarized amps?
-Regulated make a difference?
-SMPS make a difference?
 

fpitas

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Whereas code says the long slot is neutral, in reality you can't depend on that. At an old job we sent the techs out to measure 50 houses for adherence. The hit rate was about 50%. Obviously those electricians just didn't care. Now, I've known obsessive people to re-wire their house sockets. Not sure it matters in this day and age, though.

The trend these days is SMPS for size and weight. Some are regulated, some simply follow the line voltage input.
 

fpitas

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There's a thread around here somewhere where a guy replaced an old fashioned supply in an amp with SMPS. The SMPS pretty much won. Edit: see next post.
 
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fpitas

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fpitas

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To my knowledge, the new Benchmark AHB2 has an SMPS. You won't find many amps that are better.
 
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mike7877

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Whereas code says the long slot is neutral, in reality you can't depend on that. At an old job we sent the techs out to measure 50 houses for adherence. The hit rate was about 50%. Obviously those electricians just didn't care. Now, I've known obsessive people to re-wire their house sockets. Not sure it matters in this day and age, though.

The trend these days is SMPS for size and weight. Some are regulated, some simply follow the line voltage input.
I'd fire those electricians! People can be killed using older appliances. When a capacitor goes bad in some things (especially from the 30s through 50s), neutral can make its way to the case... If neutral is swapped with hot? Baaaaad news - death! (or worse!!!)
There's a thread around here somewhere where a guy replaced an old fashioned supply in an amp with SMPS. The SMPS pretty much won. Edit: see next post.
To my knowledge, the new Benchmark AHB2 has an SMPS. You won't find many amps that are better.

I checked out that thread - it looks like that amplifier's "n" of THD+n, really benefits from the SMPS
A pretty big surprise to me, as I would've thought high frequency switching noise would've kept it higher than the astonishing 160dB!
I guess the filter on the output is really good, and the transistors "ripple" rejection (sometimes a.k.a. common mode rejection I think)

I'm not looking for noise improvements (in this thread, anyway, though I always love recommendations on how to improve things).
I'm going to make a thread in the future about using my linear regulated power supply {capable of 62V /5A}) to run my LA90 Discrete (just 1.5V less than the 63.5V I measured on the above pictured Topping SMPS)


What I'm talking about specifically right now is if the supply's output is isolated from input. I'm wondering if this is the trick which allows SMPS to be used non-polarized.
 

fpitas

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I'd fire those electricians! People can be killed using older appliances.
Well, I won't argue. But that was the reality. That was in Beltsville, MD, so hardly some backwater.

What I'm talking about specifically right now is if the supply's output is isolated from input. I'm wondering if this is the trick which allows SMPS to be used non-polarized.
Yes, isolation is required with any modern supply.
 
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mike7877

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Well, I won't argue. But that was the reality. That was in Beltsville, MD, so hardly some backwater.


Yes, isolation is required with any modern supply.

I dunno, lots of supplies aren't isolated. People sometimes buy isolation transformers to fix issues with audio gear. I considered one for my Kinergetics KBA 280 a while back, but for some reason sellers charge 3-4x more than similarly sized, non-isolating transformers, and I couldn't justify more than $500 for a standard 1kva transformer (the KBA 280 is a 140Wx2 class A amplifier with an (I think) 1200VA toroidal. The 1000VA was only just on the cusp of acceptability, and since the next step up was 1500VA (+special order from China), which cost nearly twice as much, the 1kva was my only consideration.

Anyway, the first time I tripped a breaker with my oscilloscope was the last time I used it without my (now combo) 12V 7Ah battery + low-powered AC power inverter! (Actually that's wrong, sometimes I'll run the scope off the mains and float the device. Or if the device is battery powered I'll have the scope plugged in. Almost all the time, though, it's: o-scope, + 12V SLA, + 300W sine inverter. I do this to prevent potentially costly mistakes, whether they be mine entirely, or my actions as a result of another human's error (incorrect schematics for example)
 

fpitas

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I dunno, lots of supplies aren't isolated. People sometimes buy isolation transformers to fix issues with audio gear. I considered one for my Kinergetics KBA 280 a while back, but for some reason sellers charge 3-4x more than similarly sized, non-isolating transformers, and I couldn't justify more than $500 for a standard 1kva transformer (the KBA 280 is a 140Wx2 class A amplifier with an (I think) 1200VA toroidal. The 1000VA was only just on the cusp of acceptability, and since the next step up was 1500VA (+special order from China), which cost nearly twice as much, the 1kva was my only consideration.

Anyway, the first time I tripped a breaker with my oscilloscope was the last time I used it without my (now combo) 12V 7Ah battery + low-powered AC power inverter! (Actually that's wrong, sometimes I'll run the scope off the mains and float the device. Or if the device is battery powered I'll have the scope plugged in. Almost all the time, though, it's: o-scope, + 12V SLA, + 300W sine inverter. I do this to prevent potentially costly mistakes, whether they be mine entirely, or my actions as a result of another human's error (incorrect schematics for example)
The only connection allowed from the line to the circuit being powered is the safety ground.
 
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mike7877

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The only connection allowed from the line to the circuit being powered is the safety ground.

If you're talking about the third wire, the vast majority of transformers in audio gear are non-isolating. The [non-isolating] transformer lowers AC [line] voltage, which is then rectified to pulsed DC, which is smoothed with bulk capacitors. The DC voltage across the bulk capacitors is used to power the output transistors.

Also SMPS come in two flavours - isolating and non-isolating. Non-isolating are much easier to construct, and are more efficient.
Buck-boost converters (not to be confused with buck or boost converters) are all isolating because during charging, the load is not connected (and vice-versa).
For an SMPS to be isolating, it has to function similarly to a buck-boost converter. To my knowledge, most of them don't.
I could be wrong, though I don't think I am, because I've fixed friends' malfunctioning audio (ground loop problems) by lifting (isolating) them with small isolating transformers. In the past, for myself I powered the DAC in one of my systems (which used a standard wall wart), with a 6V SLA battery to keep things as quiet as possible
 

fpitas

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Well thanks anyways, but I design that kind of thing for a living.
 
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mike7877

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Something I've been thinking about is using my lab-grade linear power supply (with isolated outputs) that can provide up to 62V / 5A, to power my LA90 Discrete.
It's extremely quick! I also made it very quiet by modifying its fan circuit and replacing the 92mm fan on the internal heatsink with one of Noctua's light grey fans. At under 1A draw, the fan stays at its slowest speed, which is silent from 2-3 feet away. Since the LA90 doesn't heat up at all when idle, I'm sure it takes less than 1A.

You can see it has a brother (stacked on top) and I don't ever need all 4 channels at once, so...
Worth a try maybe! Not that the noise floor can go down much. Maybe it could, though, seeing what happened going from unregulated to regulated, maybe regulated to linear regulated could be similar (obviously not as pronounced though, IMO)

20231003_144707.jpg
 
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mike7877

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Well thanks anyways, but I design that kind of thing for a living.

I like learning. Can you explain where I went wrong instead of being sarcastic, please?
 

fpitas

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Well, good luck. Amir hasn't seen much, if any, improvement using "linear" supplies instead of SMPS.
 

fpitas

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I like learning. Can you explain where I went wrong instead of being sarcastic, please?
That wasn't exactly sarcasm. You went into a long-winded explanation.
 

fpitas

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Anyways, I think I'll let someone else handle this.
 
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mike7877

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That wasn't exactly sarcasm. You went into a long-winded explanation.
It clearly was.
And no, I didn't.

Anyways, I think I'll let someone else handle this.
It wouldn't be too hard - just correct the wrong step- at which point in the three link chain I listed above in my pretty short post
 
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mike7877

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Well, good luck. Amir hasn't seen much, if any, improvement using "linear" supplies instead of SMPS.

Why linear in quotes?

Edit: You know what? "Thanks anyways", "good luck!"
(in life being rude and dismissive to people for no reason, I heard it gets you far! {not sarcastic})
 

DVDdoug

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A pretty big surprise to me, as I would've thought high frequency switching noise would've kept it higher than the astonishing 160dB!
I guess the filter on the output is really good, and the transistors "ripple" rejection (sometimes a.k.a. common mode rejection I think)
It's easier to filter the higher frequency, and as a bonus the switching frequency is normally above the audible range.

If you're talking about the third wire, the vast majority of transformers in audio gear are non-isolating. The [non-isolating] transformer lowers AC [line] voltage, which is then rectified to pulsed DC, which is smoothed with bulk capacitors. The DC voltage across the bulk capacitors is used to power the output transistors.
Almost all power supplies are isolated, whether they are earth-grounded or not. The primary & secondary transformer windings are separate. It's illegal to sell anything non-isolated in the U.S. unless it's "double insulated" with no possible electrical connection to the power line You could build a radio that way as long as the speakers are built-in and there are no electrical connections other than the power cord. But the power supply in an amplifier has to be electrically isolated from the power line because there are connections to the outside world.

That wasn't always true. I've seen older equipment (especially tube equipment) with a 2-prong plug where the chassis is connected to neutral (hopefully). I have a live concert recording that starts with the singer saying, "I keep getting shocked off this mic." And then he says something like "Will you see if you can switch to ground them out". Apparently they get it fixed but there is a (different) story of at-least one musician killed on stage. ...But there is also that false story about Sixto Rodriguez committing suicide on stage by lighting himself on fire!
 
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