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12V power supply for amplifier
Old 9th February 2020
  #1
Gear Maniac
 

12V power supply for amplifier

I'm thinking about buying passive speakers for my tiny home studio, and although I don't yet have a studio amp I've remembered I do have a good quality car amplifier which obviously runs off a 12v battery.

I'm in the UK so mains power is 240V AC.

The amplifier is stereo with 100 watts per channel.

If I wanted to use this amplifier for my new monitors until I can save up for a studio power amp what would be the best option for powering it? Would it be better to use some sort of 12V deep-cycle battery with a charger, or some sort of 240V AC to 12V DC power supply? I don't know anything about power supplies, but presumably it would need to be something which can cope with whatever the amp is drawing at a decent volume.

The monitors I'm thinking about are ATC SCM7 (they're small, but I think they have a requirement for a beefy and top quality amp).

The amp is an old Soundstream D200 which was regarded as reference class in its time (and which did sound great when I had it installed in a car). I think it was designed by Nelson Pass. I don't know for certain, but I'm hopeful it wouldn't be completely out of its league with the little ATC speakers.

Some pointers as to how to power the amp would be much appreciated.
Old 10th February 2020
  #2
Lives for gear
 
Radardoug's Avatar
 

Maybe a better way would be to sell the amplifier, and use the money to buy a standard amp. If it was a well regarded amp it might have good value.
Old 10th February 2020
  #3
Lives for gear
"Good quality" for a car amp is seldom the same thing as "good quality" in a studio - or even a hifi - amp. So looking for a good used AC-powered amp makes sense.

Also ... if your car amp really is 100 watts per channel, continuous without clipping, that means it consumes at a minimum 16.7 amps at 12v. Probably a bit more, since nothing runs at 100% efficiency. You'll have to consider that in your power supply or run time from a battery, and also in the wiring.

On the other hand, using a car amp will mean your monitor chain can take a lot more vibration than most AC-powered amps. That might be important with some rock genres.
Old 10th February 2020
  #4
Gear Maniac
 

Thanks for that.

I wouldn't want to sell the amp, and I remember it being pretty special and at least as good as some hi-fi amps. However, I appreciate it might be more trouble than it's worth, but I'd still like to learn how I might make it work.

I've found some specs for it, and apparently maximum current draw is 26A (recommended fuse is 30A). However, apparently it's rated at 100W per channel into 4 Ohms, so would that be a problem (the ATCs are 8 Ohms). I think the amplifier could be bridged to provide 200W mono into 8 Ohms, but that obviously wouldn't be any good for this application.
Old 10th February 2020
  #5
Gear Addict
Some comments based on the schematic (beware the typo/schemo: Q4/Q14 is actually a PNP, emitter to the rail):

- This is a rather conventional audio power amp with +/-36V dual supplies preceeded by a 12VDC -> +/-36VDC converter. If I was considering using it at home, I'd go in there and replace that converter with a standard psu. It's quite a waste (efficiency, heat and noise-wise) going through that intermediate conversion step when you can go directly from AC to the DC the circuit actually requires.

- Don't expect stellar performance from it. I've simulated it just for fun and THD is quite high, even considering that LTSpice is generally over-optimistic by as much as an order of magnitude in that measurement, depending on your models.

- This doesn't mean that it doesn't sound pleasant, in fact it's likely to have been designed that way on purpose (Nelson Pass knows his stuff for sure), but accurate it isn't. It would be rather straightforward to mod for better performance, lots of room for improvement, but I don't know if you're into DIY.

- As a temporary solution until you can afford a proper amp, I'd go with a cheap and cheerful switching supply like this one (I'm sure you can find one without a fan if that bothers you). I think spending a lot more on a linear one of that kind of wattage would be overkill.
Old 10th February 2020
  #6
Gear Addict
Quote:
Originally Posted by deejayen View Post
apparently it's rated at 100W per channel into 4 Ohms, so would that be a problem (the ATCs are 8 Ohms).
No problem, just note that it will clip at around 70W with an 8 ohm load. In fact this relaxes the psu requirements a bit, something like 12V/15A will probably be more than enough.
Old 10th February 2020
  #7
Gear Maniac
 

Thanks very much for all that, Cabirio. It's very helpful and much appreciated!
Old 11th February 2020
  #8
Gear Addict
Quote:
Originally Posted by deejayen View Post
Thanks very much for all that, Cabirio. It's very helpful and much appreciated!
My pleasure deejayen. I was rather puzzled because the reason THD is so high is that the output darlingtons are epically underbiased, a strange design choice. I get an idle current of 2.9/2.0 mA (top/bottom), when the optimal value with those 0.1R emitter resistors would be around 200 mA. You can even see clearly crossover distortion in the simulated waveforms.

So, I investigated further and found a very illuminating post over at diyaudio by one Wade Stewart (from this thread). Quote:

"The original D60, D100, D200, CA40 and CA50 were designed by Nelson Pass. they had two problems. 1. They were laid out with point to point wiring which meant that they were not cost effective to build. 2. They all had thermal run away. A condition where the bias is set to high and over time the amps would overheat and shut down. All I did tto solve this problem was to re-adjust the bias to a lower setting."

I bet those p2p prototypes had the Vbe multiplier mounted on top of one of the output darlingtons, as it should be, but that's not so easy to do with a TO-92 package and probably difficult (i.e. expensive) to take to production. I also bet the commercial product has that transistor mounted on the PCB, so even if it's in contact with the heatsink, there's not enough thermal compensation and idle bias had to be lowered quite a bit to avoid thermal runaway. The problem with darlingtons is that the driver and output transistors are in the same package, so all four Vbe's have to be compensated, unlike normal EF output stages where the drivers run much cooler.

The fix would be simple: replace the Vbe multiplier with something like a BD139 screwed directly on top of one of the output darlingtons, replace the bottom resistor with a 1k trimmer and adjust. Even without touching anything else (and there's quite a lot that could be done at the input and VAS stages), simulated THD at 1W and 1kHz / 10kHz drops from 0.46% / 2.1% down to 0.0022% / 0.027% (that's two orders of magnitude) just by increasing idle current to about 200 mA (per output pair). It will run warm, about 32W dissipation at idle, but car amps usually have oversized heatsinks so that shouldn't be a problem. And Nelson Pass is well known for his "space heater" class A designs, so I'm sure that's how he intended it...
Old 11th February 2020
  #9
Gear Maniac
 

Thanks, Cabirio!

It's been many years since I used the amp, and it's currently stored away upstairs, but I'm now thinking that mine is a Mk2 version. There's a manual for the Mk2 here http://www.soundstream.com/manuals/A.../d_ca/d_ca.pdf

My electronics knowledge is extremely limited, and I don't know how much the Mk2 differs from the original. The above manual mentions "1000W of discrete Darlington output devices".

I originally bought a D60, but it was underpowered. I also remember that my first D200 kept cutting out with thermal overload - I think they said there was a bad batch of components or something. Once fixed the D200 was fantastic - I used it with a Nakamichi cassette deck, and we hit lucky when we installed it in a Saab 900 in terms of speaker placement. It was a relatively simple system, but it sounded fabulous.
Old 11th February 2020
  #10
Gear Addict
 
samwinston123's Avatar
 

I wired up a Mcintosh car amp for someone a few years ago using a 1000 watt computer power supply. The main rail in a computer is 12V and requires a lot of current, so exactly what you need. It worked pretty well. It’s not totally straightforward, you have to add a few jumpers to the supply to make it power up all the way.
Old 11th February 2020
  #11
Gear Addict
Quote:
Originally Posted by deejayen View Post
My electronics knowledge is extremely limited, and I don't know how much the Mk2 differs from the original.
I don't know, there's a lot of talk about these amps at diyaudio, as well as a bit of a cult following, but not much reliable info. Wade Stewart doesn't seem to be reachable anymore and Nelson Pass himself doesn't say much, apart from this. In fact I don't even know if the schematic I linked is for the I or II... Sorry I went on a tangent there, but since you're planning on getting a proper studio amp eventually, I think the reasonable thing to do for now would be to leave the amp as it is, get one of those cheap psu's and enjoy it!

Quote:
The above manual mentions "1000W of discrete Darlington output devices".
That's just marketing talk, those darlingtons have a maximum power dissipation of 125W, and there's 8 of them, so...
Old 12th February 2020
  #12
Gear Maniac
 

Thanks very much for that. I could look into getting a computer PSU (I work in IT). However, the PSU you linked to would probably be a bit neater. I'll ask someone I know about wiring things up.
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