A Simple 10W Direct-Coupled Class-A Power Amplifier

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ricardo
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by ricardo » Wed Jan 07, 2015 9:35 pm

I need to point out that most of the DBLTs above were on prototypes of a commercial design to a spec. 2x50W 8R bla bla.

On the basis of these tests, my contention is that with a conventional PSU for an amp between (perhaps 20W) 50W to about 100W, using the same VA transformer wound to give slightly lower power will be preferred in DBLTs even with true golden pinnae.

ie this will sound better in DBLTs and also in the shop.

But I fully agree with 'no clipping' as best of all. The main amp in our speaker listening tests was a Rotel RB5000, 2x500W 8R & 2x1000W 4R. It was the best sounding amp I've ever tested or used. I can also pontificate at length on the speakers which benefit from such large clean power and those which don't see much improvement above about 2x200W 8R.

I pontificate in another forum on what features would ensure If DBLTs were the MOST Important Tests used in the design of an amplifier.

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mediatechnology
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by mediatechnology » Thu Jan 08, 2015 8:43 am

I wanted to point out that one difference I experienced in my first test vs. the "slightly lowered VA power supply" observations was that the "clipping" was not due to the output hitting the rails but simply (gracefully?) running out of current due to being severely under-biased with an 85 mA Iq.
I suppose it's similar to what JR experienced with resistive current limiting.

In any case, transformer VA, resistive limiting or under-bias, the mild compressive odd-order distortion on high-level peaks sometimes makes stuff sound a little better.
That sort of goes against the "even order=good, odd-order=evil" dogma and musical theory.

But with a 1A or more Iq the hot little amp sounds good particularly at low level with THD levels at the generator residual.
So I want to move this project along...
I visited a friend yesterday who had two big boxes of "computer grade" electrolytics he'd saved from being scrapped for metal.
It didn't take me long to find a really nice pair of 100,000 uF 40V caps. You can weld with these.
Based on their size and the huge toroid ricardo's laptop power supply suggestion is looking more and more practical.

Until I find a nice case and decide on a linear or switcher I think I'll build up the linear supply on a piece of 1x12 white pine.
A true bread-board.
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by emrr » Thu Jan 08, 2015 10:51 am

Back to the idea of restricted current, this is definitely a factor in old tube amps, and even some preamps. If you look at cap values and consider tube rectifier properties for any given era, it becomes obvious. Many recreations of old tube equipment fall flat from the simple error of 'improving' the circuit with modern reservoir values. You get cleaner results that frequently lack the original character that was chased in the first place. If the cleaner results work for you, great, but it shouldn't be a mystery when an amp recreation behaves differently. It would follow (at least in my feeble mind) that the same effects would be felt in SS approaches, albeit with adjustment for the differences in transconductance curves. I am not a scientist, YMMV....
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Doug Williams
Electromagnetic Radiation Recorders

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mediatechnology
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by mediatechnology » Thu Jan 08, 2015 11:49 am

You get cleaner results that frequently lack the original character that was chased in the first place.
Doug how would you describe the character that's lost?
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by emrr » Thu Jan 08, 2015 3:21 pm

I have not measured anything, but much like you describe, a self-limiting aspect with what sounds like very low order harmonics contributing to a thickening and exciting quality. The larger the reservoir, the more they clipping quality becomes an obvious brickwall with harsh qualities. Usually circuit reproductions are chasing that soft exciter/compressor quality and it's no longer there, or reduced greatly.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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mediatechnology
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Off the Shelf Switching Power Supply

Post by mediatechnology » Thu Feb 19, 2015 9:20 am

ricardo and I discussed earlier in the thread the prospect of using low-cost 18V laptop switchers to make the Simple 10W Dual Class-A power amp practical.

Yesterday I received from Jameco Electronics two Meanwell GS60A18-P1JU 60W 18V 3.33A switchers. They're $19.95 each.

Link to Jameco: http://www.jameco.com/webapp/wcs/stores ... 2154951_-1

Meanwell GS-60 Data: http://www.jameco.com/Jameco/Products/P ... 154951.pdf

Right now I'm running the two channels at 10W each into 4 Ohms and these power supplies are quiet and not breaking a sweat.
The Class-A heatsinks, as you might expect, are nice hand warmers.


Despite being hooked up with clip leads I'm seeing no issues with in-band noise.
Though the supplies deal with over-current using a hiccup mode I've been unable to trigger it by clipping.
Absent from the clipped waveform is the ripple one would see using a linear supply.

I hope to have time later today to hook this up properly to monitors and listen.
This is my second very positive experience with Meanwell switchers used in audio apps.
Their 25W +/-15V +5V "bipolar" triple output supply is a good option for some of the other projects we have here.
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JR.
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by JR. » Thu Feb 19, 2015 11:57 am

I have used a few cheap switchers for my adventures in Peltier cooking... they seemed mechanically quiet and robust...

I need to resist the temptation to build my own version of class A. Actually a class AB where the outputs never turn completely off, but don't idle at full current. I have two lumps with slightly different power output but close enough for a small amp.

JR

PS: Last night was in the teens, a good time to think about class A amps :D
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mediatechnology
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by mediatechnology » Thu Feb 19, 2015 3:07 pm

I need to resist the temptation to build my own version of class A.
No, you shouldn't resist the temptation.
Last night was in the teens, a good time to think about class A amps :D
And it will sound better than the Peltier heater while it heats your food.
Seriously, I'd like to see what design you cook up.

These switchers do have a start-up time specification of up to 1000 ms and from what I see the +/- rails come up at different times.
That's one minor thing I found.
However, other than power failure recovery I don't see it being a problem given that I intend to switch the supply output rather than AC line input.
I need a DPST power switch.

Another option are two simple SPST power relays with the coil for the +18V switch powered from the -18V supply and vice versa.
With Zeners in the 12V coil lines the rails could be made to switch on around 15V or so.
With the coil supplies cross-coupled if one supply collapses or fails to come up the other is turned off.

A single DPST switch, the simplest, is probably the best.
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Covidland (The Movie) Part 1: Lockdown https://banned.video/watch?id=615f96b3aa816336dbd21792

A voice-over with Orson Welles: "What Is It That You Want?": https://waynekirkwood.com/content/Orson ... u_Want.mp3
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mediatechnology
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by mediatechnology » Thu Feb 19, 2015 7:06 pm

Thought I would post a photo of the "Simple Dual 10W Direct-Coupled Class-A Power Amplifier" powered by Meanwell 18V, 60W switchers.

This is the whole enchilada:

Image
Dual 10W Direct-Coupled Class-A Power Amplifier Powered by Meanwell 60W "Laptop" Switching Power Supplies.

The DPST power switch on the supply outputs - salvaged from an old computer - work fine.
The Dual Class-A amp has very little turn-on transient when switched this way.

The Meanwell GS60A18-P1JU power supplies are tip positive.
The 2.1 mm power connectors need to be the insulated type since the barrel of the negative supply is at -18V.
Unloaded the GS60 outputs about 18.1V, loaded about 17.6V.

As you might guess I spent some time listening to this and was quite pleased.
Owing to the switchers there was no hum when heavily-clipped.
Sonically they sounded the same as when run off the bench supply or my cobbled up linear supply.

One of the things I keep realizing over and over again with flea-power Class-A is the complete absence of low-level grit.
Monitoring at whisper levels, about 100 mV p-p into the speaker, this amp sounds far cleaner than the higher-powered Class-AB amps I usually use.
Fading the monitors down to -70 from maximum peak level I don't hear rising low-level grime.

For whatever reason, the improved low-level HF detail seems to make things image more precisely at any monitoring level.

One apparent need is gain.
The power stage is unity gain so a variable gain stage is needed ahead of the board to give maybe an extra 10-12 dB range.
I've got an active volume/balance stage inspired by Self that I like.
https://ka-electronics.com

Covidland (The Movie) Part 1: Lockdown https://banned.video/watch?id=615f96b3aa816336dbd21792

A voice-over with Orson Welles: "What Is It That You Want?": https://waynekirkwood.com/content/Orson ... u_Want.mp3
Americas Frontline Doctors Lawsuit Motion For Preliminary Injunction Of Covid-19 Vaccine Emergency Use Authorization: https://waynekirkwood.com/images/pdf/Am ... zation.pdf

ricardo
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Re: A Simple 10W Direct-Coupled Class-A Power Amplifier

Post by ricardo » Sun Feb 22, 2015 5:41 am

mediatechnology wrote:Thought I would post a photo of the "Simple Dual 10W Direct-Coupled Class-A Power Amplifier" powered by Meanwell 18V, 60W switchers.

This is the whole enchilada:

Image
Dual 10W Direct-Coupled Class-A Power Amplifier Powered by Meanwell 60W "Laptop" Switching Power Supplies.
You need a couple of LEDs on the PCB so it looks more testicle if the punter opens up the case :)

Even better if the case has a transparent window so these can be seen from the outside. Or a tube with just the filament powered up.

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