:eek: Serious SPPV (sound per pound value) Alert :eek: Power Amp Board

[Firebottle]

Firebottle does it again, quality audio, but this time I can't take the credit .

After fixing the Myst FET output amplifier for Jerry, see http://theartofsound.net/forum/showt...ower-amplifier, and being very impressed with the performance I set to research what was available in FET output power amp boards.

During the research I found that the Myst was based on a simple Hitachi FET circuit, as was the FET output board that Maplin used to sell, which had a good reputation. There are a number of boards available on line but the L12 caught my eye:



..not least because it included the psu components, all on one board.

It looks like it is based on this circuit, but as a stereo build:



So the only other items needed to build a complete amplifier are a switch, a mains transformer, some output terminals and a case. Prompt for a (not so) quick rummage around the parts stock and I came with a complete lash-up (you good folk may like to term it differently ).

I hadn't got a suitable case so the only aluminium that I could use as a heatsink/chassis was an L piece from a defunct psu. so it looks like this:



With the available borrowed mains transformers I guess it is capable of 35-40W output, but the board is rated to 100W with a suitable transformer.
Performance wise it is excellent, will be getting a second opinion tomorrow.

....and for the cost of the board............around £25, fantastic.

Alan

PS Sorry the web pictures are a bit big.

Forgot to add the link http://www.ebay.co.uk/itm/3212540900...%3AMEBIDX%3AIT

[Macca]

Are you going to bring that along tomorrow then Alan? Looks interesting.

[Firebottle]

Yes I certainly am.

[Macca]

Cool - I have a dead power amp you can have the case to build it into if you want. It's pretty big with loads of external heatsinks.

[Firebottle]

Well I've built the L12 board into my 'new' case, with a humongous mains transformer that came with it, a 500VA jobbie.

It was rather fortuitous that the secondary voltages were perfect, so it should be good for 100W into 8 ohms ppr channel



I have fitted a home brew heatsink to the on board rectifier, it's always better to keep active devices on the cooler side, rather than run them roasting hot.

Listening to it now and it rocks, great bass performance being dc coupled.

[DSJR]

Have you any intention of trying the more powerful ones such as the L20 or L25? Cheap enough as you have the transformer and basic casework.

http://www.ebay.co.uk/itm/1pc-L25-50...item33872ff9f3


May not be MOS-FET (showing my ignorance) but I have a genuine reason for asking

[Firebottle]

Sorry David but I'm not. You are correct that they are not FET output, I was specifically wanting FET output.

They are certainly cheap enough aren't they?

Alan

[Firebottle]

Well I've given the L12 a good rattle today and it goes amazingly LOUD.

Really clear and superb low end grunt, chest shakingly good. I am definitely a happy bunny with this amp. Not much waste heat either, it runs very cool.

[r100]

Well I've given the L12 a good rattle today and it goes amazingly LOUD.

Really clear and superb low end grunt, chest shakingly good. I am definitely a happy bunny with this amp. Not much waste heat either, it runs very cool.

That's cool. LJM (the designer) makes some very good amps for the DIY community. I have assembled some of the D versions with great results. I will give the L12 a try one of these days. Thank's for the report. Greets.

[Firebottle]

I've been making some very informative comparisons with this L12 amplifier.

I took it round to Rob's to see how it stacked up in his very revealing system. The amplifiers affording the reference are modified Ming Da MC805A class A monoblock valve amps, so some target to aim for.

The L12 was plumbed in and lots of different tracks played from Rob's file based system. All the following comments are describing small changes to the sound.

The word that came to my mind was a touch 'sterile', a confirmation I think of the SS versus valves flavour of the overall sound. It was not quite as involving, but that's not to say there was anything bad about the sound.

The lower bass was very good indeed, no trouble with extension and clarity.
Rob noticed a slight suck out of upper bass range, I probably wouldn't have picked up on it but Rob does know the sound of his system.
On some tracks there was a slight tizz to the cymbals, other tunes that were very busy in the mids revealed a slight 'thickening' of the mid range.

We are talking of minor differences here, a less revealing system wouldn't have shown them I'm sure.
Regarding the technicalities of the L12 design, it is my thought that matching the transistors in the complementary sections of the circuit would probably improve the performance in the mids and treble, but at not inconsiderable effort/cost to do so.

The slight suck out at upper bass got me thinking about the synergy (there's that word again) with the preamp regarding impedances and levels.
I have now modified the L12 to reduce the gain by nearly half and increase the input impedance by nearly four times.
A re-match is planned for this week to see if there is any difference

The L12 board is highly recommended to anyone wanting to build their own power amp on a budget. It runs very cool also and has very benign power up/power down characteristics.

Alan