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NATDBERG 25th July 2008 05:21 PM

Elektor mono-blocks - new purchase. Any info?
Hello people,

I just bought a couple of Elektor designed mono-blocks off ebay and would like to know more about them if I can..

They were a mystery, gamble buy as the guy wasn't even sure if they were amps but I took a chance. Here they are:

Searching on here, they appear to be either :

okt 91 - LFA-50-OA class A amp with 2SA1216/2SC2922 or

feb 94 - ulti-amp with 2SC2922/2SA1216 (better poweramp)

due to them using 2 x Sanken 2SC2922/2SA1216s. Circuit boards are EPS 880092-1, 2, 3 & 4 and is that a mains filter in the last pic?

Does anyone know anything about these designs, specs, what the pretection lights mean? Should they be good amps? Sensitivity would be good to know.

As of writing this I have yet to turn them on, do anything with them - I'm a bit of a scaredy cat when it comes to plugging in new amps with heart stopping pops, crackles, smoke etc or even my cat stepping on bit of bubble wrap.. - but may do so soon into some 10 Gale speakers.

They look fantastically well made and with extra large power supply caps.

The fuse next to the grey transformer is empty - no blown wires inside, just empty - anyone know what this protects against?

Any info would be much appreciated!

jacco vermeulen 25th July 2008 05:31 PM

Here's a builder's page, plenty more if you goggle :
Well made duo, looks like the assembler spent serious cash.

It's basically the LFA-150 with lowered rail voltages to accommodate high biasing.

AndrewT 25th July 2008 05:56 PM

Short the input with a zero ohm RCA plug and don't connect the speakers.
Start them up with a light bulb tester.
If the light stays start measuring to find out what's happening.
If it goes off then probably OK. Check the supply rail voltages, four of them.
Check output offset and output bias current.
Warm it up and check those again.
Connect speakers and check that offset again and listen for unusual noises.
Switch off and remove the input short and connect a known source.
Disconnect the speakers. Switch on and recheck the output offset with the source connected but switched off and then switched on.
Reconnect the speakers and enjoy.

NATDBERG 25th July 2008 09:46 PM

Thanks for that - when you know what you are looking for it's a lot easier!

Here is the best link I found:

a word doc of scans from a magazine?, in French, explaining it all. I can at least get the specs from this.

It says 29V rails on this, mine are marked 59V... possibly a 100W version?

Anyway, I will see if I can OCR the scans and translate parts that look interesting..

They certainly went to a lot of effort if not expense and the cases are a perfect fit fot it all too.

AndrewT, what do you mean by starting it up with a light bulb tester? And a zero ohm RCA would be a plug shorted out? They actually used BNCs for this which shows the extra effort they went to.

They smell ok which is always a good sign and look very clean inside. I've also seen matched Sankens on ebay recently too should anything go wrong.

jacco vermeulen 25th July 2008 10:00 PM

Higher rails means it's the LFA150, 150W/8 Class AB.
Pretty hefty heatsinking for a monaural AB power amp. You may have to check if both heatsinks are well enough thermally coupled.

I can scan the LFA150 schematic if it's not traceable on the web.

AndrewT 26th July 2008 09:03 AM

search light bulb tester

nigelwright7557 26th July 2008 09:16 PM

It looks very neat.

The wires are all tightly bundled together which is not good from a crosstalk point of view. Its OK to bundle PSU wires together.
But bundling signal wires can result in positive feedback.

There seems to be a lot of cctry for a power amp.
I prefer the simpler designs as there is less to go wrong.

The heatsinks do look a little on the small side as I cant see a fan.

I do gigs for upto 6 hours of playing loud and know that over time heat can build up and sometimes become a problem.
So, I tend to use PC cases and have a large heatsinbk and a fan sucking in air and another one at the opposite corner blowing it out.

scanspeakman 26th July 2008 10:23 PM

Great amp, especially when it's the lfa50-oa (instead of the lfa150). I have this amp myself so the pictures look very familiar. My lfa50-oa is working for about 12 years flawlessly. it also sounds great.

Good luck with your purchase,


AndrewT 28th July 2008 01:08 PM

each of those heatsinks for a 150W ClassAB amplifier look just fine for domestic duty in a hot environment.
If they warm up significantly between quiescent and playing loud then you are asking too much from the amp, i.e. clipped transients.

NATDBERG 28th July 2008 01:20 PM

I'm going to be a bit busy/going away as of now so won't get to test them for a week or so..

Something I read while researching said that the LFA 50-oa is based on the LFA 150 but uses lower rails for, err something to do with better biasing - it is time I confessed, I really don't know much about amp design..

Anyway, that could account for the printed boards saying 59V but this design actually using 29V rails. The LFA 150 doesn't use these Sankens whereas the LFA 50-oa does.

The top board of the stack is the protection circuit. I'm going to open up my in-need-of-repair Burmester 828 MkIIs which also uses these Sankens and compare the layout just to see if there are similarities - next week though.

I would agree with the wiring and cross talk in general but with these, it looks as though the input and output wires are only a few centimetres in length to the connectors. It would depend if the power and earthing wires bundled will produce much cross-talk. Those at the top are for the protection circuit it looks like.

The heatsink is firstly a thick aluminium bar that the Sankens are directly attached to and then this bar looks as though it is fixed to the heatsink with thermal paste. As a monoblock, hopefully the 25W of class A heat (how much is that?) per side of the chassis (npn transistors on one side, pnp on the other) is disipated enough. We shall see!

I'd be interested to hear more about how these should sound Scanspeakman. I was hoping that they might even be an answer to what I should use to power my Kef 107s (when I get around to refoaming them) - I don't listen at high levels but have heard that the Kefs need a lot of well controlled current for the bass to perform well.

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