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L20 - 350W audio amplifier

The Famous L20 – sold on eBay

There is one amplifier that seems to get more attention than most and that’s the L20. I have no have experience of the L28 which has a built in speaker protection circuit.

L20 - 350W audio amplifier
L20 200W 8 Ohms – 350W 4 Ohms at +/- 60V – advertised on eBay

Straight talk, these are mono amplifiers, there is no bias preset neither speaker protection so make sure you get one for each amplifier or the Vellemann K4700 dual channel protector.

Communica sell the K4700. I purchased mine from Velleman USA. In 2014 R300 incl. import. These are for two channels.

K4700 - dual channel speaker protector
K4700 Velleman dual channel speaker protector

Pro-Tecc in Durban sell their own brand. Last pricing was R46.00. Mono only. Spade or terminal posts – easily modified.

Mono speaker protector
Speaker protector – mono – from Protec Durban

The Kit

Easy to assemble, no schematic, painting by numbers. One can build one amplifier in an hour easily. Be careful of polarity. You must have a DMM to actually measure the resistances before populating – sometimes the colour coding can be a bit daunting especially in low light or when you are no longer a spring chicken. Board quality from most of these Eastern suppliers is very good although some of the manufacturers do complain of counterfeiting, poor quality boards and dodgy transistors.  Make sure you have PayPal protection and the supplier is authentic.

The power supply

Power supplies are obtainable from most of the exporters or kit suppliers but be warned, exporting of Toroidals or even EI transformers are very expensive due to the weight. Support local.  Swiftheat is one. See below.

Transformers for these amplifiers was supplied courtesy of a Sony audio guru in Cape Town. The STR500 series (plus others) carries a 220 : 42-0-42 transformer which seems to be adequate for one channel. I very happily became the proud owner of 4 of these.  Why? They are prone to overheating when these amplifiers are slammed hard, the heat protection sensor in these transformers goes open.  With a bit of digging one can remove the sensor and bypass.  Once full wave rectified and smoothed out you should be looking at about 60V DC per leg or +/-60V.

Smoothing caps were 2 x 10 000uFd per leg, obtained from Yebo in Bellville. Code ELC108, currently priced at just under R100.00 each.

Note:  The original power supply which I still have was made up of 6 x 400~500VA 7-0-7 (14V) UPS transformers.  I loaded the power supply so that there was a constant draw of about 25A per leg at 52V (3 in series * 1.414) without any sign of a dodgy heating pattern amongst these transformers over about 8 hours.  The problem is that the power supply weighed in at 24kg.

A really great alternative is to try Swiftheat in Johannesburg – their pricing for a 1000VA toroidal used to be about R 2 200.00 exclusive VAT.  This is a very fair price based on what it would cost for an import or through the DIY channel. This included washers and dipping for audio use.  Note that this pricing was April 2015.  Include 7 days after payment for manufacture.

Heatsinking was obtained from Mantech, code A0M0097. Purchased two 8 3m lengths. Each amplifier used a 40cm run.

L20 Sound quality

Exceptionally good. I was worried about bias, this is factory and designed and obviously we would have concerns about transistor mismatches etc. Not to be, all four amplifiers I purchased were rock steady and from what I can recall about 20-30mA.  The amplifiers were tested into 2 * Dixon 600W 4 Ohm (you know the one – 18″ bass driver and millions of piezo high frequency drivers) plus 2 x Dixon 15″ (which sound pretty shitty but for clubbing, who cares).

The quality of reproduction is amongst the best I have ever heard.  To be frank I did not trust the Sony transformers but they held out.  The original PSU bank of UPS transformers laughed off the current draw, bearing in mind that when using a program source such as music, proper music, Floyd, Deep Purple, Black Sabbath and Metallica 🙂 the current drain is really minimal compared to potential. And that potential one could nearly weld with.

Some little things to remember:

The heatsinking at full power is not really adequate once boxed. Use a fan blowing onto the output transistors ensuring each transistor gets the same treatment. You do not want to have thermal issues causing the output transistors to start current grabbing.

The original PSU using UPS transformers had sensing on each secondary rail. Because each transformer was separately fused primary and secondary this would shut off power if only one fuse blew preventing one rail of the PSU output to remain hot.

The speaker protection circuits, whilst great for peace of mind will not prevent voice coils burning in the event of a relay contact being burnt shut.  These relay contacts are not designed for professional and high powered use. Bear this in mind.

In conclusion

I still have the L20 x 4 channel amplifier which was going to be used for home theatre. I subsequently purchased the Behringer NU6000 4 channel amplifier. The L20 is in Cape Town, I am in Durban otherwise I would have posted photos. I am looking for McIntosh style VU meters though. Like most, these really add a professional finish to any home built equipment.

McIntosh Amplifier
McIntosh Amplifier – damned gorgeous

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