LJM L20 v9.2 - what do you think ? (Some questions regarding cooling + PSU)

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Dear All,

I bought 2 LJM L20 v9.2 mono boards from aliexpress about a year ago or so. I have several question to this board and just want to hear your opinion, extra advice is welcome too.

1. How do these perform regarding sound quality ? Measurements are one thing (looks good for me) but I ask those who have already built amps with these modules.

2. I might build them into a nice aluminum case with pretty big heatsinks on both sides. How much do they dissipate (in Watts, each, approximately) ? Class AB I know, but still... just an estimation for me.

3. Would 2 separate SMPS do the job of the PSU in general ? (Keeping dual mono design within the box). Properly shielded of course from the amp board.

..3a. Would you add a separate, somewhat big capacitor block between the amp module and the SMPS ? Something like 2x-3x-4x 10.000uF low ESR in parallel + corresponding little film caps on top of them - or just leave it as is and feed the modules directly from the SMPS output ?

..3b. Any recommendation for supply voltage out of experience .. ? A good balance between power (into 8 Ohms) and heat ? I assume these little beasts are heating in idle too, just like all class AB amps.

4. Any other tips & tricks ? I read somewhere for earlier versions about the need of speaker protection else a sudden mistake (shorting the outputs) and both amp + speaker might blow up.

5. Are stock, supplied isolating cooling pads good enough ? I already took off all of them and looking for some good (probably better) alternative like Kerafol (?) ..

Re: 2, see Rod Elliott. When it comes to these basic amplifier design topics, he's really got you covered.

Re: 3, there is not much to be gained but much to be lost when using SMPS in an application like this.

1. Larger SMPS tend to be PE-referenced and not stackable.
2. You need to oversize them considerably to keep them from shutting off under peak loading.
3. And no, they don't usually like it too much if you throw loads of extra capacitance at them.
4. Some voltage sagging under load is actually not undesirable... reduces stress on the outputs.

These modules are supposed to be good for 200 W / 8 ohms. That's 40 Vrms, or ~117 Vpp. You will therefore need about +/-60 V loaded... so probably a bit more in idle. (2SD1047/2SB817 are listed at either 140 V or 160 V max, so +/-70 V is about abs max, and even that tends not to be a very good idea due to secondary breakdown.) You could take it easy and go to more like +/-50 in the interest of better SOA and/or lower-impedance loading, of course... leaving it to commercial designs to squeeze the last watt out of the hardware.

Dual (floating) power supplies make your ground routing easier, but it's not like it's impossible with a single xmfr, as shown by countless commercial units. Don't go overboard with the dual mono, you are going to have a stereo input signal with grounds connected either way. Commercial designs are often using the back of the case as a star ground point - it gives good RF characteristics as well and allows for mounting any number of non-insulated RCA jacks.

Make sure you get the power right as well, with twisted transformer leads and all.

If the thing is supposed to have an unbalanced input, you are probably better off going for an IEC Class II (double insulated) kind of design, complete with shield winding in the transformer - you may find one "for medical use". There have been a fair few power amps built as IEC Class I, but do so at your own peril with unbalanced connections - any grounded / earth source will get you a ground loop. A balanced input will have more or less of a noise penalty, which is part of why pro gear tends to run higher line levels and have a variable attenuator inside the power amp.
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