Toroidal Transformer VA and Capacitance required for LJM 20.5 amplifier board?

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DaveFred

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Joined 2018
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#1
Hello,

I am looking to build a dual mono amplifier with a pair of LJM 20.5 amplifier boards,

2 PCS L20.5 Ultra low distortion 0.0015% Power amplifer board KTB817 KTD104 200W|Amplifier| - AliExpress

and I am wondering what value VA toroidal transformer (35V-0-35V) I should use?

I was going to use a transformer from Parts Express,

toroidal 35V+35V - Parts Express Ships Fast and Ships Free.

My best guess is that 250VA per side should be fine for this amplifier at this voltage.

Next question, What overall capacitance value at what voltage do you think this amplifier requires?

These are the boards I am looking at,

A pair of these,

2PCS L15D/L20D/L25D and L/MX50 Series mono rectifier power supply board With speakers DC protection board|board|board boardboard power supply - AliExpress

or,

LJM 8 X 63V 2200UF Power Supply Board DIY PSU kit for Power Amplifier|kit supply|kit kitskit diy - AliExpress

I'm thinking the first board at 63 volts should be fine.

If you have a suggestion on a single power supply board and transformer VA value, please leave a reply as I am not married to the dual mono plan.

Thoughts, comments, suggestions?

Thank you,

David.
 
#2
PS design depends on V but also on load impedance. 4? 8? 2?

At 8 ohm load an audio amp can "work fine" with 1,000uFd (or 2x2,000uFd). And since you may never clip a 150W amp, that may be fine. But it looks cheap (and you will know). Just go bigger until case is full or wallet is empty. 2x4700 is popular and fine. 2X8800u is, as you see, not a budget buster.
 
#3
A posh transformer for linear audio can be twice the nominal output. Since the amp might do 250W in 4 Ohms (but how long??) this is 500VA per channel. This will support steady FULL power abuse such as bench-test or shaker-table. In "high fidelity" speech/music the signal is peaky and we do not average even 10% of maximum power. The PT demand does not fall off so fast; however it is entirely rational to design VA equal or even a bit less than audio Watts, and ignore your lowest impedance. So as small as 150VA. I agree 250VA seems fine until testing reveals problems (sag in heavy abuse OR sag in pocketbook or shelf from PT weight/cost).
 
#4
PRR said:
A posh transformer for linear audio can be twice the nominal output. Since the amp might do 250W in 4 Ohms (but how long??) this is 500VA per channel. This will support steady FULL power abuse such as bench-test or shaker-table. In "high fidelity" speech/music the signal is peaky and we do not average even 10% of maximum power. The PT demand does not fall off so fast; however it is entirely rational to design VA equal or even a bit less than audio Watts, and ignore your lowest impedance. So as small as 150VA. I agree 250VA seems fine until testing reveals problems (sag in heavy abuse OR sag in pocketbook or shelf from PT weight/cost).
Click to expand...

Hello,

The L20.5 is supposed to be a Class AB amplifier "capable" of 125 watts @ 8 ohm, 250 watts @ 4 ohm, 500 watts @ 2 ohm with a DC voltage of ~50 (~35 AC into transformer).

This will never be use used on a speaker lower than 4 ohm.

While money is always a factor, an extra $50 or $100 spent on this project really doesn't matter. I would prefer to build it "better" rather than "cheaper". The case and my time will be the two largest costs in this build.

I don't plan on using this amp to blow the doors off the house either...
 
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