Sunday, 23 August 2015

Soundlab CPA 100 Mobile power amp, and surprising output matching.

Now, I needed a mobile amp, 100 watt-ish for a project. Didn't I have something in the loft? Yep, a cheap-and-cheerful Soundlab CPA 100. 75 watts allegedly. That'll do. It had a label on it. "Been reverse poled" ... ugh. Let's have a look.

I wasn't holding out much hope really, the fuse was intact. but I was expecting a rake of burnt out resistors, maybe some PCB tracks blown open...

I was, however presently surprised, just a handful of caps had popped their bases, as a result of being effectively connected backwards!   

Anyway, a few cursory checks, and on with the power. Great. It works. Left it running for a few mins, driving a few watts into a dummy load, when I started thinking... why two transformers?

Now normally an automotive power amp needs a few more volts to drive a larger amount of power into a 4 ohm load... Ohms law applies here again, but we're dealing with a sine wave (for RMS watts)

For example, a "proper" class A only amp will swing 6 volts into our 4 ohm load ( a little less in reality, due to the losses in the transistor), so ohms law :

Power = ((Epeak/1.414) x 2) / R where E-peak is the maximum voltage of the sine wave (it will be less than the supply in reality due to transistor losses)
= (6/1.414) x 2 / 4
=4.2 watts

A push pull amp , will swing our full 12 volts into our load, giving ~18 watts. 

There are other amp topologies, such as Class H "Rail switchers" which will give more, but the limit is usually around 50 watts.

Now normally, our big automotive power amp has a step-up switched mode supply, which generates a nice +/- supply of , say 40 volts, so it can swing 80 volts into our load... mucho power ! As long as the supply is up to it! You don't get something for nothing, so the current consumed at 12 volts from the battery would rise proportionally. If the amp with +/- 40 volt supply actually exists, it will generate about 200 watts in a 4 ohm load, and will need 200 watts (plus a bit for losses in the power supply) from our 12 volt source (That's 16 2/3 amps) In Bridge-tied load, we're looking at more like 750 watts.

So, back to our little Soundlab. Are the transformers a power supply? Nope. So how does it generate more power than it's push pull design allows for?

Those transformers are output transformers. They lower the impedance the output stage sees from the speaker. So , to swing enough current from a 12 volt supply into our load, the load needs to be small.

Let's work it out...

Power = (Epeak/1.414)x2  / Resistance , so 
Resistance = (Epeak/1.414) x 2/Power
= (12/1.414)x2/75 
= 0.226 Ohms.

So our two output transformers change our 4 ohm load to a 0.226 ohm load, which allows more current to be drawn from the output stage, and more voltage to swing into the load.

There's some quite heavy negative feedback from the speaker side to tame an otherwise cheap amplifier into giving reasonable performance, and it doesn't make 75 watts RMS, but more like 75 watts when bridged, so will probably give me sufficient power for my needs. The plus side to having a transformer coupled output is if the output transistors should fail, there will never be DC on the speaker!

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