Class D amp module build, silly power...

[Reid Malenfant]

Right, i have established that both modules are 100% working I'd have prefered to test them on a current limiting PSU but i don't have one handy, that will have to be remedied

I simply put a resistor (47ohm) in each power line except for the ground connection & connected it up to the PSU of the Fisher amplifier i mentioned earlier. No smoke, LEDs on each module lit up. Low 1mV DC offset with approximately 0.1mV of AC noise on each output. To test if they amplified i fed them a bit of good old AC hum by injecting it via a screwdriver & finger

Not very scientific but it did the job Now to get a decent PSU rigged up & some real testing

E2A:- I have purchased a 500VA 0-35V 0-35V toroidal transformer which should do adequately for the PSU. The result should be at least 120W RMS with an 8 ohm load & hopefully 200W RMS into 4 ohm per channel. I have a nice pair of 33,000uf 100V computer grade electrolytics & i know i'll have a 35A bridge here somewhere, so effectively the PSU just needs putting together now

[MartinT]

Low 1mV DC offset with approximately 0.1mV of AC noise on each output.

That's probably good CMRR, Mark. Have you looked at the output on the 'scope, I'd like to know whether you can see any switching noise? What's the frequency of operation?

Not in the same league, I know, but I'd like to do the same with the little Mini-T amp I bought for my computer. It certainly sounds good but I'd like to get the scope on it.

[Reid Malenfant]

I'll find all this out when they are connected to a decent PSU Martin I just wanted to make sure they both worked before setting up a load of kit:-

Function generator / frequency counter
Oscilloscope
4 & 8 ohm test load
AC voltmeter
Digital temperature guage

What you need to remember is that any T amp is going to vary the switching frequency depending on the output power This is entirely what class T is about, at low output power the switching frequency is likely to be 1 - 1.5MHz to decrease distortion. At higher powers they go to as more conventional 300 - 400KHz to increase efficiency as switching losses would be too high at 1.5MHz.

These as far as i'm aware are just plain old class D with a fixed frequency of switching, but they could well be free running self oscillating units in which case the switching frequency might well vary.. Whatever i'll find out & let you know.

[Reid Malenfant]

E2A:- I have purchased a 500VA 0-35V 0-35V toroidal transformer which should do adequately for the PSU. The result should be at least 120W RMS with an 8 ohm load & hopefully 200W RMS into 4 ohm per channel. I have a nice pair of 33,000uf 100V computer grade electrolytics & i know i'll have a 35A bridge here somewhere, so effectively the PSU just needs putting together now

& here are the rather large electrolytics i was refering to Everything is pretty small except for these, the toroidal transformer i bought isn't that big either. If i'm going to fit these in a case i think i'll turn them sideways so as to make things as small as possible





These are a bit overkill but that's the way i like it. They were removed from a 500W RMS per channel power amp, think i'll use some of the heatsinking from the same amp to give the switching mosfets on the modules an easier time..

[MartinT]

With the peak charging current using those huge caps, you're going to have to use some seriously rated fast rectifiers. Do you have them yet?

[Reid Malenfant]

Hundreds I have more components than a lot of repair shops Martin, i kid you not For testing i'll use a standard 35A bridge, but when i come to build a proper stereo amp i'll use MUR3040PT diodes, probably 8 rather than 4 as that will give a 60A rating.

I have also contacted an ex work colleague about some different 63V rated Capacitors, 100V is totally OTT for a +/- 50V DC supply & i might be better off saving them for something more appropriate.

[MartinT]

I used to buy those bargain bags of components back when I was a teenager and built up boxes of good stuff and experimented a lot with amps and other circuits. My favourite project was light-beam communications using converted torches and opto sensors (made from standard can transistors with the tops sawn off and filled with liquid plastic to form a lens). My mate and I could get it working across a good sized field. Many of my ideas came from Elektor, ETI and Wireless World.

All that has gone now, disposed of over a series of house moves.

[Reid Malenfant]

I know where you are coming from Martin

Unfortunately due to circumstances beyond my control () it'll be a while before i get these properly tested. I had a bit of a fit one day last week when the 230V - 0 35V 0 35V 500VA transformer arrived & it turned out to be 230 - 0 45V 0 45V I honestly couldn't believe it as the outside of the box had the bloody secondary voltages on it

Anyway, having calmed down a bit i came to realise this could actually be advantageous given a bit of ingenuity

What i'm going to do is put some incredibly low Rds ON power mosfets between the bridge rectifiers & capacitors so i can open the circuit at the desired voltage on the storage capacitors. This means i'll be able to switch between 200W RMS into an 8 ohm load at high voltage & 200W RMS into 4 ohms (maybe a tad more) at a lower capacitor voltage. The circuit is pretty much sussed out in my head so it's just a case of waiting for the power mosfets to arrive from the USA. I'll be paralleling two of them up to lower losses as the pulsed current into the capacitors will no doubt be a bit high

Sorry for the delay, back to it ASAP