Re: Need ideas for 8-channel output

Doug B. wrote:

Hello all,

I am getting started in electronics and would like some design advice
for a circuit that I have built, but am trying to improve.

This project involves a simple, 8-channel power amplifier. The input
will be a 1-10 Hz, audio-level control signal that will be either a
pure sine wave or something very close to it. The output stage will
drive a mainly resistive load (NOT audio at all, just driving lamps,
solenoids or similar). My current design and prototype uses IRF-511
power MOSFETs, driven in switching mode from a 13.2 V regulated power
supply.

The client is a artist friend of mine who uses sub-audio range signals
to control a bunch of different gadgets he has built. Currently he
amplifies these with a stack of car stereo amps that are hard to
transport and only provide four mono channels each. I believe we can
get all eight channels of amplification into a small box, as long as
the output stage is driven PWM-like. I am basically looking for a way
to provide reasonable dynamic range on the output, given the audio-
level inputs.

My current prototype uses an LM124 op amp with > 100db gain to
transform the input into a square wave, directly into the gate of the
IRF-511. This "on or off" amplification is not flexible enough.
My current thinking is that PWM is the way to go on the output, but
how to do that for 8 channels? I have looked into Class D amps, but
workable designs seem over my head in complexity and would require
absurdly large output filters for the low frequencies involved.

I have thought of using an analog Mux and microcontroller with ADC to
sample the levels on the 8 input channels. Given the low frequencies
this should be fairly straightforward.

I'm not sure how to handle the output, however. I could create a PWM
signal in the uController and analog Mux it out to an op amp buffer/
gate driver, but I would only get 1/8 of the PWM cycle on each
channel.

Ideas, anyone?
Thanks,

doug beeson

Maybe that'll help you out.
http://focus.ti.com/lit/an/sloa031/sloa031.pdf
you could make a board that holds a single Class D IC and power driving
section, out source the boards to be etched and drill and simply make a form of card rack to support the number of channels desired.

This rack would have one main power supply for it.

Just and idea..


.

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