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

With the speeds that slow, any microcontroller can demodulate the 
control signal and make 8 channels of PWM by bit banging.

VLV




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

On a sunny day (Wed, 1 Jul 2009 09:21:55 -0700 (PDT)) it happened "Doug B."
<c...@gmail.com> wrote in
<d...@n19g2000vba.googlegroups.com>:

>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

Stndard way is use 2 quad opamps as 8 comparators,
audio into one input of each, the other inputs conneted together to a 
sawtooth wave that you make with an other opamp integrator perhaps.
The output will then be 8 PWM channels with 50% duty cycle for zero
audio.



ch 1 input ---- -   
                   out -------- PWM
       ramp --- +

         8 x


.          .
.        . .
.      .   .
.     .    .     . ---- 0V
.   .      .   .
.-.------- . .-------- slice level if audio a bit negative 
.          .

    _______   ______
|__|       |__|         PWM out


Do you know how to make a ramp waveform?

The opamp output can directly drive a suitable MOSFET.

No micro needed.

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..

On Wed, 1 Jul 2009 09:21:55 -0700 (PDT), "Doug B."
<c...@gmail.com> 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

This might work:

Generate one common triangle or sawtooth, maybe 0.5 to 4.5 volts
swing, whatever PWM frequency you like.

Each channel is a comparator driving a mosfet gate, directly or maybe
through a gate driver chip.

Each comparator compares an input (in the 0-5 volt range) to the
shared triangle thing, and generates a 0-100% duty cycle as the input
ranges from 0.5 to 4.5.

Something like that, scaled to your exact needs.

John

Vladimir,

Yes, I guess I could use a controller with 8 output pins. I was
thinking of one of the smaller PICs that only has 4 or 5 output pins
available and trying to Mux the output. But a bigger controller would
be simpler, I guess.

Thank you,
doug

On Jul 1, 12:52=A0pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:
> With the speeds that slow, any microcontroller can demodulate the
> control signal and make 8 channels of PWM by bit banging.
>
> VLV
>
> 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

John and Jan,

Your suggestions of comparator + sawtooth/triangle generator are
exactly what I had originally considered, but I got stuck on how to
scale the audio properly to avoid the 50% PWM duty cycle with no audio
signal present.

I am trying to do this with a single-ended supply. I know how to bias
the audio up into, say 2.4 V (my current prototype biases the op amp
using a reference based on a 2.4V zener), but that means the output
with no audio present is also 2.4 V. Wouldn't the PWM be at 50% if I
centered the triangle wave around the bias level?

I sense that there is a straightforward, standard way around this that
I am not understanding.

Thanks for your help!

doug


On Jul 1, 2:12=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Wed, 1 Jul 2009 09:21:55 -0700 (PDT), "Doug B."
>
>
>
> <c.diffic...@gmail.com> 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
>
> This might work:
>
> Generate one common triangle or sawtooth, maybe 0.5 to 4.5 volts
> swing, whatever PWM frequency you like.
>
> Each channel is a comparator driving a mosfet gate, directly or maybe
> through a gate driver chip.
>
> Each comparator compares an input (in the 0-5 volt range) to the
> shared triangle thing, and generates a 0-100% duty cycle as the input
> ranges from 0.5 to 4.5.
>
> Something like that, scaled to your exact needs.
>
> John

On Jul 1, 7:05=A0pm, "Doug B." <c.diffic...@gmail.com> wrote:
> John and Jan,
>
> Your suggestions of comparator + sawtooth/triangle generator are
> exactly what I had originally considered, but I got stuck on how to
> scale the audio properly to avoid the 50% PWM duty cycle with no audio
> signal present.
>
> I am trying to do this with a single-ended supply. I know how to bias
> the audio up into, say 2.4 V (my current prototype biases the op amp
> using a reference based on a 2.4V zener), but that means the output
> with no audio present is also 2.4 V. Wouldn't the PWM be at 50% if I
> centered the triangle wave around the bias level?
>
> I sense that there is a straightforward, standard way around this that
> I am not understanding.
>
> Thanks for your help!
>
> doug
>
> On Jul 1, 2:12=A0pm, John Larkin
>
> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> > On Wed, 1 Jul 2009 09:21:55 -0700 (PDT), "Doug B."
>
> > <c.diffic...@gmail.com> 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
>
> > This might work:
>
> > Generate one common triangle or sawtooth, maybe 0.5 to 4.5 volts
> > swing, whatever PWM frequency you like.
>
> > Each channel is a comparator driving a mosfet gate, directly or maybe
> > through a gate driver chip.
>
> > Each comparator compares an input (in the 0-5 volt range) to the
> > shared triangle thing, and generates a 0-100% duty cycle as the input
> > ranges from 0.5 to 4.5.
>
> > Something like that, scaled to your exact needs.
>
> > John
>
>

I don't see what the big deal is? Why not use bjt's? It's not
efficient but who cares? How much power are we talking about here?

On Wed, 1 Jul 2009 17:05:10 -0700 (PDT), "Doug B."
<c...@gmail.com> wrote:

>John and Jan,
>
>Your suggestions of comparator + sawtooth/triangle generator are
>exactly what I had originally considered, but I got stuck on how to
>scale the audio properly to avoid the 50% PWM duty cycle with no audio
>signal present.
>
>I am trying to do this with a single-ended supply. I know how to bias
>the audio up into, say 2.4 V (my current prototype biases the op amp
>using a reference based on a 2.4V zener), but that means the output
>with no audio present is also 2.4 V. Wouldn't the PWM be at 50% if I
>centered the triangle wave around the bias level?
>
>I sense that there is a straightforward, standard way around this that
>I am not understanding.
>
>Thanks for your help!
>
>doug
>


Then I don't understand the transfer function that you want.

And please don't top post here. See what a mess it makes?

John



>
>On Jul 1, 2:12 pm, John Larkin
><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> On Wed, 1 Jul 2009 09:21:55 -0700 (PDT), "Doug B."
>>
>>
>>
>> <c.diffic...@gmail.com> 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
>>
>> This might work:
>>
>> Generate one common triangle or sawtooth, maybe 0.5 to 4.5 volts
>> swing, whatever PWM frequency you like.
>>
>> Each channel is a comparator driving a mosfet gate, directly or maybe
>> through a gate driver chip.
>>
>> Each comparator compares an input (in the 0-5 volt range) to the
>> shared triangle thing, and generates a 0-100% duty cycle as the input
>> ranges from 0.5 to 4.5.
>>
>> Something like that, scaled to your exact needs.
>>
>> John

On Jul 1, 8:33=A0pm, "Bob.Jones5...@gmail.com" <bob.jones5...@gmail.com>
wrote:
> On Jul 1, 7:05=A0pm, "Doug B." <c.diffic...@gmail.com> wrote:
>
>
>
> > John and Jan,
>
> > Your suggestions of comparator + sawtooth/triangle generator are
> > exactly what I had originally considered, but I got stuck on how to
> > scale the audio properly to avoid the 50% PWM duty cycle with no audio
> > signal present.
>
> > I am trying to do this with a single-ended supply. I know how to bias
> > the audio up into, say 2.4 V (my current prototype biases the op amp
> > using a reference based on a 2.4V zener), but that means the output
> > with no audio present is also 2.4 V. Wouldn't the PWM be at 50% if I
> > centered the triangle wave around the bias level?
>
> > I sense that there is a straightforward, standard way around this that
> > I am not understanding.
>
> > Thanks for your help!
>
> > doug
>
> > On Jul 1, 2:12=A0pm, John Larkin
>
> > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> > > On Wed, 1 Jul 2009 09:21:55 -0700 (PDT), "Doug B."
>
> > > <c.diffic...@gmail.com> wrote:
> > > >Hello all,
>
> > > >I am getting started in electronics and would like some design advic=
e
> > > >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 powe=
r
> > > >supply.
>
> > > >The client is a artist friend of mine who uses sub-audio range signa=
ls
> > > >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 wa=
y
> > > >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 th=
e
> > > >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 t=
o
> > > >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
>
> > > This might work:
>
> > > Generate one common triangle or sawtooth, maybe 0.5 to 4.5 volts
> > > swing, whatever PWM frequency you like.
>
> > > Each channel is a comparator driving a mosfet gate, directly or maybe
> > > through a gate driver chip.
>
> > > Each comparator compares an input (in the 0-5 volt range) to the
> > > shared triangle thing, and generates a 0-100% duty cycle as the input
> > > ranges from 0.5 to 4.5.
>
> > > Something like that, scaled to your exact needs.
>
> > > John
>
> I don't see what the big deal is? Why not use bjt's? It's not
> efficient but who cares? How much power are we talking about here?

Peak power per channel is 25-30 W. Average is much lower - I'm not
sure exactly how much - but could be in the low single digits as the
input tends to be periodic pulses.
As I mentioned, my friend already uses two fairly bulky car stereo
amps to do the job. These are BJT-based, I assume, which explains the
big heat sinks. I was hoping to get the same output (minus the audio
quality) from a much smaller board and enclosure.
I also have to admit that part of me wants to use a switched solution
simply to learn how it works.

doug