QSC RMX1450 schematic for a.a.p.l-s
 

QSC RMX1450 schematic for a.a.p.l-s
 

John Larkin wrote:

Eeyore wrote:

Stunningly bizarre!

Do elaborate. I know the circuit very well actually. I've improved it hugely in
a series of amplifiers I designed also using the 'grounded collector'
arrangement.

Graham

QSC RMX1450 schematic for a.a.p.l-s
 

On Sat, 31 Mar 2007 11:33:40 +0100, Eeyore
wrote:

Stunningly bizarre!

John

QSC RMX1450 schematic for a.a.p.l-s
 

John Larkin wrote:

Eeyore wrote:
John Larkin wrote:
Eeyore wrote:

Stunningly bizarre!

Do elaborate. I know the circuit very well actually. I've improved it hugely in
a series of amplifiers I designed also using the 'grounded collector'
arrangement.

Where to begin?

Is this amp supposed to be based on the patent you posted? It isn't.

The patent relates to a protection method, not the amplifier design. This amplifier
does indeed use that protection method.


The output stage is magnificently twisted, just to implement a silly
and inefficient transistor protection scheme.

Actually it's done that way for a completely different reason AIUI.


The same algorithm could be implemented much more quantitatively with a few opamps
and diodes, off the side of the main signal path instead of all tangled with it.

Do go on.


Beta-dependent design sucks.

I agree. Those trimpots are loathesome.


Way too many parts. Must be hell to work on.

It actually has a far lower parts count than most amplifiers of that variety.


Before-feedback output impedance must be insane, leading to
interesting dynamics.

Uhuh. It's variable shall we say ?


I've seen this phenom elsewhere. Somebody gets an idea, falls in love
with it, and bends the entire design around it, usually badly.

It was Pat Quilter's trademark for ages. They've finally adopted emitter followers
now.


I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Graham

QSC RMX1450 schematic for a.a.p.l-s
 

John Larkin wrote:

Eeyore wrote:

I've seen this phenom elsewhere. Somebody gets an idea, falls in love
with it, and bends the entire design around it, usually badly.

It was Pat Quilter's trademark for ages. They've finally adopted emitter followers
now.

Horray! Give them another 20 years, and they may discover fets!

Personally I love the lateral 'audio fets' that Hitachi originated. Hitachi no longer
make them although there are some other suppliers. They are *very* expensive though
compared to bipolars..


I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're happy
with.

Here's a question for you.

Given a classic single channel of Class AB audio amplification with a rated output
power of say 600 watts, what kind of thermal resistance would you require to allow it
to continue to operate safely in typical use with an ambient air temp of up to 40C. Use
any sensible 'assumptions' that you feel are required to calculate this..

Graham

QSC RMX1450 schematic for a.a.p.l-s
 

On Sat, 31 Mar 2007 18:07:26 +0100, Eeyore
wrote:



John Larkin wrote:

Eeyore wrote:

Stunningly bizarre!

Do elaborate. I know the circuit very well actually. I've improved it hugely in
a series of amplifiers I designed also using the 'grounded collector'
arrangement.

Graham

Where to begin?

Is this amp supposed to be based on the patent you posted? It isn't.

The output stage is magnificently twisted, just to implement a silly
and inefficient transistor protection scheme. The same algorithm could
be implemented much more quantitatively with a few opamps and diodes,
off the side of the main signal path instead of all tangled with it.

Beta-dependent design sucks.

Way too many parts. Must be hell to work on.

Before-feedback output impedance must be insane, leading to
interesting dynamics.


I've seen this phenom elsewhere. Somebody gets an idea, falls in love
with it, and bends the entire design around it, usually badly.

I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

John

QSC RMX1450 schematic for a.a.p.l-s
 

John Larkin wrote:

Eeyore wrote:
John Larkin wrote:

I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're happy
with.

But they don't do it intelligently. A proper protection scheme can
deliver a lot more usable power for the same amount of "push."

You're missing the point. The 'push' is determined by the audio signal. Mess with that and
you have distortion.

The amplifier must function unimpeded for any valid combination of input level and output
load.

Graham

QSC RMX1450 schematic for a.a.p.l-s
 

On Sat, 31 Mar 2007 18:51:09 +0100, Eeyore
wrote:


I've seen this phenom elsewhere. Somebody gets an idea, falls in love
with it, and bends the entire design around it, usually badly.

It was Pat Quilter's trademark for ages. They've finally adopted emitter followers
now.


Horray! Give them another 20 years, and they may discover fets!


I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

John

QSC RMX1450 schematic for a.a.p.l-s
 

John Larkin wrote:

Eeyore wrote:
John Larkin wrote:
Eeyore wrote:
John Larkin wrote:

I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're
happy with.

But they don't do it intelligently. A proper protection scheme can
deliver a lot more usable power for the same amount of "push."

You're missing the point. The 'push' is determined by the audio signal. Mess with that and
you have distortion.

You're missing my point. If you can get twice the undistorted output
by using a smarter protection circuit, it's still twice the output.

How can you increase the allowable dissipation with a protection circuit ? Dissipation is
determined by the audio signal and load impedance.


The amplifier must function unimpeded for any valid combination of input level and output
load.

What does "unimpeded" mean for a huge input and a shorted load?

A short isn't a valid load.

Graham

QSC RMX1450 schematic for a.a.p.l-s
 

On Sat, 31 Mar 2007 19:12:00 +0100, Eeyore
wrote:



John Larkin wrote:

Eeyore wrote:

I've seen this phenom elsewhere. Somebody gets an idea, falls in love
with it, and bends the entire design around it, usually badly.

It was Pat Quilter's trademark for ages. They've finally adopted emitter followers
now.

Horray! Give them another 20 years, and they may discover fets!

Personally I love the lateral 'audio fets' that Hitachi originated. Hitachi no longer
make them although there are some other suppliers. They are *very* expensive though
compared to bipolars..


I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're happy
with.


But they don't do it intelligently. A proper protection scheme can
deliver a lot more usable power for the same amount of "push."


Here's a question for you.

Given a classic single channel of Class AB audio amplification with a rated output
power of say 600 watts, what kind of thermal resistance would you require to allow it
to continue to operate safely in typical use with an ambient air temp of up to 40C. Use
any sensible 'assumptions' that you feel are required to calculate this..

Graham

Assume n=60%, 600 watts RMS out, the transistors dissipate 400 watts.
For Tj of, say, 140C, we need a composite Tja of 0.25 K/w.

But audio is not RMS sine waves, any more than NMR gradient pulses
are. A smart protection circuit knows the true transistor dissipation
and the actual heatsink temperature, and understands the thermal time
constants. It has benefits like allowing lots of low-duty-cycle power,
or protecting the parts at high ambients or if a fan fails or
something. There's nothing that's a better thing to limit than actual
junction temperature; anything else is a bad guess of varying badness.

John

QSC RMX1450 schematic for a.a.p.l-s
 

On Sat, 31 Mar 2007 19:57:17 +0100, Eeyore
wrote:



John Larkin wrote:

Eeyore wrote:
John Larkin wrote:

I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're happy
with.

But they don't do it intelligently. A proper protection scheme can
deliver a lot more usable power for the same amount of "push."

You're missing the point. The 'push' is determined by the audio signal. Mess with that and
you have distortion.

You're missing my point. If you can get twice the undistorted output
by using a smarter protection circuit, it's still twice the output.


The amplifier must function unimpeded for any valid combination of input level and output
load.


What does "unimpeded" mean for a huge input and a shorted load?

John

QSC RMX1450 schematic for a.a.p.l-s - Amp.jpg
 

On Sat, 31 Mar 2007 20:41:21 +0100, Eeyore
wrote:



John Larkin wrote:

Eeyore wrote:
John Larkin wrote:
Eeyore wrote:
John Larkin wrote:

I like to protect transistors by digitizing their voltages and
currents, and the heatsink temperature, running a realtime thermal
simulation, and limiting *junction temperature*.

Digitising ? That's not going to be cheap.

Digitizing is cheap, especially when it lets you safely get, say,
twice the usable power from a given mass of transistors and heatsinks.

You don't. Audio amps already push output devices pretty much as far as they're
happy with.

But they don't do it intelligently. A proper protection scheme can
deliver a lot more usable power for the same amount of "push."

You're missing the point. The 'push' is determined by the audio signal. Mess with that and
you have distortion.

You're missing my point. If you can get twice the undistorted output
by using a smarter protection circuit, it's still twice the output.

How can you increase the allowable dissipation with a protection circuit ? Dissipation is
determined by the audio signal and load impedance.

The point is to *know* the actual dissipation, not make a crude guess
about it. Using a simple current limit, foldback limit, or the trick
from the patent, if you want to make your amp reliable in actual use,
you have to set the limits low, because your junction temp
"calculation" sucks and can err wildly depending on input levels and
duration, real/reactive loads, line voltage, and ambient temp.


The amplifier must function unimpeded for any valid combination of input level and output
load.

What does "unimpeded" mean for a huge input and a shorted load?

A short isn't a valid load.

So it's OK for the amp to explode when shorted? A current limit that
protects against a shorted load will severely limit peak power output
into a normal load.

How about this?

17 KW peak power out. Power supply rails programmable from +-75 to
+-175 to match various loads. Intelligent digital
simulated-junction-temperature shutdowns. The P and N fets are
interleaved for best cooling, and the shiny bars are nickel-plated
copper heat spreaders. VF display shows everything: RMS output, power
supplies, temperatures, junction temps, all sorts of stuff.

Protecting power devices is a computational problem, and the most
accurate way to that is digitally. You *could* do the junction temp
simulation with some diffamps and multipliers and such, but since I
had a CPU here anyhow, it was a lot easier to do in code.

John

QSC RMX1450 schematic for a.a.p.l-s
 

"Eeyore" wrote in
message
John Larkin wrote:

You're missing my point. If you can get twice the
undistorted output by using a smarter protection
circuit, it's still twice the output.

How can you increase the allowable dissipation with a
protection circuit ? Dissipation is determined by the
audio signal and load impedance.

Right, but if you can push the parts to higher powers without breaking them
with a more intelligent protection circuit, then it is all good.

The amplifier must function unimpeded for any valid
combination of input level and output load.

What does "unimpeded" mean for a huge input and a
shorted load?

A short isn't a valid load.

A short is very real world. We long ago learned that audio amplfiiers that
can't tolerate shorts tend to have short lives.

QSC RMX1450 schematic for a.a.p.l-s - Amp.jpg
 

amp.jpg

Schematic, please (c:

Seriously, that's a beautiful piece of work, John.
--
John English

QSC RMX1450 schematic for a.a.p.l-s - Amp.jpg
 

On Thu, 05 Apr 2007 17:32:48 GMT, John E. wrote:

amp.jpg

Schematic, please (c:

Seriously, that's a beautiful piece of work, John.

Each one of the 16 fet pairs has its own solid-state relay that can
enable or disable that pair. So at powerup time, or on request, we
ripple down the pairs and make sure each fet has the expected
transconductance gain. The yellow things are fuses; if we ever blow a
fet fuse, we know it.

John