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Old 10th January 2006, 10:00 PM   #11
poobah is offline poobah  United States
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Good point Eva,

I just finished a design for long production... I had the choice of high voltage CMOS or a small uP with bipolar buffers. I had to choose for the new uP because the manufacturers won't promise the future of HV CMOS. Time will tell if I made the right choice.

But you are very right... some old devices were made/designed so well in the first place... there is not much much to improve.

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Old 10th January 2006, 10:15 PM   #12
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Thanks, dear members.

Although i don't have a schematic yet, the application is a switch to connect the power output transistors of a power amplifier to a higher voltage supply during audio peaks (you call this class G or class H I think).

So one input of the comparator has a reference voltage to it (DC). The other, has an audio signal coming to it.
The output would feed a MOSFET driver, driving a couple of large MOSFETS in parallel.

Because there are already some propagation delays imposed in the circuit by the mosfets and driver, I really can't afford to have too much delay in the comparator. 100ns is too much.

I was looking at the LM360, and the specs look right. But I don't have an LM360, and I do have an LMH6655. Everything about the opamp seems to be better than the comparator, in my naive evaluation.
(200V/uS slew rate, 180mA output current, 250MHz bandwidth...)

Joseph
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Old 10th January 2006, 10:28 PM   #13
poobah is offline poobah  United States
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It can (might) be done... you will have to take some measures to ensure that the output is not allowed to saturate... otherwise there will be significant delay between "a comparison" and a cooresponding output change. This will become easier to understand once you have a schematic.

And as :King:Anatech points out; check the input specifications carefully... look for "differential input voltage" and be sure not to exceed it. Doing so may not burn the amp, but the output could reverse or other crazy things could happen.

A comparator might still be your best choice though... there are ways to "anticipate" the signal and allow for some delay.

Is this all about improving dynamic headroom without the additional heat?
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Old 10th January 2006, 10:34 PM   #14
anatech is offline anatech  Canada
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Hi Joseph,
Can you do the switching in the analog domain? Most other designs use transistors. There isn't much need to be faster than the audio frequency. Unless you will go mids or highs only the large swings will be lower frequency notes. Similar to Carver, but he held his rail up for a minimum time.

Another consideration is that abrupt switching will induce spikes in the output. Avoid.

-Chris
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Old 10th January 2006, 10:45 PM   #15
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Quote:
Is this all about improving dynamic headroom without the additional heat?
Like you read my mind... Why is there a simpler way of doing this than what I was thinking?

Quote:
Can you do the switching in the analog domain?
What do you mean?
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Old 10th January 2006, 10:47 PM   #16
anatech is offline anatech  Canada
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Hi Joseph,
For ideas, look at a later model Carver schematic. No logic or comparators used at all.

-Chris
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Old 10th January 2006, 11:47 PM   #17
poobah is offline poobah  United States
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Chris & Joe,

Weren't NAD amps similar in some degree... not in technique maybe... but in the end result? I know there is another thread here where there was good discussion about headroom techniques. Although, I think the concensus reached... was screw it, build a bigger amp.

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Old 10th January 2006, 11:52 PM   #18
anatech is offline anatech  Canada
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Hi poobah,
Carver did it right. NAD, well okay, they got the effect. I was never impressed with the quality of NAD. The same can be said for others.

Properly switching the supplies requires some engineering to get right. More trouble for DIY, okay for a big lab such as Carver corp. I'd say there were other factors that determined the Carver sound that had little to do with supply switching. On earlier products you could certianly hear it.

-Chris
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Old 11th January 2006, 12:35 AM   #19
poobah is offline poobah  United States
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Oh... I was never of a fan of NAD. As far as that goes, I wouldn't like God's best amp if it were named GROIN...

I was just thinking there might be ideas for Joe. I've been pondering this... just how do you know a big power demand is coming the moment before it does?

Maybe roll off the highs (only in the trigger circuit) and set the trigger at 50-70% percent of the "regular" rail voltage?

And Chris is absolutety right; you'll have to kick in the boost supply REAL smooth to avoid audible trash going into the drains, collectors, plates etc...

hmmm... I think part of your answer lies in there Joe... this is interesting, lemme do some calcs...
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Old 11th January 2006, 12:40 AM   #20
anatech is offline anatech  Canada
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Hi poobah,
Just drive the "commutator" with the input signal raised 5 ~ 10 V above the waveform. As the signal amplitude increases, the supply voltage starts rising above a certain level. The higher level will predict the audio signal in a way.

This is really cool to watch in operation using a couple 'scopes.

-Chris
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