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"Inverted" (not 'Inverting') Output Stage
"Inverted" (not 'Inverting') Output Stage
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Old 15th November 2011, 07:56 PM   #1
Preamp is offline Preamp  Germany
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Default "Inverted" (not 'Inverting') Output Stage

Hello everybody,

I'm looking for some information regarding an intriguing output stage used by the manufacturer QSC. They equip their power amps with a circuit which allows them to connect the output transistors directly to the grounded heat sink, without the use of any insulation inbetween. I've highlighted some connections in the schematic which have been called drawing errors in a german forum, but obviously they're not (I can tell since I own two of that amps).
What I'm actually asking for is some more information about that topology. It seems interesting to me to design a smaller amp that way, but I want to understand how the whole thing works first . Temperature compensation is done with a 50 Ohm PTC which is not easily acquired - is there another way using transistors like in other amps?
I have tried a simulation run in LTspice, but it seemed that the stage is not unity gain like other o/s-es. Unfortunately I don't have enough experience to figure out how this thing does what it does, so any hints or literature would be highly appreciated!

Thanks in advance.
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Old 15th November 2011, 08:04 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Its a complementary follower pair output stage, and it can have gain.
Normal "amplified diode" thermal compensation can be used.

rgds, sreten.

Last edited by sreten; 15th November 2011 at 08:07 PM.
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Old 15th November 2011, 08:12 PM   #3
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by sreten View Post
Its a complementary follower pair output stage, and it can have gain.
I've noticed that it looks much like a CFP, but you can't simply swap the collector outputs with ground and everything works fine. What's the deal with that?
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Old 15th November 2011, 08:16 PM   #4
Wavebourn is offline Wavebourn  United States
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"Inverted" (not 'Inverting') Output Stage
This trick allows to eliminate insulators decreasing thermal resistance.
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Old 15th November 2011, 08:17 PM   #5
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by Wavebourn View Post
This trick allows to eliminate insulators decreasing thermal resistance.
Obviously . But how?
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Old 15th November 2011, 08:18 PM   #6
Elvee is online now Elvee  Belgium
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It is basically a trick "ą la circlotron", swapping active elements and power supplies.

A bit intriguing when you first see it, but not rocket science, and it does have many drawbacks to be balanced with the one small advantage: the need for floating supplies for each channel, the common mode capacitance of the supplies, etc. Not worthwhile, IMHO: Iso-cases are a much superior solution if you don't want to mess with isolating washers, etc.
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Old 15th November 2011, 08:23 PM   #7
Nico Ras is offline Nico Ras  South Africa
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I have seen this configuration in power inverters, each half is driven anti-phase with the other. Heat sink is a common supply line and the speaker is connected at a virual ground (1/2 Supply). Itis realy not that uncommon.
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Old 15th November 2011, 08:23 PM   #8
Preamp is offline Preamp  Germany
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Originally Posted by Elvee View Post
(...) and it does have many drawbacks to be balanced with the one small advantage (...)
Now this is something I didn't know, but actually would like to know about . Isn't there anybody out there who wrote an article or mentions some of the drawbacks etc. in a book?
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Old 15th November 2011, 08:24 PM   #9
Nico Ras is offline Nico Ras  South Africa
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Elvee I say SNAP
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Old 15th November 2011, 08:53 PM   #10
Frank Berry is offline Frank Berry  United States
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I've seen the same type circuit in a Jukebox amplifier.
The output transistor power supplies must be floating.
The design requires a third supply for the rest of the amplifier.
All in all, it's probably simpler to use a more standard amplifier design.
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