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Old 13th August 2011, 10:12 AM   #21
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Quote:
Originally Posted by Boscoe View Post
Could separate supply rails be used to obtain higher rail voltages for the output stage?
Yes it could, but then you would need a CFP output stage with local feedback for voltage gain. The circuit in my previous post does exactly that.

Here is the rest of the circuit, showing how to derive the lower op-amp supply voltage from the rail voltage. Apologies for the poor quality of the scan.

All of the additional components for dropping voltages and adding feedback makes the circuit more complicated than suggested by the heading for this thread.
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Last edited by ingenieus; 13th August 2011 at 10:28 AM.
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Old 13th August 2011, 11:09 AM   #22
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Another answer on tiefbassuebertr's question on where the current goes. Here is another op-amp based power amp that does it exactly the same. There seem to be nowhere for the diode current to go but into the op-amp's output!

Click the image to open in full size.

Found it on the Elliot Sound Products website:
Opamp Based Power Amp

Can somebody shed some light on this little mystery?

PS: the above topology is ideally suited to the NJL1302/NJL3281 mentioned above. Watch this space.

Last edited by ingenieus; 13th August 2011 at 11:13 AM.
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Old 13th August 2011, 11:16 AM   #23
wahab is offline wahab  Algeria
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Quote:
Originally Posted by ingenieus View Post
Can somebody shed some light on this little mystery?
There is none ...
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Old 13th August 2011, 01:38 PM   #24
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Having an op-amp that sinks current is not a problem as long as you stay within the limits. In this case the current is about 5mA which should be OK.
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Old 13th August 2011, 02:20 PM   #25
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Needs a VAS, badly!!!
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Old 13th August 2011, 02:22 PM   #26
wahab is offline wahab  Algeria
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Quote:
Originally Posted by ingenieus View Post
Having an op-amp that sinks current is not a problem as long as you stay within the limits. In this case the current is about 5mA which should be OK.
Right , but when the op amp output is in full negative swing ,
the current will double from about 6mA to 12mA,
not counting that the negative side has better
current capability than the positive side.

Also, R6 seems quite high , its value should be at most
one or two kilo ohm..

Overall, the design is lacking.

Last edited by wahab; 13th August 2011 at 02:24 PM.
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Old 13th August 2011, 02:25 PM   #27
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Tekko View Post
Needs a VAS, badly!!!
Inserting a VAS is not that trivial.
Stability would be way more problematic.
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Old 13th August 2011, 07:54 PM   #28
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Here is another variation, using Thermaltrak output devices.The power output is up, but so are the prices of the components. The first circuit was really cheap and cheerful, with the op-amp and output trannies just over US$1 each. Now we are looking at US$7 each. The circuit is however free of anything to adjust.

It looks pretty good in LTspice, maybe a little too good to be true. THD for 40W into 8 ohm (1kHz) is 0,0009%. Output into 4 ohm is 70W and for 2ohm it is 100W. There seems to be no need for any compensation.

The circuit will work without R11 or it can be replaced with a pot, as the THD depends on it. Without it the THD (measured as above) goes up to 0,007%. Too large a value and you have a class A amp on your hands. It seems quite sensitive to variation. At 56 ohm it's already in class A.

This is one of those times where simulation shows its limitations. Only real world measurements will tell us what we want to know. Perhaps R11 is best left out. The collector currents below shows the switching distortion that accounts for the difference in THD. On the other hand, measuring this sort of thing in the real world and making adjustments to get the very best possible THD probably requires equipment that few amateurs have.

How true all of this is depends in large part on the accuracy of the SPICE models. The transistor models came from Cordell Audio and the op-amp model is from Texas Instruments. It is therefore as good as it gets. The proof of the pudding is, as always, in the eating. Time to go shopping for components.
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Last edited by ingenieus; 13th August 2011 at 07:59 PM.
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Old 13th August 2011, 08:15 PM   #29
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Quote:
Originally Posted by Tekko View Post
Needs a VAS, badly!!!
Perhaps, but then it will be just another three stage Lin design. I want to see with just how few components you can build a decent amp. Ultimate performance is not important. I want something that is simple, cheap and easy to build. And I want something that I understand. Otherwise it goes like this:

Click the image to open in full size.

"Yes, that is the latest thing in audio technology. It works very well. What goes on inside it is a really complicated complicated trade secret and uhm, er....."
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Old 14th August 2011, 05:32 AM   #30
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Here is a very simple amp schematic that has excellent performance that I published here some five years ago.

Nico
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