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Old 23rd November 2012, 01:33 PM   #11
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Output buffer degrade distortion figures, and at higher output more. I tried different output buffer, lineup between others but I am back on simple JLH modified one, similar to the input buffer.
With low inputs as 0.1 V the distortion is very low at all frequencies, degraded a bit by the output buffer, but still excellent and this is no global negative feedback used. A kind of negative feed back is used in the buffers but it is very local.
Where to put volume control? It is possible to use Current conveyor’s output resistor for that, it is of low value and it will need custom volume control.
It is possible to use input volume control of 50k to 100k and those kind are ready made available. The gain between 5 and 10 is a good value for line amplifier, and more the enough to push any output amplifier into clipping.
dado
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File Type: jpg THD1k-1V-input.jpg (187.4 KB, 336 views)
File Type: jpg THD20k-0.1V-input.jpg (192.3 KB, 306 views)
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Old 23rd November 2012, 02:27 PM   #12
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If I see correctly you are running Q6 and Q8 at 20mA. These are dissipating 700mW and burning up. These can be C3503/A1381, with the Baxandall they will work equally well. Another option is to use paralleled BC5x0C for Q6 and Q8, with small degeneration resistors so they share equally, and this is fully compatible with the Baxandall as well.

A warning is in order for the current mirrors. At low currents all diode junctions have virtually perfect logarithmic conformance, but at too extreme bias the log conformance will fail. This will cause current mirrors to distort. If so you should try decreasing Iq.

However if the 20mA output Iq works, I think you can reduce distortion by increasing R3 and R4 to 2k and R5 and R6 to 1k. R12 will need readjusted for 3rd harmonic null if desired.
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Old 23rd November 2012, 03:29 PM   #13
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This one is closer to the practical implementation. Distortion is as before, used more powerful transistor for Q6 and Q8, output buffer still in class A but with less dissipation.
Kean I triad your suggestion and could not get better result. I think this here is more then good, the sound should be good.
FFT are for 0.1 and 1 input voltage.
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Old 23rd November 2012, 04:04 PM   #14
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How is PSRR?

You are right it can drive an amp to clipping. It could drive an amp to burning. I hope that is not a "feature".

Antiparallel diodes between the output buffer input and output will provide a current limit for the output, but may only work for headphones.

I would limit output voltage to 2V; I see no need for more than that. I would illustrate this with a schematic but I don't have access to a schematic program right now.

Imagine a PNP transistor, emitter at the output. Base at the same point as the base of Q3. Collector to the junction of R3 and R4. Mirror with an NPN for the negative side. Add diodes in series with emitters optionally to prevent excessive Vbe reverse bias. If it is unstable during clipping, try resistors in series with the emitters.
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Old 23rd November 2012, 04:15 PM   #15
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I did not try to simulate PSRR yet, and not sure how to do that??
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Old 23rd November 2012, 04:36 PM   #16
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Go to AC simulation. Turn off the AC for the signal source. Right click on a rail source (V1) and make it the AC source. Run the simulation and plot Vout. Plot the equation 1/Vout. Switch rail sources, try again. Finally make both rails AC source and again plot 1/Vout. This will be your PSRR in db. If your rails have a second harmonic at -30db, and your PSRR is 50db, the second harmonic will appear at -80db at the output even if the circuit has very low distortion, because the distortion is conducted from the rails. Rail filtering and/or regulation is mandatory to bring out this circuit's potential. Another reason lower rails could really help. MC7x15 are a good idea I think.
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Old 23rd November 2012, 09:31 PM   #17
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Somethink like this?
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Old 23rd November 2012, 09:44 PM   #18
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Yep, like that. In chipamp datasheets, PSRR is plotted separately for each rail. It is typically between 60db and 80db.

Is the circuit powered for that plot? It looks like you turned the rail sources into 0V signal sources. You need to enter your rail voltage as the DC offset.

Have you tried the limiter I described in post 14?
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Old 23rd November 2012, 09:51 PM   #19
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This is with simple C multipliers.
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Old 23rd November 2012, 10:06 PM   #20
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Nice. Sometimes a diode junction on the rails will generate enough of its own distortion to affect the output through the rail, so we're not out of the woods yet. If this proves an issue I fear a Kmultiplier or chip reg will be necessary. See if the Cmultipliers decrease distortion.

Again, check that V1 and V2 are actually at the correct rail voltage... The PSRR thing only works if the circuit is powered. That doesn't look right to me.
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