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Old 21st November 2008, 08:33 AM   #21
Werner is offline Werner  Europe
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"Isolate the op amp from the capacitance of the transitor base, right?"

More like ensuring that the transistor base never sees an inductive
drive impedance.

BTW, here's mine, including the rectifier and emitter-follower pre-regs. There are also SMD parts on the other side of the board.

I like the idea of crosscoupling the Vrefs, although it gets a bit costly once you employ really low noise (expensive!) references.
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Old 23rd November 2008, 12:06 PM   #22
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Time to put what I have so far up for public comment. You can click on the image to see a larger version.

Click the image to open in full size.

I've shown the circuit in a slightly odd way: please note that the non-inverting terminals of both op amps are grounded. The numbers by the ground symbol indicate the order in which the grounds should be connected to each other. For non-Eagle users: the "X" symbol indicates a physical connection to the outside, an input or an output. The -> arrows are internal : "-> V-" connects to all the other sections of the schematic marked with the same label. The inverted "T" GND is an internal ground indicator in the same manner.

See if I'm on the ball with the maths:

The gain of the op amp stage is R2 / (R1+R3) = 68/2 = 34

The reference voltage (voltage between C4 and R4) is (approximately, ignoring R6) Vin * (R1||R2) / (R4+R5+R1||R2) = Vin * 500 / 20500 = Vin 1/41

The output voltage is Gain * Reference = Vin 34/41 or 83% of the input. e.g. input 18 V output 14.9 V.

* Re. your schematic, the odd feedback loop and the unsymmetrical circuit really threw me at first. I see it now: single, highly filtered reference, noninverting for the positive output, inverting for the negative output, both configured for a gain of 2. Earlier, I missed the fact that the second op amp was inverting... your "Looking more and more like mine now" comment makes more sense now...

* Was there any particular reason to choose the BD137 series of transistors? I'm wide open to suggestions.

* Comments, suggestions, and error-reporting welcome.
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Old 23rd November 2008, 12:27 PM   #23
Werner is offline Werner  Europe
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> The gain of the op amp stage is R2 / (R1+R3) = 68/2 = 34

The gain is -R2/R1. The opamp is not aware of R3.

> The reference voltage (voltage between C4 and R4) is (approximately, ignoring R6) Vin * (R1||R2) / (R4+R5+R1||R2)

Rather (R1//R3) / [(R1//R3)+R4+R5+R6]

> * Was there any particular reason to choose the BD137 series of transistors? I'm wide open to suggestions.

Yes. It's the most widely-available medium power transistor over here.
You can buy them at the grocery store.
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Old 24th November 2008, 02:40 AM   #24
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Quote:
Originally posted by Werner
> The gain of the op amp stage is R2 / (R1+R3) = 68/2 = 34

The gain is -R2/R1. The opamp is not aware of R3.
Surely the gain is -R2/ (R1+Z) where Z is the output impedance of the previous stage, which in this case is the voltage divider that has an output impedance at DC of approx. R3?
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Old 24th November 2008, 05:51 AM   #25
Werner is offline Werner  Europe
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Then tell me why a Vin = -18V tries to bring the output
to +29.5V in simulation?

Your error is in that the apparent output impedance of the
reference ladder does not appear in series with R1.
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Old 24th November 2008, 02:02 PM   #26
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My mistake, I double dipped: Counting R1 as part of the voltage divider network, I should not have then counted the network impedance towards the op amp gain.

Fix: change R2 to 33k.

Most recent update of the Phonoclone 3 files attached. (capacitors C1 and C2 replaced with pads only as my CD silver mica caps have thick leads that don't fit in any of the standard Eagle caps in the build in libraries, and I'm too lazy to make a new part.)
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Old 25th November 2008, 11:27 PM   #27
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minor changes
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Old 25th November 2008, 11:28 PM   #28
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and a picture, top trace not shown
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Old 25th November 2008, 11:37 PM   #29
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For beta-testers:

I am ordering 5 sets of Phonoclone 3 pcbs from Olimex. Cost will be $40 US ea. including shipping, to be paid by Paypal, 2 sets are still available.

email / pm me if interested.

/R
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Old 26th November 2008, 11:26 PM   #30
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Werner,

There's a set of boards with your name on them if you want: it's the least I can do in return for the help and advice given out on this thread. Just let me know your address.

/R
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