Melcor Deconstructed

So...

I've got these nifty Melcor PCB's ( thanks Jay and Labs, and PeterC, you guys rule!), and thought that we should start the new forum off right with some educational information.

Would someone be willing to deconstruct what the parts of this opamp are doing, and why?

schemolingodingdong (from flatpicker):

*updated with a schematic that works*

Well, you see all those transistors there? Those amplify.

And all those resistors are turning electricity into heat.

And those capactors are keeping a few cycles of that electricity around for a rainy day.

:green: :green: :green:

Actually, I would love to see this deconstructed and analyzed.

Well, that's a differential pair at the input, from there I am lost!

I am uncertain if the kind of information you want so here is a stab at it:

Q1/Q2 and Q3/Q4 form two differential amplifiers.
R1 sets the current that gets split between Q1/Q2.
R3 balances currents between Q1 and Q2.
R4 makes sure that the same voltage magnitude is seen by Q4 as Q3 does.
R10 and R5 set the current for Q3/Q4.
R6 biases Q5 into conducting.
R7 provides emitter degeneration for Q5.
Q5 is the third and last voltage amplifier stage. (There is a lot of gain in this amp)
Q6 serves as a constant current source for Q5 and keeps the bias of Q7/Q8 somewhat in check.
D2 provides some biasing for Q7 and Q8, but not enough to be in Class-A.
D1 and D3 keep the output transistors from blowing up under certain conditions (forgot the mechanism at the moment).
Q7/Q8 are emitter followers and provide no voltage gain, but a lot of current gain to drive low impedance output transformers.
R11 and R12 provide emitter degeneration that improved hfe matching and stability.

Discussion of this circuit could take hours.
Hopefullly, PRR catch any errors I made here. :grin:

Tamas

http://www.passdiy.com/pdf/diyopamp.pdf

Nelson Pass wrote a pretty good article on understanding op amp design.

And my favorite the Douglas Self amp design page.
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

Tamas

that's the kind of information I was looking for :green:

Thanks!

ju

[quote author="fum"]that's the kind of information I was looking for...[/quote]Same here! :thumb:

I've seen the Nelson Pass and Douglas stuff before, but had forgotten where I'd read it.

I've been thinking about converting it to class a. Of course we've got to try it eventually. :green:

BTW, I read a good and long thread over at RAP the other day about class a vs class b and ab.

Just a few easy additions to Tamas' post & (who knows) to revive this thread:

R13 and R14 provide current limiting for the output-transistors

C2 is a feedforward-cap ('stability related')

C1 and R2 tailor the response of the first diff-pair Q1,2 ('stability related')

C3 tailors the response of the second diff-pair Q3,4 ('stability related')

C4 and C5 provide 'HF' supply bypassing

Biasing (tail-current) for the first diff-pair is simply the input-voltage (assumed to be zero) minis neg-supply minus 0.65 divided by R1:

(0V - -15V)/130k = 115uA

So through R3 and R4 flows around 58uA each (assuming equal currents), resulting in base-voltages for the seconbd diff-pair of around 2.26V below the pos-supply.

As said earlier in this thread, the tail-current for the second diffpair Q3,4 flows through R5,10.
Assume it's equally split between Q3 and Q4. The half of the tail-current flows through R6 (assume high beta for Q5). This biases Q5 for a certain collector-current. In essence this also flows through R10, above it.
This is (as I see it) a DC-feedback-loop: if the current becomes higher the voltage drop across R10 increases and the tail-current for the second diff-pair is decreased (since the voltage at the bases of Q3,Q4 is more or less fixed as we saw above - and it's not the result of this feedback-loop under discussion). This results again in a smaller voltage across R6. Voila feedback, so an equilibrium results.



Also, please correct when wrong, please expand etc

regards,

Peter

A blast from the past!.

Funny, this circuit looks so simple now...

I think there may be some transcription errors in this schematic, in the vicinity of the output stage. Was this PeterC's own schematic?

I know this one is correct : http://www.thediypill.phx.com.br/forumfiles/1731_SCH.PDF

Thanks!

I guess it is just a cutoff output stage most of the time then.

My schematic above was (correctly) copied from Peter C?s version, which I can no longer seem to find. Jay (Fallout) had boards made from Peter?s version and sold them to a bunch of us sometime back. They did not include the "Trim" pin and thus used one less resistor than the original.

Since then we discovered that C1 needed to be a 630pF. I have corrected that and have updated the schematic with notes explaining this so as not to confuse anyone in the future.

Edited to fix typo.

Since then we discovered that C2 needed to be a 630pF. I have corrected that and have updated the schematic with notes explaining this so as not to confuse anyone in the future.

Click to expand...

Flatpicker,

I believe you mean C3 (as shown in Bauman's link to the 1731 schematic). Also, this high value is experiemtal at this stage since we need to get results back from someone on the bench using this value. C2 should remain 10pf (a 630pf cap here will create another bump at 10KHz!).

Correct me if I've missed something here.

Regards,
Jeff

This all interesting info, nice to have this going. As I understand it C2 is a feedforward-cap (signal-polarity the same @ each plate) so I could understand it that increasing its value wouldn't help.

But if it's indeed C2 that needs to be boosted then all would fall into place: it agrees nicely with Jeffs simulation-results and we would have back the original Melcor-dimensioning (but it conflicts with Fabios pdf).

So... (from Flatpicker):

My schematic above was (correctly) copied from Peter C’s version, which I can no longer seem to find. Jay (Fallout) had boards made from Peter’s version and sold them to a bunch of us sometime back. They did not include the "Trim" pin and thus used one less resistor than the original.

Since then we discovered that C2 needed to be a 630pF. I have corrected that and have updated the schematic with notes explaining this so as not to confuse anyone in the future.

Click to expand...

... do I understand it correctly then perhaps that the right schematic has 630pF for C3 then - but has been lost again ?

Regards,

Peter

yea yea, I have the correct connections but not the correct value for C1, that need to be 1000pF :green:

OK, cool, hadn't even seen that one. OK, that's fine @ 1nF in the redrawn schematic. Let's keep hunting for other eventual corrections of those cap-values.

from Flatpicker:

A blast from the past!.

Funny, this circuit looks so simple now...

Click to expand...

Ehh, OK :wink: So far I've come to terms with the basic signal flow & DC-operating point of this one. Haven't been looking much at the AC-bahaviour, but it sounds like that has to be next now...

BTW, everybody doing fine here with all this 1731-jumping between here & the big API-thread ?
http://www.groupdiy.com/index.php?topic=80210#80210

I'm fine, thanks.