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Old 13th January 2009, 06:01 PM   #681
Symon is offline Symon  United Kingdom
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Dumb question, but what is Q20 doing ?

I thought the bias was set by Q10, Q9, D7 & D3
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Old 13th January 2009, 07:18 PM   #682
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Quote:
Dumb question, but what is Q20 doing ?
Q10/9 and the diodes give
you the 4 Vbe's (2.3-2.4v), which is too much(200ma bias per
device) ,Q20 shunts the extra .1 to.2v giving thermal compensation as well (it is mounted on one of the OP devices).
I might use the krill compensation but that would force
me to use 2 trannies and 3-4 diodes to achieve comparable
results.

Another good thing about this is if the Vbe fails, the worst
that can happen is overbiased outputs.
OS
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Old 13th January 2009, 08:05 PM   #683
KLe is offline KLe  Australia
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Quote:
Originally posted by ostripper
Click the image to open in full size.
Hi Pete

This is looking good

Are you going to try TimS's diamond input stage?
I saw that you posted gleefully in that thread.

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Old 13th January 2009, 08:30 PM   #684
Symon is offline Symon  United Kingdom
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Quote:
Q10/9 and the diodes give you the 4 Vbe's (2.3-2.4v), which is too much(200ma bias per device) ,Q20 shunts the extra .1 to.2v giving thermal compensation as well (it is mounted on one of the OP devices).
That's what I suspected, but as it shunts the current through Q9 and Q10 won't that tend to turn one of them off?
Wouldn't reducing the value of R15 an R24 have the effect of reducing the bias voltage but keep Q9 and Q10 biased on.
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Old 19th January 2009, 03:07 PM   #685
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Hi all.

I wish it was possible for me to follow this thread, as a man can only get so much reading done in a day.

However, I looked at one of the attachments and saw this possibility. It looks like discussion has moved forward a bit since then, but I thought I'd post anyways.

Doing this will pretty much double the gain of Q10 and Q9. Connecting the collector directly to mains is safer stability-wise, I think, but maybe we can see what this comes up with.

- keantoken
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Old 19th January 2009, 05:22 PM   #686
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Quote:
I wish it was possible for me to follow this thread, as a man can only get so much reading done in a day.
I have not even been following this thread, too busy hunting down info on diamonds

ground referencing the current sources has it's good and bad points. The plus is better regulation, while the downside is
a degradation of PSRR. In the attachment decoupling caps
are used. I still have to simulate all the mods to this basic
topology and see how they affect OP biasing,PSRR, and
loop gain.
By asking a lot of questions on the other 3 diamond buffer threads (Krill , DC300, and diamond amp) as well as
having the schema's for the dc300 and nakamichi 620
(both diamond based) I have enough to go on now.

I have the basic buffer built and running and it is very stable
so FA4 is on it's way .
OS
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Old 19th January 2009, 06:31 PM   #687
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Not what I meant, actually... Look at the emitters of the MJE340/350's. Although I'm not sure how well this works since I couldn't get the whole Q10/Q21 thing to work anyways.

If you are worried about PSRR, you can improve that by doing this... schematic attached.

I'm not sure what you mean by regulation, as I've only heard that in reference to regulators. I still have yet to learn some of my terms.

- keantoken
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Old 19th January 2009, 06:54 PM   #688
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I will double post, so I can add another attachment... Sorry.

Your two-diode CCS will lose output current as it gets hot. I don't know if this will be a problem even in a closed box, but I will say this anyways.

This is pretty much the current mirror we discussed in Dan's thread. This will have much smaller temperature drift. Don't be fooled. Even though we substituted one diode for a resistor it still has better PSRR than your original two-diode CCS without the bootstrap cap.

- keantoken
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Old 19th January 2009, 07:41 PM   #689
AndrewT is offline AndrewT  Scotland
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Hi Kean,
could you substitute a red LED for the twin diodes?

I hear that a red LED has a similar temp co to a BJT.

I would like to see both the ripple and the temp variations.
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Old 19th January 2009, 07:47 PM   #690
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Quote:
by beantoken - Your two-diode CCS will lose output current as it gets hot
That is why some opt for LED's,... better thermal characteristics.

one could also use a precision IC current source
http://www.national.com/mpf/LM/LM334.html
or zener while weighing complexity and part count against
performance..

The decoupled 2 resistor approach is standard in the blameless
design and works good. Another point, since I am wrapping
the feedback loop around the WHOLE amp, PSRR in the current stage is not going to be as big an issue as it would be for the Krill.

Quote:
Not what I meant, actually... Look at the emitters of the MJE340/350's. Although I'm not sure how well this works since I couldn't get the whole Q10/Q21 thing to work anyways.
Go to my WWW and click frugalamp link below "cheetah" and time, open FA3 folder, download [ FA3K.asc ] .. it sims purrfectly
OS
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