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Old 16th October 2007, 08:30 AM   #11
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I replaced R7 with a 10mA current source (only in sim for the moment). The output impedance drops 'a bit': flat till 20kHz at 0.2mOhms, then rising at 60mOhms at 1MHz.
Sim indicates that replacing Q1 with a 2SC1775 almost halves those numbers.
I guess that extending the bandwith (e.g. falt to 1Mhz) is not that easy?
Rüdiger
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Old 16th October 2007, 09:23 AM   #12
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oops, that were 2mOhms for B C550 and 0.5 mOhms for 2sc1775...
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Old 16th October 2007, 11:12 AM   #13
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Quote:
Originally posted by Onvinyl
I guess that extending the bandwith (e.g. falt to 1Mhz) is not that easy?
You should try to increase current thru CFP shunt transistor (as much as thru the load and more, if needed), size a CCS accordingly.
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Old 16th October 2007, 11:37 AM   #14
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Hi,
how do you make a low voltage drop CCS?

What is the Vdrop of the post1 CCS?

A 317 configured as a CCS drops 1.25V plus the Vdrop of the regulator. This can be around 2V to 4V, depending on the CCS current.
Would an LDO reg offer significant improvement in Vdrop without any other disadvantages?
What else could be used/built up?

Onvinyl,
are you averse to adding a couple of caps?
Maybe a bypass across R6 and also across the LED Vref.
Would these interfere with the sound quality?

Edit,
I think I've got the cap locations right this time
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Old 16th October 2007, 12:01 PM   #15
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Andrew, you're a genius!

Placing 100n across R6 (and 0.1R in series for simulating (a really bad) series resistance) lowers Zout to 150µOhms in the audio band and 5mOhms at 1Mhz. I filtered the led's with 1.9k/100n as well.

A different error amp (with cfp compound pair as well) simulates shocking 3µOhms in the audioband and 150u at Mhz. Not too bad for open loop...
This thing would be a pain to get stable, I fear.

Rüdiger
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Old 16th October 2007, 12:22 PM   #16
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... 100u for both is better for noise...
Rüdiger
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Old 16th October 2007, 12:59 PM   #17
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Since no-one is asking, I post it anyway...

this circuit has not been build yet! I expect it to be very nasty to calm down possible oscillations!

It sims with 9uOhms in the audioband and 250uOhms at 1Mhz.

Noise sim is 2.4nV/Hz(1/2) from 10Hz-1MHz. Did someone ask if I think these are realistic numbers?

Rüdiger
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Old 17th October 2007, 12:03 AM   #18
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Well, I breadboarded the complicated regulator and it's like I thought: It's a pain to get it working, but if you get it, you got it. ... As long as you don't think you tuned the oscillations beyond the capability of your scope ...
Seriously, I think one needs a good board with ground plane.
Did you ever make a stable cfp-input pair with a perf board?
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Old 17th October 2007, 12:08 PM   #19
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no comments? *sigh*
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Old 17th October 2007, 12:13 PM   #20
AndrewT is online now AndrewT  Scotland
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Quote:
Did you ever make a stable cfp-input pair with a perf board?
Hi,
maybe the few that have p2p this have not read the thread.
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