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Old 1st January 2009, 11:07 PM   #561
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Quote:
The theory is spot on, a simple test is to just replace the ksc1381 and ksc3503 with ztx696 and ztx796 which are higher gain devices. These arent very complementary but will do. Every transistor is more suitable for a intended purpose, here you have no cob or voltage issue, although it will help, but gain is a important issue for linearity. The higher gain might just overide some other issues.
I continued to play around with the Darlingtons and I got it..
simply awesome.. I had to model 2 transistors to
emulate a KSA 700/800 pair (fairchild describes the internals
but no stinking model )or bd 677/678...
. 3db more gain ,same unity gain/phase. plot..MUCH better LTP balance (1-2 uA).
But my concern is the massive Cob of these devices..100pf!!
All in all .. this is afrugal mod as the BD's and the KSA
darlingtons cost $.67c Vs $.40c for the 1381's..
OS
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Old 1st January 2009, 11:19 PM   #562
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Simulates so damn good I'll post it. Carlos (DX) would hate it,
He says anything with a darlington in it sounds like $#@t..
OS
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Old 1st January 2009, 11:41 PM   #563
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Hi OS,

Just a word of warning about combining a Darlington and cascoding with a VAS.

The Miller compensation capacitor makes the VAS a feedback amplifier by itself. Its output voltage is sampled and the feedback signal is the current through the Miller cap. When you replace a CE amp with a Darlington in a cascoded VAS, the loop gain of this local Miller loop is increased, and this can lead to oscillation in some cases. If it occurs, it will be in the range 10 MHz - 50 MHz or so. There's a way to look at the loop gain of this local Miller loop in SPICE, determine its phase margin, etc. I can post how to do this if you're interested.
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Old 1st January 2009, 11:48 PM   #564
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Yes , by all means..
A loop within a loop...

OS
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Old 2nd January 2009, 12:12 AM   #565
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I tried the same scheme as for measuring global gain on the
VAS (2 more nodes ,one on each end of Cdom)..
see below.. -V(D)/(C) applied to schema below..
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Old 2nd January 2009, 12:45 AM   #566
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Maybe carlos has been using the darlingtons for the wrong purpose or the wrong way, I wouldnt dream of using them in small signal areas without a cascode nor would i dream of using something like a bd140 as a vas. Carlos also says class A amps sound muffled, this is purely preferencial taste, me thinks he likes the high frequencies cross over distortion artifacts which I can hear too and is percieved as more detail and high frequency which is not necessarily that bad but it puts a edge on human voices I would prefer not to have. Yet again one mans muffled is the others normal and ones bright is anothers normal. OOps I said I wasnt going to mention anything subjective anymore.
He is a guy though, this forum needs more people like him, I love watching his clips on youtube, sometimes brings a huge smile to my face.

Even though the cascode should take care of cob issues I find it better to have these already as low as possible too, well its obvious, internal nonlinear capacitances are detrimental everywhere, havent found any suitable complementary models yet, Ill wait for the factory. The cob issues offsets some of the gains achieved with the higher gain obtained from the darlington.

That darlington you built up there will surely be on steriods

But its an extra part, if you going that route its mandotory trying a cfp too. I will do some comparisons as soon as I get some reliable darlington models from japanese factory. I think the cfp might have the legs on this one. Try it, I will first have to make a complete schem on LT. This amp has very high gain, beats my blameless but its not so linear, YET. Try using higher degeneration resitors and decrease vas miller slightly.

Some say amps with this topology sounds ^%$# Again subjective preference. In the end you must listen and decide.

BTW I notice you use trannies like mje340 for current sources, . This messes up the PSRR, a good read on this is Walt s papers on current sources. Maybe only a couple of cents difference to a 1381, lowest cod high gain device should be used, this for mirrors too.
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Old 2nd January 2009, 12:50 AM   #567
andy_c is offline andy_c  United States
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Quote:
Originally posted by ostripper
A loop within a loop...
Exactly. And that's one of the problems. I'll summarize the problems below.

1) The Miller loop is inside the global feedback loop. If you inject a signal inside the Miller loop, it will go around this loop and the global loop. The global loop feedback path will cause the simulated Miller loop gain to be completely wrong. We need to block the global feedback path, yet still allow global DC feedback to set the operating point. See the attached picture for how to do this.

2) For the case of a complementary VAS, there are two Miller loops. If a loop gain probe is put inside only one of these loops, completely wrong simulated loop gain will result. For a fix, you must artificially join the loops together per the attached picture.

3) The impedance interactions within the Miller loop make the single-voltage-source method of measuring loop gain (from the LTspice audioamp.asc example) totally inaccurate. Instead, the full-up loop gain probe from the LTspice loopgain2.asc example must be used. See this post for details.

So, to solve these, do the following:

a) Block the global loop gain for all frequencies except DC using the LC network as shown in the attached picture.

b) Artificially join the Miller loops together per the attached picture.

c) Use the full-up loop gain probe, place in the artificially-joined path as shown in the attached picture.

Remember to turn off all other AC voltage sources before simulating. If the results are correct, the Miller loop gain magnitude should look something like a bandpass filter with gain (since the Miller loop by itself has no DC feedback).

Edit: In the picture, the inductor should be 1e10 Henries. It's kind of hard to read. Also, the cap should be 1 Farad, but if you specify it as 1F, that's one femtofarad!
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Old 2nd January 2009, 12:55 AM   #568
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By homemodder - That darlington you built up there will surely be on steriods
yes, that is my high Hfe/low Cob "guinea pig" ,10-15pf/10k Hfe.
the other test was with the "dogs" BC546/MJE350.
I modeled the 2 trannies after the KSA700 ,which is more
like the 1381/992 combo.

you were right, a darlington brought the distortion down
another notch..(see below) 5-6 DB to be exact.
That is at 100V p-p output .. I did the same test before
and got 87/92/95 (H2/3/5).
OS
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Old 2nd January 2009, 01:17 AM   #569
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I did what the diagram said..(hard to read), any value in
particular for the inductor... I used .1 henry to start..
below is how I wired the probe into circuit..
Edit...I jumped the gun ..10e10 henries

OS
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Old 2nd January 2009, 01:47 AM   #570
jaycee is offline jaycee  United Kingdom
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I've been playing around myself, and after much fiddling I came to the conclusion whoever it was that designed the Cyrus One/Two knew what they were doing!

www.darkmatter.myby.co.uk/newamp (yes, a highly original name)

Check this out:

Click the image to open in full size.

This isn't one for the people who don't belive in gain, though!

Click the image to open in full size.

It's a bit of a pig to stabilise but C1 does the trick.

In case youre wondering why D2 and R3, I intend to use some salvaged 2SA1360/2SC3423, and i have only 4x of the 2SAs and 2x of the 2SCs (the Teac amp from which they were salvaged also used the diode and resistor method). If I go to PCB with this I'll be leaving pads on the PCB for adding the extra transistors.
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