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Old 4th December 2012, 05:03 AM   #1051
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bonsai View Post
Why don't you just buffer it and be done with it?
See the curves above , the buffer should have less than 1pF and more
than 50 Mohm input capacitance and resistive input impedance respectively ,
so micro meter node fets are required as it s not feasible with discrete components.

For the same reason, an eventual miller compensation must be buffered from
VAS output to its input , using a fet as well.

Last edited by wahab; 4th December 2012 at 05:06 AM.
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Old 25th January 2013, 05:24 AM   #1052
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I was considering building the STIS+AB2 amp as a prototype but I can't get it fully stable in simulation. I always end up with C24/C25 having to be 10nF or so for marginal stability, and this makes distortion abysmal.
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Old 25th January 2013, 06:03 AM   #1053
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I mean C17 and C18 in my schematic. C24/25 in Figure 12 here:

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Old 25th January 2013, 09:08 AM   #1054
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Hi Keane,

A few components are in the wrong position. C17 & C18 should be connected to the emitter of Q13 resp. Q14 and R17 (left side) should be connected to the node R9-R14. Perhaps there are more errors (I didn't check the whole schematic).

Success!
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E.
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Old 26th January 2013, 01:45 AM   #1055
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Thanks Ed. Now it makes sense!

I was wondering about the input filter. This amp must be intended to have a low-impedance source. Audio impedances at the feedback and input nodes are roughly equal, which makes sense. C2 is what I don't understand (your schematic). It is not large enough to bootstrap the input capacitance, which is still somewhere near 2-3nF. I understand the purpose of this arrangement is to achieve a 2nd-order BW limiter, but what does C2 do?
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Old 27th January 2013, 02:36 PM   #1056
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>... a 2nd-order BW limiter, but what does C2 do?
It changes it into a Sallen-Key filter.

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E.
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Old 27th January 2013, 04:44 PM   #1057
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I read as much earlier in the thread, but I can't figure out why that would be useful. It appears to increase the Q of the LP corner by applying positive feedback. Does the extra BW justify reducing the effectiveness of the LP filter?

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Old 28th January 2013, 06:24 PM   #1058
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>but I can't figure out why that would be useful.

Because most people like a clean step response, i.e. critically damped.

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E.
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Old 28th January 2013, 09:55 PM   #1059
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I think phase linearity should be the measure of "cleanliness" of AC/step response. AC response can be flat but have an anomalous step response because of phase nonlinearity. Group delay can be plotted in LTSpice using the Tg() function. The value of 22pF for C2 hardly affects response. Here are my simulated curves with and without C2 (C6 in my schematic).
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Old 28th January 2013, 10:28 PM   #1060
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If C2 hardly 'affects' the response then it should be recalculated.
BTW, in my latest design (by now 6 month old) I've deleted C2, probably for the same reason.

If this answer sounds a bit lame, I'm sorry. Reason: I'm still very busy with DiAna.

Cheers,
E.
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