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Old 12th February 2014, 08:25 PM   #3011
RNMarsh is offline RNMarsh  United States
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Sorry guys no time right now, too many other projects on the go.
Did you ever get one built and tested?

-RNM
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Old 12th February 2014, 09:21 PM   #3012
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Reminds me of other Round Tuit projects ...
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Old 12th February 2014, 09:40 PM   #3013
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Sorry, I did the schematic/layout in smt, but never fab'd the pcb. I know you would like one done in THT.
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Old 11th April 2014, 05:40 PM   #3014
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*Bump* - Another set of improvements, this time to the venerable Melcor 1731:

1) Added LTP CCS - many pros and cons, but it reduces THD20.
2) Added LTP Current Mirror - requires some care because of the differential VAS.
3) Added VAS Cascode - tried a lot of arrangements, but ultimately it is deceptively simple. If you look carefully, this is equivalent to the VAS cascode in the SWOPA, but with transistors rather than diodes (one transistor is diode-connected).
4) Re-biased the VAS to keep the VAS currents roughly the same as the LTP currents - this helps to keep the LTP collector loads roughly balanced.

Overall, it is more complex than the FET990, and does not reach the same level of simulated performance or sonics, but it uses mostly BJTs (can be made 100% BJT by using a BJT doublet for the LTP CCS). Four pairs of these can be implemented with (matched) BJT duals.

Simulated THD20 at 4V into 600R is a very respectable -130 dBr. It does pretty good at fairly high swings as well, right up to the onset of clipping. Further simulation is needed to check the overload and clipping behaviour - it may require clamp diodes like the FET990.
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Old 18th April 2014, 07:01 PM   #3015
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I think it would be much more enticeing to build if it had a JFET imput pair or cascode like Scott Wurcer's. Unless it's + input could be grounded as a summer. Ray
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Old 19th April 2014, 07:24 AM   #3016
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Originally Posted by grhughes View Post
I think it would be much more enticeing to build if it had a JFET imput pair or cascode like Scott Wurcer's. Unless it's + input could be grounded as a summer.
Although I generally show unity-gain buffers in my simulation schematics for convenience, it's actually a full-fledged generic opamp with both inverting and non-inverting outputs. It could be used for just about anything that a universal nullator-norator opamp could be used for.

Of course, you could replace the BJT input pair with a cascoded N-JFET pair, and it should work with compensation tweaks. The practical problem with a JFET input in a discrete opamp is the requirement for close matching of Vgs at fixed current(s) - which is a bit messy do to manually with SOT23 JFETs. OTOH, A generic low-noise BJT like the Philips BC849c or 850c from adjacent locations on tape generally don't differ in Vbe by more than a few mV (or lower) at fixed currents. BJT duals are also more common than JFET duals.

Of course, I use JFETS (matching them manually) in the FET990. If I could find a BJT-only or mostly BJT topology that matches the FET990 in simulated performance, I'd seriously consider it. This one gets close, but needs further work.
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