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Old 31st May 2017, 03:19 PM   #3291
linuxguru is offline linuxguru  India
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* Bump *

Just woke up from a long period of hibernation and came up with the following update to the Melcor 1731 topology. It's not the same schematic as the original, but is derived from it and has several key improvements to the classic Melcor 1731:

1. Uses a current mirror to load the LTP collectors, which gives a more useful input voltage swing for things like voltage followers and so on (the original used plain 39k resistors as the collector load).

2. The VAS is greatly modified to use the cascode structure from SWOPA. IMHO, this is a critical update to lower THD20 as well as maintain a favourable distortion profile with dominant H2.

3. Numerous biasing changes to reduce VAS and output stage quiescent current, while allowing operation over an extended +/- 6V to +/-18V rails, which is more practical for DIP footprints.

It's still work in progress, and some minor biasing changes as well as compensation tweaks are possible. However, it is unity-gain stable with the component values shown. The simulated THD20 at 4V amplitude into 600R is ~ -110 dBR with H2 dominant, and H3 is a further -12 dB below.

The crux of the low BJT count is the elaborate LTP current mirror that re-purposes existing Q5 as a voltage-follower/buffer, and also biases/drives the VAS cascode Q7. IIRC, this is the lowest-distortion Class-AB discrete opamp with only 11 BJTs that I've worked on in the last ~8 years or so. As shown, it will drive low-Z headphone loads also.
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Old 31st May 2017, 04:38 PM   #3292
grhughes is offline grhughes  United States
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Why can't it be extended to use +&-24v rails? Have you built it with real parts and tested it on a bench? Have you used it as a follower with the + input grounded and 100% feedback? Thanks Ray
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Old 31st May 2017, 05:12 PM   #3293
linuxguru is offline linuxguru  India
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Quote:
Originally Posted by grhughes View Post
Why can't it be extended to use +&-24v rails? Have you built it with real parts and tested it on a bench? Have you used it as a follower with the + input grounded and 100% feedback? Thanks Ray
It's not often that one encounters +/- 24V rails in equipment with DIP8 duals, which is what I'm aiming this design at. The limitation is mainly the Vceo rating of SMD VAS and driver-stage transistors - commodity BC8xx parts top out at around 65V, which is probably sufficient for +/- 24V rails. I'd probably check and re-check the power dissipation limits as well - the driver stage runs with an Icq of ~2 mA. If it's only going to be used for say +/- 18 to 24V rails, the driver stage can be re-biased for lower Icq in the region of 1 mA.

I have a mostly-SMD layout completed for BC8xx parts and BC639/640 outputs. It looks to be error-free and feasible to fabricate and prototype, but I can't provide a timeline on when that might happen. I'd prefer to check and recheck the layout for at least a week before sending out the Gerbers. Prototype testing will happen only after PCB turn-around and hand-assembly.

An inverting voltage-shunt configuration should work and is usually easier to stabilize than a non-inverting unity-gain voltage follower as simulated above. The topology is derived from a 50+ year old proven design, so it will probably work first-shot. The only issue is compensation/stability in the real world, which is always work in progress.

Last edited by linuxguru; 31st May 2017 at 05:26 PM. Reason: correction
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Old 31st May 2017, 05:51 PM   #3294
Carl_Huff is offline Carl_Huff  United States
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Very interesting! I look forward to seeing how this turns out.
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Old 5th November 2017, 06:29 PM   #3295
Blondie is offline Blondie  Poland
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I am interested in the results.
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Old 6th November 2017, 08:36 PM   #3296
benb is offline benb  United States
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If there's interest in the LM394 for this project (it was used in the Jensen 990 until National/TI dropped it), there's a coompany that's making it again:

Matched NPN transistor pair
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