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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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9th Jun 2011, 12:13 pm | #41 |
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Re: Stereo Demodulator issue. Baffled!
Nelson Pass is a well-known audio 'guru', but even he can't rewrite the laws of physics. A follower circuit, which is what the buffer is, works by using 100% negative feedback. Some audiophiles try to redefine 'feedback' to mean something else, so they can pretend to themselves that their amp has no 'feedback' - because they have picked up the silly idea that 'feedback' is intrinsically evil. You will find audio websites full of follower circuits, many proudly proclaiming that they have 'zero feedback'. Mostly this is due to ignorance, but Pass is not ignorant.
The uA758 circuit may be 'incomplete', as many datsheets circuits are, as it assumes that people will know about DC blocking, power supply decoupling, pilot tone filtering etc. etc. That is the difference betwen an application note and a magazine article; they are aimed at quite different audiences. It can't tell you what output cap to use, because that depends entirely on what follows. |
9th Jun 2011, 12:44 pm | #42 |
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Re: Stereo Demodulator issue. Baffled!
Nelson Pass on Negative Feedback, for your perusal:
http://www.6moons.com/industryfeatur...istortion.html |
9th Jun 2011, 1:27 pm | #43 |
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Re: Stereo Demodulator issue. Baffled!
Ah yes, that is the article where he seems to imply that emitter degeneration is not feedback. See fig 9 and the accompanying text. He mentions feedback via R2, but somehow forgets to mention that R4 also provides feedback. I guess that is how a follower can have "no feedback" - you just redefine feedback.
Most of the article is OK, an elementary introduction to feedback. What may not be clear is that Pass tends to have an idiosyncratic approach to things, so fact and opinion can get mixed together. |
9th Jun 2011, 5:00 pm | #44 |
Heptode
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Re: Stereo Demodulator issue. Baffled!
As with most of the projects the data sheets only give you the general idea of connecting the thing.
I have made a few of these types of circuits and have always added DC blocking capacitors especially when you are going to connect it to a factory built unit, as you never know what may be behind the input socket. |
9th Jun 2011, 5:31 pm | #45 |
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Re: Stereo Demodulator issue. Baffled!
Does the doubler output voltage stay the same when you get the loud hum?
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9th Jun 2011, 11:01 pm | #46 |
Octode
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Re: Stereo Demodulator issue. Baffled!
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9th Jun 2011, 11:07 pm | #47 | |
Octode
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Re: Stereo Demodulator issue. Baffled!
Quote:
John |
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10th Jun 2011, 4:22 am | #48 | |
Nonode
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Re: Stereo Demodulator issue. Baffled!
Quote:
Still, however one names what it is that the unbypassed cathode/emitter/source resistor does in a follower circuit, it surely looks like and behaves like negative feedback. Cheers, |
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10th Jun 2011, 9:36 am | #49 |
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Re: Stereo Demodulator issue. Baffled!
Yes, I was pondering this too, and thought the same.
Interestingly, Pass cites two legendary audio designers; Peter Baxandall, and John Linsley-Hood; who's reputations are without equal. I think the difference between Pass and the above two; is they were seeking progress in audio via scientific means; i.e. without a commercial interest. Whereas Pass clearly has a commercial interest, and his writings will always reflect that. |
10th Jun 2011, 12:27 pm | #50 |
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Re: Stereo Demodulator issue. Baffled!
What puzzles me is, if the tuner already had a stereo decoder fitted which was working O.K., why change it in the first place. Admittedly(and I'm not familiar with the Leak Troughline)the original decoder may well be an old discrete component design, but the
uA758 is not exactly state of the art either!!! |
10th Jun 2011, 2:53 pm | #51 |
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Re: Stereo Demodulator issue. Baffled!
The original Leak decoder is generally reckoned to be quite poor. It just uses a few transistors. Almost any chip-based PLL decoder will better it.
Emitter (etc.) degeneration is undoubtedly feedback, but it samples the output current (or a proxy for the output current) rather than the output voltage. In the case of a follower, the emitter becomes the output point so there can be no denying that feedback is involved. |
10th Jun 2011, 11:03 pm | #52 |
Octode
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Re: Stereo Demodulator issue. Baffled!
I can't work out why the Leak de-emphasis time-contants appear to be much longer than 50uS? Or have I missed something?
John |
11th Jun 2011, 6:58 am | #53 |
Nonode
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Re: Stereo Demodulator issue. Baffled!
Re stereo decoders, the attached excerpts from one of the Gordon King books show a selection of circuits, including a commercial example using the µA758, and another using an MC1310. The former has passive (L-C) output filters, whereas the latter has active output low-pass filters that use negative feedback!
Cheers, |
11th Jun 2011, 7:08 am | #54 |
Nonode
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Re: Stereo Demodulator issue. Baffled!
And some more from an old Ambit catalogue.
The comment on p.17 (Hitachi HA1196) about oscillation suggests that some care might be needed with power supply arrangements for these PLL stereo decoder ICs. Cheers, |
11th Jun 2011, 7:24 am | #55 |
Nonode
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Re: Stereo Demodulator issue. Baffled!
And a bit more yet from another old Ambit catalogue.
Cheers, |
11th Jun 2011, 3:00 pm | #56 |
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Re: Stereo Demodulator issue. Baffled!
It is a mystery why the original Leak deemphasis is wrong. One theory is that the effect of the error is to deliberately introduce an HF shelf which will reduce treble without entirely eliminating it, thus better suiting FM to the ears of those raised on AM.
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11th Jun 2011, 9:59 pm | #57 | |
Octode
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Re: Stereo Demodulator issue. Baffled!
Quote:
John |
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19th Jun 2011, 12:21 pm | #58 | ||
Nonode
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Re: Stereo Demodulator issue. Baffled!
Quote:
Quote:
Cheers, (1) Spicer, Stephen; Firsts in High Fidelity – The Products and History of H.J. Leak & Co. Ltd; Audio Amateur Press, 2000; ISBN 1-882580-31-1. |
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