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Old 27th September 2010, 01:14 PM   #131
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Originally Posted by simon7000 View Post
However when I Google "IC holder foam notebook"
That part reminded me of the "Hints and Kinks" articles which would appear in the ham radio magazine QST -- every couple of years the folks at ARRL.ORG publish a compendium of these articles and they are invaluable!
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Old 27th September 2010, 04:38 PM   #132
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I'm still waiting for mine...

Arne K
Arne, I am assuming your copy, which was shipped more than 3 weeks ago, has gone AWOL.
I've mailed a replacement copy this afternoon.

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Old 27th September 2010, 05:56 PM   #133
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Hi Joachim

I'd like to say how much I enjoyed your article on phono stages. I was particularily charmed by the use of photodiodes and a light bulb to create a truly floating power supply. Perhaps if more power was required some photovoltaic solar cells and some high-intensity LEDs would be an interesting experiment.

I was also very interested in your table of MC cartridge resistances and outputs, that showed how some cartridge designs have inherently a poorer s/n ratio, no matter how quiet the amplifier is, because of high internal resistance combined with low output. Food for thought there...

One question- in Figure 10a, there are two lots of 5 x 2200uF capacitors connected to the inputs, which don't appear to be mentioned in the text (or is that something to do with a missing page?) They seem to be bootstrapping the input in some way, but I'm not at all sure about that. Would be glad if you could clarify this.
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Old 27th September 2010, 06:30 PM   #134
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Hello Douglas !
I have all of your books and enyoy them too. To supply the stage with solar power is a good idea.
The 5 x 2200uF caps are there to realise an optional Low Z input. The gates of the Fets have be shunted to ground and the cartridge is then coupled to the sources through this caps. This is described on page 44 in some detail but the text may not be clear enough unfortunately.
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Old 27th September 2010, 06:36 PM   #135
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When i look at the circuit diagram it looks as if the elcaps short the Fets from ground to source. In reality there is a jumper.
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Old 27th September 2010, 06:38 PM   #136
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The jumper can connect the gates of the Fets to ground and then the elcaps are the input.
Puh... a bit hard to explain.
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Old 28th September 2010, 06:10 PM   #137
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Originally Posted by Edmond Stuart View Post
As in most cases, there exists more than only one view. Remember the discussion about HEC, that went on for a couple of years. Although all (three!) views were valid, not all of them were of equal value. The same applies here. In this case that means that we should look at the rationale behind Leach's approach in the first place. Let me cite Leach himself:
'Feedback amplifier stability can be improved if the output stage is bypassed in the loop-gain transfer function at frequencies above the audio band.This removes a possible pole from the transfer function caused by the output stage when driving a load capacitance. The method of bypassing the output stage at high frequencies is a feedforward technique.'
Clearly, this approach differs significantly from the Miller compensation and as such calling it an "input inclusive [Miller] compensation scheme" seems to me less appropriate.

Whether the term 'feedforward, is correct depends on which direction you are looking, IOW, rather a matter of semantics.

Cheers,
E.
Hi Edmond

I agree that the extra feedback path could be looked at as either inclusive compensation or semi-local feedback. On mature reflection, I think the latter is probably the more apt, and I really shouldn't have included it as an example of inclusion, so to speak. I can't however see it as a feedforward path; the signal at the inverting input of the differential pairs is about 20 times smaller than that at the VAS, due to the feedback network attenuation. Looking at the Leach paper again, I think my greatest qualms are over the fact that it is a simulation-only paper, with no hardware built and no measurements made. In particular, modelling an output stage as a single time-constant is really not viable.

I'd be interested to learn what HEC is- my searches were not productive. Can you give a link?

And, er, any thoughts on my article?
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Old 28th September 2010, 06:58 PM   #138
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[snip]I'd be interested to learn what HEC is- my searches were not productive. Can you give a link?
[snip]
Douglas,

HEC (or H.ec) was a term coined in this forum to identify Hawksford-style error correction.

There was a long and bloody discussion on whether HEC is a revolutionary concept or a different way to use negative feedback. Or words to that effect.
The main tread title was like 'Bob Cordell interview - feedback' or something. I'll see if I can find it, or maybe Edmond has the link somewhere?

Edit: the last 100 or so posts of the very long thread are here:
Bob Cordell Interview: Error Correction


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Last edited by jan.didden; 28th September 2010 at 07:01 PM.
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Old 28th September 2010, 07:59 PM   #139
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Thanks Jan, I'll try to find time to look through that very long thread.
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Old 29th September 2010, 05:36 PM   #140
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Originally Posted by Joachim Gerhard View Post
Hello Douglas !
The 5 x 2200uF caps are there to realise an optional Low Z input. The gates of the Fets have be shunted to ground and the cartridge is then coupled to the sources through this caps. This is described on page 44 in some detail but the text may not be clear enough unfortunately.
Hi Joachim

Thanks for the clarification. I should have been able to work it out from p44, but the absence of links on the schematic did muddy the waters somewhat.

I am however a little worried about the idea of running an MC cartridge into what is presumably very close to a zero input impedance. What effect does this have on things like the cartridge frequency response and the transient behaviour? I would have thought the coil inductance would make the HF roll off, in a rather unpredictable manner because the very low input impedance might not be well-controlled. All the manufacturers, to the best of my knowledge, specify a minimum load resistance for their cartridges. I hasten to add I know very little about the internals of MC cartridges.

Spurred on by your article, I have also been doing a little work on the inherent Johnson noise of MC cartridges. My database of cartridge parameters (96 different cartridges) shows that the best possible signal/noise ratio, assuming a wholly noiseless preamplifier,is given by the My Sonic Lab Hyper Eminent, as you say. This has a difference between its Johnson noise and its output voltage of 86.3 dB. (Mind you, it appears to cost 3000, so it ought to be good) On the other hand, the Denon DL-304 gives only 63.9 dB, which compared with the dynamic range from an ordinary CD seems very disappointing indeed. There is a difference of 22.3 dB between the best and the worst, a remarkably large range that could do with some explanation. The average S/N ratio for the 96 cartridges is 76.4 dB, which seems to me unimpressive.

It might be instructive to come up with some similar limiting signal/noise figures for moving-magnet cartridges.
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