For the cartridge DC resistance to dominate, the amplifier equivalent noise resistance needs to be 3 times lower (or less) than the cartridge DC resistance (or real part of the impedance).
Example, for a 12 ohm cartridge, the head amp has to have a noise equivalent of 4ohm or less ( 250pV/rtHz ). However, this 1/3 rule comes strictly from how noise power adds up; the reality of vinyl surface noise makes all these number totally irrelevant.
Unless you are like me, hating every bit of idle TT hiss.
The cantilever only moves a flux coupling ring so the coil and magnet can both be optimized without any mechanical considerations. Remember the coil resistance has to decrease as the square of the sensitivity ratio to maintain SNR.
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Among which, the lack of relevant frequency compensation for all examples where feedback is involved. Of course, the AC small signal analysis used to determine the noise performance doesn't reveal anything about.
Guys, this was never intended as a b all and end all set of MC head amp designs.
The LTspice files are up for anyone’s perusal. The whole idea was to get some ideas up and let people play.
I make it quite clear on slide one or two that you may need to do some comp work. If you feel strongly about it, feel free to spread your knowledge and post up comp’d circuits or whatever else you feel like.
Kindly take this thread in the spirit in which it was intended.
The LTspice files are up for anyone’s perusal. The whole idea was to get some ideas up and let people play.
I make it quite clear on slide one or two that you may need to do some comp work. If you feel strongly about it, feel free to spread your knowledge and post up comp’d circuits or whatever else you feel like.
Kindly take this thread in the spirit in which it was intended.
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Understood, although I cannot find any note about compensation. Plus that your distortion results are assuming all the feedback head amps are stable (otherwise you could not determine the distortions) which is, at best, marginally or coincidentally true.
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You position yourself as the chief inquisitor on the forum and frankly it’s quite tiresome.
I really don’t need you or anyone else’s sniping when I just try to share some info. If I decide to build one of these circuits then of course I will look at compensation. As I assume would anyone else since the LTspice files are provided.
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I had to think about this for a while until I realised where I was being dense (as usual). As I have some 3.5Ohm cartridges I can see justification for more tail chasing
I feel a mute switch is vital on any phono stage. But that won't stop me chasing lower noise if only noticable with headphones late at night.
I had to think about this for a while until I realised where I was being dense (as usual). As I have some 3.5Ohm cartridges I can see justification for more tail chasing
I feel a mute switch is vital on any phono stage. But that won't stop me chasing lower noise if only noticable with headphones late at night.
I would only add that the multiple times mentioned "noise matching" makes sense only if the Noise Figure NF or the Noise Factor F is taken as a noise metric ( same thing really, NF=10log(F) ). For an input referred noise metric, "noise matching" doesn't make any sense. Unless I'm missing what "noise matching" is about, in this thread.
To my own experience, rather than hunting down the last drop of noise, it is much useful to invest in shielding the head amp from stray magnetic fields. Down from .5nV/rtHz it's the mains 50/60Hz that dominates the audible artifacts with the idle TT. I used a 12mm thick aluminum enclosure and got the 60Hz component (input referred) down to about 50nV, about 20dB down from the unshielded version, which makes it almost tolerable for a MC gain of 67dB plus 28dB in the power amplifier (a few mV at the speaker terminal, can hear it ears on but not in any far field). I'm pondering about a head amp close to the cartridge, much like electrostatic microphones use, but it's an idea in very early stages.
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The RF concept of maximum power transfer i.e. Zsource = Zin and the resulting 3dB "optimum" noise figure is not applicable to this problem at all. I don't know where this came from.
Hence my question to Richard "Duraglit" Lee. Still waiting to learn something new.
Thanks for this Bonsai. Dick Duraglit's family will be grateful for this generous gesture and GG Baxandall will surely approveHawking co-named 'Duraglit special'
I'll explain common base separately for Guru Wurcer, Polak et al.
I have been warned off improving other people's circuits like I did for JC's. Other circuits benefit from matching but the exact matching depends on the exact circuit.
This is certainly important for MM. See eg Wayne's detailed results on proaudiodesignforum.
But it's nonsense for MC and probably low inductance MMs like the Grados. If anyone has response plots of MCs loaded with a range from their DC to 47K ... also wonky Virtual Earths like JC or even proper Virtual Earths, I would be really interested.
I'm really a speaker, microphone & DBLT guy but I was also a wannabe cartridge designer around the time of Duraglit's genesis when I had direct access to guys who designed MC cartridges for a living.
Of course there are claims that using their recommended load will result in World Peace & improve your Sex Life but I only have evidence of that with Duraglit.
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BTW, most of my meagre contributions to this thread are in my original MicBuilders document including detailed recommedations on matching
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3mA/device limit for Ye Olde Unobtainium Shoppe devices is from practical testing circa 1980. No improvement in noise beyond that even for the matched conditions.
5mA/device for ZTX is from inspection of Wayne's and H&H data. IIRC, H&H may have said something like 5mA is a sweet spot or that might have been Wayne. No surprise as we were looking at the same data.
I quote this in my MicBuilders document and it applies to specific Ortofon cartridges.
The context is that I believe Duraglit & Gerhad represent the SOTA for real-life devices though Gerhad has never tried his in a MC vinyl playing system. In the MicBuilders document, I also point out that Gerhad's amp will have a NF of 2dB for those Ortofon cartridges so that's the maximum noise 'improvement' possible.
While Duraglit does well in a Ein willy-waving comp (as shown by Bonsai) it is really the performance with real-life MC cartridges that is my main interest .. and likely the interest of those here who want to build something useful. (Cue 2 pages of Guru Wurcer on record noise drowning ...
)Actually, comparison is with Leach's original version in Audio magazine where the improvement is greater than 10dB for both noise & THD. Duraglit was the result of a friend building the Audio magazine Leach and his dissappointment. I said I would make him a better one. The story is in my MicBuilder's document.
May I ask again why you chose to build & test his current mirror version even though Leach himself admits it is inferior?
BTW, thanks for your practical experiments with the solar cells. I don't think the picture is complete yet and there are more improvements possible but its an important step forward. Need to get some beach bum stuff out of the way and see if I can still pre10 to use LTspice.
I've never claimed Zsource = Zin is the Universal answer to World Peace and improved Sex Life .. only with Duraglit.
It comes from Rnv = 1/(2 gm) for both common base & common emitter.
But Rni = 2/gm for common base and
Rni = hfe * 2 / gim for common emitter.
I'm recommending Zsource = Zin only for the common base Duraglit which has no other significant noise sources apart from rbb' & the load resistor in practice.
Lesser common base designs have other noise sources so this 'optimum' isn't so clear cut.
Baxandall's 1967 article explains the 3dB NF etc and I'm sure, his references, H&H and others too .. though perhaps not as clearly.
For THD, I'll just say that for LoZ cartridges, the MCs which demand the best noise performance, reducing Duraglit's input impedance by running more current improves THD with the cartridge
This has implications on how to 'improve' other circuits (eg the CE examples) but I've been warned off doing this.
There is a lot of truth in that.syn08 said:
The reason why Newton, the mains powered Duraglit only runs 1mA is cos, the design rapidly became nearly as complex as syn08's preamps with supadupa PSUs .. unacceptable in an inexpensive commercial amp. 1mA made less demands on PSU noise and the MC input already had lower noise, in practice, than all other commercial offerings.
Ken Farrar, my electronics mentor at Calrec, often said there is no substitute for distance .. not mu-metal etc. when dealing with hum & mains buzz
The Duraglit tin is in fact a vital part of getting simple practical SOTA noise. In the absence of distance, mu-metal bla bla, a soft iron enclosure with overlapping seams is best. Some Duraglit Specials were built into fancier expensive boxes but these required superhuman measures and much re-positioning of gear to equal the Duraglit originals. I pontificate on this in the MicBuilders document.
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My thanks to those who have simmed & highlighted the capacitor issues with Duraglit. I made some recommendations earlier in this thread but have yet to update the MicBuilders document. I'm playing hooky at the moment when I should be dealing with some beach bum issues
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Hi Ricardo
I don’t see his name in the list of the deceased in that crash.
Lydbrook - Halifax crash in WWII on 7 June 1942
Was he under another name?
George