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Mikeh wrote:
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Cock-ups With Ceramics |
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Graham Slee 
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 Posted: 29 Mar 2023 at 9:34pm |
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I've eventually settled on a load of 1k3 which attenuates the cartridge so I obtain a healthy level at an 11 o'clock volume control setting, and flicking the Accession M to RIAA isn't as bright as before, and might even be acceptable on playing other records than the brightly lit "Wind And Wuthering." So, how to make? A phono plug, a phono socket, some short bits of wire, and a 1k3 resistor (metal film), times 2 for stereo. Your own super cheap Velocitone adapters! And hum free! Perhaps call it the Bogart?
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Mikeh
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Joined: 04 Dec 2018 Location: Yorkshire Status: Offline Points: 617 |
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Posted: 29 Mar 2023 at 10:55pm |
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I remembered an old thread on Vinyl Engine discussing a similar simple approach for ceramic cartridges (https://www.vinylengine.com/turntable_forum/viewtopic.php?f=19&t=82160&start=84#p792242). There was an picture of a headshell solution..  From your findings, I take it that the resistor and plugs approach could be tried for different cartridges (I have an 8TA) to determine the appropriate match for individual cartridge characteristics. |
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Mike
Technics sl1210g with Victor U-2 or Technics EPC-205C mk4 Accession MM, Majestic + Proprius, Solo ULDE Innuos Zen mini mk3 streamer Harbeth P3ESR XD paired with 2x REL t5x |
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Graham Slee
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Posted: 30 Mar 2023 at 11:52am |
Those guys must think I'm crazy doing things with "ancient" technology. But, to be blunt, I don't care what they think. It's an interest, and if nobody has that interest, then the history went down the memory hole. But maybe this is the end of history. The **** all that throw away attitude that got us all here in the first place. Where we just sit on our fat arses doing didly squat and complain about climate change while making it happen but not in our backyard. Did I tell you I've a book on my shelf titled Calling Bullsh*t? Anyway, it was nearly 50 years ago when I started worrying about why nobody could explain to me what should have been simple - and I now want closure! End of rant  |
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Mikeh
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Joined: 04 Dec 2018 Location: Yorkshire Status: Offline Points: 617 |
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Posted: 30 Mar 2023 at 2:11pm |
I don't think there is any implied craziness, more the contrary. There appears to be a band of enthusiasts for making ceramic cartridges sound their best. The link was merely illustrating a similar approach. One pic showing additional circuit components fitted in a headhsell and another discussing exactly the same approach as you have suggested, adding in-line components.
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Mike
Technics sl1210g with Victor U-2 or Technics EPC-205C mk4 Accession MM, Majestic + Proprius, Solo ULDE Innuos Zen mini mk3 streamer Harbeth P3ESR XD paired with 2x REL t5x |
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Graham Slee
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Posted: 30 Mar 2023 at 8:27pm |
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Isn't SPICE great? Without it I'd have to make my imagination and mathematical modelling work really hard. Unfortunately, by now, not practising mathematical modelling on a daily basis, I forgot something like 90% of what I knew to pass the exams. Just what do those Velocitone adapters do? Well, so far I know that the load resistor (my 1k3 and their 10k) changes the flat output to constant velocity. But my 1k3 idea, even though good in some respects, still sounds a little bright. So, I thought I'd model the Velocitone, and in that lies the answer! See that 68k resistor and the 500pF capacitor across it? I thought that it maybe dulled the highs a bit? And it does! Drool over this graph...  See how it shelves down the upper frequencies? The green line is the reference showing constant velocity, and the red curve shows it correcting for the brightness I experienced, and it's around 5 to 6 dB, which correlates with my guess of a 6dB shelf instead of the 12dB magnetic shelf response. |
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Graham Slee
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Posted: 30 Mar 2023 at 8:42pm |
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Now, using a 1k3 load and the same Velicitone series component values (68k || 470p*), all that happens is some 18dB level reduction. So, each curve has its reference constant velocity straight curve for comparison, and the only deviation is about 0.3dB at 10kHz between the two levels, which is very acceptable.  So now to try and build the Velocitone into the head shell like that guy on Vinyl Engine! This will be fun considering my failing eyesight (* 470pF is the nearest available value to the 500pF of the Velocitone circuit diagram) Edited by Graham Slee - 30 Mar 2023 at 8:49pm |
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Graham Slee
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Posted: 30 Mar 2023 at 11:21pm |
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So, how does it work? First of all you've a capacitor driving a signal into a load. The capacitor is the cartridge, or perhaps best explained that the ceramic (or crystal) cartridge is a capacitor coupled generator. So a capacitor into a load resistor is a CR network, which is a high-pass network, and with a load of 10k ohms (like the Velocitone) and a cartridge that's around 550pF, the turnover frequency is 1/2pi * 0.01 * 0.00055 = 28,937 Hz So signals below 28.937kHz are cut at the rate of 20dB per decade, making for a slope rising at 45 degrees to the right on a 180 degree graph. And if you make that load even lower, it's still the same slope but the cut happened even further up in frequency. Either way, it doesn't matter because it encompasses the entire 20kHz audio band. Got that? And now how the Velocitone bends that upper curve... Note that 68k resistor. It's a series resistor, so the load the cartridge sees is 68k plus the 10k load, making 78k, so the curve gives at 1/2pi * 0.078 * 0.00055 = 3710 Hz Now, turnover is -3dB, so at about 4kHz you see the curve has departed somewhat, and would keep doing so until it was flat, but! The 68k is bypassed by a 500pF capacitor, so it sees a load of 10k, and the cartridge isn't far off being 500pF (550pF on average) so it divides by two! And two in dB's is 6dB, so it gradually departs and then goes parallel with the constant velocity curve on it reaching 6dB. So, it's a crafty network that compensates for the PZT (lead-zirconium titanate) upper frequency response. Now, a Rochelle salt crystal cartridge struggles at high frequencies, so would not require the high frequency (68k || 500pF) correction, so all you'd need for a old BSR ST20 (or similar) would be the 10k (or 1k3 in my case) load. And the Rochelle salt exhibits the same measured capacitance, so with the flick of a switch, you can have one or the other. And via a selector switch you can have 1. ceramic; 2. magnetic; and 3. line, from one preamp circuit, which is the one I've been developing over on the 7eventy amp blog. ------------------------------------------------ Huh, my maths fell apart as you'd note if you printed the graphs and ruled the frequency at which the lines diverge at a 3dB difference. You'd find the frequency is about 2.5kHz and not the 3.7kHz I said. Now, that's about a 1.414 difference, and that's got to be something to do with capacitive reactance - there being two in series (the cartridge and the 500pF). But, it all depends how you see the circuit. Across the 68k resistor there is one 500pF capacitor. Then there's a 550pf capacitor (the cartridge) across the 68k and 10k in series. So it becomes a complex bit of maths. But if you exclude the 10k, then there's 500pF || 550pf || 68k, so that's 1.05n || 68k, and 1/2pi * 0.00105 * 0.068 = 2229 Hz And that's a bit closer to the 2.5kHz the simulation graph shows. So, there's my correction. Edited by Graham Slee - 30 Mar 2023 at 11:36pm |
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