Guys, I bought Len Gregory's old Croft MM phonostage that he used to voice his range of cartridges. I'm in the process of critically assessing a couple of old Shure Carts on my system and I'm trying to fine tune capacitance loading. Does anyone have an idea of what capacitance loading Glenn used to design into his phonostages? I have no way of changing capacitance on his phonostage but would like an idea of what capacitance load my MM carts are, "seeing".
Glenn Croft MM Phonostage capacitance loading
- Thread starter Rexton
- Start date
As a general rule Glenn never quoted any figures for his amp's, and some of the numbers that appeared with them were very obviously nonsense (input impedence of the Micro for instance).
If you can find any cap' figure, I would treat with extreme caution unless the person quoting it has measured or calculated it.
Beware that Genn's phono stages were designed to feed a valve pre' and can sound far less than impressive if used into a SS pre'. It is beyond me, but changing that is reasonably simple (output impedence?????).
If you can find any cap' figure, I would treat with extreme caution unless the person quoting it has measured or calculated it.
Beware that Genn's phono stages were designed to feed a valve pre' and can sound far less than impressive if used into a SS pre'. It is beyond me, but changing that is reasonably simple (output impedence?????).
Tony L
Administrator
Being valve ‘Miller capacitance’ will almost certainly be a factor, so unlikely to be less than about 200pF. I had a Croft RIAA stage at the same time as an Ortofon 2M Black and didn’t really get on with it, which adds weight to this argument (the 2M sounds best with about 150pF total, i.e. including the arm cable). I know Glenn was a big Decca cartridge fan, so maybe some clues there too.
Tony, I'm currently doing something of a similar experient to yourself, I'm comparing the M55E and the M44-7 to see which sounds best in my system. I just think things might be compromised and I might not be getting the best from the carts with no ability to accurate gauge the loading capacitance, I've estimated that my carts are seeing around 125 pF before getting to the phonostage, hence the need to see how Glenn loaded these phonostages.
Tony L
Administrator
As I say I’d be astonished if it had less than 200pF at the phono stage. Glenn liked Deccas, Len Gregory made MI carts, neither care much about capacitance so there was no real need for Glenn to tackle the Miller capacitance of the first ECC83 (assuming that is even possible). As such it might be worth your adding another 100-150pf and seeing what that does (might be worth my while too with the Verdier!), but I’m pretty sure you aren’t way under.
Yes I'm starting to look into Len's history and realised that MI's and DECCA's arn't fussy about Capacitance. I've had a comment passed to me from Alan (Firebottle) who also seems to think 100pf might be the correct value and I'm sure I managed to find another unconfirmed comment that supposedly came from Glen (on PFM) that 100pf was the load he used. I have no way of extrinsically changing the load unless someone can show me an easy solution and I'm not prepared to open the phonostage up and change the resistor either. I'm ok with a soldering Iron but this phonostage is unmolested and that's the way it's staying. I'm just very anal about getting the best out of these carts. I've already got some experience with overloading the pF on the Shure M55E, I must have sensitive ears because I could easily tell when the thing was too overly loaded. Anyway, the experiment continues!!!!!
Here's a link to a thread were Glenn supposedly stated 100pf
pinkfishmedia.net
Croft integrated - phono capacitance
Hello, anyone happen to know the capacitance of the phono stage in the Croft integrated?
pinkfishmedia.net
Tony L
Administrator
I've already got some experience with overloading the pF on the Shure M55E, I must have sensitive ears because I could easily tell when the thing was too overly loaded. Anyway, the experiment continues!!!!!
To increase capacitance all you need to do is get a tiny little capacitor somewhere between the signal and return at the input. I’m trying to think of an elegant way to do this myself. These are tiny caps, they can often fit inside an RCA plug or be tacked on the back of the RCA socket. I don’t mind taking a soldering iron to my preamp as I’m neat enough to be able to put it back without anyone noticing, but I’d love to find a way to do it fast so I could properly assess small changes. Being a neat-freak when it comes to electronics I’d likely end up tacking the caps on the underside of the phono stage board, and that would be at least a half-hour job once stripping down the pre was factored-in. Far too long to assess differences.
My guess is my pre is about 250-300pF I’m basing this on how unlistenable it is with a modern Ortofon, Miller capacitance, and that I can see a 47pF cap very near the phono input on the schematic (I don’t have the education to fully understand the schematic, I certainly can’t calculate it). As such I suspect I may be a bit under for the vintage Shures. The M44/M55E sound excellent, though the V15 seems to have a bit more of a ‘saddle’ response. I’d love to be able to quickly AB another 100pF just to see what happens.
PS Neat cheap cap kit here (eBay).
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Craig B
Re:trophile
I don't expect that anyone with a vintage Shure will be looking to reduce capacitance. Tony indicated that he'd like to be able to quickly AB another 100pF, and @Rexton likely wants another 200pF (as his Croft likely has a 'bucking resistor' on the grid, assuming it is actually circa 100pF as standard).
Tony L
Administrator
Cool, that looks easy to make! I’ll order a ‘Y’ splitter (Amazon) and I’ll play soldering some caps across an RCA plug. I’ll obviously need to figure out how much capacitance the Y lead itself adds, but it should give an indication if the V15/III works better with more than it is seeing at present. If it does I’ll add it properly in the preamp somewhere.
PS Amazon also have a wealth of more conventional Y-splitters than the big gold plug I linked above, e.g. this one. Not sure which to buy, maybe this one even though it is likely higher capacitance itself as it will no doubt put less stress on the RCA sockets as I’m fiddling about. Doesn’t really matter as it is just an experiment, neither would be permanent. I just want to hear what happens with another 50/100/150pF and the V15. I’ve got a load of unused Neutrik Rean RCA plugs that I can stick capacitors across. The problem is I have no idea where they are…
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Craig B
Re:trophile
I prefer the conventional short cable style for the same reason.
Suggest putting shorting plugs in the 'Y' splitter female ends and measure capacitance across pin and ring at the male end. Then it's just a matter of including this in your capacitance calculations.
Suggest putting shorting plugs in the 'Y' splitter female ends and measure capacitance across pin and ring at the male end. Then it's just a matter of including this in your capacitance calculations.
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Thanks! I could do this but I've already got the splitters and loading plugs from another phonostage, don't really fancy opening things up and buggering about. Yep, I'm lazy!
Craig B
Re:trophile
If the splitters are male on the single plug end and female on the dual plug ends like those pictured above then, yes, these will do without having to include any additional adapters.
If your loading plugs consist of capacitors soldered across +ve and -ve within RCA male plugs, then these will also do.
Ideally, one would take into consideration the capacitance of the 'Y' splitter and include this in the calculation of what value of additional capacitance sounds best.
Tony L
Administrator
Quoting my own post for an update. I ordered the second type, but being an idiot only ordered one, which wasn’t much good for anything, but did allow me to measure the capacitance, which seemed very high at 66pF (using my cheapo meter thingy, Amazon, an amazing little thing I’d recommend to everyone). As such I reordered the first gold plug type, which does come in pairs, hoping that had a lower capacitance, which it does at 22pF. They seem nicely made too, very solid. Heavier than I’d like, but in reality no worse than many overpriced audiophile/bling-fi cables so shouldn’t damage my preamp’s RCA sockets with careful use.
That’s as far as I’ve got as I’m now waiting for the cap kit to turn up, and when that turns up and I can see the size I’ll think about what RCA plugs to mount some values in. I’m thinking of buying the cheapest lightest plastic body RCA plugs just to keep weight down. They aren’t really in the signal path, the goal is just adding-in capacitance. I plan to build the equivalent of the ‘Phono Equalisation Kit’ pictured upthread with say three values up to 100pF (which would be about 122pF with the Y plugs) so I can easily experiment. My he is the plugs themselves don’t have any detrimental effect as thinking more about it I don’t really want to mod the preamp despite spotting a perfect place to stick caps under the phono board that I could do invisibly and reversibly without leaving the slightest trace. The reason being it would screw things right up for the MP-500, and I want to be able to switch that back in with a simple headshell or arm change (depending which 3009 I’m using). Using plugs or flying leads makes way more sense.
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Craig B
Re:trophile
I suspect the 66pF comes as much from the 3 x RCA plugs as it does from the cable. The dB Systems kit colour codes the loading plugs according to the combined value of the 'Y' adapter capacitance (70pF) plus the actual polystyrene cap within, i.e. their 250pF plug (red) contains a 180pF cap.
The included loading plugs are coded as follows:
- 150pF Brown
- 200pF Green
- 250pF Red
- 350pF Orange
- 450pF Yellow
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