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woodturner-fran 24th March 2006 10:20 AM

cartridge load check
 
This may be a stupid question, but,

how can I simply check whether my amp is presenting the correct load to my cartridge?

Background - the amp has a phono stage that is switchable from high to low impedance and is marked that it can take either MM or MC. I want to check that at the high setting it is actually giving 47K and to find out what the low setting gives.

Is it as simple as putting a multimeter across the pos and neg of the RCA terminal?


thanks in advance,

Fran

woodturner-fran 24th March 2006 11:42 PM

Eh.... don't know if I'm measuring the right thing exactly, but I put a multimeter across the RCA outputs.
At the high setting I'm reading 42K and 14K at the low setting. I'm assuming that the 42K is a little off the 47K quoted for MM, but a lot of the Mc carts I see talk about 100 or maybe 160 ohms. Would running these with the 14K harm them? not sound as good?

any help much appreciated..

Fran

lndm 25th March 2006 02:04 AM

AFAIK, the capacitive loading of a cartridge is significant, maybe more so than the resistance. The capacitance resonates with the coils inductance, and lifts the high frequency response. Too little capacitance and the highs may be gently rolled off but this can be better than too much (and a peak).

When your response is peaked from too much capacitance, the resistance will damp this resonance. AFAIK, it is best to avoid needing to rely on this.

Don't know how this relates to phono, but generally speaking, a source and load should be matched for the purpose of maximum power transfer.

Ghianni 25th March 2006 07:35 AM

Hi,

Perhaps you have been informed about this, but if you dont take a look at it: http://www.hagtech.com/loading.html

Ghianni.

EC8010 25th March 2006 10:58 AM

Jim Hagerman is correct in showing that the inductance of the cartridge combines with the load capacitance to form a resonance which is damped by the load resistance. What he doesn't mention is that moving magnet cartridges rely on this resonance to equalise the transducer's falling nechanical response.

MosfetOwner 26th March 2006 03:31 PM

It depends what is meant by your amp’s assertion that it is suitable for MC cartridges. In general, there are 2 types of MC cartridges, high output ones, which produce a voltage comparable to MM cartridges (approx. 5 mV), and low output ones, which produce a very low voltage (0.2mV), but which have the potential to sound better, due to less cartridge to preamplifier ‘effects’. Those effects are due to the higher inductance in coils wound with more turns of wire, interacting with the phono preamp. You should be able to measure the input load using a multimeter. However, chances are it won’t be exact, as it’s referring to a 47k impedance (AC resistance), rather than a 47k DC resistance. That 47k impedance will be made up with components such as resistors or capacitors, and IC, valve or transistor inputs, and may vary with changes in frequency. So, the resistance reading is just a rough estimate of the impedance. For instance, if you measure a nominal 8 ohm impedance speaker, it will most likely have a DC resistance of about 6 ohms, with the rest of the impedance made up of capacitance and inductance, varying (sometimes widely) with frequency! You should be able to plug a MM or high output MC cartridge, such as a Denon DL-160 or Ortofon MC-3 Turbo, into a standard 47k input load phono preamp and produce acceptable sound.

Incidentally, another common loading that was often used was 100k instead of 47k, which was used for quadraphonic capable cartridges. The 100 ohms you quote is often quoted as a minimum preamp loading for (for instance) a Denon DL-103 low output MC cartridge. With those, it’s a matter of matching the impedance from the preamp input to the cartridge, using a step up transformer to match the impedance, or a preamp with ‘more than 100 ohms’ input load, according to Denon. So, your amp seems to provide quite a bit more load than the 100 or 160 ohms required by many of those cartridges, even in low impedance mode. If it provides enough gain to amplify the extremely low voltage produced by the cartridge (0.3 mV) it may be OK. To provide the best performance, you would need a transformer with a 40 ohm impedance, to match that of the cartridge, and step it up to the 47 kilohm impedance of the preamp, OR a preamp with plenty of gain, and somewhere slightly more than 100 ohms input load. I’ve heard people quote a wide range of preamp input loads, varying from 100 to 300 ohms, used to load a Denon DL-103 cartridge properly. My own MC amp provides a load of less than 220 ohms, as it has a 220 ohm resistor in parallel with other components, and it sounds OK, although I tend to use step-up transformers most of the time. Whether a much higher load (14k) is acceptable I don’t know – you’d need to check what the amp and cartridge manufacturer says is acceptable for them.

woodturner-fran 26th March 2006 07:29 PM

thanks to all who responded - especially mosfet owner for the laymans terms.... the current cart I have is a grado gold MM - using the 47K load. As above, when I measure this, I get 42K, so I suppose I shouldn't be worried about this from what you're saying.

I've also just bought a Denon 110, lower output (1.6mV) and the lit says 160ohms. When I measured the "lo" I got 14K.... much more than the 160 quoted. Will it a) do the denon any harm? and b) is the 160 quoted a kind of minimum figure, or should I be investigating swapping out some resistors?

again, thanks in advance

Fran

anatech 26th March 2006 07:34 PM

Hi Fran,
Do you have the owner's manual for your preamp? Many times there are sockets for user defined MC cartridge loads.

-Chris

woodturner-fran 26th March 2006 08:55 PM

Hi,

no, don't have the manual - not only that but this is an integrated valve amp (using 2 12AX7 and 1 12AU7 for the preamp). Really I know no more than the phono input being labelled MM/MC and a little toggle switch beside it labelled input impedance Hi/Low.

I have it set to hi with my MM and its fine - I also have a preamp with a MM phono stage and I put that in the loop and could tell no difference in sound between using the preamps phono or the phono stage in teh integrated.... just to make life complicated!

I have this feeling that I won't know until I try it with a MC. FWIW I'm nearly certain that the guy I bought it off was using it with a dynavector MC - I think it might have been a 10X5 which uses 150 ohm and 2.5mV output.


This is a lot more complicated than it looks!!!!!

Fran

anatech 27th March 2006 01:00 AM

Hi Fran,
Okay, help us out and tell us what it is. If I missed it, I apologize.

-Chris


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