My moving coil cartridge output is .4mv . I am able to change the cartridge loading resistor in the phono stage in my hybrid vacuum tube preamp( There are instructions for doing so). The instruction for the cartridge says to use a 20 ohms if using a transformer and 100 ohms if using a headamp. I will not be using either but connecting direct to the phono input on the preamp Do I remove the default 47k resistor in the preamp and use the 100 ohms resistors?
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No, load resistor is for you MC pre-pre or MC preamp, you can't remove the 47K MM load resistor. I mind you need a pre-pre 17.5 dBs gain or a step-up 1:7.5 to achieve 3.0 mV need for your MM phono preamp.
The gain will probably be insufficient for direct use with your new cartridge unless your phono stage has a gain of 70dB or more.
The gain from phono input to main is 67db.The preamp manual (ARC SP9mk2)states that 47k is wired in for most high-output moving coil or moving magnet cartridges. The cartridge specs (ATOC9) call for 20 ohms recommended load impedance. The preamp manual also recommends using the appropriate value (extra resistors provided ) and illustrates how to solder the resistors on a part of the circuit board for the match. So assuming I did not have a phono input stage ..to boost the MC output I could do it with a transformer or a "headamp"..so this is where I am a bit confused..is the phone stage of my preamp then a headamp? If so the cartridge manufacturer recommends 100 ohms load.
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Helpful to know that you have an SP9..
When I referred to the gain required in the phono stage I was referring to that of the phono stage only, it may be that if the SP9 phono stage is sufficiently quiet and if the power amp sensitivity for full power is 1Vrms or less you have more than sufficient gain available as things stand. So the first thing to try is adding a shunt resistor as mentioned in the manual - in this case I would start with 100 ohms and see if you have sufficient gain and an acceptable noise floor.
You may find that you do not have quite enough gain or the noise levels are a bit excessive in which case either a step up transformer or head amp is indicated. (Personally I prefer transformers with my 200uV SPU GM E II and use Lundahl LL1941s, the rest of my hardware is my own diy stuff.)
You'll have to try it and see how it works out. Generally I would suggest additional gain ahead of the phono stage for any cartridge with an output of <1mV @ 5cm/sec lateral recorded velocity.
Ok I will do that in the morning and report back...I suppose then that I should remove the stock 47k and put the 100 ohms in place...thanks
No don't remove the 47K, just put the extra resistor in parallel with it. Bear in mind that in some cases (I am not familiar with the SP9) there will not be a physical 47K anyway, but this will be made up by other resistors etc. You just want your resistor in parallel with the input, on the cartridge side of any DC blocking cap.
I agree with kevinkr, it seems your ARC SP9 it's MM (so you need more gain as stated) I don't change or remove the 47K load resistor, if you go SS route pre-pre I suggest Salas pre-pre MC 20dBs
An externally hosted image should be here but it was not working when we last tested it.
RE cartridge loading
Ok everyone ..here is the circuit and res. connection ..for one channel phono section ARC SP9 mk2
R1 is 54.9 ohms
R2 is 49.9 k
RC is variable (here is where I would add the appropriate vale )
C36 is .560 pf...the manual says not to remove these
Also DF96 I believe you are right regarding the 47ks and will probably do as you suggest
regards
Ok everyone ..here is the circuit and res. connection ..for one channel phono section ARC SP9 mk2
R1 is 54.9 ohms
R2 is 49.9 k
RC is variable (here is where I would add the appropriate vale )
C36 is .560 pf...the manual says not to remove these
Also DF96 I believe you are right regarding the 47ks and will probably do as you suggest
regards
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You could put a 100 ohm resistor in parallel with the 47K and try it. You probably won't have enough gain and won't be able to turn the volume control up far enough to get to listening volume, and if you do, you might have too much noise. In fact, you could just plug it together as is - just the 47K - and see if you have enough gain and low enough noise. Your preamp probably expects on the order of 4mv.
If not, you can either use some MC step-up transformers or a MC pre-preamp which will give you enough signal to drive the MM phono input.
Jim
If not, you can either use some MC step-up transformers or a MC pre-preamp which will give you enough signal to drive the MM phono input.
Jim
I put in the 100 ohms and I was surprised that I had enough gain. I was able to rotate to the 11-12 o'clock position with a comfortable listening level. Beyond that it started to get real loud...then again my listening space is not large. The noise floor is excellent. I think I can live with that. Thanks for the options.
I am not sure of the sensitivity , nevertheless the speakers are Thiels CS3.5.and I use one of two amps at a time ..Adcom GFA 555 or a MCintosh MC2105...regards
As per Stereophile archive Thiel CS3.5 have 88dB/W/m, MCIntosh 105W 4 ohms, Adcom GFA 555 325W 4 ohms, small listening space can help.
For MM load input Salas moderator suggest to put a 100K trimmer instead the fixed 47K load resistor to trimm by hear best result & after change for a very high quality fixed resistor.
Overall gain at 67dB with your power amps seems to be sufficient. My set up has 30dB gain via SUT, 48dB via phono stage and 4dB via the line stage, my power amp has 20dB of gain - I could loose 10dB and still be fine.. So it seems you have a relatively sensitive amp.. (two actually)
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