Moving Coils and Moving Magnets: Stepping Up & Stepping DOWN

[The Grand Wazoo]

Here's a question for you!
Has anyone ever tried converting a moving coil phono stage into a moving magnet one by wiring SUT's in back to front?

I need to find a way to use a MM cartridge as a temporary measure. I have a noisy MM stage in my pre-amp which I need to get seen to - hence the problem.

Before that can happen, I'm going to be playing with a different turntable/arm/cartridge combination - the cartridge is a Decca. Recently, I've been using my LFD MC1 phono stage (MC only). So what I'm interested in is whether I can take my Bent Audio Mu SUT's and wire them backwards so they reduce the Decca output to moving coil levels. I know this is not going to be anything like ideal, but I can't think of any other way of doing it without buying another phono stage.


So, what do you reckon - will it work?
The key thing is that this is meant to be temporary.

It'd be nice to hear some thoughts on this.
Cheers folks!

[Reid Malenfant]

I see no reason why not, electrically there is nothing i can see that'll prevent it working. Quite how it'll sound is another matter. But as a stopgap does that really matter?

[MartinT]

It should work but it's difficult to predict how it would sound. Transformer impedance coupling often works well so maybe it'll be a revelation!

[The Grand Wazoo]

I see no reason why not, electrically there is nothing i can see that'll prevent it working. Quite how it'll sound is another matter. But as a stopgap does that really matter?

That was exactly my reasoning. I've already got all the components & it's got to sound better than some cheap MM phonostage, surely?

[The Grand Wazoo]

It should work but it's difficult to predict how it would sound. Transformer impedance coupling often works well so maybe it'll be a revelation!

That's also something that occurred to me - it's something so ridiculously stupid that no-one's tried it before (I mean why on earth would you!) & it could, just maybe, sound incredible!

.......probably not, though!

[Reid Malenfant]

I think you may well be surprised, thinking about it the only thing that's likely to happen is that the cartridge or should i say the phono stage will just appear to be a little bit noisier. Impedance matching is still there so i wouldn't expect anything untoward to happen to the music.

If anything it'll be an interesting experiment :) You also keep listening to LPs while your other stage gets fixed, in fact if it was noisy then this might well quieten things down a tad.

[The Grand Wazoo]

I think you may well be surprised, thinking about it the only thing that's likely to happen is that the cartridge or should i say the phono stage will just appear to be a little bit noisier. Impedance matching is still there so i wouldn't expect anything untoward to happen to the music.

That's good to hear


If anything it'll be an interesting experiment :) You also keep listening to LPs while your other stage gets fixed, in fact if it was noisy then this might well quieten things down a tad.

Actually, no because it's integral to the preamp & sooner or later, I'll have to forgo records (& tape) for a bit while I have it fixed. This is, of course, the reason I've not got around to it yet - because I had another way of playing MC's, it just didn't seem so vital to address the problem.
At least my CD player & tuner have their own volume pots, so I suppose I can make do!

[DSJR]

I wish my maths was good enough to do some calculations as regards input impedance as seen by the Decca ("normal twangy ones" like 30 - 40K instead of 47K - mine doesn't "twang") and some moving magnet types prefer summat higher than 47K IMO (maybe there was some sense in Quad using 68K for the 33 pre phono stage back then).

Good luck :)

[The Grand Wazoo]

Cheers Dave, I'll report back after the weekend.

[Barry]

Here's a question for you!
Has anyone ever tried converting a moving coil phono stage into a moving magnet one by wiring SUT's in back to front?

I need to find a way to use a MM cartridge as a temporary measure. I have a noisy MM stage in my pre-amp which I need to get seen to - hence the problem.

Before that can happen, I'm going to be playing with a different turntable/arm/cartridge combination - the cartridge is a Decca. Recently, I've been using my LFD MC1 phono stage (MC only). So what I'm interested in is whether I can take my Bent Audio Mu SUT's and wire them backwards so they reduce the Decca output to moving coil levels. I know this is not going to be anything like ideal, but I can't think of any other way of doing it without buying another phono stage.


So, what do you reckon - will it work?
The key thing is that this is meant to be temporary.

It'd be nice to hear some thoughts on this.
Cheers folks!

Don't worry Chris, it'll work fine as a temporary 'stop gap'. Have sent you a PM (and e-mail) spelling out chapter and verse for you.

Regards

[The Grand Wazoo]

Thanks Barry - I'll check the various in-boxes.

[Barry]

Here is the 'chapter and verse' to which I referred.

Using a Bent Audio Mu transformer 'in reverse' to match the Decca cartridge to your LFD MC-1 phonostage.

First some technical facts:

LFD MC-1 phonostage
input impedance: 50 Ohm

Bent Audio Mu transformer (using Stevens & Billington TX 103 devices)
Gain 20:1 (26dB)
primary winding resistance: 130 Ohm
secondary winding resistance: 16 Ohm

Decca H4E cartridge
output voltage (at 5cm/s): 3.75mV (min, more likely 5mV)
recommended load impedance: 50kOhm

So if you use your SUT "in reverse", assuming you can sort out the opposite sex connectors, the 50 Ohm input impedance of the LFD will load the Decca with 26.5kOhm. (50+16).400 + 130 = 26530 Ohm.

This is lower than recommended, but some users prefer a loading of 25 -33kOhm for their Deccas.

The output of the Decca will be reduced from say 5mV to 5/20 = 0.25mV; comparable with that of your Koetsu at 0.36mV.

So on paper it ought to work; certainly no harm will be done, but I'm sure you understand it's not an optimum arrangement - only a 'stop gap'.

You will suffer an increase in thermal noise, but since I cannot lay my hands on data concerning the coil resistance for the Decca Mk IV cartridge, I'm unable to calculate what this increase will be.

[The Grand Wazoo]

Well, it sounds like it's worth persevering with. You're correct, I will need to sort out the opposite sex connectors because the Bent Audios have a captive output cable..........

http://img43.imageshack.us/img43/780/imgp1683.jpg


.........(or input in this case) These should be easy enough to deal with - I've got some male:male adaptors in my box of bits.
Also, I'm normally able to load the input with a resistor of any value, but presumably, as the input will now be the output, that's fairly irrelevant?

http://img39.imageshack.us/img39/7164/imgp1677.jpg

[MartinT]

Don't load the 'input' any more as Barry has already calculated a fairly low 26.5k ohm output impedance. It's hard thinking backwards!

[spendorman]

I wonder what capacitance the transformer will present to the moving magnet cartridge.

It is interesting that the Radford SC22 preamp can accept low output moving coil or moving magnet cartridges on the same input without any adjustment.

[Rare Bird]

Just fix the MM stage yourself? :scratch:

[The Grand Wazoo]

Just fix the MM stage yourself? :scratch:

That word there 'just' is a pretty big deal Andre!

[Rare Bird]

Is it the ARC? Which one?

[The Grand Wazoo]

It's an early SP8

[Rare Bird]

Do you know which revision it is Chris?

[The Grand Wazoo]

It's the original - people call it 'Revision 1' but I don't understand how something that hasn't been revised can be called a revision!

[Barry]

I wonder what capacitance the transformer will present to the moving magnet cartridge.

It is interesting that the Radford SC22 preamp can accept low output moving coil or moving magnet cartridges on the same input without any adjustment.

Using the transformer 'in reverse' will increase the capacitive reactance, just as the impedance will be increased, i.e 20:1. So the capacitative loading of the LFD MC-1 phono stage will be reduced by 20:1.

Some additional capacitance might be needed; connected across the terminals of the Bent Audio Mu SUT, but it will be case of 'horses for courses'. In general Deccas do not need additional capacitance (despite the fact that I found with my Mk. VI in one particular set up required an enormous amount of capacitance (3,900pF) to tame the top end).

Obviously the first course of action will be see how the 'lash up' works with the Decca 'au naturel', before experimenting with additional capacitance.

If anyone can advise me of the capacitance loading of the LFD MC-1, I will be able to do the necessary sums.

[The Grand Wazoo]

Quick update - the turntable is here, the Bent Audios are connected up in reverse gear to the LFD and all sounds fine.
There is a bit less gain that the Koetsu through either the LFD or the SP8+SUT. There's a little hum but not too significant & I'll try various different earthing possibilities shortly, but in the meantime there's too much music to be listening to!

[Barry]

Quick update - the turntable is here, the Bent Audios are connected up in reverse gear to the LFD and all sounds fine.
There is a bit less gain than the Koetsu through either the LFD or the SP8+ SUT. There's a little hum but not too significant & I'll try various different earthing possibilities shortly, but in the meantime there's too much music to be listening to!

The Decca's output will be reduced to ~ 0.25mV. Compared to the Koetsu at 0.36mV that's -3.2dB; not too bad - certainly liveable.

Happy listening! :)

[Reid Malenfant]

It's an early SP8
Oh yes, get the ARC fixed ;) I'd be lost without my MP1 now i have lived with it for a while.