MC impedance matching

[RobHolt]

If I understand things correctly, with 1:10 ratio MC step up Tx 100k on the secondary reflects as 1k on the primary, 50k as 500R, 25k as 250R and so on.
For a true value presumably the DC resistance of the windings needs to factored in?

I'm using Sowter MC step ups in my Phono 2 and want to add the facility to change impedance by having a switchable load across the secondary.

Anyone done this?

cheers,

Rob.

[Richard]

Hi Rob

Not done it but yes you'd expect to have to add the resistance of the TX primary to the load reflected from the amp input it is plugged into.

The problem with your (sensible) ideas may be the lack of info available as to how the manufacturer's of your cart specify the exact load. For example you'd expect inductance and capacitance to have a bearing also. This is all part of the black art which is vinyl. An engineering approach would be to check the overall frequency response for whatever load and riaa/amp you use.

In practical terms I think switchable shunts across the input would at least show how sensitive/worthwhile small changes of loading might be.

Rich

[RobHolt]

Thanks Richard,

Cartridge is a Lyra Argo and Lyra recommend a very low TX load to match the 3 Ohm cartridge impedance. Driving from a 3 Ohm source ought to make capacitance changes fairly benign in effect on the frequency response.

I use medium impedance Sowter stp-ups in the Phono II and have been loading the secondary with the standard 47k which with the Lyra makes the top end a tad fierce at times. I changed the secondary loading to 10k a few days ago which ought to make the Lyra see about 125 Ohms and it certainly sounds more neutral as a result.

While I run the Argo for most stuff I also have a mono wired Denon 103 for old mono vinyl and this needs very different loading, hence the need for switchable loading.

I'm thinking of making loading steps of 50R, 125R, 250R, 600R and 1K available. I have a spare WAD chrome knob, appropriate selector switch and some well screened cable. I'll give it a do and report back

Rob.

[NealG]

Rob, I've found that loading a cart with 7-10 times the internal impedance puts you in the right area, you could try 20-50 ohms. I've recently loaded my KB at 47R and it sounds more natural. I also found with the DL103 and 103R this ratio worked well. YMMV.

[RobHolt]

Originally Posted by NealG

Rob, I've found that loading a cart with 7-10 times the internal impedance puts you in the right area, you could try 20-50 ohms. I've recently loaded my KB at 47R and it sounds more natural. I also found with the DL103 and 103R this ratio worked well. YMMV.

Thanks Neal,

The Lyra sounds incredibly open and dynamic on a high (>500R) loading but I'm convinced some the extra zip and life is being added due to a little top end lift. Lower loading certainly gives a better balance.

[RobHolt]

Job done.

Six selectable impedances:



....and a nice new knob & LED on the front of the phono II (panel marking will go on when the transfers arrive):



Selector switch and resistors simply connects to the space vacated by R13/14 on the pcb. Keep the cable short (6 inches will do it) and uses screened coax to prevent hum.

[RobHolt]

and to finish things off........