Changing Reduction input impedence

[Burch]

Got my Reduction last Thurs. and was listening to great tunes by Friday night. Smooth build, no problems, it seems to be working great! However, I use a Grado Statement Platinum (.5mV) with a step-up going into the Reduction. The input impedance of the Reduction would be a perfect match for the Grado (they recommend ~47kohm), but through the step-up (24dB) I think the Grado "sees" a much higher load, around 186 ohms. So, two questions:

1. I think I need the input Z to be around 12Mohms for the Grado to be loaded at 47kohm...if I swap out the 47.5kohm resistor, would it affect the circuit in any other way?

2. I can easily clip the 47.5kohm resistor out of the Reduction, but putting a new one in (or a couple in series) may be more of a chore. I could quite painlessly/quickly add these to the output side of my step-up though, which seems to me would be electrically equivalent. The only exception is the fact that the resistor would effectively be removed if the input cables were disconnected. Would/could this harm anything?

 

[Paul Joppa]

No transformer actually works at such a high impedance level. Most stepups will have a specified preferred load, usually no more than 47K, which will optimize their treble response - a higher load impedance will expose a treble resonance in the transformer.

The specified inductance of the cartridge is 2mH, which gives a reactance of 186 ohms at 15kHz. That would then be the -3dB point. So a high load impedance is desirable (I'd say 1000 ohms instead of 47K, but they may have other reasons for their spec). It just can't be done with a transformer unless the stepup ratio is fairly small. At a guess, 10:1 or less is the best you can do with a transformer. If your transformer has other taps, I would certainly experiment with them first.

All that said, with a transformer connected the transformer's winding resistance provides the necessary grid bias on the first stage, and the 47K can be removed without harm. I'd suggest carefully unsoldering one end of the resistor and listening for an extended period, before re-soldering it for another extended listen. By then you will have a good idea of the effect in your system, with your music, and using your ears - the only ones that really count, after all. All this theory stuff is useful but it only takes you so far!

 

[Burch]

Thanks Paul!
Yeah, after the original post I did some more digging in the Eros/Seduction forums and realized, mostly from some of your posts, that the SUT may be more critically sensitive to changing the load than the cartridge. So, the cartridge will be -3dB at the frequency where the reactance is equal to the load, am I understanding that correctly?

My SUT uses the Lundall LL1931 (purchased as a kit from K & K Audio). Right now it's set up for 1:16, but I can go with 1:8 as well so I'll try that. Interestingly, K & K also has a phono pre that uses the same LL1931 transformer in the MC input. That same input has switch selectable cartridge loading that ranges from 20-47k ohms, so perhaps the LL1931 is less sensitive to high loads? I'm not really sure...I inquired to them about it, so I'll post when I hear back...

At any rate, I'll absolutely take your advice and disconnect the 47.5k and do some listening

 

[Grainger49]

Burch,

First congratulations in proving and posting that the instructions are right.  Well, that is what your post indicated.  I have confidence in Bottlehead, have for a long time (like 10+ years).

Paul Joppa has posted some formulas for the reflected impedance of SUTs.  I think I have a link. Nope.  Sorry.

 

[Burch]

Hi Grainger,

You're correct, I didn't find anything wrong with the instructions (mine had the 0V for T15, but that's been fixed)...IMO they were pretty outstanding, actually!

Really enjoying the finished product as well, although I can already sense the "upgrades" spectre looming....