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Koetsu Urushi + MC step up transformer

After years of listening to my Koetsu Urushi fed directly into the 47K MM input of a Conrad Johnson PF1 I have started using a step up transformer. It is built around a pair of Lundahl transformers and I have tried some different loading resistors but I find the sound is quite harsh and has lost some of the air and space it had before.

I was told that the transformers would need 50-100 hours of bedding in and they would loose the harshness... Hmmm. I've never been a great believer in the burn-in philosophy for entirely passive components - like bits of wire - will a MC transformer burn-in to any extent or should I continue playing with the loading to find the best match or just toss the whole thing out the window?
68spider
68spider OP
6 posts
 
It is a transistor pre-amp, I don't have a schematic of the PF1, the instruction manual only states that it is the industry standard 47,000 ohms shunted by 100 pF. It describes how to lower the impedance with a parallel resistance but not how to increase it.

I cannot find a Zobel network diagram on the Jensen website but their MC transformer diagram adds a capacitor in series with the modifying resistor - that may be worth trying.

At the moment I'd rather not dig inside the pre-amp without some guidance, perhaps I should try contacting CJ for a schematic. Otherwise I'll wait and see if it burns with more use.
lewm
13,093 posts
 
Have you yet tried connecting the trafo to the input with the standard 47K-ohm resistor in place, in other words with no other changes? Should at least work well. The networks etc are for touching up the response to make it as flat as possible; a Zobel or added capacitance is certainly not a requirement.
68spider OP
6 posts
 
Yes, I've tried pretty much every combination of set up including directly connected with no other loading - which would be a 278 ohm load on the cartridge - it all sucks. And pretty much to the same effect, there doesn't seem to be much variation whatever I try.

Have I set up the rest of my system to over compensate for connecting the Urushi into a MM load, such that doing the "correct" thing into a MC load is now throwing the setup so far out of whack?
lewm
13,093 posts
 
Can you be a little more descriptive? In what way does it "suck"? Do you lack gain even with the SUT? Is it bass heavy or exaggerated in the treble? Is it grossly distorted? Are both channels behaving in identical fashion? Etc...

Since I believe K&K are the importers of Lundahl, you may be able to get good advice from Kevin. He is very knowledgable.
68spider OP
6 posts
 
The "sucky sound" is a lot of harshness in the upper treble - quite unpleasant. Also, the sound stage has gone completely flat, all the air and space around the instruments has disappeared. Bass seems OK. There is no problem with the gain, the 13:1 ratio is just right.

I'm in the UK and didn't get the Lundahls from Kevin, although he did send me his spreadsheet for calculating the load resistance.

Both Sowter and Jensen recommend a Zobel network to reduce ringing in the transformer. It is just a resistor in series with a capacitor across the secondary windings. I have added such a network to the Lundahls and, on initial listening, this sounds very promising.

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