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Old 18th August 2012, 11:40 PM   #1
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Default Step up transformer

Hey Anyone?

I was trying to build a step up transformer for a friend to get some output from an old B&O MMC to match the input level on his newer Sony receiver, (no phono preamp). It was to go with a PAS 3x phono section. In the end I got him to buy an NAD PP2, problem solved.

But now I have these Shure microphone transformers and a really nice Mahogany case I built. So what do I do with them?

The thought occurred to me that I might use them for my own system? I have a Rega Planar II with a Bias 2 cartridge. The Bias 2 has an output of 7.2mv which is fine but doesn't match the final output of my Planet through the system. The transformers are good for a 1/10 voltage gain which would give me 72mv volts out of the transformers. My Audio Research SP3a-1 has a maximum 300mv on the low level phono input. So unless my decimal brain is wrong I'm OK here?

I have two questions. First, if the cartridge has an output impedance of 47k, and the AR has an input impedance of 47k do I still need a 47k load resistor across the step up outputs?

Second, the Planar 2 doesn't have a ground wire, the tonearm is 'internally grounded' whatever that means. So will I need to run a ground from the step up transformer to the preamp? On the Shures the transformer case is grounded to the signal ground on the pc board. Should the signal ground be connected to the SUT chassis and then the chassis to the preamp? Or would it be better to separate the transformer case from the signal ground. And then ground the cases through the SUT chassis to the preamp?
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Old 19th August 2012, 12:08 AM   #2
kevinkr is offline kevinkr  United States
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These transformers expect to be driven by the very low source impedance of an MC cartridge, with an MM the primary inductance is not going to be high enough for decent bass performance. In addition the level from your cartridge is likely sufficient to saturate the transformer.

To reflect the correct load impedance to your MM cartridge would require a 4.7M resistor at the input of your phono stage, assuming that your transformers had sufficient inductance to work properly which they don't.

All in all not a good idea. Sell the transformers or get a MC cartridge with suitable compliance for your arm, and a suitable source impedance and output level for your transformers.

I live on a daily basis with MC cartridges (4 of them) and a couple of SUTs as a point of reference.
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Old 19th August 2012, 02:56 PM   #3
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Thanks Kevin, I had a feeling it wouldn't work but didn't know enough about it to know why. I'll finish the SUT and sell it on Ebay.

I'm assuming a 47k load resistor is in order? And should I provide a chassis ground on the SUT? And if a ground is recommended should the transformer case ground and/or the signal ground be connected to it?

Last edited by mr2racer; 19th August 2012 at 03:17 PM.
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Old 19th August 2012, 03:38 PM   #4
kevinkr is offline kevinkr  United States
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Keep the grounds separate for each SUT, and provide a separate chassis ground. No load resistor required internally - the end user may need to select a different value.
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Old 19th August 2012, 04:38 PM   #5
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Thanks Kevin. I just looked on Ebay. Thy get stupid prices for these things! If I get my $45 bucks back I'll be happy.
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