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Old 14th September 2009, 07:53 AM   #1
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Default Input trasformer for ICEPOWER based amp

Hi All,

I built an ICEPOWER based amp (200ASC) a while ago. I want to add input transformers to the design to raise the input impedance and make the amplifier easier to drive by a preamp. I am looking into something like what Jeff Rowland or Murano are using. In their case they are using the Lundahl 7902 which is very expensive.

I want to try to use an input trasformer from Edcor (www.edcor.com) which I used successfully in a DIY DAC project I did. The problem is that I am not sure which model to select. I know I need a 1:1 transformer. I also know that the static resistance of the Lundahl is 28 Ohmper side. The problem is that the spec for the Edcors is available as impedance and not static resistance and I am not sure which model I need. For my DAC project I used a 600:600 ohm model to couple the DAC directly to the output with no active circuit and it was magnificent.

Can anyone assist?

Thanks
Ronen
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Old 14th September 2009, 08:19 AM   #2
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Correction: the link to the Edcor site is www.edcorusa.com
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Old 14th September 2009, 08:26 AM   #3
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A 1:1 transformer will not raise the input impedance. For that you'll need a stepdown turns ratio. Perhaps something like 5:1 if you can afford to lose some gain. If you can't afford to lose the gain, perhaps an IC buffer will do the trick.
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Old 14th September 2009, 12:01 PM   #4
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I am not sure how the input transformer is used in the Jeff Rowland and Murano amps but they Lundahl 7902 that they use is a 1:1 transformer. I guess this is more complicated then I thought.
Anyone can shed some light on this topic?

Thanks
Ronen
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Old 14th September 2009, 12:20 PM   #5
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Quote:
input impedance
I guess that the Rowland used an input transformer for different reasons than input impedance.

So I guess that you should get exactly the same Edcor transformer you used for you DAC. 600R:600R
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Old 14th September 2009, 03:08 PM   #6
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Frank is right. You don't get something for nothing. Without an active circuit you'll lose gain to get a higher input impedance.

That may be OK if your preamp can output a healthy level. Can it supply enough current to drive the 8~10K of the Ice? If not, can it supply a high voltage level that you can step down?
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Old 15th September 2009, 09:11 AM   #7
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OK, so I guess its not impedance matching for which the transformer is used.
I this case how would I translate the static resistance of 28 Ohms of the Lundahl transformer to an impendace figure which is what Edcor has in their spec.

Thank you to all who helped so far,
Ronen
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Old 15th September 2009, 09:28 AM   #8
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An input trafo is most often used to get galvanic separation of the source and the amp. Most often to ensure that no stray ground voltages comes neither in nor out of the device. Normally a 1:1 trafo would be used for that.

It is also often used as an output trafo to make a balanced signal out of a unbalanced signal, an in the input then reversing that. Both ends will normally use a 1:4 step-up/down trafo, reversed on each end.

You can also use a 1:4 step-up trafo to achieve passive signal processing that would otherwise result in level loss, such as a passive line level filter.
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Old 15th September 2009, 04:26 PM   #9
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Ditto the above.

Which model Lundahl do you have? Does it state a ratio?
If your preamp is capable of high voltage output - like a tube preamp - then the matching transformer could work very well for you.
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Old 15th September 2009, 05:18 PM   #10
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HI,
if you have a look here http://www.jeffrowland.com/M501-detail.htm you can see that they use an active circuit together with the input transformer. Search Jenssen sites for info on input circuits where you may find some suggestions. I am not sure, however, if this is exactly what JR uses.
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