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Old 3rd August 2008, 09:12 PM   #1
Sandor is offline Sandor  Italy
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Location: Milan
Default Transformer Volume Control

I have found a transformer for volume control with the following specs:
primary 1200 ohm center tapped, secondary 10 Kohm with 23 taps.
The manufacturer suggests to put a 50 ohm series resistor on input.
Using center tap on input provides a 6dB gain.

It seems to me that input impedance is too low to provide a good match with the source. Do you agree? Or that input impedance is typical for that kind of component?

Best regards.

Paul
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Old 4th August 2008, 02:20 PM   #2
oshifis is offline oshifis  Hungary
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I am just guessing:

Are these impedance values measured at 1kHz or at DC? Or do they come from the data sheet? Are the taps logarithmically arranged? Do you have any further info from the manufacturer (at least the type or part number)?
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Old 4th August 2008, 02:58 PM   #3
Sandor is offline Sandor  Italy
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Values given by the manufacturer refer to impedance (frequency not known) and not to DC resistance. No further information. I have found the transformer on ebay, item #110217168156.
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Old 4th August 2008, 04:31 PM   #4
oshifis is offline oshifis  Hungary
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Hm, it looks interesting. Probably it has the flattest or widest frequency response if the primary is driven by 600/1200 ohms and the secondary is loaded with 10k. That fits for studio environment, I think. Altough the schematics shows a 30-50R source and 50-100k load, which is more suitable for home DIY use. You will need also a good quality 24-pole rotary switch.
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Old 4th August 2008, 09:42 PM   #5
Sandor is offline Sandor  Italy
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Thank you for you answer.
Don't you think that input impedance is way too low for use in a preamplifier? not to mention the fact that it should have a voltage gain of about 16...
The use that comes to my mind is as a step-up xformer in a resistor-based I-V converter for current output DACs, such as TDA1541A. Naive idea?
Best regards.

Paul
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Old 18th August 2008, 07:40 AM   #6
ondesx is offline ondesx  France
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May we know more on this Tranny ? Manufacturer, refs, specs, availability, price, etc.

TIA.
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Old 23rd September 2008, 11:55 AM   #7
ondesx is offline ondesx  France
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I think there is some misunderstanding with the provided data. There is no sense to speak of the input or output impedance of a tranny, since it "reflects" to the input the impedance of the load and to the output the impedance of the source weighted by the square of the number of turns !

A more interesting data would be the inductance, the attenuation curve and the voltage input before saturation at 20 Hz !

I'll ask the vendor these infos before buying any...

I designed several TVC with gear from different countries (England, Thailand, USA...) most of them are trannies but the present one which is an autoformer !

I'm still looking for a very good price/quality ratio for such devices since the previous trannies remain expensive...
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Old 1st October 2008, 09:51 PM   #8
ondesx is offline ondesx  France
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The vendor doesn't answer to any question... May we trust with this guy ? Humm...
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