Understanding TAP-X input impedence

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willamp

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Understanding TAP-X input impedence
« on: 9 Jan 2012, 01:44 pm »
Hi All, I recently started investigating alternative volume controls and find TVCs interesting. I use SUT for my phono cartridge so am familiar with secondary load based equations to calculate primary impedance.

If I apply this equation ( Rp=Rs/(Gain^2) to the TVCs it appears that at full volume (gain=1) my preamp would see the impedance of my power amp input (150K) and as you turn the volume down the input impedance would rise very quickly. For example -6db (gain of 0.5) would give an input impedance of 600K (150K/(0.5^2)). Is my thinking correct here? If so this seems like a very good situation.

Is there such a thing as source impedance of the secondary of the TVC attached to the amplifer that needs to be considered relative to the power amp input load impedance? This where I keep getting a bit confused. Do you look backwards through the transformer and consider the source impedance of the pre-amp and apply the transformer gain equations? If this does need to be considered does it decrease as volume decreases?

thanks in advance for any enlightenment

willamp

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Re: Understanding TAP-X input impedence
« Reply #1 on: 9 Jan 2012, 04:54 pm »
Just to clarify, when I refer to preamp in my above post I'm referring to my phono preamp. Probably should have said source, instead.

Phil

John Chapman

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Re: Understanding TAP-X input impedence
« Reply #2 on: 11 Jan 2012, 09:15 pm »
Hello!

The real transfomer guys (like Dave Slagle who makes the autoformers) can run circles around me with the details. There is more to it than the strict 'square of the turns ratio' math but in ball park terms that is the action when calculating the output impedance of the pre-amp. As you turn the volume down that results in a lower and lower output impedance driving the amp. Because it is the turns ratio squared it drops rather quickly.

From the sources point of view it is not just that ratio that dictates the load seen by the source. The inductance of the winding is a large factor in what that impedance is. For example even with the high amp input impedance you list the source will see an impedance lower than that - particularly at lower frequencies. This is why these devices end up being pretty large and heavy - it takes some turns around the core to get inductance up high enough to be an easy load for typical sources.

Hope that helps a bit.....

Thansk!

John