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In Reply to: Why are TVCs not more popular posted by Geofdillon@gmail.com on June 19, 2025 at 07:32:09:
TVCs, since they use inductors, have a number of problems.
As any designer knows, inductors tend to ring (distort) unless critically damped. There is one specific resistive value that can do that.
A transformer transforms impedance. What this means is that if you have a number of taps on the output or input of the device, the correct value of critial damping (such that it neither rings or rolls off) will be different. As you change the source impedance (which might be the output impedance of a DAC) the critical damping value changes with it.
This all means you have to have a switch that not only accommodates the taps of the transformer but also has provisions for loading at each step.
The second problem is bandwidth, which is affect by a number of factors; interwinding capacitance (which, if the device is not properly loaded, tends to act as a coupling capacitor in parallel with the transformer, from input to output, causing frequency response to not be flat). The device itself will roll off on the bottom and top frequencies. In order to get proper 20Hz bass response with no phase shift, 2Hz is the recommended low frequency cutoff. That's tricky for most transformers. Similarly the high frequency bandwidth should extend to 80-100kHz so as not to sound 'dark' due to phase shift.
A third problem is distortion caused by hysteresis. When the transformer passes an AC signal, the transfer from input to output is not linear, instead is affected by a sigmoid curve reflecting the hysteresis in the materials and design. See link for the math.
On these accounts alone, its very possible to use an active line stage that has less colorations with greater transparency.
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Follow Ups
- technical issues TVCs have to overcome - Ralph 06/26/2509:14:57 06/26/25 (0)
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