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Power transformer DCR sets the dynamic impedance.

Most folks on this forum understand that there is a balance to everything. Most also know that copper losses are not the only factor in power transformer and choke design. While I have seen chokes designed to the point where core loses become the major culprit, I’ve yet to see this in a power transformer. The low DCR power transformers I have used and seen thus far have ran very cool and quite, had nice clean sinusoidal waveforms, and had very small external magnetic fields. The power transformers I use are simply over specified for the current demand I need. I just over specify the required current until the DCR becomes low while keep ALL other parameters within good engineering ranges. I don’t think anyone here is advocating throwing all other parameters into the wind over a singular parameter. That would be nonsense. I think what is being advocated here is the use of a properly designed power transformer with a low DCR secondary.

The point that cannot be argued here is the fact that the power transformer DCR sets the stage to where the power supply’s dynamic impedance will fall. (Active regulated power supplies excluded here) It is very difficult to get below 400 ohms dynamic impedance on a power supply that uses a 255 ohm secondary winding. Drop the secondary DCR from 255 ohms down to around 20 ohms and all a sudden you are able to get below 100 ohms dynamic impedance for the power supply. Elementary electronics teaches us that a power output section needs a low impedance power source. If the power source has significant impedance, then power losses occurs in the source instead of being transferred to our load. If you take the plate impedance of the 45 tube to be about 1650 ohms, which power supply is better for playing unimpeded dynamic music, a 400 ohm one or one that is less than 100 ohms?

If you are using a high impedance power supply in your power amplifier then you are leaving a lot laying on the table. I try very hard to keep my power supply impedance below 1/10 of the plate impedance for my output tube. This makes a world of difference in the dynamic capability of the amplifier. I like clean dynamic sound. Some people like the overly romantic sound of a high impedance power supply. I believe this is where one of the splits occurs between camps. Right, wrong or indifferent, not everyone likes the same thing.


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