ronaldw441 wrote:Please describe the best way to measure the bandwidth of an output transformer when out of circuit. I would like to compare a Hammond 1650N to an Edcor CXPP60-MS-4.2K.
Here are some results from several transformers I've tested. Unfortunately I've been inconsistent. Some were measured using a variac, others with a signal generator. None have been loaded and none at 400hz. I'll have to go back & try dcriner's method.
I'm still learning this too, but here is my stab at it:
1) Go ahead and yes, re-test the transformers loaded. Test them on the tap you will be using with a resistive load that most closely matches your intended speaker load. Note impedance.
2) Increase frequency to find the HF -3dB. Take the nominal output - say you are measuring 300mV - and multiply it by .707. Increase the frequency until your output drops to 212mV. That's your HF cutoff. Higher is better, and will permit higher levels of feedback and stability.
3)
Difficult step you might omit, since values will normally be under 5Hz and that gets difficult to read, even if your function generator can go down that low. Decrease frequency to find the LF -3dB. Take the nominal output - say you are measuring 300mV - and multiply it by .707. Decrease the frequency until your output drops to 212mV. That's your LF cutoff. Lower is better, and will permit higher levels of feedback and stability. Maybe look for the -1dB at LF instead, since that will be much easier. Multiply by .89.
4) Shoot a 10kHz square wave through the tranny and monitor the loaded output on your oscilloscope. Less ringing is better. It is possible you might have a tranny that has a higher HF cutoff than another, but has more ringing. I would then select the transformer that rings less as the superior transformer.
These tests don't test saturation/output power capability. For that, just go with the boat anchor test.
I'm very interested in your Edcor test - please let us know your results.
Shannon