[AMRadio] Audio Driver Transformer advice |
Donald Chester
k4kyv at charter.net
Fri Jan 16 01:45:13 EST 2015
----- Original Message ----- >>>>From: Paul Baldock <paul at paulbaldock.com> I'm building a modulator with 2 x 813's in pushpull (strapped as triodes) driven by a 20W Solid State audio amplifier. I need a driver transformer to go between the 813 grids and the 20W amp. The 20W amp can drive about 20V peak to peak. I think I need about 200V peak to peak for the 813's, so a 10:1 turns ratio ratio would be about right. So based on that an audio transformer designed to go from 600Ohms center tapped to 4 or 8 ohms would probably do the job. Have I got the math right? Does something like this exist? Thanks Paul >>>>> For best results, use the least amount of step-up possible to just barely get the required peak-to-peak audio voltage, with a little bit of additional head-room. Ideally, the driver amplifier will go into saturation (flat-top) at the same instant that the modulator is driven to its maximum capability. IOW, the driver and modulator stage reach saturation at the same time. I would design in a little extra margin, so that the driver is capable of driving the modulator slightly beyond saturation, just to make sure you get the maximum undistorted audio power out of the modulator before the driver starts clipping. A typical class-B grid impedance varies widely over the audio cycle, from infinite ohms (on the negative half of the cycle when the tube is not conducting, down to a minimum of about 500 ohms at the crest of the positive peak. The grid voltage / grid impedance curve will likely be non-linear. The total impedance of the whole mid-tapped winding would be something like 4 times 500 ohms, or 2000 ohms. Using the 8-ohm tap, the impedance ratio would be 8 ohms to 2000 ohms, or a 1:250 Z ratio. That would be equivalent to a 1 : 15.8 turns ratio. The 4-ohm tap would require more step-up, 1 : 22.7. I doubt that a 600 ohms to 8 ohms transformer would give enough voltage swing. It may be hard to find an output transformer with a primary impedance as low as 2000 ohms at that power level, although I'm sure such a thing exists somewhere. In the real world, I would look for a good quality hi-fi output transformer rated for at least 20 watts, and preferably 30 watts or more, with as low a plate-to-plate impedance on the primary as you can find, but nothing much below 2000 ohms. A (backwards wired) transformer with more step-up will deliver more peak-to-peak voltage to the grid, but the effective internal resistance of the source voltage will be increased, giving more distortion as the audio source voltage regulation is degraded. Putting it another way, using too much step-up, the grids of the tube will cause the audio voltage output from the transformer to sag. Ideally you might find a transformer with multiple taps so you could experiment to find the optimum turns ratio, but you'll have to make do with the best you can find. Also, a transformer rated for higher power (and lower primary impedance) will tend to have lower winding resistance, which would reduce the audio voltage sag. Once you have found what looks like a suitable transformer, try both the 4-ohm and 8-ohm tap and see which one gives the most satisfactory results with the least distortion. Don k4kyv --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com
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