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Torroids as audio

Posted by Paul Barker on February 5, 2002 at 02:23:36

I’ve been experimenting with power torroids for audio, like Steve Bench. Time to update folk with my findings.

Like Steve I used the Talema 3.2 watt 115+115:22+22 on the input as a phase splitter. My Wayne Kerr bridge has broken so I cannot tell you any inductances easily, and don’t have time to set anything else up. But I can sweep accurately from 20hz to 20khz, and here are the findings in that range. The input, phase splitter transformer is completely flat for the entire range. That was encouraging.

I am not used to push pull building so many failed attempts to make use of this and head scratchings later, what I ended up with was a pair of 6em7’s, oscillator to maximise gain after phase splitting transformer, rc coupled to power sections, which are rc coupled to drive ½ each of a 6080.

Output transformer is a 60va 115+115 into 6+6 volts. Secondary paralleled for about 3k pk to pk into 8 ohm.

Measuring the whole, we start light -3db at 20hz gradually and smoothly rise in response to 100hz where we remain flat to 20khz beyond which I cannot generate an accurate signal at the moment (using a test CD, the valve signal generator has decided to become a radio receiver and plays Yorkshire Coast Radio quite nicely. Something in there is oscilating I guess.)

Where are we losing the base? Well I set a fairly high role off point at the final drive, around 8hz, thinking to protect the output’s from low level saturation. This is the point where power begins to fall. I measure a 1/12th peak to peak voltage drop at the point where the output tube is driven at 20 hz. Well I wouldn’t say that was much of a problem. Changing the cathode bypass cap on the output stage makes no difference to base response. I think the only explanation is the limitations of the output transformers alone.

How does it sound. Excellent. I am driving the output stage deep in class a, a long way from any possibility of AB and the sound is good. Very nice tone, good air, loudspeakers are impossible to visualise if you shut your eyes, it’s a room full of depth and breadth. The fact that it is base light does not show, -3db at 20hz is acceptable. It depends on your use, and how you have set your hearing expectations. You can tune your ears to accept this. Room acoustics play a much larger part in the picture.

These would be fine for mid/high frequency bi-amp, or for Lowthers with an active sub.

If I wanted a one for all amp in my very best listening room, I would buy the correct output transformers, but for 95% of applications these torroids put in a first class performance, the overall package is far better than most other things I have heard.

Power capability opf the torroids is your enemy, I am driving mine to just 3 watts, for Lowthers that is overkill. If you need much more I think again you’ll need proper output transformers.
I took on board Dave Slagle’s suggestion of increasing the turns per volt on the torroid, but for me to achieve that is about as much work as winding myself a C core, so I left it alone. I would have to hand thread each turn, I recon on about 10 turns per volt, so I’d be there all day, on each one and it would look like a bird’s nest at the end.

Regards

Paul Barker

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