Using Windings Of Output Transformer To Provide Negative Feedback To Cathode Of A Single-ended Output Stage

Purpose: to improve output speaker damping in order to reduce loudspeaker bass resonance ('boominess') that is normally suppressed by closed loop negative feedback coupled with high open loop gain. This does not exist in 'minimalist' single-ended valve amplifiers of few valves, low gain and having little or no negative feedback at all.

Test Circuit
Output valve: KT66 as triode
Driver valve: EF36 pentode (gain = 40dB [x100])
Transformer: DB1087 (A, B); modified DB1090 (C)






Performance Of Test Circuit Into Loudspeaker Model


'A': no feedback, Ra = 5,000Ω 
'B': using 16Ω speaker winding, Ra = 5000Ω 
'C': using 1300Ω section of primary, Ra = 3700Ω 

Results
It's interesting that the resonance with no feedback is still quite low at only +3dBV, so some damping is evident, mostly due to the low resistance of the speaker winding, but also because the Ra increases in proportion and so the power goes down. This is not a complete picture of course, since for a real speaker the effect is exacerbated by mechanical resonance of the speaker cone, resulting in exagerated sound output at the resonant frequency.

Best results were obtained using the full (16W) speaker winding in series with the cathode ('B'). This reduced the resonance peak to just +1dBV. Surprisingly, using a 1300Ω section of the primary in its place was worse, producing +2dBV ('C'), not very different from 'A'!

Total Voltage Gain Of Output Stage
I.e. input grid to 8Ω speaker tap:

'A': x 0.234 (best) 'B': x 0.2 'C': x 0.143

i/p sensitivity for max. o/p (10W): 385mV 450mV 630mV

Note the 1300Ω winding added an extra 37Ω to the cathode circuit, which of course could not be bypassed, and so this mostly accounted for the reduced gain for 'C', in addition to the feedback. Of course the speaker winding option ('B') has a negligible impedance by comparison.

My personal conclusion is that option 'B' offers the best compromise (and doesn't involve hacking the transformer primary!). I tried the amp with a small, very simple, closed-box 'music-centre' type speaker and couldn't discern very much in the way of resonance.

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