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Old 17th October 2005, 04:40 PM   #1
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Default EL34 Feedback circuit

I am considering this circuit for a SE EL34 integrated audio amplifier. This circuit incorporates the OT's + side secondary winding as noted on the schematic.

What is the purpose and effect of this connection?

Click the image to open in full size.

Thanks for your responses.

dr._sleep
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Old 17th October 2005, 05:24 PM   #2
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Mis-labeled OT as "primary" should be "secondary"

Click the image to open in full size.

dr._sleep
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Old 17th October 2005, 05:42 PM   #3
kopite is offline kopite  New Zealand
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Read the articlal that goes with the schemetic on the Lundahl website.
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Old 17th October 2005, 06:01 PM   #4
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Puting the speaker winding in series with the cathode like the schematic that you show provides a small amount of local negative feedback. This performs the usual feedback functions, lowering the output impedance, lowering distortion, improving frequency response. Since the feedback is local to the output tube only, the sound deadening properties of global feedback are avoided.

I have used this type of feedback on SE triode amplifiers. it will usually take a few tenths of a percent distortion away with no ill effects. It can also go a long way toward improving the sound of cheap output transformers. I will use the 16 ohm tap for feedback (connected to the cathode) even though the speaker is connected to the 8 ohm tap. If the amp oscillates reverse the polarity of the feedback by swapping the speaker and ground connections. You can not swap the primary leads because of the ultralinear tap.

This technique can be applied to a push pull amplifier in some cases. Look at the schematic for the Audio Research D76. It will not work with most transformers. The transformer must be wound symmetrically for this to work on a push pull amp. It seems to work well with some of the old UTC transformers. It should work with a toroidal OPT, but I haven't tried it.
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