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Dynakit ST70 Restoration Project

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jcrubin:

--- Quote from: TimFox on August 13, 2019, 04:03:02 pm ---With respect to voltage vs. current, the usual dynamic loudspeaker in an enclosure is a electromechanical resonant structure operating (mainly) above the free-resonant frequency.  The mechanical and acoustic parts of the total system dominate the purely electrical parts:  the dc resistance of a nominal 8 ohm speaker is less than the impedance.  Above resonance, the speaker naturally wants a constant-voltage drive.  If you use a “voice coil” mechanism at lower frequency (below resonance of the complete mechanism), then a constant current drive is more appropriate.  Virtually all audio power amplifiers are configured as constant voltage sources, with output impedance substantially less than the rated load impedance.  The Dynaco tube amplifiers achieve this by voltage feedback from the output transformer secondary to the cathode of the input pentode.

--- End quote ---

This is why when I went to the new rear posts I was sure to tie the 16 ohm outputs to internal standoffs so the they could be incorporated into the feedback circuit. My understanding was that the 16ohm taps were generally used for this.

bob91343:
I had an associate many years ago who wanted to make a better audio system.  Instead of feeding back the output voltage of the amplifier to get flatter response and lower distortion, he instead wound a tickler coil on the voice coil and used its output for feedback.  He called it motion feedback, since the coil output was somewhat proportional to the movement of the voice coil.

I don't have any more details but he had the thing set up with a rather small speaker system and it sounded wonderful.  Crisp and clean and wider range than one might expect from a little speaker.

I haven't heard of anyone else doing anything similar.

GK:

--- Quote from: bob91343 on August 14, 2019, 05:25:26 am ---I haven't heard of anyone else doing anything similar.

--- End quote ---



The concept has been around for decades and Philips marketed active speakers using the technique in the seventies. Just google "motional feedback".

https://www.google.com/search?source=hp&ei=b7tTXcbHDY3c9QPij6boBw&q=motional+feedback&oq=motional+feedback&gs_l=psy-ab.3..0l2j0i22i30l8.1262.5477..5692...1.0..0.303.4429.0j3j15j1......0....1..gws-wiz.......0i131j0i3j0i10.4klykOX3_6U&ved=0ahUKEwiGu5e27oHkAhUNbn0KHeKHCX0Q4dUDCAc&uact=5

jcrubin:
 Having been requested upon restoration of my newly restored Dynaco Dynakit ST70 Amplifier, I've decided to test out the Amp by directly connecting the Commodore to it, and attenuating the signal instead of using a pre-amp.  A visual output of the signal will further be my restored 1950's Heathkit laboratory oscilloscope.  After setting the volume I just roll right into the demo songs. Enjoy.

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