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'Remote sense' feedback loop

M

mandoblast

New Member
A bit of searching didn't turn anything up, so I thought I put this out there. Anybody tried this? Background: I'm finally getting around to wrapping up rehab/updating of a Grommes receiver model C502 (not real common, I guess).
Unit was in good original condition. Replaced coupling caps and electrolytics under the deck. Big cans are alright. Changed to individual fixed bias. 330v on the EL84 plates, 300 on screens.
I was pretty happy with the sound, except for the slightly flabby bass. I tried some wild output-cathode feed-back schemes and did get tighter bass out of them, but wasn't really happy. I was kind of just goofing around with it, so I'm not sure I ever did it the 'best' way. But I was also aware of how this would increase the drive requirement to the tubes, so whatever gains I made in tightening the output would be erased by distortion in the early stages as they tried to overcome the cathode feedback. Chasing a moving target... I think this is what I got.
Fast forward: With the output stage returned to optimal, I disconnected the feedback resistor from the 16ohm tap and ran a pair of wires straight from the backs of the speakers to the feedback cathode resistor, top and bottom. Well, through a 100k pot. Wow. Done. Sound is better with LESS feedback (dB's) than with the feedback coming from the transformer tap.
'Remote sense' is the term used in power supplies for a provision to bring a small wire back from the load to show the regulator the actual voltage across the load. Since the remote sense is high impedance/low current, it isn't affect by resistive losses over the run.
Roger
Oh. After all this, I guess I should do better than just sticking stripped wires into the backs of my speakers. Hmmm.
 
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Great input. This reminds me of an experiment published some years ago where a dual voice coil driver was configured to have one coil actively driven, while the other provided the feedback -- very similar to what you are doing, only with a traditional single voice coil. As I recall, the results were from those experiments were very favorable as well.

Thanks for sharing!

Dave
 
I've had a few amps that did better with less feedback. Usually it wasn't flabby bass, it was just no bass, no treble, and overall dead sound.

Just out of curiosity, how is your connection different than just running it from the 8 ohm tap on the transformer + a 100k pot? I'd figure that you should be able to tap it straight off the transformer and get the same results. The pot sound sort of like Fisher's "variable damping" control or whatever they called it on their earlier mono amps.
 
Different wire

Good question, Gadget. I tried that, using the same resistor (just moving it) gives 3dB less feedback ( think). You could reduce the resistor to compensate, and WITH ZERO LOSS CONNECTIONS it would make no difference. But my wires and connection are far from ideal. This scheme gives the amp the information it needs to compensate for that.. to up the voltage when it needs to in order to stop the woofer, start it, etc. It also gives ME the information that I need better speaker wires/connections.
Might be the same as Bogen's adjustable damping, minus the remote connection.
 
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