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Old 26th February 2013, 04:45 PM   #1
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Default Altec 1570B Feedback

Greetings all! After having recently restored a pair of Altec 1570B monsters back to original spec I was looking for opinions on the elimination of the capacitor in the feedback loop as I have read this will enhance the sound quality for hifi use. I assume it is C1 (.0002uF) on the schematic. I have heard either to bypass the cap and to leave the R3 (100K) in place and to remove the cap AND R3. What to do? Fire away fellow tube people I am interested an any knowledgeable input you may have.

Link To Schematic:www.dvq.com/hifi/images/1570a.pdf
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Old 26th February 2013, 04:56 PM   #2
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C1 and R3 stabilise the feedback loop. If you want instability with your music then remove them. It might just be that they are not needed with a well-behaved load; only experiment will tell you. Measure the frequency response into the low ultrasonic region with and without them, or look for ringing on square wave drive.

This amp appears to be intended for PA, not hi-fi. It appears to use 'zero bias Class B triodes', which are normally used for PA and AM modulator application where reliable power is far more important than low distortion.
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Old 26th February 2013, 06:57 PM   #3
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If you remove the cap don't complain when your tweeter burns out due to ultrasonic oscillation...the amp was designed with feedback for a reason - to stabilize gain, reduce distortion, reduce output impedance for speaker damping.
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Old 26th February 2013, 08:56 PM   #4
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Right on. I thought about that too. I just got them back to original so I am in the "playing around" stage now. Actually, for an amp that was originally designed for PA service they are quite musical. The imaging is nice and they seem to be able to "command" a woofer like no other.

Now to just make these things a bit easier on the eyes because they have got to be some of the ugliest amps I have ever seen. It is like the army built them: sacrifice form for function but they will last forever!
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