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Old 15th March 2005, 03:24 AM   #1
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Default Help with Tube Phono Output Impedance

I've been having some trouble with my tube phono when its plugged into an active crossover (which has a high impedance input buffer stage). The cartridge also picks up the noise generated when the turntable rotates (in addition to the music) ! With phono connected directly (ie no active crossover in between) to the tube amp everythings fine.

The phono is basically a standard 3 stage design with the eq over two stages and a cathode follower output with at 22K. The original design had no output load (ie simply signal out from cathode via 2uF cap to RCA). Without this output resister there's no signal generated into the active crossover whatsoever. With a 1Meg resistor there horrendous noise generated.

I've currently connected a 2K output resistor. This has reduced the noise significantly but not totally.

Can someone explain what's actually happening and the best way to address this problem.

Thanks.
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Old 15th March 2005, 05:25 AM   #2
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What kind of noise are you getting? Scratchy noises, buzzing, or hum (possibly at 50 or 100Hz)?

Using 2K at the output means that the AC load for the cathode follower is greatly decreased. This increases distortion and reduces maximum voltage swing from that stage.
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Old 15th March 2005, 06:14 AM   #3
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Quote:
Originally posted by audiousername
What kind of noise are you getting? Scratchy noises, buzzing, or hum (possibly at 50 or 100Hz)?

Using 2K at the output means that the AC load for the cathode follower is greatly decreased. This increases distortion and reduces maximum voltage swing from that stage.
Hi Jason,

It's definitely not hum associated with poor grounding. The system (w/active XO) is silent when the needle is not on the LP. At the same time, there's no issue with phono connected directly to amp.

Even when the cleaning brush touches the arm of the needle (not the needle point which is on the LP) it picks up the brush friction ?!# (which it normally does not).
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