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Old 18th March 2015, 01:35 PM   #1
apnneto is offline apnneto  Brazil
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Default Distance between two audio input valves

What is the shortest distance between two audio input valves? My concern in this new layout is to avoid crosstalk.

I am using the input stage 12AX7 tube and the inverter is the 12AT7 tube. I want to make the holes in the chassis, but need this information to avoid future problems.

This is a stereo amplifier, so the question is about the distance between the tube in the two channels and not between the tubes of the same channel.

Regards.

Last edited by apnneto; 18th March 2015 at 01:38 PM.
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Old 18th March 2015, 01:55 PM   #2
rayma is online now rayma  United States
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Originally Posted by apnneto View Post
What is the shortest distance between two audio input valves? My concern in this new layout is to avoid crosstalk.
This is a stereo amplifier, so the question is about the distance between the tube in the two channels and not between the tubes of the same channel.
The wiring and/or the pcb layout will contribute much more to crosstalk than any coupling between two different tube envelopes.
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Old 18th March 2015, 02:13 PM   #3
apnneto is offline apnneto  Brazil
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Okay. Thanky you rayma.

My case is as follows: the two channels are completely independent, only sharing the ground star. When I touch the coupling capacitor of one of the channels, the crosstalk disappears. If I use sockets with shields also crosstalk disappears. The distance between the inlet tubes of each channel is 10cm. I wonder where the problem lies?
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Old 18th March 2015, 02:14 PM   #4
rayma is online now rayma  United States
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Originally Posted by apnneto View Post
I wonder where the problem lies?
Could be the coupling caps themselves. Some people have used shields over large caps to prevent coupling from the outer foil to other parts of the circuit.
It is possible to have coupling between the plate structures of the two tubes envelopes if they are very close. It's odd that either installing
a shield OR touching the cap will kill the crosstalk, though.

Last edited by rayma; 18th March 2015 at 02:25 PM.
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Old 18th March 2015, 02:29 PM   #5
apnneto is offline apnneto  Brazil
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I tried to use capacitors of different brands and types but behavior was the same.

Below is a photo of the plate. The distance between the paths of audio signals is 1 mm, being in parallel. I tried to connect directly from the ECC 83 socket to the KT88, but without effect.

I am developing this board only for learning. For this reason I would check what is going to be able to learn and redo the layout.
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Old 18th March 2015, 02:35 PM   #6
rayma is online now rayma  United States
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Originally Posted by apnneto View Post
I am developing this board only for learning. For this reason I would check what is going to be able to learn and redo the layout.
I would add at least an inch of space between the two channels, or make two separate, identical boards, mounted some distance apart.
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Old 18th March 2015, 02:43 PM   #7
apnneto is offline apnneto  Brazil
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Ok. I'll split the plate in half and find what the distance that the problem disappears. After post here my findings. Grateful for the support.
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Old 18th March 2015, 02:45 PM   #8
rayma is online now rayma  United States
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Originally Posted by apnneto View Post
Ok. I'll split the plate in half and find what the distance that the problem disappears. After post here my findings. Grateful for the support.
Yes, the channels are rather close together, and capacitive coupling is certain. A shield strip between the channels could help as well.
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Old 18th March 2015, 05:04 PM   #9
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Quote:
Originally Posted by apnneto View Post
...My concern in this new layout is to avoid crosstalk...This is a stereo amplifier...
Then the crosstalk does not need to be more than some 35...40 dB.
If you look the crosstalk specification of top quality MC cartridges, it's something
between 20 to 30 dB.

http://ortofon.com/images/hifi/broch...5_brochure.pdf
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Old 18th March 2015, 05:16 PM   #10
rayma is online now rayma  United States
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Originally Posted by artosalo View Post
Then the crosstalk does not need to be more than some 35...40 dB. If you look the crosstalk specification of top quality MC cartridges, it's something
between 20 to 30 dB.
Cross coupling on a pcb is quite different from that in a phono cartridge, and cannot be compared in that way.
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