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'shielded wire+lead dress'
Author:Gareth Barry (guest: search)
Date: Tue, Oct 16th, 2007 @ 05:49 ( . )

Hi guys-am trying to finish my channel switching amp (lead and simple preamps+20 watt p/pull) and am wondering-is shielded wire to tone pots and between tone pots a good idea, especially since the arrangement of my amp has both preamp's tone pots close to each other? Should the shield be grounded to at one end if I implement this? Would be good to prevent any possible problems before I really start debugging.

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'shielded wire+lead dress'
Author:CarlB (guest: search)
Date: Tue, Oct 16th, 2007 @ 06:22 ( . )

Gareth,

Wouldn't be a bad idea to shield those.

But even more so, pay close attention to your coupling cap placement. On more than one amp I've found cross-talk and similar problems come down to a cap picking up signal from something close by.

An example, the right-hand side of a PI splitter was being used as the means to mix in a second channel. The input coupling cap for that side of the splitter was about an inch from the Master Volume pot of the first channel.

Crank up the first channel, and even with the Master Volume on zero, the second channel would bleed through the distorted tones of the first channel.

I disconnected the cap from the board, and played with its location. Put the cap about a centimeter from the pot, and the cross-coupling was amazing. You can use this setup instead of an opto-isolator or a coupling transformer, it's that good.

But bad for the intent, which was to have two independent channels, without the second picking up distorted tones from the first.

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'shielded wire+lead dress'
Author:Gareth Barry (guest: search)
Date: Wed, Oct 17th, 2007 @ 03:26 ( . )

Are there any downsides of using a hell of a lot of shielded wire? (i.e. from every screen/grid and to every tone pot)

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'shielded wire+lead dress'
Author:Evan (guest: search)
Date: Wed, Oct 17th, 2007 @ 10:34 ( . )

On 10/17/2007 @ 03:26, Gareth Barry wrote :
Are there any downsides of using a hell of a lot of shielded wire? (i.e. from every screen/grid and to every tone pot)
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Man, I'd go mental if I had to do that. I guess that's a downside :-D.

What I'd do:

- Heed Carl's advice w.r.t. layout of caps. Also search for an old post by CarlB on layout for high gain amps (maybe it's been faq'ed?) although in addition to feedback (instability) you need to be concerned with bleed through (separate issue).
- ground the signal in the idle channel.
- build it with non-shielded wire
- if it's possible, build just the hi gain channel first (divide & conquer in reverse, so to speak)
- if problems occur, try to sort out where they are (chop stick)
- THEN add shielded cable only where necessary.

Shielded cable does seem to suck a bit of life out of an amp, imo, tho' based on the (small) capacitance involved it's hard to see why. I am not the only one to make this observation. But it is demonstrably true that a good layout with unshielded wire will sound *much* better than the identical circuit with a poor layout that requires a mile of shielded cable to stay stable.

I'm rambling. Sue me :-).

HTH

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'shielded wire+lead dress'
Author:Gareth Barry (guest: search)
Date: Thu, Oct 18th, 2007 @ 02:59 ( . )

Ok, will do then. When grounding the shield I have been told that the shield needs to be grounded at the 'source' end. Does this mean the source of the hum/problem or the signal source? Does is matter if it's grounded at both ends?

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'shielded wire+lead dress'
Author:sharkboy (guest: search)
Date: Thu, Oct 18th, 2007 @ 03:32 ( . )

Usually it creates a ground loop to ground it at both sides. Doing so will often create more hum than using unshielded cable.

Think of signal path direction- the driving side is the source, and it usually makes more sense to ground it there, but really one side or the other is the overriding rule.

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'shielded wire+lead dress'
Author:Gareth Barry (guest: search)
Date: Thu, Oct 18th, 2007 @ 06:40 ( . )

O.K then, but am I then misreading the chassis layout son all the projects? If I look say at the input wire to tube 1, I can definitely see that it is grounded at both ends! See why I'm confused?

Will go a bit easier on the shielded wire but will send the unused channel's signal to ground-I guess that's the right idea?

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'shielded wire+lead dress'
Author:Evan (guest: search)
Date: Thu, Oct 18th, 2007 @ 10:08 ( . )

On 10/18/2007 @ 06:40, Gareth Barry wrote :
O.K then, but am I then misreading the chassis layout son all the projects? If I look say at the input wire to tube 1, I can definitely see that it is grounded at both ends! See why I'm confused?
:
: Will go a bit easier on the shielded wire but will send the unused channel's signal to ground-I guess that's the right idea?
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I recommend using layouts for component location, but wiring by reference to the schematic. You aren't the first guy to get tripped up.

As far as how to ground the signal... I'm a single channel amp kinda guy :-). Maybe someone who does channel switchers can offer up some experience. The obvious approach would be a DPDT relay, but perhaps there's something better.

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'shielded wire+lead dress'
Author:Gareth Barry (guest: search)
Date: Fri, Oct 19th, 2007 @ 04:11 ( . )

O.K guys, I have a HUGE apology to make-I am OFICIALLY a MORON!!
I had a relook at the diagrams, and can see that the shielded wires are only grounded on one end. I realise that I must have gotten confused with some of the uses of the shield itself as a convenient grounding point, such as in the hi-octane.

Have fixed this problem now, and I think I am ready to fire her up

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'shielded wire+lead dress'
Author:Gareth Barry (guest: search)
Date: Fri, Oct 19th, 2007 @ 04:16 ( . )

I used DPDT relays at the beginning and end of the signal path of both preamps (tried to follow the method in the Renegade) and routed the un-used channel's signal path to ground, also at the beginning and end of the signal path.

There seems to be quite a few different opinions on whether shielding is good thing, or a necessary evil that needs to be minimised-Have a look at Paul Ruby's page.

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