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Old 18th January 2009, 04:46 PM   #1
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Default Input switching gainclone, layout, general technique

So I'm moving my GC from breadboard to enclosure. It's using BrianGT's LM1875 amp PCB's and a self made PS board. I have 3 sets of isolated RCA input jacks I'm going to select between, found a nice 3P3T mil surplus switch on ebay. I have some small diameter Co-ax that I can use. If I can attach a photo, you'll see that I intend to mount selector switch a couple inches from the inputs, then run up the right edge and over to the attenuator mounted in the middle of the front panel. From there to each input and signal ground.
1) My question is should I use that small coax, or twisted pair wiring?
2) On tube amps, I used to join all the RCA grounds together with a buss wire, and use coax shield as gnd from there to selector then to input. Should I do this, or run individual grounds to star ground and star ground to signal ground on PCB
Click the image to open in full size.
any other comments on my tentative layout?
thanks a bunch
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Old 18th January 2009, 07:31 PM   #2
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My '2 cents'

I would say mount the sink and amps along the back panel, boards as close to the back as possible for shortest signal path. Drill a hole in the sink and mount the attenuator at the rear between the boards, use an extension shaft. Same applies (assuming you'll use a rotary switch) with the input selector. The sink then provides a bit of shielding from the trafo and PSU, and you signal wiring is a short as possible.

Im no expert so hang on for the 'experts'.
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Old 18th January 2009, 10:32 PM   #3
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Those are good ideas, but I already drilled and mounted the heatsinks, and the cooling slots in top and bottom cover lined up perfect with the fins. Admittedly the heatsink is overkill, but later I may upgrade to a different chipamp IC. But you are right, that would have been a good way to do it. I'm sweating the actual signal wiring mostly at the moment. I hate drilling more holes in the enclosure, but I'm going to have to for the PS, a different hole for the toroid, and the mtg bracket for the selector switch.[IMG]Click the image to open in full size.[/IMG]
Rear View, probably won't use da feets.
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Old 18th January 2009, 11:08 PM   #4
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Hi Sparky,
No such thing as too big of a heatsink. Another way to lay things out would be move the toroid to the front and use shaft extensions on both att and selecter switch, that puts all the input wiring in one place and you could use unshielded wire. Probably should use shielded from the att to the boards though.
A common ground wire thru the RCAs should be fine.

Bill
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Old 19th January 2009, 07:15 AM   #5
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Default Re: Input switching gainclone, layout, general technique

Quote:
Originally posted by Sparky OR
should I use that small coax, or twisted pair wiring?
No big difference. Some people claim that solid wire sound better. That would speak in favour of the coax.

Quote:
Originally posted by Sparky OR
On tube amps, I used to join all the RCA grounds together with a buss wire, and use coax shield as gnd from there to selector then to input. Should I do this, or run individual grounds to star ground and star ground to signal ground on PCB
What do you need a ground at the selector for? I vote for a ground wire from the RCA's bus wire to the PCB's signal ground.
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Old 19th January 2009, 01:26 PM   #6
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pacific blue, thanks for your input
If I use a buss wire to connect RCA jacks together, would I then run the coax to the selector switch, which is about 3" away, then join the coax shields to the run that goes up to the attenuator, joining it to the run to Signal input ground? (a continuous ground from buss through switch, att, to PCB). Trying to avoid the stereo ground loop issues.
My star ground bolt is going to be right in the middle of the chassis, and some safety lift components to AC mains ground.
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Old 19th January 2009, 01:37 PM   #7
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Quote:
Originally posted by Sparky OR
....................My star ground bolt is going to be right in the middle of the chassis, and some safety lift components to AC mains ground.
The Safety Earth will be permanently bolted or welded to the chassis with no joint or connector in the third wire connection from the mains socket.
The Audio Ground (my star ground bolt) can be near the centre of the chassis (or other convenient location) but should be electrically isolated from the chassis.
Then you use a disconnecting network to link the Audio Ground to the Safety Earth.
This disconnecting network can be a parallel set of:-
a.) inverse parallel Power Diodes,
b.) Power resistor,
c.) optional HF capacitor,
d.) optional ground lifting switch.
Note that opening the ground lifting switch still leaves the remainder of the disconnecting network components linking the Audio Ground to the Safety Earth.
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