Switching attenuated signals

bart11073 · « **on:** 1 Mar 2003, 09:58 pm »

I have almost finished my AKSA 100 case, and I was thinking about the music system. The only source where I listen to serious is the cd-player so a TLP would be a perfect match. But I have also other sources and a medium quality preamp. If I can switch between the output of the TLP and the output of the preamp everything would be perfect. Is is possible to switch between attenuated signals without degration of the sound? And how much would that cost me?

Bye,
Bart

AKSA · « **Reply #1 on:** 2 Mar 2003, 12:31 am »

Bart,

A high quality C&K Dual Pole Dual Throw (DPDT) switch, with gold plated contacts, would do the trick. These switches have two lines of three terminals each side; center is movable, top and bottom are make/breaks. You'd be switching at line level - around 1V rms - so you'd need to take care with shielding, preferably by building a little ferrous box around the switch. Use CAT5E cable to carry the signal, and switch the earth as well as the hot terminal, so left side center to amp, left top to TLP, left top to preamp; right side center to amp, right top to TLP earth, right bottom to preamp earth.

Such a switch would cost you about $US4. But the time to plan, connect and implement it would be much, much more of course.

Cheers,

Hugh

bart11073 · « **Reply #2 on:** 3 Mar 2003, 01:23 pm »

I was thinking that it's maybe easier to use relays so that the signals can be switched at the back of the cabinet. Are there any good relays?

Thanks, Bart

Malcolm Fear · « **Reply #3 on:** 3 Mar 2003, 07:17 pm »

I tried relays to switch between a moving coil and moving magnet cartridge. Whenever the relay was triggered, it iduced great amounts of hum. I bought big see through, gold contact relays. Perhaps the coils were too big.
I have mounted a selector switch near the rear of the cabinet, then run a shaft extension out through the front.

tg3 · « **Reply #4 on:** 5 Mar 2003, 06:10 am »

Quote from: Malcolm Fear

I tried relays to switch between a moving coil and moving magnet cartridge. Whenever the relay was triggered, it iduced great amounts of hum.

Was the relay AC or DC? Not sure how a DC relay with a well filtered DC source could induce hum.

Malcolm Fear · « **Reply #5 on:** 7 Mar 2003, 03:08 am »

Hi Thurman
I used a DC relay. I drew the poewr from the power supply of the GK-1.
I tried a battery to see if it would reduce the hum. It didn't.
The relays are large (like a matchbox, only bigger).
I was playing around with phono signals.

tg3 · « **Reply #6 on:** 8 Mar 2003, 03:46 am »

Quote from: Malcolm Fear

Hi Thurman
I used a DC relay. I drew the poewr from the power supply of the GK-1.
I tried a battery to see if it would reduce the hum. It didn't.
The relays are large (like a matchbox, only bigger).
I was playing around with phono signals.

The next thought is that you induce a ground loop when the relay is on. Any way to post a schematic of your circuit? I doubt the relay per se is causing the hum.

AKSA · « **Reply #7 on:** 9 Mar 2003, 12:08 am »

It's actually possible the relay coil is inducing the hum, by acting as an inductive trap for any circulating AC fields from the transformer, and radiating them, now greatly magnified, into the adjacent line level switch.

The problem may be solvable by enclosing the relay in a mu metal shield, and indeed relays are available with such shielding to render them insensitive to closely packed identical relay coils right alongside.

Cheers,

Hugh

SamL · « **Reply #8 on:** 9 Mar 2003, 09:45 am »

Anyone in NZ/OZ know where to get mu metal?
Does anyone know if microwave or computer case are made off mu metal?

Sam