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Old 14th May 2002, 12:40 PM   #1
Nisbeth is offline Nisbeth  Denmark
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Default Relays as input switch?

Can anyone explain to me how I use relays for switching inputs in a preamp? (specifically how to make sure only one input is selected at a time). Are there any circuits available online where this is shown?

Any help appreciated!
/Uffe
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Old 14th May 2002, 09:39 PM   #2
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How you wire the relays will be determied by how you intend to tell them to switch:
--Using a front panel switch to route DC to the coils--piece of cake. (Yes it's nice to use a front panel switch <i>and</i> relays, because you can use small relays with nice, gold-plated contacts and a cheap switch. The DC doesn't effect the signal.)
--Using a logic chip to take charge of the process--this will depend on the chip you choose.

Grey
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Old 14th May 2002, 09:59 PM   #3
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Default DC doesn't effect the signal

That is true. However, magnetic field from the coil might degrade the sound. Many preamp constructions pass audio input and output signals through an activated relay and the magnetic field of its coils. The better way for input switching is to do it in the opposite manner by conducting the signal through an inactive relay while all non-conductive relays are activated. Thus, the magnetic field cannot adversely affect the sound quality. It is also a good idea to switch not only the input signals but also corresponding grounds.
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Old 14th May 2002, 10:08 PM   #4
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As long as you're using DC on the coil, I'm not sure I see how the coil field will effect the signal. If you get into a steady state magnetic field causing audible differences, then we need to start accounting for the Earth's magnetic field, the magnets in phono cartridges, magnets in CD drive motors, etc. etc. etc.
AC, of course, is a different matter.
I'd leave all grounds hooked up at all times. All you need is one chassis at a different ground potential (not at all uncommon) and you're likely to get a substantial <i>pop!</i> as you switch inputs. Not to mention the unfortunate results should you get a bit of dust or corrosion on the ground contact so that it doesn't "make," leaving the hot the only thing that connects...

Grey
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Old 15th May 2002, 01:28 AM   #5
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In my experience relays DO do damage, or at least alter the audio output.
To prove this, add a wire (wires) to short across the relay signal contacts, and then alternately connect or isolate the relay coil connection.
IME you get two (slightly) different sounds.
Also try this with amplifier output protection relay.

Conclusion - the coil current induced magnetic field has an audible effect.

BTW - Earth's magnetic field is much lower level than that in a relay.

Regards, Eric.
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Old 15th May 2002, 01:48 AM   #6
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Yes i have had this argument before and it seems that because of the AC nature of the signal some modulation of the magnetic field occurs and hence does minor damage to the sound. Given the close proximity to the coil, the magnetic effects are far greater than that of the Earth's magnetic field.
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Old 15th May 2002, 02:47 AM   #7
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Of course you are right. Given the simple choice of using active and not active relay why would anybody risk the chance of degrading the sound by running the signal through magnetic field? But as always the choice is yours.
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Old 15th May 2002, 04:07 AM   #8
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Default Bi Relays

Bi-stable relays, though expensive are available.
These require a set or reset coil current pulse to operate.
Perhaps an alternative worth considering ?.

Eric.
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Old 15th May 2002, 04:37 AM   #9
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Default Affect of DC on signal paths nearby.

Although purists may object, I think there is merit in using the sonic change (should there be one) resulting from DC on the relay coil to help tailor the sound of your amplifier.
I remember owning a Sony CD player where the wires leading to the analog output jacks were bundled with the power supply wires for the analog ouput stage. On first seeing this arrangement I thought this was just a way of making the internal wiring look cleaner. After re-routing the wiring; keeping the output and power lines as far apart as possible I noticed that the sound became fuzzy and overly warm. Returning the wiring restored the original sound. It seems Sony knew what they were doing and were tweaking the sound of this CDP using the nearby supply lines.
It is true that this method (if it's true and I'm not just imagining things) is only distorting an already distorted signal, and the best thing would be to clean the original signal. But as long as it sounds better I have nothing against it.
Trying both active and inert relays for the selected source to see which one you like sonics-wise may be a good idea.
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Old 15th May 2002, 04:53 AM   #10
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The bundling of cables you speak of is to minimise conductor loop areas.
This is an old technique.

Eric.
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