Need an input selector and/or volume controller for my amp - Google ain't much help!!

Evening all!

Well I've spent the last couple of hours Googling to try and find a 3 or 4 source input selector (well at least 2 source, possibly with integrated volume control) for my amp but so far it's been pretty fruitless.

Can anybody help? Ideally I need one with a PCB layout/component list and I don't need tone or balance controls.

Thanks!

John.
 
Last edited:
Well those were actually my initial thoughts and I have a dual gang pot for the volume control, however I've read a number of articles and it seems that the general concensus is that rotary switches can introduce noise to the circuits...? I don't need remote controlled but I wouldn't object to it! The simpler/cheaper the better!
 
Last edited:
Well those were actually my initial thoughts and I have a dual gang pot for the volume control, however I've read a number of articles and it seems that the general concensus is that rotary switches can introduce noise to the circuits...?

I don't see why a properly wired selector switch would add any noise; it never has for me anyway.
 
Do I really need a switch?

Been pondering my options over a bottle of Fursty Ferret and it occurred to me that maybe I'm just making things more complicated than they need to be. Would really appreciate your opinions on the below...

My amp will primarily be used for a CD player with the occasional bout of vinyl indulgence, and there's not going to be a lot of source switching, so is it not a viable option to take the switch out of the signal path altogether and wire two inputs into one?

Will there be any detrimental effect to the signal/amp this way, and is it simply a case of connecting both inputs (one via a pickup pre-amp) to the PCB connectors or is there more two it? What would happen if two signals were accidentally played at the same time?

Thanks,

J.
 
Last edited:
Hello johnalexwarren, it's not a good idea to connect two different outputs from different devices to the same input. You could potentially blow up the output of both devices. I would recommend that you use at least a two way switch for your inputs, if you have switching. You could get away with a common DPDT switch here, as the ground wires should be common, thus don't need switching. If you desire more inputs, you could also try using an old printer switch from a computer. There are ones out there that had 4 positions. They usually have a lot of decks, more than necessary, but you can ignore them.
And last but not least, if you aren't going to be switching inputs often, you could always just build the amp with one input, and then swap the cables from the source as necessary. If you do this, remember to turn the amp off first.

Peace,

Dave
 

sandyK

Member
2007-04-27 12:54 am
Sydney
It's also not a good idea to have 2 input cables in parallel due to the increase in the total cable capacitance. Best results will be achieved by also switching the earth side of both inputs as well, especially since you are using vinyl.
The "earth" side of a typical modern CD/DVD player's output will have >100VAC when referenced to mains earth (230V mains) and measured with a DMM, due to a typical 100nF capacitor between the primary and secondary sides of the player's SMPS. This will degrade performance. In fact, if you use several SMPS powered components such as a STB etc, you can get quite a nasty bite when fiddling with the plugs, especially if the amplifier is earthed.

SandyK
 

benb

Member
2010-04-24 1:52 am
Been pondering my options over a bottle of Fursty Ferret and it occurred to me that maybe I'm just making things more complicated than they need to be. Would really appreciate your opinions on the below...

My amp will primarily be used for a CD player with the occasional bout of vinyl indulgence, and there's not going to be a lot of source switching, so is it not a viable option to take the switch out of the signal path altogether and wire two inputs into one?

Will there be any detrimental effect to the signal/amp this way, and is it simply a case of connecting both inputs (one via a pickup pre-amp) to the PCB connectors or is there more two it? What would happen if two signals were accidentally played at the same time?

Thanks,

J.
Outputs (the CD out, the phono pre out) are not designed to drive the low impedances of other outputs. What you would do is make a "passive mixer" with resistors. I'm thinking a value of 4.7k ohms should work okay. Have a resistor from each of the "hot" outputs, one from the CD, one from the phono pre, to the "top" connection of the volume control (which would be what value, 25k or 50k?). Duplicate this to do two channels, of course using a dual-gang volume control (all grounds are connected together, including the "bottoms" of the volume control). The ground and the slider or "mid" connection(s) of the volume control go to (each channel of) the amplifier.

This has disadvantages, though I think the main one is that of any passive preamp - if the cable between the volume control and the amp is more than a foot or two, the cable capacitance could cut down on the treble, and worse, any such effect would change with volume, as the source resistance going into the cable would change with the control setting.

But I'm guessing it will all work perfectly fine.
It's also not a good idea to have 2 input cables in parallel due to the increase in the total cable capacitance. Best results will be achieved by also switching the earth side of both inputs as well, especially since you are using vinyl.
The "earth" side of a typical modern CD/DVD player's output will have >100VAC when referenced to mains earth (230V mains) and measured with a DMM, due to a typical 100nF capacitor between the primary and secondary sides of the player's SMPS. This will degrade performance. In fact, if you use several SMPS powered components such as a STB etc, you can get quite a nasty bite when fiddling with the plugs, especially if the amplifier is earthed.

SandyK
Just the letters SMPS is enough to suggest looking for an older CD player with a linear power supply (or, as a not-quite-science-project, replace a current player's smps with a linear supply). As far as the floating voltage, I'd think having that much voltage differential between units is reason enough to keep the grounds connected, as long as there's no hum or noise caused by having them all connected.
 

sandyK

Member
2007-04-27 12:54 am
Sydney
"Just the letters SMPS is enough to suggest looking for an older CD player with a linear power supply (or, as a not-quite-science-project, replace a current player's smps with a linear supply). As far as the floating voltage, I'd think having that much voltage differential between units is reason enough to keep the grounds connected, as long as there's no hum or noise caused by having them all connected. "

benb
Good luck with finding a CD player that doesn't use SMPS, and hasn't seen better days. !
Although that AC is from a very high source impedance, it does degrade overall SQ if the earths are connected together. It will come down to just how good the rest of your system is, as to whether it is noticeable or not.
My own preamp uses a rotary switch that switches both the signal and earth connections for that reason.

SandyK
 
SandyK has a valid point here that I had not considered. However, from the point of view here in the states, it would seem to me that the sort of power supply that SandyK is suggesting exists regularly in modern devices would violate various standards and codes. I would think that the ground would still need to be commoned to the neutral of the power supply, if nothing else for the safety reasons that can be deduced.

Peace,

Dave
 

sandyK

Member
2007-04-27 12:54 am
Sydney
Dave
I am probably incorrect on the actual value of these capacitors. In some DVD players it is actually 2 capacitors in parallel, as shown in the service diagrams of a couple of Pioneer DVD players.(U.S. versions too.)
To quote from Silicon Chip magazine P.15 October 2007 :
"....the capacitors on the primary side of the switchmode supply couple the mains input through to the signal ground,which connects to the case."

The current is quite low, so it passes requirements.
The problem is exacerbated when you use several components with SMPS, as the typical leakage current is multiplied by the number of components using these supplies.
Alex