newbie making an Input/Output switch

dalius98

Member
2016-04-13 9:50 pm
Hello everyone. I've just registered here to see all the nice build photos you're posting, but I figure it won't hurt to ask some questions too :)

I plan on building an Input/Output selector with inbuilt speaker (and probably headphone) amplifier. The plan is to use 3 pole, 4 position rotary switch to select an input... and the following parts get complicated...

I want to be able to switch between speakers and two headphones, but have the ability to use either one set of headphones, or both at the same time.
The way I see it, I could use a 3PDT switch to select between speaker or headphone amp, and then have additional switches on the amp output side to select between the headphones. That's quite a lot of switches...

Anyway, I have a lot of questions:
1. Do I need to switch ground on output side, or only on the inputs? (trying to prevent ground loops here - what other safety measures I can take?)
2. What kind of cable would you recommend for wiring up the internals with? I have some Cat6 cable, with 4 individually shielded pairs, of twisted wires. But I am worried that it's too thin... I suppose I could use any kind of shielded headphone cable too?
3. Most importantly, do I even need separate headphone amp? I'm currently using Sennheiser HD558, (the other set is nothing special), which have a relatively low impedance of 50ohm, and they sound decent with the integrated audio on my PC (really have nothing better to compare them to). Would running them off a speaker amp designed for 4-8ohms induce any problems? What about max power? I wouldn't want to blow the drivers or anything.


4. What to look for when buying an amplifier board? I am not hardcore enough to make everything myself - I plan on buying something requiring just a PSU and a case, like the tons of TPA3116 boards on ebay... While I am doing some research on the chips themselves, there's a lot more that makes an amplifier sound good than that. I just don't know what. I see a lot of different implementations of the same board, using different kinds of capacitors, inductors (SMD and through hole) or board layout...

I suppose I should be asking amp-specific questions in other parts of the forum, but being new here, I'm not too sure I should spam that much :D

Summing this up, does this kind of project look like something reasonable? Would two different amps being close to each other, tons of switches or anything else introduce any problems?
 
A circuit gets "broken" when ONE wire of the two wire connection is opened/broken.
You only need to open the Signal Flow to cut off that circuit.

I would separate your switches so that one switch operates on inputs and the other switch operates on outputs.

If you plug in a headphone you have that connected. That usually requires the speaker output to be switched to OFF.
If you have two headphone outlets, you could have them switched to always ON or select between one or other. It is not usually necessary to have the switch to turn both heaphones to OFF.

The 1/4" TRS socket used for the "big" headphone plug can be wired to turn OFF the speaker circuit when a headphone is plugged in. (I don't know if the little 1/8" socket has this "turn OFF" facility).
That requires the speaker output to be wired through the TRS socket and you end up with speaker voltage across the headphone output/s. Not good for headphones and ears. It is better to have the speaker outputs and the headphone outputs both reproducing at a sensible volume with the same setting of the vol pot.

Another INPUT alternative is to have a "mixer" input, instead the usual one, or other, switch.
The mixer would allow you to listen to two (or more) sources at the same time on your speaker and/or headphone.

The input switch could be configured to have Input A, or Input B, or Both Inputs, fed to the amplifiers.
 
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dalius98

Member
2016-04-13 9:50 pm
Thank you for your input. Although that got me even more confused :)
When talking about a two wire connection, we're talking about a typical + and - connection, like on speakers - those would get disabled when one wire is not connected. But on a phone/TRS connection, I would still need to switch off both L and R channels to cut the audio, right?

The input and output switches WILL be separate ones, I can't even figure out how to do it with one...
I agree with not needing to completely turn off both headphones - although I plan to keep mine connected at all times, if I want to use speaker output, I will have to route the "audio path" through speaker amp instead of the headphone one, and that would "switch" them off...
And I don't need a mixer input as I will use only one input source at a time for the foreseeable future...

I am attaching a crude schematic to better convey my idea. Still need to figure out the best way to switch between headphone outputs...

Then there's the more complicated stuff of choosing at least decent amps and whatnot...
 

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Thank you for your input. Although that got me even more confused :)
When talking about a two wire connection, we're talking about a typical + and - connection, like on speakers - those would get disabled when one wire is not connected. But on a phone/TRS connection, I would still need to switch off both L and R channels to cut the audio, right?
Yes, the right channel input would require to be broken and the left channel input would require to be broken.
The input and output switches WILL be separate ones, I can't even figure out how to do it with one...
I agree with not needing to completely turn off both headphones - although I plan to keep mine connected at all times, if I want to use speaker output, I will have to route the "audio path" through speaker amp instead of the headphone one, and that would "switch" them off...
And I don't need a mixer input as I will use only one input source at a time for the foreseeable future...

I am attaching a crude schematic to better convey my idea. Still need to figure out the best way to switch between headphone outputs...

Then there's the more complicated stuff of choosing at least decent amps and whatnot...
Your diagram shows 4 inputs. Is that two stereo channels? or do you intend to have 4 stereo channels (equals 8 channels)?

Combine the two sets of input switching.

The outputs from that switch can connect to both the stereo power amplifier for the speakers and to the stereo headphone amplifier. You don't need a separate switch. The source impedance at the other end of the 1/8" TRS should have sufficient drive capability to drive the cables and switches and the two input impedances of the head & power amplifiers.
 
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dalius98

Member
2016-04-13 9:50 pm
Your diagram shows 4 inputs. Is that two stereo channels? or do you intend to have 4 stereo channels (equals 8 channels)?

Yep, that would be 4 stereo channels. Well, to be honest, I really need only two at the moment, but I see myself *maybe* needing additional inputs in the future so there's that. I have some 3Pole 4position switches anyway, might as well use them.

Combine the two sets of input switching.

The outputs from that switch can connect to both the stereo power amplifier for the speakers and to the stereo headphone amplifier. You don't need a separate switch. The source impedance at the other end of the 1/8" TRS should have sufficient drive capability to drive the cables and switches and the two input impedances of the head & power amplifiers.

as for the "amp selector" switch - I see that's not needed, not sure what I was thinking :D I would still need a switch to turn on the power to either one of the amps...