AB Headphone Switch for Zumo / MP3

[Caba]

Ok,

I'm building an A-B switch with a volume control to plug the Zumo and the MP3 player into. I want more options than the Zumo MP3 has available and I'm stuck in the middle of nowhere with limited ability to buy "specialty" items...so I'll take a shot at making what I want. Below is a rough schematic of what I'm building and I have three specific questions:

1) Will tying all of the grounds together cause feedback to the source that is not being listened to or other problems?

2) The potentiometer that I have is 0-50k Ohms. I have read (somewhere on the internet so it MUST be true!) that you should use 0-200 ohm (not k ohm) potientometer for volume control to avoid high impedence and possibly damage the source. (The article was on guitar amps so I don't know how well it applies.) Could someone enlighten me on this as to the potential problems?

3) Are there any other huge missteps in what I'm doing??? I have no experience in this, just my engineering classes 20 years ago (which I slept through most of) and a general skill for taking things apart.

 

[ponyfool]

Or, you could just buy this one for $23.

https://electronicsusa.com/mk1and2.html

 

[DocFJR]

Here are some thoughts from an old analog electronics tech:

Tying the grounds together is fine; that's what all mixers do. Just be sure to use shielded cables and electrical solder, not plumbing solder.

Without specific knowledge about the impedance of each source output, picking the right pot is more a matter of experimentation. Try a variety of pots to see which offers the best range of volume. However, you want to use log pots, not linear pots. Log pots are the type used by audio equipment. They mimic the way we hear. Logarithmic pots are harder to find; check audio catalogs. A linear pot would not sound as though it's changing the volume in a smooth range.

On the technical side, an imput impedance that matches the output impedance transfers the most power, but you are not designing an amp here. Listening to each is the best practice. Neither selection you note can hurt the source.

Finally, each of your sources may have a different impedance. Analog mixers typically use 4.7K ohm pots at the front end, and 10K or higher in later stages, but in each case, they are driving downstream op-amps. You probably want to drive speakers, so you really can't go wrong with any choice, but each may affect how useful the range appears to be. the worst that could happen with a 200 ohm pot is that it may not attenuate enough (you may not be able to totally shut off the volume).

I hope this helps,

Doc

 

[Caba]

Doc,

Thanks for the input. It's going to be extremely difficult for me to tell if I've got a Log Pot or a linear one. I can't ask the guy from the parts store because he doesn't speak english and the local version of "potentiometer" was his response to my fumbling with rudimentary slovak. I also don't have access to any cataloges in english or know anyone who speaks both english and slovak with enough technical electrical knoledge to translate.

Good to know I'm not going to hurt anything. I'll give it a shot and post the build / review here when I'm done.

Ponyfool - I'd found those on line, but the $23 to order + the $70+ shipping plus customs would have it to me in about 8 weeks. At this point I'm in for about 140 Slovak Korun or about $6.50

Thanks again.

Caba

 

[OhioFJR1300]

Check into an Ampli -rider. Its about $65.00 US and made by a super guy in Toronto. It has a remote volume control and works well. I have 2 of these and they let me use 3 audio inputs and adjust the volume as I ride.

Ampli-rider link here

 

[ponyfool]

Sorry about that Caba, didn't see you were in Eastern Europe.

I can't help you with the potentiometer aspect, don't know enough about that. Also can't help with whether or not grounding all of them is ok. However, since you have two remaining input sources with two separate connections, all leading to a common output the way your diagram shows, your switch is actually called a DPDT (double pole, double throw) and are very common (I'll bet they're even common in Eastern Europe) and should work just fine.

 

[jekirby]

There is a much simpler way. You do not have to use a switch and can have your Zumo and MP3 on and playing thru the same earphones/amplifier. I will describe, but I could email you a diagram as I do not have a photo posting site I use. Here goes:

+L source 1 (mp3)---------10 ohm resistor--------|

|

+R source 1 (mp3)----------10 ohm resistor-------)---|

| |

| |

+L source 2 (gps)-----------10 ohm resistor------------)------------------- +L amp/headphones

|

+R source 2 (gps) -----------10 ohm resistor---------------------------------- +R amp/headphones

All -'s are tied together L and R respectively.

Basically take both L+'s and R+'s from sources throught 10 ohm resistors and then tie the corresponding +'s together and all the -'s together. What this does is protect the amp of both the gps and mps but let them share one amp/headphones/speaker whatever. Use the volume controls on the mp3 and gps to act as your 'mixer'. I scavenged enough parts to make this for less than $5.00.

Edit >>all my little vertical marks got all screwed up, so you an pm me and I will email you diagram.

 

[ponyfool]

There is a much simpler way. You do not have to use a switch and can have your Zumo and MP3 on and playing thru the same earphones/amplifier. I will describe, but I could email you a diagram as I do not have a photo posting site I use.

Send it to me, I'll host it.

 

[Caba]

Ponyfool - Not a problem. I'm sure everything is available here as well, the problem is finding it and getting enough communication to understand the details.

Jekirby - Thanks for the link in the PM. Good information. I've bookmarked for later reference.

I'm about half way through building the switched unit. I'll post up when done (w/piccies).

Thanks all,

Caba

 

[bramfrank]

You're dealing with speaker levels - you do NOT need shielded cable.

I suggest you might not bother with the potentiometer - each of the devices has a volume control - set the volumes on the devices. But if you MUST use a volume control, use something with perhaps 500 ohms MAX - 150 will do the job even better. You wire the potentiometer across the output leads and connect the headphone to the wipers and ground.

The resistive combiner is a pretty decent idea - again, I suppose it depends on whether you have enough drive to deal with the 3 dB loss in the combiner.

I use Westone UM2 ear-canal (in-ear) monitors. They are very sensitive.

 

[Caba]

Ok - Got it done and it works!! (ok at least technically)

BEFORE you look at the pics below, please remember that this is my first pass at soldering connections :blushing: A lot of it was ugly, but I learned a lot.

What I would have done differently - All I had was 18 Ga wire - way overkill and made the soldering and fitting much harder, but I shouldn't have to worry about resistance! Would have been much better to use 22 or less.

The best I could get for the potentiometer was 22k ohm, and yes, 90% of the volume control is in 20% of the range. Below 22k, the store only had units with single sets of contact - i.e. - no stereo. I'll probably remove the volume control, but half of this was to see if I could, no other reason. Final version was wired exactly as I had originally drawn.

I was able to get good solid connections and I found a solid flux (well, I think it's flux, can't tell for sure, but it worked)

Parts Starting - Lucky I could find these here. (Tic Tac's included for size reference only - :glare: )

Everything wired and in the box (pardon the overkill on the wires and the solder)

Closed up and working! - The two verticle sockets are the inputs and the third is the output. Switch is oriented so the connector in the same direction is active.

I did end up with the following process for putting the wires to the connectors.

First I would dip the iron in flux, heat the post on the switch (or other connector) and just put a small excess of solder to the post, but well wetted.

Next I did the same to the stripped end of the wire.

Then it was easy to get both hot and let the solders join. Didn't have to worry about holding the switch, gun, wire, and solder all at the same time.

Thanks all for the input (the last thing I wanted was to toast the Zumo or MP3 here). Would have been much easier back home...Hell, back home I'd have just ordered the $23 part and be done, but this was fun.

 

[Fred W]

Hey, not that bad of a soldering job considering experience and the tools you had available.

I've seen EE's do a whole lot worse with professional equipment...

 

[DocFJR]

You're dealing with speaker levels - you do NOT need shielded cable.

I agree that normally you don't need shielded cables, since RFI is not a problem with speaker-level signals, I only suggest them since it would allow the output to drive a downstream preamp if desired (an example of this is the single audio out circuit of a laptop. It is designed to drive relatively low impedance headphones (speaker level), and also serves as a preamp out to drive external playback or recording equipment. Internally, the audio circuit for this jack is shielded). In addition, shielded cables would protect against picking up spark plug noise or any noise that may result from cables in parallel with the audio lines. Again, not a critical concern, but with such short runs, shielding has no down side.

Doc

 

[jekirby]

You're dealing with speaker levels - you do NOT need shielded cable.
I suggest you might not bother with the potentiometer - each of the devices has a volume control - set the volumes on the devices. But if you MUST use a volume control, use something with perhaps 500 ohms MAX - 150 will do the job even better. You wire the potentiometer across the output leads and connect the headphone to the wipers and ground.

The resistive combiner is a pretty decent idea - again, I suppose it depends on whether you have enough drive to deal with the 3 dB loss in the combiner.

I use Westone UM2 ear-canal (in-ear) monitors. They are very sensitive.

Both Ipod and Nuvi have enough guts to overcome db drop, but then again I use in-ear monitors.

 

[jekirby]

Here is the diagram. Please pardon sloppy drawing. I use a tablet pc as an etch-a-sketch.

 

[sthames]

Great Job CABA,

That's Engineering!! :clapping: