Transistors for an Audio Switch - Page 1
Forum section: electronics / beginners

author: 5Beans6
author_rank: Newbie
timestamp: May 15, 2017, 07:46:17 pm
content: I want to make a custom audio switch box to be able to change between my headphones whenever i want just by the flick of a switch. I was wondering if i could just use regular npn transistors to direct the current from my computer or if i have to use a relay as audio is AC?

author: fable
author_rank: Regular Contributor
timestamp: May 15, 2017, 07:52:38 pm
content: Why not use 4052 or 4053 analog switch ic ?you can find them everywhere

author: 5Beans6
author_rank: Newbie
timestamp: May 15, 2017, 07:56:34 pm
content: Because those are multiplexers, doesn't that mean the signal will be turning on and off rapidly as that is what multiplexing does? This is very undesirable for an audio situation

author: fable
author_rank: Regular Contributor
timestamp: May 15, 2017, 08:01:42 pm
content: No.You control which switches are on by driving select pins high or low...read datasheet.Sorry for my bad english.

author: 5Beans6
author_rank: Newbie
timestamp: May 15, 2017, 08:05:13 pm
content: Oh ok, cool! I was not aware of their existance. Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2017, 08:05:21 pm
content: An analogue switch is no good for switching power, i.e. the current flowing through headphones or a speaker. They are only suitable for small signals.To switch headphones or a speaker, use a relay.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: May 15, 2017, 08:05:43 pm
content: don't you have to take the headphones on/off your head? then switching a plug is too hard?quality switching dynamic headphone directly could require handling up to few 100 mA (Orthodynamic headphones) and blocking few 10s of Vrelays are the obvious candiates for mOhm metal-to-metal distortion free switchingfor solid state there are opto isolated SS Relays with bidirectional MOSFET switchesbjt transitor bidirectional switching isn't pretty

author: 5Beans6
author_rank: Newbie
timestamp: May 15, 2017, 08:07:49 pm
content: Quote from: f5r5e5d on May 15, 2017, 08:05:43 pmQuote from: Hero999 on May 15, 2017, 08:05:21 pmSo relays it is then?

author: edavid
author_rank: Super Contributor
timestamp: May 15, 2017, 08:30:34 pm
content: Quote from: 5Beans6 on May 15, 2017, 07:46:17 pm"Change between my headphones" and what?If you want to be able to switch between speakers and headphones without unplugging the headphones, an easy way is to take an audio plug and 2 audio jacks, and wire them to a manual DPDT switch. No relay is required.You could also buy this:http://www.ebay.com/itm/2-PORT-3-5mm-STEREO-Manual-Sharing-Switch-BOX-Audio-Speaker-selector-way-2-1-/222343690109As you guessed, bipolar transistors are hard to use as audio switches.

author: 5Beans6
author_rank: Newbie
timestamp: May 15, 2017, 09:20:45 pm
content: Sorry, yes i do mean speakers and headphones. That looks like a viable device, however the quality kinda worry's me. I was also thinking about including some extra features too that I can only achieve by doing this DIY style due to the way my amplifier is and such.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2017, 10:20:23 pm
content: Quote from: 5Beans6 on May 15, 2017, 08:07:49 pmYes, a relay is your best bet. A small SPDT (Single Pole Double Through) unit will do.

author: BrianHG
author_rank: Super Contributor
timestamp: May 15, 2017, 10:29:16 pm
content: Quote from: Hero999 on May 15, 2017, 10:20:23 pmStereo?Why not DPDT?

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2017, 09:45:50 am
content: Quote from: BrianHG on May 15, 2017, 10:29:16 pmYes, that would make much more sense.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 16, 2017, 09:50:14 am
content: I would argue that a more important question here is whether or not it's a good idea to drive headphones and speakers from the same source. Something built to drive external speakers might be expecting 4-8 ohms, but depending on the headphones, you could be looking at anywhere from 32 or lower to 350 ohm load.

author: edavid
author_rank: Super Contributor
timestamp: May 16, 2017, 03:05:03 pm
content: Quote from: TheUnnamedNewbie on May 16, 2017, 09:50:14 amI think we are talking about powered speakers.

author: Zero999
author_rank: Super Contributor
timestamp: May 16, 2017, 04:46:30 pm
content: Quote from: TheUnnamedNewbie on May 16, 2017, 09:50:14 amHeadphones will work fine off the same amplifier designed to drive low power speakers, i.e. under a couple of Watts into 8 Ohm. If it's a much larger amplifier >10W or so, then use an attenuator, consisting of a potential divider (a couple of resistors) to drive the headphones. A common solution is to use bridge outputs to drive the speakers and single ended to the headphone socket.

author: mikerj
author_rank: Super Contributor
timestamp: May 18, 2017, 10:57:40 am
content: There are analogue switches that are suitable for switching headphone level signals e.g. MAX4762.

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2017, 02:25:33 pm
content: Quote from: mikerj on May 18, 2017, 10:57:40 amThey look good and reasonably priced too.I wonder how they handle negative signals? Perhaps there's a built-in charge pump to generate negative voltages?