Transistor arrays for switching audio? - Page 1
Forum section: electronics / projects

author: iampoor
author_rank: Frequent Contributor
timestamp: September 22, 2013, 10:03:02 pm
content: HiDoes anyone know of any SMD transistor arrays that would be suitable for switching audio signals? Most of the arrays I have found seem to have protection diodes to protect from inductance (Im assuming when used with an electromagnet)?

author: David_AVD
author_rank: Super Contributor
timestamp: September 22, 2013, 10:04:24 pm
content: You'll need to detail exactly how you intend to use a transistor to switch audio first I think.

author: iampoor
author_rank: Frequent Contributor
timestamp: September 22, 2013, 10:10:04 pm
content: Quote from: David_AVD on September 22, 2013, 10:04:24 pmGotcha. I will draw a small schematic/diagram!

author: dannyf
author_rank: Super Contributor
timestamp: September 22, 2013, 10:14:00 pm
content: QuoteAnything can be suitable, depending on your particular implementation.One low-tech approach is to use the transistor as a leg on a T-network of resistors: saturating the transistor mutes that channel it is on.

author: zapta
author_rank: Super Contributor
timestamp: September 23, 2013, 01:02:54 am
content: Quote from: iampoor on September 22, 2013, 10:03:02 pmDoes it have to be a SMD transistor array? How about switches like this one?http://pdf1.alldatasheet.com/datasheet-pdf/view/26850/TI/CD4016B.html(there are many others).Tell us what you want to achieve. This way you will get better answers.

author: Paul Price
author_rank: Super Contributor
timestamp: September 23, 2013, 03:09:34 am
content: You could use a transfiguration diode driving unwanted audio signals into the bit bin with careful biasing.

author: iampoor
author_rank: Frequent Contributor
timestamp: September 23, 2013, 03:32:14 am
content: Okay thanks for all the suggestions so far.Basically I want to build a Pic controlled switching array. Instead of using regular switches Im using rotary encoders and a pic. I want to be able to select inbetween 10 different values of resistors and switch individual resistors into the circuit. In this particular application I am using it as a primitive gain control. I want to try using it in an opamp feedback loop, and as an attenuator after amplification). Voltage's will range inbetween 9v and 30v. My number 1 design priority is to minimize additional noise (which will really come mostly down to circuit board layout I believe) and to eliminate noise while switching resistor values.I know there are other ways to implement this with premade IC chips, but I am really looking to do this as a learning expierement....

author: David_AVD
author_rank: Super Contributor
timestamp: September 23, 2013, 05:16:36 am
content: What have you tried so far? How did it work out?

author: iampoor
author_rank: Frequent Contributor
timestamp: September 23, 2013, 07:21:17 am
content: Quote from: David_AVD on September 23, 2013, 05:16:36 amI havent tried anything yet, in the brainstorming phase right now. HOping to try a few different topologies and compare them in person, since each seems to have significant drawbacks so far.The bank of resistors goes from 100 to 1000 ohmns, so Im concerned that the bilateral arrays might interfere on the lower values.

author: David_AVD
author_rank: Super Contributor
timestamp: September 23, 2013, 09:51:17 am
content: I think you should experiment with just a couple of gain ranges to start with then scale up from that.

author: nctnico
author_rank: Super Contributor
timestamp: September 23, 2013, 05:23:05 pm
content: I'd recommend a digital potmeter or 74HC4316 analog switches. The resistance seems a bit low. I'd choose 1000 Ohms for the smallest resistor.

author: dannyf
author_rank: Super Contributor
timestamp: September 23, 2013, 11:37:04 pm
content: DC biased diodes are fairly good as switches too.

author: David_AVD
author_rank: Super Contributor
timestamp: September 24, 2013, 12:38:46 am
content: Quote from: iampoor on September 23, 2013, 03:32:14 amIs this the signal amplitude or supply rail?

author: zapta
author_rank: Super Contributor
timestamp: September 24, 2013, 03:54:40 am
content: Will a digital volume control works for you? There are many, here is an arbitrary onehttp://www.ti.com/lit/ds/symlink/pga2310.pdf

author: iampoor
author_rank: Frequent Contributor
timestamp: October 29, 2013, 01:55:04 am
content: Finally had a chance to revist this.Im not looking to go the Digital Volume control route, I want to find a solution that can span across multiple applications, and that route is too limiting right now.Ive een looking at using switched resistor ladders, since Ive realized most of the controls I want to simulate are logarithmic. Ive also realized that my initial idea was far too complex. Seems like a digitally controller resistor ladder will maximize components.http://www.eijndhoven.net/jos/attenuator-calculator/index.htmlNow these usually use relays, but Im interested in simulating this with transistors. I was thinking of using NPN switching pairs to simulate the "on-on" relays. Thoughts? Im still looking for an IC transistor array with 8 transistors without diodes on the collector!Thanks for all your help! And yes David, going to start simple :-)

author: minime72706
author_rank: Frequent Contributor
timestamp: October 29, 2013, 02:13:16 am
content: Audiophiles (cringe) tend to like using signal relays to make variable gain (or attenuation, if you will) systems.http://twistedpearaudio.com/control/jt.aspxThis probably isn't helpful, but it took very little effort on my part to write these sentences.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 29, 2013, 03:48:05 am
content: Quote from: minime72706 on October 29, 2013, 02:13:16 amIm thinking aout implementing a similar resistor ladder, albeit with Transistor Arrays. Not sure if a suitable part exists... (Im a noob). I have found many transistor arrays with 8 common collector darlington pairs, but Im wondering if an array with 8 transistors with no common collector exists. I have found one with 3, but that doesnt help too much...

author: edavid
author_rank: Super Contributor
timestamp: October 29, 2013, 04:25:01 am
content: Quote from: iampoor on October 29, 2013, 01:55:04 amSo use a digital pot.QuoteMy thought is that using NPN transistors to switch audio is hopeless.QuoteThere isn't one.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 29, 2013, 04:36:41 am
content: Quote from: edavid on October 29, 2013, 04:25:01 amOkayWhy is it hopeless? Please, eat my newie rain into a pulp, Im so lost!Thanks you! Just wanted to make sure I am not missing something.

author: David_AVD
author_rank: Super Contributor
timestamp: October 29, 2013, 07:11:14 am
content: Using transistors is not going to work very well at all. I suggest a simple experiment with one transistor to confirm.

author: Whuffo
author_rank: Regular Contributor
timestamp: October 29, 2013, 12:23:03 pm
content: You could use FETs to shunt signal to ground; low RDS on parts have on resistances less than an ohm.

author: c4757p
author_rank: Super Contributor
timestamp: October 29, 2013, 01:03:11 pm
content: Quote from: David_AVD on October 29, 2013, 07:11:14 amI've done this before and it works pretty well.V3/R4 bias the collector positive when switched off so the negative dips don't forward bias the B-C junction. You can even use a transistor array (with the diodes) without problems. You'll probably want resistor arrays as well for the collector bias and base resistors.If you want something generic to switch signals in and out rather than simple attenuation, you'll need something more complicated of course.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 29, 2013, 01:25:28 pm
content: I think what people are worried about is unexpected effects on the input audio signal - you can probably kiss fidelity goodbye. I suppose if the resistor value swamps the transistor effect, it might work okay. Discrete transistor circuits are still somewhat of a weak point for me.

author: c4757p
author_rank: Super Contributor
timestamp: October 29, 2013, 01:33:17 pm
content: Try it then. You probably wouldn't do it in an "audiophile-grade" system, but with the right parts you could probably make it work quite well at a consumer grade or a bit above.

author: edavid
author_rank: Super Contributor
timestamp: October 29, 2013, 03:29:09 pm
content: Quote from: iampoor on October 29, 2013, 04:36:41 amDistortion. Why fight it when you could just do things the right way in the first place?

author: Frant
author_rank: Regular Contributor
timestamp: October 29, 2013, 09:16:58 pm
content: Quote from: iampoor on October 29, 2013, 03:48:05 amHow about optocouplers or MOSFET solid state relays?