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Solid state switching
Solid state switching
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Old 26th March 2020, 06:45 AM   #1
R1234 is offline R1234  Australia
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Default Solid state switching

I would like to build a push-button solid state input selector with lots of inputs. Never done this before after having terrible experience with a Quad pre-amp about 25 years ago. I think it used 4066 c-mos switches and was quite un-reliable.

Just wondered what the latest thinking is for about 6 Hi-Z 10K line inputs with low THD. feeding straight into a volume control.

I don't want to go over complicated with arduino etc preferring simplicity with no pulse artefacts. - Any suggestions, - latest IC's etc would be welcome.

Thanks.
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Old 26th March 2020, 09:05 AM   #2
mchambin is offline mchambin  France
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DG413 CMOS switches
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Old 26th March 2020, 09:14 AM   #3
jaddie is offline jaddie
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40 years ago (can't believe I'm typing that) I worked with a number of different analog switches in audio applications. The results were routing switchers for pro audio, intercom systems, etc. What we had back then were the 4066 (which we summarily rejected as trash very early on), and a line of much nicer switches from a company called Siliconix, who produced the very first analog switch in the 1960s. Time has marched forward and Siliconix is now part of Vishay, but a brief look around for current product also landed me on Analog Devices. Both seem to have quite a range of very nice analog switches. Start with the Siliconix DG1408, then have a look around. The AD stuff seemed to have very flat V/R curves.

The application rules are not much different today than 40 years ago, though the switch specs have improved. Drive them with a low Z buffer, load them with a termination resistance high enough that the voltage vs resistance curve doesn't introduce distortion, low enough to realize near perfect off leakage (10K worked then, and works now too). And watch their maximum voltage swing vs supply voltage figures. Today's chips have better headroom, lower on resistance, and better off isolation that we had, but what we had worked really quite well and would have been good enough for any audio today, so today's stuff is more than adequate. Both AD and Siliconix have several 8X1 multiplexers that can be controlled with 4-bit word presented in parallel. We used an 8-bit priority encoder to develop that word from discrete switches (like a CD4532B, still current), and the encoders could be chained for more switches. Add a 4-bit latch, perhaps a decoder/driver to light up individual switch indicators, audio buffers on the output and a pair could drive your volume control just fine. Of course, back in those days, real tracking stereo controls were a bit hard to get (you had to buy like 10K pieces, or use some really expensive P&G units) so ours ended up being a 1.5dB step digitally controlled attenuator chip, which also worked quite well, driven from an up/down counter. Even built in a reset reference level button. I saw some by TI, pretty nice and much better today. So yeah, no Arduino, no PC, no programming, it can all be done with logic chips.

We had one big advantage: DIP chips. You're probably going to have to deal with SMDs now. Have fun!

edit: Ha! One of our old favorites, the DG308 (one of the few chip numbers I still remember), is now in a B version, and still made! Wrong style for what you're doing, it's 4 switches individually controlled the 8x1 would be simpler to use as an input selector.

Last edited by jaddie; 26th March 2020 at 09:18 AM.
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Old 26th March 2020, 09:37 AM   #4
R1234 is offline R1234  Australia
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Thanks for your detailed response jaddie. Oops, - I forgot about SMD's - Yuk ! (Shows my age doesn't it !)

Maybe I'll just use relays.
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Old 26th March 2020, 09:49 AM   #5
jaddie is offline jaddie
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Relays? Yuk. Some things get simpler, some more complex. They aren't a free ride either.

SMD's are not impossible, just a task. Need a tiny iron and a good magnifier. But if you look around you may actually find some DIP product from Siliconix. They've been at it SO long, they certainly have retrofit parts for the old product. I'd look for you, just not now. The logic chips are certainly still available as DIP. You're doing a PC board anyway, right?
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Old 26th March 2020, 10:54 AM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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How about a 74HC4051 which has 8 inputs and one output.
A set/reset button circuit driving a counter which drives 4051 s1/2/3 inputs to select device.
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Old 26th March 2020, 11:34 AM   #7
R1234 is offline R1234  Australia
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I was trying not to bother with a PCB but it looks like I might have to, as this looks like being a lot more involved than I was hoping.

Thanks for the link Nigel, I have express PCB but have not used it, is the MP one better ?
Does the 7HC4051 arrangement have good THD/overload specs ?
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Old 26th March 2020, 12:01 PM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by R1234 View Post
I was trying not to bother with a PCB but it looks like I might have to, as this looks like being a lot more involved than I was hoping.

Thanks for the link Nigel, I have express PCB but have not used it, is the MP one better ?
Does the 7HC4051 arrangement have good THD/overload specs ?
I use the 4051 on a USB scope I designed and it works well.

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Old 26th March 2020, 12:27 PM   #9
sgrossklass is offline sgrossklass  Germany
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R_on looks lower than for a CD4066 at least. In return, it's only good for +11 V ab max instead of +20 V, but either should do for 2 Vrms and change. R_on nonlinearity looks similar between 74HC4051 at 9 V and CD4066 at 15 V, with probably an edge to the latter when give about a +1 V offset (or correspondingly asymmetrical supplies).

As previously stated, drive with a buffer for each input (and give that some minimal input protection, too) and load with as high an impedance as you possibly can... a 15 ohm R_on nonlinearity into 100 kOhms is still good for 0.015% p-p when it comes to distortion, so you want at least 100k, preferably 1 Meg.

When it comes to +/-15 V capable options, NJR has the NJU7301 that may be worth looking at, as well as the Siliconix-compatible NJU201A and 211. Unfortunately, their datasheets are quite useless and look like they haven't been updated for over 30 years. Toshiba's TC9163AN used to be quite popular, I don't think this one has been made in quite a while though.
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Old 26th March 2020, 03:43 PM   #10
mlloyd1 is offline mlloyd1  United States
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Solid state switching
Whenever this question pops up, I recommend to study schematic for Hafler 915 JFET preamp. You can find the relevant bits even searching on this site. The circuit there does what you want, performs well and is not terribly expensive.

Also, I agree with the recommendation to look at DG413 analog switch.

good luck!
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