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 Forum index » DIY Hardware and Software
Voltage-controlled switch for audio
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rutgerv



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PostPosted: Sat Nov 27, 2010 2:34 pm    Post subject: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Hey everyone!

As I've always wanted to have a nice analog DIY poly-synth I'm currently investigating if it's possible to design a voice-card that contains an entire mono-synth with a completely analog audio path, but with al it's features under CV control. I'm hoping to get a series of these voices controlled by a STM32 processor running on MIOS (see www.ucapps.de) and a series of DACs.

To be able to change the audio routing in the voice-card (for instance to different filter inputs) or to enable/disable the a hard sync option between two VCO's I'm looking for a nice, simple, cheap, but high-quality voltage controlled switch.

I saw the clever use of the multiplexer in the Bergfotron Advanced VCO: http://hem.bredband.net/bersyn/VCO/vco_advanced.htm , but it made me wonder how it would affect signal quality?

Also, I've found the SSM2404 audio switch, but it seems to expensive and hard-to-find for my design.

Does anyone have experience with this type of VC-switching? And can you recommend what to best use for audio-path switching or CV-path switching? Maybe just a simple LM13700-based VCA would do the job? Or is the VCA capability overkill, since I only need on/off?

Kind regards,

Rutger
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Tim Servo



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PostPosted: Sat Nov 27, 2010 3:41 pm    Post subject: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Hey Rut,

Most programmable poly synths in the past have used analog switches for routing audio and CV signals. If you are reasonably careful about signal levels, you can get by with the relatively inexpensive 4066 and 4051 switches. You'll see the 4051 used a LOT in older analog synths, as this is an analog mulitplexer (can select one of several signals under digital control).

If you need to control the level of a signal (as opposed to just turning it on or off), then a 13700 is a good solution. You might also want to consider the Analog Devices 2164, which contains four exponential VCAs. The 2164 can be easily configured as two linear VCAs also, and is quieter and has a greater dynamic range than the 13700.

For research, I'd suggest looking over the schematics for programmable polyphonc synths like the Memorymoog, the Oberheim OB-X and OB-Xa, and the Sequential Circuits Prophet 5. All of these can be found on line with a bit of searching and will be good sources of inspiration.

Hope this helps.

Tim (multiplexing quite a bit these days) Servo
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bugbrand



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PostPosted: Sun Nov 28, 2010 9:07 am    Post subject: Reply with quote  Mark this post and the followings unread

For cheaper analogue switches than AD have a look at Intersil - in particular the DG 4xx range etc.

These are good if you want to use regular modular bipolar swings - I think the 405x that Tim suggests can be set up for bipolar operation (I usually think of them as used for positive voltages only due to being CMOS4000), but I've always used the DGs. The 405x chips are much cheaper so...

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fonik



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PostPosted: Sun Nov 28, 2010 10:24 am    Post subject: Reply with quote  Mark this post and the followings unread

in the past i used bott, DGxxx and 405x series ICs for voltage controlled switches.
this cheap solution has one major drawback in my point of view: you will get 'pop'-sounds when switching the audio.
on way to come around with this 'pop' would be to use VCAs and to ad some slew to the pulse that switches the signals...

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Boogdish



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PostPosted: Sun Nov 28, 2010 11:21 am    Post subject: Reply with quote  Mark this post and the followings unread

In guitar amps vactrol ( a light dependent resistor coupled with an LED) based switches are often used because they don't pop. You also wouldn't have to worry about exceeding voltage ranges. The drawback would be price.
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rubendelacosta



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PostPosted: Sun Nov 28, 2010 3:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

maybe some optocouplers would work well forthis situation.
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nerdware



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PostPosted: Sun Nov 28, 2010 4:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Would a CGS55 be overkill? Too expensive?
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Tim Servo



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PostPosted: Sun Nov 28, 2010 6:22 pm    Post subject: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Well, I think the principles of the CGS switch might be worth looking at, but if you're designing your own voice card from the ground up, it probably is a little more complex than you'll need in most cases. Also, when designing a voice card, you should be able to ensure that signal levels stay within +/-5V, which can be easily handled by a 4066 or 4051. Now if you're going to use these switches within a modular, then you need to be a lot more careful about overvoltage protection.

As far as the popping problem. what causes this? Is it just situations where there's a DC offset? The 40xx series switches are used in so many programmable analogs, that there must be a reasonable way around this. I've never heard any popping or clicking problems in a Prophet or OB-X.

Another alternative might be the H11F1 Optocoupler. Can handle high signal levels, reasonably priced (about $1.90 at Mouser), and you can put a very slight 'slew' in the control signal if you experience any popping or clicking problems.

Tim (clickity pop) Servo
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fonik



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PostPosted: Mon Nov 29, 2010 12:04 am    Post subject: Re: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Tim Servo wrote:
As far as the popping problem. what causes this? Is it just situations where there's a DC offset?

i don't think so. it is because of the steep edge of the controlling signal, cutting off the waveform (glitch? i don't think this is the correct term). if you add some slew the 'pop' goes away.
some people like to use gates (i.e. from dividers) to control VCAs. for this purpose you can use passive slew circuits, as seen here:
http://navsmodularlab.blogspot.com/2010/07/diy-quad-slew.html

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Tim Servo



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PostPosted: Mon Nov 29, 2010 4:18 pm    Post subject: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Hey Fonik,

Actually, I was wondering about what caused a pop when switching audio with an analog switch. In that case, you can't really "slew" the control signal. It has to be either on or off.

Tim (Iffy Pop) Servo
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Blue Hell
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PostPosted: Mon Nov 29, 2010 4:41 pm    Post subject: Re: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Tim Servo wrote:
I was wondering about what caused a pop when switching audio with an analog switch.


When the momentary voltage of "channel from" differs from the one at "channel to" and the switch is "fast" ... it will "pop".

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Scott Stites
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PostPosted: Mon Nov 29, 2010 8:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

There are switches which actually work to slow down the transition to eliminate pops. Google "analog switch pop" and you'll probably find the application note. IIRC, they're thoroughly modern (read tiny SMD). Mouser has them, along with pictures of how tiny they are.

There is this rather convoluted method (a VCA would probably be easier) that one might use to reduce pops with DG/CD parts. Interesting nonetheless...

http://www.google.com/patents/about?id=vVAGAAAAEBAJ&dq=6157726

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Blue Hell
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PostPosted: Tue Nov 30, 2010 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Dont know how fast the CD thingies switch ... but if its really fast a PWM trick could be used too.
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fonik



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PostPosted: Tue Nov 30, 2010 1:16 pm    Post subject: Re: Voltage-controlled switch for audio Reply with quote  Mark this post and the followings unread

Tim Servo wrote:
Hey Fonik,

Actually, I was wondering about what caused a pop when switching audio with an analog switch. In that case, you can't really "slew" the control signal. It has to be either on or off.

Tim (Iffy Pop) Servo
i was refering to the VCA approach. sorry, have not been that clear...
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rutgerv



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PostPosted: Thu Dec 02, 2010 2:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Cool! Thanks for all the nice suggestions. I will experiment with a few of them and see what gives the best result for a decent cost!

Regarding the SMD stuff, I'm not particularly drawn towards soldering that Smile. However, I found out that it can be a lot cheaper sometimes! I already intend to use an SMD set of matched transistors with an SMD tempco mounted on top and glue everything onto a tiny PCB that would fit in a DIL socket.

Best,

Rutger
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elektro80
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PostPosted: Sun May 25, 2014 12:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

I recently acquired some of the Analog Devices SSM2404s. One of the main features is that the design is pop free.

http://www.zmitac.aei.polsl.pl/Electronics_Firm_Docs/Analog_pdf/ssm2404.pdf


Quote:
Switch control is realized by conventional TTL or CMOS
logic. Guaranteed “break-before-make” operation assures that
all switches in a large system will open before any switch
reaches the ON state.

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fonik



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PostPosted: Sun May 25, 2014 1:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

wow. scott told me about these months ago, alas, they are obsolete and i have not been able to source them.
may i ask where you got them?

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PostPosted: Sun May 25, 2014 4:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

A friend gave them to me. 2pcs.
He bought them not that long ago on eBay. I dunno what he paid for them. I guess he gave them to me because he figured he would not get around to build whatever he had planned to build.

Are these supposed to be hard to find? I know they are no longer being produced, but shouldn't there still be quite a number of them in stock somewhere?

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