Audio relay switcing audio.

[retrotubeguy]

Hi guys, I was wondering if anyone had any opinions on switching low level audio such as guitar signals with relays. The signal would be buffered and transformer isolated so that the one mono signal could be fed to two amps The plan is to switch between amps or blend them . I would like to transformr isolate them to avoid any interaction between  the two amps. This is commonly done. I was planing to drive the audio transformers with a small gain stage just to skwirt a little juice into them to prevent any loss.  The big question is ,if I add gain before a set of relay contacts would I escape any capcitance issues what about contact resistance. Dont want to use mercury wetted relays. I have used standard signal relays before with no problem but I am concearned about  noise, reliabillity, etc. What about pickering reed relays. Bit perplexed. I know just about any decent relay will do but is there a better way.
Thanks.

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[johnkenyon]

Have a look at http://control.com/thread/963420679 for some ideas...

[Richard Crowley]

If you are going to amplify (or at least buffer) the guitar signal, then you have isolated yourself from effects of the high-impedance parts of the circuit.
Hard-switching could be as simple as muting (shorting) the unused output. Shorting across a low-level signal is typically cleaner than trying to interrupt the signal path with a series switch.
Blending would be a matter of making a "balance" or "pan" type circuit. And you wouldn't expect to be using hard contacts like switches or relays.

[macboy]

Use a good quality telecom relay and you are all set. These are designed to switch low voltage audio type signals. At least you seem to know better than to use any old cheap ebay-special power relay.

Since you are buffering anyway, why not use two buffers, one for each amp, in order to effect the isolation that you want? Then you can ditch the transformer.

[PChi]

Reed relays are good and last a long time. I built an audio pre amplifier with some reject reed relays that is still going 35 years later.

[LaserSteve]

The phone company flows small amounts of DC current thru older style switches, relays and spliced wires, including ISDN/ADSL.  This "sealing" current of a few milliamps insured a good connection at AC.  This reduced the need for a full time person to stay at stepping switch and relay switched Central Offices.  This person's job would be mostly cleaning/adjusting  contacts, for eight hours at a time.   Sealing current reduces the effects of water vapor and penetrates thin oxidation on the contacts.   That was until  they developed the glass reed relay which is filled with inert gas.

After a glance at Wiki, this is also known  as sealing, fritting, or wetting current.

Modern small relays rated for audio have the right kind of "rare metal" contacts that are self cleaning to a degree. However reed relays designed for audio excel at this task. They also switch quickly with little bounce.

Off topic: test fixtures often use mercury wetted reed relays to produce pulses with sub nanosecond rise time and no bounce. But that is a story for another thread.

Worth a read: http://www.ospmag.com/issue/article/062013-McCarty

Steve

[retrotubeguy]

Thanks for the reply's. I think I will use the reed relay or Telco relays. They are for A/B Selection. The transformers are staying. They will be at the output stage I just prefer to have them no ground loops etc. People tend to like them also. I have used them just seem to sound better. I will be using a dual gang balance pot for blending. Any thoughts on discrete Jfets Vs Op amps for my buffer. I was considering using the LM833 or TL072 or similar garden variety audio Op amp. Never really had a bad experience with either OP amp but there is a lot of talk about using Jfets over OP amps for guitar. I think its silly. Some say Op amps are harsh sounding.  What do you guys think

Thanks

[Paul Price]

You can accomplish this task most easily using any of the CMOS 4xxxx series of analog switches, with excellent isolation, low cost and a small circuit footprint,  and using very low power, especially compared to any relay-based circuit.

Think CD4051BE, 4052, 4053, 4016, 4166 part numbers.

[SeanB]

As the relays do not need to provide isolation, and you already have +-15V rails for the opamps the CMOS 405X series switches will work best. Use optocouplers for the control lines if you are worried about digital signal interference, and simply terminate all inputs and outputs with 100k resistors to analogue ground to keep the switches safe. Arrange so the DC output of the buffer can be nulled if the slight noise on switching is an issue, but the CMOS switches are cheap and reliable.

[Paul Price]

Or give a look at a Texas part: TS12A4451x

For every cheap and simple CD4xxxx solution, TI, Maxim and LinTech offers a more expensive and harder to obtain solution, for example:

[retrotubeguy]

MMm OK I will give that a look. Cmos sounds like the way to go. Thanks