Relay

[SamiBami]

Hi all,
I got an effect box which consists of 4 potentiometers, I want to extend my effect presets by buying 3 sets of 4 potentiometers and connect one set of potentiometers each time by relays or some other kind of switching
that way I get 3 more predefined effect presets

I know that relays can add noises to audio, so can you please recommend me a proper switching mechanizem and model

Sami

[SamiBami]

any one??
what about the CD4066? could this "pass" the resistance I'll connect it to?

[Enzo]

RElays add noise when the circuit points they switch are left unterminated. In other words if you want to switch between two different caps, you can't just leave the cap soldered to the relay contact. There needs to be a high value resistor or some other accomodation to maintain any charge in that cap. Otherwise it will pop when switched. I mean make a pop sound, not actually burst. SO simple good practice eleminates most noise issues.

Are these unrelated pots? Or are they grouped as in a tone stack in a guitar amp? If they are unrelated, then I guess they all have to be switched individually. But if you are selecting two sets of tone controls, then beter to just make up the whole set and have a relay at the entrance and at the exit, rather than switching each pot.

Little relays would work, but so would probably a 4066 or other analog switch IC.

And just a handful of JFETs would also work. Use one wherever you want a switch. And Pchannel and Nchannel would allow one control line that went positive to negative and back would let you use one control signal to both turn on and turn off the appropriate JFETs. Look at how Crate switches channels on a lot of models. Or look at something like a Peavey Classic 30, how they turn the reverb on and off.


DUmmy up some small circuit with a pot in it, and then add switching for an alternate pot, just to test the approaches.

A relay will need a power supply for the relay coils. and switching would be simple on off. The 4066 would need a supply for the IC itself. And logic level control inputs. JFETS would need control voltages. Each method would need some extra voltage unless you can use the supplies the signal circuits use.

[SamiBami]

Tnx Enzo
I thought setting it up for the Boss DD-7 which all 4 pots pins are connected together (in parallel) besides the middle pin, the effect is digital, so I think that the pots are some kind of voltage reference for ADC, in that case relay's noises won't be a problem

regards the 4066, I saw in datasheet that it passes up to few tens of uA,do you think it is enough?

what FET would you recommend?

Tnx

[Enzo]

Oh, control voltages. SO no signal to worry about, no distortion to worry about. I doubt much current is flowing in your pot circuits. Look at where those pot wipers go. I'd wager they go to some pins on an LSI or even some CMOS switcher themselves.

Switches like JFETs or that 4066 will go to a low resistance. I don;t think it would affect control voltages, but look at a data sheet, if having say 200 ohms instead of a direct circuit matters, better to know up front.

A 4066 offers multiple sections, but you could try one, and if it burns out, you are only out 50 cents.

I don;t have favorite JFETs, I run a maintenence and repair operation, I don;t hobby build much anymore. I replace existing ones when they fail, so I stock many types. But real common ones would be J111, J112, or J175. There are a bunch of 2Nxxxx types and others. GO to www.jameco.com and do a search for "JFET". A bunch of options come up. And certainly any other general supplier like Mouser would have plenty.

[indianajo]

Look at CD4052s also, 4:1 mux IC. If you are running a low impedance circuit, TI makes CMOS analog switches as low as 3 ohms resistance now. The trouble with relays is that the turning off of the coil makes a big "POW" in audio. Putting backwards fast recovery diodes across the coil helps some, but not a lot. Also at low voltages and currents you have to use silver or rhodium or gold plated relay contacts. Look up "dry circuit". I'm finding surplus rhodium plated relays for $2 in the US, but list price is over $30 each.
The trouble with modern low resistance CMOS analog switches is they are in SMD packs and are harder than **** to solder. The old ones can be bought in DIP packages. MCMelectronics.com in US has a nice $2 project board 21-4580 to solder DIP sockets to, it is sourced in taiwan so there might be an oriental source that has them easier for you to access. Label on the PCB is Datak 12-617b or CIC 21-111.
I recently bought some J176 JFET's from newark.com (US subsidiary of farnell.com). Fairchild division TI has recently changed from the old National Semiconductor pinout to the Siliconix division of Vishay pinout.

[SamiBami]

Tnx indianajo
I'll try the IC..
regards FET it's problem because I need to create also negative gate voltage control (relative to FET's source)

[Enzo]

Oh, good idea, I forgot about the 4051/4052/5053 family of useful analog selectors.

You do need a diode across the relay coil, but the most common reason for big pops is unterminated caps in the signal path.

[KatieandDad]

In HIGH-ENED world. Can anyone recommend a decently priced relay that costs pennies.

RS do several at about £1.29 each but they recommend that the minimum switched voltage is 100uV and the minimum switch current is 20uA. Not ideal for audio where the voltage and current can be 0.

I'm going to be using nearly 100 in a decent Switched Ladder attenuator so cost is of prime importance. DPDT would be nice but DPST would be OK.

[AndrewT]

did you look at the Glassware product.
It could be implemented with relays rather than rotary switches.
This could reduce the number of relays required very substantially.