Switching between speaker attenuators?

[FunkyGibbon]

Hi,

I am considering a project in which I switch between a master volume, and a speaker attenuator to achieve both cleanish, and super compressed sounds, using relays.

Should I anticipate problems with using relays (or any other switching method) to switch the speaker attenuator in and out or, indeed, switch between two speaker attenuators at different levels?

I know that the rotary selector switches in attenuators are usually of the make-before-break type. Would a MBB type relay be required to do this safely for the amp output transformer? Is there such a thing as a MBB relay?

Thanks,

Christopher

[FunkyGibbon]

Does anyone know ANYTHING about switching considerations between the output transformer and the speakers??...

Thanks.

[FiveseveN]

Would a MBB type relay be required to do this safely for the amp output transformer?

Tube amps tolerate the momentary lower-impedance load better than an infinite impedance. So yes.

Is there such a thing as a MBB relay?

Pretty sure there aren't. But you can build such a switch using two relays and a slight delay (maybe a cap to keep the coil energised) so they're both "on" (both loads connected) for a brief moment.

[FunkyGibbon]

Thanks for your replay, FiveseveN.

Please see the circuit I've used below. I'm not sure how to rewire this with a delay between the coils of two relays as you suggest. Any tips?



Thanks again.

[pinkjimiphoton]

FWIW,
i've been switching between my amp's speaker and my talkbox for literally decades now with zero issues or problems.

if you read a speaker with a meter while it's got a guitar going thru it, you'll see it swing all over the place, for an 8 ohm speaker it could go 20 r or more on some peaks, or zero ohms on others...at zero ohms, it's not an iresistance/inductance anymore, it's a conductor. but it's so brief, it doesn't damage anything.

you should be able to switch the output load with no problem.

ymmv.

[FunkyGibbon]

Thanks, pinkjimiphoton for your assurances.

However, if I were to play it really safe, I think I now understand how you use two relays for MBB operation: connect the contacts in the same configuration as before, but now to two relays instead of one. One relay always switches first, and the other second.

I just don't understand how to delay the second relay. Would a cap between the control signal and the second coil work? What value? See below:



Thanks again.

[slacker]

Someone else asked almost exactly the same question a few weeks back, the thread is here http://www.diystompboxes.com/smfforum/index.php?topic=100257.0 might help.

[FunkyGibbon]

Hi Ian,

Thanks. I did search furiously, but didn't use search terms to come up with that thread.

However, in that thread they suggest a permanent load resistor parallel to the speakers/ switching arrangement, with break-before-make switching. That way the output transformer always sees a load, and never an open circuit. That's one way of doing it, and it seems they settled on that as it is much simpler than the make-before-break solution. But is the MBB solution that hard? Using the permanent load solution, the output transformer sees a high load (180R in Traynor amps, 470R in the Headswitch) briefly during switching. Although that's fine, it's not ideal. Using the make-before-break solution, the OT sees a low load momentarily (4R for a pair of 8R attenuators), which is better.

However, I'm not good enough to work out how to delay the second relay. If I can't work it out, of course the permanent load solution is good and I'll go with that, but I'd like to find a MBB solution if I can.

What about this solution from Geofex?:

http://www.geofex.com/article_folders/rmtswtch/rmtsw.htm

See the diagram "Remote switched relay bypass with LED." C1 is designed to slow down the change in voltage in the inductor of the relay. If I had one of these networks for each relay coil, both being fed by the control signal from the 4584 circuit, I could use a larger C1 for the 'second' relay. Any thoughts? Perhaps I'll need to drive each transistor network with a separate inverter, rather than sharing the output of one??..

The Geofex circuit uses 12V. I presume I can use all the same values for 9V?

Thanks.

[FunkyGibbon]

Like this?:




Note the different sizes of C1 and C2 to produce a delay in the second relay. Any comments?

Do I need separate inverter stages to drive the transistor networks for the two attenuator relays, or can they be driven by the same inverter stage (I'd prefer not to have to send THREE trigger signals from the footswitch box)?

What about noise? Do I need to shield all the 9V stuff from the signal path in the speaker attenuator?

Christopher

[kodiakklub]

i have to believe MBB relays exist. We needed them at my old job and used them with industrial automation buttons. the PLC code we wrote needed them as opposed to BBM. so i have to believe they exist as a type of relay somewhere. look into the industrial automation suppliers.

[FunkyGibbon]

Thanks, Karl.

I'll keep looking for MBB relays. In the meantime, any thoughts on the above circuit, shielding, component location, etc. would be most appreciated.

Christopher

[FunkyGibbon]

Like this?:




Note the different sizes of C1 and C2 to produce a delay in the second relay. Any comments?

Do I need separate inverter stages to drive the transistor networks for the two attenuator relays, or can they be driven by the same inverter stage (I'd prefer not to have to send THREE trigger signals from the footswitch box)?

What about noise? Do I need to shield all the 9V stuff from the signal path in the speaker attenuator?

Christopher

So... any thoughts?...