Head switcher project: some questions

[janou195]

Hello

I am working on a project of head switcher: 1 guitar, 1 cab, 1 chain of effects to be in an effect loop, 2 tube heads. I think about a box into which all these things are plugged, with a footswitch to select one head or another. It would do about the same as a radial headbone VT, but (hopefully) without sucking tone, and handling the effect loop.

I order to have at any time a proper load on each head, I am planning to use some relays whose switching sequence is controlled by a microcontroller. Furthermore, in order to avoid ground loop problems, the two heads will be tottaly isolated (no common ground).

Here is the design I am thinking about at this time (the realys references are wrong and the power supply is not represented):
http://olivier.jamond.free.fr/headswitcher.bmp

To switch from one head to another, the switching sequence will be something like:
- shunt of both amps inputs (relays RL8 and RL9 on the schematic)
- shunt of both amp loop return (RL1 and RL2)
- switch of the loop send (RL7)
- switch of the amps outputs (RL4, RL5, RL10)
   assuming on switch from amp A to amp B (initial state: A linked to LS, B linked to its fictive load R2 on the schematics):
   * connection of R1 on A in parallel with the LS (RL4)
   * switch of the LS from A to B (RL5)
   * detach R2 from B (RL10)
- link of the loop return to amp B (RL2)
- link of the guitar to amp B input (RL9)
This whole switching sequence is handled by a microcontroller.

I still have some questions related to this project:
1) What do you think about this project/design? Maybe I am missing something?
2) When I look the aiken headswitcher schematic in the link following, I don't understand what is the purpose of the 470Ohm resistors R1 and R3. What do they do?
http://web.archive.org/web/20030718021444/http://aikenamps.com/HeadSwitch.pdf
3) Will this design generates some relay switching "pop" problems? In particular, during the first stage of the sequence, as shunting the guitar signal for both amps, the amps input will be connected to nothing during the switch of RL8 and RL9. Will it be problematic?

Many thanks!

[GGBB]

Not an expert on this topic by any means - I'm sure some will join in soon - but ...

2) Dummy load

3) Probably. There are ways to deal with that, but you are using a lot of relays so that's a lot of dealing with that.

1) If I were to take a stab at this, my approach would be to use relays for only the speaker switching. That's RL4, RL5, and RL10 in your diagram, but I don't see the point of RL5. The speaker could connect to the spare terminals of both RL4 and RL10 simultaneously. Better yet, implement the dummy load the way Aiken does it and use just one DPDT relay to switch the speaker. As far as everything else is concerned, I would essentially just run everything in parallel, using buffered splitters and joiners to have both amp's loops connected through the same effects chain at the same time, and using JFET switching to mute amp inputs and effect returns, and effect sends if you wish. The JFET switching is noiseless, an as long as everything is properly buffered, you shouldn't have problems and you can do it all with only one relay.

Again, I'm no expert so maybe I'm missing some fundamentally important detail, but I'm sure someone will correct me if I did.

[janou195]

2) But what the point of these two resistor with respect to the dummy load (R2,R4,R5,R8,R9)? They must affect the active amp tone, no?
3) What reassures me is that with the switching sequence, I think that the relays will switch null current  (except the first and last in the sequence, as shunting and reconnecting the guitar signal, and maybe the switch for the loop, because of time effects..)? I am wrong?
1) the relay R5 is used because I cannot find some "make-before-break" relays. So I do it "manually" with another relay. Regarding buffers, I would attempt not using one for total tone preservation. I plan the final box to be only "wire" for the active amp.

Thanks

[GGBB]

2) But what the point of these two resistor with respect to the dummy load (R2,R4,R5,R8,R9)? They must affect the active amp tone, no?

I am guessing but I believe they are for providing some dummy load during break-before-make switching, and possibly help with switching noise. EDIT: Would have the effect of 470ohm in parallel with the speaker, which drops the load that the active amp sees very slightly - worst case is 16ohm to about 15.5ohm. Theoretically that would change the sound, but probably not enough to be noticeable.

1) Regarding buffers, I would attempt not using one for total tone preservation. I plan the final box to be only "wire" for the active amp.

I can understand that for the guitar input, but a quality buffer should not cause any tone loss in the effects loop.

[Lukes0n]

Hello, did you tried your head switcher layout? I am looking for building something similar. I guess it should work with those relays switching the load and the speaker, but better to have it confirmed that is working safely. Thanks for reply

[PRR]

......head switcher layout? ...

May not be what you want? Still a fascinating catalog.
https://enablingdevices.com/product/head-switch/

[GGBB]

OP only ever made the two posts here, and hasn't logged in since March 04, 2015.

[Lukes0n]

......head switcher layout? ...

May not be what you want? Still a fascinating catalog.
https://enablingdevices.com/product/head-switch/

That one is great head switcher !

I just really want to know, if is possible to use relay switching as in layout without doing some damage to the amplifier. Because from my perspective its just changing one load to another in miliseconds.