Twin Preamp channel switching and other head scratchers

[Jennings]

Morning folks!  I wonder whether I could petition the hive mind to help guide me through getting my little design over the line...this is as far as I've got with the schematic so far.


As you'll see, it's largely based on the familiar Dual Lite topology (kudos to Steve there  ), except I'm attempting a number of adaptions:

• Single jack input, but panel switchable hi/low sensitivity.
• Relay driven foot-switchable channel switching and jumping together.
• Output impedance selector, with switched jack connecting a resistive dummy load when speaker cable unplugged.
• Standby or mute switch.

Please do feel free to make comments, suggestions and corrections...but I'd also love a bit of help with the following head-scratchers too:

• Does my relay setup and associated power supply look OK?
• How do I go about adding indicator LEDS to my preamp switching, located on my dual footpedal?  I'm a bit stumped!
• Can I incorporate panel mounted manual switching too?  My brain hurts already after my drawing got this far!
• Is there a "better" way of muting the amp with a manual panel switch than by implementing the standby as-drawn?

As an aside, I'm aiming to build this as compact as I can too...so ideally I'd love to combine the on and standby / mute switching into one three position switch.  Or is that just likely to up the noise if I'm bringing part of the circuit too close to AC mains?

[Merlin]

I doubt you'll have anough headroom to run a 7805 from the heater supply. But you don't need a reg anyway, just put a suitable dropping resistor in series with each relay coil.
EEK! Don't put a standby switch directly in series with a choke! The flyback will kill the choke. Node 'A' needs to move.
You can't easily put LEDs in the footswitch unless you use transistors to switch the relays.
You could mute the amp by grounding the EL84 grids, but it would not be a good idea to combine that with the power switch (hum). A simpler option might be to turn off the EZ81 heater, if you don't mind a bit of warm-up delay?

[acheld]

You can't easily put LEDs in the footswitch unless you use transistors to switch the relays.

The Dumble clones do so successfully . . .  maybe I'm missing something.  But I've built several amps with LEDs in the foot switch. 

[Merlin]

Are you thinking of putting the footswitches in parallel with the relay coils instead? Yeah I guess that's an option. Could be tweaky though.

[Jennings]

Thanks so much for the help...really useful, as this is the first time I've looked at anything other than manual toggle switching. 


Point noted on the standby...personally I don't use standby switches at all (most of my builds have valve rectification), but a friend wants me to build him one too and is adamant, so I'm having to look at a way of keeping him happy...I think he mostly uses it to mute the amp really.


What about if I use a couple of series IN4007 diodes to drop the voltage (by about 1.4V) for the relays, and just use LEDS mounted within the amp's front panel to at least get a visual on my switching status (see attached)?  Incidentally I've never looked at how to implement transistor switching...would it be significantly more complicated, out of interest?


Once I get the remote switching sorted I'll try and get my head round how to also have manual switching too (in case I forget the footswitch at a session!)

[Merlin]

Yeah that looks OK. Your power supply caps are unsually small though, can you even buy 8uF caps these days?

[Jennings]

Thanks Merlin...help and sanity checks much appreciated.  I do tend to build this kind of thing with low filtering...I like the looser feel, and with a fairly low gain approach and careful layout I don't usually have an issue with noise floor.  Even if I sub the choke for a resistor instead.  In practice I can usually easily (currently) get hold of 8 or 10uF caps for those preamp sections.

[tubenit]

Lotsa useful info on relay switching in ARCHIVES under frequently asked questions.


Relay circuit & layout board & ft switch


With respect, Tubenit

[Jennings]

Thanks Tubenit...great info there, and much appreciated.


So I've made some revisions based on the advice so far...and a couple of tweaks from my end too (see attached up-revved schematic). 

• Added a wiring scheme for a switched TRS stereo footswitch jack - so panel switch or foot switch activated relays.
• Choke and standby omitted (for space reasons predominantly) and Vox-style parallel first stage power supply.
• Mute switch added - I'd rather mute than switch the power supply really...would it work to link EL84 grids for phase muting rather than grounding?

[Merlin]

Looks OK to me, except your diagram shows the volume control wired as a variable resistor, which I assume is not what you intend.

would it work to link EL84 grids for phase muting rather than grounding?

Grounding the grids will give a more reliable mute, but it will probably work ok.

[Jennings]

Ah yes! Great spot, cheers. I removed the high pass cap (I don’t get on with them) and inadvertently added a jumper. Point taken on the mute switch…I’ll change that for a grounding rather than phase cancellation.


Only point left for me to ponder is the channel selection indicator (jumper indicator is fine as-is)…currently I have one LED which will be off when the pentode channel is activated, and on when the triode channel is selected. Shame I don’t have two LEDs, one for each preamp, but maybe I’ll have to live with it that way.

[Jennings]

Circuit diagram updated.  Mute switch ground the EL84 grids, plus various bits tidied, annotated and completed/corrected.  LED status descriptions also added (nothing is pentode, yellow lit is triode, blue lit is jumpered).


I'll move on to build this in coming weeks...just finalising the layout of the chassis first and grabbing a couple of components.  Plan is for a compact 12" (30cm) by 9.5" (24cm) aluminium C section chassis, into a head shell cabinet.  I'll start a new thread as it progresses, but I hope the schematic here is also of general interest to anyone mulling over preamp foot and manual switching, particularly for the first time.

[Merlin]

Only point left for me to ponder is the channel selection indicator (jumper indicator is fine as-is)…currently I have one LED which will be off when the pentode channel is activated, and on when the triode channel is selected. Shame I don’t have two LEDs, one for each preamp, but maybe I’ll have to live with it that way.

Do you have room for another LED? You can do this

• Red = pentode
• Yellow = triode
• Blue = jumpered

[Jennings]

Ah yes!!! Thanks so much! I really struggled getting my brain round all that switching with the indicators as well. I think I can locate the LEDs below each volume knob…the front panel’s going to be quite busy, but should all fit.