mhuss wrote:I have had good success recently using NEC MOSFET opto switches. They're small, efficient, and have ratings up to 400v. I can do a writeup if anyone is interested.
--mark h
Mark - Thanks for the circuit. Do the MOSFET opto's have any problems with signal bleed like optoisolators?
When the footswitch it open, does the MOSFET opto behave in the "Normal" functions - Normally Open, Normally Closed? And when you close the foot switch does MOSFET opto flip - ie the Normally Open switch change to Closed?
ampdoc1 wrote:Here's a simple circuit I think I got from a Boogie MK 1. I've used it in channel switching applications on Fenders, and it works pretty well. The relay indicated is 5Vdc, 167 Ohm coil, 150mW. If you can't find this specific item, you can probably use something close.
DEH
ampdoc
Wow, that look like a really simple circuit. Is there any switching noise associated with the relay? Is the idea of the circuit that when the footswitch is open the coil is energized therefore closing the switch?
The MOSFETs are about 1 gigaohm (1000 meg) when off, so leakage is not a problem even in high impedance tube amps. The switching is instant and silent, and there are no contacts to wear out.
Yes, the SSR is NO/NC as labeled when nothing is connected. Closing the jack connection "un-normals" them.
The relay I spec-ed is DPDT. I (and I think most amps that use relays) used the Normally Closed (relay is inactive to engage) switches for the high gain circuits and the Normally Open (relay is energized to engage) switches for the clean; that way when the relay is activated the signal thru the relay is low gain and not as liable to pick up any hum or noise.
As relay circuits go this is pretty quiet. But, in my experience relays ALWAYS have some "pop" when switching (unless there are corresponding "mute" FETs in the circuit. This is not something that bothers me, as the "pop" is not noticeable when playing live. However, if you were changing channels in a recording situation this would definitely be a no-no!
Myself, I'm gonna try Mark's posting. Looks simple enough, and NO noise???? Whata' deal. Thanks guys!
ampdoc1 wrote:Here's a simple circuit I think I got from a Boogie MK 1. I've used it in channel switching applications on Fenders, and it works pretty well. The relay indicated is 5Vdc, 167 Ohm coil, 150mW. If you can't find this specific item, you can probably use something close.
DEH
One thing that you might want to add to that circuit is a back diode (1N4001) and .01uf cap across the coil, mounted at the relay. Solves any potential switching noise from the coil fields collapsing.
Gary
Located in the St Croix River Valley- Afton, MN
About 5 miles south of I-94
aka K0GWA, K0 Glas Werks Amplification www.glaswerks.com
mhuss wrote:The MOSFETs are about 1 gigaohm (1000 meg) when off, so leakage is not a problem even in high impedance tube amps. The switching is instant and silent, and there are no contacts to wear out.
Yes, the SSR is NO/NC as labeled when nothing is connected. Closing the jack connection "un-normals" them.
--mark
Hi Mark,
that looks like nice little circuit! Can it be extended by adding another mosfet?
My next problem is to get the right parts in germany, are there any substitutes you know of ?
I've only built one channel switcher and the 'architecture' of the circuit and switching is similar to a SLO, except I added an extra opto to shunt the clean channel when in Dirty mode (in the SLO, the clean channel is always 'running' but the Lead circuit swamps it; however there is still some very slight cancellation). It uses optoisolators (VTL5C1), which are 50M ohms resistance when OFF, so IMO bleed is not a problem. It depends on how much current you put through the LED for the ON resistance, but when run right the resistance when ON is 200-500 ohms or so. The switching is very fast and there are no clicks/pops. I just used the 5v tube rectifier winding for my power source and rectified that.
It uses four optos total:
VR1 - Sends output of V1a to Dirty channel (V1a always goes to Clean channel)
VR2 - Connects output of Dirty channel to V4 (where Clean and Dirty are summed)
VR3 - Shunts output of Clean channel to ground
VR4 - Grounds the pre-PI MV. When in Clean mode the ground for the MV is 'lifted'. This makes the pre-PI MV a 'Lead Master' so you can match the volume of the Clean and Dirty channel. The FX loop Return Level control is really the global Master Volume.
All four optos turn ON simultaneously (very simple).
I would think long and hard about making things footswitchable but not having a corresponding panel mounted switch. IMO, you shouldn't have to have a footswitch to operate the amp but that's just a personal pet peeve.
