I've seen a few schems out there of amps with with switchable negative feedback, usually with a toggle on the back panel of the amp
q1 - is it harmful to switch it in and out while playing? i.e. pop or other potential harmful issues
A pal of mine wants me to build a 2 channel amp based on some fender circuits where if I stick reasonably true to the original circuits one uses negative feedback and the other does not - Blackface Pro reverb normal channel, Blonde Bassman channel with added reverb and trem, footswitchable between the 2.
Project is probably a bit over my pay grade in terms of my building skills but I thought I would at least take a serious look at it as it would be a really good learning experience. I know there are a variety of issues trying to do this because of differences in the circuits in general but the first thing the stuck out to me was the negative feedback/presence on one channel but not the other
In theory I could rig up a switching scheme that switches it in and out when the channels switch but thought I would ask some of you more experienced guys if it is even a good/safe idea to switch feedback in and out when the amp is being used before I explore the intricacies and compromises needed to try shoehorn these channels in the same amp
thanks!
Footswitchable negative feedback?
Moderators: pompeiisneaks, Colossal
Re: Footswitchable negative feedback?
Why don't you use a 250k log pot instead of the switch?
I use a 33k with a 250k log pot in series, like Rivera's Focus.
It won't pop and so they can dial the right amount of feedback they need.
I use a 33k with a 250k log pot in series, like Rivera's Focus.
It won't pop and so they can dial the right amount of feedback they need.
Re: Footswitchable negative feedback?
the idea was to footswitch between the 2 channels one with feedback and one without. The pot idea would work but require some knob twirling when switching channels
Re: Footswitchable negative feedback?
I would try this:
Put two resistors in parallel, 1 resistor is connected to a LDR / vactrol. When the ldr is "off", you have the higher resistance value, when the ldr is "on" you have the lowered paralleled resistance.
Put two resistors in parallel, 1 resistor is connected to a LDR / vactrol. When the ldr is "off", you have the higher resistance value, when the ldr is "on" you have the lowered paralleled resistance.
Re: Footswitchable negative feedback?
If you want it strictly switchable the vactrol is the best way, I agree.
Re: Footswitchable negative feedback?
You mean something like the Trex? http://mhuss.com/18watt/schematics/18wTRex-S.pdf
Just a newbie looking for that perfect sound...
Re: Footswitchable negative feedback?
Espjr215 -Yes the T-rex and 36 watt Richie Hall layouts were 2 layouts I had seen with a switch for negative feedback - I was mostly curios if it would be harmful to switch it while playing - pop or other noise??
Roberto/John are your suggestions for vactrol/ldr because they tend to switch a little slower (for pop), or some other reason vs a relay or Jfet.
John when you suggest to switch 2 parallel resistors as you do, I assume you meant to switch between a typical neg feedback value and a value that results in very little feedback vs switching between a typical value and no resistor at all? Is this correct?
Are you suggesting this to avoid pop or simply to make it easy to do?
Roberto/John are your suggestions for vactrol/ldr because they tend to switch a little slower (for pop), or some other reason vs a relay or Jfet.
John when you suggest to switch 2 parallel resistors as you do, I assume you meant to switch between a typical neg feedback value and a value that results in very little feedback vs switching between a typical value and no resistor at all? Is this correct?
Are you suggesting this to avoid pop or simply to make it easy to do?
Re: Footswitchable negative feedback?
You can use fet too, and to soften the switch you can add a capacitor between the control signal and ground (set the RC constant around 10ms).
Re: Footswitchable negative feedback?
Lindz,
I truly do not think that you will notice the switching speed of the vactrol in this application.
Most Jfets usually need a negative rail to operate, also the voltage swing of the output may exceed most common jfets, depending on how you implement them and what xformer tap is used. Vactrols, yes cost dollars more, but are generally simpler to use in this situation.
My comment about the resistors in parallel, let's look at some simple values:
If we use two 100k resistors, and when the ldr is "off" we see a value of 100k in the feedback line. If the ldr is "on" 100k and 100k in parallel is now 50k - more or less standard values. Two fixed, switchable values.
You could switch this any way you want with higher values, lower values, a pot in series with the ldr and resistor to season to taste. The series resistor is needed with the pot to set a minimum resistance.
Or just the LDR and a single resistor IF you only want a feedback "on" "off" approach. But then you lose your presence and resonance depending on your circuit....
Just a few thoughts Monday morning before coffee....
I truly do not think that you will notice the switching speed of the vactrol in this application.
Most Jfets usually need a negative rail to operate, also the voltage swing of the output may exceed most common jfets, depending on how you implement them and what xformer tap is used. Vactrols, yes cost dollars more, but are generally simpler to use in this situation.
My comment about the resistors in parallel, let's look at some simple values:
If we use two 100k resistors, and when the ldr is "off" we see a value of 100k in the feedback line. If the ldr is "on" 100k and 100k in parallel is now 50k - more or less standard values. Two fixed, switchable values.
You could switch this any way you want with higher values, lower values, a pot in series with the ldr and resistor to season to taste. The series resistor is needed with the pot to set a minimum resistance.
Or just the LDR and a single resistor IF you only want a feedback "on" "off" approach. But then you lose your presence and resonance depending on your circuit....
Just a few thoughts Monday morning before coffee....