Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on May 02, 2015, 08:44:50 am

Title: Serial FXLoop - How to add a parallel function ?
Post by: kagliostro on May 02, 2015, 08:44:50 am

This FXLoop is a kit that you can find in the web, it is a serial FXLoop, but a friend would like to add a parallel switchable option

the problem (to me) to have things clear in mind, is the 2.2M resistor (I think it acts as a virtual earth)

so I'm not able to decide exactly how to arrange the added part to obtain parallel function

(http://i.imgur.com/BMuK4k2.jpg)

I was thinking to connect the junction of the 22n cap / 500p cap (send side)

with the junction of the 47k resistor / 22n cap (return side)

all with an in series switch to insert or disinsert the parallel and an in-series resistor (lot of doubt about proper value)

But, as told, the virtual earth presence confuses me a lot

Which will be your idea about ?

Many Thanks

K


p.s.: I hope the circuit was correctly reverse engeenered

Title: Re: Serial FXLoop - How to add a parallel function ?
Post by: VMS on May 02, 2015, 06:15:55 pm

Aiken has article that shows the summing feature for this type of feedback-amplifier:

http://aikenamps.com/index.php/designing-single-stage-inverting-feedback-amplifiers (http://aikenamps.com/index.php/designing-single-stage-inverting-feedback-amplifiers)

Title: Re: Serial FXLoop - How to add a parallel function ?
Post by: kagliostro on May 03, 2015, 02:59:02 am

Many Thanks for the link VMS

I hope I'll be able to understand all that math :huh:

Thanks

Franco

Title: Re: Serial FXLoop - How to add a parallel function ?
Post by: VMS on May 03, 2015, 03:40:11 am

If you compare your loop to this loop, then for unity gain you would use 2,2M resistor, I think.

(http://i26.photobucket.com/albums/c149/VacuumVoodoo/Miscelanea/FX_Loop4.gif)

Title: Re: Serial FXLoop - How to add a parallel function ?
Post by: kagliostro on May 03, 2015, 06:14:49 am

Yes, may be correct

I think I'll try with a trimmer as to establish a right value for the in-series (on parallel signal) resistor

As injection poit I think (as told) to the junction of the 47k resistor / 22n cap, but not sure of that at all

EDIT: I modified "my" schem and seems more readable, I added the mod for parallel/serial action

do you think it is correct ?


(http://i.imgur.com/lxyovS1.jpg)

what about to connect the 2.2M resistor to ground via the switch when in serial mode ?

in the schem you posted there is a ground connection on the switch, but to me didn't seem indispensable

Franco

Title: Re: Serial FXLoop - How to add a parallel function ?
Post by: VMS on May 03, 2015, 08:11:50 am

Quote from: kagliostro on May 03, 2015, 06:14:49 am

Yes, may be correct

I think I'll try with a trimmer as to establish a right value for the in-series (on parallel signal) resistor

As injection poit I think (as told) to the junction of the 47k resistor / 22n cap, but not sure of that at all

EDIT: I modified "my" schem and seems more readable, I added the mod for parallel/serial action

do you think it is correct ?

I'm not really an expert on these but to me it looks similar to the picture I posted.

Quote from: kagliostro on May 03, 2015, 06:14:49 am

what about to connect the 2.2M resistor to ground via the switch when in serial mode ?

in the schem you posted there is a ground connection on the switch, but to me didn't seem indispensable

Franco

You have to try it, probably you wont here any difference.  :dontknow:

Title: Re: Serial FXLoop - How to add a parallel function ?
Post by: kagliostro on May 03, 2015, 10:36:13 am

Thanks Again VMS

Franco