Author Topic: Adding a buffer/buffered bypass to an LPB1  (Read 641 times)

Liaztraht

Adding a buffer/buffered bypass to an LPB1
« on: February 19, 2021, 02:25:34 AM »
I've been learning how to layout circuits, and wanted to combine an LPB1 with a simple buffer using the AMZ schem for a common drain jfet.

If I want to to have the buffer active in bypass, would I just route the input jack to the in of the buffer, the output cap from the buffer to footswitch where the input normally goes, then route the LPB half like normal? Also would a pulldown resistor be a good idea into the LPB half?

I have a Marshall Origin that needs a kick in the pants that sounds better than the built in boost, and want a buffer to drive the line when not boosting. I'm making a small rock and punk board for it and don't want to rip the Boss pedals off my main board.

kaycee

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #1 on: February 19, 2021, 03:01:46 AM »
Certainly, yes to your plan to wire the always on buffer. Also, I'd say yes to the pull down resistor on the LPB, but I may be wrong on that one.

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #2 on: February 19, 2021, 03:08:40 AM »
Certainly, yes to your plan to wire the always on buffer. Also, I'd say yes to the pull down resistor on the LPB, but I may be wrong on that one.

Cool! Thats how I had it set up in diptrace.
I don't think a pull down will be too bad even if not needed. Worst case is it does nothing. Better to have it and not need it :P

kaycee

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #3 on: February 19, 2021, 04:10:29 AM »
It would be easy enough to put the pull down on loose and see what difference it makes. It depends whether the buffer is designed as a stand alone unit, with correct output impedance for a following pedal or amp, or if it is meant as an input stage to an effect. I would suspect that if it has a resistor to ground on the output it is the former.

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #4 on: February 19, 2021, 04:21:40 AM »
It would be easy enough to put the pull down on loose and see what difference it makes. It depends whether the buffer is designed as a stand alone unit, with correct output impedance for a following pedal or amp, or if it is meant as an input stage to an effect. I would suspect that if it has a resistor to ground on the output it is the former.

I am planning on using the first buffer here http://www.muzique.com/lab/buffers.htm

Reason I am putting it onto the same board as the LPB is to even out the sound. I prefer the lpb after a buffer,  so I'm doing it 2 in 1.
« Last Edit: February 19, 2021, 04:23:31 AM by Liaztraht »

kaycee

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #5 on: February 19, 2021, 04:34:21 AM »
I think you will probably be alright either way you do it, and it will be easy enough to snip out one resistor to ground if you find an issue. By all means build it and learn and enjoy! Later you might like to look at building a Boss FA1 to do the job. An excellent pedal that can boost or cut bass and treble and give good volume boost. I have one always on at the front end of one of my boards. You can get a PCB or do the vero at tagboard effects.

antonis

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #6 on: February 19, 2021, 05:18:58 AM »
Just bear in mind input impeadance domination due to pull down resistor.. :icon_wink:
(it's been effectively placed in parallel with 1M JFET Gate bias resistor..)
Of course, by making it 4M7 or bigger, it shouldn't have any practical effect on total input impedance..


Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #7 on: February 19, 2021, 05:34:59 AM »
I think you will probably be alright either way you do it, and it will be easy enough to snip out one resistor to ground if you find an issue. By all means build it and learn and enjoy! Later you might like to look at building a Boss FA1 to do the job. An excellent pedal that can boost or cut bass and treble and give good volume boost. I have one always on at the front end of one of my boards. You can get a PCB or do the vero at tagboard effects.

Thanks!

For this Marshall, a simple front end push is all it needs. I usually use an SD1 always on, but I need a little more bottom end. The onboard boost is like throwing a blanket on it. Then I can leave my main board at home, and carry a 2-3 pedal board. Boost, Muff, and a Rat.

Tagboard Effects is awesome. They are how I got in to building and moding effects. I've done plenty of vero from them, but I am moving towards PCB for a cleaner and faster build. Every time I build something nifty, and tweak it to my taste, friends and locals want me to make them a copy. I'm not comfortable with using someone else's layout if I am going to sell it, even with permission. I made a layout of Fancy Lime's Minisagverket as a first board to push myself, but I'm in the process of laying out different tweaks of mine from some fuzzes and this boost I have on breadboard, and doing a couple small runs to fund the hobby.

I seriously need to stop taking cool pedals to gigs... And my Orange... and my Drum kit. I keep getting offers on my gear every time I play.  Even had one guy pull out his wallet and flash a grand after a drumming gig. Brand new $2k shells. He didn't leave happy   ::)

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #8 on: February 19, 2021, 05:37:57 AM »
Just bear in mind input impeadance domination due to pull down resistor.. :icon_wink:
(it's been effectively placed in parallel with 1M JFET Gate bias resistor..)
Of course, by making it 4M7 or bigger, it shouldn't have any practical effect on total input impedance..
Even when I am placing it at the beginning of the LPB circuit? It would be after the buffers output cap, and before the LPB input cap.

BJM

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #9 on: February 19, 2021, 05:43:35 AM »
Don't know if it's still there but AMZ also had a few schematics for it's Multi-Purpose Opamp PC Board wit a combination of a buffer and booster, with separate outputs for the buffer and the booster. That may even be simpler for switching.
« Last Edit: February 19, 2021, 05:45:55 AM by BJM »

antonis

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #10 on: February 19, 2021, 06:50:44 AM »
Just bear in mind input impeadance domination due to pull down resistor.. :icon_wink:
(it's been effectively placed in parallel with 1M JFET Gate bias resistor..)
Of course, by making it 4M7 or bigger, it shouldn't have any practical effect on total input impedance..
Even when I am placing it at the beginning of the LPB circuit? It would be after the buffers output cap, and before the LPB input cap.

Wherever you place a pull down resistor, it's set in parallel with succeding input impedance.. :icon_wink:
Of course, for LPB1, it shouldn't make big difference due to booster's relatively low input impedance..
(less than 50k or lower, depended on particular transistor hFE, Emitter resistor value and voltage divider bias resistors values..)

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #11 on: February 19, 2021, 07:22:07 AM »
Just bear in mind input impeadance domination due to pull down resistor.. :icon_wink:
(it's been effectively placed in parallel with 1M JFET Gate bias resistor..)
Of course, by making it 4M7 or bigger, it shouldn't have any practical effect on total input impedance..
Even when I am placing it at the beginning of the LPB circuit? It would be after the buffers output cap, and before the LPB input cap.

Wherever you place a pull down resistor, it's set in parallel with succeding input impedance.. :icon_wink:
Of course, for LPB1, it shouldn't make big difference due to booster's relatively low input impedance..
(less than 50k or lower, depended on particular transistor hFE, Emitter resistor value and voltage divider bias resistors values..)

Always learning something new! Thanks!

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #12 on: February 19, 2021, 07:51:58 AM »
This is the schem I worked up in Diptrace.


And the PCB layout so far. Just a Gerbv screen shot.


Comes out to a nice 1.33 x 0.73 inches. I would do a proper pot pad, but it would make it bigger, or go to double sided to get all the pads to line up.

antonis

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #13 on: February 19, 2021, 04:22:07 PM »
IMHO, C4 value should be equal to or higher than C2 one unless you're sure about succeding effect input impedance higher than 50k or so..

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #14 on: February 19, 2021, 04:54:47 PM »
IMHO, C4 value should be equal to or higher than C2 one unless you're sure about succeding effect input impedance higher than 50k or so..
What kind of issues could this cause?
This is the first time I've messed with bufferes, and set it up the same way it would be running a separate buffer into a stock LPB.

antonis

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #15 on: February 19, 2021, 05:17:01 PM »
LPB1 output cap forms a High Pass Filter, together with next stage input impedance..
(just like the one formed by C1/R1)

Corner frequency (-3dB cut-off point) is found according to f = 0.159/(R*C), where R = Shunting to GND impedance in Ohms and C = Series cap in Farads..
(e.g. Q1 input HPF corner frequency is 1.6 Hz..)

So, for an unknown Q2 coupled effect input impedance, C4 value should be ten times bigger, at least..
(unless, of cource, you want a "bright" booster ..)

P.S.
You realize I'm talking about Booster (Q2), not Buffer (Q1), don't you..??
« Last Edit: February 19, 2021, 05:19:08 PM by antonis »

iainpunk

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #16 on: February 19, 2021, 05:26:21 PM »
IMHO, C4 value should be equal to or higher than C2 one unless you're sure about succeding effect input impedance higher than 50k or so..
What kind of issues could this cause?
This is the first time I've messed with bufferes, and set it up the same way it would be running a separate buffer into a stock LPB.
you could lose some bass and low mids if the input impedance (complex resistance) of the next pedal or amp is low.

cheers, Iain

Liaztraht

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #17 on: February 19, 2021, 05:57:27 PM »
LPB1 output cap forms a High Pass Filter, together with next stage input impedance..
(just like the one formed by C1/R1)

Corner frequency (-3dB cut-off point) is found according to f = 0.159/(R*C), where R = Shunting to GND impedance in Ohms and C = Series cap in Farads..
(e.g. Q1 input HPF corner frequency is 1.6 Hz..)

So, for an unknown Q2 coupled effect input impedance, C4 value should be ten times bigger, at least..
(unless, of cource, you want a "bright" booster ..)

P.S.
You realize I'm talking about Booster (Q2), not Buffer (Q1), don't you..??
Ah. Was a little confused at first. Shouldn't read anything technical right after waking up!

That is a quirk of the original, but may be a good idea to up the output cap.

Should be easy to slide everything over to fit an electrolytic on the board.

ElectricDruid

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #18 on: February 19, 2021, 08:30:50 PM »
Corner frequency (-3dB cut-off point) is found according to f = 0.159/(R*C), where R = Shunting to GND impedance in Ohms and C = Series cap in Farads..

When Antonis says "0.159" what he's talking about is 1/(2*Pi), which is the way you usually see it written out: 1 / (2 * Pi * R * C).

Same thing. Antonis' version is quicker to work out on your calculator, which is why he's done it that way rather than in full.
  • SUPPORTER

antonis

Re: Adding a buffer/buffered bypass to an LPB1
« Reply #19 on: February 20, 2021, 11:10:54 AM »
Antonis doesn't consider himself a privileged guy who owns Greek keyboard... :icon_wink:
(or else, he could easily use ω/f instead of 2π..)

P.S.
Cheers, Tom..