JHS Prestige Buffer / Booster

[Gargaman]

I want to make a buffer / booster with a TL072.
Is it possible to make it with this chip? One side (non-inverting amp) for buffer and the other side (non-inverting amp) with gain around 4.
One gain control pot that would be on the feedback loop at the second amp.
All the way down --> 0 gain (buffer)
All the way up --> gain of 4 (booster)

I searched for it and seems like the Prestige JHS is what I'm looking for but there's no schematic on the web.
Thanks

[ElectricDruid]

The buffer part is pretty simple:



(From http://www.muzique.com/lab/buffers.htm)

The booster part is only marginally more complicated - it's the classic non-inverting op-amp circuit from a dozen (million?!) tutorial pages. Here's an example of an input buffer/booster with fixed x4.3 gain from my Hard Bargain distortion:



You could make this variable by replacing the 33K resistor with a pot. If a 33K pot is hard to find, you can change the values, but keep the same 3:1 relationship (so 100K/33K or 10K/3K3 for example).

HTH,
Tom

[Gargaman]

Thks Tom
I'll try to scratch a schematic justaposing the two blocks.
Meanwhile I found this thread that looks like what I wanted. (schematic on reply#11)
https://www.diystompboxes.com/smfforum/index.php?topic=109288.0

[Gargaman]

Maybe something like this?

1M5 resistor is a pulldown. Will the input impedance be 1M5 paralelled with 1M?
If so, I might use 2M2 in both places to get a input impedance about 1M, right?
On the second part, should 15K resistor go to Vref instead of ground?

[antonis]

Maybe something like this?

1M5 resistor is a pulldown. Will the input impedance be 1M5 paralelled with 1M?
If so, I might use 2M2 in both places to get a input impedance about 1M, right?
On the second part, should 15K resistor go to Vref instead of ground?

Something like this without IC1a should also work fine..
(don't bother too much about pull-down resistor as long as you wire 3PDT switch with IN grounded when effect in by-pass mode..)

15k resistor MUST go to Vref (as it is) to implement 22μF cap as part of HPF and to roll-off DC gain to unity..
(in case of a series cap of your taste added, it could easily go to GND also..)

[Gargaman]

Tks, Antonis

15k resistor MUST go to Vref (as it is) to implement 22μF cap as part of HPF and to roll-off DC gain to unity..
(in case of a series cap of your taste added, it could easily go to GND also..)

Makes sense. My taste is to maintain the guitar tone unaffected. May I calculate 15K with 22uf and see whats the cutoff frequency?

Something like this without IC1a should also work fine..
(don't bother too much about pull-down resistor as long as you wire 3PDT switch with IN grounded when effect in by-pass mode..)

So I can try with a single opamp (741) but will be a poor choice given its lower input impedance?
If using a dual should I wire the other side like this?

If I leave the pulldown resistor off, the input impedance will be the resistor bias (1M)?
I'm wiring 3PDT like this:

I finished a kind of a bass fuzz with TL072 recently, used this scheme and a pulldown resistor at the input.
It's popping.
May I try a pulldown on the output?
tks

[antonis]

Alright..
(one thing at a time..)

Of course you can calculate 15k/22μF cut-off frequency..
(which, by the way, is subsonic  -  about half of Hertz..)
Better make 15k -> 1k5 and lower 50k pot down to 5k one..
(or keep them as it is and add to 15k resistor a series cap of your taste..)

Op-amp input impedance doesn't depend of single or dual  or quad..
(it primarily depends on inverting or non-inverting configuration and secondarily on BJT or FET input..)

Yes, if you leave pull-down resistor out, input impedance will be the value of bias resistor..
(but, for the sake of peace of mind, place a2M2 to 10M pull-down resistor together with 3PDT switch IN grounded..)

IMHO, No..
As far as there is 100k resistor at the output, there is no need for extra pull-down resistor..
(the above also stands for all effects exhibiting a Volume/Level output pot..)

In your 3PDT switch board, IN is grounded but OUT is floating..
(can't post a wiring diagram for the moment but you can search for 3PDT switch IN+OUT grounded..)

[Gargaman]

Of course you can calculate 15k/22μF cut-off frequency..
(which, by the way, is subsonic  -  about half of Hertz..)
Better make 15k -> 1k5 and lower 50k pot down to 5k one..
(or keep them as it is and add to 15k resistor a series cap of your taste..)

Right. I'm out of 5K pot now, maybe keep as it is or lower to 10K pot and 3K resistor.

Op-amp input impedance doesn't depend of single or dual  or quad..
(it primarily depends on inverting or non-inverting configuration and secondarily on BJT or FET input..)

Got that. Maybe I wasn't clear. The dual I have is TL072. Single I have is 741. I read somewhere that 741 input impedance would be lower than 072, that has FET input. I'll look the datasheet for the 741 model I have.

Yes, if you leave pull-down resistor out, input impedance will be the value of bias resistor..
(but, for the sake of peace of mind, place a2M2 to 10M pull-down resistor together with 3PDT switch IN grounded..)

IMHO, No..
As far as there is 100k resistor at the output, there is no need for extra pull-down resistor..
(the above also stands for all effects exhibiting a Volume/Level output pot..)

In your 3PDT switch board, IN is grounded but OUT is floating..
(can't post a wiring diagram for the moment but you can search for 3PDT switch IN+OUT grounded..)

Ok, I'm scratching a schematic with a single opamp (741), will use 1M bias resistor without pulldown resistor.
For this case, I won't have the volume pot (100k) at the output so I may incorporate a resistor on the output.
I'll search for the 3PDT diagram.
I'm not clear about the 2M2 to 10M position you are talking...

[ElectricDruid]

If you just need a buffer/booster, the dual is basically a waste of one op-amp, since you can do it fine with a single op-amp, like the second schematic I posted (which is pretty much exactly what you need, luckily enough). If you're choosing a single op-amp to build it with, the 741 is an authentic "vintage" choice and will work fine, but won't offer such a high input impedance (high, but not *as* high) as a FET input amp like the TL07x series and it might be a bit noisier too. But that's the price of "mojo"! If you want something cleaner and clearer and more modern sounding, find a TL071 and be done with it. Or build it with the 741 for now, and make sure you use a socket so you can experiment with alternative op-amp chips later on.

There often isn't a right answer for "which op-amp?" It's a question of taste and what you're trying to achieve.

[Gargaman]

Tks, Tom
All right, it's working. Gain unity with pot at 0, boosting when dial up. Nice!
But popping like a mother@#$%er.
I left the pulldown resistor but it's on a breadboard that has the same 3PDT wiring diagram posted below.
Antonis said something about piece of mind that I don't get it.
Anyway, tried to stick 2M2 resistor from input to ground, and from output to ground, but still pops.
I've been using this switching diagram for a while and no popping with fuzz face, dod250, marshall guvnor. I've been using 1M5 pulldown on input all buildings.




The chip is a UA741CP (don't know if the manufacturer it's TI or ST). Whatever, the datasheet for UA741CP says input resistance typical 2M and minimun 0,3M. I'm going with this untill have some 071 around.

[ElectricDruid]

Tks, Tom
All right, it's working. Gain unity with pot at 0, boosting when dial up. Nice!
But popping like a mother@#$%er.

Yeah, with a pot in it, there's a DC voltage across the pot so it might (ok, *will* ) crackle a bit. You could fix that by removing the connection of the 15K resistor to Vref, and putting an 1uF electrolytic cap from there down to ground instead. That blocks the DC but makes an AC ground, so it should crackle less.

I left the pulldown resistor but it's on a breadboard that has the same 3PDT wiring diagram posted below.
Antonis said something about piece of mind that I don't get it.
Anyway, tried to stick 2M2 resistor from input to ground, and from output to ground, but still pops.
I've been using this switching diagram for a while and no popping with fuzz face, dod250, marshall guvnor. I've been using 1M5 pulldown on input all buildings.

See how it goes with the 1uF cap in it. It might fix it. Otherwise the exact value of the pulldown resistor won't make any odds - 1M, 1M5, 2M2 are basically all the same for that job. It's either there or not-there, and that's pretty much it.

The chip is a UA741CP (don't know if the manufacturer it's TI or ST). Whatever, the datasheet for UA741CP says input resistance typical 2M and minimun 0,3M. I'm going with this untill have some 071 around.

Yeah, good plan. You're not ruling anything out, and you can try stuff or leave it as-is later on as you wish.

[Gargaman]

Thks again, Tom!
It was popping when switching on an off. The pot is running clean, no crackle.
I stick pulldown resistor at input and output, seems the pop is gone!
I tested with 20feet cable and seems like it's doing it's job.
Gonna build.
Tks again!

[Gargaman]

Ok, found that input/output grounded 3PDT wiring diagram
https://www.diystompboxes.com/smfforum/index.php?topic=104109.0
Reply#8
If I use this, can I leave pulldown resistors forever?
Am I using input pulldown resistors in vain (since I'm grounding the input so far)?

[ElectricDruid]

It was popping when switching on an off. The pot is running clean, no crackle.

Ah, ok. That's all fine then.

I stick pulldown resistor at input and output, seems the pop is gone!

Excellent! Nice work.

Ok, found that input/output grounded 3PDT wiring diagram
https://www.diystompboxes.com/smfforum/index.php?topic=104109.0
Reply#8
If I use this, can I leave pulldown resistors forever?
Am I using input pulldown resistors in vain (since I'm grounding the input so far)?

Yes, if you're grounding the input, I don't think you need the pulldown resistor as well. That said, I often include it on my circuit designs because the pulldown resistor goes on the PCB (and is therefore something that I control) whereas the switch wiring is offboard, and people might do that however they like - including many ways that won't ground the input. But I'm thinking about designing boards that are as beginner-friendly as I can make them. For a one-off for your own use, it's a different story and you don't have to worry about the myriad ways people might find to do it wrong!!

[PRR]

...the datasheet for UA741CP says input resistance typical 2M and minimun 0,3M. ....some 071 around.

That 0.3Meg is *before feedback*. Since you will have more than 100:1 of excess gain for feedback, the input will be over 30Meg raw, so essentially whatever ?Meg resistor you use for bias.

If you need DC accuracy (here you don't), you won't like a 1Meg bias resistor. The 100nA input current times 1Meg is about 1/10th volt of DC error. But for single-supply audio we do not care if we end up at 4.5V or 4.6V.

The TL07x series have MUCH less input current and can have low DC error even at 10Meg (I've seen them run hundreds of Meg). It is perfectly legal to use half a TL071 TLO72 as a single opamp, if that is all the work you have for it. It is a "waste" of half a 19 cent chip but it is not worth spending hours to think-up a way to use the other 9 cents of chip. You do want to strap it as follower and bias half-way, because an idel wide-open opamp will find a way to make trouble. (And the JFET input TL07x types can drift in and out of trouble from stray static in the air.)

[ElectricDruid]

It is perfectly legal to use half a TL071 as a single opamp, if that is all the work you have for it. It is a "waste" of half a 19 cent chip but it is not worth spending hours to think-up a way to use the other 9 cents of chip.

You mean a TL072, Paul. But yeah, the point still stands - wasting half an op-amp really *isn't* the biggest deal in the world.

Still, it doesn't half make me twitchy. I'm not sure I could actually leave a circuit with half an op-amp just tied up as an unused buffer. It'd drive me insane. Other people may find their OCD manifests in alternative ways...

[Gargaman]

Tks for the info Paul
I saw the 072 datasheet and input bias current is about pA.
I dont like the idea of wasting half of amp or any other part neither, Tom; or using a 3PDT if the job can be done with a DPDT.. Especially if the part will be missing in another project. I started messing with chips now, I'm running at low units of opamps, and gonna save some 072 for obsessive compulsive drives.
At the end, this buffer / booster would not be appropriate if someone wants to keep the buffer always on. It would be better to wire the buffer in line with the boost part activated by the switch. I presume that this can be done with DPDT and Led for each part

[Gargaman]

Maybe adapting this (reply #1). I'm not sure.
https://www.diystompboxes.com/smfforum/index.php?topic=120480.0
DPDT OFF will be shorting the feedback loop and turn off Led. (just buffer)
DPDT ON will put 15k and pot into the path and turn on Led. (boost)
Meanwhile, another Led (buffer led) will be always on as soon as you connect dc jack.

[Elijah-Baley]

I searched for it and seems like the Prestige JHS is what I'm looking for but there's no schematic on the web.
Thanks

The JHS Prestige is very probably a JHS Mini Bomb modified (it's the same PCB). To my ears it has more gain than the Mini Bomb, and maybe less low cut, (maybe an effect caused from the more gain).
The JHS Mini Bomb is a clone of the Escobedo Duende jfet boost.