Guitar onboard pre-amp

[uki]

Been making this for a long time and recently decided to modify it a bit, with very good results, this preamp is from the Guitar Handbook.
Had to draw the schematic for a better understanding, in the book there is only the layout, if anyone feel like modifying it and sharing that would be great.
The original pre-amp is clean and very bright and give a big gain which is adjustable by a VRI.
The IC can be replaced by the upgrade TLE2071 which is much better than the TL071, the 741 and 748 does distortion.
To reduce a bit the brightness I've been experimenting and reclaced the 50K VRI for a 100K VRI and C1 with different values, the ones I had available, 300nF, 100nF, 56nF, 33nF, (also tried 15nF(strident) just to see how bright it goes). Default value is 22nF. So far 33nF sounded very good.
Also I have changed C3 for a 150pF, this was running all the time with the other changes, another change done was the resistors to metal film 1% with those the circuit is quieter.

Red dots are cuts in the board.
Blue lines are links G2-K2, H3-J3 and B5-F5.
Follow some pics:
1st pic is the original, 2nd is the new layout I've made.

[tubenit]

Wow!  That's really cool!  Thanks for sharing it.

With respect, Tubenit

[HotBluePlates]

... The original pre-amp is clean and very bright ... To reduce a bit the brightness I've been experimenting and reclaced ... C1 with different values ...

Because there is a feedback loop from the output of the TL071 to the inverting input, and C3 is inside that loop, I believe C3 is what makes the circuit sound bright.

However, I don't know if C3 affects circuit stability. Try removing C3, then try using larger values in its place. I'm guess you'll resolve the brightness issue between those choices.

[uki]

... The original pre-amp is clean and very bright ... To reduce a bit the brightness I've been experimenting and reclaced ... C1 with different values ...

Because there is a feedback loop from the output of the TL071 to the inverting input, and C3 is inside that loop, I believe C3 is what makes the circuit sound bright.

However, I don't know if C3 affects circuit stability. Try removing C3, then try using larger values in its place. I'm guess you'll resolve the brightness issue between those choices.

I've actually changed(I forgot to mention) the C3, replaced the 100pF for  a 150pF I don't know if it effected the tone, but I didn't changed it alone, gonna try it alone thou to see how it goes, have done all 3 components C1, C3 and the VRI at once,  when lowering the C1 the tone got smoother and smoother as it was going down.
So far the 100k VRI and the 33nF put the tone about same as the guitar without the OP-AMP. But that will vary from one guitar to another and the amp must be considered as well, I got a darn bright amp, with the normal input , tone controls are at T: 4,5; M: 2; B: 10. Haven't found yet the TLE2071 which is much better to put in the circuit.

Also I have found  some other similar circuits with the same IC some with tone controls which I'm considering to borrow the idea and see how it goes into this one. Follow a pic attached.

Question: The VRI have one non used leg could that be put to some use, or grounded  ?

Wow!  That's really cool!  Thanks for sharing it.

With respect, Tubenit

Thanks Tubenit !!!

[sluckey]

Question: The VRI have one non used leg could that be put to some use, or grounded  ?

Don't connect it to ground. Instead, connect that leg to the wiper terminal. That's a common practice when connecting a pot as a variable resistor. The advantage is if the wiper should fail, the pot will revert to the full 50KΩ resistance rather than an open circuit.

[uki]

Question: The VRI have one non used leg could that be put to some use, or grounded  ?

Don't connect it to ground. Instead, connect that leg to the wiper terminal. That's a common practice when connecting a pot as a variable resistor. The advantage is if the wiper should fail, the pot will revert to the full 50KΩ resistance rather than an open circuit.

The schematic in the 1st post, I think the wiper is in the ground , or Am I seen this wrong ?

[sluckey]

I didn't follow the schematic. Dark blue on a black background is hard on my eyes. But, if the wiper is connected to ground then when you connect that leg to the wiper it will also be connected to ground.

[uki]

I didn't follow the schematic. Dark blue on a black background is hard on my eyes. But, if the wiper is connected to ground then when you connect that leg to the wiper it will also be connected to ground.

Thanks Sluckey ! Those colors are probably hard for some other people eyes as well, here a black on white version:

[HotBluePlates]

Question: The VRI have one non used leg could that be put to some use, or grounded  ?

It doesn't work like that; the unused leg is being "used" as VR1 overall is wired as a variable resistor. That variable resistor sits within a feedback loop and is the control which alters the gain of the circuit.

So you can do what Sluckey said: if you have to do something with the unused leg, connect it to the wiper of VR1.

Also I have found  some other similar circuits with the same IC some with tone controls which I'm considering to borrow the idea and see how it goes into this one. Follow a pic attached.

That circuit is a Baxandall tone control. The 2 tone controls are inside a feedback loop around the IC. Consider this a 2nd circuit block, which you might put after your other complete circuit block.

[uki]

It doesn't work like that; the unused leg is being "used" as VR1 overall is wired as a variable resistor. That variable resistor sits within a feedback loop and is the control which alters the gain of the circuit.

So you can do what Sluckey said: if you have to do something with the unused leg, connect it to the wiper of VR1.

Wouldn't that make the VRI act like a volume control ?

That circuit is a Baxandall tone control. The 2 tone controls are inside a feedback loop around the IC. Consider this a 2nd circuit block, which you might put after your other complete circuit block.

That using the same IC ? Adding the tone stack right after the feedback loop?

[uki]

I forgot to mention, there is a pop when turning the DPDT switch, but this doesn't happen in all guitars, some will some not, what can be done to fix this pop?

[HotBluePlates]

It doesn't work like that; the unused leg is being "used" as VR1 overall is wired as a variable resistor. That variable resistor sits within a feedback loop and is the control which alters the gain of the circuit.

So you can do what Sluckey said: if you have to do something with the unused leg, connect it to the wiper of VR1.

Wouldn't that make the VRI act like a volume control ?

Sorry, just getting back to this thread.

Yes, VR1 acts something like a volume control. It increases or reduces feedback around the IC, so it changes the gain of the IC. A normal volume control would just be a passive voltage divider instead.

That circuit is a Baxandall tone control. The 2 tone controls are inside a feedback loop around the IC. Consider this a 2nd circuit block, which you might put after your other complete circuit block.

That using the same IC ? Adding the tone stack right after the feedback loop?

Use a 2nd IC.

R6 and C4 are the output of your existing amp circuit, right? Add your complete new Baxandall tone circuit after those, and before the bypass switch you now have between the amp circuit & the output jack.

[uki]

Use a 2nd IC.
R6 and C4 are the output of your existing amp circuit, right? Add your complete new Baxandall tone circuit after those, and before the bypass switch you now have between the amp circuit & the output jack.

How about using a "double" IC like the TL072? It would reduce a lot the size.  How about the pop noise when using the switch is there a way to fix it ?

[HotBluePlates]

How about using a "double" IC like the TL072? It would reduce a lot the size.

Sure, if your circuit has 2 opamps, then getting those in a single package is quite acceptable.

How about the pop noise when using the switch is there a way to fix it ?

I don't know for certain, but the popping is probably voltage on C1 & C4 being discharged when switching. There is usually a resistance to drain the charge from caps like this, to prevent switch popping. You may try something like several-MegΩ between the switched contacts (say 4.7MΩ or so).

[uki]

You may try something like several-MegΩ between the switched contacts (say 4.7MΩ or so).

I'm not sure what you mean,
I found this link: http://www.geofex.com/Article_Folders/box_pop.htm
It says:

For mechanical switches, put a 100K to 4.7M (exact value does not matter) resistor from the "outboard" end of both the input and output capacitors to ground. This keeps the ends of the capacitors pulled to the right voltage all the time, and cures the leakage problem. No capacitor clicks.

Is this what you mean?

I've added R7 and R8 to the schematic , see if this would work.

[HotBluePlates]

Yes, those added 4.7MΩ resistors were what I meant.

[uki]

It is been some time since I last fiddled with it, I've tried the above with some 1M resistor and it did the trick, no more pop, now I can move on mass production  !!!

[Willabe]

Good deal. 

[jojokeo]

It is been some time since I last fiddled with it, I've tried the above with some 1M resistor and it did the trick, no more pop, now I can move on mass production  !!!

It's called a pull-down resistor uki. Common practice is to put this directly on the circuit's input. Also you can tailor the brightness using a small cap placed across the feedback loop. In general practice this is done even if the circuit is not bright because it adds stability to the feedback loop gain.

[uki]

It's called a pull-down resistor uki. Common practice is to put this directly on the circuit's input. Also you can tailor the brightness using a small cap placed across the feedback loop. In general practice this is done even if the circuit is not bright because it adds stability to the feedback loop gain.

Cool stuff, thanks for this, it is a good preamp, the guitar sound gets very solid and constant, does give also better responsiveness. I did find some ways to work around the brightness, turning down treble a bit more and not using the bright resource on amps.

Another problem I came across is, in my guitar, other ones doesn't do that, the pots(new pots) when turned to the ends(just in the ends) up or down does a scratch sound but only when the preamp is engaged, how to fix that?

[HotBluePlates]

The scratchy sound is probably due to the fact your preamp/buffer flows direct current through items connected to the output jack.

[jojokeo]

Uki, you have an incorrect schematic with several errors. I drew your corrections as shown in various colors. If you have questions then ask, but with the signal shown shorting to ground at the wiper of the feedback gain control pot, you really shouldn't be getting much or very little sound out. So this tells me that your schematic and actual circuit do not match? See attachment:

[uki]

Uki, you have an incorrect schematic with several errors. I drew your corrections as shown in various colors. If you have questions then ask, but with the signal shown shorting to ground at the wiper of the feedback gain control pot, you really shouldn't be getting much or very little sound out. So this tells me that your schematic and actual circuit do not match? See attachment:

You are right, there are error, hmm I didn't catch them before, there is more errors, C1 and C2 are indeed incorrect, about the gain control pot, Im not sure which leg is the wiper it is one of those little blue mult turns pot, did you see the pics in the 1st post?

The circuit works and it is very loud, you are right again layout works, the schematic need some work, I'm glad you did see it, I've followed the layout(that is all what the book gives) the schematics was drawn by myself, no surprise why there are erros 
gotta work it out !!

Thanks !!

The scratchy sound is probably due to the fact your preamp/buffer flows direct current through items connected to the output jack.

I don't follow please explain, the preamp output goes to the jack nothing in between, all guitar controls are before the preamp.

Thanks

[jojokeo]

Im not sure which leg is the wiper it is one of those little blue mult turns pot, did you see the pics in the 1st post?

The "wiper" is touching the resistors R2 & R6 & switch/ground point which is also designated by the red dot/black triangle on the pot's edge. You don't want all of this connected to each other. You want the - side of C4 going to the switch & R6 only. The other side of R6 goes to the wiper of the pot only. The other side of the pot goes to R4 only. The other side of R4 goes to the + of C2. The - of C2 goes to pin 2 of op-amp / along with the 4.5 bias voltage which comes from the middle point of R1 & R2. Also, this is a crude way to get the bias voltage - it normally has a 47uF or 100uF capacitor across R2. Also R1 and R2 are unnecessarily large for a voltage divider when 10k - 47k will work fine.

[uki]

The "wiper" is touching the resistors R2 & R6 & switch/ground point which is also designated by the red dot/black triangle on the pot's edge. You don't want all of this connected to each other. You want the - side of C4 going to the switch & R6 only. The other side of R6 goes to the wiper of the pot only. The other side of the pot goes to R4 only. The other side of R4 goes to the + of C2. The - of C2 goes to pin 2 of op-amp / along with the 4.5 bias voltage which comes from the middle point of R1 & R2.

Wow thanks for this    I did a major redraw on the whole schematic, man dunno where I had my head when it was 1st done    I think now it is correct, would look again please ?!

Also, this is a crude way to get the bias voltage - it normally has a 47uF or 100uF capacitor across R2. Also R1 and R2 are unnecessarily large for a voltage divider when 10k - 47k will work fine.

Cool, I had no idea about it, never studied electronics besides what I read on my own, I'm just an adventurer on this stuff and I do like it a lot !! Ok gonna try smaller values for the voltage divider, I might have some usable resistors about the suggested values, but what voltage divider does ?

Here is the new schematic:

[jojokeo]

You still have errors in that schematic uki. My suggestion is to lose the fancy design work with the switch and other non-needed stuff so it's not confusing to look at. Also stick to common words such as input, output, ground, etc. - I don't know where you get the words "core" and "screen"?
 
I re-drew the schem correctly as it should be so it's much easier to follow. I included a proper voltage supply including Vref / bias, and also an LED with a current limiting resistor. In the circled area you don't "have" to include this for a successful circuit if you don't want to but you can at least see a more proper one that will be much more stable. Where there's the 1n5817 diode this is a reverse voltage / low drop fast switching type of diode but you can use a simple cheap 1n4001 or 1n914 type. The 100r provides decoupling of the circuit from the power supply.
 
Now I would suggest a few things to be done differently like having gain, volume, and tone controls but this is enough for now without complicating things even further.
 
Hope this helps- Joe

[uki]

Oh man I keep bumping my head with it!!      

I understand what you say and to keep things simples are much better, I did use those words because that is what is in the layout, I was just following the layout, my goal is just to share it with people, and provide an schematic just as the layout is, if anyone want it. Did I do it better this time ?!   

Man thanks so much,     I really appreciate  the heads up and help.

[uki]

... I included a proper voltage supply including Vref / bias, and also an LED with a current limiting resistor. In the circled area you don't "have" to include this for a successful circuit if you don't want to but you can at least see a more proper one that will be much more stable. Where there's the 1n5817 diode this is a reverse voltage / low drop fast switching type of diode but you can use a simple cheap 1n4001 or 1n914 type. The 100r provides decoupling of the circuit from the power supply.

Thanks again Joe!!

Now that I got some chance to come back to it I have few questions.

1) What is a Vref/bias  ?

2) That switch in the circled area is the bypass switch ?

3) What is this means? 100r provides decoupling of the circuit from the power supply.

Thanks in advance.

[jojokeo]

1) What is a Vref/bias  ?

2) That switch in the circled area is the bypass switch ?

3) What is this means? 100r provides decoupling of the circuit from the power supply.

1) Voltage reference is appx 1/2 Vcc or supply voltage. Using a 9v battery it places appx. 4.5Vdc at certain points in a circuit to provide this voltage or bias to the circuit for proper or optimum operation.

2) No, that switch is for the LED & it's current limiter when the circuit is activated. To fully bypass the circuit would mean via a switch that when off the signal would be re-routed from input directly to output. Good circuit design also simultaneously shorts out the circuit's input to ground at the same time.

3) This means separating or disassociating the power supply and the load (circuit). It helps assure reliable component operation, performance, and also helps prevent or reduce noise which can be caused by various power supplies.

[uki]

Good circuit design also simultaneously shorts out the circuit's input to ground at the same time.

The bypass switch in the original circuit isn't putting the circuit input to ground is it, if not how to do that ?

[jojokeo]

I forgot that it's an onboard preamp. I was thinking it was more like an effects pedal. So it's always "on" isn't it? Therefore no need to worry about that or maybe even the 100r decoupling resistor?

[uki]

If the battery is too low, the signal becomes distorted, so the is why the bypass switch.

[jojokeo]

These pics show various ways to wire up whatever you want. You don't need an LED for your guitar to tell you if you have the circuit on or not so a DPDT toggle will be all that's needed. You will want to change the guitar's 1/4" jack to a stereo type. Connect the battery's negative wire to the "ring" terminal. When you insert a normal mono guitar jack into it, it will short this to the ground where you will hook up the guitar's neg part of the pickups and the circuit's neg part also. This will activate the battery automatically when you plug a chord into your guitar and eliminate the need for a power switch. You will have to install and hookup a bypass switch somewhere though in your pickguard if you must have the need to bypass the circuit. OR a better solution is to replace your Tone control pot with a pushpull type. Then no added drilling or switch that's rarely used is needed to clutter or devalue your guitar. If any problems with battery or circuit occurs - simply pull out your Tone control and you're back to stock mode.

*uki - just come on down to HB in SoCal and I'll do it for you, we have the US Open of Surfing going on right now. Enjoy the sun, surf, sand, & hotties while getting your set-up all done at the same time. Speaking of which (what am I doing on the computer?) 

[uki]

Man cool pics thanks !! I would like very much to show up there one of those days... but you know I'm in Brazil !!! Far, far away... LOL

I got in my guitar lots of stuff , 3 DPDT one selector switch, volume and tone, I got the stereo jack too, it is been there for decades also the circuit, before I was just following the layout without trying to understand how it works or working on a schematic, now that I'm hooked on electronics I've realized the importance of the schematic, also to share it with others.

Not sure if you read the posts above I got a noise problem the pots scratch at the ends of the turns only when the OPamp is on, been acting like this for some time, after 25 years of use could be a bad component?

The scratchy sound is probably due to the fact your preamp/buffer flows direct current through items connected to the output jack.

What do you think ?

I got that new model of the IC TLE2071 , it does have a much cleaner sound, I did run a test today and with all new components with this circuit the guitar's pots doesn't do the scratch at the ends of the turns.   

How to trace down such problems in circuits ?

 

[jojokeo]

... and with all new components with this circuit the guitar's pots doesn't do the scratch at the ends of the turns.   

How to trace down such problems in circuits ?

It was very likely due to a leaky coupling capacitor, especially since you are using electrolytic types where you don't need to? The way to test is after a coupling cap there should be no DC measured. They are supposed to block DC and allow AC to pass which carries your signal - just like in tube amps. You should've also been able to measure for it at the wire going into your pot that was scratchy.

[uki]

... and with all new components with this circuit the guitar's pots doesn't do the scratch at the ends of the turns.   

How to trace down such problems in circuits ?

It was very likely due to a leaky coupling capacitor, especially since you are using electrolytic types where you don't need to? The way to test is after a coupling cap there should be no DC measured. They are supposed to block DC and allow AC to pass which carries your signal - just like in tube amps. You should've also been able to measure for it at the wire going into your pot that was scratchy.

No electrolytics I think, there are ceramic,  polyester and tantalum ones, are the tantalum ones like electrolytic ? So then one of those are leaking DC ?! And if I measure for DC after those I will find the defective one ?!

[jojokeo]

No electrolytics I think, there are ceramic,  polyester and tantalum ones, are the tantalum ones like electrolytic ? So then one of those are leaking DC ?! And if I measure for DC after those I will find the defective one ?!

No - you fixed it didn't you and you said you replaced all the parts?

I got that new model of the IC TLE2071 , it does have a much cleaner sound, I did run a test today and with all new components with this circuit the guitar's pots doesn't do the scratch at the ends of the turns.

because you said circuit was:

I got in my guitar lots of stuff , 3 DPDT one selector switch, volume and tone, I got the stereo jack too, it is been there for decades also the circuit,

and

I got a noise problem the pots scratch at the ends of the turns only when the OPamp is on, been acting like this for some time, after 25 years of use could be a bad component?

[uki]

I got the stuff in the guitar and a new circuit that Im testing in the breadboard ! 2 circuits 

Haven't replaced anything in the guitar circuit, but I have been testing different caps in the breadboard, sorry if I wasn't very clear about that. 

I have plugged the guitar in the breadboard to test the new circuit and send the output to the amp, that way I was able to compare both, with the bypass switch in both circuits I can turn one or other and hear one at a time, or both together !

[jojokeo]

I was confused a bit but now I understand what you got going :)   Yep, usually things go pretty well working with stuff exposed like that and makes quick changes easy and fun. One last comment on the values of the tantalum/electro's is that if it's in the signal path 1uF is more than plenty to get full signal through. Sometimes .1uF and like tube c-caps 0.002uF (2nF) and 0.02uF (20nF) are fine too - it only depends on where and how they are used and what you like or need?

[uki]

I think this time the schematic is correct and exactly as it is in the layout, it is in the 1st post.
 
I've found the an schematic for this circuit !