Question about minimum input voltage using Voltage Regulator L7809?

[Jasonmatthew911]

Thanks!...So, for the 2nd stage, do you think it's a better idea for the Volume wiper to go into the inverting input (Pin 6 instead of Pin 5) in order to have lower noise with higher gains, and then I just switch the 2 Gain resistors/Cap going from pin 5 to pin 7 instead?

It's not less noise *at* higher gains.  It's less noise if the circuit has the capability to be set for higher gain but the gain is actually set to a low level.  Confusing  I know.   In the non-inverting case the second stage has to be set for the maximum gain and you adjust the signal going into it to be lower to get lower gains.    The high gain of the non-inverting second stage will boost it's own noise by that gain factor.  In the case of the second stage being inverting the volume control adjusts the gain itself so it's not boosting its own noise as much.

When the second stage has low gain the it's the non-inverting stage which often ends up with lower noise.

It's one of those devil in the details things.   So it's a matter of picking the right circuit for job.

For the non-inverting case you can also reduce a bit of noise using a 10k volume pot instead of the 100k pot.

Hey Rob, I just breadboarded the attached TL072 Pre-Amp going into my 386 Headphone Amp...Something isn't right with the PreAmp, because I have to play the guitar hard in order for it to sound, it's almost like it's super Gated, and when it does sound, it's like a nasty sputtery Fuzz, and when I feed my 555 mini synth into it, I have to raise the mini synth volume almost all the way up before it's square oscillation starts to sound, weird because that 555 synth has a loud  output signal, also when it sounds it's not clean like it should be...Something is wrong with the PreAmp as is...Any idea what it could be?...I have the first stage of gain set to 2, and the 2nd stage is around 3...What could be gating and sputter fuzzing the signal?...Think I should go back to the inverting input design maybe?...BTW, the Headphone amp works fine on its own with the mini synth and guitar, so we know the issue is in the pre-amp...Let me know your thoughts when you get a chance, thanks.

[Rob Strand]

Hey Rob, I just breadboarded the attached TL072 Pre-Amp going into my 386 Headphone Amp...Something isn't right with the PreAmp, because I have to play the guitar hard in order for it to sound, it's almost like it's super Gated, and when it does sound,

Sputtery usually means a biasing problem.

When I look at the schematic is looks OK.  So maybe the build doesn't match the schematic 100%.


Do you have a multimeter?   
If you can measure the DC voltages on the output of each opamp it will tell us if the opamps as biased OK.  You should see about half supply at the opamp outputs:  so 4.5V with 9V supply and 9V with an 18V supply.

Electrolytic caps around the wrong way stuff-up the bias.

This unlikely to be the issue but the 2x100k + 10uF on Vref might be better as 2x10k + 47uF or 100uF.

[Jasonmatthew911]

Hey Rob, I just breadboarded the attached TL072 Pre-Amp going into my 386 Headphone Amp...Something isn't right with the PreAmp, because I have to play the guitar hard in order for it to sound, it's almost like it's super Gated, and when it does sound,

Sputtery usually means a biasing problem.

When I look at the schematic is looks OK.  So maybe the build doesn't match the schematic 100%.


Do you have a multimeter?   
If you can measure the DC voltages on the output of each opamp it will tell us if the opamps as biased OK.  You should see about half supply at the opamp outputs:  so 4.5V with 9V supply and 9V with an 18V supply.

Electrolytic caps around the wrong way stuff-up the bias.

This unlikely to be the issue but the 2x100k + 10uF on Vref might be better as 2x10k + 47uF or 100uF.

All the 10uf Caps are BiPolar and in the direction as you see in schematic...I had also changed out those 2 resistors to 10K, but that wasn't it, I'll try a bigger 100uf cap there and see what that does...About lowering the input voltages, I feel like that defeats the idea that I wanted it to run on the 18V for the extra headroom...Remember I had made a TL072 PreAmp before, but using the inverted inputs and it gave me more headroom at 18V, but I had accidentally put the Pot at the input, so it only gave me unity gain, no extra gain, but it sounded good/clean...Wondering if I should go back to that, and start making changes from there maybe

[Jasonmatthew911]

I meant lowering the voltage on the outputs, sorry...As I wanted the 18V headroom

[Jasonmatthew911]

Hey Rob, I just breadboarded the attached TL072 Pre-Amp going into my 386 Headphone Amp...Something isn't right with the PreAmp, because I have to play the guitar hard in order for it to sound, it's almost like it's super Gated, and when it does sound,

Do you have a multimeter?   
If you can measure the DC voltages on the output of each opamp it will tell us if the opamps as biased OK.  You should see about half supply at the opamp outputs:  so 4.5V with 9V supply and 9V with an 18V supply.

Hey Rob, so I just measured the DC voltage at the outputs...This is what I got...

With 18V supply, Pin 1 is at 8.2V, and Pin 7 is at 17.51V

With 9V supply, Pin 1 is at 4.26V, and Pin 7 is at 8.85V

So this could be the problem, right?...What do I need to do for proper Biasing?

[Rob Strand]

All the 10uf Caps are BiPolar and in the direction as you see in schematic...I had also changed out those 2 resistors to 10K, but that wasn't it, I'll try a bigger 100uf cap there and see what that does...About lowering the input voltages, I feel like that defeats the idea that I wanted it to run on the 18V for the extra headroom...Remember I had made a TL072 PreAmp before, but using the inverted inputs and it gave me more headroom at 18V, but I had accidentally put the Pot at the input, so it only gave me unity gain, no extra gain, but it sounded good/clean...Wondering if I should go back to that, and start making changes from there maybe

Without checking the DC voltage on the opamp output it's not possible to get an idea is there's a problem with the build or the design.

You can try changing the second stage to inverting if you like.  Perhaps changing the volume control to a feedback type like the " Reckless Abandon" preamp.     There are certain types of overload which can make a non-inverting stage sound bad - you would need an oscilloscope to check that though.

FWIW, those caps are Electolytics.  They are polarized caps.   BiPolars are yet another type of cap.

[Rob Strand]

Hey Rob, so I just measured the DC voltage at the outputs...This is what I got...

With 18V supply, Pin 1 is at 8.2V, and Pin 7 is at 17.51V

With 9V supply, Pin 1 is at 4.26V, and Pin 7 is at 8.85V

So this could be the problem, right?...What do I need to do for proper Biasing?

The pin 7 voltage is bad.  Definitely going to cause problems.

With your circuit as per the schematic I can't see why the biasing would be off.  There's no *cause* for it.

If the cap on the resistor + cap from pin 6 to ground was shorted it would cause that problem.

[Jasonmatthew911]

All the 10uf Caps are BiPolar and in the direction as you see in schematic...I had also changed out those 2 resistors to 10K, but that wasn't it, I'll try a bigger 100uf cap there and see what that does...About lowering the input voltages, I feel like that defeats the idea that I wanted it to run on the 18V for the extra headroom...Remember I had made a TL072 PreAmp before, but using the inverted inputs and it gave me more headroom at 18V, but I had accidentally put the Pot at the input, so it only gave me unity gain, no extra gain, but it sounded good/clean...Wondering if I should go back to that, and start making changes from there maybe

Without checking the DC voltage on the opamp output it's not possible to get an idea is there's a problem with the build or the design.

You can try changing the second stage to inverting if you like.  Perhaps changing the volume control to a feedback type like the " Reckless Abandon" preamp.     There are certain types of overload which can make a non-inverting stage sound bad - you would need an oscilloscope to check that though.

FWIW, those caps are Electolytics.  They are polarized caps.   BiPolars are yet another type of cap.

I'm using electrolytic Bipolar caps, would that cause a problem?

[Rob Strand]

I'm using electrolytic Bipolar caps, would that cause a problem?

No, they should be fine.

Maybe check for a short on the board around that "pin 6" cap.

[PRR]

> Pin 7 is at 17.51V

That capacitor is shorted. Voltage checks around it, and a sharp eye under a bright light, should show where.

[duck_arse]

...... my 555 mini synth into it, I have to raise the mini synth volume almost all the way up before it's square oscillation starts to sound, weird because that 555 synth has a loud  output signal, .....

this, of interest. is there listenable sounds anywhere of this?

also, it never hurts to post a photo/s of your built (preamp) board.

[Jasonmatthew911]

I'm using electrolytic Bipolar caps, would that cause a problem?

No, they should be fine.

Maybe check for a short on the board around that "pin 6" cap.

So weird...I built the attached schematic  using the 2 inverted inputs, and the PreAmp worked pretty good, except for minor background noises here and there, also I didn't feel that it gave me much gain...Connecting a guitar directly to amp sounds just as loud as the PreAmp on Full Volume, almost seems like a unity gain PreAmp...I did mess with the resistors between input and output pins, sometimes I'd get more gain with saturation, at one point I made a pretty decent accidental heavy distortion...

Should I be using 2 Opamps for each PreAmp in order to get a louder clean sound or should I technically be able to do it fine with 1 opamp?

I went back and wired everything again going into the non inverting input for 1st stage and inverting input for 2nd, and that didn't even give me a sound...I don't know why it works well for me when I use the inverting inputs in both stages, but as soon as I use pin 3 input it never works right for me...I even changed out my TL072 chip just in case, but that pin 3 input just doesn't want to work right for me...

Does anyone here have a fairly simple TL072 Clean PreAmp circuit that I can use as reference to build something that works a bit better than what I've managed to achieve with the inverting inputs?...If not, maybe help me improve this attached circuit that at least works fairly well, I just feel it needs a little bit more clean gain to be above unity gain, and if there's any other tip to eliminate more background noise?...The minor background noise could be because the circuit isn't shielded in a box, not sure...Any extra help with this is appreciated, thanks.

[Jasonmatthew911]

Nevermind, I moved a component and now the non inverting inputs are working good for me...I got the attached Pre-Amp to work pretty well...On 9V it gives slightly beyond unity gain, but at 18V is where it gets really loud, and with volume beyond half way it turns into an overdrive, but the extra headroom and gain on 18V is great...I may have to lower the gain stages a bit for 18V, but then for 9V it may not be too loud...The volume difference from 9V to 18V is huge...Let me know if there's any recommendations on this schematics...Weird that it worked with the 1M resistor off of input pin 5, but when I connected that 1M resistor that used to be at the pin 3 input, it just didn't work...This is the final schematic that worked the best for me, and at 18V is giving way too much volume, at 9 o'clock it's loud and clean, beyond that the distortion starts to come in, and with 9V it's loud and clean around 3 o'clock, almost full, and full volume at 9V gives a slight almost unnoticeable saturation...But it sounds much louder and better than connecting direct to amp, which is what I was aiming for...Just gotta perfect this circuit and I can move on to finishing my PCB's...Any extra suggestions on my Gains or how to balance output with 18V and 9V would be great, thanks.

[Jasonmatthew911]

Nevermind, I moved a component and now the non inverting inputs are working good for me...I got the attached Pre-Amp to work pretty well...On 9V it gives slightly beyond unity gain, but at 18V is where it gets really loud, and with volume beyond half way it turns into an overdrive, but the extra headroom and gain on 18V is great...I may have to lower the gain stages a bit for 18V, but then for 9V it may not be too loud...The volume difference from 9V to 18V is huge...Let me know if there's any recommendations on this schematics...Weird that it worked with the 1M resistor off of input pin 5, but when I connected that 1M resistor that used to be at the pin 3 input, it just didn't work...This is the final schematic that worked the best for me, and at 18V is giving way too much volume, at 9 o'clock it's loud and clean, beyond that the distortion starts to come in, and with 9V it's loud and clean around 3 o'clock, almost full, and full volume at 9V gives a slight almost unnoticeable saturation...But it sounds much louder and better than connecting direct to amp, which is what I was aiming for...Just gotta perfect this circuit and I can move on to finishing my PCB's...Any extra suggestions on my Gains or how to balance output with 18V and 9V would be great, thanks.

Sorry, I didn't label the resistor going into pin 3 it's 1K, and I forgot to connect the pin 8 Cap to the Power rail in this last schematic...It's working pretty good so far... I'm gonna try and play with the gain stages a bit...Any suggestions to make this better are appreciated, thanks again.

[Jasonmatthew911]

Nevermind, I moved a component and now the non inverting inputs are working good for me...I got the attached Pre-Amp to work pretty well...On 9V it gives slightly beyond unity gain, but at 18V is where it gets really loud, and with volume beyond half way it turns into an overdrive, but the extra headroom and gain on 18V is great...I may have to lower the gain stages a bit for 18V, but then for 9V it may not be too loud...The volume difference from 9V to 18V is huge...Let me know if there's any recommendations on this schematics...Weird that it worked with the 1M resistor off of input pin 5, but when I connected that 1M resistor that used to be at the pin 3 input, it just didn't work...This is the final schematic that worked the best for me, and at 18V is giving way too much volume, at 9 o'clock it's loud and clean, beyond that the distortion starts to come in, and with 9V it's loud and clean around 3 o'clock, almost full, and full volume at 9V gives a slight almost unnoticeable saturation...But it sounds much louder and better than connecting direct to amp, which is what I was aiming for...Just gotta perfect this circuit and I can move on to finishing my PCB's...Any extra suggestions on my Gains or how to balance output with 18V and 9V would be great, thanks.

I just replaced that 100K in 1st gain stage for a 10K, and now the 9V and 18V are both pretty loud and more balanced out with each other, what a big difference that made, now 9V and 18V are both loud and clean up until around 12 o'clock at least, after that I just get a little bit of saturation, but I also barely have any background noise anymore...With this last change, I think this is it, unless anyone has something for me to try that could improve it, just let me know, thanks.

[PRR]

Do a checklist. Does every grid (device input pin) have a grid-leak (resistor etc) to tell it where to bias-up at? Is every electrolytic capacitor working with right polarity?

[Jasonmatthew911]

Do a checklist. Does every grid (device input pin) have a grid-leak (resistor etc) to tell it where to bias-up at? Is every electrolytic capacitor working with right polarity?

Ok, I took out the 1K at Pin 3 input, you're right it wasn't really needed, did nothing to sound, but if I add a large 1M resistor at the Pin 3 input down to the (2) 10K resistors, the circuit won't even make a sound...I also thought it was necessary, but it seems to have been the reason my non inverting inputs weren't working in the first place, when I got rid of that big 1M resistor on pin 3 is when the circuit finally started working...Is that odd?...Lastly, if I take out that 10uf Electrolytic Cap going into Pin 5 input, it also stops working/sounding, so it looks like that Cap is necessary, I switched the polarity, but that doesn't really make a difference cuz I'm using 10uf Bipolar Electrolytic Caps...

Anyhow, Is that normal that the circuit is working ok without that 1M resistor at the Pin 3 input or strange?...Anything else look strange to you?

Also, I had a question, in the first Gain stage between Pin 2 & 1, I had to use just the 2 resistors, because if I connected a Cap to the 10K resistor that goes to GND, it would actually attenuate my signal a bit, so it sounded louder/better leaving it out...Is that normal as well?

[Jasonmatthew911]

Do a checklist. Does every grid (device input pin) have a grid-leak (resistor etc) to tell it where to bias-up at? Is every electrolytic capacitor working with right polarity?

BTW, I can add that 1M grid leak resistor right from Pin 3 to GND, should I?...I just can't connect that large resistor on Pin 3 to the (2) 10K resistors below, it just doesn't work if I do, but it does if I just connect it to GND...Let me know if I should keep it connected at Pin 3 to GND, and if 1M is good, or should I go smaller maybe?...I added it to GND, and it works the same, but if it's something to improve the performance, then I'd keep it...Were you suggesting that I need a large grid-leak resistor only for Pin 3, or did you mean to make sure that I have them on every input of the rest of my circuits, like on input of Headphone Amp, Input of Cab Sim, and input of DI Transformer as well?

[duck_arse]

photos, so we can see what you are doing, cause it doesn't add up right. you need to bias pin 3 - bias to half the supply and the output will swing as much each way about halfway. so, 1M from pin 3 to the two 10k resistors forming the bias divider. pin 5 is also needing bias, so you need that other 1M already shown. and between pin 1 and pin 5, you seem keen on breaking the DC connection between, so use a cap - only one cap needed, and 10uF is waaaaaaaaaay big enough - and no 10k there because you already have the correct 1M connection shown.

oh, and I've just seen your pin 2 connection. it needs to go through the resistor to bias volts, as does pin 3 - OR - pin 2 resistor can go to ground, as you've drawn, only IF it has a capacitor to block DC. otherwise, the ground connection tells the opamp "GROUND", and nothing gets thru.

but you are telling us what you've done, instead of showing us what you've done. we'll get you there sooner or later.

[Jasonmatthew911]

photos, so we can see what you are doing, cause it doesn't add up right. you need to bias pin 3 - bias to half the supply and the output will swing as much each way about halfway. so, 1M from pin 3 to the two 10k resistors forming the bias divider. pin 5 is also needing bias, so you need that other 1M already shown. and between pin 1 and pin 5, you seem keen on breaking the DC connection between, so use a cap - only one cap needed, and 10uF is waaaaaaaaaay big enough - and no 10k there because you already have the correct 1M connection shown.

oh, and I've just seen your pin 2 connection. it needs to go through the resistor to bias volts, as does pin 3 - OR - pin 2 resistor can go to ground, as you've drawn, only IF it has a capacitor to block DC. otherwise, the ground connection tells the opamp "GROUND", and nothing gets thru.

but you are telling us what you've done, instead of showing us what you've done. we'll get you there sooner or later.

Ok, I moved things around a little bit on my breadboard, cuz I suspected maybe the actual breadboard was the issue, and it was, some bad connection holes on my breadboard...I made the changes and now I was able to put the 1M from Pin 3 to the (2) resistors and all is working fine, I also added the Cap in 1st gain stage, and took out the resistor and cap that were extra going into Pin 5...It's working fine now...I don't send pics of the Breadboard, because it's messy looking and you probably wouldn't see all the connections very well...See my attachment, this is where I'm at...After hearing it for awhile on Guitar and Bass and comparing the sound with the direct sound to the amp, I feel it sounds loud enough, and with a bit more low end, at full volume it saturates a little bit, and might lack a little better definition as compared to my direct sound from guitar/bass to Amp...Through the Headphones it sounds pretty decent with guitar and bass...Anyways, now that I've come this far with the PreAmp and heard it long enough, I feel like I want to make it sound better if possible...Any suggestions to  EQ this, what parts I could add to give this a fixed EQ more to my liking, and maybe even give it slightly more clean gain without distortion, by compressing it or adding a simple clean boost circuit to it, so it gives a nice clean boost beyond unity gain...I'm a little beyond unity gain on this schematic, but when I go beyond unity gain, it saturates the signal a bit, and I'm not 100% convinced with the clarity definition, as I'm used to the clarity/definition of the Klon, it's my always on pedal for guitar, but I'm trying to make this good keeping it as simple as possible due to the limited space I will have on my 2 Boards and in Box, and I don't do SMD parts, all my parts are through hole...Anyway I could clean it up a bit or make it a louder clean boost...Should I maybe add like a simple clean boost circuit, something like the LPB-1 in front to increase definition?...Please let me know how I can make this PreAmp better with a few more parts?