Neve Tone Stack

[SprinkleSpraycan]

I want to play with the Neve tone stack as seen in the https://tonestack.yuriturov.com/ calculator. Thanks by the way to why-trv for the upgraded calculator.

Problem is i cant get it to work yet. There is a picture of the default values and how i used a TL072 for it.
There is only a very gated, sputtery, low volume signal getting through.
I used 680r instead of 620r because that's what i have. I'm treating Rin and Rf as jumpers.

I tried a TL071 and it sounded a bit clearer but with VERY little volume. The TL071 only worked with a 100n at the very front as shown in my drawing. I can put a boost in front and CRANK it to get more through and even that doesn't do much.

So what am i missing? I would like to try this with overdrives, fuzz's. Whatever might be cool. Please educated me on the next step.

[Eddododo]

Did you set up a Vbias network? In this schematic, all of the 'ground' EXCEPT FOR THE V- ON PIN 4 is actually a 'virtual ground,' 4.5v in this case.

If you are unfamiliar with this, I can elaborate, But basically we buy us an off amp halfway between the +9 V and 0 V ground, And use this halfway point as the "virtual ground"

Anytime we are using a single supply like a typical 9v arrangement in pedals, we need to establish this bias voltage

[fryingpan]

Did you set up a Vbias network? In this schematic, all of the 'ground' EXCEPT FOR THE V- ON PIN 4 is actually a 'virtual ground,' 4.5v in this case.

If you are unfamiliar with this, I can elaborate, But basically we buy us an off amp halfway between the +9 V and 0 V ground, And use this halfway point as the "virtual ground"

Anytime we are using a single supply like a typical 9v arrangement in pedals, we need to establish this bias voltage

You mean on the V+ pin. It's an inverting opamp.

Do this. From 9V, take two equal valued resistors (say, 100k), connect them in series to ground, then connect the middle point to the V+ pin. In this configuration, there shouldn't be any AC on the V+ pin so you don't need a bypass capacitor (a large value from the middle point to ground, ie. in parallel to the second resistor of the string). You will probably need one in any design you make.

EDIT: and add a large value capacitor (like, 47u) after R6 before ground.

[SprinkleSpraycan]

If you are unfamiliar with this, I can elaborate, But basically we buy us an off amp halfway between the +9 V and 0 V ground, And use this halfway point as the "virtual ground"

I'm listening...I understand creating the 4.5v with a voltage divider. Where do I apply that in this case? I don't see anything marked with the typical vb for instance.

[fryingpan]

If you are unfamiliar with this, I can elaborate, But basically we buy us an off amp halfway between the +9 V and 0 V ground, And use this halfway point as the "virtual ground"

I'm listening...I understand creating the 4.5v with a voltage divider. Where do I apply that in this case? I don't see anything marked with the typical vb for instance.

As I wrote in my previous post, the offset goes into the V+ pin. And any ground connection in the opamp's network needs to be DC-blocked by a large capacitor. (47u and higher, although with guitar smaller values may be beneficial).

[Eddododo]

If you are unfamiliar with this, I can elaborate, But basically we buy us an off amp halfway between the +9 V and 0 V ground, And use this halfway point as the "virtual ground"

I'm listening...I understand creating the 4.5v with a voltage divider. Where do I apply that in this case? I don't see anything marked with the typical vb for instance.

As I wrote in my previous post, the offset goes into the V+ pin. And any ground connection in the opamp's network needs to be DC-blocked by a large capacitor. (47u and higher, although with guitar smaller values may be beneficial).

I want to clarify to avoid confusion- by V- and v+, I'm referring to ground and 9v+ respectively, on pins 4 and 8 also respectively.

OP- generally to convert ground reference to virtual ground, , you create the 4.5 bias as you mentioned, and use THAT in place of every ground EXCEPT for the true ground that goes on pin 4.

In other circuits, you would also use 'true ground' for anything outside of the active part of the circuit; usually this would just be pulldown resistors before the input cap and after a final output cap

[Eddododo]

So the red circles are Vbias grounded.

Note the added 'pull down' resistor prior to the input cap- it is actual grounded, since it's not part of the active circuit and therefore not referenced to 4.5v

Also the voltage divider should have some largish cap from Vb to Ground, like 10u+

[SprinkleSpraycan]

So the red circles are Vbias grounded.

Note the added 'pull down' resistor prior to the input cap- it is actual grounded, since it's not part of the active circuit and therefore not referenced to 4.5v

Also the voltage divider should have some largish cap from Vb to Ground, like 10u+

Ahhhhh okay. I'll make these changes tomorrow, see what happens and report back. Thank you thank you.

[fryingpan]

You can (probably should) connect R6 to ground through a capacitor as well. Technically it's Input+ (yeah, sorry about that) that provides the offset, and that alone. If you connect R6 to Vbias, and you don't bypass that through a capacitor, it's going to mess with your bias a bit, as well as producing a different response.
So at least add a capacitor in parallel to the biasing resistor to ground. Keep in mind that Vbias is a DC matter, while the opamp is operating upon AC (the audio). You could just connect R6 to a series capacitor to ground, and technically Vbias would be a bit more stable (but it probably doesn't matter that much).

[duck_arse]

But basically we buy us an off amp halfway ....

He's been eaten by the AI!!!

also, now that your input is biased, your opamp output is biased, and you need an output cap to the outside world.

[amptramp]

There is an interesting mod to a Fender amp where you can add one SPST switch that goes from Fender tone to Tweed tone.  It disconnects the tone stack from ground and it is shown here:

BTW Coleen Fazio has an excellent series on amp repair and modification and I recommend watching her (easy on the eyes) and listening to her (very soothing voice).

[SprinkleSpraycan]

Ha-zah! It works. Thanks everyone for new wrinkles in my brain. 🧠

[merlinb]

You can (probably should) connect R6 to ground through a capacitor as well.

R6 is already isolated with capacitors so there is no DC on it, so R6 can (should) go directly to ground.

[fryingpan]

You can (probably should) connect R6 to ground through a capacitor as well.

R6 is already isolated with capacitors so there is no DC on it, so R6 can (should) go directly to ground.

True. I didn't notice that.

[Mark Hammer]

There is an interesting mod to a Fender amp where you can add one SPST switch that goes from Fender tone to Tweed tone.  It disconnects the tone stack from ground and it is shown here:

I did this to a Blackface Tremolux head I had, some 35 years ago, maybe longer.  It was presented in an article in POLYPHONY in the early '80s.  Lifting the ground connection of the Middle control, or fixed resistor that replaces it in Treble/Bass-only amps, can result in a jarring change in tone and/or level.  So the alternative is to insert a higher resistance than what the resistor or Mids control provides, and use a footswitch to bridge it (for normal functioning) or unbridge it.  The connection to ground remains intact at all times.

Here's an example.  The green curve is what you'd get with the treble, middle, and bass controls centred, in a standard Fender amp.  Adding 33k in series with the mid control, gets you the red curve, also with the controls set to midpoint.  You can see that there is still a bit of midscoop, just much less, and an overall increase in level, since there is less passive signal bleed via the midrange path.  Good for a solo boost that doesn't radically change the character of your tone, or result in huge level jumps.  The yellow curve shows that the treble control still functions, although adjustments to the bass control result in much smaller changes. The Mids control still works, and can produce clear scoops, although the purple curve shows the resonant cut is moved somewhat lower in the spectrum.  (I hope the attachment is viewable)

One can play with the values, using the tonestack calculator of your choice, and see what the impact is.

[fryingpan]

There is an interesting mod to a Fender amp where you can add one SPST switch that goes from Fender tone to Tweed tone.  It disconnects the tone stack from ground and it is shown here:

I did this to a Blackface Tremolux head I had, some 35 years ago, maybe longer.  It was presented in an article in POLYPHONY in the early '80s.  Lifting the ground connection of the Middle control, or fixed resistor that replaces it in Treble/Bass-only amps, can result in a jarring change in tone and/or level.  So the alternative is to insert a higher resistance than what the resistor or Mids control provides, and use a footswitch to bridge it (for normal functioning) or unbridge it.  The connection to ground remains intact at all times.

Here's an example.  The green curve is what you'd get with the treble, middle, and bass controls centred, in a standard Fender amp.  Adding 33k in series with the mid control, gets you the red curve, also with the controls set to midpoint.  You can see that there is still a bit of midscoop, just much less, and an overall increase in level, since there is less passive signal bleed via the midrange path.  Good for a solo boost that doesn't radically change the character of your tone, or result in huge level jumps.  The yellow curve shows that the treble control still functions, although adjustments to the bass control result in much smaller changes. The Mids control still works, and can produce clear scoops, although the purple curve shows the resonant cut is moved somewhat lower in the spectrum.  (I hope the attachment is viewable)

One can play with the values, using the tonestack calculator of your choice, and see what the impact is.

But by making this footswitchable aren't you introducing a shitload of noise into the signal? I mean, it's like 6 or more metres from the amp to the footswitch and back.

[Mark Hammer]

I've never played that far from an amp, so I couldn't tell you.  But it's not something that should pose great risk if one uses decent shielded cable.

[amptramp]

I've never played that far from an amp, so I couldn't tell you.  But it's not something that should pose great risk if one uses decent shielded cable.

Or you could use a relay for control.  No signal leaves the stompbox.

[fryingpan]

I've never played that far from an amp, so I couldn't tell you.  But it's not something that should pose great risk if one uses decent shielded cable.

Or you could use a relay for control.  No signal leaves the stompbox.

Of course for an on/off switch that's the sensible thing to do.

I was asking because I had an idea of adding an expression pedal to a reverb control on an amp, and the only thing I can come up with is an AC signal (an oscillator, for instance) being sent over to the expression pedal, with the pot modulating its amplitude, then rectified and smoothed once back into the amp, and made into a control signal.