Help me understand this NFB schematic.

[VaporDemon]

In this schematic, can someone elaborate on it further and help me understand how the thump and hair switches are affecting the negative feedback, Obviously the the 4.7nf on the pot is a resonance control, but how do the 39K and 47K resistors and 10pf caps affect things? I would like to try this circuit on an amp with a 100K NFB control pot in series with a 27K NFB resistor, so I have a range ok 27K to 100K. Would I still run my NFB pot off the speaker tap and then into this circuit and eliminate the 27K resistor or is this circuit going to change my minimum NFB resistance? Ultimately I'd like to keep my range the same but also implement this circuit as well. Thanks in advance!

 

[RedPlated]

The 39k and 47k are in series so there’s a total of an 86k NFB resistor. They are split so that the high end can be controlled via each separate resistor. The 10pf over the 39k is cutting high highs in the power section.

The 10pf on a switch over the 47k will allow more highs when open, thus the, “hair” nomenclature. When it’s switched closed, the 10pf parallels the 47k and cuts more high highs.

Leave the 39k replace the 47k with a 100k pot. You’ll have 39k minimum NFB and 139k at max. Or if you want a lot of range, use a 22k with a 250KA pot.

I have tested caps over NFB resistors extensively and have never found a scenario where they improve the tone. Cameron uses them frequently. But I don’t like the way they neuter the high end bite. There’s better places to cut highs.

The 47k in the depth circuit is reducing the effect of the depth cap. When the switch closes, the cap bypasses the resistor and is fully in circuit, significantly increasing its effect and “thump.”

 

[VaporDemon]

The 39k and 47k are in series so there’s a total of an 86k NFB resistor. They are split so that the high end can be controlled via each separate resistor. The 10pf over the 39k is cutting high highs in the power section.

The 10pf on a switch over the 47k will allow more highs when open, thus the, “hair” nomenclature. When it’s switched closed, the 10pf parallels the 47k and cuts more high highs.

Leave the 39k replace the 47k with a 100k pot. You’ll have 39k minimum NFB and 139k at max.

I have tested caps over NFB resistors extensively and have never found a scenario where they improve the tone. Cameron uses them frequently. But I don’t like the way they neuter the high end bite. There’s better places to cut highs.

The 47k in the depth circuit is reducing the effect of the depth cap. When the switch closes, the cap bypasses the resistor and is fully in circuit, significantly increasing its effect and “thump.”

Awesome info, thanks man! I realize now I was wrong and my range was actually 27K to 127K on my existing NFB control. I will try the circuit with and without the high frequency caps and experiment a bit to see what the difference is.

 

[FourT6and2]

The 47k in the depth circuit is reducing the effect of the depth cap. When the switch closes, the cap bypasses the resistor and is fully in circuit, significantly increasing its effect and “thump.”

Isn't this a weird version of the the Thump control? All it's doing is reducing NFB resistance. It's not actually affecting the depth capacitance value.

Normally, you have that 47K and 4700pF in series after the switch, with the 470pF always in circuit. And when you flip the switch, it puts the additional 47K in series with the NFB resistor, raising the total value, along with the additional 4700pF, again raising the total value of the depth cap. I've not seen it used in this way, where the switch is bypassing just the additional resistor. It's kind of weird unless someone is trying to LOWER the total NFB resistance instead of RAISE it when the switch is engaged, while NOT affecting the depth capacitance. So it's not really a Thump control. It's just a NFB resistor switch.

Typically on Fortins and Camerons and Gowers and other hot-rodded builds, you've got a 470pF cap always in-circuit on the pot. Then the switch ADDS (not subtracts) an additional resistor AND cap.

As it looks in this schematic, total NFB resistance is 133K, with 470pF + 4700pF depth caps (totaling 5,170pF). Then when switch is engaged, all it does is reduce NFB to 86K?

OP where did you find this schematic? It looks like it's from the Fortin Cali schematic from SLO Clone. And are you sure it was drawn correctly and the switch shouldn't actually be in series before the 47K instead of bypassing it? You gotta be careful when you use schematics drawn by other forum members. There are often simple mistakes like this. That 47K should be in series after the switch, not in parallel with it IMO.

 

[FourT6and2]

Here's how I believe it should look:

 

[FourT6and2]

Other examples:

 

[RedPlated]

Isn't this a weird version of the the Thump control? All it's doing is reducing NFB resistance. It's not actually affecting the depth capacitance value.

Normally, you have that 47K and 4700pF in series after the switch, with the 470pF always in circuit. And when you flip the switch, it puts the additional 47K in series with the NFB resistor, raising the total value, along with the additional 4700pF, again raising the total value of the depth cap. I've not seen it used in this way, where the switch is bypassing just the additional resistor. It's kind of weird unless someone is trying to LOWER the total NFB resistance instead of RAISE it when the switch is engaged, while NOT affecting the depth capacitance. So it's not really a Thump control. It's just a NFB resistor switch.

Typically on Fortins and Camerons and Gowers and other hot-rodded builds, you've got a 470pF cap always in-circuit on the pot. Then the switch ADDS (not subtracts) an additional resistor AND cap.

As it looks in this schematic, total NFB resistance is 133K, with 470pF + 4700pF depth caps (totaling 5,170pF). Then when switch is engaged, all it does is reduce NFB to 86K?

OP where did you find this schematic? It looks like it's from the Fortin Cali schematic from SLO Clone. And are you sure it was drawn correctly and the switch shouldn't actually be in series before the 47K instead of bypassing it? You gotta be careful when you use schematics drawn by other forum members. There are often simple mistakes like this. That 47K should be in series after the switch, not in parallel with it IMO.

Doesn’t work like that the way it’s drawn.

If the switch is closed, the NFB would bypass that 47k on the switch and just keep going through to the depth pot. Path of least resistance. It’s not going to add anything to the series resistance. The 47k on the switch would have to be inline with the depth pot to do what you’re saying. There wouldn’t be any point in putting it where it is if you were trying to add another 47k to the NFB resistance.

It works like initially said. 470pf always on depth. With a limited amount of the 4700pf in parallel, reduced by the 47k. Closing the switch removes the 47k and adds the 4700pf, now fully in circuit. Again, think path of least resistance. You’ll get a deeper, more mid scooped depth frequency. Hence the, “thump” designation.

If the 4700pf was in series, it would reduce the total capacitance, not increase it.

Try wiring this up as drawn and see if you get any additional NFB resistance by closing the switch. It won’t change.

 

[glpg80]

Doesn’t work like that the way it’s drawn.

If the switch is closed, the NFB would bypass that 47k on the switch and just keep going through to the depth pot. Path of least resistance. It’s not going to add anything to the series resistance. The 47k on the switch would have to be inline with the depth pot to do what you’re saying. There wouldn’t be any point in putting it where it is if you were trying to add another 47k to the NFB resistance.

It works like initially said. 470pf always on depth. With a limited amount of the 4700pf in parallel, reduced by the 47k. Closing the switch removes the 47k and adds the 4700pf, now fully in circuit. Again, think path of least resistance. You’ll get a deeper, more mid scooped depth frequency. Hence the, “thump” designation.

Try wiring this up as drawn and see if you get any additional NFB resistance by closing the switch. It won’t change.

Agreed. The 4.7nF is always in circuit regardless of switch position. When the switch is open the 47k acts as ESR on the 4.7nF detuning its Q. Close the switch and the 47k is out of circuit no longer dampening the 4.7nF which increases its Q.

 

[RedPlated]

Here you go. I made the circuit and tested it for resistance and capacitance:

 

[VaporDemon]

Isn't this a weird version of the the Thump control? All it's doing is reducing NFB resistance. It's not actually affecting the depth capacitance value.

Normally, you have that 47K and 4700pF in series after the switch, with the 470pF always in circuit. And when you flip the switch, it puts the additional 47K in series with the NFB resistor, raising the total value, along with the additional 4700pF, again raising the total value of the depth cap. I've not seen it used in this way, where the switch is bypassing just the additional resistor. It's kind of weird unless someone is trying to LOWER the total NFB resistance instead of RAISE it when the switch is engaged, while NOT affecting the depth capacitance. So it's not really a Thump control. It's just a NFB resistor switch.

Typically on Fortins and Camerons and Gowers and other hot-rodded builds, you've got a 470pF cap always in-circuit on the pot. Then the switch ADDS (not subtracts) an additional resistor AND cap.

As it looks in this schematic, total NFB resistance is 133K, with 470pF + 4700pF depth caps (totaling 5,170pF). Then when switch is engaged, all it does is reduce NFB to 86K?

OP where did you find this schematic? It looks like it's from the Fortin Cali schematic from SLO Clone. And are you sure it was drawn correctly and the switch shouldn't actually be in series before the 47K instead of bypassing it? You gotta be careful when you use schematics drawn by other forum members. There are often simple mistakes like this. That 47K should be in series after the switch, not in parallel with it IMO.

Here you go. I made the circuit and tested it for resistance and capacitance:

Fantastic man! Appreciate you taking the time to do that video. Nice to see the circuit represented in actual components. Once I see it that way it’s much easier to process.

 

[RedPlated]

Fantastic man! Appreciate you taking the time to do that video. Nice to see the circuit represented in actual components. Once I see it that way it’s much easier to process.

Glad it helped…

 

[FourT6and2]

Here you go. I made the circuit and tested it for resistance and capacitance:

Good video. And yes, that's what I'm saying. Except all the schematics and physical amps I've seen with this particular setup have the resistor and cap in series AFTER the switch. Not a switch bypassing the resistor, and then a cap in series after. Look at all the schematics I posted. Those are showing a different setup than the one OP posted, are they not?

What's going on in those then?

 

[RedPlated]

Well that’s sort of complex because they are all doing different things.

The schematic you posted in post #5 is different. The 4700pf is 100% out of circuit with switch open. In contrast to the original post, where when the switch is open, it’s still affecting the depth pot a small amount, through the 47k to the entrance of the depth pot.

When you close the switch on your schematic in post #5, it becomes electrically identical to the schematic in the original post when the switch is open. Yours doesn’t have the capability to apply the 4700pf fully to the depth pot, like the original schematic with switch closed. The one on post #5 is very close to the Cameron depth setup.

Your first schematic in post #6 is identical to the one in post #5, just with different depth values. Plus, it’s cutting the NFB from 100k to 50k simultaneously when you add the .0033 depth cap through the 27k. It puts a second 100k in parallel with the first 100k NFb resistor. This is the Cameron depth circuit. Not the NFB part, the depth section.

Anyway, there’s a ton of schematics you posted. I’ll write a novel describing them all. But none of them is doing exactly what the one in the original post is doing. It’s not right, wrong or better or worse. Just a different way of doing it.

 

[FourT6and2]

Well that’s sort of complex because they are all doing different things.

The schematic you posted in post #5 is different. The 4700pf is 100% out of circuit with switch open. In contrast to the original post, where when the switch is open, it’s still affecting the depth pot a small amount, through the 47k to the entrance of the depth pot.

When you close the switch on your schematic in post #5, it becomes electrically identical to the schematic in the original post when the switch is open. Yours doesn’t have the capability to apply the 4700pf fully to the depth pot, like the original schematic with switch closed. The one on post #5 is very close to the Cameron depth setup.

Your first schematic in post #6 is identical to the one in post #5, just with different depth values. Plus, it’s cutting the NFB from 100k to 50k simultaneously when you add the .0033 depth cap through the 27k. It puts a second 100k in parallel with the first 100k NFb resistor. This is the Cameron depth circuit. Not the NFB part, the depth section.

Anyway, there’s a ton of schematics you posted. I’ll write a novel describing them all. But none of them is doing exactly what the one in the original post is doing. It’s not right, wrong or better or worse. Just a different way of doing it.

Exactly. This is what I was explaining in post #4.

I'm just curious if the one in the OP is drawn incorrectly. Because it comes from a schematic posted on the SLO Clone forum, drawn by a member there attempting to blueprint an amp he had in front of him. And mistakes happen. It usually takes 3-5 attempts on that forum before someone gets it drawn right. I have a hunch the 47K and 4700pF should be in series after the switch... just like in all the other schematics of Fortins, Camerons, Gowers, etc. They all have it drawn the same, except this one (which is a Fortin schematic).

 

[RedPlated]

Exactly. This is what I was explaining in post #4.

I'm just curious if the one in the OP is drawn incorrectly. Because it comes from a schematic posted on the SLO Clone forum, drawn by a member there attempting to blueprint an amp he had in front of him. And mistakes happen. It usually takes 3-5 attempts on that forum before someone gets it drawn right. I have a hunch the 47K and 4700pF should be in series after the switch... just like in all the other schematics of Fortins, Camerons, Gowers, etc. They all have it drawn the same, except this one (which is a Fortin schematic).

All the schematics you posted have it setup the same way. In parallel over the smaller depth capacitor. None of those put the added depth resistor/ cap combo in series with the NFB.

They may be “after” the switch but they’re doing the same thing. Adding the resistor and cap in parallel. But they're doing it in different ways with different values. But the net effect is similar. I think you’re getting hung up on the way that 47k is setup in the first schematic. Electrically it’s the same.

I know I said none of them are doing what the original schematic is doing, because they do have different values and the resistors are in different places. But they are all parallel.

The exception is #5 in post 6. Depending how those connections are made (passing over each other or connected) those two 1nF’s on the main line may be in series. Hard to tell but I think the intent is parallel.

 

[VaporDemon]

Exactly. This is what I was explaining in post #4.

I'm just curious if the one in the OP is drawn incorrectly. Because it comes from a schematic posted on the SLO Clone forum, drawn by a member there attempting to blueprint an amp he had in front of him. And mistakes happen. It usually takes 3-5 attempts on that forum before someone gets it drawn right. I have a hunch the 47K and 4700pF should be in series after the switch... just like in all the other schematics of Fortins, Camerons, Gowers, etc. They all have it drawn the same, except this one (which is a Fortin schematic).

Sorry I thought I replied to you already. I got that schematic online somewhere, I am a member of the SLO Clone Forum but I didn’t get it there. It was listed in a bunch of links to different amp mod schematics and labeled as the Fortin Cali Mod. I do understand that there are a bunch of incorrectly drawn schematics floating around but I will set them up as a temporary mod on my workbench and clip them in to try them and experiment with the values and layout. This is how I’ve been doing it since I started my journey. I appreciate you sharing your schematic examples as well. Thanks for the info!

 

[FourT6and2]

All the schematics you posted have it setup the same way. In parallel over the smaller depth capacitor. None of those put the added depth resistor/ cap combo in series with the NFB.

That's not what I'm saying. Something's getting lost in translation but no big deal.

 

[VaporDemon]

Here you go. I made the circuit and tested it for resistance and capacitance:

Just wanted to clarify something from the video. When you say the "NFB in" going to the 39K resistor, is that point attached to the phase inverter or is that where the speaker tap is connected? Attaching an image I made with DIYLC based on the schematic if you have a minute to look. I attached the extra pot terminal to the wiper on both pots.

 

[RedPlated]

Just wanted to clarify something from the video. When you say the "NFB in" going to the 39K resistor, is that point attached to the phase inverter or is that where the speaker tap is connected? Attaching an image I made with DIYLC based on the schematic if you have a minute to look. I attached the extra pot terminal to the wiper on both pots.

Yes, NFB in from PI to the 39k.

 

[scottosan]

Good video. And yes, that's what I'm saying. Except all the schematics and physical amps I've seen with this particular setup have the resistor and cap in series AFTER the switch. Not a switch bypassing the resistor, and then a cap in series after. Look at all the schematics I posted. Those are showing a different setup than the one OP posted, are they not?

What's going on in those then?

I do this on a push pull, essentially shorting the resistor the less scooped with the resistor is series