# Thread: Negative feedback, and resonance question?

1. ## Negative feedback, and resonance question?

Hi everyone!

I'm trying here to understand the negative feedback circuit of my Marshall dsl 15(see schematic bellow).

My first question is when i engage the deep switch the signal of the negative feedback passes trough R91 then R94? or goes from R91 to the R93 in parallel with C69 path ?

How can i now what frequencies are boosted with the resistor R93 in parallel with the C69 capacitor? there is somekind of graphic calculator for this?

What is the purpose of the R91 resistor, is just to reduce the signal, or it has effect on the deep switch?

Also alot of amp schematics dont have a R58 resistor, in the negative circuit this resistir R58 is just to reduce the signal strenght?

2. When the switch is open it's in 'deep' mode.

The deep corner (-3dB) frequency is 1/2/pi/R91/C69 = 1/2/3.14/22k/22nF = 330Hz. The gain is highest at low frequencies and decreases at -6dB /octave to level out after 330Hz. R91 sets both the upper corner frequency and the gain above that frequency.

R93 stops the gain from rising below 33Hz.

IMHO R58 is superfluous. It would only do anything if CON2 was disconnected and even then nothing especially useful.

3. 1/(2pi*220k*22n) =33Hz, not 330Hz.

What do you mean with "R91 sets both the upper corner frequency and the gain above that frequency"?

4. Originally Posted by Rod
1/(2pi*220k*22n) =33Hz, not 330Hz.

What do you mean with "R91 sets both the upper corner frequency and the gain above that frequency"?
It's 22k not 220k so 330Hz is correct

Upper corner freq depends on R91 * C69, HF gain depends on R91/R57 thus R91 affects both.

5. R58 should have no affect on audio, but it does keep C44 charged, possibly kills potential pops when the deep switch is pushed or at least when that connector is opened.

In the schematic bellow, in the negative feedback circuit we have a resistor R50 in series after the resistor R51 in parallel with the capacitor C36.

Putting the resistor R50 i series after or before the resistor R51 in parallel with C36 it's the same thing? If it is diferent, it's diferent in what?

Thanks.

7. Originally Posted by Rod

In the schematic bellow, in the negative feedback circuit we have a resistor R50 in series after the resistor R51 in parallel with the capacitor C36.

Putting the resistor R50 i series after or before the resistor R51 in parallel with C36 it's the same thing? If it is diferent, it's diferent in what?

Thanks.

The order of the R50 and R51//C36 make no difference.

8. Originally Posted by nickb
The order of the R50 and R51//C36 make no difference.
So putting R50 before or after R51//C36 the result is the same?

9. Originally Posted by Rod
So putting R50 before or after R51//C36 the result is the same?
Yes. It depends on the total feedback impedance. In this case R50 + Zx; where Zx is the impedance of R51 and C36.

Hopefully you will agree than 2+4 = 4+2 so, R50 + Zx = Zx + R50

For completeness Zx = R51*Zc/(R51 + Zc); Zc= 1/2/3.14/f/C36

10. Originally Posted by nickb
Yes. It depends on the total feedback impedance. In this case R50 + Zx; where Zx is the impedance of R51 and C36.

Hopefully you will agree than 2+4 = 4+2 so, R50 + Zx = Zx + R50

For completeness Zx = R51*Zc/(R51 + Zc); Zc= 1/2/3.14/f/C36

Thanks nickb for the help, it's still confusing for me but it's a little more clear.

If i understand it right so if increase the resistor R50 for exemple to 100k, it will reduce the high freaquency and the gain in the high frequencys right?

What is the caculation method to see at what frequencys the gain will lower?

11. Originally Posted by Rod
If i understand it right so if increase the resistor R50 for exemple to 100k, it will reduce the high freaquency and the gain in the high frequencys right?
R50 and R51 are the negative feedback resistors. If you increase the resistance the negative feedback is reduced and the gain will therefore increase. At high frequency C36 is a short circuit so the gain is set by R50. Doubling the value of R50 will double the gain (if you ignore the finite open loop gain).

12. If you increase R50 the gain at all frequencies will increase, but the effect is greater at high frequencies.

To get a feel of what is going on, look at the circuits at the extremes of zero and infinite frequency. The impedance of a capacitor is 1/2/pi/f/c. So at zero the capacitor has infinite impedance and the gain is (p to (R50 + R51)/VR9. At infinite frequency the capacitor is a short and the gain is R50/VR9.

So lets think about inbetween. Start at zero and imagine the frequency is increasing. The capacitor's impedance is big compared to R51 so the gain is steady. Soon the impedance of C36 will equal R51 and this is our first "corner" frequency i.e. when 1/2/pi/f/C36 = R51 (or rearranging f= 1/2/pi/R51/C36). After that the gain will be reducing at the capacitor's lower impedance dominates R51//C36 ( i.e. R51 in parallel with C36). Go higher in frequency and we reach a point where C36//R51 is small enough compared to R50 that we have a second, upper, corner frequency. After that the gain levels out again.
The upper corner frequency is f= 1/2/pi/c/[R50.R51/(R50+R51)]

I plotted a graph for you - green is R50 = 47k and blue is =100K

BTW, I am pretending the amplifier has infinite gain to keep it simple. The truth is a long way from that but I don't think that helps your understanding of the concept at this stage.

The gain equation, with the infinite amplifier gain assumption is

(R51+R50)/VR9 * SQRT ( 1+ (2 * pi * f * C36* R50 * R51 /( R50 + R51))^2)/(1 + (2 * pi * f * C36* R51)^2 ( if I did it right )

If am afraid that if I give you the step by step calculation it will only serve to confuse you. You really need to read up on reactive circuits theory, know about imaginary numbers and negative feedback theory.

Nickb did you use some program to plot the graph or online calculator, a calculator like that will be very helpful

Just one more question about a mod i'm thinking in the negative feedback, going back to the schematic attached of the Marshall.

If i remove c44 and put a wire in it's place, then remove R94 and in it's place, put the capacitor C44 in series with the R94 resistor, will this mod have the same result of leaving everything has it is, or not doing this mod?

14. Originally Posted by Rod

Nickb did you use some program to plot the graph or online calculator, a calculator like that will be very helpful

Just one more question about a mod i'm thinking in the negative feedback, going back to the schematic attached of the Marshall.

If i remove c44 and put a wire in it's place, then remove R94 and in it's place, put the capacitor C44 in series with the R94 resistor, will this mod have the same result of leaving everything has it is, or not doing this mod?

It won't work. C44 is needed to prevent DC from flowing.

I used LtSpice, a free circuit simulator, to do the plots.

PS: Instead of 50 questions how about you tell us what you are trying to do?

15. But there is negative feedback loops or circuits schematics, that dont have a capacitor in the circuit.

16. Originally Posted by Rod
But there is negative feedback loops or circuits schematics, that dont have a capacitor in the circuit.
True, but they are different circuits. This one needs it as the DC operating conditions of the LTPI will be changed if it were omitted and most annoyingly they would change when the deep switch is operated leading to a nasty pop sound on the output.

17. What is the LTPI?

Ok but if this circuit didn't need the capacitor, putting a capacitor after or before the presence control, would it change the behavour of the presence control? or is the same?

18. Long tail phase inverter.

19. Originally Posted by Rod
What is the LTPI?

Ok but if this circuit didn't need the capacitor, putting a capacitor after or before the presence control, would it change the behavour of the presence control? or is the same?
It would be completely different as R93 would be bypassed by the 4.7uF so you would have low gain always i.e it's much the same as if the deep switch were closed. The presence control would not be affected.

20. HI guys.

What is the best way to decrease the mids when the the deep switch is activated, i think that i have to change values in R93 and C69, right ?

21. Something like this could be a starting point for your experimentation. I doubt it's the best way

But it is the best I could come up with without spending days thinking about it...

22. HI Nickb!

It's seems a ltspice schematic, right?
Do you know or have somekind of tutorials to start learning LTspice for the amp world?

23. Right! I use it to sketch schematics quickly as it's the fastest tool I know for it. Very easy to use GUI. Download it, play and learn. There is decent built-in help and an active Yahoo group where you can ask questions. For simple networks like these you should be up and running in no time.

24. Hi, nickb.

One more question, what happens if i lower or raise the value of the resistor R93? of the schematic bellow.

25. R93 is like a place holder. The circuit there is R94, which is either switched in or out. For highs the cap in parallel is always there.

26. Originally Posted by Enzo
R93 is like a place holder. The circuit there is R94, which is either switched in or out. For highs the cap in parallel is always there.
But when the deep switch is activated the circuit is in open mode, so the signal of the negative feedback will pass trough R93, so my question is when changing values of R93 what results will i obtain?

27. The whole circuit is a voltage divider. The NFB is inserted at the top of R57 4k7. Feedback comes through R91 22k. When it is on we can mostly ignore the 100 ohm resistor, so the R91/57 divider knocks the speaker signal down to approximately 1/6. When the switch opens, R93 220k is added. SO now we have (R91+R93)/R57, so 242k/4k7. SO we are left with about 2% feedback instead of 17%.

Tack a 250k trim pot in place of it and twiddle, then report to us what it does.

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