Negative feedback, and resonance question?

[nickb]

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.

[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?

[Enzo]

Long tail phase inverter.

[nickb]

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.

[Rod]

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 ?

[nickb]

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...

[Rod]

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?

[nickb]

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.

[Rod]

Hi, nickb.

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

[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.

[Rod]

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?

[Enzo]

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.