Preamp/ Crossover design

[Michaelaudio]

Ah okay.

I guess now this should Work. I delayed the Turn ON/Off time a little, so that the fets can Turn ON or Off the signal paths before Power is removed.

[Michaelaudio]

Here we go again. It doesnt work.  I get a weird Waveform Out of the First Stage, and only ~ 2.5Khz and above. Why? I can only imagine that its the biasing, but what is wrong with it?

[mskeete]

Looks like a mistake around your input buffer.
The input signal should connect to the + input. The 47K resistor should connect between the + input and 7.5v

Your input signal is being shorted to the output of the buffer but above 2.5KHz the output impedance of the opamp rises and some distorted signal gets through

[Michaelaudio]

Oh dear ofc! As a voltage follower should be..

Why doesnt it Work like that with AC voltage? BC there is no voltage drop in the Feedback Loop?

[mskeete]

I don't understand your question. Can you rephrase it?

[Michaelaudio]

Why u cant use the - Input with the voltage follower. I die some Research and i got it now.  Refreshing my knowledge, the Output would Go lower, when the - Input is a Higher voltage than on the + Input, therefore u wouldnt get much in the Output, Just how it is in my circuit rn.

[Michaelaudio]

Filter works now, but there 2 Problems left.

1. No Output in the LPF, after the 2 10K resistors, the Signal is pretty weak, and in the Output u have no Signal at all. But No short anywhere.

2. When i Connect R28 to the +Batt (i had to replace R28 with a Diode, ofc) then the BSS84 wont turn off when OUT (p6/U6) is high. Its still 20V at the Output. When i Connect the Diode (R28) to the source of Q2, then BSs84 turns Off, but then the Signal Mosfets are only switching Off, after Q2 switched Off, and i want it to be vise versa. But for that i have to Connect R28 to the +Batt.  Verry Strange

[mskeete]

You have a direct connection from C22 to 7.5v
That's shorting your signal out. Replace it with a large value resistor. That should get you sound.

The whole arrangement is a little strange though

[Michaelaudio]

Oh, i placed c22 bc i didnt want that current is flowing constantly theu r22, but shouldnt be a Matter i guess, i will try that.

Yea its Something different, but since i didnt know how to get rid of the Turn off pop with another way, i came up with this, and it should Work. But somewhere the BSS84 gets a Voltage on the Output, when there shouldnt be any Output .

[mskeete]

By "strange" I was refering to the filters
why dont you build your 33Hz filter around U1C/U1D?
I think it will be more benificial to have that filter in the common signal path in terms of overall phase response
ethier way , R19/R20 should be connected directly to U1C/U1D and not after C3/C8 which is part of another filter - sort of as you had it before but increase the value to 20K

For active sallen and key filters you dont need to scale one set of values to be 10 times the oter like you'd do with passive filters

[Michaelaudio]

Tbh,  that didnt csme to my mind, yes would have been Vetter to do it that way.

Oh my Bad, i corrected that in the current schematic, there ofc connected directly.

Now im Sure i'll get the LPF to Work, Just the weird Mosfets behavior left.

Edit: Nope, C22 isnt the fault. Somehow the 7,5V are shorting the Signal out. When i connect R19/20 directly to R22, the Signal is completely gone. I dont get it, thats the Same with the other 2 LPF of the midrange, and its working fine there.
It must be the OP amp, bc in this Case the 7,5 arent Busses with the audio Signal. Maybe i'll Change the verry First Input high Pass to the 33hz, then Connect R19/20 directly to R22 and removed the 7,5V from R22, then it should Work.

[mskeete]

If you look at your schematic closely, you'll see it's not the same
I mentioned in an earlier post that your shorting the signal out with that connection to 7.5v
You dont need R22. You can kill two birds with one stone by using R19/R20 to mono the signal and act as the first resistor to the LPF
You do need a DC path from the + input of the opamp to 7.5 and the easiest way to do it is

remove C21, R21, C22, R22 (and that 7.5v connection) and connect the common of R19/R20 to C23/R23

[Michaelaudio]

Got it, thanks. Will Test it tomorrow, but i think it will work finally.

[Michaelaudio]

So the Filter works now, but BSS84 doesnt turn off when i Press the Push button. The Gate gets imidiatly a high Signal, but the drain stays high. I removed R29, but that didnt Changed anything. Seems +5V isnt enough to Switch it Off, guess i need the Same voltage thats in the source.

Edit: i had a brainfart, i need to do it Just Like the mosfet that switched the Power... That solches this Problem.

Only one Thing left: The preamp is really noisey! When i Put a 220uf in the Output of the Voltage follower (for biasing) it Just gets rid of the higher noise. With resistors deviders and a 10uf in each one, i hadnt noise at all. Why is this one so noisey?

[Michaelaudio]

I'll Post that Here too, See the updated schematic below.


When i push the Button in the On-state, Q8 doesnt turn off emidiatly, its drain stays at 7.5V till C38 is discharged. But that Happens ONLY when i connect any Output of a Filter to an amp, when there is so Connection to an amp, it Just works fine, and Q8 Turns off emidiatly. WHY? i dont get it, it makes No sense

[mskeete]

I think you could simulate your "amp only" problem by connecting a resistor of say 10K across one of the output