Preamp/ Crossover design

[Michaelaudio]

So i build a BT speaker with a Selfmade preamp and 3 TPA3116 boards. 2x a stereo Board 1x Mono Board from eBay. When i Connect the Outputs of my Filter to the Inputs, the noisefloor is really loud and when i Turn Off power, i have a Pop. When there is nothing connected to the boards  its all quiet (besides u turn the pot. in the middle then its noisy too, dont know why) and i have no turn off pop. So i guess the noise is loading some Caps that are causing the pop when turning Off, but why is it so noisy?  I guess i missed something on my Preamp, but what?

Boards:
https://www.ebay.de/itm/173916201431
https://www.ebay.de/itm/284035213328

[Jan Audio]

You can make a screenshot with hold alt + print screen button.
Good luck

[Yansi]

Not sure what you have tried to achieve with those sallen key filters, but it is useless if what you wanted was an active crossover. Will not simply work as you've drawn it by multiple reasons!

Mainly because:

a) HF, MF and LF channels after combining back together will not result in the original signal. 
b) three-way crossover needs compensation of phase delay, can't use separate filters, need to use 2 chained two-way splitters
c) can't feed a Sallen-key filter with a high impedance source, if the filter is not designed with that in mind - which I guess it is not (I mean your SUB LPF).
d) can't supply the bias to the Sallen-key filter by slapping a random voltage divider to one of the resistors
d) lacking any  radio interference input protections.

Look & learn:
https://www.linkwitzlab.com/xo_eq.htm
https://sound-au.com/project125.htm
https://sound-au.com/project78.htm
https://sound-au.com/project09.htm
https://sound-au.com/project148.htm
https://sound-au.com/project81.htm
https://sound-au.com/project123.htm

Designing an active 3-way crossover is not just slapping some filters together. Disapointment guaranteed otherwise! 3-way analog crossover has zero "simple solutions".

But you could definitely make a simple 2-way one, just by using a highpass filter and difference signal for the LF path, such as that:  https://sound-au.com/articles/derived-xovers.htm

[Michaelaudio]

I quick read some articles, and i have some stuff to do i guess.

I still have some quesions:

- what do u mean with high impedance source in the Bass Filter? The 2 resistors that tie l and r together?

- guess U mean the resistor devider in the Bass filter? I guess its done that way If u have to use single supply? read it in a PDF from TI about Filters with single supply, and it works

I def. need a 3 way Crossover, since the speaker is complete and works fine, beside the noise, that i causing a Turn of Pop. It even seems that the Outputs sonst have much noise, which is weird, so im not sure If the Filter ist the Main Problem. But maybe it is, and i have to make a new one.  I'll draw something and Show my schematic here then.

[Benta]

What Yansi said.

I'm at a loss at what your filter circuit is supposed to do. Just at a glance I see:

HP
HP
HP-LP   ->probably BP?
HP-LP   ->probably BP?
HP-LP   ->probably BP?



Please explain your intentions, that will make helping you possible.

[Michaelaudio]

Right, its 2x a High Pass (L+R) and 2x a band Pass  for the mids and one for the Bass.

[Benta]

Right, its 2x a High Pass (L+R) and 2x a band Pass  for the mids and one for the Bass.

Understand.
But why do you have a BP for the bass? Do you have a subwoofer as well?

[Benta]

OK, you need to do two things first:

1:
Swap the lower LP and HP filters. This will eliminate the (c) problem that Yansi mentioned. The parallel connection of R29 and R30 will then replace R35 of the LP filter.

2:
Your single-supply approach with a distributed "virtual ground" through several voltage dividers is not the right solution. You need to make one low-noise bias source, preferably active, that is stable and low noise.
In general, using dual supplies and a real ground works much better.

[Michaelaudio]

Isnt that the way how u tie left and right Signal together? At least i have seen it in every Subwoofer Preamp so far. Also when i swap LP (170hz) and High Pass (27hz) then i would just get frequencies below 27HZ wouldt i?

Some kind of low noise regulator?  But i would have to use many of Them, since i cant Just tie them all together.

I had the noise Problem before with another Crossover, but instead of deviders i used Z diodes for the biasing. I seapped all diodes with resistors and the noise was gone. Maybe the 7815 is too noisy

Id prefer using a real ground, but its battery powered speaker with single supplied amp boards.

[Benta]

Isnt that the way how u tie left and right Signal together? At least i have seen it in every Subwoofer Preamp so far. Also when i swap LP (170hz) and High Pass (27hz) then i would just get frequencies below 27HZ wouldt i?

Some kind of low noise regulator?  But i would have to use many of Them, since i cant Just tie them all together.

I had the noise Problem before with another Crossover, but instead of deviders i used Z diodes for the biasing. I seapped all diodes with resistors and the noise was gone. Maybe the 7815 is too noisy

Id prefer using a real ground, but its battery powered speaker with single supplied amp boards.

Yeah, had a brain fart, sorry.
Just scrap the Bass HP filter completely and stay with the LP. Then you can sum L+R easily.

For the DC biasing, use an opamp (you have one spare now) as a voltage follower with a nicely decoupled input voltage divider.
This will supply the bias for the two input amps and R8, R15, R19 and R25. R33 is out anyway.

Scrap R2, R3, R7, R9, R14, R16, R18, R20, R24, R26 (and of course R32, R34)

[Michaelaudio]

When i tie R8,15,19, 25 together, wouldt that have an effect in the other Filters? The Signal in R8 is a different one than from R19, wouldt they influence mutually?

[Benta]

When i tie R8,15,19, 25 together, wouldt that have an effect in the other Filters? The Signal in R8 is a different one than from R19, wouldt they influence mutually?

In a normal circuit with bipolar supplies, those resistors would be grounded. What you need is to provide a "virtual ground" for them, and this is done through a voltage follower with low output impedance supplying a bias voltage to them.

Single-supply design is not easy.

[iMo]

I would add at least the blocking capacitors at the dividers, something like 10-100uF..

[Michaelaudio]

Yes, when i add decoupling Caps, the noise is reduced. I could live with that amount of noise, its pretty quiet. But the problem ist i have still s huge turn off pop, and dont know how to get rid of that.

@Benta, yes but when i tie all the Points together to one Voltage followern Output, that wouldt be so good bc of the Signals of the different stages, i guess?

[Zero999]

Yes, when i add decoupling Caps, the noise is reduced. I could live with that amount of noise, its pretty quiet. But the problem ist i have still s huge turn off pop, and dont know how to get rid of that.

That's caused by the capacitors charging, when the power is applied. Using a bipolar power supply will help, but the only way to eliminate it is to use switches (relays or analogue switch ICs) and a timer to only connect the output, after the power has been applied for long enough for the capacitors to charge. Even then, there still may be a faint click, as capacitors never really charge to 100%.

[Benta]

@Benta, yes but when i tie all the Points together to one Voltage followern Output, that wouldt be so good bc of the Signals of the different stages, i guess?

If your voltage follower is low impedance output (as it should be), the different signals won't see each other. This way to do it is standard.

[jonpaul]

Suggest to original poster to research and read the many fine texts and papers on crossover and filter design

All of this is very well documented.

Finally most well designed audio preamps and crossovers use a split power +/- 12 or 15V and have an FET, relay or other turn on transient muting ciircuit.

 
Bon Chance


Jon

[Michaelaudio]

@Zero999, i dont have a Turn ON pop at all, its a turn OFF pop, probably couse some Caps are discharching at the Inputs, but it only Happens when i Connect the outputs of the Crossover.

@Benta, now i got it.

As i said, i cant use dual supply, and i have a Chinese amp board that works with single supply too, and it hast absolutely No Turn ON or Off pop

[SiliconWizard]

@Zero999, i dont have a Turn ON pop at all, its a turn OFF pop, probably couse some Caps are discharching at the Inputs, but it only Happens when i Connect the outputs of the Crossover.

Don't you think it may be due to the output caps (C4, C8, C13...)?

[Michaelaudio]

I think so, but i cant get rid of them ofc, or maybe use 1uf instead of 10?

Edit: Just shorted C4, still a turn off pop, doesnt seem to be the Output caps.

[glentek]

Here's my preamp circuit - HP and LP filters at around 80Hz for a subwoofer. Digital volume control - I specified too slow optos - should have been 6N137, and the main channels could benefit from an additional gain stage on the output. Note the half supply bias generator, and the shunt resistors on the outputs.

[Michaelaudio]

That Looks more professional I have digital Volume Control too, but its a litle wondom (Sure) Board, which comes at verry first, so the Outputs of this Board are going into the filter Board. I'll try the shunt resistors, If that isnt going to Work, i'll have to make a new Board i guess, then with a low impedance source for the bias.

Edit: i Just found Out my amp boards already have a shunt, 1k in the stereo Boards and 20k on the mono board.  I wanna know what is causing this Pop, its not the noise and cant bei the Caps with a 1K shunt across them. What is it?

Why isnt ur Input ground actually going to ground? I dont quite understand the First Input Stage.(yea its a voltage follower but whats with the Inputs ground and C9/7 and R22/23)

And how did u safe that Pic? when i use snipping tool the pic is too tiny and U cant read anything.

[Michaelaudio]

I thought about switching the outputs off before Power is removed, would this Work? Note that the Der in the Signal path is a N-ch ofc.

[Michaelaudio]

Maybe this will do it.

[glentek]

Why isnt ur Input ground actually going to ground? I dont quite understand the First Input Stage.(yea its a voltage follower but whats with the Inputs ground and C9/7 and R22/23)

And how did u safe that Pic? when i use snipping tool the pic is too tiny and U cant read anything.

The input stage is to avoid ground loops with a head unit - it's basically a differentail amplifier. I copied it from a commercial car amplifier circuit.

Kicad schematic has File - Export - Drawing to clipboard. I then paste into a new image in Irfanview, save as png