Beginner's blues, weird active filter behaviour

Hi all,

I have built up my first 2 way L-R filter based on the schematic on Rod Elliot's website, project 81, with component's selected to give a crossover point of around 300Hz.

12dB / Octave Linkwitz Riley Crossover

It has a few issues with phase and distortion.

The first issue is that is distortion rises as the frequency rises, please see the photos below.

At ~70 Hz the waveforms are clean though out of phase by something like 3ms.

At ~1kHz the output from the high pass section start to show distortion, looking a little like crossover distortion but offset to the negative rail slightly.

At ~20Khz the output from the high pass section looks terrible and the low pass section shows weird spike things in the waveform.

The Op amps I used were TL071s, resistors were hand matched 1% metal film and the capacitors hand matched polyester film types.

What gives, is it the breadboard or have I missed something?
 

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My component values are all as per Rod's original design, i.e. 470nF for the filter capacitors and 11.2K for the filter resistors.

I have no load on it at all, just connecting the probes directly to the outputs via jumper wires.

I asked the question about decoupling capacitors and the consensus seemed that they weren't necessarily needed for TL071s.

I've decoupled evey IC's supply pins with 103 (10nF I think) ceramics, I only had 2kV high voltage ones if that makes a difference but the problem remains.

I've attached a photo of the circuit, it seems a bit messy I know... :D

Red and Black are +V and -V, green is earth, white is signal between stages, blue is filter to output buffer, yellow is feedback and orange is output.

decoupling caps are the little blue ones.
 

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Something's definitely wrong, hard to figure out with that cramped layout, but you have a lot of loops of wire talking to each other above the breadboard which isn't great.

The rails need bulk decoupling to ground somewhere, and your ground wiring needs to run linearly along the signal chain.

You didn't mention what your power source was nor what the driving signal was from or what source impedance.

Check every IC is getting both power rails at the chip itself (you'd be surprised how well a circuit can behave with a power pin floating!).
 
Check the signal on the output of the first opamp, and scope the rails at each opamp to make sure your power supply is working properly.

I checked the power supply and it works as it should, perhaps a more bit noisily that it should thinking about it.

Probing the signal between the stages seems to show it gets progressively worse rather than it going haywire at a particular stage.
 
OK. Enjoy prototyping. Its interesting. Me the weekend will try an interesting tube circuit: I will try to make an 6AL11 double pentode compactron behave like a 40106 or 74HC14. Let's see what happen.

Very cool, I don’t what that is and googling has left me more confused but it sounds cool! :D

I’ve been trying to avoid getting into valves, I have only one valve amplifer (a set of ancient Quads) buy so far have worshiped at the altar of linearity and hence have avoided getting any more.

I do like the way they look though... :D
 
Something's definitely wrong, hard to figure out with that cramped layout, but you have a lot of loops of wire talking to each other above the breadboard which isn't great.

The rails need bulk decoupling to ground somewhere, and your ground wiring needs to run linearly along the signal chain.

You didn't mention what your power source was nor what the driving signal was from or what source impedance.

Check every IC is getting both power rails at the chip itself (you'd be surprised how well a circuit can behave with a power pin floating!).

Thanks for your thoughts Mark.

The layout is cramped as I rather childishly hoped to build two channels in the breadboard, cut up a RCA cable, plug it all in and test it in the living room that afternoon... :D

I shall rebuild it spaced out and test it again to see.

The wires are the only ones I have so far, I found this breadboard/project lab thing on eBay and ordered a generic set of wires as I had no clue otherwise.

It’s a Knight portable mini lab with built in power supplies, signal generators and some digital stuff I haven’t a clue about. I thought it would be a perfect compact thing to do electronic stuff on that can be closed up and packed away to avoid incurring the wrath of the Better Half at home, is it possible that all the stuff under the breadboard is interfering with the circuit?

The power supply I used is the variable LM317/338 supply built into the box. It seems ok on the oscilloscope, better than the other power supply built into the thing, being TO-3 based LM7812/LM7912 +- 12V. I Set the variable power supply to +- 15V for the test above.


When you say bulk decoupling somewhere, does that mean an electrolytic capacitor of some description and if so how would I go about calculating it’s value?

When you say the ground should run linearly along the signal chain, again please forgive my ignorance but what does that translate to in terms of wire routing?

The source is a XR2206 based signal generator kit with some improvements based on stuff I found on YouTube about removing the DC offset from the output with a 1uF electrolytic cap on the output and adding trim pots to adjust the sine wave. I can’t hazard a guess at its input impedance I’m afraid.

I’ll stop trying to fit two channels on one board and rebuild it with short lengths of hook up wire instead of the long loop wires and see what happens.

Thank you for your help.