VCO output clipping, what do I need to change?

[nigeld]

Hi,

I’m building a VCO based on the 1222 but now with a lot of mods (the design can be found here) but I’ve hit a problem with my output where connecting a speaker ‘clips’ the output. Here is the output before I connect a speaker to the output jack:

And here is the same output after connecting the speaker to the jack:

So not good! What can be causing this clipping? The same happens on the triangle out and I suspect the PWM but that’s hard to see. The vpp is dropping from a nice 9.44V down to 6.96V I was aiming for close to 10vpp.

More importantly what do I need to do to stop this?

I appreciate any advice on this, the circuit is currently breadboarded so I can test this all out.

[Marty]

Hi,

I think you can’t connect just a “speaker” to the output. You need to connect the VCO output to a mixer first, and from the mixer to your speakers. Hope this helps.

[nigeld]

Ok, looks like I need to build a mixer next

Thanks for the response!

[Marty]

Here a schematic of a simple but good mixer.

And this is how my final mixer module looks like:

I’ve build a 6 channel mixer by copying the signal inputs by adding another opamp.
The schematic shows a 3 channel version using 1 TL074 (quad opamp), but I have used 2 TL074 opamps to have 6 channels, and I have used one left over opamp as a second output which u can use to connect a oscilloscope for instance.

happy soldering

[nigeld]

That’s super useful, thanks !

[ThijssjihT]

Why a mixer? Just an op-amp buffer will do the same thing as a mixer, right?

Furthermore, I would like to emphasize the output resistor. In the schematic, there is 1k written at the output, but it is not drawn. It will be necessary. An 4 or 8 ohm speaker will clip the output very hard again, and it might destroy the op-amp. The 1k resistor will drastically decrease the volume though. If it is too quiet, you might want to decrease the value of the transistor, but be careful to not draw more current than the datasheet for your particular op-amp specifies.
With this solution, you are effectively connecting your speakers to a pre-amplifier. To drive speakers correctly, you need an output stage. Although I like the mixer solution (it is quick and easy, and although less quick and easy than an op-amp buffer, you will end up with a useful mixer), if you chose to not make the mixer right now, you might want to use a “good” output stage.

It has been a while (12 years) since I last tried to build a simple quick and easy end stage. IIRC it was something like this.

The 8 ohm resistor at the output plays the role of your speaker. Don’t include it in your design.
The power rails must be at least ±3V, but you probably have standard ±12V available, so I put that in. The push-pull transistors should be relatively higher power. TIP31 and TIP32 are a bit overkill for this, but examples of some transistors I always have at hand.
The input resistors are just to scale down voltage to something a headphone speaker might want. You can play around with those values. Don’t put the headphone speakers in your ears before turning it on, just in case the volume is too high.
Push-pull configurations heavily distort the sound, but the op-amp feedback loop compensates for that. Any modern op-amp will be good enough in normal audio ranges.

You can connect this directly to your VCO, or other modules, or to the output of any pre-amplifier like a mixer.
There are better output stages than this one, but if I didn’t make any mistakes, this should do in most cases.

[nigeld]

Sorry for the loooong delay, I was working on this mixer design then building it and a 5 week break to the UK and France took place, but finally I got it working, so thanks for the help!

Mixer test

[K.ostas]

I assume you mean a powered speaker, right? If you connected directly to a speaker, then the 9V DC voltage of the oscillator would have destroyed the speaker. Speakers work with tiny AC voltages and need plenty of current to move. Your oscillator outputs exactly the reverse: high DC with little current. The circuit suggested by ThijssjihT will turn the voltage to current to drive the speaker, but you don’t need and you don’t want that if you have an amplified speaker.

If you indeed, connected the input to an amplifier/speaker, what you observe is not clipping per se, but filtering due to the AC coupling capacitor at the amplifier input and reduction in VPP due to the impedance mismatch. Typical desktop speaker amplifiers will have an impedance of about 20k.

If you want to hear the output, and you have indeed an amplified speaker and not just a speaker, you need a buffer with a high input impedance and low output impedance. At JFET (TL07* LF3*) type op amp hould work very nicely.

[nigeld]

Yes this was into a powered speaker, here is what the output stages for the different wave shapes look like at the moment:

[K.ostas]

This means the outputs are buffered. It looks like the impedance of the powered speakers is the bottleneck, despite 1k on the output of the op amp being lower than the 10-20k of the speakers. Not much you can do about that. No mixer or voltage to current converter will solve the issue.

Edit: If you can look on the manual of the speakers you can see their input impedance.

[craigyb]

My only thought is this, the sawtooth output is 10v from the as3340, then you pass it through an opamp with a gain of 1.22 making it 12.2v at the output which is probably at the limits of what you can successfully pass through the opamp capabilities. I would remove the gain stage from the opamp and have it purely as a buffer and keep the output at 10v which is standard.

It might look fine under the scope with no load but as soon as you add an output to it then it could affect the opamp operation. I’m no expert but that clipping looks like an opamp limit.