Quest for an alternative method to control tremolo depth

[~arph]

Ok, this might be a bit vague, apologies beforehand.

I am looking for a way to add a depth control to a tremolo signal. Normally one would do this by adjusting the magnitude of modulation and that's just not possible here. I have an analog multiplier with an internal modulation signal that is modulating between 0-1 times the input signal. No depth control possible with this particular ic. There is no modulation input. Now what I tried is various mixing stages to blend between the original and tremolo signal, but in the end there is always an area in the blend range where you get intermodulation and as a result a perceived doubled tremolo rate with a funny waveform. Ok so I am reasonably sure this is caused by phase cancelation and so I wonder if it is at all possible to fix. ( if we can we have a much overlooked application for an ic that is already known to us for something else )

[ElectricDruid]

If you're using a multiplier IC to get 0->1 times the original signal to get a tremolo'd signal, you ought to be able to crossfade between that and the dry signal to vary the depth.
Phase shifts or other weirdness means something odd is going on - is it inverting? That might make it more akin to a ring mod or full-wave rectifier, I suppose, if you start mixing in an amount of the dry signal. Have you got a schematic or a scope shot?

Tom

[~arph]

Thanks,

Setup is this:

Inverting input stage, into the multiplier and into the mixer (geofex panning for fun). Output of the multiplier also into the mixer.
I don't know if the multiplier is shifting the phase of the input signal. I tried inverting the clean path and mixing that, same result.

I have a scope, but my workbench is my kitchen table for now, so I haven't been using that for a while.
I will probe around a bit more and double check that the effect on the mixed sound is not originating from the multiplier alone. (I checked this before, but I could have made a mistake).
I can also purposely mix in the carrier signal by adjusting the null trimmer. If I choose an audible carrier freq. then I can scope the output and see if the multiplier does not act funny.

It is probably something obvious that I am missing. I will see if I can draw up a schematic. The IC btw is the XR2206

[ElectricDruid]

Have you got the biasing for the amplitude modulation sorted out? I found this in the datasheet:

Amplitude Modulation:
Output amplitude can be modulated by applying a dc bias and a modulating signal to Pin 1. The internal impedance
at Pin 1 is approximately 100k. Output amplitude varies linearly with the applied voltage at Pin 1, for values of dc bias at this pin, within 14 volts of VCC/2 as shown in Figure 6. As this bias level approaches VCC/2, the phase of the output signal is reversed, and the amplitude goes through zero. This property is suitable for phase-shift keying and suppressed-carrier AM generation. Total dynamic range of amplitude modulation is approximately 55dB.

This explains where your weird phase shift is coming from, but I don't understand it clearly enough to see what to do about it. Figure 6 is crucial. This is a ring modulator (4 quadrant multiplier) not an amplitude modulator (2 quadrant multiplier)
Are you using the "Multiplier output" (pin 3) or the "Sine Triangle output" (pin 2)?

Tom

[~arph]

Pin 1 is the signal input.
Pin 2 is the output.

I am using the IC as both a ringmod and tremolo in this case. (you can change the cap between 5 and 6 to alter the carrier frequency)

input is like this:

signal from op amp though a cap. Then a 10k to a voltage divider for (10k trim pot with two 100k resistors. one to gnd the other to vcc.) Then into a 1k5 resistor, into input 1.
The trimpot is used to trim out the carrier from the signal (as per figure 6. no signal means vcc/2 hence no output)

Pin 3 is actually used to set the output level. This needs a resistor to Vref.. (see fig 11.)

The strange this is that when I have R.G. panning circuit in there I get a gradual increase of doubling of the rate while dialing towards the WET end (it starts to get noticable at about 75% wet) Whilst when I listen to the wet end only (disconnecting the clean signal from the mixer) this doubling is not there and the circuit works fine.
What I don't understand is that if this is phase related that the effects is most prominent when the mix is at the extreme wet end and not when 50/50.

I have tried separate Vrefs for the opamp and xr ic. Also different divider resistors for different available vref currents. All to no avail.

[ElectricDruid]

It sounds like your levels aren't right. A ring mod can easily give a doubling effect.

Here's an image which shows what I think is going on:



Your crossfade is changing the reference level for the waveforms and thereby fading between AM and RM. As you can see, that doubles the apparent modulation frequency. It could be either the audio waveform or the modulation waveform (since multiplication is the same both ways around) but it's the unipolar/bipolar that you need to think about, I reckon.

The good news is that I think it's working as it's supposed to!

HTH,
Tom

[Mark Hammer]

The old Craig Anderton tremolo uses an optoisolator as part of the input resistance of an inverting op-amp stage.  The invertor-based LFO cranks out a fixed-amplitude modulation wave to illuminate the LED half of the opto.  The amplitude of the modulation is dictated by adjusting how much those variations in the LDR resistance contribute to changes in the gain of the op-amp.

[~arph]

Thanks, I will see if I can hunt down any DC level changes on any of the multipliers inputs when messing with the blend.
I fully understand what you are saying. I still think it is strange, given that the blend circuit comes after the signal modulation and the DC offset is set by the trimmer at pin 1. (which is isolated from opamp DC by a cap). The effect you describe seems to implicate that the offset inside the IC shifts in DC. (from unipolar to bipolar modulation) when I mess with the blend, I have to investigate as how this is possible.
Also wouldn't shifting the DC level of the modulation wave also cause an overall volume change? this is not the case.
I think it is working as it is supposed to and that I'm simply missing something obvious. To be continued.

@Mark. Yes that is a nice way to do depth control. But the thing here is that I do not have a modulation signal, but an already modulated signal (I can understand the confusion as the XR2206 is a function generator too after all) that I need to add clean signal to. Look at it as a tremolo pedal with full depth that you want to add a variable amount of clean signal to in order to add depth control.

[ElectricDruid]

I still think it is strange, given that the blend circuit comes after the signal modulation and the DC offset is set by the trimmer at pin 1. (which is isolated from opamp DC by a cap). The effect you describe seems to implicate that the offset inside the IC shifts in DC. (from unipolar to bipolar modulation) when I mess with the blend, I have to investigate as how this is possible.

Don't forget *how* ring mod is done - usually it's a "balanced modulator", where an amount of the signal is cancelled by an inverted amount coming via a VCA. E.g. it's all in the mixing. Without that mixing, it's just normal AM. I think that's what's going on in your crossfader, not that it's somehow amazingly causing offsets in the inputs.

Tom

[~arph]

Hi Tom,

It is indeed all in the mixing. I tested with the scope and a signal generator to see what is going on. In the end I simplified the mixing arrangement so that in tremolo mode the mixing range is limited so I am not getting the doubling and in ring mode mode the limiting is removed. It's a simple matter of using a DPDT instead of a SPST switch. Now on to the schematic. This will be a simple two IC ringmod / tremolo with adjustable blend.

Thanks for the help guys!

[ElectricDruid]

Excellent, nice work! That sounds like a simple and powerful circuit.

I look forward to seeing the schematic.

Tom

[~arph]

It's here now:

http://www.diystompboxes.com/smfforum/index.php?topic=117603.0