LM13700 Tremolo?

[The_Armadillo]

My eyes were just recently opened to the versatility of the lm13700. I see there's a schem labeled amplitude modulator... could I use that side of the OTA as a tremolo and make an oscillator out of the other? Anyone ever tried it? Does it sound good?

[R.G.]

The 13700 can do that. I haven't designed one of those, but I see no real problems doing it.

There are two problems with OTAs in general.
1. They have a limited signal level they can do without distortion. For the old school ones like the 3080 and 3094, this was around 25mV. The special predistortion diodes on the 13700 and others lets this get up to about a volt.
2. As a result of the low signal level, they tend to be noisy. This may or may not be an issue with any given pedal effect. The people who play only in their bedroom will be the worst critics, as it's hard to hear the noise on stage.

[The_Armadillo]

I was just a little hesitant about diving into it because I've heard that it was uneven or something along those lines...

[frequencycentral]

The single OTA VCO on the data sheet is a little 'imperfect' IIR.

[The_Armadillo]

I drew up a layout a while back using the single OTA VCO.... Kinda new here, so how do I post a picture?

[The_Armadillo]

Just answered my own dumb question...
http://s1246.photobucket.com/albums/gg602/dsarmbrust/?action=view&current=13700tremolo.jpg

Edit: messed up link

[gritz]

Hi The_Armadillo 

I'm having a bit of trouble viewing your layout (my fault - it's on a tiny screen here), but I'm pretty familiar with the 13700 datasheet so here goes:

Remember that the datasheet schematics are designed around a +-15 volt power supply. As you will be using a +9 volt supply some of the resistance values will need to be scaled - the linearising diodes should be fed from a 3k9 resistor to give a diode current of 1mA and the resisrors to the amp bias pins should be scaled for an absolute maximum of 1 mA (the control inputs typically sit at about 1.2 volts depending on the current through them). Incidentally, be very careful when prototyping with the 13700 - if you accidentally short a bias pin to +V without a current limiting resistor in series it will fry. They are pretty robust apart from this though.

It appears from your layout that the output resistor of the left hand ota (R5) is connected to 0V - it should be connected to a 4.5V rail, as should the wiper of VR1.

I've never tried the VCO but I imagine C1 will need to be a fair bit bigger for lfo type speeds,

Finally, for guitar circuits I feel that low noise is better than low distortion (the LM13700 distortion isn't particularly intrusive imo) so my personal perference is to not bother with the diodes and to scale the resistor dividers at the amp inputs to give a maximum differential input of about 100mV. Oh yeah. the ota has a pretty low impedance input so you'll need some sort of buffer between yout guitar and the trem to provide the necessary current.

Hope this helps a bit.

[merlinb]

My eyes were just recently opened to the versatility of the lm13700. I see there's a schem labeled amplitude modulator... could I use that side of the OTA as a tremolo and make an oscillator out of the other? Anyone ever tried it? Does it sound good?

An easy way to get started is to take the signal path of the Ross compressor and throw away everything on the right-hand side of the diagram after Q2.


What you're left with is just the voltage-controlled amplifier (yes, the Ross used a CA3080, but the same circuit works with an LM13700). You can then play around with driving the control pin. Hook it up to an LFO and you've got tremolo...

[The_Armadillo]

Thanks for the replies!
I wasn't quite sure what to do about the +/- 15 volt supply, so I just assumed that I could use ground at all the -15v points, but I see that wouldn't really work... If I was using 9v, in reference to the data sheet, would i use 4.5v where it says ground and ground where it ways -15v? I'd probably have to draw a new layout. I think I messed up more than the +/- 15v conversion.

Would I be able to use this as a ring mod type effect? I noticed on the data sheet that the only difference between the AM and a four-quadrant multiplier is one resistor... would it be a dry mix type of resistor?

[merlinb]

Would I be able to use this as a ring mod type effect? I noticed on the data sheet that the only difference between the AM and a four-quadrant multiplier is one resistor... would it be a dry mix type of resistor?

Ring Mod:

[The_Armadillo]

Wow. Thats pretty awesome.

[The_Armadillo]

Another thing, if I slowed down the ring mod in to LFO range it would act as a tremolo, correct?