ad3208 LFO mod

[objectx]

Hi everyone, I've been thinking about making a mn3205 based analog delay but I really want the option of adding a lfo to the delay time for chorus and vibrato a la memoryman. Can anyone one give me some advice about hooking up a lfo into this circuit?

cheers.

[Mark Hammer]

Best place to start is with the LFO circuit for the Boss BF-2 flanger, or a similar 3-control circuit.  The way these things work is by mixing an LFO output and DC voltage.  The Depth pot essentially adjusts the balance between the LFO and the DC voltage.  Ultimately they both feed the clock-generator circuit that steps the BBD.

Now, at a conceptual level, that entire BF-2 circuit could be easily applied to an MN3205 chip instead of an MN3207 chip.  But there are a few initial stumbling blocks:

1) The clock frequency range that the circuit produces needs to be adapted to producing slower clock rates to suit the production of long delays, rather than the fast clock rates needed for flanging.  Not enough to simply quadruple the number of BBD stages.  That would only shift the delay time from 1-13msec to 4-52msec.  You would probably want to increase the 47pf cap labelled C30 in most BF-2 schematics to perhaps 270pf, which would give a maximum delay time of 298msec and a min delay of about 23msec.

2) Since they are intended to reduce clock-related noise, the lowpass filters have to address the range of clock speeds covered.  Slowing the clock down means moving the cutoff frequencies of the filters downward proportionally.

3) The longer the actually delay time, the less tolerable modulation is.  Consequently, there would need to be a fixed resistor between the output of the LFO section and its side of the Depth pot to assure that only a fraction of the full LFO output signal possible is ever used under any setting.  If we were using the BF-2 circuit as our starting point, I'd stick 68k resistor in series with the input to the LFO side of the 50k Depth pot.

Now, none of this is intended to be an obstacle.  Rather they illustrate the various aspects to be attended to in adapting the different circuit fragments you may borrow from here and there to accomplish what you want.

Hope this is helpful info.

[objectx]

thanks mark, that's really useful info. I had been looking at the boss ce-3 schematic which uses the mn3205, but I'll have a look at the bf2 as well.

[objectx]

After looking at both the ce-3 and bf-2 schematics they seem quite similar at least in the LFO area so that should be a good starting point for the modulation. I'm still a bit confused as to how these two circuits couple the modulation to the clock chip. I'm probably going to show my ignorance here but I guess that changing the c30 cap for increased delay time and leaving the rest of the circuit the same with the "depth" control now acting as the delay time control is the way this works? I'll no longer need the pot and trim set up as used in the ad3208 and other schematics?

[StephenGiles]

You might also want to look at EH delay circuits with chorus, they tend to use a simple (he says!!) clock using a 4047 cmos which is forced to be voltage controllable by an EH diode trick.

[objectx]

Yes, I was wondering if I could just lift the lfo and clock directly from the dmm and use it with the 3205 bbd. Is that possible? The datasheet says the 4047 can run off 3 to 15 volts but the dmm runs off -15 volts so I'd imagine I'd have to do a bit of work to get it working off +9v?