how to trigger tap tempo with a clock?

[dblondin]

Hi, first post I'm new to building/modifying pedals and I've found this site immensely useful for my first projects. So thanks!

There's something I'm trying to do and I'm not sure where to start. I have an Empress Tremolo and I'm trying to keep it in time with another device which sends out a 9v clock signal. I'm wishing to use the clock to keep the tremolo in time (instead of using my foot). What type of switching circuit might be best to effectively toggle the tap tempo with a clock?

[earthtonesaudio]

It all depends on the details of the Empress and the clock signal you want to interface together.

You should determine:
-If the Empress behaves normally with a continuous stream of tap inputs (assuming your clock source is continuous)
-What the empress does to turn a button press into a tap signal internally.  Generally the switch is used to provide a logical high voltage in one position and a logical low voltage in the other position, but the only way to know what the Empress does is to trace that portion of the circuit with a voltmeter.
-What the characteristics of the 9V clock source are.  Is it a square wave?  Is it exactly 0-9V amplitude or something in between?  Is the duty cycle 50%?  That sort of info is helpful.

Once you have a good sense of the details you can begin the interfacing.  That part is actually the easier bit and there are many options.  For example, say the Empress tap switch includes a resistor connected to +9V, and pressing the switch brings the end of the resistor to ground, providing a 9->0 transition to the tap tempo circuit.  Further assume your clock source is a 0V/9V square wave.  You can connect a NPN transistor in parallel with the switch, add a resistor from base to clock source, and be done.

[dblondin]

Excellent, thanks so much. Here are the details about the Empress tap and the clock signal:

If I tap the switch along with the pulse of the clock signal, it stays in time all day long.

The Empress tap tempo switch is isolated, latching, SPDT. One of the leads is open, the center is connected to ground and the other I'm getting an alternating measurement of 3.4v or 0v each time the switch is pressed.

The clock is from a Grendel Drone Commander. It's a square with 50% duty cycle. I read that someone measured it ramping from 0 to 8-9V at the lowest tempo, and 0V to only about 1-2V at the fastest tempo setting.

So with that info in mind, what would you suggest? Of course, I'd like the mod to coexist with the original tap tempo switch.

[earthtonesaudio]

That's all good news.  This is what I propose: link

The added parts are an NPN transistor and a 10k resistor.  The emitter shares a ground connection with the Empress tap switch (center), the collector goes to other lead (3.4v or 0v lead) and the base is connected through the resistor to your clock signal.

This scheme will only work when the Tap switch is thrown to the open position side, so for convenience it may be a good idea to add an LED indicator circuit to let you know the Empress is "clock enabled".  Since one of the switch lugs currently connects to nothing this is also easy: link

Finally, I don't know what hardware you may use to connect the clock to the Empress, but if for example you use a 1/4" guitar cable, it would be a good idea to use a normally closed jack plug so that removing the clock cable puts the added circuitry in a definitively "off" mode and makes it immune to noise pickup from wherever.

If you're into synth stuff you probably are quite familiar with normalled jacks, but if not read on...
For instance, if you use a jack like this one:

There are 3 solder tabs.  One is for the 1/4" shield, one is for the 1/4" plug tip, the third is for that extra bit of metal which connects to the tip when the plug is removed.  When you plug in the 1/4" cable to bring the clock signal in, you break that connection and it behaves as a normal mono 1/4" jack.  When the plug is removed, however, it makes that connection and this is what we will use to mute the transistor when the clock is not used.  Simply wire the extra solder lug to ground through a resistor.  The value is not critical, but I would suggest in the neighborhood of 1k.  The reason for the resistor is that inserting the clock plug will temporarily short the Grendel's output to ground, which it may not enjoy.  But you don't want to use a large value resistor because that will make the transistor more susceptible to noise pickup.

[dblondin]

Thanks for your meticulous response - very helpful for a fledgling circuit builder...and someone who is very concerned about damaging some expensive devices!  Okay, this all makes perfect sense and the parts I'll need are on order. I found a 1/8" normalled jack at Mouser that should fit into the crowded tremolo enclosure with little fuss. I'll report back with the final results.

By the way, that applet from Falstad is very cool. I was just looking for such a tool that would work on Mac. It seems like it will be good for my education.

[earthtonesaudio]

If you like Falstad's circuit sim you may also like the analog filter applet:
www.falstad.com/afilter ...a nice quick way to visualize the frequency response of an analog circuit.

You can export/import between this and the circuit simulator too, but it only does linear components (i.e. no diodes).

I have a Mac at home and Falstad is my go-to sim now.  Good luck with the tap tempo mod!  By the way I checked some videos of the Grendel, it looks very fun. 

[dblondin]

I've got it on the breadboard now and it works great, sounds amazing. I don't think I'll need the added LED because it's easy enough to tell if it's working by tweaking the divider control on the tempo - if the tempo changes on the Empress the external clock is doing it's thing. Now I just have to integrate the parts in, that should be easy enough.

The bridge-t filter on the Grendel, I've never heard anything quite like it. Combined with LFO modulation it's just alive. And it doesn't sound to resonant or synthy, it's just very organic and alive. The designer showed me how to install an external audio input so that's next. Combined with something like EH Freeze, things should get real interesting. I wonder if there are any other filters out there of this bridged t sort.

I can't even express how thankful I am for your help on this. I'm actually making progress and nothing has gone up in smoke!

[earthtonesaudio]

Awesome!  Glad to hear it.  I like bridged-T filters too.  There are a few projects that use them in stompbox-land but they don't get as much attention as some of the other filter types.  I actually made a wah a while back that uses one.
I think there's something extra going on in that Grendel, it really does sound quite musical.
Just out of curiosity is there a schematic available for it?  I don't remember seeing one on their website.

[dblondin]

No, it hasn't been made available as far as I know. I certainly would be interested in learning what's going on in there. I hope to make the divider go one division lower, slow it down more. Tweaking the filter even slightly changes the sound quiet a bit so the slower LFO settings are actually the most interesting. And it doesn't go slow enough in my opinion. It's possible to remedy that through CV input but CV replaces LFO1 only. And since LFO2 controls cutoff and resonance differently than LFO1 (as far as I can tell) one can't really get the filter moving around in the same way just via CV. I've tried to no avail but have made some interesting sounds in the process.

Anyway, you should ask Santa for one and probe that filter for ideas!