Building the tap tempo tremolo

[bside2234]

Can someone explain the external sync a little? It just hooks up to the tip of a jack? Jack sleeve should be grounded or isolated?

[ElectricDruid]

Yes, it just connects to the tip of a jack, and yes, the sleeve should be grounded. It'll accept any voltage input between about 5V and 15V or more. It works best with nice regular square pulses at the sorts of tempos that humans like, though the range is pretty wide (wider than the tempo knob allows).

HTH,
Tom

[bside2234]

Thanks Tom. I kind of guessed the tip thing but just wasn't sure if the jack had to be isolated from ground or not. Better to ask then to assume.

[Hoffmann]

So what causes that "clicking" sound when effect is in bypass mode? Is it coupled through a ground plane ( obviously not through power line as it clicks in bypass...) , and can it be avoided some other tehniques other than sending PWM to ground( better isolation of the ground planes- better separation of digital and analog circuitry on the board ect.) ?
'

[Taylor]

The PCB has separate ground planes for the sections which are all coupled only at the ground wire. IMHO it can't be decoupled much better short of having entirely separate PCBs. Having built a number of them, and never having an issue with ticking, my best suggestions would be to pay attention to your wiring and lead dress, swap the 330p for a 1n, and get your trims set up correctly. I don't know of any real downside to the alternate bypass on page (3?) if you need to use it.

[9aul]

Hi Taylor,
I have already made your tiny giant amp (which by the way is great! I'll post some pictures soon) and I have now just got the tap trem and the Echo base. Ive ordered and received all my parts and was halfway through making them when I realized I'd bought a surface mount chip version of a 78L05 rather than just the three leg one. Here in England there aren't a great many suppliers of components and delivery charges are always a pain. I was wondering if it would be possible to just wire this chip into the circuit (I know a bit messy, but oh well). The chip has 8 legs; 4 ground legs, 1 V-IN and 1 V-OUT and also 2 NC (im guessing I shouldn't connect these?) Would it be OK to just wire all the grounds together and solder them to the ground and then the IN and OUT as specified on the board?

Thanks for your help.

[Taylor]

Yes, that will work if you can manage to do it. You're right that NC means no connection.

The only caveat would be to make sure the part you have can handle similar amount of current, or at least what the PIC draws (maybe .Mike knows that number without having to scan through the PIC datasheet?)

[.Mike]

The PIC16F684 operating current is measured in microamps. The PIC feeds a maximum of 0.5mA to the transistor buffer. I can't imagine needing more than 5mA apiece for the LEDs. So, 0.5mA + 10mA + <1mA = 11.5mA max.

By the way, you can order 78L05s from Tayda for $0.09 plus shipping, which shouldn't be more than a buck or two. I think there is a $5 minimum order.

Also, don't be afraid to scavenge. You can probably find a 5v regulator in an old radio, VCR, TV, etc.

Good luck!

Mike

[timotheee]

Just finished the tap tempo trem build - works like a champ and sounds excellent! I used the alternate bypass routing listed waaayyy back on page 3 along w/ a 2.2nF cap (in lieu of the 330pf) and no ticking here. Thanks Taylor and everyone involved in this!

[Taylor]

Cool build. 

[Beo]

If you want CV instead of a pot, connecting that way will work, assuming you have a CV source putting out 0v-5v. If you want both CV and pots active simultaneously, you'll need some resistive mixing. The CV source should have a 10k resistor in series, and the pot should have a 10k coming from its wiper. Then connect the two 10ks together and wire them to where the pot's wiper went previously.

Taylor, I have an old optical expression pedal that I'm trying to make work. I have a stereo switching jack wired up such that the wiper switches over from the pot to the CV input when a jack is inserted. 5V and Ground is routed to the expression pedal (tip and sleeve respectively) just as it is to the pot. So, I'm not trying to have both the pot and the expression pedal CV source active simultaneously. I've modified the expression pedal to use an LED instead of a bulb, so current should be minimal. In isolation testing, my expression pedal outputs 0-5V properly, when fed ground on sleeve and 5V on tip. However, when I plug it in, the output is pegged around 2.5V, with very little effect on voltage when I sweep the pedal. Any idea what might be going on? Should I source 5V separately to drive an LED/LDR?

[Taylor]

If you can draw a schematic of how you've done it might help to understand. But, as far as I know there are no optocouplers that work like voltage dividers (two separate CDS cells that change resistance in opposition to each other) so I'm not sure how you're dividing the voltage.

[Beo]

If you can draw a schematic of how you've done it might help to understand. But, as far as I know there are no optocouplers that work like voltage dividers (two separate CDS cells that change resistance in opposition to each other) so I'm not sure how you're dividing the voltage.

In taking photos and drawing up a diagram, I discovered my error. I had the wiper of the pot and the return to the board backwards on my switching jack. I swapped the wires and now my 5V control voltage from my pedal is feeding the tempo input. Two problems now, first of all the sweep isn't very good (I have to get about half way to toe down before the voltage starts climbing). Second, when I heal down, the LDR is very slow dropping back to 0V from 5V. Seem's to take about 3 seconds or more to settle down to ground. Definitely not responsive, compared to twisting the pot back and forth.

Is this an inherent trait of optical expression pedals, or is this just a very old pedal? As I said, I swapped out the bulb for a jury rigged LED, and this thing looks likes it's from the seventies. Maybe it's old tech LDR meant for some other application and a new faster LDR would make a difference. It's one of those shark fin light blocker types.

[Taylor]

The sweep being less than ideal doesn't surprise too much. You could probably play around with some series and parallel resistors on the LDR to get it more ideal - see "the secret life of pots" on geofex if you need some ideas.

LDRs are slow. 3 seconds is pretty extreme, but not entirely surprising. I expect that this would be used as a volume pedal for an organ, where speed might not matter too much for the average family organ. You could try getting a modern LDR with faster response time like the NSL-32 used in this project or the vactec series (datasheets with on and off time are available for both).

[Beo]

Thanks Taylor. I have some used wah shells on the way, which I plan to use for several expression pedal applications, so I think I'll have better solution in the future. A resistive pot will be much more responsive. At one point, I was thinking of reusing an old Morley pedal for the Tap Temp Trem... the footprint of the Morley allows for a nice layout of the additional knobs and would make a great all in one Trem pedal. However, Morley's use optical as well, and I'm not sure if the response is fast enough for these type applications (vibe, trem, phaser speed control).

[isher1992]

What is the 330 pF Capacitor for?  For some reason, I accidentally ordered a 300 pF cap.  Would this impact it tonally?

[Taylor]

That cap has to do with the frequency response in the output gain stage. A slight difference in the cap value will not be noticeable, and many people end up using a significantly larger value there if they have ticking when it's built up.

[isher1992]

That cap has to do with the frequency response in the output gain stage. A slight difference in the cap value will not be noticeable, and many people end up using a significantly larger value there if they have ticking when it's built up.

Thanks, was worried for a minute but went ahead and installed it anyway.

[isher1992]

Well, I had everything installed and built and wired together...went to plug it in to a 9v DC, about 100-200 mA, and had no sound for a few seconds, then next thing I knew, a resistor started to burn and turn black.  I can't remember off the top of my head which one specifically, but just one of them.

I used a TL082 instead of a TL072, could this be the cause of that?

[Gaetano Capuano]

Must have had your diode backwards