Original handbuilt guitar effects pedals. Manchester, UK.
All circuits copyright Wraa Defects, 2018.

ISD1420 LoFi Looper / Sample Rate Reducer

February 3, 2019

Demo of my Lo Fi Looper



There are a few LoFi loopers around using the old ISD series answering machine chips, notably the Z_VEX LoFi Loop Junky and the Tone God's Payback.


Like any curious circuit bender, the first thing I tried was reducing the 5v to the ISD chip. This lowers the pitch & tempo of the loop being played back, but if you push it far enough the sample rate drops to audible frequencies and you get sweet-to-vapourised bitcrushed sounds. Too much resistance (~1k5) causes the chip to fail, but thankfully the first ~0.5k ohms of resitance does nothing to your signal, so I set this amount with a trimpot as a limiting resistor in series with the 1k Downsample pot. Note that you can only record when the Downsample pot is turned counter clockwise.


I assume that the Z-VEX pedal alternates the amount of power going to the chip to create the vibrato effect. How Mr Vex achieved the depth knob, I'm not sure.


You may be infuriated by my seemingly superfluous use of op-amps, but I found the LM386 to be a superior pre-amp for the ISD chip. I'm all in for including more chips if they're better for the job (as my other sprawling designs will attest).


The ISD requires a specific series of HIGH & LOW logic inputs at particular terminals to set it up as a looper. This is described in the datasheet,




My schematic




 (I'm currently working on a much improved version of this design, so I'd recommend waiting a week or two if you fancy building it! It will be simpler with lower parts count and have various other improvements/fixes! I will also make and sell PCBs for it, for a more reliable build.)




The three footswitches are:


Right - Record. Latching switch.. Pressing a second time stops recording.


Centre - Play. Momentary switch. The idea was that pressing this at the same time as turning off the record switch would set the loop playing immediately after recording, but it doesn't work. I'm fairly convinced that using a latching switch here would allow this to happen. The Payback uses a clever little logic circuit to begin playback as soon as you stop recording.


Left - Play trigger. This is pretty cool, lets you play the loop for as long as you keep your foot on the switch, making for fun bursts of sample. The loud click lets it down though. The Tone God mentioned that the clicking could be overcome with a transformer but I've yet to figure out exactly how. Tapping and releasing this switch also functions as the stop playback switch.


The EQ knob sets a pretty radical filter. As the ISD kills most of your high end, the EQ's high pass setting is incredibly tinny to compensate. Also, it's a LoFi Looper, it needs tinniness! (The EQ sounded a bit different on the breadboard and needs slightly reworking to balance the centre setting, but my stripboard is caked in duct tape so it will have to wait.)


The LED setup works pretty well on my design, with a tri-colour (RGB) common annode LED wired to turn red while recording and flash green at the start of every loop cycle. A limiting resistor (~3k) connects the annode to +9v. The Red cathode is connected to ground via a pole of the Record 3PDT footswitch. The Green cathode is sent to pin 25 of the ISD as shown on the stripboard layout. The Blue cathode is unused. The power LED dims slightly as the Downsample is turned clockwise.


I didn't think of adding an effects loop to the loop playback, as the Tone God did. This could be a lot of fun. I also like the idea of an input volume, allowing the signal to get clipped and distorted while recording.


The "save" mode on the Z-VEX & Tone God designs has the exact same function as the Record footswitch. The key difference being you can tell which setting it's in so you don't accidentally record over a stored loop, which is easily done if the Rec ftsw is down when you plug in the pedal. Add one with an ON-OFF in series with the Rec ftsw.



The ISD chip gets called 'LoFi' for a reason. It's not going to satisfy tonal puritans and is best for bedroom use, but the sample rate reducer aspect makes it something else. It's great that you can knock up a loop pedal for not very many pennies, but it's not going to contend with your Ditto or DL4.

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