Ibanez EM5 and DL5 schematic comparison

     After redrawing the Ibanez DL5 schematic, I moved on to the EM5 from the same line. The EM5 has become sought after, and commands hundreds of dollars. The DL5 goes for tens.

    There is an official schematic available for the EM5, but I thought it would be interesting to rearrange it to match my DL5 layout. This highlights differences and similarities. So, here they both are for a-b-ing.

Redrawn EM5 schematic

DL5 schematic

Delay Line

    The biggest visible difference is the delay chip, M50195 vs M65831. The DL5's M50195 uses external RAM and an external comparator, but it functions the same way. Both chips are built around the same adaptive delta modulation converters and 1-bit delay line. The external RAM is larger, at 64kb vs 48kb.

    The input filters got moved around a little, but they're identical 5.8kHz lowpasses. The integrator caps in the modulator/demodulator are also the same. The M65831 datasheet specifies that it needs 30Ω series resistors for some reason.

Clock

    The clock is exactly the same, and validates what we found when investigating that part of the DL5.

    For some reason, the extra gates from the inverter weren't used for the bypass logic this time. Instead, they added transistors set up as inverters. These pedals are a bit unreliable when it comes to switching on/off, so maybe this was an improvement to the circuit.

Filters

    The input stage, preemphasis, and deemphasis stages are all basically the same. The frequencies that are boosted and attenuated haven't changed.

    We already compared in the delay line's input filters, and found that they're the same. We also have output filters though, and they're different. The DL5 cuts at 5.8kHz, matching its input filter. The EM5 cuts lower at 3.9kHz. Immediately after that, there's an additional lowpass formed by R16 and C19. Together these give the EM5 a darker sound, and probably help filter the additional noise that comes from using less memory for the same length of delay.

    The EM5's output filter also has soft-clipping diodes, presumably to prevent runaway echos that get louder and louder. There's more gain in the feedback loop (lower R17 value) to compensate for this lower output. Strangely, R20 has been more than doubled, limiting the maximum volume of the repeats.

In Closing

    The two pedals don't have any major differences that explain the 10x price gap, at least not to my satisfaction. I suspect people happen to like the darker sound, and maybe the clipping of the EM5. It should be trivial to make the DL5 sound the same though. Since it's built on the same tech, I believe a humble PT2399 delay could also sound the same.

Ibanez DL5 schematic

    

     I've been looking at various delays, digital and analog. I came across the Ibanez DL5, from their "Soundtank" line. There is a reverse-engineered schematic, but it just didn't make sense to me.

Tic Toc, Ya Don't Start

    The thing that I was most interested in was the clock source for the delay chip. It vaguely looks like an oscillator, but not really. There are two inverting stages made from IC6. You need an odd number of inversions to make an astable oscillator though. Also, what is TR6 doing? It has two fixed voltages going into it, and a current flowing into C29?

Bad clock schematic

    I wanted to validate this, so I opened up my pedal, took some pictures, and started tracing.

Traced PCB

    It turns out that TR6 was labeled incorrectly, and it's actually TR3. R28 is actually R26. R24 and R25 don't go to different voltages; they both connect to what is probably +5V.

    The key mistake was the omission of R23. This connects TR6(TR3) to the second inverter, and totally changes what's going on.

    Here it is redrawn. TR5 helps to bias the CV going into TR4. TR4 acts as an exponential current sink, pulling current out of C29. The inverters monitor the capacitor voltage at pin 1, and use TR3 to recharge the capacitor when it gets low.

Corrected clock schematic

    I made a version of it in Falstad, so you can experiment with it. Link here

Falstad simulation

    It's actually very similar to the Monotron's VCO that we looked at previously. 

Monotron VCO

I Can't Be Your Lover

    The number of mistakes in this one area calls into question the whole schematic. What else can we find wrong?

Incorrect schematic

    The "Ref" pin (24) of the M50195 delay chip is incorrectly labeled "OP1+", making it look like it's an input for one of the internal opamps. It's acutally a reference voltage that's used by the opamps.

    The opamp on the delay's output (IC5B) is connected backwards. What should be its output is instead its input.

    Long story short, I decided to redraw the schematic. This let me make some more conventional layout choices too.

Redrawn schematic

    I skipped a few things that didn't interest me, like the LED, some power filtering, and jacks.

    Now we can see what all the blocks of the circuit are, and how the signal flows through them.

    At the top left we have an input buffer, and a preemphasis filter that feeds into the delay line. We also have a deemphasis filter that mixes the dry and wet signal together.

    Below that is a lowpass filter that cleans up the wet delay signal. After that is a FET that bypasses the effect. Then comes the repeat control that mixes the signal back into the delay line.

Input, filtering, and mixing

    At the bottom left is the delay line itself. It has a similar lowpass filter that's built using one of the M50195's internal opamps.

Delay line and RAM

    Bottom right is the latch that controls the FET. Pressing the tact switch toggles the state of the latch, and the FET, turning the effect on or off.

Bypass Latch

Bonus

    Here are the traced and redrawn boards. It's not a faithful recreation, but good enough to make a schematic from. The 70mil-spaced 36 pin DIP was "fun" to recreate.

Koogo Analog Delay schematic (Donner Yellow Fall)

Update: This appears to be a clone of the PT-80 Delay by Scott Swartz from 2002. The Donner Yellow Fall came out around 2014. In Scott's notes, he mentions taking the circuit of a Boss DM-2 and inserting the PT2399.

    I've been experimenting with controlling analog effects pedals, like this chorus. I bought a cheap "Koogo Analog Delay" to try to repeat my success. Turns out that it's not analog at all. It's the same digital PT2399 chip that seems to be in every delay. It does have a leg up by adding a sa571 compander though. Turns out this is the same design as the Donner Yellow Fall pedal; It's just been rebranded.

    I decided to draw a schematic in case there was anything to learn from it. I didn't add any capacitor values since they're not marked, and would have to be desoldered and measured. I did put in a few speculative values though.

    The PT2399 lives on a daughterboard and follows the datasheet design pretty closely. So closely that I just copied and pasted the last schematic I made from a PT2399 board.

PT2399 daughterboard

    

    The main board isn't too interesting either. It uses variations on the compressor and expander from the SA571 datasheet.

    

    An extra lowpass filter has been added before the expander. This is probably to help knock down some of the aliasing noise that would throw off the envelope detector of the SA571.

    A bypass FET has been put in the feedback loop so that you'll still get "tails" when turning off the circuit. This means that the delayed signal won't be immediately cut off; The last repeat will ring out, but no additional repeats will be created.

    The input buffer adds pre-emphasis, boosting the high frequencies. The summing amp is the complement to it, doing de-emphasis duty. These cancel each other out, and have no real effect on the dry signal. The benefit is to the signal-to-noise ratio of the delay line. By de-emphasizing (low passing) the output of the delay line, the high frequency noise can be greatly reduced.

Mainboard schematic

    I think these additions were inspired by actual analog pedals, despite the delay line being digital. You can see the pre/de-emphasis, lowpass filter, compander, and feedback bypass in the Boss DM-3

Boss DM-3

PWM Chorus Control

    I'm a fan of synths that have analog choruses built in. The Juno 106 and Poly 800 are two popular examples. Strangely, neither gives you proper rate or depth controls for the chorus. I thought it would be interesting to not just include these controls, but to store them in a synth's patches. This means that the CPU needs a way to interact with the chorus circuit.

    In the modern day, we have a number of options for adding digital control to an analog circuit. A DAC can bridge the gap if the circuit is already voltage controlled. A digipot can do the same if it's resistance controlled. A less common, and more interesting approach, is to pulse width modulate(PWM) a CMOS switch. We already looked at Pearl's use of this in their Drum-X.

Switches as resistors

    A CMOS switch can act somewhat like a variable resistor when turned on and off very quickly. The more time it's on (relative to off) the lower the effective resistance. That's all PWM is: a change of the on-time relative to the off-time (duty cycle). Two of these variable resistances can be put together and used like a potentiometer. The trick is to increase the resistance of one, while decreasing the other. The idea being that the total resistance should stay roughly the same, and only the ratio of resistances changes.

PWM

    The nice thing about pulse width control is that it's easy for most microcontrollers to generate. Virtually all of them have timers with special facilities to output a variable pulse width signal. At the same time, it's quite easy to generate a PWM signal in an all analog circuit. You can simply compare a CV source to a saw carrier wave. The output will be a pulse wave where the width is proportional to the CV amplitude.

PWM diagram from PCBhaven

Chorus circuit

    I'm working with a clone of the Ibanez CS9 chorus pedal, branded as a KMISE Classic Chorus. The pedal generates a triangle wave that it then uses to modulate the clock of a delay line. The rate control adjusts the frequency of this triangle wave LFO, while the depth adjusts the amplitude. Here's the corresponding portion of a very similar chorus circuit:

LFO with rate and depth potentiometers

Switches as potentiometers

    Notice that both the rate and depth are controlled with potentiometers set up as voltage dividers. This means we need to fake four separate resistances and maintain the ratios we discussed before. We can make this much simpler by using double-throw CMOS switches, like the ones in the CD4053 IC.

    Each throw is provided by its own switch internally, but the two are set up to have opposite states: one is open while the other is closed. So, they will also maintain opposite duty cycles when we PWM the pair. If one is closed 25% of the time, the other will be closed 75%, always "adding up" to 100%. This equates to resistance, meaning they will always maintain the same total resistance between them. This makes it easy to use the two throw switches together as one potentiometer.

Rate control

    Let's apply this to the rate knob. You can think of one throw as giving the maximum rate, and the other giving the minimum rate. You can see a small hitch when the PWM signal is low. This is the time spent at the minimum speed. If we do this very quicky, the hitches become negligible, and the rate averages out based on the ratio of time spent at the two extremes.

 

Depth control

    Now we'll look at the depth knob. If we use the same trick, we don't get the same tiny hitches; We get deep bites taken out of our signal.

slow carrier PWM

    This is exaggerated by the slow PWM carrier frequency used to illustrate the result. In practice we use a much faster carrier, and the results are less dramatic. Bites are still take out, but they're much narrower, and more rapid. More rapid means they're high(er) frequency noise, and we can lowpass them away. Notice that the schematic already has a lowpass filter on the output of the depth knob: R34 & C20. If we probe after this lowpass, we see (in yellow below) a much more normal triangle with a lower amplitude based on the PWM duty cycle. You can think of the low pass as having stolen some of the peaks to fill in the valleys.

fast carrier PWM with filtering

 

Hardware

    The ubiquitous Arduino can give us the two PWM signals. The analogWrite() function can be used to generate 980Hz PWM signals on pins 5 & 6. This is quite slow in the audio world, but still fast enough relative to our slow LFO, and lowpass filter. So, if you use an Arduino as the brain for your synth, it would be able to store and recall your chorus settings. It would only need a humble CD4053 to actually control the chorus. No passives required.

    The 4053 includes three double-pole switches, meaning we have one free after controlling rate and depth. This last one can be used to switch the effect on and off. It could be PWM'd to mix the wet and dry signal, but it would require a very high carrier frequency to allow the full audio spectrum to pass through unaffected. Note that some choruses already include a way to turn the effect on/off via a digital signal. This clone happens to use "true bypass" instead.

 

Other ideas

    Instead of controlling the LFO, we could have replaced it entirely. This might be a good option if your micro is well equipped to generate its own LFO output. This also opens up the possibility of new waveforms.

    Or we could have generated CV and converted it into PWM instead of generating the PWM signal directly. This would make more sense for a micro that can't generate any (additional) PWM outputs, or is already set up to generate a large number of CV signals. Many synths multiplex one DAC to multiple outputs, and it would be easy to tack on two more for the chorus.

Ibanez FL5 Flanger Schematic, PCB and Mod

If you look around, a lot of pedal sites describe the FL5 as a cutdown FL9. Strangely, no one seems to have a clean schematic for the FL9 or any schematic at all for the FL5.

Working off of the blurry FL9 schematic and an FL5 board, I drew up my own.

For the most part, everything is the same, including component values. The nice, matched transistors that drive the MN3102 clock have been replaced with regular, discrete transistors. "R11" and "T3" seems to be overlooked and just aren't present. The bypass circuit is a little different, but is best replaced with true bypass anyway. The biggest difference is the missing "Delay Time" pot from the FL9. I've added this back in as a daughterboard.

For simplicity's sake, I've chosen to leave out the 1/4" jacks, 9V clip and LED. Here's the result.



Eagle files are on my GitHub.