Cracking Open The Behringer JT-4000 Micro

[@francescomanentihq]

Any ideas about the bottom left pins? Are they just for debug/serial port? Maybe there is room for mods and hacks if there are some labels on the back side of the pcb. Otherwise a multimeter can be useful to find out more about them

[@3dims]

I'm afraid that side of things is a little beyond me. I can crack open the case and have a look again if it helps?

[@SpeccyMan]

That display is NOT LCD. It is an OLED! The firmware will be in the microcontroller. The EEPROM will almost certainly be patch memory. Never assume anything.

[@3dims]

Of course it’s an OLED! Thanks for the spot.

[@realmchat6665]

Nice video, been wanting to see what's in there making all those awesome sounds.

[@3dims]

Thanks. Glad you approve!

[@Wulfhard1965]

Now at which spot to solder an audio input for the analog filter?

[@3dims]

Beats me!

[@fiachrahackett]

This is really cool, I would love to see someone (who has the knowledge and skills) to circuit bend and mod this a bit. a pitch shift would be extra fun but proper midi support would be great too. The additions of the firmware updates have been excellent so perhaps there is still hope for further firmware support and updates, a way to un latching the parameter knobs to 1/4 turns would be ideal. I'm really enjoying mine as my entry point to hardware synths (vintage casio keyboards not withstanding) . I've paired it with a sonicake sonic ambience reverb+delay pedal to get some really dreamy sounds. Looking forward to more of your videos on this topic :)

[@3dims]

Thanks, and totally agree some circuit bending would be ace. The tiny surface mounted soldering means that’s beyond my basic skills in this department, but perhaps someone will take up the challenge. As a bargain intro to hardware synths I think you’ve chosen well, I’m still discovering more about the JT-4000 Micro, so will share what I can when new, cool things occur to me.

[@fiachrahackett]

@@3dims absolutely I don't really even know what's possible, but I've seen people modify all sorts of devices, but I haven't got a clue myself. haha

[@rabmccudden683]

They’re just bringing out a midi version now, they posted a video a few days ago. I’d literally just bought 1 & half an hour later I noticed the new midi version 😅

[@jacobrosen]

That mcu is most provably an stm32g431KB or maybe K8, and seing as those has either 64k or 128k flash build in i assume the external eeprom is used for parameter storage or something. But it is a bit weird that they use an external eeprom imstead of using an mcu with bigger flash and store it internally

[@3dims]

Thanks for sharing. It’s a bit difficult to know for sure, but I think you probably are right.

[@SianaGearz]

So the processor is an STM32L4. The EEPROM Chip, i can't see it, but is it a 24c series? I say user setting data. The reason is that STM32 contains program storage flash, and will indeed not execute software from external memory, and it's generally got a whole oodle of it, depending on which variety it is. But the flash in there isn't made to be constantly rewritten, it can fail, and you can't just change a byte, you need to clear a page every time you change something, and you need a ton of measures to manage and stave off failure. So a common design trait is to use a non-page EEPROM, an i2c-connected 24c series, to store the settings, and that's good for approximately a lifetime of bytewise writes, and if it goes bad, you can still swap it out, you can even swap it with Fujitsu FRAM which is basically immortal. These EEPROM are slow for bulk loading, they're best used to load and store individual bytes or small byte sequences. In contrast if you want to store and load bulk data, you use a 25c or 25q series SPI flash chips. The TI AIWI chip is actually TPA6132A2 stereo headphone amplifier. A popular alternative that is used by just about everyone else is MAX97220, feistier, usually comes out cheaper (except during pandemic), that one is usually marked AIG or AIF, good to remember for the next teardown, you're going to find them. I know, maddening, you just have to have an idea who made the chip (easy with this TI, not so easy with most chips) and then use the manufacturer's topcode/topmark search, except the websites are broken a lot of the time, and then verify from the datasheet, it generally has this data as well, except when it doesn't. I happen to have memorised some of these. The 4580 is a classic opamp that one might use in a filter. Won't make a call whether that's it without probing the circuit. Honestly if i was Behringer i would just cheat and not have an analogue filter at all, just do it digital on the L4, nobody takes the product claims literally anyway right? The display is an OLED. These small blue ones are basically the cheapest text/matrix display that you can get today, they cost like half a dollar i think, cheaper than an LCD.

[@SianaGearz]

The suggestion that the Holtek HT16... chip is an LCD driver... well yeah there is a 1621 but that would be weird, given it's the wrong kind of driver, it's a segment LCD driver, not a text/graphics one. I'd like to suggest that this is instead the HT16K33, a keypad scanner and LED driver with i2c. This way they can use a cheaper low pin count microcontroller (processor) IC, and then multiplex the i2c EEPROM, the i2c SSD1306 display, and all the LEDs (those in the push button switches) and the switches themselves plus possibly a rotary push encoder all on a single i2c bus.

[@3dims]

Thanks for your input, it’s really fascinating to see what others know and can unpick from just a video. My knowledge is limited but is growing through useful contributions like this one. Thanks again.

[@3dims]

Thanks for the correction. Every day is a school day. Cheers!

[@SianaGearz]

@@3dims I come from friends of Woody's Piano Shack, i'm their resident electrical engineer basically. They were looking at your vid. Found those guys when i ran into Floyd Steinberg's somewhat painful attempt to repair an Alesis Micron a few years ago. We ended up fixing it successfully.

[@3dims]

@@SianaGearz that's so cool and really happy to have you most excellent input.

[@koncreteto2758]

Scary how close the screen ribbon is to the pot wonder if it would pose a risk to being worn down by friction over time

[@3dims]

Good spot… I’ve just opened up the JT-4000 and tucked the ribbon cable away from the moving pot. I suspect the ribbon cable got free-d up with me messing…

[@BeepsAndBeats]

I think I might try putting a lipo battery in this

[@3dims]

That’s an interesting idea. Let us know if you do.

[@infindebula]

While one or both of the ICs you mentioned as candidates are likely part of the filter, there should be absolutely no reason for it to be anywhere near the cutoff and resonance controls. Those controls should be connected to the processor, so the only thing the filter needs to be close to is the DAC - for controlling filter with voltages, and also to generate audio to be filtered.

[@3dims]

Thanks for the info. Really helpful insight. Cheers!

[@SianaGearz]

Why? If it's a classic analogue filter, then the pots specifically wouldn't be connected to the processor at all, the signal or opamp feedback just runs directly through the pots. Trying to make the filter processor controlled introduces a bunch of complexity not really desired in a toy-like synth, since you basically end up needing at least a shitty analogue multiplier plus more effective DAC channels, you can of course use the microcontroller outputs but end up needing to condition that signal, and you're quite space constrained in a device like that. At that point i'd be tempted to just completely get rid of the analogue filter and just implement a digital one, there is a DSP and FPU in the microcontroller.

[@infindebula]

@@SianaGearz are the filter settings programmable, and/or modulatable by the processor-controlled envelopes, LFO, or key tracking?

[@SianaGearz]

@@infindebula Good call. There IS a connection to the microcontroller that i missed, the filter knobs are also used to adjust settings. They also appear to be single track potentiometers. The cutoff and resonance are indeed completely automated, they have MIDI direct control, and LFO and envelope generator drive the cutoff frequency. So you're completely correct in your original assessment.

[@infindebula]

@@SianaGearz I’ll even go a step further… people bitch and moan when they see modern clones of analog monosynths - they say “why didn’t you make it programmable?” or “why didn’t you make the knobs respond to MIDI?” There is an inherent disadvantage to doing that, which we’ve just touched on. It makes every panel control subject to the resolution and latency introduced by the digital circuitry. Then some would counter “well you won’t hear stepping with 16-bit values” which is theoretically true, but the inherent noise in the whole system generally negates resolutions better than about 10 bits. Unless of course you attempt to dIgitally de-noise, which adds latency. For me, there is an unquantifiable connection between the player and synth when the analog controls (and keyboard) are actually in-circuit. Digital controls open up a whole new level of versatility at the cost of … well as I said it’s unquantifiable, but there is something there!

[@majobecher7835]

AIWI=TPA6132A2=HPamp, DAC most likely inside STM32 as cheep as it gets..., RC4580=OpAmp=Filter?

[@3dims]

Superb. Thanks for chipping in.

[@straighttalk2069]

The single filter is the reason I'm selling mine and my Pro VS mini. I would've paid the extra had they included the three extra filters required for true polyphony which these are not. Instead we're left with a weird sort of paraphony.

[@3dims]

The single filter doesn't bother me TBH. Also, I'm not sure where Behringer would fit another 2 analogue filters into this miniscule box.

[@straighttalk2069]

@@3dims Apart from the JT-4000 and Pro VS mini, which are without a doubt toys, all of my hardware synths are fully polyphonic. My only paraphonic synth is my Waldorf Pulse 2 and even that clearly states that it's a Monophonic synth. Behringer could easily have made the JT-4000 large enough for four analog filters. Look at the Pro 800 eight voice synth with eight analog filters for £300 and not a large synth. That works out to £37.50 per analog voice and analog filter so £150 for a completly analog and fully polyphonic version of the JT-4000. I would've bought a Pro 800 if I didn't already own a DSI tetra and a Rev2 16 voice.

[@3dims]

@@straighttalk2069I have a Pro 800 and it’s a class synth, but totally different form factor to the JT-4000.

[@koncreteto2758]

Quit moaning and let us enjoy them 😂 ​@@straighttalk2069

[@Wulfhard1965]

@@straighttalk2069 The Korg Poly-800 also had only one filter, and it became a top seller not only for his price point. No one would call that a "toy", just because it was only paraphonic.

[@OMNI_INFINITY]

*Hmmm...is it BPSynth modified?*

[@3dims]

Similar platform, so maybe inspired by BPSynth….???