Building a Roland Juno-60 clone Audio Chorus processor and could use some help - Page 1
Forum section: electronics / projects

author: Midphase
author_rank: Regular Contributor
timestamp: January 17, 2025, 07:50:09 pm
content: Howdy, sorry for the noob questions. I have come across a DIY project to recreate the fabled Chorus module from the Roland JUNO 60 and I have some questions and curiosities that I'm hoping some of you might be able to shed some light on.My first question concerns power. The schematic calls for 15V AC, but this doesn't make sense to me as the original synthesizer would have been outfitted with a transformer and power supply circuit to convert all of the internal power to DC power rails. Does this schematic effectively include a rectifier/power supply conversion section? But if so, wouldn't make more sense to just feed it a 15v DC supply from a wall wart in the first place? If that's what I wanted to do, would it make sense for me to bypass the D1 and D2 diodes and C1 and C2 caps and go straight into LM7815 and LM7915 regulators? (sorry if this sounds like an inane idea...just trying to understand this stuff one step at at a time).Let's start from there, and then I'll follow up with more questions. Many thanks in advance for your help and knowledge, this is a fantastic forum!Here are the relevant links and files:https://github.com/gligli/juno-chorus-clone

author: moffy
author_rank: Super Contributor
timestamp: January 17, 2025, 10:38:14 pm
content: 1. 15VAC: The schematic has D1 and D2 which do the recification and C1 and C2 which do the initial filtering and storage. It needs the AC because it has +/-15V supplies. If you want to feed in external DC power you would need three pins and at least +/-18V supplies(min) with GND to feed into the +/-15V regulators. The extra voltage is because the regulators need a minimum of about 3V across them to regulate.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 17, 2025, 11:29:51 pm
content: Yes, it does the rectifying. It's a dual half-wave rectifier. Which is why it requires relatively large capacitors relative to the current draw of this circuit.What's the point? To get a dual supply from a single winding transformer. If you want to use a DC input supply you'll need a dual DC supply.

author: Midphase
author_rank: Regular Contributor
timestamp: January 17, 2025, 11:33:03 pm
content: Got it, ok that makes sense. All that I can find for power is a 16v AC to AC supply, do you think that would be ok to feed into it as opposed to 15v exactly? I assume the regulators can deal with an extra volt coming in?This is what I had in mind:https://www.amazon.com/Adapter-Linear-Transformer-1000mA-Straight/dp/B00B8865BG/ref=sr_1_4?crid=1YF10UFUVAGWG&dib=eyJ2IjoiMSJ9.Jv6w52AdSaKyG2ZjZQcTB-UidrICbg_yj5eAw_jOlUFHDOcOEElKngvU1IUnshfEMEFSLQsmtt0SbXHQqiXrt1cSmaJnQj86pFwrMs4111qWLm2HLIOa3vpMwdjkbQCUwgspIkqFqw_i3nuezP-yvGsS7rAgQTkUJ6qUv-LrhGqaTx_zVq8A2UKJ8BYH3r_IBWgmlw82wAXR4ee_GJWp9NVxg1gNRQuFr6xC3qc4y8A.iZiFFikVcwmOn9XOqXbX484lJ6-mkDIRIZXDnfb70t0&dib_tag=se&keywords=power+supply+ac+to+ac&qid=1737155370&sprefix=power+supply+ac+to+ac%2Caps%2C176&sr=8-4Otherwise, I was thinking of using a switching power supply like a Mean Well, but in that case would I even need to go through the rectifier/electrolytic caps circuit at all since I'd be feeding the circuit clean +15v and -15v right out of the supply?I was thinking something like this:https://www.mouser.com/ProductDetail/MEAN-WELL/PD-2515?qs=V9a8iPeg90wj6pKOH0Z1PQ%3D%3D

author: Analog Kid
author_rank: Super Contributor
timestamp: January 17, 2025, 11:40:59 pm
content: 16 VAC vs. 15 VAC = no never mind; no harm, no foul.(But you might want to measure that transformer's open-circuit voltage just to be sure it actually is about 16 volts and not, say, 24 volts.)

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 18, 2025, 12:52:46 am
content: So yes, you can replace this with a dual power supply that outputs DC. But I would keep the linear regulators, as those power supplies like Meanwell are pretty noisy. Meaning that you'll need to keep the input voltage at least 2.5V above the output voltage of these regulators (the 78/7915 have a dropout of around 2.5V). So you'd need +/-17.5V min. I didn't look at the schematic in details though, but I'm sure you can go for +/-12V instead of the +/15V (so you could swap for 78/7912 instead) and thus use one of these Meanwell which output +/-15V DC.If you go for the original approach, the 78/7915 can handle up to 35V so there is very little chance a 15V AC transformer would lead to > 35V once half-rectified. I would have a closer look at the input capacitors - you may want to go for 35V ones instead of 25V, to be on the safe side and not operate them close to their limit.

author: Midphase
author_rank: Regular Contributor
timestamp: January 18, 2025, 02:04:01 am
content: Great info. Thank you all.I will have some more questions once I complete this build that hopefully you can shed some light on.Thanks!

author: AussieBruce
author_rank: Regular Contributor
timestamp: January 18, 2025, 03:09:14 am
content: Hi, Just one more possibility that it will pay you to be aware of, sometimes arrangements like the double half-wave rectification in your schematic can suffer from ‘latchup’, where one half of the setup starting on the first half cycle of the AC input forces the other half of the circuit into a state where it locks up. What you usually see is one of the regulators not delivering anything. This condition can be easily fixed by adding some reverse protection diodes, there are plenty of people here who can provide you with the details. So it’s nothing to be concerned about for now, but if you’re designing your own layout you might want to leave a bit of spare room around the power supply.Latchup can be intermittent, you may not see if for a while.Nice project, you'll enjoy it (and the result).

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 18, 2025, 08:52:21 am
content: To improve on the power supply you might want to add some small decoupling capacitors before and after the voltage regulators. Typical values are 330nF before and 100nF after. Place them as close as possible to the regulators.

author: Midphase
author_rank: Regular Contributor
timestamp: January 18, 2025, 07:30:11 pm
content: All of this new info is leading me to thing that perhaps using the Mean Well switched power supply with the +-15v output might be the wiser choice. The price is basically the same.Regarding the decoupling caps, that's a good idea. I think the original designer of this board was trying to stay as true to the original design as possible, despite it having some potential flaws.

author: moffy
author_rank: Super Contributor
timestamp: January 18, 2025, 09:34:48 pm
content: Quote from: Midphase on January 18, 2025, 07:30:11 pmMean Well make pretty good switching supplies, but they produce significant switching noise which can be audible, and hard to filter, the 16V AC adapter would be a lower noise choice.

author: helius
author_rank: Super Contributor
timestamp: January 18, 2025, 10:06:19 pm
content: I thought "chorus" effect as seen on Solina String Ensemble was a mix of multiple bucket-brigade delays?

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 18, 2025, 10:57:49 pm
content: Quote from: helius on January 18, 2025, 10:06:19 pmI haven't looked closely at the schematic above, but a quick view made it hard to see how this would create a "chorus"-like effect, I'm curious to analyze this circuit more closely.Yes, a chorus effect is a modulated delay and thus requires delays in one form or another (can be 100% digital or "hybrid", like the "bucket-brigade" approach). Not saw where the delay is in this above circuit, but again just had a very quick look, so I'm curious.

author: themadhippy
author_rank: Super Contributor
timestamp: January 18, 2025, 11:24:39 pm
content: theres a bucket full of delay around u5,6 ,and 7

author: Midphase
author_rank: Regular Contributor
timestamp: January 18, 2025, 11:43:50 pm
content: Quote from: helius on January 18, 2025, 10:06:19 pmYes, this circuit uses BBD IC's. It's a similar Chorus design to the one used in the Akai AX60 synths. It uses two MN3009 which are 256 stage BBD IC's in combination with an MN3101 which is a clock driver for said IC's. CabinTech has the IC replicas which should be a drop-in replacement for the original ones (I'm hoping).

author: Midphase
author_rank: Regular Contributor
timestamp: January 18, 2025, 11:51:05 pm
content: One of the things I haven't figured out yet is exactly what the switch is supposed to control? I assume it switches between Chorus 1 and Chorus 2 settings, but on the original Juno 60 it was also possible to press both buttons together to achieve a 3rd option. Not sure if this is able to do that, or how I would go about achieving that effect.In addition, this circuit adds Rate and a Depth adjustment potentiometers that the original Juno didn't have. There are also two trimmers that I assume are for tweaking the rate and depth settings more finely...but I'm not 100% sure.My plan for this build is to:1. Build it exactly how it currently is and test it out.2. See if there is an option to improve the design (as some have already suggested above such as decoupling caps and safety diodes).3. Correct and clean up the schematics, as well as provide a comprehensive BOM and instructions so that I can share it with whoever might want to tackle this project in the future.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 18, 2025, 11:53:48 pm
content: Quote from: themadhippy on January 18, 2025, 11:24:39 pmAh ok, told you I only had a very quick look and to be fair, the schematic is not the cleanest I have seen - it wasn't immediately obvious there were any ICs in there apart from opamps due to the mess of wires and the odd symbols they used.So, these were the BBDs:https://xvive.com/audio/product/mn3009/

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 19, 2025, 07:29:37 am
content: Quote from: SiliconWizard on January 18, 2025, 11:53:48 pmYeah, you have to zoom in and notice those rectangles with four connections on each side to deduce that there are some 8 pin ic's used in this thing. With my older eyes I thought the marking was NN3009, which resulted in some ball bearings.Needed a closer look to see it says MN3009.There are also some wires missing, one between R25 and U6 and another between R26 and U8.If I wanted to build some audio effect like this, I would get myself a STM32 MCU that has two proper I2S interfaces with it's own PLL, like the STM32F411 for instance, and a PCM1808 module for the audio input and a PCM5102 module for the output and do it all in software. To simulate the analog properties just generate some noise and add it to the mix.This way you can create much more effects than just the simple chorus.Quote from: Midphase on January 18, 2025, 11:51:05 pmThe button in the provided schematic switches between effect on and off. It controls the two 2SK30 FET's that are in series with the output of the BBD's.I guess that the potentiometers provide control over the type of effect. RV1 is for modulation speed, RV2 for modulation depth, and RV3 and RV4 I'm not sure about. Either some kind of feedback or amplitude modulation. Would need a closer investigation of the circuit and reading the datasheets of the MN3009 and MN3101.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 19, 2025, 09:19:35 am
content: Quote from: pcprogrammer on January 19, 2025, 07:29:37 amYes, but those vintage delay chips have actually made a comeback especially in guitar pedals and many appreciate the kind of sound you can get out of them, so... and one "benefit" is that there is zero software involved. For some, that's a benefit. The chip that is most used currently in guitar pedals that are marketed as "analog delays/chorus" is the PT2399. Very popular.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 19, 2025, 10:24:03 am
content: I'm not sure if it still works, but I have a very old BBD based effect. Was a project in the Elektuur (elektor) way back in the maybe late eighties.It is on my long list of projects to make a digital version of it.Some pictures of the device. It uses two SAD1024A BBD's. The second board is a function generator, also from the Elektuur, based on a XR-2207 waveform generator.

author: themadhippy
author_rank: Super Contributor
timestamp: January 19, 2025, 01:17:37 pm
content: Code:If I wanted to build some audio effect like this, I would get myself a STM32 MCU that has two proper I2S interfaces with it's own PLL, like the STM32F411 for instance, and a PCM1808 module for the audio input and a PCM5102 module for the output and do it all in software. To simulate the analog properties just generate some noise and add it to the mix.and wot sort of latency are yer talking about? the ear n brain get a bit confused if things get too far outa wack

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 19, 2025, 01:42:36 pm
content: Quote from: themadhippy on January 19, 2025, 01:17:37 pmFor direct input to output very low, like a single sample.There is no need to create a buffer in between for the pass through, just for the delay you need to buffer them. Mixing the two streams together is peanuts as long as you are not doing digital filtering on the signals.

author: Bud
author_rank: Super Contributor
timestamp: January 19, 2025, 04:11:13 pm
content: Quote from: Midphase on January 18, 2025, 11:51:05 pmThat would be appreciated. Most of the time people make promises to only disappesr later and never follow thru.

author: Midphase
author_rank: Regular Contributor
timestamp: January 19, 2025, 05:37:10 pm
content: This thread has been incredibly informative so far and I'm deeply appreciative.One of the things that I learned was that in order to get positive and negative voltages into the circuit I need to start out with AC voltage. However, I do have a couple of other musical devices which I know require positive and negative rails, one of them is a Eurorack case made by MOOG, and the other is a development board made by Erica Synths called Labor. Both of those devices are fed from a wall-wart style power supply which is feeding DC into the device.So my question is, how do those devices achieve positive and negative rails if they are being fed a DC supply?Some links:https://www.moogmusic.com/products/powered-eurorack-casehttps://www.ericasynths.lv/shop/diy-kits-1/edu-diy-labor/

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 19, 2025, 06:18:55 pm
content: There are several ways to make it work. One can use a boots converter to make the negative voltage or a resistive voltage divider with a buffer for the ground rail. Another option is to just use a single rail and use a capacitor in the output to get rid of the DC component.Some info here:https://www.ti.com/document-viewer/lit/html/SSZTAI6With digital devices you can get mostly by with just 5V or even 3.3V and lower. Modern electronics has changed the playing field. Think back to the first home computers where plus and minus 5V and 12V were needed to make it work.

author: Bud
author_rank: Super Contributor
timestamp: January 19, 2025, 10:46:15 pm
content: By looking at the photos, they are using DC-DC switching converters to generate positive abd negative rails.

author: Midphase
author_rank: Regular Contributor
timestamp: January 20, 2025, 03:12:58 am
content: BTW...if anyone is interested, here are the schematics for the original Juno 60 Chorus module that the one I'm building is derived from. I've been meaning to analyze the original schematics to figure out exactly how much of the original design was maintained, and how much was changed to turn this into a standalone unit.Roland Juno 60 Chorus Original.pdf(376.18 kB - downloaded 60 times.)