DX7/DX9 schematics or service manual?

The Lounge Other Gear
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I recently got hand and ears on a Yamaha DX9. I just love the noise print and how it renders the digital artifacts and sounds so round and balanced mighty voluminous at the same time.

I thought this must have to do something with the used DAC (R-2R ladder DAC circuit?) and the following LPF. What I understood so far is that the LPF is not a chip but a circuit. After that, the specifics of the amplification stage like with op-amp, etc. will probably influence the sound, too.

I would like to plan some little electronic tinkering with either a breadboard or with the output stage itself to find out and send other signals through it. Maybe even just inject signals into the original hardware.

However, I only found the truncated service manual of the DX7/9.

Does anyone of you have the schematics?
That would be wonderful.

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I don’t have the schematics but I hope you can find them so that I can see and hear the results of your experiments here :wink:

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You have the 16 page version? I think this one with 37 pages is the service manual in it’s entirety but you’ll have to verify. Good luck with your tinkering.

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Thanks for the link, could not download without giving financial information for the free test period. Will do if devastated :sweat_smile: Great to know a complete version is available. :smiley::+1:

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I finally found high resolution schematics, but for the DX7 seven only. Then I spent some time with some AI to understand what’s happening and this is the result. Please don’t overvalue these results.

Btw., this is no automatic response, it needed a lot of back-and-forth and biological reasoning, too. I will check this later by hand, however, I already ordered these these DACs :slight_smile:

This is the result for the DAC section only. (After that to explore: LPF section and the AMP section.)

DX7 DAC Section - Summary

BA9221 DAC

β€’	Type: 12-bit DAC
β€’	Standard Usage: Can directly convert 12-bit digital audio without issues
β€’	DX7 Special Trick: Uses an 8-bit signal + 4-bit dynamic scaling (SF0-SF3)
β€’	Clocking: No fixed clock, updates when DACCE (DAC Chip Enable) pulses
β€’	Voltage levels:
β€’	DAC input logic: Standard TTL (~0V = LOW, ~5V = HIGH)
β€’	Analog output range: Dynamically scaled by SF0-SF3

SF0-SF3 Dynamic Scaling

β€’	Purpose: Adjusts DAC reference voltage dynamically to enhance resolution
β€’	Effect:
β€’	More resolution in quiet sounds
β€’	Less resolution in loud sounds
β€’	Mechanism:
β€’	Scaling done via resistor network, analog switches, op-amps
β€’	Fully analog process, no digital computation
β€’	Behavior:
β€’	Similar to RIAA curve β†’ Expands quiet details, compresses loud parts
β€’	Ensures perceived high resolution despite 8-bit data

Signal Path

1.	Digital input (8-bit) β†’ From Operator chip (IC) to BA9221 DAC (IC42, Pins 1-8)
2.	Scaling control (SF0-SF3, 4-bit) β†’ From IC41 to BA9221 DAC
3.	DACCE pulse β†’ From IC44, Pin 3 to BA9221 DAC, Pin 10 (Triggers update)
4.	Analog output β†’ From BA9221 DAC, Pin 14 β†’ Through LPF circuit β†’ Final amplifier stage

Reproducing DX7 Sound Signature for Sample Playback

β€’	This is a 12-bit DAC, but to reproduce the DX7’s characteristic sound, 8-bit samples must be used.
β€’	These 8-bit samples must already contain the characteristics of the dynamic curve, which is normally applied via SF0-SF3 scaling.
β€’	To achieve this, samples must be pre-processed to match the DX7’s dynamic behavior before being sent to the DAC.
β€’	SF0-SF3 control signals must still be applied to allow the analog circuit to handle dynamic range processing.
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LPF section

These are the results of the analysis of the LPF part with some kind of advanced AI. It ran a simulation, and this resulted the frequency response printed here.

I post this here for note keeping and also later reference. I hope I can manually check the stuff and rerun the simulation on my computer at the weekend.

Maybe it’s interesting for someone.

===============================================
 Yamaha DX7 Post-DAC Low-Pass Filter (LPF) Analysis
===============================================

### 1. Circuit Overview
- **Type:** Active Low-Pass Filter (LPF)
- **Configuration:** Sallen-Key Topology
- **Purpose:** Smooth DAC output, remove high-frequency artifacts, prevent aliasing
- **Filter Order:** 2nd-order (12 dB/octave roll-off)
- **Filter Response:** Butterworth (flat response, no resonance)
- **Estimated Components:**
  - **R1, R2:** 1kΞ©
  - **C1, C2:** 8.8 nF
  - **Op-Amp:** Likely used in a non-inverting configuration

### 2. Circuit Diagram (UTF-8 Box Drawing)

       β”Œβ”€β”€ R1 (1kΞ©) ───┬── C1 (8.8nF) ───┐
  IN ───               β”‚                 β”œβ”€β”€ OUT
       β”‚               β”‚                 β”‚
       β”‚               β”‚                 β”‚
      GND            R2 (1kΞ©)            β”‚
                       β”‚                 β”‚
                       β”œβ”€β”€ C2 (8.8nF) β”€β”€β”€β”˜
                       β”‚
                      GND

### 3. Frequency Response Simulation Results
- **Cutoff Frequency:** ~18 kHz
- **Roll-Off Rate:** ~12 dB/octave
- **Behavior:**
  - Flat response in the passband
  - Smooth transition to attenuation above 16 kHz
  - No significant resonance or peak
  - Effectively attenuates frequencies > 40 kHz

### 4. Interpretation & Practical Impact
- Ensures a **clean analog output** by filtering DAC stepping noise
- Preserves **full audio range up to ~16-20 kHz** while removing digital noise
- The 2nd-order Butterworth response guarantees **a smooth transition**
- Suitable for FM synthesis output, ensuring clarity in harmonics & dynamics

===============================================
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