I know I've disrupted the very essence of this fully analog design, but what can I say? Old habits die hard. If you don't banish me to the archives, maybe what I've just finished might interest you. Since I knew nothing about preamps, I spent the first year studying all the most renowned schematics. After selecting the G9 as my starting point, I dedicated another year to analyzing it piece by piece, creating submodules to measure everything I could, and understanding the logic behind it all.
A little side note: I'm an amateur musician and an electronics engineer with 30 years of experience, specializing in high-frequency systems. Anyway, I then spent six months designing the circuit and another six months assembling, programming, and testing it. All in all, it took me around four years to create this.
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It's a dual G9, meaning a dual mono or stereo version, controlled by an open-source ATmega/Arduino microcontroller.
Power supply
There are some rather crude additions, but they are the result of extensive research and experimentation. The switching power supplies may have shocked you, but after countless tests, it was difficult to integrate a regulated power supply without facing EMC issues, overheating, or space constraints. These power supplies are well-filtered (I spent a long time searching for these models and tested a lot of gear that ended up in the trash). I also monitor the power supply temperatures and can certify that all components can operate up to around 60°C ambient.
I also spent a lot of time studying the current draw cycles and refining the NTC circuits and consumption setups to avoid putting too much strain on the power supplies at startup. I’m very satisfied with the results of this work.
Parallèle Drive
Of course, you've probably noticed the six tubes, which also required some power. That's because I added two parallel channels without dry/wet blending — you'll see what I mean. These channels use ECC83 tubes in a typical British setup from the 70s/80s, allowing me to add a bit of grit to the upper mids of vocals or wind instruments.
G9 channel
Since I have extensive experience in EMC, I also explored the possibility of opening the feedback loop. I tested many different tubes, and I can confirm that even with the high gain of ECC83 tubes, I can open the G9's feedback loop without any issues using relays. It was a bold move initially, but the stability is solid, delivering a punch of around +11 to +12 dB on this channel.
Level setting
Each tube is driven differently on each channel. The clean channel is controlled via the cathode, like in the original schematic, using digital potentiometers. The drive channel is controlled by its attack, which is more conventional. The output tube, an ECC82, is also driven via its grid through an impressive PGA circuit from Texas Instruments. This circuit also manages the insert input level for each channel (though I admit I've encountered some circuit issues here, and it doesn't work exactly as I want).
HPF:
I almost forgot, I added a real bypass with a relay (there are 32 relays on the board) going to a HPF (High-Pass Filter). You select the cutoff frequency, and voilà...
Noise & zero crossing
Of course, I had to address the noise issues with the potentiometers. I initially designed a homemade zero-crossing circuit, but I wasn't convinced by the results. In the end, on the CLEAN channel, since it's controlled via the cathode circuit, it's completely transparent. No issues on the output circuit either, as the Texas Instruments chip handles it smoothly, even at high gain. For the drive channel, my solution was less than ideal; since the PCB was already made, I couldn't change it. If I were to create another version, I would definitely use the Texas Instruments chip without hesitation.
PCB & control
Le PCB est une conception à 4 couches, avec la section numérique en dessous et la section analogique au-dessus. Les alimentations électriques sont sur la couche 3, tandis que la couche 2 est un plan en cuivre massif pour éviter les interférences. Il y a environ 1 000 composants. L'ensemble du système peut être contrôlé soit via des boutons, pour ceux qui connaissent les versions analogiques traditionnelles, soit via l'écran tactile, voire via un VST intégré à votre DAW - parfait pour les débutants.
Mecanic:
Mechanically, there's nothing particularly special, it's just tight, that's all. I made the front panels myself over time, I have a small machining center for that.
Input selection:
For each channel, you can choose between the front or rear combo input. Then, on the screen, you select the type of source: instrument, mic, or line. A relay matrix routes the signal either through transformers or directly to the tubes. Bypassing is, of course, possible, and there's also a ground lift option. Each channel has a phase inverter and the possibility to supply 48V phantom power, but only when the mic input is activated.
If you select stereo mode, the settings from channel 1 are copied to channel 2.
Tube calibration
Given my background in automatic measurements for high-frequency systems, I added a program that calculates the gain of each tube to equalize both channels in stereo mode. This also led to the creation of a feature I hadn't initially thought of, which ended up taking a lot, a lot,
a lot of time.
Auto Output level
In fact, you can activate the auto-gain option for each channel (1 or 2). How does it work? You set your desired output level, then you touch the clean or drive channel, and thanks to the magic of math and tube correction curves, your output level stays constant. Even if you open the feedback loop and suddenly add +10dB, the output level will remain unchanged. Essentially, the SRPP output becomes dynamic. This saves a tremendous amount of time when shaping your sound. You tweak the clean or drive channels, adjust the feedback loop, or try one last option that I’ll introduce... and nothing changes at the output.
BassBoost on Drive channel
The drive channel is equipped with a reduced bandwidth on the lower end of the spectrum to allow the clean channel to shine, as well as to enable the feedback loop to open. This keeps the sound very round, yet airy. It avoids the overly "liquid" effect on the channels. However, if you want to go a bit lower, with the bass boost, the drive channel switches to a different capacitor, allowing the ECC83 to open up the entire low end. As a bassist, I find this feature quite interesting.
Bien entendu, cette option est également compensée par l'auto-gain lorsqu'il est actif. L'incertitude actuelle est d'environ 0,3 dB pour ce système, et je pense qu'elle restera à ce niveau.
Options
You'll notice available space at the bottom of the motherboard. On each channel, there's the possibility to add, in thru bypass mode of course, processing cards – digital or analog, your choice. There are all the necessary connections to control them via multiplexed SPI links, along with audio inputs and outputs going to the mechanical relays, and the 5V, ±15V power supplies.
I admit, for now, I'm going to stop here, unless someone crazier than me comes out of their cave to improve this :-O
VIewer
There's also a page I’ve called the "viewer," which allows you to monitor the tube parameters (supply voltage, heating voltage, and the temperatures of the power supplies). It also tracks the usage time of the tubes. I don't have enough data yet, but I thought it would be cool to add. Regarding voltage control, it also manages the heating time – when they’re stable, it means everything is good.
The sound
Let's talk about what really matters – the sound. Well, I haven’t heard other G9s in person, so I visited all the studios in my area to compare. I was lucky enough to try some really nice gear, like Neve, UAD, Manley, and others that are a bit less known. Let’s just say I wasn’t too nervous, although at first, in the big studios, I was holding my breath (as we say around here). In the end, the sound here is a bit drier. The drive channel and the opening of the feedback loop help to emulate quite a few competitors. I can't fully describe it, but for vocals, it's proven to be very practical and versatile, less so for guitars, but really great for bass.
thank you
That was a fascinating topic to discuss. I have many other projects in the works, and I'm happy to share this with the community I've been reading for four years. I'm just a small French guy living in the mountains, and I'm about to take this toy to our rehearsal space. I've never spent so much time on a project, even when I get paid for it. Your feedback will be my reward, but please, don't be too harsh on the digital side – I know it doesn't win everyone over.
Thank you for your attention, I look forward to your comments. And a big thank you to the passionate people who have shared their knowledge – much appreciated!
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