My First “UPVERTED” Design
In my earlier post about taking the plunge and forcing myself to learn how to use Upverter, I mentioned the project I was working on was a new guitar overdrive / distortion pedal I named “Sausage Factory”.
Well, some friends got back to me and requested to see what was in it – just out of curiosity, so although my original intention was to keep this a private project, I figured it would do no harm to share and describe more about it.
Every design should begin with a design specification. Mine was very simple:
- Marshall style high-gain front-end.
- Fender/Vox/Marshall passive tone stack (Bass, Mid, Treble)
- 7-band Graphic Equalizer similar to the Mesa/Boogie Mark IIC
- Individual footswitching for the OD section and Graphic
- Fit in a single hammond 1590BB metal box
- As low noise as possible.
I used an opamp based graphic. I knew I could probably do all this in DSP based on Eli Hughes’ Monkey Jam, but since I’m learning Upverter at the same time I figured this design would stay in the analog realm so I could have more going on with the PCB. This will be harder and take longer to assemble my prototypes by a long way, since there’s a lot of 0603 parts in a graphic EQ!
This one was done first as a module in Upverter so that I could place this graphic EQ into any future designs very easily. I like the the physical design reuse aspects of Upverter – this is not just a device sheet or sheet symbol but a full hierarchical reuse block which includes the pre-routed PCB layout for it.
I’m specifying LM4562MAX/NOPB parts in this project because they are the best low-noise opamps right now which are affordable and useful at lower gains (such as in the EQ section where the gain may be 0dB or up to +/-12dB)
The opamp gyrators were set to have Q factors and frequency bands similar to the MESA/Boogie Mark II head’s graphic, but there’s no way to get it exactly the same because it’s not the same circuit. The target center frequencies are 84Hz, 240Hz, 400Hz, 1KHz, 2KHz, 4KHz and 8KHz. The Boogie was designed a long time ago, before opamps were cheap enough or low noise enough to be used as gyrators, so that circuit uses tank circuits with real inductors and caps and resistors in series. There’s no end to the audible nuance in such things, but for my intents this is enough to make great tone, and finding the right inductors to use to faithfully copy the Boogie EQ circuit would render this project prohibitively expensive.
I created a new project after the GEQ7 re-usable module for the main overdrive pedal. This is the Sausage Factory – I named it this because it’s meant to be a meat-grinding face-melting distortion capable unit.
As with the EQ modules, I’ve used LM4562MAX/NOPB opamps throughout. I’ve used those before in other high-gain designs like the “Screamin’ Dolly” and they sound amazing, yet have extremely low noise. The Screamin’ Dolly next to a TS-808 will give the same tone but way less hiss because of these opamps. Similar to the TS-808 and TS-9 in this design I’m using an input buffer with a 2N5088 transistor configured as an emitter follower.
You can see the gain stage is followed by a higher voltage clipping circuit as used in Marshall Preamps, where they used a diode rectifier bridge, shorted with an additional rectifier (Silicon) diode between them. This provides a clipping voltage of about 1.5-2V as opposed to many overdrives using a single pair of back-to-back signal diodes which will clip hard at about 0.6V. The circuit here is better for a more natural head-like overdrive, and for driving the following passive tone stage. After the tone stack is a buffer with a small amount of additional voltage gain (6dB), a Master volume control pot, and then into the EQ.
Both sections – the preamp/overdrive and the graphic EQ, have pin-headers for ribbon cables to go off the board and be soldered to the 3-pole double-throw try bypass footswitches. I have a big bag of these footswitches from China ready to go…
I keep hearing about more and more PCB prototyping fab services, one of which is PCBWay. They seemed to be fairly low cost and promised a very fast turnaround time, so I went ahead and ordered the bare boards to be made there. I had them back in 2.5 weeks which was pretty fast for an off shore fab.
The quality seems good, though my silkscreen coudn’t easily be edited to show all the graphic EQ reference designators properly – so I’m just going to have to use Upverter interactively while I put these prototypes together, as a “living” assembly drawing.
The boards were vacuum packed well, and I ordered their minimum prototype run of 5. I also ordered the stainless steel solder paste stencil – I’ll do another post and video when I use it to show how to use a stencil and desktop reflow oven for soldering the parts onto the boards.
So, this is an honest quirk of Upverter when you use re-usable modules like I did for this EQ section – it prefixes each module reference designator with the hierarchical parent designator of the module instance. So all these reference designators begin with GQ: that’s not a bad way to handle this hierarchy. However I was not able to move the silkscreen texts in the main board, so many of them ended up cropped by the solder mask openings…
Oh well, one more thing we will improve with Upverter I guess!
Stay tuned for more posts when I put these together and do some testing.