Design of a Korg Nutube Amplifier Part 3: PCB Computer Aided DesignFollow article
This is the third post in a series about creating a guitar pedal style amplifier. Last time in Part 2 we took a look at creating a breadboard prototype. Following which we probably have enough data to commit a circuit design to CAD.
Long gone are the days of drawing PCBs using sticky tape or a pen; yes this was the way in the distant past. However, the modern way is to use a computer program to create both schematics and then a PCB from these schematics. Just like your favourite word processing app, there are many CAD programs out there.
There are CAD programs that are quite eye-wateringly expensive (the price of a high-end car) and requiring a reasonable amount of training and experience to use, down to completely free versions that can again need training or be simple to use. In our case, we will use the free DesignSpark PCB.
Having used quite a few CAD packages over the years I would say that most have a similar system and operation. DesignSpark PCB is no different and felt quite familiar and intuitive, plus there are also many excellent tutorials that help fill gaps in knowledge.
As with most CAD processes, we will need to go through a few stages: schematic capture, part creation and final layout.
One of the great features is that footprints and symbols are provided for a good proportion of the RS catalogue, with a system in place that can help create footprints for some of those that do not. Below is an example: we did not have an LM317T part or footprint, so we followed a link in DesignSpark PCB to http://rs.componentsearchengine.com/ to create both schematic and layout symbol/footprints.
As can be seen above there are several parts available. If we wished to make one to match the Fairchild LM317T we could click build/request and follow the guides giving a part within a few minutes.
It would be remiss at this point if we did not point out that, while this is a great feature, a lot of these parts are unverified and therefore not guaranteed to be entirely error free. This is far from unique and would be true of any unknown library — even with the most expensive CAD packages. So as with any CAD library and even those you make yourself — maybe especially those you make yourself — verify!
If you want a custom or specific CAD cell, there is no better solution than creating one from scratch. There are several tutorials out there and once you get the hang of it you can start creating some exotic CAD cells.
If you have read the previous articles in this series you may have noted that the schematics previously shown were from CAD packages, most of which were created in DesignSpark PCB. Given there are several excellent tutorials on how to get started, we will not cover how to use the package and instead just the results we get from it.
First, we need power. For our design, we decided to go with a 12V “brick”, but this could easily be a 9V battery (it won't last very long) or 18-24V laptop supply. While the circuit we have designed should work to 40Vdc; the parts we are using have an upper limit and we chose capacitors carefully as overvoltage on electrolytic capacitors can result in rather drastic consequences. Another point of caution with higher voltages is the regulator will get hotter, due to the reasons discussed last time.
We also added a small CLC filter on the power input. With a large diode for reverse voltage protection, this will rely on either an external fuse or the plug top/power brick current limiting but should save the tube and design in the event of accidental power reversal. We would not advise use with a 12V battery without a fuse, for example.
The input circuit looks complicated, but if you ignore the DNI (Do Not Insert) impedance buffer at the input, there is only a short in R22, a capacitor in C1 and 10k series impedance, before the bias which is also buffered by 10k. These 10k resistors provide a little current limiting from the variable supply created by the potentiometer a.k.a. “pot” (this is close to 0R once set to limits).
We have left the buffer out as we assume our source to be another pedal and therefore low impedance, but if you wish to have a high impedance input just fit the impedance buffer.
To cascade the amplifiers (for additional gain) we have added an impedance buffer. Since the Nutube has such a high output impedance it struggles to drive the second Nutube without a little impedance trickery.
We’ve also added the first in our 3 control pots here; acting like a variable resistor this pot effectively allows us to remove the gain from the first amplifier or add it to the second resulting in around 5-25 times gain from a pair of tubes.
For our output we have another impedance buffer to guarantee a low output impedance, AC coupling to remove the bias induced by the tubes — which also has an output impedance adjustment resistor set by R24 (we used 10k because we had some already) — and followed by two adjustment controls.
The first control is an RC filter, which will allow us to remove HF by varying degrees. With R14 at 100k there is little effect from the tone circuit, however, as R14 drops the circuit becomes more dominant and will attenuate the output more, especially higher frequencies. By altering C18 this roll-off can be adjusted to suit and the smaller C18 is the higher the frequency before the effect becomes dominant.
Finally, a volume control which effectively divides down the output from 1/11 to 11/1. When the wiper is at pin 1 of R13 the output has 110k to gnd and a 10k source impedance (loud); when the wiper is at pin 3 the output has 110k source impedance and 10k to gnd (quite).
CAD layout has many methods and pitfalls, with each engineer having their own methods and styles. A key to success is to floor plan correctly before you start, as knowing where everything is going to go helps a lot and will make the CAD layout easier. Use of the “rats nest” can help here, but the first design is going to be daunting.
The very first thing to do is think where the boards are to be made since this will influence the layers, track and gap parts and vias used. Different PCB fabricators have different ability’s and technology’s they can deal with, so this is a key step that should be not be ignored.
Getting everything placed before routing is a good strategy for smaller boards. Sometimes this is not possible but it can help.
Another item commonly missed from designs is probably the most important nets of all, ground and power. It is often put in as an afterthought or thin track, whereas the impedance of the ground plane is key to keeping the system noise down and in the case of the amplifier, keeping noise out of the amp. To this extent we allocated most (as much as possible) of layer 2 of our PCB to ground. This will keep everything common and gnd induced noise low.
The use of “polys” can help. The design shown above is using these flooded copper areas extensively to give a reliable and low impedance supply. Use caution however, as these floods do add a lot of complexity to the design and require some lateral thinking during the layout and floor planning. Close to our hearts, being Yorkshire born and bred, is the side benefit: a board being near 100% copper means less waste as less copper is etched off, resulting in get more copper for your money.
Having routed and placed your parts there are three things left to do:
- Create your manufacturing files in the format required (Gerber most likely)
- Check check and check again everything, including the Gerbers!
- Submit these files to the PCB fab of your choice. DesignSpark PCB has a link for one which is convenient but there are much more out there.
PCB’s ordered, we need to kit the BOM (Bill Of Materials). DesignSpark PCB has a link for this and if the parts are set-up correctly it can be ordered directly from the PCB package.
It will then be a waiting game to see if the PCB we just designed works, or if it’s a bust!
Next time we’ll build up this PCB and check our circuits work. There may be a slight issue, given our guitar playing experience is somewhat lacking and so we may have to cheat and order a 3.5mm – ¼ inch cable.