Circuit Layout Tips
This was not meant to be a complete guide, but rather a bucket of advises and observations I collected over years. That stuff helps me get quiet and reliable layouts, so someone might find it useful.
What I usually try to achieve is a layout that follows signal path from left to right (or the other way round). Also, I try to keep AC as far as possible from the signal, so in most cases, the power supply will be on the opposite side of the board from the circuit input and DC supply will flow in the opposite direction from the signal. As an example, below is my layout for Deacy germanium amplifier with germanium buffer stage added at the front. Note that signal flows from left to right (except for the purple NFB lead that goes the other way round) and DC supply flows from right to left. Components are usually placed orthogonal to the signal flow to make the best use of space.
When laying out components around an IC, like in a typical tubescreamer-like overdrive, I still try to follow the same logic, but it’s not as easy as with discrete components. I usually put the IC in the center and lay out all other components in “bug” formation around it, like shown below on a point-to-point Timmy overdrive layout.
For both amps and effects I tend to use a variation of bus grounding scheme. Input jack is grounded either through direct contact with the chassis or with a short lead. All grounds from each amplifier stages are grounded close to each other (somewhat like star grounding) together with the corresponding filter caps. Control pots are grounded to the bus close to the stage they belong to.
As far as the lead dress goes, I try to stick to these rules. Some contradict others, so again you have to compromise.
- Keep leads as short as possible, longer leads usually means bigger chance of catching the noise.
- Try to keep leads carrying AC current as far as possible from the leads carrying the signal. If you absolutely have to intersect AC and signal leads, make sure they are orthogonal to each other.
- Like with components, I always try to make leads follow the signal path and avoid hi-level signal leads running close to lower-level leads. Again, if you have to make an exception, use shielded wire (see the next section) and/or try to keep the leads orthogonal.
Shielded wire can be useful for removing noise and oscillations but is noticeably harder to work with than unshielded wire, so a reasonable compromise should be made. The most common place to utilize shielded wire is circuit input. That’s typically the lead that carries the low level signal, so any noise introduced there will be amplified many times in the rest of the circuit, so it’s crucial to make sure that it gets to the first stage of the amplifier/effect as clean as possible.
Typically, only one end of the shielded wire should be grounded to prevent ground loops, but sometimes I use the shield as ground lead which requires grounding on both sides. A good example would be gain pot on overdrive/dist pedals or amplifiers, as shown below. Using shielded wire reduces risk of noise and makes it look cleaner because one shielded lead replaces two unshielded leads.
My Dr Boogey build may give some ideas how to do it in practice, as it turned out to be pretty quiet and oscillation-free. Signal leads go from left to right. The first pot is gain and there are no leads from the following stages anywhere near. White input lead is shielded and orthogonal to the orange output lead to minimize interference. All the leads going to the volume pot (far right) are tied together, which is OK because they are all carrying the same output signal. The same goes for the two orange output leads going to and from the bypass switch.
As far as choosing the shielded wire, I cannot speak highly enough about teflon coated wire. The biggest risk when working with PVC shielded wire is that you’ll melt the inner insulation while soldering the shield and make contact between the main lead and the shield. Teflon doesn’t melt in contact with the soldering iron and will take much more abuse than PVC. On the other hand, it’s a bit tougher so it’s harder to bend, but I think that there are more pros than cons. Below is the pictorial that explains how I typically use shielded wire.
Twisted wire pair
This is a very popular technique that works well with AC leads, such as tube heater leads, high voltage secondaries, etc. The idea is to twist the leads tightly and make the radiation from one lead cancel out with radiation from the other lead. To maximize noise rejection, twists should be right and uniform. To achieve that, I put two leads in the bit socket of a hand drill and (with help of another person) make it rotate slowly while keeping the leads about 5 inches apart at the other end. It takes literally 10 seconds to make a perfect twisted pair like that. Some like to use zip wire which is essentially a perfectly parallel pair or leads and should cancel out noise nicely.
Shielding using components
Not only shielded wire can block the noise from getting into the signal. Often I place components or even pieces of metal strategically in the circuit so they can block the noise. Going back to the Deacy layout, note that the output transformer is placed between rectifier diodes (that can emit noise because they are dealing with AC current) and the rest of the circuit. By grounding the transformer cover we’re effectively turning it into a shield, so noise can’t get into the transformer or go through. The same goes for the phase splitter transformer that goes between the output stage and stage 3 of the preamp.
Another component that can often be used as a shield is a electrolytic capacitor typically found in amps. One side of it is always grounded which means that it can act as a shield. Below is a photo of my 2W Trainwrack clone that uses a large JJ capacitor placed between the rectifier diodes and the rest of the circuit to block any AC noise.
As far as using sheet metal for shielding, I typically use it to shield power transformers. In my SLO style build I used aluminum L profile to mount the transformer and make it act as a shield at the same time.
In my Mark IIc+ style build I used aluminum L profile to mount tubes, but at the same time it shields the circuit from the transformer. Note that large white capacitors act as a shield between the tubes.
Another component that may act as a shield is a 9-pin or 7-pin tube socket. Most of them have a metal tube running through the center of the socket that can be soldered to. When grounded, this piece of metal will help shield the noise between tube pins. It’s not much, but every little bit counts!
Pots are another good candidate for shielding. When pot body makes contact with the chassis it will be grounded making it into a shield. When I use powder coated chassis I always strip some paint around pot holes to make sure there’s contact between the pot and the chassis. Again, it’s not much, because you’re only shielding pot internals, but it’s a good practice. Some solder a bus wire to backs of the pots and ground it to the circuit ground. You can do that, but I would make sure that there’s no contact between pots and chassis because it would introduce a ground loop.