Beginner confused with BOM

oobs

oobs

New member
Hi all. I just got my soldering set together, and I have made two pedals from a kit earlier. Now I am looking at the Sabbath Distortion but I am a bit confused with the rather simplistic BOM.
  • Resistors: should I buy carbon or metal film ones? Does it matter?
  • Same with capacitators, can I order whatever fits the number?
  • Semiconductors: I found the rest but the BOM says BC184 and what I have available is BC184C NPN?
Thanks in advance!
 
oobs said:
Hi all. I just got my soldering set together, and I have made two pedals from a kit earlier. Now I am looking at the Sabbath Distortion but I am a bit confused with the rather simplistic BOM.
  • Resistors: should I buy carbon or metal film ones? Does it matter?
  • Same with capacitators, can I order whatever fits the number?
  • Semiconductors: I found the rest but the BOM says BC184 and what I have available is BC184C NPN?
Thanks in advance!
Click to expand...
Congratulations on joining the solder sniffers club!

Resistors: I use metal film. You can use carbon if you want that "old school mojo".
Capacitors: Look at the footprint on the board. If it's rectangle, use film caps. If it's a circle, use electrolytic. Generally speaking...if the value in in (N) than it's a film cap and if it's in (U) than it's electrolytic.
Semiconductors: You "should" be fine with the BC184C NPN that you have, but I'm sure someone that is much more knowledgeable than myself can speak to this point with authority.
 
The letters at the end semiconductor names tell you things about how it has been packed for distribution and tells you if its consumer or industrial grade (the latter usually can stand higher temperatures, and tends to be slightly more expensive). For our purposes, if the initial letters and numbers are the same, you're fine... What the letters at the end mean specifically can be found in datasheets.

Then again, as with many things, these seem to be suggestions and manufacturers might do their own oddball things once in a while. Though I don't seem to have encountered these myself, I am sure examples can be brought up by others here...
 
To add to the other comments:

For electrolytic caps (and maybe some other caps too, but less commonly - I think mylar caps might be one, they're usually an "alternative" for film box) you need to keep the voltage rating in mind too. Your best bet is getting 25V or 35V (or even 50V) rated electrolytic caps, but size increases along with the rating, so for smaller sizes (like 1uF up to maybe 100uF?) you can easily get 35V or even 50V caps which will be fine in every common pedal, but for higher values than that you really need to look at the size and pick the one that fits. 6x11mm is a pretty common size that fits pretty much everywhere, and if there's a larger value it usually has more space on the PCB too.

Also to add to the rule that nF is going to be film box, uF is going to be electrolytic, and pF is going to be ceramic - but 1uF could be either electrolytic or film box, you need to check what specifically is called for. Also tantalum is an option, but I'm not very well versed in it.

For semiconductors, like others said you can usually ignore the extra letters in the end, but do verify you're buying the DIP-8 package and not surface mount version (unless you want/need surface mount, of course), and for some specific ones like charge pumps the full letter name could be important (at least the ICL7660SCPAZ comes to mind, you want the S there).
 
Lots of great advice, especially the build reports.

One thing to add, buy spares.

Nothing worse than breaking or cooking a component and having nothing to replace it. Or worse, losing one. I dropped a diode on the floor, spent 20-30 minutes searching for it, couldn’t find it. Asked the wife for help, spent another half hour and nothing. It was then that I knew legends of the component thief trolls was true.
 
Regarding the transistor, the letter at the end of the transistor often indicates the gain range of the transistor.

Manufacturing transistors is somewhat inexact and the gain can have a pretty large range of values. Manufacturers
will do 'binning' where they test parts from each batch and sort them into ranges. So for example, the
"plain" BC184 will have a gain (hFE) between 240 and 900. But some manufacturers will sort those into
BC184B (240-450 gain) and BC184C (450-900 gain). See https://www.jameco.com/Jameco/Products/ProdDS/2277551.pdf
for an example of a datasheet where they call this out.
 
z2amiller said:
Regarding the transistor, the letter at the end of the transistor often indicates the gain range of the transistor.

Manufacturing transistors is somewhat inexact and the gain can have a pretty large range of values. Manufacturers
will do 'binning' where they test parts from each batch and sort them into ranges. So for example, the
"plain" BC184 will have a gain (hFE) between 240 and 900. But some manufacturers will sort those into
BC184B (240-450 gain) and BC184C (450-900 gain). See https://www.jameco.com/Jameco/Products/ProdDS/2277551.pdf
for an example of a datasheet where they call this out.
Click to expand...
Thanks,this is interesting! So would I be well advised to buy a few of them and test which falls on the lower end and which on the higher? Can that even be tested? And how big would the practical difference be in a pedal?
 
oobs said:
Thanks,this is interesting! So would I be well advised to buy a few of them and test which falls on the lower end and which on the higher? Can that even be tested? And how big would the practical difference be in a pedal?
Click to expand...
If the BoM doesn't specify, it's probably fine. You could socket the transistor if you _really_ wanted to try to tune it, but
for most pedals, something in the general vicinity of the gain range will be fine. There are some designs where it's important
to have one transistor with less gain than the other, for example, but in those cases the BoMs just specify different transistors
with different gain ranges. Some fuzz pedals in particular tend to be more particular about their transistor selection.

If you're interested in measuring the gain, there's are some pretty cheap testers like the FNRISI LCR-P1 or these
Generic LCR-T1s.

re: buying a few of them, that's generally not a bad idea in any case, if for no other reason that they're cheaper in quantity
and buying more stuff amortizes shipping. But not necessarily because you want to try a bunch of them out.

One place where you might need to buy a bunch of transistors to test is for phasers - when you're making a XC Phase,
for example, you need to test and match the JFETs so they all have very similar parameters, or buy an already matched set.
(for JFETs, you need to match Idss and Vgs rather than hFE)
 
One gotcha on the film capacitors, these can change width as you go to higher values, and in the middle range (approx 0.1 µF) you can find both 2.5 mm and wider. When you look at your PCB, if the capacitor rectangle is about the same width as a resistor, you want a 2.5 mm width capacitor.
This is the only place I made mistakes in one of my early builds when I bought film cap that wouldn't fit in the space available.

I buy nearly all my capacitors from Mouser, and you can filter for this property easily to make sure you get the size you need.
 
Resistors: metal film have a tighter tolerance at 1% whereas carbon film is 5%. I use metal film.

Capacitors: there are a few different kinds that we commonly use for pedals - film, electrolytic, and ceramic. Film are the best for sound quality, electrolytic are usually in the power section where the audio signal is not present, and ceramic are for really low values you can't find in film.

Some more details - film lead spacing and size are important. A lot of the PPCB boards are laid out for a certain size. You want to look up the specific component on Tayda or Mouser and check out its size - for example a 10nF film is a good standard size and value - check out the body height, length, thickness, and lead spacing. Most film caps up to 100nf will be 6.5 high, 7.2 across, and 2.5 deep, with a lead spacing of 5mm. Above 100nf and they start to get bigger, but most of the boards are laid out with this in mind. The lead spacing is always the same though, at 5mm. Some of the higher voltage rated caps are bigger, you always want to double check the dimensions. Anyway, when you look for film caps with values 100nf or less, look for those dimensions.

Electrolytic caps - Find caps that match with these dimensions. With higher value caps like 100uf, you may need to go up to the next size, 6.3mm diameter, but you will still want 2.5mm lead spacing. Pay more attention to voltage rating with these, 16v is nice and small and good for most boards, but some have a charge pump which increases the voltage to 18v and you will need to order 35v caps for those.

Ceramic - these aren't that great for audio. Try to find MLCC versions instead of ceramic as they are supposed to be better. Size isn't as important for these as the component is small enough to fit pretty much anywhere. The general rule of thumb is if the cap is in the pF scale, it's going to be ceramic or MLCC, if it's in the nF scale, it's going to be film, and if it's uF it's going to be electrolytic

Transistors - For example a BC108 has different suffixes - A, B, and C. These denote the amount of gain the transistor outputs. If it's not specified, we usually use B or C, C being higher gain than B. Higher gain doesn't necessarily mean better sounding though. Just get whatever you can source, either will be fine. If you want to experiment, you can make a socket for the transistor and try the two different types.
 
CheapSuitG said:
Lots of great advice, especially the build reports.

One thing to add, buy spares.

Nothing worse than breaking or cooking a component and having nothing to replace it. Or worse, losing one. I dropped a diode on the floor, spent 20-30 minutes searching for it, couldn’t find it. Asked the wife for help, spent another half hour and nothing. It was then that I knew legends of the component thief trolls was true.
Click to expand...
Apparently they especially love a good little SMD component
 
gtfields13 said:
One gotcha on the film capacitors, these can change width as you go to higher values, and in the middle range (approx 0.1 µF) you can find both 2.5 mm and wider. When you look at your PCB, if the capacitor rectangle is about the same width as a resistor, you want a 2.5 mm width capacitor.
This is the only place I made mistakes in one of my early builds when I bought film cap that wouldn't fit in the space available.

I buy nearly all my capacitors from Mouser, and you can filter for this property easily to make sure you get the size you need.
Click to expand...
Even then, you can just bend cap legs to fit. The cap will sit proud, but as long as the legs aren't touching each other/other metal, it will work fine. I've not paid attention to sizes and ended up having to make "capacitor gardens" before.
 
Another reason to buy spares is that a lot of places will give you a pretty significant price break at Qty 10 or 25 vs. 1.

Good advice on paying attention to capacitor size. If you don’t want to scrutinize the dimensions of every one, physical size is generally proportional to Voltage rating. I generally stick to a range of 25-100V. That way it should work in any pedal, but you shouldn’t have any honking gigantic stuff (like 600V rated) that won’t fit on the PCB.

Re: component letter codes, start looking at the Data Sheets, Mouser and comparable sites have a link right on every product page. There’s usually a chart near the top that breaks down all the prefix/suffix variables. Sometimes it’s something electrically significant, sometimes it’s just to do with physical dimension, or lead spacing.
Knowing which variables matter and which ones can be ignored is a good skill to start building up.

This is a pretty good ‘Parts Sourcing 101’ from the Aion site:
aionfx.com

Parts Sourcing: Where to Buy What

After giving a general overview of what stores are out there, it’s helpful to give an overview of parts sourcing from the component perspective—the best place to buy resistors, the best place to buy…
aionfx.com aionfx.com
* though it’s worth noting that the WIMA MKS2 line of film caps that he recommends has, sadly, been recently discontinued 😔
 
Last edited:
comradehoser said:
Even then, you can just bend cap legs to fit. The cap will sit proud, but as long as the legs aren't touching each other/other metal, it will work fine. I've not paid attention to sizes and ended up having to make "capacitor gardens" before.
Click to expand...
Fair point - some of us may be a little but too, umm, uptight about appearances. As you know, this impacts the toan.
 
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