In our electronics (I) lab they told us to never use resistors in parallel or series and choose a value near your calculations. but never explained why. Is it ok to just buy the range of 1ohm to 1M and use them to get the resistance I want?
I'm starting to build a guitar amplifier. while buying the parts I may end up with lots of spares.
thanks
The pros about using a single resistor over either parallel or series:
The pros about using a combination of resistors:
However, in any professional (and serious hobby project), I would use only one resistor when possible, taking the closes/best resistor value and power into account.
If you are purchasing parts for an amplifier design that you will be tweaking resistor sizes to optimize performance I strongly suggest that you invest in a resistor assortment kit. Such a kit will prove useful for you for many future projects as well. I still regularly use resistors from an 0805 SMT kit that I purchased over 16 years ago.
Kits such as these give you a lot of flexibility to stick with the single resistor philosophy or in selecting unique in between values either by doing components in series or parallel.
You can get kits such as this one with axial leaded resistors:
Alternatively kits are available with SMT components of a specific physical size. Here is one kit with 0603 sized resistors:
What they should have said, was that in practice you work with what you got and it often isn't necessary to use networks just to get a non-standard value. For that, you really want to get used to the E12/24/48/96 series.
For example, if you calculated you need 18800 Ohm, a 18k7 would usually be good enough. Often enough 18k will do fine. Not even talking about tolerances on those values (although the problem of tolerances does add-up if you start combining resistors, make sure you use those that are accurate enough if your application needs it), often enough simple is better than accurate.
The intent behind their statement was fine, but a bit too absolute. Sometimes, you do need a very specific value. However, in that case it could be wiser to put a single E192 resistor in it instead.
For prototyping purposes, having the entire E48 or better available is quite useful. Don't let anyone stop you from getting a range of resistors.
If you don't have a list already, find your E-series lists here.
Generally speaking combining resistors is either irellevant or self defeating, but when prototyping one does what one needs to so of course it is done extensively. For example it is easy to solder an smt resistor on top of another and saves a step vs desoldering if I want to quickly try a value smaller than what I chose .
Consider that resistors values are laderred on a logarithmic scale with steps based on tolerance of the series , however many applications just need to be in the ballpark and even 5% is overkill
For example , e.g. 10/100/1k/10k/100k/1M +/-50%will cover 90% of resistor cases.
For those applications you should just use the closest common value since you don't much care anyway (e.g. all pullups to 10k). So stacking parts isn't needed except if you are simply short some particularly low high or low value
However there are situations where one precisely calculates the resistor value, combining resistors is self defeating because the errors combine , so better tolerance parts are needed when using esistors in parallel or series, this is both unreliable and more expensive
For example if you calculate a value such as 105.2k , if the requirement is +/-10% simply round to 100k 5% part , if it is +/-1% use the nearest value in that series, if you need finer control, you may choose to combine 0.1% parts at a cost. In the most extreme a trim resistor can be laser trimmed to control impedance during assembly (or a trimpot if it needs regular adjustment). Only you know what the requirement is for that application
There are second order optimizations, especially when considering design for cost , but those are case by case wierdness usually as a result of askew logic of volume production
There is no universal answer on your question.
Normally in low power electronics you try to use available space as efficient as possible by taking in consideration possible interaction of closely located components.
For example in radio circuits you would try to shield oscillators from amplifiers, make traces as short as possible to avoid antenna effect by picking up electro-magnetic energy in the conductors.
In power electronics very often you have to use series or parallel conducted resistors to dissipate heat (otherwise you are looking into purchase of quite expensive resistors).
As you mention the guitar amplifier (pre-amplifier) then probably you will use opamps. In this case we deal with low current, low voltage circuit and you will end up with a few potentiometers (variable resistor).
Now depending on selected technology (through hole or SMD) you have to select electronic components. SMD is most space/cost efficient but not easy to work with without proper equipment (depending what you have in your workshop -- with proper equipment SMD is easy to work with).
If you refer to pre-amplifier then probably it would be sufficient to use 0.125 Watt resistors.
If you are referring to guitar power amplifier then for a start you need to define what power output is expected. Based on this information you will need to decide on class of amplifier based on percent of harmonics (distortion effects create significant percentage of harmonics). You can use predesined specialized ICs or go for discrete components.
Specialized ICs significantly simplify the circuit and with good components guaranty stated specification (please pay special attention to quality of capacitors).
At this point you will need to look into power consumption by power amplifier and what complexity it will involve to design power supply. PWM power supplies give most efficient conversion from high to lower voltage -- but known for significant radio emission of radio harmonics (even sufficient shielding not a solution).
Class D amplifiers are among most power efficient amplifiers but will poorly fit for radio electronics (PWM switching amplification produces a lot of radio-noise) and audiophiles will say 'no-no-no' as switching amplifier introduces a lot of radio interference which will 'leak through' into pre-amplifiers.
Just a few thoughts from the top of the head to take into consideration.
