I’ve seen pictures of diy guitar pedals that appear to use 630v polyester caps, like those that would be used in tube amps when there is no high voltage present. I’ve also read different things about how different types of capacitors work at different voltages and how the value of capacitance can change. Will a high voltage rated capacitor perform the same way as a low voltage capacitor where there are low signal voltages? I’m getting confused about the need for low Leakage, derating, low ESR….? Are there any good resources that you recommend. For a simple diy self taught hobby it’s not critical, but what is the point in looking at schematics and trying to understand design, if the practice choice of components leads to poor outcomes, and characteristics that don’t match the expected results? Please point me in the right direction for both pedals and solid state and tube guitar amps.☺️
Depends on the application
Inside a 9V-powered guitar pedal any cap rated 15V or more will do.
There is no upper voltage limit, but too high voltage means bulky case - to be avoided if possible.
As a rule of thumb use film caps for less than 1uF, above use electrolytics.
Avoid MLCC in coupling applications as these are the worst and prone to microphonic noise pickup.
For supply voltage blocking any ceramics like X7R/X5R will do.
And last but not least: Ignore any threads discussing "the sound of capacitors".
Inside a 9V-powered guitar pedal any cap rated 15V or more will do.
There is no upper voltage limit, but too high voltage means bulky case - to be avoided if possible.
As a rule of thumb use film caps for less than 1uF, above use electrolytics.
Avoid MLCC in coupling applications as these are the worst and prone to microphonic noise pickup.
For supply voltage blocking any ceramics like X7R/X5R will do.
And last but not least: Ignore any threads discussing "the sound of capacitors".
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Someone asked the same question on high voltage caps used at low voltage. I set up a 100k/1.5k/12AX7 stage and bypassed the cathode with a 22 uF cap rated at 6.3V and one 250V. Measured the low 3 dB point and they were the same. Other than the ceramic issue and tantalum capacitors I do not worry too much on the type.
I agree, avoiding ceramic caps that can be microphonic or possibly poor thermal stability in the signal path is a good idea. For low capacitance values in the pF range, you could look at polystyrene. Otherwise film caps rated 25 or 63V are pretty common, and not too big. I like the Kemet 5 mm box capacitors, for the mechanical construction and footprint. The outer box provides support on the PCB and they are available up to 2.2 uF I think.
EDIT: Of course you will need capacitors with higher voltage ratings than 63 V for most tube amp applications, except in the case of cathode bypass where it is common to see lower rated electrolytics.
Also quite common in tube amps is ''Orange Drop'' film capacitors for signal path and tone controls. Usually rated about 300 V but check the working voltages of the various stages to be safely above.
EDIT: Of course you will need capacitors with higher voltage ratings than 63 V for most tube amp applications, except in the case of cathode bypass where it is common to see lower rated electrolytics.
Also quite common in tube amps is ''Orange Drop'' film capacitors for signal path and tone controls. Usually rated about 300 V but check the working voltages of the various stages to be safely above.
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There are huge differences between ceramic capacitors. Ceramic capacitors in the pF range are always class 1, nowadays usually NP0, also known as C0G. Those are actually quite good.
Higher values can be ceramic class 2 or 3 capacitors (*), which are quite poor. They have dielectric codes like X7R, X5R, Z5U, Y5V and many others.
(*): That is, there are NP0/C0G multilayer ceramic capacitors up to 220 nF or 470 nF nowadays, but those nanofarad values can also be X5R. Values in the microfarad range are always X7R, X5R or worse, as far as I know.
Higher values can be ceramic class 2 or 3 capacitors (*), which are quite poor. They have dielectric codes like X7R, X5R, Z5U, Y5V and many others.
(*): That is, there are NP0/C0G multilayer ceramic capacitors up to 220 nF or 470 nF nowadays, but those nanofarad values can also be X5R. Values in the microfarad range are always X7R, X5R or worse, as far as I know.
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Capacitors are far from ideal components and prices/availability can be crazy, so we need to choose the capacitor "good enough" according to the circuit needs.
Examples:
ESR: Why do we need very low ESR it if the circuit has a high impedance at that point? On the other hand, if we the impedance is low around the frequency range to be used, we do need low ESR.
Parasitc inductance: it depends on the frequency range and use. If you are building a rectifier for a battery charger, basically ANY capacitor can be used considering this parameter. On the other hand, your circuit works with high frequencies (e.g. 500kHz) or the capacitor is in the feedback of an audio amplifier, ok, you really need to worry about it.
Robustness: in a small signal circuit, pre-amp for example, we have mA so not much to worry. But if you are building a SMPS, a passive crossover for 300W etc, you need robust capacitors to have a reasonable life span.
Leakage: it depends on the current level involved in the circuit and the purpose. In a timer circuit (RC in a 555 timer) or in a small signal high impedance input (MOhms) it might be important, but if you are using the capacitor for in a 4ohm crossover for the woofer section, sub mA leakages are negligible since we are talking about some Ampers.
Voltage limit: In general, you need just a margin to not exceed maximum voltage. In the case of electrolytic capacitors used for high current audio applications such as crossovers, sometimes it's good to choose a much higher voltage just to have a higher AC current capacity - higher voltage capacitors are, in general, bigger and thus, able to better handle more current and better heat dissipation. When I build a crossover with eletrolytic caps for, let's say, 100W and up, even if I only need 50V limit, I normally take the 160V or 250V due to the better AC current handling - this improves the life span.
Capacitor material: for audio you always want linear frequency behaviour, but if the application is not audio, sometimes linearity means nothing - e.g. an RF oscillator only works in one frequency.
Many other parameters which must be engineered so as to achieve a good final equipment, which achieves the goals and doesn't waste money.
Examples:
ESR: Why do we need very low ESR it if the circuit has a high impedance at that point? On the other hand, if we the impedance is low around the frequency range to be used, we do need low ESR.
Parasitc inductance: it depends on the frequency range and use. If you are building a rectifier for a battery charger, basically ANY capacitor can be used considering this parameter. On the other hand, your circuit works with high frequencies (e.g. 500kHz) or the capacitor is in the feedback of an audio amplifier, ok, you really need to worry about it.
Robustness: in a small signal circuit, pre-amp for example, we have mA so not much to worry. But if you are building a SMPS, a passive crossover for 300W etc, you need robust capacitors to have a reasonable life span.
Leakage: it depends on the current level involved in the circuit and the purpose. In a timer circuit (RC in a 555 timer) or in a small signal high impedance input (MOhms) it might be important, but if you are using the capacitor for in a 4ohm crossover for the woofer section, sub mA leakages are negligible since we are talking about some Ampers.
Voltage limit: In general, you need just a margin to not exceed maximum voltage. In the case of electrolytic capacitors used for high current audio applications such as crossovers, sometimes it's good to choose a much higher voltage just to have a higher AC current capacity - higher voltage capacitors are, in general, bigger and thus, able to better handle more current and better heat dissipation. When I build a crossover with eletrolytic caps for, let's say, 100W and up, even if I only need 50V limit, I normally take the 160V or 250V due to the better AC current handling - this improves the life span.
Capacitor material: for audio you always want linear frequency behaviour, but if the application is not audio, sometimes linearity means nothing - e.g. an RF oscillator only works in one frequency.
Many other parameters which must be engineered so as to achieve a good final equipment, which achieves the goals and doesn't waste money.
Capacitors in guitar pedals are typically not chosen for hi-fi characteristics. Rather they are chosen to give an interesting guitar tone. I have seen tantalum caps used for coupling and in filters, without question because they are nonlinear and the pedal designer thought it made for an interesting guitar sound effect. Also, often guitarists like pedals that have some "swish," some change in the sound with volume level, and or in other ways that change dynamically as the guitar is being played. OTOH, too much in the way of hi-fi sound can be boring for guitars. So, its fine to try different parts and see what sound you like.
Sure, as I said, it depends on the goals. You mentioned on more parameter to consider: in this case you want the non-linearity effects.
In musical instrument amps, most of times the circuit plus the speaker are chosen to color the sound, to add harmonics and help to create the final sound. That's why guitar players love tubes. It is the non-linearity effect and the saturation smothness of tubes that make them happy.
In musical instrument amps, most of times the circuit plus the speaker are chosen to color the sound, to add harmonics and help to create the final sound. That's why guitar players love tubes. It is the non-linearity effect and the saturation smothness of tubes that make them happy.