I have not considered multilayered ceramic caps but I will look into them. Certainly it would be worth switching them in and out to see how they compare to the more (sexy) appealing film caps.
Thanks to everyone who helped me to understand this schematic better. I feel that I have a better understanding of signal amplification now.
Film caps are not appropriate for power supply decoupling (bypassing) because they have relatively high inductance. You are risking RFI problems in your amp if you replace the ceramics with film.
Not to adversarial here because I do not have a firm grasp of all of this concepts. I am just trying to go on the numerous things that I read and then try to make an educated decision. I believe that MKP type capacitors have been widely tested and reviewed in power supplies, like the Solen FastCap & Mundorf MKP (10uf value). Mundorf states that “The practically induction-free type of construction and the low loss factor of the MCap® results in a very "quick“ capacitor." Also the RELCAP RTE Polystyrene caps (.01uf value) have an extended foil construction and were constructed for this purpose. What am I misunderstanding here?
I was referring to typical film caps, not super fancy ones that have been specially designed to have low inductance. However, it is not clear to me that even these caps have low enough inductance to be effective at RF frequencies. Bypassing DC power supplies is for one thing to suppress RFI in the circuit. I don't see any advantage in using film caps in place of good ceramic ones in the power supply decoupling.
Last edited:
Most film caps will have significantly less inductance than an equal value of electrolytic cap.
Some will have an inductance that is so much lower than equivalent or very special electrolytics that you can almost ignore it.
Bypassing at the PSU is only needed if the PSU will perform better with it.
What is needed is decoupling at the receiver end of the power rails. The big current consumers are the output devices. They can turn on demand in an instant and turn off almost as fast. They need decoupling. That's where to spend your time designing.
Some will have an inductance that is so much lower than equivalent or very special electrolytics that you can almost ignore it.
Bypassing at the PSU is only needed if the PSU will perform better with it.
What is needed is decoupling at the receiver end of the power rails. The big current consumers are the output devices. They can turn on demand in an instant and turn off almost as fast. They need decoupling. That's where to spend your time designing.
Yeah, I agree it's a good idea to add capacitance at the output transistors and bypassing those with a good low inductance capacitor. I did that in my Crown amps.
I think some of this internal resonance adds to the so called sonic signature of the film cap giving the perception of a more tube like (distortion) sound or feeling of open space in the music.
So getting back to my original inquiry then. If I use a 10uf cap at the output transistors would it be beneficial to also have a lower value cap like a .01 to address higher frequencies specifically or is it redundant and unnecessary?
Decoupling has to handle a variety of frequencies.
The highest harmonics due to switching on a BJT from Zero current to output current go way above the audio frequency range, not just 10 times our accepted 20kHz hearing limit.
Expect the harmonics to demand current right up to the MHz range for fast transients.
These can only be met by ensuring the trace lengths are kept very short.
The round circuit route from the output device to the decoupling cap to the PCB Power Ground and back to the PSU (the other half of the decoupling pair) must be in the mm range. A 10uF cannot be placed in a route that short.
A 0.1" pitch ceramic (x5r) is very small and the leads can be trimmed quite short, 1mm each.
The highest harmonics due to switching on a BJT from Zero current to output current go way above the audio frequency range, not just 10 times our accepted 20kHz hearing limit.
Expect the harmonics to demand current right up to the MHz range for fast transients.
These can only be met by ensuring the trace lengths are kept very short.
The round circuit route from the output device to the decoupling cap to the PCB Power Ground and back to the PSU (the other half of the decoupling pair) must be in the mm range. A 10uF cannot be placed in a route that short.
A 0.1" pitch ceramic (x5r) is very small and the leads can be trimmed quite short, 1mm each.
Thanks. This was helpful. It may be beneficial to use multiple bypass caps in parallel but I am going to insert them one at a time so that I can listen for any differences, good or bad. Starting by returning the amp to its original circuit design. I was trying to save myself a step by inserting multiple bypass caps at once but its not worth not knowing what kind of perceivable change I made if any.
Hey Osvaldo - I hope you had a good vacation!
I was hoping you or someone else would be so kind as to explain to me what the consequences might be if I were to increase capacitance on the Driver Board for C1 & C6? Right now C1 calls for a 100uf 25V but I increased it to a 220uf Jensen because I had read that it was o.k. to increase the input cap. However, I left C6 at the specified 470uf 16V. I found a Blackgate that isn't too expensive that is 680uf / 65v and would like to use it for C6 but I don't want to mess with the balance of anything by changing the the value of the NFB. The Schematic is already posted if anyone would be so kind as to educate me. I would like to know how the circuit works.
I would leave C6 at the specified value but use a good audio quality electrolytic there.
Thanks for the reply. I did use a Nichicon KZ Muse. Generally speaking though, do these 2 caps contribute to frequency roll off or do they have a relationship to one another in a push pull fashion. If I increase it will there be too much negative feedback? How does the circuit work. I know it is a lot to explain but if you could point me to a paper to read that would be cool.
C1 blocks DC at the input and C6 provides, in combination with the feedback resistors, the low frequency high pass for the amplifier. In theory, increasing C6 will lower the low frequency cut off but it's already probably down around 1 Hz or so already.
Thank You Again! I am tired of not knowing how to make my own decisions. It is time to find a book or something, maybe classes
Ask and yea shall receive:
http://www.amazon.com/Designing-Aud...024X/ref=sr_1_1?ie=UTF8&qid=1328738282&sr=8-1
http://www.amazon.com/Audio-Power-A...=sr_1_1?s=books&ie=UTF8&qid=1328738316&sr=1-1
I don't have either of these yet...
C1 and R2 form a high pass filter which rolls off the low frequency response at the input. C6 and R13 form another high pass filter which also rolls off the low frequency response.
In each case the corner frequency is given by: F = 1 / (2 * Pi * R * C) where R is resistance in ohms and C is capacitance in Farads. It's easier to work with Kilohms and microfarads though, in which case the formula is: F = 1000/(2*Pi*R*C).
So for C6 = 470uF and R13 = 1.5K, F = 0.23Hz i.e. The frequency response is 3dB down at 0.23Hz (and 1dB down an octave higher at 0.46Hz). Plenty low enough, IMHO.
For C1 = 100uF and R2 = 30K, F = 0.053Hz! When Dirk guessed 1Hz, he was thinking "normal". It's actually 20 times lower. You could drop C1 all the way down to 1uF and the response would still only be 1dB down at 10Hz.
For fast mental computation it s even more easier to make the calculus
using the units normalized to 1 to 9.9... and then multiplying by the relevants
powers of 10 wich will allow to simply add negative and positive exponents.
Dirk, I found a good website here on circuits. Here is a review article on input coupling Input and output coupling : BIPOLAR JUNCTION TRANSISTORS and Feedback Feedback : BIPOLAR JUNCTION TRANSISTORS It gives nice simple explanations of some basic things which I need, but I am still going to look into the books you suggested as well. Thanks.
Godfrey, Thanks for explaining the low frequency roll off. Now that I understand that C1 & C6 define the bandwidth I just remembered that this was part of the manufacturer's design philosophy - "Ultrawide Bandwidth". Should I be concerned that I increased the capacitance of C1 from 100uf to 220uf under the assumption that it would increase the bass response or is it just basically over kill. I certainly wouldn't want to do any harm from increasing the bandwidth even further but I am thinking I should return it back to spec.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.