Hello,
I am learning about coupling capacitors and how to determine the correct values to use. I've been reading a number of threads throughout Diyaudio that discuss this topic, and I believe I'm starting to understand.
For the purposes of this discussion, let's assume the coupling capacitor at the preamp output (C1) is 10uF, and the coupling capacitor at the power amp input (C2) is 1uF and has an input impedance of 100k.
To determine the -3db corner frequency, I would use this calculation: F = 1/(2pi*C*Z). In this example, because the C1 and C2 are in series, C equals (C1*C2)/(C1+C2) or 0.91uF. Doing all the math, the corner frequency would be 1.75 Hz.
My first question is: is the above correct?
Secondly, I believe that I read that because C1 and C2 are in series and because C1 is much larger (10x) than C2, that the capacitance of C1 is essentially negated. Does that mean that C1 could be 100uF and still result in a very similar -3db point (1.61 Hz)?
Finally, I've seen many people suggest that a good rule of thumb is to use 1uF coupling capacitors on inputs and 10uF on outputs. Is that a "safe" approach, perhaps because you can assume most consumer gear follows similar guidelines?
Thanks!
I am learning about coupling capacitors and how to determine the correct values to use. I've been reading a number of threads throughout Diyaudio that discuss this topic, and I believe I'm starting to understand.
For the purposes of this discussion, let's assume the coupling capacitor at the preamp output (C1) is 10uF, and the coupling capacitor at the power amp input (C2) is 1uF and has an input impedance of 100k.
To determine the -3db corner frequency, I would use this calculation: F = 1/(2pi*C*Z). In this example, because the C1 and C2 are in series, C equals (C1*C2)/(C1+C2) or 0.91uF. Doing all the math, the corner frequency would be 1.75 Hz.
My first question is: is the above correct?
Secondly, I believe that I read that because C1 and C2 are in series and because C1 is much larger (10x) than C2, that the capacitance of C1 is essentially negated. Does that mean that C1 could be 100uF and still result in a very similar -3db point (1.61 Hz)?
Finally, I've seen many people suggest that a good rule of thumb is to use 1uF coupling capacitors on inputs and 10uF on outputs. Is that a "safe" approach, perhaps because you can assume most consumer gear follows similar guidelines?
Thanks!
The calcs look about right.
Basically you size output caps to give a -3dB point way below (A decade or two) the audio band when driving into whatever you have decided should be the lowest expected load impedance, and you size input caps to give a -3dB point way below the audio band given your known load impedance.
Note that a coupling cap does NOT usually have much signal voltage imposed on it, there are NOT filter caps in any sense that matters, so going large has no real downside.
Regards, Dan.
Basically you size output caps to give a -3dB point way below (A decade or two) the audio band when driving into whatever you have decided should be the lowest expected load impedance, and you size input caps to give a -3dB point way below the audio band given your known load impedance.
Note that a coupling cap does NOT usually have much signal voltage imposed on it, there are NOT filter caps in any sense that matters, so going large has no real downside.
Regards, Dan.
That is about right, if it accounts for all the resistors to ground after the first coupling capacitor.
If there is, say, a 10k to ground in the preamp after the 10uF, that will change the results.
If the preamp has a 1M resistor to ground at the output instead, then not so much.
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Yes.
Not "negated", but "negligible".
As you yourself showed by your 1.75Hz-1.61Hz difference. A single-pole roll-off is a VERY b-r-o-a-d thing. For all practical purpose this is "2Hz". To cover part-slop, in a Spec Sheet I might put "-3dB @ 5Hz" and let the user be thrilled to actually get 2Hz or 1.75Hz.
You want "enough". You want to be generous because a complete audio path (studio to speaker) has a dozen or a thousand coupling networks. You usually want to be "much below 20Hz". In DIY home gear where you know you only have a few C-R nets, 2hz is generally ample. A large studio console may have many-many couplers aimed below 0.2Hz, so the whole-chain response is <1dB @ 20Hz.
"Most" consumer gear is not "ALL" consumer gear. =>10K is a fairly safe bet for 99% of inputs, but I have seen 2.4K and 800 and (on oddballs) even lower.
And that is today. In vacuum tube gear even 100K was low.
On your output, what is the output able to drive well? If it is a 12AX7 then even 50K load will be a strain mid-band. So a 0.2uFd cap will give <2Hz on any load it should be driving. If it is an audio op-amp chip it can probably drive 500r comfortably, and >10uFd would ensure no shy bass even with heavy loads.
Is there a need to keep both caps in picture?
I would simply jumper the pre-amp cap if the power amp already has it.
especially if power amp has a high quality film cap and pre has a lowly electrolytic..
I would simply jumper the pre-amp cap if the power amp already has it.
especially if power amp has a high quality film cap and pre has a lowly electrolytic..
That's not safe, since DC would appear on the preamp output jack, the cable, and power amp input jack.
Also, the preamp capacitor may need a higher voltage rating than the one in the amplifier,
so the amplifier's capacitor could be damaged.
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Gear has to be designed stupid-proof. All DC blocked in and out.
Yes, you can jump your own caps. But remember "stupid proof". What if you need to make a change, in a great hurry, before enough coffee or after too much wine? Will you always remember the special conditions?
There are always exceptions. Tube guitar amps traditionally have no input cap because a passive guitar never has DC coming out. (And this gives trouble in a pedal-filled world, so we now see in-caps even on "re-issues" of classic designs.) Loudspeaker output caps must be huge/costly, so much design focuses on omitting them.
The preamp does have 10k to ground after the coupling cap. How do I factor that into the equation?
Also, the power amp has 1M to ground on it's input before it's coupling cap. Does this need to be factored in, or is it considered as part of the input impedance?
Thanks for all your help!
thanks rayma, but I mistakenly assumed its an integrated amp...its what i am building
if they are separate entities, yes, both needed..
if they are separate entities, yes, both needed..
Then you have 10uF/10k in the preamp, and 1uF/100k in the amp.
There's a 1M in between, but that doesn't change things significantly.
To a rough approximation, you have two coincident poles at 1.6Hz.
This isn't surprising, but it should be avoided if possible.
I'd increase the 10k to 100k instead. What is your preamp, or if diy,
is it tube or ss?
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Even in integrated amps, separate coupling caps are often used on the preamp
and the power amp pcbs. That way there's no DC on the internal connectors or wiring.
In your diy integrated amp, that likely isn't needed, and should work better.
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The type of capacitor also matters. If you're using electrolytics, it's common to overspec the capacitance by an order of magnitude or more than you'd strictly need from a bandpass perspective. This is done to ensure the ripple across the electrolytic is negligible at audible frequencies, which in turn ensures it's contribution to distortion is very low.
If you're swapping electrolytic caps for film ones, you can usually use much smaller film caps to do the same job. As an example the pre out on my NAD3020 had 100µF electrolytics, giving me something like 200mHz cutoff for a nominal 1KΩ power amp input impedance. I substituted a 4µ7 film cap (physically around the same size), which increased the cutoff to ~5Hz with the same load. I could do this because the distortion contribution of film caps is significantly less than that of electrolytics.
If you're swapping electrolytic caps for film ones, you can usually use much smaller film caps to do the same job. As an example the pre out on my NAD3020 had 100µF electrolytics, giving me something like 200mHz cutoff for a nominal 1KΩ power amp input impedance. I substituted a 4µ7 film cap (physically around the same size), which increased the cutoff to ~5Hz with the same load. I could do this because the distortion contribution of film caps is significantly less than that of electrolytics.
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Yamaha CX-1000U preamp.
The reason for these questions is that I'm in the middle of a recap and the pre and rec outputs all use 10uF/50v, while the inputs use 1uF/50v. All electrolytic. I've replaced the 1uFs with 1uF Wima film, but 10uF runs into space constraints. So, I've been trying to understand how and why these values are calculated as they are.
Thanks very much for your help.
I see. So, in my example, I could use something like a 3.3uF film on the preamp output in this example, which would set my cutoff point to like around 2Hz, and the optimal low frequency response around 20Hz.
Any values less than 3.3uF I'd start jeopardizing the low frequency response, correct?
Thanks for your help!
You can go a fair bit lower than 3µ3 before a 20Hz cutoff goes awry. I generally use a 2 octave rule of thumb, hence the 5Hz. That said, if a 3µ3 film cap fits nicely, go for it.
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