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Author Topic: Output Coupling Cap Sizing  (Read 732 times)

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Deke609

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Output Coupling Cap Sizing
« on: April 20, 2021, 05:46:18 AM »
Is the size of the Beepre 1 and II output coupling cap simply determined using the -3db cutoff frequency equation, i.e., f = 1 over 2*pi*R*C, where R is the input impedance of the amp that the Beepre drives?

If yes, then

(1) the 10 uF stock cap size was chosen so that the BP can drive anything having an input impedance of 10K and up?

(2) 1 uF will suffice when driving a power amp with a 100K input impedance (e.g., my modded Kaiju)?

many thanks, Derek

Offline Paul Birkeland

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Re: Output Coupling Cap Sizing
« Reply #1 on: April 20, 2021, 06:07:50 AM »
The 10uF cap may have been in our inventory and seemed like the most appropriate choice when we put the kit out.  That isn't all that uncommon.

In BP2, that cap needs to be fairly large in order to properly work with the upcoming upgrades.  10uF is a solid choice for loads down to about 2-3K.
Paul "PB" Birkeland

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Deke609

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Re: Output Coupling Cap Sizing
« Reply #2 on: April 20, 2021, 06:21:51 AM »
Thanks PB. So, just to be clear, in the BP1, when driving my 100K input impedance Kaiju, 1,0 uF would be fine, right?

cheers and thanks, Derek

Offline Paul Birkeland

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Re: Output Coupling Cap Sizing
« Reply #3 on: April 20, 2021, 07:01:16 AM »
Sure, 1uF could be fine with that combination.  If you ever add a powered subwoofer with a line level input into the mix, then it won't be fine.
Paul "PB" Birkeland

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Deke609

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Re: Output Coupling Cap Sizing
« Reply #4 on: April 20, 2021, 07:37:01 AM »
Awesome. VCap CuTF output caps here I come!

... If you ever add a powered subwoofer with a line level input into the mix, then it won't be fine.

Unlikely, unless I figure out a way of strapping a subwoofer to my head for headphone listening   ;D  Then again, my JVC SZ-2000 headphones pretty much have that covered.  With sub 100Hz bumped up 15 dB, they are bass monsters!

cheers and thanks, Derek

Offline Toobuzz

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Re: Output Coupling Cap Sizing
« Reply #5 on: April 28, 2021, 03:50:30 AM »
I’m trying to understand this a bit more. If a 1uf cap would be sufficient for a load of 100k ohms, what would a 15uf cap bring to the mix?  Would going above the stock 10uf only be advisable with lower impedance loads?

I’m half trying to understand the logic and half trying to see if I can use this extra 15uf cap I have  ;D.  I’m initially using the BeePre with a 47k ohm set of solid state mono amps.

Offline mcandmar

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Re: Output Coupling Cap Sizing
« Reply #6 on: April 28, 2021, 05:36:04 AM »
Handy little site here to do the math for you, have a play with different capacitance values and see how it effects the low frequency roll-off.    In a recent preamp build i found i wasn't happy with .47uf into a 50k load, but closer to 1uf sounded much better.  I believe having the -3b point around 3hz is preferable.

http://sim.okawa-denshi.jp/en/CRhikeisan.htm

@Paul, have any words of wisdom in relation to phase shift, how important is it, how audible is it?    I tried Googling it and the answer i kept getting is what happens in a speaker crossover is way worse so don't worry about it.

M.McCandless

Offline Paul Birkeland

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Re: Output Coupling Cap Sizing
« Reply #7 on: April 28, 2021, 05:48:54 AM »
I’m trying to understand this a bit more. If a 1uf cap would be sufficient for a load of 100k ohms, what would a 15uf cap bring to the mix?  Would going above the stock 10uf only be advisable with lower impedance loads?

I’m half trying to understand the logic and half trying to see if I can use this extra 15uf cap I have  ;D.  I’m initially using the BeePre with a 47k ohm set of solid state mono amps.
If you ever plug a powered subwoofer into your BeePre, the load will suddenly drop substantially.  In the stock BeePre 2, the minimum load after the coupling cap is 10K.
Paul "PB" Birkeland

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Deke609

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Re: Output Coupling Cap Sizing
« Reply #8 on: April 28, 2021, 07:07:28 AM »
I am trusting that PJ or PB will correct me if I'm wrong here -- but I think it's worth pointing out that in both the BP1 and BP2 the output coupling cap is in series with two parallel resistances/impedances: (1) the input impedance of the amp it is driving; and (2) either a 475K resistor (BP1) or a 10K volume pot (BP2).

If the amp that is driven by the BP (1 or 2) has an input impedance of 50K to 100K, things are simple. For example, 475K || 100K is 82K-ish, and so pretty close to 100K and you can pretty much pretend as if the 475K wasn't there and a 1uF cap will suffice (I've replaced my 10uF ODAM cap with a 1 uF CuTF cap in my BP1 and am happy with the results). But if the power amp has only a 10K resistance, you can completely ignore the 475K resistor, but now you need an 8uF output coupling cap if you want good low end. (I've also read that there are weird phase/distortion effects at the low frequency roll-off region that are best avoided by choosing a -3dB cutoff of around 2Hz.  I've not done the math to check that this is true, and so am just parroting what I've read.  I also recall reading that is is a bigger issue for amps that, unlike the BP 1 and 2, use feedback.)

Likewise in the BP2, 10K || 100K is 9K-ish and you can pretty much ignore the 100K, but now your choice of output coupling cap is a bit more limited - it needs to be around 9uF-ish. But if the power amp has only a 10K input impedance, you're down to an effective 5K impedance (two 10K impedances in parallel), and you'll want the cap to be 16uF or so. I have no idea how all this might change with the future upgrades for the BP2.

In my most recent tweaking of my BP I removed the 475K resistors. If I correctly understood one of PJ's posts in another earlier thread, the purpose of the 475K resistor is to allow the output coupling cap to charge - e.g., even where the BP1 isn't connected to a power amp. But since mine is connected to my modded Kaiju, the BP1 output coupling caps can charge via the 100K input resistors in the Kaiju -- seems to work fine. In the BP2, the volume pot performs this function.

cheers, Derek

Offline Paul Birkeland

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Re: Output Coupling Cap Sizing
« Reply #9 on: April 28, 2021, 07:25:01 AM »


In my most recent tweaking of my BP I removed the 475K resistors. If I correctly understood one of PJ's posts in another earlier thread, the purpose of the 475K resistor is to allow the output coupling cap to charge - e.g., even where the BP1 isn't connected to a power amp. But since mine is connected to my modded Kaiju, the BP1 output coupling caps can charge via the 100K input resistors in the Kaiju -- seems to work fine. In the BP2, the volume pot performs this function.

If you remove those resistors, you are removing what keeps the output side of the coupling cap at ground potential and you are relying on your interconnect to do that.  If you power up your BeePre with no interconnects connected, then go to plug them in, bad things are going to happen.  Please put those resistors back!
Paul "PB" Birkeland

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Deke609

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Re: Output Coupling Cap Sizing
« Reply #10 on: April 28, 2021, 07:38:04 AM »
If you remove those resistors, you are removing what keeps the output side of the coupling cap at ground potential and you are relying on your interconnect to do that.  If you power up your BeePre with no interconnects connected, then go to plug them in, bad things are going to happen.  Please put those resistors back!

yup, many thanks, I figured as much. I didn't have spare 500k-ish resistors on hand, so I omitted them and ordered some 560K resistors that arrived yesterday. But I'm quite careful about making sure the BP1 is connected to the Kaiju before powering up the BP1 - so I am trusting that I'll be fine until I get the resistors installed sometime this week.

[Edit: and just to clarify: I omitted them b/c I was installing new output rca jacks, and b/c of the modified layout in my modded BP1, the 475K resistors were attached across the center pin to the ground tab of the jacks and weren't salvageable]

cheers, Derek
« Last Edit: April 28, 2021, 07:44:09 AM by Deke609 »