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June 16, 2019, 09:43:50 pm

Author Topic: Corner frequency question  (Read 1762 times)

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Offline Strikkflypilot

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Corner frequency question
« on: September 24, 2015, 11:33:05 pm »
If I am not completely wrong, the coupling caps e.g. in the Crack is part of a highpass filter on the output. The other part is the 2.49kohm resistor on the jack?

So if I want to keep the cutoff frequency and swap out the cap for a slightly larger or smaller one, I could if I also change the jack resistor, thus keeping the corner frequency but altering output impedance and time constant?
Home system:
Sources: Ibasso DX90, Google Chromecast Audio optical out
DAC: Schiit Gumby
Amp: Bottlehead Mainline
http://bottlehead.com/smf/index.php?topic=7463.0
Phones: HD800S

Office:
Sources: Iphone/ Ipod
DAC: Dragonfly Red+Jtrbug
Amp: Crack/Speedball heavily modded
Phones: HD580,HD600 grilles

Offline Strikkflypilot

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Re: Corner frequency question
« Reply #1 on: September 26, 2015, 06:45:36 am »
Thanks for the reply Grainger.
Slowly I am grasping elements of understanding:)
Home system:
Sources: Ibasso DX90, Google Chromecast Audio optical out
DAC: Schiit Gumby
Amp: Bottlehead Mainline
http://bottlehead.com/smf/index.php?topic=7463.0
Phones: HD800S

Office:
Sources: Iphone/ Ipod
DAC: Dragonfly Red+Jtrbug
Amp: Crack/Speedball heavily modded
Phones: HD580,HD600 grilles

Offline Paul Birkeland

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Re: Corner frequency question
« Reply #2 on: September 26, 2015, 06:50:48 am »
If I am not completely wrong, the coupling caps e.g. in the Crack is part of a highpass filter on the output.
Yes.
The other part is the 2.49kohm resistor on the jack?
No - see Grainger's post.
So if I want to keep the cutoff frequency and swap out the cap for a slightly larger or smaller one, I could if I also change the jack resistor,
The resistor on the jack will be sized to keep the voltage spike at the output of the amplifier decent when turning it on.  If you put in a higher value capacitor in, that resistor value should go down.  Doing so will load the amplifier a little bit more, regardless of which headphones you use.
thus keeping the corner frequency but altering output impedance and time constant?
The corner frequency is the headphone impedance in parallel with the resistor value in conjunction with the C value.  Output impedance is is not altered with these modifications.  (See the 6080 data sheet for the relationship between operating points and impedance)
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Strikkflypilot

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Re: Corner frequency question
« Reply #3 on: September 26, 2015, 08:33:58 am »
This is a great forum!
I really appreciate You guys taking Your time with this.
I will compare what You are writing with some schematics so I can visualize.

I hope it is ok to post this image here, it doesnt give away much I think. I thought this was the high pass filter.
« Last Edit: September 26, 2015, 08:45:17 am by Strikkflypilot »
Home system:
Sources: Ibasso DX90, Google Chromecast Audio optical out
DAC: Schiit Gumby
Amp: Bottlehead Mainline
http://bottlehead.com/smf/index.php?topic=7463.0
Phones: HD800S

Office:
Sources: Iphone/ Ipod
DAC: Dragonfly Red+Jtrbug
Amp: Crack/Speedball heavily modded
Phones: HD580,HD600 grilles

Offline Doc B.

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Re: Corner frequency question
« Reply #4 on: September 27, 2015, 04:21:04 am »
The point here is that the impedance of the headphones is so much lower than the 2.49K resistor in parallel with it that it's value will dominate in the filter calculation. For example, a 2.49K parallel with 300 ohm headphone yields an impedance of about 268 ohms. so your HP filter is composed of the output coupling cap and a 268 ohm resistor.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline Strikkflypilot

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Re: Corner frequency question
« Reply #5 on: September 27, 2015, 04:25:17 am »
Thanks Doc.

That was the final piece!
Now i got it.
Home system:
Sources: Ibasso DX90, Google Chromecast Audio optical out
DAC: Schiit Gumby
Amp: Bottlehead Mainline
http://bottlehead.com/smf/index.php?topic=7463.0
Phones: HD800S

Office:
Sources: Iphone/ Ipod
DAC: Dragonfly Red+Jtrbug
Amp: Crack/Speedball heavily modded
Phones: HD580,HD600 grilles

Offline theGopher

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Re: Corner frequency question
« Reply #6 on: December 23, 2018, 05:32:19 am »
Doc - doesn't the output impedance of the Crack also come into play in determining the -3dB corner frequency or is it only the bleeder resistance and headphone impedance after the output coupling capacitor that matter?

Offline Paul Birkeland

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Re: Corner frequency question
« Reply #7 on: December 23, 2018, 06:51:49 am »
Doc - doesn't the output impedance of the Crack also come into play in determining the -3dB corner frequency
The output impedance can influence where the -3dB corner is, but you have to take into account the actual impedance curve of each headphone you are trying to analyze (and the impedance usually moves pretty wildly at low frequencies, so you may dip down -5dB, then come back up to your reference level, then dip back down again).
or is it only the bleeder resistance and headphone impedance after the output coupling capacitor that matter?
This is a sufficient approximation, and is accurate if you simplify the load that your headphones present as a resistive one. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline theGopher

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Re: Corner frequency question
« Reply #8 on: December 23, 2018, 09:03:35 am »
Thanks Paul.

I'd seen mention elsewhere in the forums about the effect of Crack's ~120 Ohms output impedance on corner frequency. I could see how it might affect the overall gain at DC since it would form a voltage divider with the output cap's bleeder resistors in parallel with the headphones, but not how it could affect the AC frequency response of the C-R high pass filter former by the output cap and the output cap's bleeder resistors in parallel with the headphones.