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xrk971 Desktop Class A (DCA) Headphone Amp
xrk971 Desktop Class A (DCA) Headphone Amp
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Old 16th May 2018, 04:13 PM   #21
twocents is offline twocents  South Africa
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xrk971 Desktop Class A (DCA) Headphone Amp
Thank you BDHM for the info and link most interesting. Wish I saw this before, but already have a stepped attenuator on the way for my Lender preamp.

X, note what Rod Elliot says: A linear pot used for volume is quite unsatisfactory.
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Old 16th May 2018, 08:02 PM   #22
badboygolf16v is offline badboygolf16v
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Hi X

Please could you clarify the required input and output caps for me.

Which of C101, C101A, C101B, C101C and C101D are mandatory. I'm guessing one of the 10uF is mandatory and one smaller value as bypass is optional.

Is C113 mandatory and C111/A/B bypass optional?

Thanks
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Old 16th May 2018, 09:08 PM   #23
xrk971 is offline xrk971  United States
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xrk971 Desktop Class A (DCA) Headphone Amp
On the input cap, at least one cap of minimum 4.7uF is mandatory. It can be electrolytic or film. Or you can use a big one and a small bypass.

On output cap, minimum is 2200uF and if you want bypass with filter. Use a good name brand one here rated at least 35v.
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Old 16th May 2018, 11:26 PM   #24
pelayostyle is offline pelayostyle  United States
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Hi xrk. Will be primarily using my HD-650's which are 300R. What value would you recommend for R124 and R125?
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Old 17th May 2018, 02:04 AM   #25
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by pelayostyle View Post
Hi xrk. Will be primarily using my HD-650's which are 300R. What value would you recommend for R124 and R125?
Click the image to open in full size.

R124 and R125 are load resistors that help to drain the output coupling capacitor so that it doesn't hold a high voltage that might cause a pop when you plug phones in. Since this amp can drive 30ohm loads just fine, and your cans are 300ohms, having say, 300ohm or 270ohm R124/125 would be equivalent of a 150ohm load on the amp, so that is not a problem. I would use anything from 270ohms to 680ohms 1/4w or 1/2w if you have it.

The HD560, Btw, is the *Ideal* headphone for this amp. You will love it.
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Last edited by xrk971; 17th May 2018 at 02:10 AM.
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Old 17th May 2018, 02:29 AM   #26
xrk971 is offline xrk971  United States
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xrk971 Desktop Class A (DCA) Headphone Amp
Default Balanced Drive SE Class A HPA

In case anyone has a DAC or preamp with balanced outputs (+ve and -ve phase balanced line driver), and they have headphones modified to accept balanced drive amps, there is an easy way to make yourself a unique, and very cost effective balanced SE Class A HPA. Just use two DCA boards, one for each earpiece (driver). By doing this, you get 4x the power, and twice the slew rate. Best of all, because this is a SE Class A amp that is cap-coupled, you never need to worry about the amp going DC on you and blowing out your expensive cans. I have tried this with the PCA before and it sounds fantastic. So much dynamic reserve, and everything sounds so much more powerful.

Here is schematic:
xrk971 Desktop Class A (DCA) Headphone Amp-dca-balanced-drive-hpa-png

So, if you were to run each amp with Vcc of 25v, and 150mA bias current, with 42ohm cans you can get about 2.2wrms and +/-20vpp swing from an effective Vcc of 50v. This is useful for super inefficient 74dB planar headphones.
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File Type: png DCA-Balanced-Drive-HPA.png (140.1 KB, 1040 views)
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Last edited by xrk971; 17th May 2018 at 02:33 AM.
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Old 17th May 2018, 08:38 AM   #27
abraxalito is offline abraxalito  United Kingdom
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In balanced mode you have the two 2200uF output blocking caps in series, giving you an effective reduction to 1100uF. Best short one out or increase their size to 4700uF to maintain the same bass response.
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Old 17th May 2018, 10:25 AM   #28
lineup is offline lineup  Sweden
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Quote:
Originally Posted by xrk971 View Post
On the input cap, at least one cap of minimum 4.7uF is mandatory. It can be electrolytic or film. Or you can use a big one and a small bypass.

On output cap, minimum is 2200uF and if you want bypass with filter. Use a good name brand one here rated at least 35v.
I calculate that 470uF output is good enough for my 32 Ohm phones.
This gives a roughly 10Hz lower frequency.
Of course anything more, like 2200uF, will not hurt.
Especially if you have 8 Ohm phones you need 2200uF.
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Old 17th May 2018, 11:08 AM   #29
yourownbuzz is offline yourownbuzz  Australia
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Woah x just got home and saw that the boards have arrived. What kind of postal magic did you do? That was crazy quick!
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Old 17th May 2018, 11:12 AM   #30
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by lineup View Post
I calculate that 470uF output is good enough for my 32 Ohm phones.
This gives a roughly 10Hz lower frequency.
470uF will give you around -3dB at 10Hz. Electrolytic caps will add distortion at this frequency, making the cap a bigger value will lower distortion. There's also the matter of LF phase shift which is increased with smaller valued caps. I myself prefer to use at least 2200uF with my 56ohm Superluxes.
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