xrk971 Desktop Class A (DCA) Headphone Amp

[dbaldock9]

One other question about the DCA - It's something that you may have mentioned, and I just missed it - What input voltage swing does it take to drive the maximum output voltage swing (when set up with a 24Vdc linear supply as the input to the regulator section)? Thanks!

 

[xrk971]

Gain is about 12dB (4x) IIRC, max possible output is 20v-4v or 16Vpp. But that is badly distorted - I would stay below 12Vpp. So 3Vpp would be max needed on input.

 

[Keetesyflush]

Hey XRK - just wanted to say thanks for the awesome project. I just finished the build and can't wait to start cap rolling. I put my last two matched 2sk170's in and it sounds glorious.

 

[xrk971]

Nice work!

 

[xrk971]

Threre is a new variant of DC setpoints that uses an ultralow 118uA bias current on BF862 JFET. It is predicted to give great sound and nice dynamics. I have tested it on the pocket class A and it sounds great.

xrk971 Pocket Class A Headamp GB

Here is predicted FFT for 700mV into 55ohms (about 0.03% THD):

 

[lasercut]

Im putting together another DCA for balanced drive without any output caps,
to balance the DC on the output I was going to replace a 1k1 resistor (r106/109) with a trimpot, is there any particular reason to do this with the 22k (r104/105)below JFET instead like you mention in post 218?

 

[xrk971]

Yes that can work too. You might want to use a 1k resistor and 200 ohm pot set at midpoint 100R nominally.

 

[lasercut]

thanks, will report how it turns out

 

[xrk971]

Hey XRK - just wanted to say thanks for the awesome project. I just finished the build and can't wait to start cap rolling. I put my last two matched 2sk170's in and it sounds glorious.

Did you ever box this amp up? Curious what caps you settled on after rolling them?

 

[pinnocchio]

Hi X,

Just a quick question... Why is there no solder mask on top of those vias? I'm sure you can ask the PCB house to put a mask on them. It just safer against shorts insulation.

Just a suggestion

Thanks
Do

 

[xrk971]

Believe me I have asked. There is something with the order of the processes. Maybe I need to ask again.

 

[xrk971]

I think we found the solution - in Eagle it is called "tented vias" switch on. Hopefully future board fabs will have this, and the boards will lose their characteristic gold-riddled look. But will be much less prone to solder bridges.

 

[xrk971]

A compact version of this amp as a daughterboard for the
Yarra:

The YARRA Preamplifier/HPA for Melbourne DB Group Buy

 

[jwjarch]

X, can we assume that the SK209’s could be used in your pocket or desktops HPA’s as well? I’ve already struck out twice with fake BF862’s from EBay and I don’t care to try again. I have 100 2sk209’s on the way though.

 

[xrk971]

Yes, but resistor setpoints need adjustment.

For DCA (or DCA for Yarra) use this:
The YARRA Preamplifier/HPA for Melbourne DB Group Buy

I still have to run simulation for the pocket amp to get new setpoints with 2SK209.

 

[lasercut]

Also got fakes from ebay, the last few on RS dried up last year. ended building 2 IRF610 buffers instead for a balanced amp and left DCA as is.

 

[xrk971]

If you have an Aksa Lender preamp alaready and want to try the DCA or PCA, here is your chance - JPS64 just made an Aksa Lender preamp format card for the DCA/PCA:

The YARRA Preamplifier/HPA for Melbourne DB Group Buy

Your choice of JFET (BF862, 2SK209, 2SK170BL).

 

[ferret]

Hi X,
On the BigBottle thread Bigbottle MC only Valve/JFET phonostage Group Buy they were also using the BF862 which has dried up. They ended up recommending the BF861C, which has very similar (though the equivalent noise input voltage is double that of the BF862 at 1.5nV/root Hz) specs. This could maybe be used on the HPA?
Kevin

 

[adydula]

What external power supplies will work with this amp?
I have several wal-worts but want to make sure one of them might be a good candidate?

Any links to some would be greatly appreciated.
Alex

 

[xrk971]

It has a built in capacitance multiplier followed by a CRCRC that will give it very clean power despite what is coming into it. However, for best results, I have found that a 12v linear Class 2 wallwart circa 1000mA (typical ones found with wifi routers and access points, etc) work well to power a DC-DC step-up board (white aluminum PCB 10A variety). You could put a CLC (1000uF and 22uH 1000uF) after this, but find that it is not needed. The DC-DC allows you to adjust the output voltage to be about 22 to 24v so that you get about 18v to 19v into the amp at the Drain pin of the MOSFET.

If you wanted to, you could use a linear transformer (10VA to 25VA) and a bridge and CRC to get circa 22v to 23v, feed that into the cap Mx of this board to get the 18v to 19v needed. But this is complicated

Here are some examples of PSU's that can work:
Westell 085-200054 Class 2 Power Supply 12V DC 1000mA Adapter Tested | eBay

The strange thing is don't use a SMPS to feed a SMPS DC-DC - it gets noiser. A linear feeding the DC-DC step up uis quieter.

For the DC-DC use these:
DC Boost Converter-Constant Current Mobile Power Supply 250W 10A LED Driver 853879987626 | eBay

If you look at DrPro's example, this is the way to do it: