BF862 are nice JFET transistors.
I have downloaded the datasheet into my library.
They have very good current for being JFETs.
Another thing,
could you display the currents in your schematic.
It is helpful for understanding.
Also specify voltage, especially the power supply.
I have downloaded the datasheet into my library.
They have very good current for being JFETs.
Another thing,
could you display the currents in your schematic.
It is helpful for understanding.
Also specify voltage, especially the power supply.
Hi Lineup,
Yes, the BF862 is really nice sounding. Super low noise, and very inexpensive. Has 20v limit though. Here is a typical DC setpoint for the DCA when running a hot 150mA bias. There is also a 125mA setting that has a bit more H2 vs H3 but overall higher THD. Still, 0.02% to 0.05% THD for driving 30ohm cans is not bad for a two transistor amp with no global feedback.
Cheers,
X
FFT for 2vpp into 30ohms:
Yes, the BF862 is really nice sounding. Super low noise, and very inexpensive. Has 20v limit though. Here is a typical DC setpoint for the DCA when running a hot 150mA bias. There is also a 125mA setting that has a bit more H2 vs H3 but overall higher THD. Still, 0.02% to 0.05% THD for driving 30ohm cans is not bad for a two transistor amp with no global feedback.
Cheers,
X
FFT for 2vpp into 30ohms:
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For C113/114 there is room for a 16V silmic, do you think a higher voltage cap might be a better choice?
also would 240 ohm be the optimal value for R124/125 when using 45ohm headphones?
edit: I see you already answered this, sorry.
also would 240 ohm be the optimal value for R124/125 when using 45ohm headphones?
edit: I see you already answered this, sorry.
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For the Pocket amp with 16v in batteries that can work but desktop amp with higher voltage, use 25v minimum on cap. Technically 16v can work but is too close you may get more distortion.
240 ohm should be ok.
240 ohm should be ok.
In preparation for testing out the balanced drive setup, I just built a second DCA. Works like a charm upon initial power up. This is an easy to build amp - guaranteed to run if you don't make any mistakes in stuffing it.
I only had some smaller stubby heatsinks, and they work fine still. In the foreground are some nice SE Class A, unbalanced-to-balanced drive phase converters made with just two BJTs. Those are powered by 4x9v batteries for 36v Vcc. My DAC has balanced outputs so not absolutely needed, but useful for plugging in any regular unbalanced source.
I only had some smaller stubby heatsinks, and they work fine still. In the foreground are some nice SE Class A, unbalanced-to-balanced drive phase converters made with just two BJTs. Those are powered by 4x9v batteries for 36v Vcc. My DAC has balanced outputs so not absolutely needed, but useful for plugging in any regular unbalanced source.
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Some hand adjustments of resistor values is giving more balanced output. The R106/R109 1k1 resistor was producing early negative clipping. Changing to 910R improved things a lot.
Adjustments of R107/R111 to 56R seems to give a good balance of overall low THD and good proportions of H2 vs H3.
Vcc is set at 20.07v at the MOSFET drain.
For 1,0vpp into 270ohms, this gives 0.023% THD and H3 is -18dB below H2.
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It sort of depends on your particular JFETs and MOSFETs, but if you are using the JFETs I provided, yes. One way to check is to adjust the R106/R109 up or down a bit to get about Vcc/2 at the output node above the power resistors and on the source side of the MOSFETs. Within 0.5 to 0.7v is ok but if it is more than 1v off, then adjust it. The default 1k1 was giving me 8.3v with 20v Vcc, so it was clipping way early on bottom. With new value, it was 9.4v. As for the balance in H2/H3, you need to look at FFT but using a value between 51R and 67R should be ok - you can adjust to taste. I did find that Vcc of about 20.2v was the sweet spot and 21.5 give very low THD but H2 and H3 are about equal.
Use the above values as a starting point and I think you will find it sounds very good.
Use the above values as a starting point and I think you will find it sounds very good.
It is a bit funny.
The BF862 is not having gate refered to ground.
Instead it is coupled as a constant current source.
And of course it works.
It is a clever circuit and with very good results.
I have looked here in Sweden
but not found BF862 🙁
EDIT:
Correction
I found one BF862.
But it is a small SMD variant SOT-23:
Kop MOSFET N, 20 V 0.025 A 300 mW SOT-23, NXP, BF862,215 | Elfa Distrelec Sverige
The BF862 is not having gate refered to ground.
Instead it is coupled as a constant current source.
And of course it works.
It is a clever circuit and with very good results.
I have looked here in Sweden
but not found BF862 🙁
EDIT:
Correction
I found one BF862.
But it is a small SMD variant SOT-23:
Kop MOSFET N, 20 V 0.025 A 300 mW SOT-23, NXP, BF862,215 | Elfa Distrelec Sverige
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Hi Lineup,
The circuit was originally developed with help from Aksa, so of course it works well 🙂 BF862 is only available as a tiny SOT23. That’s the only SMT part on this DCA amp. I have through holes available for a TO92 like 2SK170 but as you know, those are hard to come by. Sadly BF862 is at EOL and only available from places like eBay or Aliexpress or boutique electronic shops. Fortunately they are cheap and no one bothered to fake them. Interesting thing about BF862 is that D and S are interchangeable. This makes layout simpler. It acts like a gate controlled variable resistor going either direction.
The circuit was originally developed with help from Aksa, so of course it works well 🙂 BF862 is only available as a tiny SOT23. That’s the only SMT part on this DCA amp. I have through holes available for a TO92 like 2SK170 but as you know, those are hard to come by. Sadly BF862 is at EOL and only available from places like eBay or Aliexpress or boutique electronic shops. Fortunately they are cheap and no one bothered to fake them. Interesting thing about BF862 is that D and S are interchangeable. This makes layout simpler. It acts like a gate controlled variable resistor going either direction.
I have a supplier in Sweden who sells 2SK170GR.
https://www.bhiab.se/index.html
But I will not buy any 2SK170GR from them.
I will buy 2SK246BL and 2SK103BL to get complementary pairs.
They are good. Takes 50 Volt.
BL is the high current variant. 6-14 mA.
GR is 2.6-6.5 mA.
That supplier even sells 2SJ74. But has not this in store. But 2SK170 is in store.
Same supplier sells 2SK216 and the complementary 2SJ79.
I will probably invest in those, too.
https://www.bhiab.se/index.html
But I will not buy any 2SK170GR from them.
I will buy 2SK246BL and 2SK103BL to get complementary pairs.
They are good. Takes 50 Volt.
BL is the high current variant. 6-14 mA.
GR is 2.6-6.5 mA.
That supplier even sells 2SJ74. But has not this in store. But 2SK170 is in store.
Same supplier sells 2SK216 and the complementary 2SJ79.
I will probably invest in those, too.
For this DCA amp, you don’t need the high voltage offered by hose 2SK’s. I think noise performance of BF862 is better than 2SK170. If you get the DCA, I include a set of BF862’s at no charge.
If anyone for this build or others you might be building, I recently scored some NOS Fairchild IRF 610. Got a hundred.
1usd for two needed in the DAC, shipping etc covered. If you need more, we can figure it out.
I will also VGS match them to best of my ability.
Thanks
David
1usd for two needed in the DAC, shipping etc covered. If you need more, we can figure it out.
I will also VGS match them to best of my ability.
Thanks
David
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If it is better than 2SK170 ... then it is really good 🙂
Thanks for your offer. But I will not build DCA. I am not even sure I will build my own headphone amp. That one with BF245 in the input.
I have recently made a new start of my electronics. There are plenty of things to buy before I can start building some experiment. But I have bought a full set of 0.6W metalfilm resistors. That's a good start.
For DCA, I think a simple LM317 CSS can great improvement in efficiency and output voltage swing.
I think he means LM317 instead of the array of four 270R parallel load resistors. It’s a cheap and easy constant current source. But my only misgiving is that it is tantamount to adding an opamp in the audio path. An LM317 is an opamp comparator with high current capability. Alternatively, a 50mH choke can work very well too. No opamps.