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BF862 based SE Class A Headamp without the HEAT
BF862 based SE Class A Headamp without the HEAT
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Old 17th November 2017, 05:44 PM   #501
Mayday is offline Mayday  Sweden
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BF862 based SE Class A Headamp without the HEAT
Quote:
Originally Posted by xrk971 View Post
The top is at 19v but their source is at 10v (as can be seen as input to drain of bottom FETs), so net result is only about 9v Vds on top FETs.

So do you have first sound yet?
I see what you mean. You are of course correct.
No, no sound yet as I don't like the high DC-offset with the volume all the way down.
I'm trying a different things to see if I can remedy that.
Is this something you have come across?
I may have brainf@rted, with a capacitor on the output, it needs a load...right?
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Last edited by Mayday; 17th November 2017 at 05:55 PM.
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Old 18th November 2017, 12:41 PM   #502
Mayday is offline Mayday  Sweden
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BF862 based SE Class A Headamp without the HEAT
I put the oscilloscope, well the closest thing to it I have atm, a DSO138 on the DAC output and Edcor secondaries.
I got nothing but jibberish. I sent 1Khz sine & square wave via audacity.
Conclusion, the error I made that caused the PS to not reach full voltage killed the CS4397 IC. I've got a new one on the way.
While having the thing apart, I desoldered the 470uF/25V ROA Cerafine output caps and the Silmic II 100uF/25V PS cap. I soldered the Cerafine back in but as PS cap.
I'll put a 1000uF/10V Cerafine as output cap and bypass it with a nice 1uF MKP.
I also ordered some more of the 33R Takman 0.25W metal film resistors to put in parallel with the ones already on the headamp boards. That would put bias from about 8mA to around 10mA per jfet. That, and the bigger output cap with bypass cap should be an improvement with regards to SQ.

I really need at least a Hantek DSO5072P or similar, hard to justify the expense with the missus though as "I have too much crap as it is " lol.
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Old 22nd November 2017, 05:39 AM   #503
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Quote:
Originally Posted by Mayday View Post
with a capacitor on the output, it needs a load...right?
Yes, something like 270ohms to 470ohms should work to bleed off the cap due to initial turn-on. Otherwise your cans will go thump when you plug them in.

Nice thing with cap coupling is it is inherently safe for your cans.

I got an Owon SSO8192V 100Mhz 1G/s 8" LCD Display 1M Depth Memory VGA Port Oscilloscope for about $270. Works great - 1M depth is very nice and huge screen compared to Hantek.
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Last edited by xrk971; 22nd November 2017 at 05:43 AM.
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Old 27th November 2017, 12:59 PM   #504
Mayday is offline Mayday  Sweden
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I got the Cerafine 1000uF/10V caps and the additional 33R Takman resistors today.
Haven't really had much time for diy the last couple of days, but I hope to get the headamp fully populated again in the next few days.
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Old 27th November 2017, 03:49 PM   #505
Mayday is offline Mayday  Sweden
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BF862 based SE Class A Headamp without the HEAT
Well I got a little more done today
I populated the output capacitors and the extra 33R Takman resistors(on one of the boards as I wanted to test bias in both setups).
I used a 9V battery as power supply.
33R in parallel for 16.5R gave 6.97mA, 33R as is gave 5.30mA.
I did put a Wima MKS2 4u7 across each output capacitor.
I got a 0.001mA difference between all output pairs. I'm ok with that result.
The pics show only one, the first, resistor I put in parallel with the ones already on board.
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Old 15th December 2017, 12:56 PM   #506
xrk971 is offline xrk971  United States
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BF862 based SE Class A Headamp without the HEAT
Mayday,
Are you playing music yet?
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Old 15th December 2017, 02:48 PM   #507
Mayday is offline Mayday  Sweden
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BF862 based SE Class A Headamp without the HEAT
Nope, unfortunately not.
Got the 22Vdc supply the other day, it rattled and measured at 0Vdc...
My painlevels hasn't really allowed much diy lately either.
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Old 5th April 2018, 03:36 AM   #508
sarathssca is offline sarathssca  India
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can we have this amp modified for dual rail power supplies say +/- 7v or +/-9v instead of around 18v currently being used?
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Old 5th April 2018, 05:23 AM   #509
sarathssca is offline sarathssca  India
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Quote:
Originally Posted by sarathssca View Post
can we have this amp modified for dual rail power supplies say +/- 7v or +/-9v instead of around 18v currently being used?
and the purpose is to use it as a preamp before a amp like ACA and would not be using as a standalone headphone amp. I am looking for dual power supply option because the volume control IC (DS1882) needs a +/-7v supply and would like to pair it up wit this before the ACA or any other power amp.

example of DS1882 is given in :
Does anyone know how to use the digital potentiometer DS1882
where its using opamps buffers following.
DS1882 needs very high input impedance stage after it in the range of 1Mohms (buffer or preamp).

Last edited by sarathssca; 5th April 2018 at 05:27 AM.
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Old 9th April 2018, 10:23 PM   #510
mlackey is offline mlackey  United States
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Mayday, XRK,

Be careful about adding a small resistor to the output of the cap. The output feeds back into a resistor divider on the input, which sets up the gain of the amp. Adding a small value resistor to the output will change the divider, and effectively reduce the gain to near zero. As designed, the 100K in the divider also functions as a pull-down, bleeding off the output cap's DC leakage. If you need a larger pull-down, scale the divider appropriately, but also be aware that doing so will decrease the input impedance.

On another note, I was intrigued that Mayday was actually building the year-old board, so I dug up my old boards out of the box. But after listening I made a change. I replaced the single 470uF blocking cap with 2 ea 1000uF bipolars (Nichicon UES 1000uF 25V). Its my opinion that the sound is much improved, and actually very, very good. It is generally accepted that when it comes to output caps, bigger is better. Two in parallel doubles the capacitance and halves the ESR. Also, here's a link to an interesting read:

Nichicon Muse ES bipolar caps measured: <-120dB THD, <-140dB IMD
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