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PostPosted: 23 Dec 2012, 01:27 
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Joined: 28 Dec 2010, 22:07
Posts: 319
Hi all,

Attachment:
HDAMP6.GIF


I'm trying to build a preamp/headphone amplifier reminiscent of Gilmore's Dynamic amplifier. It is similar because I'm using four transistor pairs in push-pull-parallel to drive headphones, or even small speakers. All transistors are in the TO-92 package and don't dissipate much power, but this is not going to be a powerhouse amp. As said before, I intend it to be a preamplifier or headphone amp in a small chassis to sit next to my power amp and provide a little gain. Power will be given from a 24v adapter and probably filtered.

Here's my problem...I measure the output voltage versus the "ground voltage" and it usually measures within 100 mV. I want to be able to adjust it to zero, or provide feedback for it that automatically keeps it at zero. I tried putting a 100 ohm potentiometer between Q12 / Q13 but that didn't seem to work so well.

Also, I would measure the + and - rails and they would measure +/- 6v with a few mV of deviation (I was using a 12v supply for the test). When I hooked the headphones up to the output, the rails would sometimes shift to +7.2 and -4.8v. I measured the emitter resistors on the output transistors and not one of them was conducting super-heavily.

My power supply splitter was a pair of 1500 ohm resistors and a pair of 47 uF capacitors connected across the supply voltage with the 0v point taken in the middle. Perhaps I need to make a better rail splitter?

Although the JFETs used in the simulation are 2N5486, I used MPF102 for the current sources. It took a bit to find the best resistor values. The first current source delivers about 4 mA and the second delivers about 25mA.

Does anyone have any ideas of what I'm doing wrong? There's a specific reason I'm going with such a complicated design: this layout is pretty much the same way high-power amplifiers are set up. You can see what my next project might be.

Ed


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PostPosted: 24 Dec 2012, 16:28 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Is the part of the circuit around D1 and D2 drawn correctly?

Good listening
Bruce

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PostPosted: 24 Dec 2012, 16:57 
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Joined: 28 Dec 2010, 22:07
Posts: 319
Facepalm,

D2 is drawn backwards...it should have the cathode connected to the drain of Q10. I wired it correctly on the board but in this drawing it should be switched left to right. I will correct it and post a new drawing later.

Ed

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PostPosted: 24 Dec 2012, 23:22 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Speaking of Q10.... isn't it supposed to be a CCS with the resistor in the source and the gate grounded?

Cheers!

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PostPosted: 25 Dec 2012, 00:22 
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Joined: 28 Dec 2010, 22:07
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Ah, another error...I guarantee I wired it correctly because, aside from the problems the amp sounds good. Maybe that's why my simulation wasn't working too well, though...

Ed

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