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Old 24th May 2002, 06:30 PM   #1
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Default BJT buffer design, can anyone help

Hi all!

Thanks for all the humor on the forum: Jocko, Dirty Harry and all the rest!
(how's that rich kid doing?)... but now serious:

I found out that I use only very little voltage when listening in my small room in the evening. Thus I wanted to design a class A voltage folower using BJT's capable of driving 8 ohm speakers and 32 ohm headphones at 0.7 volt RMS. I have come up with this circuit.

Can anyone help me with a improvement on the topology?

Greetings,

Thijs
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Old 24th May 2002, 06:58 PM   #2
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Yes, but I am tied up for all while. Sit tight, and I'll whip something up.

Jocko
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Old 24th May 2002, 07:59 PM   #3
DarkOne is offline DarkOne  Slovakia
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I think that your idea is not ideal. Try this
If you want, I can post resistor values next week.
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Old 24th May 2002, 09:00 PM   #4
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Exclamation DarkOne

His speakers are going to love the ~-2V DC offset. Don't try this one at home kids.....
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Old 24th May 2002, 09:29 PM   #5
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Thanks Jocko.. looking foward to it..I'll sit tight :-)

Hi Darkone: thanks for the schematic, since AC coupling at the ouput is OK for me the offset is no problem, but I would like to try to DC couple the buffer some how..

Hi papa Harry .. us kids allways simulate before firering up.. but thanks for the warning. I just finished reading app note 227 of Nat Sem and this might give some inspiration.. I'm still aiming for a buffer with low component count and reasenable performance. Maybe you've got any ideas?

gr,
Thijs
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Old 24th May 2002, 10:57 PM   #6
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How low is the parts count you are aiming for, and how high can the output impedance be? BJT or MOSFET?

A month or so ago......I believe "CAPSLOCK" asked about the so-called diamond buffer. I think the thread faded away. That is an option.

Jocko
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Old 24th May 2002, 11:24 PM   #7
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Hi Jocko,

Thanks for your interest..

Output impedance needs to be low enough to drive speakers whitout dissipating to much be may be sub-optimal (1 ohm ?). For headphones this doesn't matter that much.

I'm trying to make a small buffer-amp (in size that is). I would like to keep dissipation low and parts count low to keep the whole thing small. I was also thinking about the Hiraga Monster topology I aim at more relaxed spec's I figured I could do with less parts somehow. I was actually also aiming at a lower supply voltage than the +/- 15 Volts as shown in my schematic..that's why I used BJT's, less dissipation, less voltage needed, etc.

I'm really currious about how such a thing would sound. Loud enough?, Bi-amped? It could turn out a nice thingy if it stays simple, cheap, maybe even tri-amping, paralelling for 4Ohm use.. battery driven.. you name it.. it could be very flexible and handy to try things out..

I hope you understand my aim a little bit, I hope it makes sense...

gr,
Thijs
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Old 25th May 2002, 01:15 AM   #8
sonnya is offline sonnya  Denmark
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Default Re: DarkOne

Quote:
Originally posted by HarryHaller
His speakers are going to love the ~-2V DC offset. Don't try this one at home kids.....
Do you think so!? hahhahaahhahahahaaaaaaa ... hhmmrmrmrpppfff hahaha (I am laughing my self to dead!!!!)

Low distortion from a BJT buffer (diamond) => highly linear current gain and a high idlecurrent. of course without feedback!

Sonny
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Old 25th May 2002, 02:04 AM   #9
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Hi Sonnya and Jocko,

I just read everything I could find on 'diamond buffer'. Great stuff ...nice reading!

I now realize I should use high linear devices (no 2N3055 this time .. I sometimes learn stuff here too) .. and bias heavely... but that's what I want to avoid..

I am aming at 1.6Volt peak to 8 ohm: that's 200mA. In push-pull Class A this needs at least 100mA idle..I don't wanna go higher than 150mA.. else I will need real heatsinks at so on.. I would like to keep idle dissipation under 2 watts.. maybe I'm not realistic.. hmmmm

You see: I have this picture of a nice little, no large heat sink, no complicated circuit, easy regulated power supply, above all great sounding buffer-amp in my head..and it wants to get out!

I wanna build six of them , 4 to try a low volume Bi-Amp set-up and 2 for the Grado's..

oke .. I won't anoy you any more .. back to the simulator again..

gr,
Thijs
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Old 27th May 2002, 03:17 PM   #10
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I think I'm gonna settle with this one ..

What do you guys think?

Iq is 140mA,
Pq is 660mW/ouput device
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