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Old 15th April 2014, 11:27 AM   #1
Preamp is offline Preamp  Germany
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Default Headphone amp on perfboard: layout critics welcome

Since I'm having a hard time getting any layout done for months and years, I suddenly changed my mind to go with perfboard. I've got some lying around and this way I can simply get going with it and finally build something!

So I sat down yesterday and created a 100mil-spaced layout. I wanted it to be rater space-efficient and not widely spread over a giant, mostly empty board. I also didn't want to have much wire links in there, so I decided to leave the power supply lines unrouted till the end and finally wire them directly to the PSU.
Trying to obey the rules of proper grounding, I ended up with a power ground (dark blue trace) and a signal ground (light blue trace). Please have a look at the attached picture. The red square is the point that connects directly to PSU ground. I'm not sure however whether I can safely connect both grounds together on-board in the area encircled in red or if it's mandatory to run a second wire to the PSU from the red X.
Probably it's best to ultimately test it out, but I'd still like to hear some suggestions beforehand. Do you think that my overall thoughts are OK so far or did I even do something terribly wrong?

In case you might wonder: the output transistors are supposed to be mounted to the case, lying flat beneath the board.

Any comments are greatly appreciated .
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Old 15th April 2014, 07:43 PM   #2
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Here's the schematic.
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Old 16th April 2014, 02:17 AM   #3
agdr is offline agdr  United States
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Quote:
Originally Posted by Preamp View Post
Here's the schematic.
Just off the cuff... you might be offending the symmetry gods here. I would be expecting a LED, or two 1N4148s in series, on either side of the feed. One going to each transistor base. It should work but the IC4B op amp output would settle at a Vbe drop off center. I'm no discrete wizard though, better get some more opinions.
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Last edited by agdr; 16th April 2014 at 02:25 AM.
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Old 16th April 2014, 08:07 AM   #4
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Not sure whether it's correct, but this is supposed to be the schematic of a well-known commercial amplifier .

Quote:
Originally Posted by agdr View Post
It should work but the IC4B op amp output would settle at a Vbe drop off center.
Wouldn't that mean that IC4B is effectively working in Class A?
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Old 16th April 2014, 11:52 AM   #5
jcx is offline jcx  United States
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the op amp output isn't reliably biased Class A with symmetric opposed CCS of the same value

I wouldn't use both CCS

the schematic is pretty generic - could be expected to find similar circuits 30 years ago

some in audio do make a fetish of brand or design by a "big name" - but audio design, EE is much deeper in history than most impressed by the audio ad copy may appreciate



but of course the 1st question hasn't been addressed - what headphone(s) - they vary way more than loudspeakers in I,V, power required - to the point that some amps are really bad matches to some headphones
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Old 16th April 2014, 11:59 AM   #6
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I'm going to use it with my DT-990 250Ohm or maybe even the DT-770 80 Ohm.

Since the amp is from Beyerdynamics (or beyerDIYnamics in this case ), I suppose them to play quite nicely together.
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Old 16th April 2014, 12:26 PM   #7
jcx is offline jcx  United States
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for those headphones you can just use small signal Q, don't really need the BD139/140 - 20 mA will drive the 770 80 Ohms to 120 dB SPL

for deep Class A output stage even t0-92 would work if you drop your supply V by 1/2 - the 880 250 Ohm only needs 2.8 V to reach 120 dB SPL


I also don't know where you're going with all the inverting, the inverting input low Z input makes it poor match to a volume pot

the "always invert" recommendation really only applies when you try to get to single digit ppm distortion levels - and you won't with those op amps
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Old 16th April 2014, 04:34 PM   #8
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Well, thanks for your comments.
Guess I'll now be looking for another design and still won't start building anything soon...
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Old 16th April 2014, 05:32 PM   #9
jcx is offline jcx  United States
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with perfboard you aren't committed - can easily add, undo, incrementally increase complexity

Designing An Opamp Headphone Amplifier | HeadWize shows some options for op amps, buffers

even just a resistor pull down single emitter follower op amp buffer with ~30mA would get you started if you use R, Q that can handle 1-2 W
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Old 16th April 2014, 05:56 PM   #10
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by jcx View Post
with perfboard you aren't committed - can easily add, undo, incrementally increase complexity
Sure, but this time my plan was to start out with a known (good?) design and not go through all that trial-and-error. Well, but now I'm getting rather intrigued - that's what makes diy audio a great hobby after all, isn't it?
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