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Old 30th May 2012, 01:51 PM   #11
jkorten is offline jkorten  United States
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If this is a headphone amp, the transistor as it is configured may supply a DC offset which would not be good. Sending the signal through a 220 ohm resistor to the headphone pretty much defeats the purpose of the output transistor. I would use two NE5534's in parallel (non-inverting buffers) with a 50Ohm resistor on each output instead of Q1. Use feedback from the ouput of U1 instead and the rest stays the same. This circuit as posted is neither high fidelity nor safe for headphones in my opinion. Am I missing something?
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Old 30th May 2012, 02:06 PM   #12
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^ That's why the whole shebang is inside the feedback loop.

That 220 ohm resistor still is overly large. It only makes the loop unstable more easily and needlessly limits voltage swing. Something in the order of 47 ohms (minimum 10 ohms or so) should do just fine for short-circuit protection.

The basic circuit is sound - that 1-transistor affair is about the simplest output buffer you can build. Linearity should be much improved by replacing the 100R resistor with a CCS though, and even more can be gained by going with the common A/AB push-pull buffer.

Speaking of resistors, a "base stopper" resistor of several 10 ohms between opamp output and buffer input may be a good idea. Common collector circuits have this habit of breaking into very high-frequency oscillation when fed from inductive source impedances, and an opamp output exhibits just that.

Last edited by sgrossklass; 30th May 2012 at 02:17 PM.
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Old 30th May 2012, 03:00 PM   #13
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Here's a circuit with a CCS (OP parameters for 5532):
Click the image to open in full size.
Distortion components are about 10 dB lower throughout when compared to the basic circuit at same current, though even the latter is quite well-behaved.
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Old 30th May 2012, 03:52 PM   #14
jkorten is offline jkorten  United States
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@sgrossklass - did you mean to have a pullup resistor on the base of Q1? Or did you forget to remove those connections after you decided it wasn't necessary?

I would be even more worried about the original circuit's DC offset if the 220 resistor was changed to 10 ohms. Because there would be that much more current flowing through the headphone coils.
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Old 30th May 2012, 04:32 PM   #15
jcx is offline jcx  United States
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Quote:
Originally Posted by sgrossklass View Post
...
Speaking of resistors, a "base stopper" resistor of several 10 ohms between opamp output and buffer input may be a good idea. Common collector circuits have this habit of breaking into very high-frequency oscillation when fed from inductive source impedances, and an opamp output exhibits just that.
additionally a "Zobel" RC to AC gnd (or directly across the base-collector leads) at the output Q may be better for RF stability and allows reduced series base stopper values for less phase shift

the op amp output Z will be "inductive" where loop feedback is working but falling off at a single pole rate, near output Q ft
way beyond typical op amp loop gain intercept I wouldn't bet on what the op amp output looks like so the series stopper can "isolate" from the unknown op amp output Z and the added Zobel gives more reliable setting of the RF Z seen by the output Q
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Old 30th May 2012, 08:26 PM   #16
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Quote:
Originally Posted by jkorten View Post
I would be even more worried about the original circuit's DC offset if the 220 resistor was changed to 10 ohms. Because there would be that much more current flowing through the headphone coils.
Uh... why?

Assuming everything is working, output DC offset is very nearly zero. Hence, no DC current flowing through the load at idle.

Look where the feedback connects to. It's not the opamp output, it's after the output series resistor. The opamp will do its best to keep this spot at input voltage times voltage gain.

Obviously this may be sufficient to fry a load if the level is set high enough, but the original circuit with 220 ohms definitely doesn't manage an awful lot of output into 32 ohms - hardly 2.2 Vpp or 780 mVrms, or 19 mW. My little Sansa Clip+ manages about that much, too. Granted, this is about as loud as you'll ever need with normally-sensitive headphones (or even good for ear-damaging levels with more sensitive ones), but a 5532 could have done that "barefoot". With 10 ohms, maximum output comfortably climbs to 100 mW, or 200 mW for the CCS version.

While in-ears commonly can take no more than a few 10 mW, their "full-grown" cousins are usually rated at several hundred mW, and some DJ cans can even take a full watt or more. Since audio material usually has a higher crest factor than pure sines, you need some extra power headroom anyway.
Quote:
Originally Posted by jcx View Post
additionally a "Zobel" RC to AC gnd (or directly across the base-collector leads) at the output Q may be better for RF stability and allows reduced series base stopper values for less phase shift

the op amp output Z will be "inductive" where loop feedback is working but falling off at a single pole rate, near output Q ft
way beyond typical op amp loop gain intercept I wouldn't bet on what the op amp output looks like so the series stopper can "isolate" from the unknown op amp output Z and the added Zobel gives more reliable setting of the RF Z seen by the output Q
Seems reasonable. Pulling down loop gain at RF definitely isn't a bad idea both in theory (simulation) and in practice, hence why Zobels are common in power amps.

Last edited by sgrossklass; 30th May 2012 at 08:30 PM.
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Old 19th June 2012, 02:09 PM   #17
eimis is offline eimis  Australia
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Thanks. Would this amp sound better with lm317 CCS instead of that 100r resistor, in theory (or your experience)?
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Old 21st June 2012, 12:36 PM   #18
eimis is offline eimis  Australia
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Quote:
Originally Posted by sgrossklass View Post
Here's a circuit with a CCS (OP parameters for 5532):
Click the image to open in full size.
Distortion components are about 10 dB lower throughout when compared to the basic circuit at same current, though even the latter is quite well-behaved.
Could I use LM317 CCS instead? If so, what value resistor is needed in this circuit? Is 5.1 ohm okay? Thanks.
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Old 21st June 2012, 04:09 PM   #19
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Quote:
Originally Posted by jcx View Post
additionally a "Zobel" RC to AC gnd (or directly across the base-collector leads) at the output Q may be better for RF stability and allows reduced series base stopper values for less phase shift
Do you mean the conventional "zobel" as commonly seen 10R-0.1uF on power amps from emitter to ground? Or is this suppose to be a zobel on the output Q base to ground (or collector), if so, what values might you suggest?
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