If you double the headphone resistance, the voltage goes up by about
40%.
The OPA2604 stage has a gain of just about 10 as it is. The 1K and
10K in the second OPA2604 section sets the gain of that part.
If I was designing this, I would have made the two stages of the OPA2604 share the work of providing gain differently. The second section is also canceling the distortion of the output pair.
No, no, no! That was a joke. They are more than 10 times the transistor you want there.
3W is more than more than enough power to blow your ears up.
I see. So use a film capacitor there instead of electrolytic. Polyester would be fine then I guess, as Wimpie suggested (this one might be too small for that perhaps).
On the other hand, MooseFET says the 10uF at the input is the only one worth changing. But as others mentioned, electrolytic does fine for DC coupling (zero bias coupling) & no filter purposes, which is what this
Polyester would probably be good enough that its flaws are unhearable. My point is, that if the capacitor in question is not in the signal path (e.g. power supply storage or decoupling) or in the signal path, but having essentially no voltage change during signal swings (e.g. coupling capacitors) or are involved in the signal path but only for high frequency, ultrasonic functions (like opamp stability) then their imperfections cannot be heard in the output.
For the few, rare capacitor applications that are in the signal path and also have significant part of the signal voltage appearing across them, then the imperfections in their operation will impress alterations on the signal. Tone control and other filter functions fall in this category. Here is a good capacitor tutorial site that compares and contrasts the various kinds of imperfection various dielectrics suffer.
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I think this cap falls in the category of, "in the signal path, but seeing insignificant voltage changes with signal swing" so its dielectric flaws are not important. You just have to make sure it has enough capacitance to remain in this category. So the only reason to consider another dielectric would be if the environment (high temperature or low pressure) or long lifetime requirement might cause this capacitor to dry out and loose capacitance.
I withdraw that comment. I was assuming the output stage was operating, class AB, where the base capacitance would have to be charged and dumped each signal cycle. But now that I have read the description and see that the amplifier is biased for class A operation, the base capacitance remains essentially charged and those caps are big enough. It was also a comment made more with hunch than calculation. Sorry.
I doubt the noise contribution for any functional alternatives is hearable, let alone, measurable.
I just chose the TIP29, 30 pair because they have fairly flat gain out to more then the expected peak current, where the MJE340, MJE350 pair does not. This would lower the output current requirements of the opamp driver and improve the linearity during the peaks. But those high gain Zetex transistors would really lower the opamp load, but could not handle the heat of full class A operation, without being embedded in a heat sink (perhaps being inside a heat pipe). Their die area is about the same as the TIP29-30, so their safe area is fine, if you can keep their temperature down.
As to the suitability of the 2SD1763, 2SB1186 pair, here are the data sheets:
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With a 1.5 amp rating and designed as complementary pairs, they would work just fine. They each have a minimum current gain of 60, maximum gain 200 at 1 amp, and I suspect might have a bit more at the half amp expected peak current. But from this data sheet, I would prefer the narrower gain range and higher minimum gain pair 2SD2400A, 2SB1569A (minimum gain 100, maximum gain 200 at 1 amp). I would like to see the actual gain versus collector current and safe operating area curves, though. All of these are much better choices than the MJE340, MJE350 pair in the original design, in my opinion.
"Tim Williams" wrote in news:RBCei.114$Ct1.77 @newsfe03.lga:
I played few records with it, however my other amp (without the maketing gimmic non hearable filter) played lots of records while sounding as good or better...
I can't think there could be that much input below 20Hz from a 33.33... rpm record. Clicks, pops, and rumles certainly but I can hear those...
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The 10uF at the input and the about 22K it is loaded with makes for a cut of frequency of about 0.7Hz. At reasonable volume settings, this defines the low frequency cut off of the whole circuit. If any coupling capacitor has an AC voltage on it, this is the one that does.
The 22uF, which I said served no purpose, ends up setting the low frequency limit when the gain is all the way up.
I looked to the circuit diagram, although it can be used as a headphone amplifier, I have some doubts about the safety of the design.
When the connection from the potentiometer to the output breaks, you will have full output (about 13Vp) that may be harmful for your hearing. Are you using headphones with a sound level protection? If not, I would not be happy with a circuit that can deliver so much output.
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