[Solved] How to lower the gain of this headphone amp
Hi.
Currently with these values the gain of this amp is 11:
The problem is, I've no idea how to change the gain to, for example, 2? Which resistors to what value should I change..? I hope someone understands this better than I do.
Thank you.
Hi.
Currently with these values the gain of this amp is 11:
An externally hosted image should be here but it was not working when we last tested it.
The problem is, I've no idea how to change the gain to, for example, 2? Which resistors to what value should I change..? I hope someone understands this better than I do.
Thank you.
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For lower noise, you can change both R2 and R3 to 1k. If you do this, you should increase C2 to something really big like 470uF.
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R1 = 220k is really huge.
Is this a circuit board you got from China?
Depending on which op amp you use, you may not need the input coupling cap. If not, then I'd replace the coupling cap with a 1k resistor, and replace R1 with 220pF capacitor. This will form a first order filter to keep RFI out of the op amp.
Since the input impedance is determined largely by VR1 here, you can easily decrease R1 to something like 50k. Then make R3=50k as well for low offset, and adjust R2 for the gain you want.
The noise is determined by the parallel value of R2//R3 and since R3 is largely limited to whatever R1 you use, those are your play limits.
The small cap across R3 is also a good idea.
jan didden
The noise is determined by the parallel value of R2//R3 and since R3 is largely limited to whatever R1 you use, those are your play limits.
The small cap across R3 is also a good idea.
jan didden
It's a DIY clone of a popular headphone amp that I've already built. It had some serious hum issues at first but I solved them by putting capacitors between ground and +/- rails. I also put a cap between ground and smoothing capacitor plate. That shut up all hum. I've no idea what happened there because I'm not an engineer but I'm happy it solved the issue. At the moment the amp has original values, except for R3 which is 9.1K and the amp sounds absolutely fine.
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?
^ 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.
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.
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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.
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.
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
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.
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.
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Here's a circuit with a CCS (OP parameters for 5532):
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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