Opamp Audio question

Thanks for the tips - I'm definitely getting a lot of distortion when I turn up the bass or turn down the impedance of the phones (turn the volume pot in the headphones to max). The distortion is very unpleasant, it sounds like a bass played through a cheap fuzz box.

I've tried the amp with my old Sennheiser headphones which have 600ohms and I also get distortion when I turn up the bass. The volume level seems about the same as with the 40ohm phones which is a bit confusing to me. Maybe I'm already current limited with the higher impedance.

So far I haven't had the scope and the bass in the same room (one is at work, one at home) so I can't really tell what's going on but headphone impedance isn't the only thing.

By the way, I'm a total beginner on the bass so I'm sure I won't have "appropriate source material" for quite a while yet.

Cheers, Andrew

Hi Walter,

Speaking of caps - how does the size of the decoupling caps come into play. I know that the capacity affects the frequency response but I don't quite understand how it affects distortion. When I had very small decoupling caps in my signal path I got a clean signal on the scope but when I hooked up the phones the amplitude dropped to near zero and I hear a faint, highly distorted, high-frequency signal. When I placed a much larger cap in the path I got a much cleaner signal.

I understand that low frequencies pass through the large cap but I don't quite understand whether the small cap introduces distortion by clipping. I was, probably incorrectly, assuming that the lower frequencies are attenuated but if I think about it in terms of charge I imagine that with high amplitudes the cap being charged up quickly and then saying "hey, I'm full, I can't get the remaining 50% of your signal". That would result in the kind of fuzz I'm hearing.

Thanks, Andrew

Although caps do introduce distortion, it's much subtler than what you're hearing.

Don't try thinking of the caps' effect in time domain ("charging and discharging"), it'll just confuse you. Think of it in frequency domain: the cap is a resistor, whose resistance is different for low- and high-frequency signals. The resistance is Z = 1/(2 * pi * f * C). So if you combine that with the resistance of the headphones (which is reasonably constant for all frequencies in the audio range), you'll see that you get two things: first, a voltage divider which passes more or less signal to the 'phones depending on frequency, so a small capacitance means the 'phones see less signal; second, the total resistance that the opamp sees depends on frequency as well, so a small capacitance means the opamp isn't loaded as heavily.

As a quick rule of thumb: the capacitance you want, for coupling between stages (or to a load), is C = 1/(2 * pi * R * f) where R is the load resistance and f is the lowest frequency you want to pass. So for 16 ohm phones, to pass signals of 40Hz and above, C = 250uF. Notice that it has nothing to do with how much power is involved: it's the same for a milliwatt or a thousand watts. (In truth, there are some issues there; this is just a first approximation.)