Opamp Audio question

Hi Graham,

My several headphones are in the 25-60 ohm range. Not sure I understand your comment, though. I'm a relative noob, as you can probably tell. I thought that by running the two amps in parallel I'd effectively get twice the "available" current at a given voltage. I went with a 10:1 amplifier based on the schematics I was looking at and it turns out to be a good ratio for my bass, which has a pretty low level output voltage. Now I'm wondering if I can use a second parallel amp to get more output power for the lower impedance headphones.

Thanks, Andrew

If you google on "headphone-amp output level" you'll find a related thread in rec.audio.pro, May 2004. In that thread, an article by Douglas Self was mentioned,

In that article's section on "driving heavy loads" he discusses paralleling opamps to achieve better current capacity. However, it's not a great technique. As Self's article makes clear, most opamps get pretty lousy with loads below 600 ohms; to get down to driving the 32 or 16 ohms typical for many modern phones and earbuds, you'd need to parallel quite a few opamps. For instance, I own a commercial headphone amp that uses four parallel opamps to drive each output channel. I have measured the distortion of this rig and it climbs steeply up with low (30 ohm) loads.

Your TLC272 is a poor choice for this application; if you look on its datasheet you'll see that, characteristic of CMOS opamps, it is specified only down to 10k loads (so you're off by a factor of 500 or so), and it is not stable with capacitive loads (the typical capacitance of a long-ish headphone cord is enough to set it oscillating). Its current drive capability is very weak - although it's specified for a max current of 30mA, the amount of current it can drive while still having any sort of gain, bandwidth, and distortion spec is more like a tenth of that. You would be far, far better off with something like an OPA2134 (or even a TL072), though IMHO these still do not really have enough current drive, even with two sections paralleled.

Rather than paralleling opamps, you might consider the common approach of adding a discrete class-B buffer to the output. Self gives an example circuit in his article. One caution: although Self says that he did not need any compensation to avoid oscillation, in my own experience I've found it necessary to put a small (47pF or so) capacitor in parallel with the feedback resistor.

Hi Walter,

Thanks for the info and link, guess I'll have some reading to do this weekend.

I read some articles by Chu Moy and he used the OPA too. We have a lot of different opamps in our bin stock here but no OPA so I picked one with a decent output current but I didn't understand the relationship to distortion and gain. My plan was to learn about opamps with the stuff on hand and then order some parts that are more appropriate. I'm actually surprised that the sound is pretty good to my ears given that I've committed other sins like using cheap caps for decoupling.

This sounds like a good idea - I might go for that.

Andrew

Ahh, I see, I was using the wrong terms. I meant to ask about paralleling the inputs and outputs but I said bridging instead. I'll try the voltage follower.

Thanks, Andrew

Thanks, Martin, I'll add those parts to my shopping list. Looks nice and compact so it'll fit into the Altoids box I've been using for my amp.

Andrew

A hex invertor is another option.

NT

Frankly, if you don't hear the sound clearly in the headphones with just the one amp, then you seriously need medical attention - you're suffering from hearing loss.

Get thee to the clinic!

Best of Luck, Rich

Depending on the load impedance you may actually be better off increasing current rather than voltage. Headphones may actually measure anywhere from 8 ohms to 600 ohms IME for example.

Without knowing that info it would be speculative to suggest anything.

Graham

Google LM386 if you need a low cost easy to use amplifier to drive headphones down to 8 ohms quite easily.

Jim

Hi Andrew,

I assume the TLC272 is a 'rail to rail' amplifier. That means it should be able to swing +/- 4.5V at the output. Into a 25 ohm load the load current would be

You'll therefore get more output by operating 2 sections in 'parallel'. You don't need more volts ( as you'd get by bridging ).

Configure the second half of the op-amp as a voltage follower connected to the first half's output and connect the outputs together via a couple of low value Rs ( say 10 ohms ) in series with each output pin ( for current sharing ).

Graham

fig 5 in

martin

I would recommend that a considerable amount of skepticism be applied when reading articles by anyone who writes about audio without the benefit of either a distortion analyzer or randomized double-blind testing.

On the other hand, it is also true that some of the flaws introduced by running an opamp into a too-low impedance are of the sort that is not always easily heard by untrained ears. Hearing different sorts of distortion is a trained skill; one has to learn what to listen for, and in fact some kinds of distortion are often perceived as positive (as being more pleasant than the undistorted sound) by naive listeners. For instance, a little bit of low-order distortion of low frequencies can make bass sound more "rich" and "full". Clipping (what happens when the volume exceeds the voltage that the opamp can supply, thus turning sine waves into sorta-square waves) is easily audible, and ugly-sounding, when it affects more than a few percent of the wave; things like intermodulation distortion and slew-rate limiting are less easy to hear unless you have appropriate source material and know what to listen for.

The input impedance of an LM386 is too low for it to be directly driven by a bass (the OP's signal source). He would need an input stage of some sort; his existing opamp stage would probably be appropriate for this.

If you use a buf634, watch out: they have quite a lot of DC offset (up to

Or you could try to include the buf634 within a feedback loop; again, watch out, because then you've got too much phase lag and it will oscillate. The datasheet has some notes on how to control it, but personally I found it very touchy.

I understand your goal here is to learn about opamps. But if you're interested in a headphone project in particular, you might also take a look at ADSL drivers. There are some interesting project ideas at

You might also take a look at the TI TPA6120 headphone amp chip. It's surface-mount so a bit hard to work with, but I've had good results with it.

He's not a drummer

martin

bridge in the context of amplifier outputs usually means having both output terminals live but with opposite phase, (so while one is high the other is low...)

This gives twice the power into twice the impedance.

could be, what sort of phones are you using?

if they're 32 ohm phones it probably is, if a much higher resistance it could work.

maybe add an LM386 for the output stage. if that's not loud enough add two and bridge them.

Bye. Jasen

connecting outputs directly together is generally a bad idea.

ok try this.

connect the non inverting inputs together and use separate feedback sections for the non-inverting inputs connect both outputs to 4.7 ohm resistors and the other end of both resistors to the headphones,

drive the headphones in series. (drive the left channel via a capacitor, ground the right leave the common terminal unconnected)

Not necessarily. The LM386 has an input impedance of 50K and a voltge gain of 200. Keeping in mind that the guitar can put out as much as 1Vpp (at least my one does), there's pleanty of room for a 500K to 1meg series input resistor.

I built a headphone amp for my guitar using an LM386, and I find it has all the volume I could ever want, with both my bass and six string. But you do need to avoid those inefficient 99c store headphones.

LM386s are acceptable for answerphones, but not much else. They sound horrid. You cant get much simpler than a 386, but you can certainly do better.

NT