Opamp Audio question

[andrew queisser]

I built myself a headphone amplifier for my bass. It's mono using one half
of a single-supply dual opamp (TLC272A). I'm using a very simple circuit
from the data sheet (AC-coupled non-inverting) and so far it's working ok.
I'm not expecting high fidelity out of it.

My question: can I simply bridge the inputs and outputs of the two amps to
get twice the output power? I somehow feel that's the wrong approach, is it
and why? If it's not ok, how much of the circuit do I have to duplicate? I
basically have a voltage divider to offset the input to 4.5V (9V battery),
10:1 resistors for feedback and caps everywhere to decouple input, output
and the feedback divider.

Thanks,
Andrew

 

[Pooh Bear]

I built myself a headphone amplifier for my bass. It's mono using one half
of a single-supply dual opamp (TLC272A). I'm using a very simple circuit
from the data sheet (AC-coupled non-inverting) and so far it's working ok.
I'm not expecting high fidelity out of it.

My question: can I simply bridge the inputs and outputs of the two amps to
get twice the output power? I somehow feel that's the wrong approach, is it
and why? If it's not ok, how much of the circuit do I have to duplicate? I
basically have a voltage divider to offset the input to 4.5V (9V battery),
10:1 resistors for feedback and caps everywhere to decouple input, output
and the feedback divider.

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

 

[andrew queisser]

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

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

 

[Walter Harley]

I built myself a headphone amplifier for my bass. It's mono using one half
of a single-supply dual opamp (TLC272A). I'm using a very simple circuit
from the data sheet (AC-coupled non-inverting) and so far it's working ok.
I'm not expecting high fidelity out of it.

My question: can I simply bridge the inputs and outputs of the two amps to
get twice the output power? I somehow feel that's the wrong approach, is
it and why? If it's not ok, how much of the circuit do I have to
duplicate? I basically have a voltage divider to offset the input to 4.5V
(9V battery), 10:1 resistors for feedback and caps everywhere to decouple
input, output and the feedback divider.

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, http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm.

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.

 

[Pooh Bear]

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.

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
+/- 180mA but the chip will current limit long before that.

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

 

[andrew queisser]

Hi Walter,

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

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.

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.

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.

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


Andrew

 

[andrew queisser]

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
+/- 180mA but the chip will current limit long before that.

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

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

 

[Rich Grise]

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.

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

 

[[email protected]]

I built myself a headphone amplifier for my bass. It's mono using one half
of a single-supply dual opamp (TLC272A). I'm using a very simple circuit
from the data sheet (AC-coupled non-inverting) and so far it's working ok.
I'm not expecting high fidelity out of it.

My question: can I simply bridge the inputs and outputs of the two amps to
get twice the output power? I somehow feel that's the wrong approach, is it
and why? If it's not ok, how much of the circuit do I have to duplicate? I
basically have a voltage divider to offset the input to 4.5V (9V battery),
10:1 resistors for feedback and caps everywhere to decouple input, output
and the feedback divider.

Thanks,
Andrew

A hex invertor is another option.

NT

 

[RST Engineering \(jw\)]

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

Jim

 

[Jasen Betts]

["Followup-To:" header set to sci.electronics.basics.]

I built myself a headphone amplifier for my bass. It's mono using one half
of a single-supply dual opamp (TLC272A). I'm using a very simple circuit
from the data sheet (AC-coupled non-inverting) and so far it's working ok.
I'm not expecting high fidelity out of it.

My question: can I simply bridge the inputs and outputs of the two amps to
get twice the output power?

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.

I somehow feel that's the wrong approach, is it and why?

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.

If it's not ok, how much of the circuit do I have to duplicate?

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



Bye.
Jasen

 

[Jasen Betts]

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.

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)

 

[Walter Harley]

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.

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.

 

[Walter Harley]

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

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.

 

[Blake]

The input impedance of an LM386 is too low for it to be directly driven by
a bass

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.

 

[[email protected]]

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

Jim

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

 

[andrew queisser]

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.

I checked out RadioShack's catalog, since there's a store next to my work
and I did pick up a LM386 on Friday. Haven't gotten a chance to play with it
yet but it will be one of my next experiments. My bass has active
electronics so it's probably going to be ok.

My current amp works at low volumes but starts distorting rather severely
when I turn up the output of the bass OR the volume control on the
headphones (these are cheapo 40ohm headphones with volume pot built in.)

Thanks,
Andrew

 

[andrew queisser]

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.

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

 

[andrew queisser]

[snip]

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,

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

 

[Walter Harley]

[snip]

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,

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.

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.)