The blameless Audio Buffer
or a perfect headphone amplifier
A story about dancing with single electrons.....

APN 001
Date:    2010-04-10

Link to shematic (Buffer)
Link to shematic (Pre-LowPass)
Link to shematic (System/Grounding)
Link to measurements part I
Link to measurements part II

Link to overcoming measuring limitations

Whats to do anymore:
- pictures of the breadboards
- deeper measurement (THD/THD+N)
- THD vs. freq

1.   Long journey to find the right.....

A long time ago, I started looking for a ultimate Headphone amplifier. But all circuits I found were tradeoffs between distortion, noise, complexity and cost.
Some weeks ago, I found the design idea of this buffer.........
First, I hacked it into a simulation programm.... and was impressed about it's performance and that it would cost almost nothing.......
I started designing the stage so that a first prototype could be built on a breadboard...... because that's the difference in real world and a simulation... it's not the same !

2.   Prototypes

The prototype was build up on a grey Tuesday morning in almost 1/2 hour in my home-lab from components I found there (e.g. NE5532).....
First I set the current up to only 25mA ( not to destroy components by too much current in the first step)
In the evening, I started measuring the prototype at the lab at work... after the first results, I take into account, that the buffer has a performance worth measuring into the deep wich means measuring things right and not fast..... and that are the results I got......

4. Simulate again

After this results, I wanted to know wich was the limiting factor is the circuit. After replacing the Opamps in the simulation it became clear that the Opamp is the limiting factor in noise and distortion domain... so a NE5532 is good, but not good enough to get best results....

So there are some nice Opamps you could use.. some are best for Noise but  not for distortion, others for distortion but not noise and some are a tradeoff......

122 dB max @ 1V
not more than 110dB
120 dB max @ 1V not more than 115 dB
2 / 4
120 dB max @ 1V
110 dB possible
THD good, Noise "bad"
1 / 2 / 4
119 dB max @ 1V
130 dB possible (???)
too expensive and noisy
very expensive

Noise good, THD ?


1 / 2




1 / 2

Opamp table (diff. input, 1Vout in 27 ohms)

3. More current

Next step was increasing the output current by reducing R1/R2 to 7 Ohms and to mount some heatsinks on T3/T4 beacuse of Powerloss would be approx 1W per transistor......

4. Adding a differential input

To get the best performance out of the buffer you must be able to get it in (that sounds worried but that's it !).
With the differential input, you have a chance to make a single GND point in your circuit and isolate GND noise from outside the buffer circuit.
Keep in mind, that 120dB signal-2-noise ratio at 1V output level means that the noise into the buffer must be lower than 1µV !
( And thats why the article is calles dancing with electrons)
On the other hand, the additional resistors make noise measurement worse (about 3-5dB). So keep them low or remove them if not needed......

5. Measuring until the instrument gives up

If you look at the results (here), you will see that you can reach the measurements system  limitations (if using the OPA2211 e.g.) .....

Noise (2Vref)
THD+N 10mV/27Ohms THD+N 100mV/27Ohms
THD+N 1V/27Ohms THD+N 2V/27Ohms THD+N 3V/27Ohms
122,5 dB
68 dB / 0.04 %
87,5 dB / 0.0042 %
106  dB / 0.0005 %
106 dB / 0.0005 % 105 dB / 0.00056 %
119,5 dB
65 dB / 0.056 %
86 dB /  0.005 %
105 dB / 0.00056 % 107,5 dB / 0.00042 % 105 dB / 0.00056 %
OPA2211 125,0 dB 71 dB / 0.028 %
92 dB /  0.0025 %
110 dB / 0.00032 %
113 dB / 0.00022 % 112,5 dB / 0.00023 %
> 140 dB
102 dB / 0.00079 %
114 dB / 0.0002 %
115 dB / 0.00018 % 115 dB / 0.00018 %

Also I had problems with the first breadboard version, therefore I had to build a new one.....

6. Overcoming measurements limitations

Seeing point 5, I had to add a low pass filter to supress k2 and k3. But this only makes sense with OPamps capable of generating lower THD than the oscillator.....
Using OPA2211 should give the answer....

7. Perspective......

So long, here the project stops.... next step will be to build a better oscillator (and even a complete system that's able to measure soooo deep)
In the consumer/users point of view, I must say that the results are much better than needed (and expected). I think a SNR of ~120 dB (or even 100 dB) and THD+N ~ 90 dB should be sufficient for an headamp amplifier. Even there's no source I know wich would deliver lower THD than ~100 dB  (Live concert with Headamp direct  connected to mixer perhaps ?) in home use (ok, my DAC has ~105 dB, but this is MINE and will not be borrowed :-)