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BamboszeK 5th January 2020 11:18 PM

High performance OPA1602 + TPA6120A2 Composite Headphone Amplifier
 
3 Attachment(s)
Hello guys,
About half a year ago I decided to design my own high performance headphone amplifier. The idea came more from some kind of engineering challenge rather than serious need - my O2 was probably more than enough.
My designs considerations were pretty simple:
  • Challenge myself and check what lowest distortion can I archive
  • Decent power with ability to drive almost any headphones
  • Near zero output impedance
  • Safe for headphones, no DC at output and stability in any conditions
  • Relatively cheap and easy to build - no unobtainable, expensive or exotic parts
After a bit of reading, tinkering and simulations I settled with composite amplifier architecture. I've considered lot of solutions including discrete buffer, LME49600, numerous models of opamps, ADSL line drivers. Every solution had its pros and cons like price, stability, output impedance, DC characteristics, output power, minimum load resistance, complexity, power dissipation limit, temperature and so on.

Finally I decided to nest already very good TPA6120A2 inside feedback loop of another operational amplifier. At this time I'm using OPA1602 but want to experiment with something else like OPA1652 or OPA1656. Especially the last one seems promising but first I need better test equipment to analyze such small distortion levels.
TPA6120A2 is almost perfect candidate apart from one thing - stability. It is current feedback operational amplifier and its stability is not so easy to tame. High attention must be paid to minimize stray capacitances at PCB.

Apart from that TI recommends at least 10 ohms of series output resistance. But this means rather high output impedance. I tried to address this issue in two ways - complex feedback, post output resistor or output inductor.

At this time I'm using ~300nH air core inductor at output. TPA6120A2 seems happy with it and output impedance is very low, about 10 miliohms. There is downside tho. Such inductors are hard to get off the shelf so I had to hand wound them. Inductors with other that air cores probably would introduce high distortions.

But there is another way. PCB can be configured to take feedback post resistor/inductor. This configuration is not so popular way of isolating capacitive load from amplifier while maintaining low output impedance. I haven't tried it yet but possibility of omitting inductor is very promising.

For power supply good, old LM317 and LM337 regulators are used with half wave rectifier. Board can be powered by AC-AC wall wart or transformer. Those wall warts are cheap and easy to use as there is no contact with potentially dangerous mains wiring.

Instead of using more common voltage divider to set output voltage I decided to use zener diodes. Dynamic resistance of zener diode is lower than required resistor which should result in lower output noise. By default 13V zeners yields about +/- 14.5V at output with less than 10mV difference between negative and positive rails.
Cadj capacitors are of course present. There are two LEDs at both rails. Besides being indicators they run at relatively high current, providing constant load to regulators which keeps low output impedance of power supply.

Whole PCB was designed in free and open source software - KiCad. Simulations were done in LTspice.

3D render, real photo taken before mounting my ugly hand wound inductors and schematics could be found in attachment.

What are my plans for future?
  • Learn CAD software to design enclosure and order it at CNC workshop
  • Acquire/build equipment allowing me to measure true performance of amplifier (aiming <-130 dB THD)
  • Adding micro controller (probably STM32) used for monitoring amplifier - temperatures, DC offset, failure, relay control and etc.
  • Maybe bigger version with XLR input, preamp output, switchable gain and Alps RK27 volume control?

BamboszeK 5th January 2020 11:20 PM

Everything was measured with Audio Precision APx525. Unless otherwise noted given conditions were used: 1kHz, 2Vrms, 150Ω load, AC - 20kHz bandwidth, no weighting, maximum power at 1% THD+N, both channels driven.

Gain: 12dB (x4)
Frequency response (▒0.1dB): 12Hz – >80kHz
20Hz – 20kHz ▒0.05 dB
Output impedance: <10mΩ
DC offest: <300ÁV
THD+N 1kHz: <-110dB
THD 1kHz: <-117dB
SNR: >124dB
Power, 32Ω: 3W per channel
Power, 150Ω: 700mW per channel
Power, 600Ω: 185mW per channel


As you can see I'm limited by performance of Audio Precision APx525. Unfortunately I don't have analog oscillator option installed in my AP so SINAD measurements are limited to about 105-110dB. Measurements were done without any enclosure in very noisy lab with cheap cables.

BamboszeK 5th January 2020 11:28 PM

9 Attachment(s)
FFT 1kHz, 256k length, 8x avg.
Everything looks same like direct APx525 loopback, maybe apart from 50Hz peak coming from mains. Lab in which tests were performed is very noisy.
https://www.diyaudio.com/forums/atta...1&d=1578270346

Frequency response
There is low pass and high pass filters at input of amp for filtering out possible DC voltage or radio frequency intererences.
APx525 is limited to only 80kHz. I also have access to Audio Precision System Two with higher bandwidth but I think such high frequencies are not meaningful anyway.
Note the scale used - 1dB max with 0.1dB step.
https://www.diyaudio.com/forums/atta...1&d=1578270346

THD+N 1kHz vs measured power with 150Ω load
https://www.diyaudio.com/forums/atta...1&d=1578270346

THD 1kHz vs measured power with 150Ω load
Measurement similar to previous one but only harmonic products, excluding noise
https://www.diyaudio.com/forums/atta...1&d=1578270346

THD+N 1kHz vs measured power with 32Ω load
https://www.diyaudio.com/forums/atta...1&d=1578270346

and for comparison
THD+N 1kHz vs level - blue channel is amplifier and red is Audio Precision loopback
https://www.diyaudio.com/forums/atta...1&d=1578270346

THD+N ratio vs frequency
https://www.diyaudio.com/forums/atta...1&d=1578270346

THD ratio vs frequency
Again, just THD without noise.
https://www.diyaudio.com/forums/atta...1&d=1578270346

IMD: SMPTE 60 Hz + 7 kHz @ 4:1
https://www.diyaudio.com/forums/atta...1&d=1578270346

xrk971 6th January 2020 12:57 AM

Nice work! Looks like you have a super amp. 3W into 32ohms is amazing.

:cheers:

proid 8th January 2020 04:02 AM

Look great! Can you make measurement with 32R load? I think that will tell more, 150R is relative easy.

BamboszeK 8th January 2020 03:48 PM

5 Attachment(s)
Measurements with 32 ohms load. There is some strange anomaly in THD+N vs freq graph. As I said electromagnetic environment in this bulding is very noisy and measurements were done with bare board without any shielding.

Lots of antennas are mounted on top - two radio stations, a few GSM and other devices laboratories.
THD+N vs freq looks fine on 50R so fortunately this is not problem with amplifier.


BTW. If anyone is interested I have a few spare PCBs.

ivanlukic 8th January 2020 05:11 PM

BamboszeK,

How would you describe sound of this composite amp compared with O2?

proid 9th January 2020 03:57 AM

Quote:

Originally Posted by BamboszeK (https://www.diyaudio.com/forums/headphone-systems/347613-performance-opa1602-tpa6120a2-composite-headphone-amplifier-post6036835.html#post6036835)
Measurements with 32 ohms load. There is some strange anomaly in THD+N vs freq graph. As I said electromagnetic environment in this bulding is very noisy and measurements were done with bare board without any shielding.

Lots of antennas are mounted on top - two radio stations, a few GSM and other devices laboratories.
THD+N vs freq looks fine on 50R so fortunately this is not problem with amplifier.


BTW. If anyone is interested I have a few spare PCBs.

According to this, i think thd is limited by amplifier with 32r load, right?

BamboszeK 11th January 2020 01:13 PM

Quote:

Originally Posted by ivanlukic (https://www.diyaudio.com/forums/headphone-systems/347613-performance-opa1602-tpa6120a2-composite-headphone-amplifier-post6036945.html#post6036945)
BamboszeK,
How would you describe sound of this composite amp compared with O2?

I'm probably the last person who you would ask for describing sound. I didn't tried to compare them directly but I don't except any audible diffrences untill O2 starts to clip. My amplifier delivers consedaribly more power which may be useful with some headphones.
Quote:

Originally Posted by proid (https://www.diyaudio.com/forums/headphone-systems/347613-performance-opa1602-tpa6120a2-composite-headphone-amplifier-post6037659.html#post6037659)
According to this, i think thd is limited by amplifier with 32r load, right?

APx525 without AG52 analog generator option is oficially specced only to -105dB THD+N. To be one hundred percent sure that distortions comes from DUT I need better analyser. I want to get Victor's oscillator and build notch filter. With this hopefully I would be able to dig deeper in distortion. Or maybe some kind member with APx555 wanna help with measurements?

BamboszeK 1st February 2020 12:38 PM

4 Attachment(s)
@Dave Zan suggested me some interesting stability simulation method with so called Tian Probe. Technical details could be found here and complete step by step guide for multiloop feedback analysis could be found here.
If I'm right my amplifier should have 39 dB of gain margin and 51░ phase margin with 300 Ω || 10 nF load (Figure 2). With 300 Ω || 1 nF load PM is 46░ and GM is infinite (Figure 3). Last graph presents Bode plot with closed loop (Figure 4).


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