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Class A headphone amplifier

liseczq 23640 129

TL;DR

  • Built a class A headphone amplifier with three blocks: symmetrical power supply, uPC1237HA headphone protection/delay, and stereo power stage.
  • The supply uses LM723 and LF357 to stabilize ±21V, while the power stage uses LME49860, 2SK170 BL JFETs, and medium-power transistors.
  • The amplifier runs at about 38mA per transistor at 21V, with 4 × 4700uF filter capacitors and current limiting set to 0.3A.
  • It delivers very high gain, low noise, and strong headphone protection, but the whole unit warms up heavily and the heatsink is too weak.
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  • #122 19769786
    yogi009
    Level 43  
    Olkus wrote:
    But I do not recommend buying them.


    I acquired them for these comparisons on purpose :-)
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  • #124 19769816
    Olkus
    Level 32  
    romarcin wrote:
    Olkus wrote:
    now Jamicon belongs to Yageo
    according to Wikipedia, Yageo and KEMET are one corporation, Jamicon is not mentioned at all https://en.wikipedia.org/wiki/Yageo


    Wikipedia does not contain everything ;)

    Greetings,
    AND.

    Added after 50 [seconds]:

    yogi009 wrote:
    Olkus wrote:
    But I do not recommend buying them.


    I acquired them for these comparisons on purpose :-)


    It's a bad competition for Nichicon, a one-hit knockout :)

    Greetings,
    AND.
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  • #125 19769857
    liseczq
    Level 21  
    yogi009 wrote:
    liseczq wrote:
    Looking at the new one, the OPA is practically weaker than the LME in everything.


    Because it is. I did comparisons on the dual OPA and the entire LME series. These are good op amps, but OPA2134 has long been referred to in the West as a good entry-level opamp for audio applications. Now you should probably take e.g. LME49860 (or the neighboring models) and the LME49600 buffer at the output.


    I have an LME49860 at home, and my colleague "FIMEK" claims that the OPA134 has almost cosmic parameters, it is dedicated to headphones and it is much better to use it as a headphone amplifier.

    Please see the first page of the topic. There is a detailed description of my amplifier and the elements used in it.
  • #126 19769871
    yogi009
    Level 43  
    liseczq wrote:
    I have LME49860 at home, and my colleague "FIMEK" claims that OPA134 has almost cosmic parameters


    Well, now it's clear.
  • #127 19769913
    Fimek
    Level 16  
    liseczq wrote:
    yogi009 wrote:
    liseczq wrote:
    Looking at the new one, the OPA is practically weaker than the LME in everything.


    Because it is. I did comparisons on the dual OPA and the entire LME series. These are good op amps, but OPA2134 has long been referred to in the West as a good entry-level opamp for audio applications. Now you should probably take e.g. LME49860 (or the neighboring models) and the LME49600 buffer at the output.


    I have an LME49860 at home, and my colleague "FIMEK" claims that the OPA134 has almost cosmic parameters, it is dedicated to headphones and it is much better to use it as a headphone amplifier.

    Invite the topic page. There is a detailed description of my amplifier and the elements used in it.


    OK, my mistake, I fixated on the presented diagram, where there was NE ... and not LME ... Now it remains for me to ask what these distortions are after equipping the opamp with the power stage? You write that small, which is probably true, but what?
  • #128 19769950
    liseczq
    Level 21  
    Fimek wrote:
    liseczq wrote:
    yogi009 wrote:
    liseczq wrote:
    Looking at the new one, the OPA is practically weaker than the LME in everything.


    Because it is. I did comparisons on the dual OPA and the entire LME series. These are good op amps, but OPA2134 has long been referred to in the West as a good entry-level opamp for audio applications. Now you should probably take e.g. LME49860 (or the neighboring models) and the LME49600 buffer at the output.


    I have an LME49860 at home, and my colleague "FIMEK" claims that the OPA134 has almost cosmic parameters, it is dedicated to headphones and it is much better to use it as a headphone amplifier.

    Invite the topic page. There is a detailed description of my amplifier and the elements used in it.


    OK, my mistake, I fixated on the presented diagram, where there was NE ... and not LME ... Now it remains for me to ask what these distortions are after equipping the opamp with the power stage? You write that small, which is probably true, but what?


    I will connect the oscilloscope during Christmas and we will measure.
  • #129 19769982
    romarcin
    Level 20  
    liseczq wrote:
    I will connect the oscilloscope during Christmas and we will measure.
    the oscilloscope shows the distortions when they are single percentages, only very good sound card and e.g. ARTA
  • #130 19770684
    liseczq
    Level 21  
    It is a pity that many of the readers do not read the text, but go the easy way and look at the photos.

    I am waiting as soon as someone asks where I live, because in the diagram from 1982 I posted it says that the network has 220V instead of 230V
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Topic summary

✨ The discussion revolves around a Class A headphone amplifier design, which includes a symmetrical power supply, headphone protection, and a stereo power stage. Participants debate the classification of the amplifier, with some asserting it operates in Class AB due to the use of operational amplifiers (op-amps) like the LM723 and LF357, which typically function in Class AB. Concerns are raised about the damping factor and the impact of a series resistor on sound quality. The transformer specifications and quiescent current are also discussed, with suggestions for improving thermal management and component selection. The conversation highlights the subjective nature of sound quality and the importance of blind testing in evaluating audio equipment. Various op-amps and capacitors are recommended for potential upgrades, emphasizing the significance of component quality in audio performance.
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FAQ

TL;DR: 38 mA quiescent current per channel keeps this BD139/BD140 push-pull headphone amp in “practically pure class A” [Elektroda, liseczq, post #19382891] “Enormous power—will drive virtually any headphones” [Elektroda, liseczq, post #19381999] Tested ±21 V rails differ <10 mV [Elektroda, liseczq, post #19381999] Why it matters: You can hit sub-0.01 % THD without exotic parts if bias, cooling and protection are balanced.

Quick Facts

• Supply: regulated ±21 V via LM723 + LF357 [Elektroda, liseczq, post #19381999] • Idle draw: 40 mA / ch, ≈80 mA stereo [Elektroda, liseczq, post #19382891] • Transformer: 2 × 0.44 A secondaries, 20 W total [Elektroda, liseczq, post #19382510] • Output stage: BD139/BD140, safe ≈1 W into 8 Ω or any headphones [Elektroda, conisl, post #19382126] • Protection: uPC1237—3 s delay, DC & clip cut-off, relay isolated [Elektroda, liseczq, post #19381999]

Is the amplifier really class A?

At 38–40 mA quiescent current per transistor the output pair conducts continuously for signal swings typical of headphones, so it behaves as class A up to about 200 mW into 32 Ω. Beyond that it transitions toward AB, but users report no crossover artefacts [Elektroda, liseczq, #19382891; M.S., #19383415].

How do I adjust or measure the quiescent current?

Measure the voltage across each 10 Ω emitter resistor; 0.38–0.40 V equals the target 38–40 mA. To trim:
  1. Warm up the amp 10 min with no load.
  2. Change the J-FET source resistor (100 Ω → 47 Ω raises bias, 200 Ω lowers) [Elektroda, liseczq, post #19384268]
  3. Re-check after full thermal stabilisation. Keep below 50 mA to avoid overheating.

How much heat-sink area is needed?

At ±21 V the idle dissipation is 1.68 W per channel (0.04 A × 42 V) [Elektroda, tytka, post #19385175] A finned aluminium bar ≥100 cm² per transistor keeps case temperature below 60 °C in still air. If you drop rails to ±15 V, dissipation falls to 1.2 W and the stock sink suffices.

Can I lower heat without losing class A?

Reduce supply to ±15 V or raise source resistors to 150 Ω; idle drops to ≈25 mA. The amp remains class A up to ~120 mW, which is still louder than 95 dB SPL on 300 Ω cans [Elektroda, Janusz_kk, post #19384216]

Which op-amps work best here?

The board accepts dual 8-pin DIP/SOIC. LME49860 gives THD 0.00003 % typ. [Elektroda, liseczq, post #19769640] OPA2134/OPA134 measure 0.00008 % but draw less current. Both are unity-gain stable at ±22 V. Avoid NE5532 if you need input noise below 5 nV/√Hz.

What alternatives exist to the LM723 PSU?

You can swap in ±15 V LM317/337 regulators, but you lose the tracking accuracy (≤10 mV mismatch) provided by the LF357 error amp [Elektroda, liseczq, post #19381999] Keep 0.3 A current limit regardless.

How does the uPC1237 protection work?

The IC monitors output DC > ±1 V, clipping level set by R-divider, and supply drop. It opens the relay within 10 ms on a fault and delays initial connect about 3 s to mute turn-on thumps [Elektroda, liseczq, post #19381999]

Can the amp drive 4 Ω loudspeakers?

It can produce sound, but the 20 W transformer limits current to 0.44 A, so dynamic peaks will sag the rails. Use only for brief tests at low volume [Elektroda, liseczq, post #19382510]

What distortion figures can I expect?

With LME49860 and bypassed output resistor, users report ≤0.005 % THD+N at 1 kHz, 150 mW into 32 Ω, measured via sound-card and ARTA [User measurements]. Phase shift stays under 2° to 20 kHz.

Which capacitors minimise audible colouration?

Electrolytics in the signal path (2 µF) give smoother highs than X7R ceramics, which introduce microphonic piezo effects [Elektroda, yogi009, post #19769482] Low-ESR Nichicon FW/FK or Panasonic FR show the lowest measured 3rd-harmonic at –110 dB [Trioda Forum tests].

What failure could still destroy headphones?

A shorted output transistor may dump full rail voltage before uPC1237 reacts. Fit a 100 mA fast fuse in each rail as secondary protection, and mount thermal pads to avoid runaway at > 80 °C junction.

How would I re-layout for OPA134 single op-amps?

  1. Cut traces between dual-op-amp pins 1-8 and 4-5. 2. Add adapter boards that parallel two OPA134 per channel. 3. Route pin 5 space to relay driver so ±22 V rails remain for coils. Keep decoupling within 5 mm of each IC.

Is a class D headphone amp a drop-in replacement?

Modern ICEpower ASX modules hit 0.002 % THD and 94 % efficiency, but need post-LC feedback for <0.5 Ω output Z. Some listeners call the sound “sterile”, yet blind ABX tests rarely show significance [Elektroda, CodeBoy, post #19611287]
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