4
\$\begingroup\$

I am a self-study noob to electronics (and audio electronics) but diligently learning every day :) Some things I understand well, others I'm fuzzy on.

This device is a headphone amplifier using NE5532 opamps in single-supply biased mode, biased with +6V DC. My understanding is that it is a bad thing to push any amount of DC through headphones whatsoever, so that's why one must put the coupling capacitors C25 and C26 on the output. 1000uF is a fairly large cap, but the value was chosen to maximize the frequency response of the audible range from the HPF it produces with the headphone impedance (32, 90, and 320 ohms). I was testing this completed design today and noticed that, on first power-on of the device, the output capacitors C25 and C26 have to charge up before they are able to fully block DC. This is expected, but for a long ~4-5 second window (this was wrong, the length of time is much shorter, see edit), there is 2-3V DC flowing through the output into the headphones that is reduced over time. That raises a warning flag to me that something has to be done before the user can safely plug in the headphones!

First thing that comes to mind is to reduce the output capacitance so the DC blocking state is reached more quickly. But, this would modify the high-pass to attenuate low bass notes significantly. Is there another more common solution for how to address this problem? Surely this can't be ignored and left as-is, right?

For reference, I mostly modified this design for an NE5532-based headphone amplifier from the Phil's Lab YT channel, which is based on the Doug Self NE5532 headphone amp. My design is more-or-less the same as the Phil's Lab design except it has reduced buffering on the output. https://www.youtube.com/watch?v=Z2GUoi63pJs

Oh, and if you see other obvious issues in the schematic as you're viewing please comment on them! This is my first ever non-trivial circuit and I'm sure I've messed up quite a lot of things.

EDIT: In the original question, I had been testing the charge time without a load. With a 32 or 250 ohm load, the RC charge time is very fast, but the length of time it's taking to charge without load is due to the high R38 and R39 values (10K). I also fear that quick burst of current might pop a headphone speaker. 10K was chosen to not severely impact the output impedance for the headphones when a load is attached, but that value is pretty high for even a 32 ohm load. Reducing that value lets the capacitor charge more quickly to a steady state with no load attached, but it doesn't entirely solve the problem as there is still a non-zero amount of DC passing through at the start.

Schematic Design Pg1 Schematic Design Pg2

\$\endgroup\$
10
  • 2
    \$\begingroup\$ One option is to have a mute circuit with time delay, which connects the output to phones after the caps have charged. Sometimes a relay is used. Changing to bipolar supply would eliminate the charging time. Although the 1000uF with assumed 15 ohms in series and 32 ohm phones will result to 3dB point being at 3 Hz, would maybe a 220uF be more reasonable to charge faster? \$\endgroup\$
    – Justme
    May 3 at 19:34
  • \$\begingroup\$ Thanks for the suggestion. I will try out the 220uF in my ltspice simulation and see what the response is for that. It could just very well be that 1000uF is just way too high. I might have been a bit overzealous with the size in fear of attenuating any part of the audible subbass notes. \$\endgroup\$
    – Gbps
    May 3 at 19:40
  • \$\begingroup\$ I'd prefer not to make a mute circuit since I know that's bothersome for people who like flipping on and off the amplifier before joining voice calls and such. \$\endgroup\$
    – Gbps
    May 3 at 19:41
  • \$\begingroup\$ @Gbps also note that with the headphones disconnected the output cap can only charge through the 10k resistors R38 and R39, so the charge up time will be very slow. with a 32 ohm headphone load at the output it would be much faster - you should include that in your simulation. \$\endgroup\$
    – Bryan
    May 3 at 19:43
  • 1
    \$\begingroup\$ Then use a mute circuit that charges the caps faster. Like short circuit the output to ground directly and release them. Should not take more than 0.1 seconds. \$\endgroup\$
    – Justme
    May 3 at 19:50

1 Answer 1

0
\$\begingroup\$

I don't think you need worry that the charging current pulse when you switch the amplifier on will damage the headphones. However, if you are still worried, add a 6 V regulator L7806 to the power supply, using a 220µF cap at its output and connect your headphone 'earth' to the 6 V output.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.