Author Topic: Audio amp help  (Read 2059 times)

0 Members and 1 Guest are viewing this topic.

Offline jeffheath

  • Regular Contributor
  • *
  • Posts: 55
  • Country: us
Re: Audio amp help
« Reply #25 on: February 17, 2019, 06:28:16 am »
...I updated the schematic to put the capacitors in the correct location. 

The correct location for the capacitors is between the pot and the input resistors.

The capacitors should be after the potentiometers, otherwise the bias point of the circuit will shift, as the volume is adjusted. A capacitor is needed to block the DC voltage to the potentiometer.

You may not notice it immediately but the DC from the op-amp will cause premature deterioration of the pot with increased noise over time.
Oops.. I thought he was saying I had it the wrong way and didn't even process the rest of his post.. Fixed for (hopefully) the last time... Will see what it sounds like.
 
The following users thanked this post: Richard Crowley

Offline bson

  • Supporter
  • ****
  • Posts: 1606
  • Country: us
Re: Audio amp help
« Reply #26 on: February 17, 2019, 09:34:50 pm »
If you attenuate the output and reduce the gain accordingly instead of the input, you 1) reduce the amount of gain needed, 2) provide a fixed input impedance, 3) make the load more resistive (although HPs are already mostly resistive), and 4) makes the input offset voltage non critical: little gain is applied, and it's divided down on the output by the volume control, so output can be DC direct.  It's only power amplifiers where output attenuation isn't possible, but for µW type loads like HPs it's the go-to.  If you attenuate first you push the signal down further in the noise floor, only to pull it back up again with gain which invariably pulls up noise with it.  (See Friis' formula.)

Here's a super simple HP amplifier, but you'd be surprised how good it sounds with a monitor-like headphone like the AKG K102 and a quality source.  I sell a slight variation on it, purely based on word of mouth, to some very picky people.

Thumb:


Full PNG:
http://www.rockgarden.net/download/eevblog/hpa.png

KiCad v5 schematic:
http://www.rockgarden.net/download/eevblog/hpa.sch

Enjoy!
 

Offline paulca

  • Super Contributor
  • ***
  • Posts: 2068
  • Country: gb
Re: Audio amp help
« Reply #27 on: March 16, 2019, 10:57:00 am »
You could also consider a Baxandall configuration which produces a stable logarithmic active volume control.  ie. the potentiometer is in the gain loop.

https://istem.engineering.osu.edu/sites/istem.engineering.osu.edu/files/uploads/5557volumecontrolsproject.pdf
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 13454
  • Country: gb
  • 0999
Re: Audio amp help
« Reply #28 on: March 16, 2019, 02:00:20 pm »
...I updated the schematic to put the capacitors in the correct location. 

The correct location for the capacitors is between the pot and the input resistors.
That's what I said:

The capacitors should be after the potentiometers, otherwise the bias point of the circuit will shift, as the volume is adjusted. A capacitor is needed to block the DC voltage to the potentiometer.

Quote
You may not notice it immediately but the DC from the op-amp will cause premature deterioration of the pot with increased noise over time.
Th normal issue is wiper noise, when the potentiometer is adjusted, but in this case it's worse, because the DC bias point of the amplifier will change, as the potentiometer is adjusted.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf