Go Back   Home > Forums > >

Headphone Systems Everything to do with Headphones

directly coupled class A buffer for low-voltage headphones (work-in-progress)
directly coupled class A buffer for low-voltage headphones (work-in-progress)
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools
Old 22nd October 2020, 04:29 AM   #11
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
Welp, the boards arrived. Unfortunately they have a buzz problem.

When I have the laptop plugged into the wall, the amp is picking up buzzing noise. This noise is affected by moving around the mouse.

If I unplug the laptop from the wall, the buzz completely disappears.

The puzzling thing is, my copper clad prototype was completely free from this problem.

I tried placing a piece of copper clad beneath the PCB (soldered to power supply negative), but it had no effect.

I tried switching from directly connected input to 1uF film caps, but it didn't affect the buzzing.

Very interesting, time to debug!
Attached Images
File Type: jpg IMG_5897.jpg (358.7 KB, 121 views)
File Type: jpg IMG_5898.jpg (737.3 KB, 123 views)
  Reply With Quote
Old 22nd October 2020, 07:21 AM   #12
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Westende Resort, BE coast
I wonder how accurate the -4.6V at the output is between all three sections. What is the voltage difference between the 'ground return' section and the two active sections? That is not shown on the sim.
It may be OK in the sim (all same name devices are identical in a sim but not in reality), but I wonder how it works in real life. Did you measure that?

Nice PCB layout BTW.

Jan

Last edited by jan.didden; 22nd October 2020 at 07:29 AM.
  Reply With Quote
Old 22nd October 2020, 09:27 AM   #13
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
Hi Jan, thanks for the kind words!

Yes, it is critical to adjust the output offset of both channels to zero (or within a few millivolts), using the 10-turn potentiometers.

With the PCB version of the buffer, I actually did not "match" any of the 2N7000's while assembling the board, and I had to fudge around with the current sense and trim resistors a bit (I dialed one of the 10-turns all the way and it wasn't quite enough, oops!). This is why you can see a few of those resistors sticking up so high off the board.
  Reply With Quote
Old 22nd October 2020, 09:50 AM   #14
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
Hmm, I made a mistake with the R26 and R27 1meg input capacitor drain resistors. I should not have tied them to the sleeve, as that makes it impossible to galvanically isolate the amp from the input, if you choose to use input capacitors (and omit the R28 10k ground-tie).
  Reply With Quote
Old 22nd October 2020, 03:42 PM   #15
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
Ok, I figured it out.

The "ground link" resistor (which joins audio ground to buffer "positive" rail) was too high in value. Lowering it to 1k reduced the buzz volume, and lowering it to 100R reduced the buzz beneath the level of my tinnitus. Of course, a dead short also eliminates the buzz.

I must admit I chose 10k as a "I'm not sure about what I'm doing" value. And also in case someone decided to use a non-floating power supply (which would feed 5 volts back into your DAC ground).

What surprises me was that something else I tried earlier did not fix the buzz: I modified the circuit such that the buffer circuit was galvanically isolated from the audio input:
  • populate the input capacitors C3, C4
  • remove (open circuit) the "ground-link" resistor R28
  • remove the cap drain resistors R26, R27
  • rig two new cap drain resistors, 1meg from TP14 to TP18, and 1meg from TP16 to TP22

surprisingly, the buzz persisted past this "isolated" version.
  Reply With Quote
Old 22nd October 2020, 03:51 PM   #16
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
The next step will be to do some distortion analysis. I have ordered vicnic's 1kHz ultra-low distortion oscillator, which should arrive soon. Excited!
  Reply With Quote
Old 22nd October 2020, 04:25 PM   #17
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
Updated schematic with a few fix-ups:
  • fix LED polarity
  • change R28 "ground-link" resistor value to 100R
  • connect capacitor drain resistors R26, R27 to buffer "positive" rail
Attached Files
File Type: pdf class-a-2n7000.pdf (70.8 KB, 34 views)
  Reply With Quote
Old 23rd October 2020, 11:45 PM   #18
Rick PA Stadel is offline Rick PA Stadel  United States
diyAudio Member
 
Join Date: Apr 2017
Location: greater Kansas City
-- Very pretty board!

'Floating supply' always seems like a much better idea than it turns out in practice -- you still need a return path for the input signal .. but, then, you've already discovered that.

The input signal could be referred to Vg in the original schematic. But that would make it more vulnerable to connection mishaps.

100mA still seems a little heavy for TO-92 devices -- they're gonna get pretty toasty dissipating around half a watt each quiescent.

Cheers
  Reply With Quote
Old 24th October 2020, 01:39 AM   #19
cellularmitosis is offline cellularmitosis  United States
diyAudio Member
 
Join Date: Jul 2013
Thanks Rick!

The circuit is currently running off of 5.0 volts, with about 2.7 of that across the fets. So the to-92’s are dissipating about a quarter watt each.
  Reply With Quote
Old 26th October 2020, 04:30 PM   #20
laserscrape is offline laserscrape
diyAudio Member
 
Join Date: Dec 2017
Have tried you tried simming (or listening) with typical biasing+cap coupling compared to virtual ground?
  Reply With Quote

Reply


directly coupled class A buffer for low-voltage headphones (work-in-progress)Hide this!Advertise here!
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Directly-coupled buffer with only one supply rail? cellularmitosis Analog Line Level 8 8th October 2020 03:03 AM
TBJ's Class D - a discrete class D amp that anyone can build! (Work-In-Progress) tbj Class D 19 18th December 2019 12:45 PM
very simple class A buffer for headphones? laserscrape Tubes / Valves 39 12th December 2019 06:55 PM
bias voltage buffer series resistor on ac coupled opamp bnae38 Digital Line Level 42 11th March 2018 01:51 AM
DAC that will drive headphones directly jkeny Digital Line Level 7 27th January 2010 12:00 PM


New To Site? Need Help?

All times are GMT. The time now is 10:44 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki