diyAudio - Entries for January 9, 2013
Go Back   Home > Forums > Blogs
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Headphone amps are hard.

Posted 9th January 2013 at 10:41 AM by rjm (RJM Audio Blog)
Updated 21st January 2013 at 07:12 AM by rjm

The $50-from-China-via-ebay NJM4556-based headphone amp, pictured below, fails the "Is thing thing on?" test. With 16 ohm, 104 dB/mW headphones there is a faint-yet-audible background hiss when the unit is powered up and the volume is turned all the way down. The hiss increases only slightly as the volume is turned up (input disconnected).

The output noise of an op amp is normally estimated from the input-referred voltage noise density multiplied by the gain, and the current noise density multiplied by the gain and the sum of the input impedances on the inverting and noninverting inputs. The NJM4556 datasheet does not give a figure for current noise, but a reasonable guess is 1 pA/sqrtHz. The voltage noise density is estimated at 8 nV/sqrtHz.

Total noise is

Vn (output) = sqrt [(i_n R_s)^2 + e_n^2 + 4kT R_s] * Gain

Working from a source impedance of ~10k, I get about 18 nV/sqrtHz or -138 dB.

[Revised according...
Attached Thumbnails
Click image for larger version

Name:	DSC_2499 1600.jpg
Views:	1355
Size:	107.3 KB
ID:	834   Click image for larger version

Name:	sApVI schematic.jpg
Views:	3967
Size:	244.7 KB
ID:	835   Click image for larger version

Name:	DSC_2486 sm.jpg
Views:	496
Size:	224.8 KB
ID:	836   Click image for larger version

Name:	sapvi.png
Views:	352
Size:	37.7 KB
ID:	837  
Attached Files
File Type: zip op amp (9.8 KB, 86 views)
rjm's Avatar
diyAudio Member
Posted in The Lab
Views 2494 Comments 10 rjm is offline
Hide this!Advertise here!

New To Site? Need Help?

All times are GMT. The time now is 05:32 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2