Please register or login. There are 0 registered and 831 anonymous users currently online. Current bandwidth usage: 326.30 kbit/s May 26 - 12:53pm EDT 
Hardware Analysis
Forums Product Prices

  Latest Topics 

More >>


  ExtremA, a reference class-A DIY amplifier. 
  Feb 22, 2007, 07:30am EST 


By: Sander Sassen

The graph below shows the THD+N (total harmonic distortion + noise) at 1kHz and 6kHz, the measurement bandwidth is limited to 20kHz. Clearly the noise, at 30uV, dominates here. Only at 30W at 6kHz the distortion starts to rise above the noise floor. What is also clear is that fact that although we set the bias current to allow for 100W/8-ohm, the transformer used for this test left us twenty watts shy of the design goal.

THD+N at 1kHz and 6kHz

Fig 8. THD+N graph at 1kHz and 6kHz with 20kHz measurement bandwidth.

The next figure shows the THD and THD+N at 40W and 20W, the measurement bandwidth is now limited to 80kHz. The top two graphs clearly show that by using a larger measurement bandwidth the noise now fully dominates, rendering the plots rather meaningless. To be able to make a proper THD measurement we have used a function of the AP measurement system that uses band filters to measure only around the harmonics.

THD+N versus frequency

Fig 9. THD and THD+N at 40W and 20W, 80kHz measurement bandwidth.

Next up is the residual THD at 40W/1kHz. The 2nd harmonic is due to the fact we have not used Hfe matched transistors throughout. What can be read from this figure is that at 40W the true THD is 0.00008% (-122dB). A figure you will be hard pressed to find with any commercial amplifier. The nature of class-A distortion and its harmonic distribution allows us to extrapolate down to lower signal levels. At 1W the distortion would be around 0.000002% (-154dB) something which canít be measured directly.

Residual THD at 40W/1kHz

Fig 10. Residual THD at 40W/1kHz.

1. Introduction
2. Transistors
3. Concept and design
4. Scalability and construction
5. Measurements
6. Copyrights, Updates etc.

Discuss This Article (13 Comments) - If you have any questions, comments or suggestions about the article and/or its contents please leave your comments here and we'll do our best to address any concerns.

Rate This Product - If you have first hand experience with this product and would like to share your experience with others please leave your comments here.



  Related Articles 

A weekly newsletter featuring an editorial and a roundup of the latest articles, news and other interesting topics.

Please enter your email address below and click Subscribe.