Editing Audio power amplifier (section)

==Further developments==
For some years following the introduction of solid-state amplifiers, their perceived sound did not have the excellent audio quality of the best valve amplifiers (see [[valve audio amplifier]]). This led [[audiophile]]s to believe that "tube sound" or [[valve sound]] had an intrinsic quality due to the vacuum tube technology itself. In 1970, [[Matti Otala]] published a paper on the origin of a previously unobserved form of distortion: transient [[intermodulation distortion]] (TIM),<ref>{{Cite journal | doi = 10.1109/TAU.1970.1162117| title = Transient distortion in transistorized audio power amplifiers| journal = IEEE Transactions on Audio and Electroacoustics| volume = 18| issue = 3| pages = 234–239| year = 1970| last1 = Otala | first1 = M.| s2cid = 13952562}}</ref> later also called [[slew-induced distortion]] (SID) by others.<ref>{{cite journal|author=Jung, Walter G.; Stephens, Mark L. and Todd, Craig C. |date=June 1979|title=An overview of SID and TIM|journal=Audio}}</ref> TIM distortion was found to occur during very rapid increases in amplifier output voltage.<ref>{{cite journal|title=Circuit Design Modifications for Minimizing Transient Intermodulation Distortion in Audio Amplifiers|url=http://www.aes.org/e-lib/browse.cfm?elib=2065|author=Otala, Matti |journal=[[Journal of the Audio Engineering Society]]|volume=20 |issue= 5|date= June 1972}}</ref>

TIM did not appear at steady state sine tone measurements, helping to hide it from design engineers prior to 1970. Problems with TIM distortion stem from reduced open loop frequency response of solid-state amplifiers. Further works of Otala and other authors found the solution for TIM distortion, including increasing [[slew rate]], decreasing preamp frequency bandwidth, and the insertion of a lag compensation circuit in the input stage of the amplifier.<ref>{{cite journal|title=Distribution of the Phonograph Signal Rate of Change|url=https://www.aes.org/e-lib/online/browse.cfm?elib=2906|author=Lammasniemi, Jorma; Nieminen, Kari |journal=[[Journal of the Audio Engineering Society]]|volume= 28 |issue= 5|date= May 1980}}</ref><ref>{{cite journal|title=Psychoacoustic Detection Threshold of Transient Intermodulation Distortion|url=https://www.aes.org/e-lib/browse.cfm?elib=4002|author1=Petri-Larmi, M. |author2=Otala, M. |author3=Lammasniemi, J. |journal=[[Journal of the Audio Engineering Society]]|volume=28|issue=3|date=March 1980}}</ref><ref>Discussion of practical design features that can provoke or lessen slew-rate limiting and transient intermodulation in audio amplifiers can also be found for example in {{cite book|chapter=Ch. 9|author=Hood, John Linsley |title=The Art of Linear Electronics|isbn=978-0-7506-0868-8|doi=10.1016/B978-0-7506-0868-8.50013-8|publisher=Butterworth-Heinemann|place= Oxford|year= 1993}}</ref> In high-quality modern amplifiers the open loop response is at least 20&nbsp;kHz, canceling TIM distortion.

The next step in advanced design was the Baxandall Theorem, created by Peter Baxandall in England.<ref>Baxandall, Peter (February 1979) [http://hifisonix.com/audio-power-amplifier-design-peter-j-baxandall/baxandall_audio-power-amplifier-design/ "Audio power amplifier design"] {{Webarchive|url=https://web.archive.org/web/20220609031924/http://hifisonix.com/audio-power-amplifier-design-peter-j-baxandall/baxandall_audio-power-amplifier-design/ |date=2022-06-09 }}, ''Wireless World magazine''</ref> This theorem introduced the concept of comparing the ratio between the input distortion and the output distortion of an amplifier. This new idea helped audio design engineers to better evaluate the distortion processes within an amplifier.