...found all over diyaudio. What is "home-made" exactly, when these schematics and the expertise of the designers rivals designers at high-end companies.
Hi OldDIY,
I noticed the No 3 1991 circuit uses current drive for the power transistors since there's NO base emitter resistors on the power transistors.
I re-discovered this method 2017 and published an amplifier in Linear Audio Vol.13. But I was not aware that it was used by anyone else other than in the PE Texan amp (Practical Wireless may 1972).
I'm interested to know the name (in English script) of the author for this article so I can add him (or her) to my references?
Since I can't read the script of the article in the above link I'd be interested to hear from you or someone who can read it whether the author mentions a benefit from current drive of the power transistors when base-emitter resistors are not used?
BTW A snippet from my Linear Audio article on diyAudio explains the advantages that I found with current drive of the power transistors when base-emitter resistors are not used (snippet PDF download). The main obvious one for audio amps being stable Class-AB biasing
Subsequent to my LA Vol.13 article several ways were used to overcome the charge removal issue with current driven power transistors with no base-emitter resistors. There seems no reason (at least to me) why this technique shouldn't be used to make very good audio amps.
Hello Ian,
Author's name can be transcribed as M. Dorofeyev, and the article could be quoted as: "The class B regime in power amplifiers for audio frequencies", Radio, No. 3, pp. 53-56, 1991, Moscow.
The journal started publication in 1924 and is still in print.
I've read the entire article and enjoyed the clear explanation of the circuit's concept. However, the author does not explicitly mention the absence of base-emitter resistors.
If you're interested, I'll be glad to translate the part of the article where he explains the circuit function.
Regards,
Braca
Update
The spreadsheet has been updated for NPN/PNP mismatch and Re mismatch. Still a beta version.
Also as an xls version is here PAK402-spreadsheet-PP-CC-wingspread.xls - Google Drive
An explanation PDF is here PAK402 Spreadsheet-PP-CC-Wingspread-Explanation.pdf - Google Drive
Any queries and feedback would be appreciated.
Cheers, Ian Hegglun
Hi edbarx, All,
The spreadsheet has been updated for NPN/PNP mismatch and Re mismatch. Still a beta version.
Also as an xls version is here PAK402-spreadsheet-PP-CC-wingspread.xls - Google Drive
An explanation PDF is here PAK402 Spreadsheet-PP-CC-Wingspread-Explanation.pdf - Google Drive
Any queries and feedback would be appreciated.
Cheers, Ian Hegglun
Hi edbarx,
Bob Codell's book is helpful in Ch. 24 (1st Ed) or Ch. 28 (2nd Ed) "The Negative Feedback Controversy".
Is global feedback inherently bad? I'd say the short answer is No
. It isn't Spectral Growth Distortion or re-entrant distortion from Baxandall's analysis that global feedback inherently bad. Bob Cordell's book shows it is irrelevant to our audio amplifiers; transistors invariably use degeneration and this pushes the level of local feedback over the Spectral Growth bump (1st Ed. Fig. 24.4, 2nd Ed. Fig. 28.4). So adding global feedback around the usual follower output stage doesn't cause a Spectral Growth bump (see Bobs 1st Ed. 24.5, 2nd Ed. Fig. 28.5). He shows any amount of global feedback can be used, starting from 0dB in our power amps, without upsetting the harmonic structure of a typical follower power stages.
And it isn't global feedback that make an amplifier oscillate - even single stage amplifier can be made to oscillate - like the Colpitts oscillator which is a single stage emitter follower that is sufficiently capacitively loaded.
There are a few other debunked reasons for not using global feedback in Bob's book that I won't mention here.
The main problems I believe with global feedback are
1) recovery from clipping, and
2) sharpness of the clip region.
Bob Cordell says amplifiers sound different because they recover from clipping differently. Bob also says soft clipping power amplifiers can be over-driven more than hard clip amps before the clipping becomes obvious and this gives soft-clip amps a better listening experience (my paraphrase of Bob).
Bob built a clip-power meter and demonstrated it at a HES workshop and showed under typical listening levels power amps will clip even though most are unaware it of it CordellAudio.com - Home Entertainment Show 2007. Many power amp designers and builders assume clipping doesn't happen on their system and therefore think soft-clipping isn't a big issue. Actually, it depends a lot on your speaker's sensitivity; if your amp is in the usual 100W-200W range you should consider adding soft-clip to your amp if your speakers have less than 90dB/W/m sensitivity.
Bob says not using any global feedback won't necessarily give an amplifier soft-clipping, nor will it guarantee its free from clip-recovery problems. So using global feedback is not the culprit per se for killing soft-clipping or the clip-recovery problems. Global feedback just makes them worse. But there's no inherent problem here with global feedback.
When you look at the equations for feedback in a power amplifier there's no distinction in the equations when global feedback is used over local feedback - the equations don't differentiate between the two types of feedback. Obviously when more stages are enclosed by global feedback the number of poles and zeros that are enclosed by this feedback increases, which means the difficulty of maintaining an adequate gain and phase margin increases. But opting for no global feedback does not make things much easier - eg read Arto Kolinummi's thesis "Audio Power Amplifiers - towards inherently linear amplifiers" here on designing amplifiers with no-global feedback.
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