bitrex wrote:> > I was looking over the schematic for the Roland Jazz Chorus amplifier, > and the power amp section seems pretty standard, except for the fact > that the designers apparently chose to use a "PNP" configured Sziklai > pair as the bias voltage source, to set the quiescent current in the > output devices (biased into class AB I'd guess.) > > Bottom right: > > http://tinyurl.com/h4jnsg2 > > Any ideas as to the reason behind this decision? Thermal stability? >** The Vbe multiplier pair is mounted on the main PCB so only tracks ambient temp changes. Soon as the upper output device heats, the bias current will rise. .... Phil
Sziklai pair power amplifier bias
Started by ●March 11, 2016
Reply by ●March 11, 20162016-03-11
Reply by ●March 12, 20162016-03-12
On 03/11/2016 04:59 PM, Jim Thompson wrote:> On Fri, 11 Mar 2016 15:29:49 -0500, bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> On 03/11/2016 02:55 PM, John Larkin wrote: >>> On 11 Mar 2016 11:00:44 -0800, Winfield Hill >>> <hill@rowland.harvard.edu> wrote: >>> >>>> bitrex wrote... >>>>> >>>>> I was looking over the schematic for the Roland Jazz Chorus amplifier, >>>>> and the power amp section seems pretty standard, except for the fact >>>>> that the designers apparently chose to use a "PNP" configured Sziklai >>>>> pair as the bias voltage source, to set the quiescent current in the >>>>> output devices (biased into class AB I'd guess.) >>>>> >>>>> Bottom right: http://tinyurl.com/h4jnsg2 >>>>> >>>>> Any ideas as to the reason behind this decision? Thermal stability? >>>> >>>> The output stage needs three Vbe drops for good class-AB >>>> biasing, and they only provide two. What's more, the use >>>> of a Darlington or Sziklai pair for the bias voltage, and >>>> with no base resistor on Q22 to establish a predictable >>>> modestly-high operating current for Q21, further reduces >>>> the generated bias voltage. This means there's an output- >>>> stage deadzone with exaggerated crossover distortion. >>> >>> I designed a guitar amp once, for a chain of music shops in the South. >>> It was too good, and people called it "flat." So I added an adjustable >>> amplitude tracking distortion generator, which they loved. They >>> advertised it as "Bell Tone", meaning I guess that it made a guitar >>> sound more like a bell, not a telephone. >>> >>> Hey, somebody has heard of it! >>> >>> http://vintageamps.com/plexiboard/viewtopic.php?f=11&t=96883 >>> >>> >> >> Dare I say it, but I think Win is wrong on this one. R47 and R48 and Q11 >> Q12 form a voltage source which is set to around 3.5 volts, which brings >> the output stage out of the area where crossover distortion would be a >> factor. >> >> The output stage uses a standard Darlington configuration made from two >> transistors each for the upper NPN, and a power Sziklai for the PNP, >> which is a configuration you see a lot even in power amplifiers that >> aren't on a chip. >> >> I recall reading somewhere that using a Sziklai for both sections of the >> output stage has some advantages, but I guess power PNPs were expensive >> and not that great back in the day. > > Yep. It's sort of class-AB... but suicidal if there isn't good > thermal tracking.Sziklai pairs have local feedback, which Darlingtons don't. Local feedback is like butter--it makes everything better. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●March 12, 20162016-03-12
On Sat, 12 Mar 2016 10:23:09 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 03/11/2016 04:59 PM, Jim Thompson wrote: >> On Fri, 11 Mar 2016 15:29:49 -0500, bitrex >> <bitrex@de.lete.earthlink.net> wrote: >>[snip]>> >> Yep. It's sort of class-AB... but suicidal if there isn't good >> thermal tracking. > >Sziklai pairs have local feedback, which Darlingtons don't. Local >feedback is like butter--it makes everything better. ;) > >Cheers > >Phil HobbsI sort of agree with that. I tend to build with blocks that have local feedback, then join them up... but this amplifier is a bit on the _gross_ side ;-) ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●March 12, 20162016-03-12
On 03/12/2016 10:25 AM, Jim Thompson wrote:> On Sat, 12 Mar 2016 10:23:09 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 03/11/2016 04:59 PM, Jim Thompson wrote: >>> On Fri, 11 Mar 2016 15:29:49 -0500, bitrex >>> <bitrex@de.lete.earthlink.net> wrote: >>> > [snip] >>> >>> Yep. It's sort of class-AB... but suicidal if there isn't good >>> thermal tracking. >> >> Sziklai pairs have local feedback, which Darlingtons don't. Local >> feedback is like butter--it makes everything better. ;) >> >> Cheers >> >> Phil Hobbs > > I sort of agree with that. I tend to build with blocks that have > local feedback, then join them up... but this amplifier is a bit on > the _gross_ side ;-) > > ...Jim Thompson >Aside from the Sziklai weirdness, it otherwise looks pretty much like a textbook differential-voltage gain-output stage SS amp to me... They use a bootstrap load for the VAS. Woohoo, don't have to shell out for another transistor!
Reply by ●March 12, 20162016-03-12
On Sat, 12 Mar 2016 10:29:15 -0500, bitrex <bitrex@de.lete.earthlink.net> wrote:>On 03/12/2016 10:25 AM, Jim Thompson wrote: >> On Sat, 12 Mar 2016 10:23:09 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 03/11/2016 04:59 PM, Jim Thompson wrote: >>>> On Fri, 11 Mar 2016 15:29:49 -0500, bitrex >>>> <bitrex@de.lete.earthlink.net> wrote: >>>> >> [snip] >>>> >>>> Yep. It's sort of class-AB... but suicidal if there isn't good >>>> thermal tracking. >>> >>> Sziklai pairs have local feedback, which Darlingtons don't. Local >>> feedback is like butter--it makes everything better. ;) >>> >>> Cheers >>> >>> Phil Hobbs >> >> I sort of agree with that. I tend to build with blocks that have >> local feedback, then join them up... but this amplifier is a bit on >> the _gross_ side ;-) >> >> ...Jim Thompson >> > >Aside from the Sziklai weirdness, it otherwise looks pretty much like a >textbook differential-voltage gain-output stage SS amp to me...Sziklai szucks for all but a few specialized applications >:-}> >They use a bootstrap load for the VAS. Woohoo, don't have to shell out >for another transistor!Yep. I did that in my car amp to maximize the output swing (ran straight off of 12V, no inverter). ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●March 12, 20162016-03-12
bitrex wrote:> > > Aside from the Sziklai weirdness, it otherwise looks pretty much like a > textbook differential-voltage gain-output stage SS amp to me... > > They use a bootstrap load for the VAS. Woohoo, don't have to shell out > for another transistor! >** Bootstrapping the class A stage load does way more than can be done with just an extra transistor. You would also need a boosted rail above the positive one to get a similar result. .... Phil
Reply by ●March 13, 20162016-03-13
In article <C2BEy.28788$%_.1066@fx03.iad>, bitrex <bitrex@de.lete.earthlink.net> wrote:> I was looking over the schematic for the Roland Jazz Chorus amplifier, > and the power amp section seems pretty standard, except for the fact > that the designers apparently chose to use a "PNP" configured Sziklai > pair as the bias voltage source, to set the quiescent current in the > output devices (biased into class AB I'd guess.) > > Bottom right: > > http://tinyurl.com/h4jnsg2 > > Any ideas as to the reason behind this decision? Thermal stability?It looks like it's to regulate the current on R117 and set the bias. Maybe it's flawed in a way that sounds interesting. -- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
Reply by ●March 13, 20162016-03-13
In article <mcmurtrie-66898D.22594612032016@news.sonic.net>, mcmurtrie@pixelmemory.us says...> > In article <C2BEy.28788$%_.1066@fx03.iad>, > bitrex <bitrex@de.lete.earthlink.net> wrote: > > > I was looking over the schematic for the Roland Jazz Chorus amplifier, > > and the power amp section seems pretty standard, except for the fact > > that the designers apparently chose to use a "PNP" configured Sziklai > > pair as the bias voltage source, to set the quiescent current in the > > output devices (biased into class AB I'd guess.) > > > > Bottom right: > > > > http://tinyurl.com/h4jnsg2 > > > > Any ideas as to the reason behind this decision? Thermal stability? > > It looks like it's to regulate the current on R117 and set the bias. > Maybe it's flawed in a way that sounds interesting.The combination looks like they are being used as the diode multiplier. That config gives a little more than a normal forward bias volts on trannies, around .7 to .8 volts. When things warm up abit, I do think the saturation level drops and also reduces the bias in that circuit for Q20 and Q18 to compensate. Looks like a good place for it and a typical quasi output. Jamie
Reply by ●March 13, 20162016-03-13
Kevin McMurtrie wrote:> > > > > http://tinyurl.com/h4jnsg2 > > > > Any ideas as to the reason behind this decision? Thermal stability? > > It looks like it's to regulate the current on R117 and set the bias. > Maybe it's flawed in a way that sounds interesting. >** It's not flawed and the same circuit is used in many amplifiers orginating from Japan - eg Yamaha. More often, the first device of the pair is in a flat pack ( TO126 or TO220 ) and bolted to the heatsink to provide better tracking. .... Phil
Reply by ●March 14, 20162016-03-14
On Sun, 13 Mar 2016 20:23:27 -0500, M Philbrook <jamie_ka1lpa@charter.net> wrote:>In article <mcmurtrie-66898D.22594612032016@news.sonic.net>, >mcmurtrie@pixelmemory.us says... >> >> In article <C2BEy.28788$%_.1066@fx03.iad>, >> bitrex <bitrex@de.lete.earthlink.net> wrote: >> >> > I was looking over the schematic for the Roland Jazz Chorus amplifier, >> > and the power amp section seems pretty standard, except for the fact >> > that the designers apparently chose to use a "PNP" configured Sziklai >> > pair as the bias voltage source, to set the quiescent current in the >> > output devices (biased into class AB I'd guess.) >> > >> > Bottom right: >> > >> > http://tinyurl.com/h4jnsg2 >> > >> > Any ideas as to the reason behind this decision? Thermal stability? >> >> It looks like it's to regulate the current on R117 and set the bias. >> Maybe it's flawed in a way that sounds interesting. > > The combination looks like they are being used as the diode >multiplier. That config gives a little more than a normal forward >bias volts on trannies, around .7 to .8 volts. > > When things warm up abit, I do think the saturation level drops and >also reduces the bias in that circuit for Q20 and Q18 to compensate. > > Looks like a good place for it and a typical quasi output. > >JamieFor class AB bipolar amps (which are pretty much ancient history) a good scheme is to use biggish emitter resistors paralleled with power diodes. No thermal problems, no bias pots. Better is to use mosfets, each with its own closed-loop opamp to drive its gate. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
