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?
Sziklai pair power amplifier bias
Started by ●March 11, 2016
Reply by ●March 11, 20162016-03-11
On Fri, 11 Mar 2016 09:58:40 -0500, 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?Naaaah! It's just to add more "warmth" to the sound >:-} ...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 11, 20162016-03-11
On 03/11/2016 01:07 PM, Jim Thompson wrote:> On Fri, 11 Mar 2016 09:58:40 -0500, 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? > > Naaaah! It's just to add more "warmth" to the sound >:-} > > ...Jim Thompson >Actually, apparently the reason the Jazz Chorus is popular among recording artists is that it's sort of the opposite of tube amp "warm" - it sounds terrible distorted but for "clean" tones it's very "precise" and "clinical" (audiophool terms.) For certain styles of music that's exactly what you want.
Reply by ●March 11, 20162016-03-11
On 03/11/2016 01:07 PM, Jim Thompson wrote:> On Fri, 11 Mar 2016 09:58:40 -0500, 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? > > Naaaah! It's just to add more "warmth" to the sound >:-} > > ...Jim Thompson >The built in "chorus" effect is based on a bucket brigade delay IC (Panasonic MN3007 series I think) and Roland used that circuit a lot in the 1980s, in their analog synthesizers as well. It sounds fantastic! Though it can be a little noisy.
Reply by ●March 11, 20162016-03-11
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. -- Thanks, - Win
Reply by ●March 11, 20162016-03-11
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.Warm sound ?>:-} ...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 11, 20162016-03-11
On Friday, March 11, 2016 at 11:00:57 AM UTC-8, Winfield Hill wrote: ...> 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. > > > -- > Thanks, > - WinSurely the ratio of (R47+R48)/R47 (4.7+12)/4.7 = 3.5. Gives more than the required 3 Vbe drops so it would be over biased. The low current in Q11 because of the extra gain of Q12 would affect it somewhat and may bring it back to a reasonable bias. Kevin
Reply by ●March 11, 20162016-03-11
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 -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 11, 20162016-03-11
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.
Reply by ●March 11, 20162016-03-11
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. ...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.
