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:
Any ideas as to the reason behind this decision? Thermal stability?
Naaaah! It's just to add more "warmth" to the sound >:-} ...Jim Thompson
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| James E.Thompson | mens |
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I love to cook with wine. Sometimes I even put it in the food.
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.
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.
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
Warm sound ?>:-} ...Jim Thompson
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| James E.Thompson | mens |
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| San Tan Valley, AZ 85142 Skype: skypeanalog | |
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I love to cook with wine. Sometimes I even put it in the food.
On Friday, March 11, 2016 at 11:00:57 AM UTC-8, Winfield Hill wrote: ...
Surely 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
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!
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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
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| 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.
** 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 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
[snip]
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
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| 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.
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!
Sziklai szucks for all but a few specialized applications >:-}
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.
** 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
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.
** 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
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
For 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
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