New Class A, Super-A, Non-Switching : need a revival ?
would there be an intellectual property infringement if a company sells three small PCBs, or carrierboards, about 1.5 by 1.0 inch tall, fitted with SMDs, ready to use, providing each one a specific Class AB biaising scheme :
- "New Class A" bias module (see Technics folklore - synchro bias)
- "Super-A" bias module (see JVC folklore)
- "Non Switching" bias module (see Pioneer folklore)
The idea is to help hobbyists designing their own audio power amplifiers, making an effortless revival of the three non-switching class AB biaising methods. Hobbyists and designers would integrate the module of their choice onto their own main board.
Technics "New Class A" is relying on fast diodes, implementing a dynamic switch that is automatically chosing between the voltage that's coming from the driver (this is when the output device is pushed to drive the load), and between a fixed voltage that is preset and defining the minimum bias current (this is when the output device gets pulled out, with the load being driven by the other half).
Looks fine, apart from the inherent switching nature of the process. Technics engineers used to call it "synchro-bias", but the marketing rejected that name, and coined "New Class A" for very understandable reasons. I guess that nowadays ultrafast diodes can be used, specified with a less than 10ns switching time, an order of magnitude faster than the output devices.
JVC "Super-A" monitors the sum of Vbe of all output devices, plus the voltage drop that occurs on the 0.22 Ohm emitter resistance. This voltage gets then compared to a fixed threshold (like 2 Vbe in case of a Darlington), and when the total voltage gets beyond this threshold, plus a little something, a transistor fitted into the Vbe multiplier arrangement acts to gently increase the multiplication coefficient.
Looks weird, like a positive feedback ! The increase rate must remain well controlled.
Pioneer "Non Switching" monitors the sum of Vbe of all output devices, plus the voltage drop that occurs on the 0.22 Ohm emitter resistance. This voltage gets then compared to a fixed threshold (like 2 Vbe in case of a Darlington), and when the total voltage gets beyond this threshold, plus a little something, a transistor fitted into the Vbe multiplier arrangement acts to gently increase the multiplication coefficient.
In essence, this is exactly as JVC "Super-A". Pioneer scheme needs a supplementary current source, which would make transistor count higher than JVC "Super-A", but on the other hand Pioneer takes some shorcuts regarding the Vbe multipliers, saving transistors over there.
What about the LT1166 ? It is an integrated circuit from Linear Technology specially designed for biaising Class AB amplifiers. Would be a 4th module welcome, embedding a LT1166 ?
Don't you think that with the advent of Class D audio amplifier, there will be some hobbyists wanting to confront Class D sound with some other (older) technologies, that one cannot find anymore, like those three fancy Class AB biasing schemes dating back from the early eighties, and possibly the LT1166 way also, which is more recent ?
thanks for taking the time for reading
Sansui "Tanaka Non Switching" monitors the sum of Vbe of the last two power devices, plus the voltage drop that occurs on the 0.33 Ohm emitter resistance. This simple schematic seems to be effective as explained by Tanaka, the author of the design, in the AES publication he made in 1980.
AES E-Library: New Biasing Circuit for Class-B Operation
There are, however, some differences between the circuit that got described at the AES, and the practical implementation in the Sansui AU-D5 amplifier.
Don't know if there are practical Sansui implementations, closer to the circuit that got described at the AES.
See attached pictures.
You are very welcome...has deep knowledge and wants to teach people
We are needing you urgently, as the ones knows more....here...are teaching less....keeping some advantage to themselves....in the reality, i think, they have not real knowledge, and do not teach us things to avoid us to discover they are empty inside.
I am glad you have "landed" here...... we need you.
Excepting for design with diodes, others only work as advertised into resistive load.
A reactive load can source or sink away the reserve current, causing the affected
power device to still shut off sometimes.
I like Schottky diodes to moderate the crossing: square law curves, low dissipation,
and fast clean recovery. A resistor across the same fixed voltage span, but not in
any way connected to the load, draws the constant reserve current. The diodes
insure the load never has a path to source or sink away this reserve.
I slave the power devices to the curve of the diode crossing. I do not curve upon
the law of hot devices that may or may not always be well matched compliments.
Much easier to trust two cold Schottkys be our good quasi-complimentary match.
Here you will find more such historical approaches:
The only not pure classA amp with pure classA sound, that I know is the Yamaha HCA amp MX-10000:
Pioneers approach - "New Biasing Circuit for Class-B Operation" - the patent:
The making of an effortless revival from all non-switching class AB biaising methods is indeed an interest idea for the aim of evaluation. Perhaps anybody realize this and offer such an evaluation amp kit.
Far as I know: The easiest way to assure operation in this class is to cross
deliberately underbiased Schottkys. Thus forcing them to operate upon the
square law curve. You then use this law to re-curve your power transistors
for a super-smooth B crossing! Plus the reserve current always bled by R1.
It doesn't matter that Schottkys operate in Class B and turn off. There is
no significant tail current, and they recover fast. The slow transistors are
always conducting the reserve current, and thus in Class A at all times.
It doesn't matter the exact quiescent current of an underbiased Schottky
B crossing. Its always more than zero, and less than would blow anything.
Will probably drift a bit with temperature. You can bond Q3 Q4 to heatsink
if you want to assure hot drift always floats deeper into B rather than AB...
It probably isn't necessary.
Remember: We are talking about a deliberate underbias. No more than one
half the Schottky stack can ever be fully on at once. We strictly regulate
the voltage sum across the stack, so that could never happen. Runaway
just isn't a likely event, even if you did nothing to thermally compensate.
Whats a Schottky diode gonna dissipate in this application? Less than 2W.
TO-220 packages are common, so thermals are going nowhere! Hot power
devices only do the work, but are never trusted to make the decisions.
The smoothness of the crossing is assured by the quasi-complimentary
square laws, and not by a specific bias current that has to be controlled.
No attempt is made to define what actual small B crossing current might
be? The extra reserve current bleeder assures the power devices will be
operating well above the small crossing of the Schottkys.
Wow, that looks truly impressive Kenpeter - dirt cheap and dirt simple. I like it. Do you have any practical measurements/builds to share with us?
I have two questions :
1) Is there a patent infringement in case one manufactures complete non-switching power amplifiers or tiny bias sub-assy boards working as described above : Technics New Class A (Synchro Bias), JVC Super-A, Pioneer Non-Switching, or Sansui Tanaka Non-Switching ?
2) Is there a patent on the Sansui Tanaka Non-Switching technique presented at the AES ?
Page 133, about the non-switching amplifiers, it reads :
"This involved a kind of positive feedback, and carrying the risk of blowing-up the output stage ! It also relies on the switching of small diodes, which ameliorated and displaced, rather than overcoming the switching problem".
I guess Ben Duncan didn't wanted to write explicitely that the Technics SynchroBias, marketed as "New Class A", was displacing the problem instead of solving it. The Technics "New Class A" is thus a little bit apart. It should be clear and remembered that JVC "Super-A", Pioneer "Non Switching" and Sansui "Tanaka Non-Switching" are not relying on diodes to be switched.
It is amazing to see the LT1166 biasing integrated circuit, being ignored. Were there commercial power amplifiers using a LT1166 biasing integrated circuit ?
Like Margan, what's worrying me is the behaviour of the output stage when driving a complex load like a loudspeaker. Some optimizations like the ones discussed here may prove unable to cope with capacitive, inductive, or varying impedance loads, especially on non-periodic, non-static, non-symetric input signals like music is. In this context, is there a LTspice standard test gig I can use, for better knowing the behaviour of the output stage ?
Regarded your first question it is for me in generall the problem to find the right google-keywords, because for the same amplifier idea/topology there are much different names resp. terms. If I don't know all terms therefore, it is impossible, to find all exist informations (sometimes only about chance).
Regarded Sansui's developer Mr. Tanaka I found about
Sansui Au111 [Archive] - AudioKarma.org Home Audio Stereo Discussion Forums
in post from BeatleFred (01-26-2003, 12:14 AM) informations concerning an AES (JAES) article, unfortunately without the exact title of this paper. But perhaps this information is useful for an additional websearch.
The LT1166 is an advice, that is very good for servo biased topologies (and new for me). Unfortunately I don't know commercial amplifiers, where this IC is inside. And I haven't never heard an amplifier inside this servo.
Here some ULRs therefore and some discrete servo bias/automatic bias topologies:
for the death link there go to
Gary Pimm's DIY audio pages
Auto bias circuit for power amplifier using power MOSFET - Patent 6683500
Madrigal Library: Adaptive Bias
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