very first "blameless" amplifier prior 1969

[steph_tsf]

hello,

do you know any commercial audio power amplifier prior 1969 architectured this way :

- full silicon
- first stage is a long tailed pair (differential pair arrangement)
- second stage is the voltage amplifier stage (VAS) with the collector being loaded by a constant current source (not the bootstrap capacitor arrangement)
- symetric power supply
- no coupling capacitor at the output (OCL arrangement)

In a nutshell, I would like to trace the very first "blameless" audio amplifier.

Such "blameless" audio amplifier must have been a (silent ?) milestone in a context where virtually all silicon audio amplifiers were variations on the theme initiated by H.C. Lin (RCA fellow) in 1956 : input transistor acting as VAS, feedback on the emitter, capacitive bootstrap in the collector.

At the moment I have the impression that the first "blameless" audio amplifier may have been the Sinclair Z30 and Sinclair Z50. If you dare removing R7. See attached picture. Quite regrettable is the rather imperfect class AB biaising arrangement. Actually, for this reason, it doesn't deserve the qualification of "blameless".

Or, maybe, the "blameless" concept showed long time before 1969 while designing audio integrated circuits ?

Any clues ?

[wahab]

That is no so called blameless , it has no vas enhancement...

A 1969 model of this brand seems to satisfy the requirements though :

Sonab amplifier information?

[steph_tsf]

hi Wahab, can you provide the schematic of the U700 circuit board that's inside the Sonab R4000 receiver, the 1969 version ? You say the topology I am researching is not so called "blameless". How would you call it ? Make a proposition.

[steph_tsf]

Worth checking is a possible "USA" origin of the "blameless" topology. Need to check authors like Daniel Meyer, Don Lancaster, Gary Kay. There may be articles from them in Popular Electronics. Will somebody check ?

I guess the "Leach amp" came much later, actually it can be viewed as a symetric version of the "blameless" : one NPN "blameless" loaded by one PNP "blameless". Right ? What year is the "Leach amp" ? Is it true it delivers a dull sound compared to "blameless" ? How possible ?

[john_ellis]

The best amplifier from 1968 was Bailey's "laboratory amplifier" which had many of the features of a modern circuit, but not all mentioned in the "blameless".
It had a differential input pair; medium impedance VAS load (moderate value resistor taken to a high voltage supply rather than bootstrap) and fully complementary output stage. Beats most of the circuits around at the time for audio, but was not presented as an audio circuit.

[wahab]

Originally Posted by steph_tsf

hi Wahab, can you provide the schematic of the U700 circuit board that's inside the Sonab R4000 receiver, the 1969 version ? You say the topology I am researching is not so called "blameless". How would you call it ? Make a proposition.

I took this schematic on the thread about the SONAB brand..
It s exactly the same schematic as the one popularized by D. Self
but with inverted polarity transistors ,i.e, a NPN differential..

The subsequent link display another version , with no vas buffer.
SONAB R4000 receiver annual service....

[steph_tsf]

Hi wahab, thanks for the schematics. Yes indeed the Sonab R4000/U700 is a "blameless" ancestor dating back from 1969. A nice milestone. Amazing to discover such quality in a box containing a tuner, a preamp and an amplifier. What do you think about the fuse in series with the output ? Is it there in both versions ?

[steph_tsf]

Originally Posted by john_ellis

The best amplifier from 1968 was Bailey's "laboratory amplifier" which had many of the features of a modern circuit, but not all mentioned in the "blameless". It had a differential input pair; medium impedance VAS load (moderate value resistor taken to a high voltage supply rather than bootstrap) and fully complementary output stage. Beats most of the circuits around at the time for audio, but was not presented as an audio circuit.

And the Bailey's 30W amplifier dating back from 1968 ? Not a "blameless" ancestor ? Nothing on the radar from Heathkit, Hewlett Packard, Schlumberger or Tektronix ? Some oscilloscope vertical amplifier maybe ?

[forr]

I do not remember a Bailey amp having a diff input pair.

Nor pre 1970, neither commercial, but worth saying some words about it :
One of the most looking like a Blameless for its low power stages, but published fourteen years sooner, was the circuit due to B.J. Codd in Wireless World, October 1979, pages 81-85. I've never seen it mentionned anywhere since.

Its features :
- Diff input pair, 2 * 1 mA current source driven, 2 * 120 Ohm emitter degenerated, loaded by a Wilson 3 current mirror.
- Then a cascoded VAS loaded by a 3 mA current source, with 39 pF Miller compensation.
- Class A push-pull emitter followers buffering the output stage.

There was a clear search for very low distorsion.

[wahab]

Originally Posted by steph_tsf

Hi wahab, thanks for the schematics. Yes indeed the Sonab R4000/U700 is a "blameless" ancestor dating back from 1969. A nice milestone. Amazing to discover such quality in a box containing a tuner, a preamp and an amplifier. What do you think about the fuse in series with the output ? Is it there in both versions ?

Curiously , the second version has a simpler VAS but seems to use
a relay instead of fuses.