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FERNSEH · 5th Mar 2018, 10:46 am

From Piet's Home-built Television website:
A description of the operation of the transformerless audio output stage:
Output TransformerLess (OTL) audio power amplifier. This was an entirely new concept, where the bulky output transfer could be deleted. Although the first publication that triggered this development was not from Philips - by Petersen and Sinclair in 1952, although their publications was still using transformers - it was picked up and further optimized by the company. The basic idea is to make a push-pull output stage but without the availability of opposite polarity devices (like today an NPN and PNP transistor). There are two ways to do this. The first is to use two power pentode output valves (here PL84) which are connected in series, and drive the two gates in anti-phase. The point between the anode of the lower valve and the cathode of the upper valve is then much lower ohmic than a classical anode output. This "full-OTL" would come later in the early sixties, but the first implementation was a "light-OTL" where the upper valve was not driven and effectively acted as a constant current source load of the lower valve. The common node between the two output valves was AC connected to g2 of the upper valve, which drove the series connected loudspeakers. The output impedance was much lower than in a classical amplifier, but still 800Ohm. The breakthrough was that Philips developed 400Ohm speakers, of which two were serially connected. One was a large bass speaker mounted on the sidewall of the cabinet, one was a smaller treble speaker mounted on the front panel below the picture tube. As can be seen from the circuit diagram, the voltage on the cathode of the upper valve is 107V, and this requires a valve that can withstand such a high cathode-to-filament voltage. Later Philips would introduce the EL86/PL86 specially designed for this parameter. This concept provided excellent sound volume and OTL would remain a Philips high performance differentiator for many years to come. OTL was almost simultaneously introduced in the Philips radios and audio amplifiers from 1954 onwards.
Visit: https://www.maximus-randd.com/piets-...ision-pt2.html
And page one: https://www.maximus-randd.com/piets-...ision-pt1.html

DFWB.

5th Mar 2018, 10:46 am	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Interesting TV audio amplifier. From Piet's Home-built Television website: A description of the operation of the transformerless audio output stage: Output TransformerLess (OTL) audio power amplifier. This was an entirely new concept, where the bulky output transfer could be deleted. Although the first publication that triggered this development was not from Philips - by Petersen and Sinclair in 1952, although their publications was still using transformers - it was picked up and further optimized by the company. The basic idea is to make a push-pull output stage but without the availability of opposite polarity devices (like today an NPN and PNP transistor). There are two ways to do this. The first is to use two power pentode output valves (here PL84) which are connected in series, and drive the two gates in anti-phase. The point between the anode of the lower valve and the cathode of the upper valve is then much lower ohmic than a classical anode output. This "full-OTL" would come later in the early sixties, but the first implementation was a "light-OTL" where the upper valve was not driven and effectively acted as a constant current source load of the lower valve. The common node between the two output valves was AC connected to g2 of the upper valve, which drove the series connected loudspeakers. The output impedance was much lower than in a classical amplifier, but still 800Ohm. The breakthrough was that Philips developed 400Ohm speakers, of which two were serially connected. One was a large bass speaker mounted on the sidewall of the cabinet, one was a smaller treble speaker mounted on the front panel below the picture tube. As can be seen from the circuit diagram, the voltage on the cathode of the upper valve is 107V, and this requires a valve that can withstand such a high cathode-to-filament voltage. Later Philips would introduce the EL86/PL86 specially designed for this parameter. This concept provided excellent sound volume and OTL would remain a Philips high performance differentiator for many years to come. OTL was almost simultaneously introduced in the Philips radios and audio amplifiers from 1954 onwards. Visit: https://www.maximus-randd.com/piets-...ision-pt2.html And page one: https://www.maximus-randd.com/piets-...ision-pt1.html DFWB.