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Nederlands Forum over Oude Radio's |
Re: Plano en bi-ampli
Geschreven door Rob Krabbendam op 15. Dec 2004 12:25:12:
In reactie op: Plano en bi-ampli geschreven door Henk Waardenburg op 13. Dec 2004 21:38:00:
Ik heb ooit een korte tekst hierover geschreven op onderstaande pagina. Misschien kan dit een beginnetje zijn van een mooi RHT-verhaal.
En dit stond jaren geleden ook op mijn toenmalige site:Real Bi-ampli
It the fifties Philips introduced Bi-ampli radio's. These radio's have two separate amplifiers. One has high tones filtered out and is connected to a big speaker, the other one has low tones filtered out and feeds a small speaker. This way one gets a better sound reproduction, because high and low frequencies have less chance to interfere with each other and (in theory) the total output power is doubled. An example of a 'real' Bi-ampli is my B6X63A. It has a double triode ECC83 as a pre-amplifier and two (penthode) EL84's as output stages. The two 4 Ohm speakers are connected via two separate output transformers.
Fake Bi-ampli I: two pentodes forming one output stage
While real Bi-ampli was only used in more expensive sets, you can also find a Bi-ampli logo on middle class radio's from the second half of the fifties. Here it is more like a marketing trick though. Examples of such radios are my B5X82A and B5X84A. These sets do have an extra small speaker to reproduce high tones, but it is simply connected to the same amplifier as the bigger speaker via a capacitor acting as a high pass filter. Pre-amplification is done here by the triode part of an EABC80. The single output stage is of the transformerless EL84/EL86 series balanced type. So you won't be surprised to find high impedance speakers in these sets.Fake Bi-ampli II: abuse a stereo amplifier
In the late fifties there came another reason two build two separate ampifiers in a radio: spatial sound reproduction by using as much sound channels as we have ears, simply called 'stereo'. By the way, in those days you needed a stereo record player to get a stereo signal in the first place. Philips had just introduced the 'plano': a radio equipped with two speakers, one on the left side and on the right side. Soon the plano got as much amplifiers as it had speakers, so stereo playback became possible.While listening to the radio, the left channel always had the high tones filtered out so you got a Bi-ampli-like effect like there was with the B5X84A. Examples of such radio's are my B5X94A and one of my B6X04A's (both with an ECC83 and two EL84's). When I was restoring this B6X04A, I simply cut out the filtering capacitor because I thought it was a wiring fault!
Fake Bi-ampli III: just two amplifiers and a logo on the dial
Fortunately, by 1961 Philips discovered that the plano 'Bi-ampli' filter could not really be called an improvement in sound quality. So they simply kept the Bi-ampli logo on the dial glass but did no more funny stuff with the tone filters. Examples: some of my B6X04A's, my B5X04A, B6X12A, B5X23A and B5X43A. By the way, the last mentioned has an FM stereo decoder. By 1965 the Bi-ampli logo finally disappeared (B5X42A, B5X44A).