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 High fidelity switch.

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dr pepper

Posts : 3
Join date : 2010-10-14

PostSubject: High fidelity switch.   Thu Oct 14, 2010 11:06 pm

I have a hmv 1121, a uk model.
It has a function similar to what I beleive some of the airline sets have a high fidelity position on the tone control.
Looking at the circuit this switches a couple of caps around the first if trans, looks like it might reduce the selectivity of the set.
Is this true, presumably the set must be so selective that frequency response is limited.
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jack shirley

Posts : 67
Join date : 2010-10-12
Location : SE USA

PostSubject: Re: High fidelity switch.   Fri Oct 15, 2010 4:59 am

Yep. In fact some sets used a continuously variable bandwidth to achieve selectivity vs fidelity. One such scheme used mechanical cables to adjust the spacing between the windings of the IF transformers.
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Tom Albrecht

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Join date : 2010-10-15

PostSubject: Re: High fidelity switch.   Fri Oct 15, 2010 10:31 am

In fact, "normal" selectivity with two 455 kHz IF transformers restricts audio bandwidth a lot. Selectivity is good, but the sound is far from "high fidelity." Having a means to open up the IF bandwidth is absolutely necessary to getting good high frequency audio response.

I have an Airline Movie Dial with a hi-fi setting which works beautifully to improve the high frequency audio response. Selectivity is much wider in the hi-fi setting, as it should be.

This feature was only available on a few high-end radios. Almost all radios have standard high selectivity, and the familiar "AM sound" with a strong roll-off of high audio frequencies, regardless of the setting of the treble control.

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PostSubject: Re: High fidelity switch.   Fri Oct 15, 2010 11:31 am

When CRO's became available to the radio service trade, 'wobulators' or primative sweep generators were used to set the flatness of the IF response In sets with just one IF stage
tuning the convertor transformer slightly higher and the diode tranformer slightly lower could improve the fidelity but more likely just trimmed response to make it sound wider by reducing the overall gain making the listener turn up the volume control. These commonly were tapped
to accomodate the loudness effect.

In the expereimental days of stereo broadcasting around 1960, AM stations sometimes paired with FM stations for special programs. The link was far from perfect since most stations used dedicated "order wires", telephone circuits, to interconect the master control boards of the studios. This brought to the public's attention the contrasting sounds of AM and FM. Shortly afterwards the multiplex was chosen and convienience was swapped for a host of multipath
buzzes and swizzling noises as station exploited the SCA options to increase revenue.

How the radios worked was linked to what tubes were being made and better tubes needed better coil manufacturing techniques. After WW2 American radio manufacturers switched over to television but Germany produced a large variety of radio/radiogram sets that did provide very wide control over the amplifier equalization and bandpass of of the AM IF.

In the US the high end radios shifted over to component tuners and amplifiers.
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dr pepper

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Join date : 2010-10-14

PostSubject: Re: High fidelity switch.   Sun Oct 17, 2010 2:26 am

Well thats interesting.
Here in the Uk this is the first set I've come accross of its age that has a hi fi switch, I'd read on this site that airlines had them but didnt know how they worked.
This set also has an acoustic lens over the speaker which also happens to be the tuning scale, each lens slat being a diffrent band.
There was 2 versions of this set, one late 40's and the other is mid 50's, my drawings are for the late 40's version, I wonder if the later one has this feature.
The BBC didnt start broadcasting in stereo untill 1974, and then they used PCM encoding.
Here it is, its a little ugly:

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