Mystery switch


I have an old Audio Reflex receiver (Model AR-800) which has a 'DE. EMP' switch on the back: 75uS to the left and 25 uS to the right.  The 'u' actually has a preceding tail on it, so its probably a symbol. 
What does this do?
Also, it says on the back 'Do not use speakers less than 8 ohms'.  Is this as imperative as it sounds?

Thanks, 
128x128sound22card
The power supply rails ,transformers are small that is why it cannot handle lower ohm loads .never heard of this receiver. How old is it 
I would not invest in it ,no where to modify it properly ,which you 
would need ample space on any decent aftermarket parts which are all much bigger.
My mid 70’s Pioneer receiver also had switchable Dolby demphasis but that was a dead end 

my marantz 4 chanell reciever from the 70's had dolby built in. never used it because i don't remember any fm stations transmitting in dolby. 
I thought it'd be like the wall switch that Steven Wright mentioned in one of his skits. He'd flip it on and off and nothing would happen. It did for over a week until someone called from out of state and asked him to stop it.
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That switch was used for Dolby FM broadcast many moons ago!
Dolby FM

In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR,[2] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.[3] Dolby FM was based on Dolby B,[4] but used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise.

A similar system named High Com FM was tested in Germany between July 1979 and December 1981 by IRT.[5] It was based on the Telefunken High Com broadband compander system, but never introduced commercially in FM broadcasting.[6]

Yes it is important that you use 8 ohm speakers.

That switch was used for Dolby FM broadcast many moons ago!
Dolby FM

In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR,[2] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.[3] Dolby FM was based on Dolby B,[4] but used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise.

A similar system named High Com FM was tested in Germany between July 1979 and December 1981 by IRT.[5] It was based on the Telefunken High Com broadband compander system, but never introduced commercially in FM broadcasting.[6]

Yes it is important that you use 8 ohm speakers.

That switch was used for Dolby FM broadcast many moons ago!
Dolby FM

In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR,[2] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.[3] Dolby FM was based on Dolby B,[4] but used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise.

A similar system named High Com FM was tested in Germany between July 1979 and December 1981 by IRT.[5] It was based on the Telefunken High Com broadband compander system, but never introduced commercially in FM broadcasting.[6]

Yes it is important that you use 8 ohm speakers.

That switch was used for Dolby FM broadcast many moons ago!
Dolby FM

In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR,[2] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.[3] Dolby FM was based on Dolby B,[4] but used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise.

A similar system named High Com FM was tested in Germany between July 1979 and December 1981 by IRT.[5] It was based on the Telefunken High Com broadband compander system, but never introduced commercially in FM broadcasting.[6]

Yes it is important that you use 8 ohm speakers. You will damage the amp if you don't!


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