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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Jun 2018, 11:39 am   #41
John10b
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Default Re: Why 2.5 ohms impedance?

Excellent stuff.
Let’s imagine this situation, you have sitting around the table the Design Engineer, the Production Manager and the Accountant.
Who would get their own way? I suppose as in life it has to be a compromise.
Cheers
John
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Old 28th Jun 2018, 12:21 pm   #42
ms660
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Default Re: Why 2.5 ohms impedance?

Quote:
Originally Posted by G8HQP Dave View Post
The optimum turns ratio depends on the speaker impedance and the optimum load impedance required by the valve (not the anode resistance),
Maybe you misunderstood what I was trying to say, the optimum load impedance for the valve is ra (anode resistance) dependant..."the power output versus distortion from the valve" To get an optimum turns ratio means matching the loudspeaker to the valve, the loudspeaker impedance is known and the valves ra is known but it's not as simple as transforming loudspeaker impedance to the valves ra, for triodes the impedance will be higher and for pentodes the impedance will be lower.

On top of that there's the reactive elements of the load and its effects to consider, it gets very complex, I think 3 ohm was chosen as an all round fit all on an electronic/sound/cost basis for the radios of the day.

Lawrence.
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Old 28th Jun 2018, 12:23 pm   #43
vidjoman
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Default Re: Why 2.5 ohms impedance?

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Originally Posted by merlinmaxwell View Post
I would have thought that two layers would be optimum, you have to get the wire back to the start after all.
Many speakers do not have a paper former that covers the complete circle and there is a gap which allows the return wire to sit in the gap and return to the top of the coil. There are/were some speakers made that had a double layer coil structure and often wired in parallel. This way they could make a longer coil = longer throw of the cone, and halve the resistance and also make the coil take more power.
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Old 28th Jun 2018, 3:02 pm   #44
David Simpson
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Default Re: Why 2.5 ohms impedance?

Back in the early(1920's) days of radio, a TRF might have a P220 Triode as the audio o/p valve. Headphones were used back then - straight in the Anode HT. No Transformer ! So a valve which requires an RL of approx. 4Kohm, gets a pair of phones at approx. 2000 ohms each(= 4Kohm). The valve is happy with that, tickerty boo.
Later TRF's & certainly Superhets, had more powerful triodes & pentodes which required a range of audio transformers in the anode HT circuit - to match their required theoretical RL's to a range of manufacturer's speakers. This is where impedance (Z) matching comes in. Plus the fact that these later valves required higher HT's, and no one wants a couple of hundred DC volts across headphones or a delicate wee speaker coil.
If folk are keen on the audio side of vintage radios - consider buying a cheapo LCR Meter, and something like - a CT44, a Marconi TF893A, a Heath/Sch. Wattmeters. All 3 of which cater for a range of Impedances of between 3 & 600 ohms.

Regards, David

PS. Valve theory-wise :- ra is different from Ra(or RL as its sometimes known). ra is for Static measurements. Ra is for Dynamic measurements, roughly speaking.

Last edited by David Simpson; 28th Jun 2018 at 3:08 pm. Reason: Add'l info
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