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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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28th Jun 2018, 11:39 am | #41 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,884
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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 |
28th Jun 2018, 12:21 pm | #42 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Why 2.5 ohms impedance?
Quote:
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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28th Jun 2018, 12:23 pm | #43 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
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Re: Why 2.5 ohms impedance?
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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28th Jun 2018, 3:02 pm | #44 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,853
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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 |