100V audio transformer - UK Vintage Radio Repair and Restoration Discussion Forum

ValvoStef · 24th Jan 2016, 11:59 am

Morning all,

At work, our site electrician gave me two old Tannoy speakers. The speakers have an impedance of 16 Ohms and the transformer connected to it is a 100V transformer. Connecting 50 V to the primary I measure 0.4 V on the secondary winding. This woulr result in a ratio of 125. Assuming the output impedance of the transformer matches the speaker is 16 Ohm, therefore the primary impedance would be 125 * 125 / 16 Ohms = 970 Ohms? Repeating this test with 100V results in 108 ratio and 729 Ohm primary impedance. I would expect a linear increase. What have I overlooked?

BR

Stef.

Radio Wrangler · 24th Jan 2016, 12:09 pm

100v line systems use transformers of different ratios at different speakers in order to choose the amount of power which that speaker gets. Otherwise, if the ratio of that one speaker is matched to the amplifier driving that line, then that speaker will take the entire amplifier output.

David

Herald1360 · 24th Jan 2016, 12:20 pm

If that's 50V @ 50Hz, it's probably too low in frequency for the transformer design, so the ratio will appear to vary with level- the core is probably saturating at the higher voltage input.

If the "Tannoy" speakers are just for PA work, they probably won't do much below about 200Hz anyway.

ValvoStef · 24th Jan 2016, 12:31 pm

Yes I understand this, David. But I used the same tappings with different voltages, I wouls expect the impedance change in the same ratio as the voltages. Odd, really, this doesn't make sense to me.

Hartley118 · 24th Jan 2016, 12:37 pm

As Herald1360 says, most of these PA speaker transformers aren't designed for full level at 50Hz, if you have an audio oscillator, it's worth repeating the test again at a higher frequency - say 400Hz to avoid core saturation. Assuming you don't have a spare 100V PA amplifier around to produce a big enough test signal, you'd be best feeding into the secondary and measuring what comes out of the primary. That ought to give a more sensible result for the ratio calculation.

If your speakers are actually marked with the Tannoy logo, they might well be collectable!

Martin

ValvoStef · 24th Jan 2016, 12:44 pm

Yeah, OK, I did not put the frequency into consideration. I have a signal generator which has a 0 to 10 v output. With This connected to the secondary winding I'll repeat the test.

Radio Wrangler · 24th Jan 2016, 12:48 pm

Impedance ratio is the square of voltage ratio, so they will change differently.

David

ValvoStef · 24th Jan 2016, 12:51 pm

Damn, yes, of course. I did not put the square into consideration.

Herald1360 · 24th Jan 2016, 12:56 pm

But you still had a non linear apparent winding ratio- 108 vs 125- that should have been constant.

ValvoStef · 24th Jan 2016, 1:09 pm

Yes, I think this may be due to the fact that at 50V on the primary winding the secondary winding gave 0.4 V, this is where instrument accuracy may have an effect as well. I measured. This with a Fluke 114, some years old and never been recalibrated since purchase day.

24th Jan 2016, 11:59 am	#1
ValvoStef Hexode Join Date: Mar 2014 Location: Fareham, Hampshire, UK. Posts: 482	100V audio transformer Morning all, At work, our site electrician gave me two old Tannoy speakers. The speakers have an impedance of 16 Ohms and the transformer connected to it is a 100V transformer. Connecting 50 V to the primary I measure 0.4 V on the secondary winding. This woulr result in a ratio of 125. Assuming the output impedance of the transformer matches the speaker is 16 Ohm, therefore the primary impedance would be 125 * 125 / 16 Ohms = 970 Ohms? Repeating this test with 100V results in 108 ratio and 729 Ohm primary impedance. I would expect a linear increase. What have I overlooked? BR Stef. __________________ Failing to prepare is preparing to fail ...

24th Jan 2016, 12:09 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,869	Re: 100V audio transformer 100v line systems use transformers of different ratios at different speakers in order to choose the amount of power which that speaker gets. Otherwise, if the ratio of that one speaker is matched to the amplifier driving that line, then that speaker will take the entire amplifier output. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Jan 2016, 12:20 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 100V audio transformer If that's 50V @ 50Hz, it's probably too low in frequency for the transformer design, so the ratio will appear to vary with level- the core is probably saturating at the higher voltage input. If the "Tannoy" speakers are just for PA work, they probably won't do much below about 200Hz anyway. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

24th Jan 2016, 12:31 pm	#4
ValvoStef Hexode Join Date: Mar 2014 Location: Fareham, Hampshire, UK. Posts: 482	Re: 100V audio transformer Yes I understand this, David. But I used the same tappings with different voltages, I wouls expect the impedance change in the same ratio as the voltages. Odd, really, this doesn't make sense to me. __________________ Failing to prepare is preparing to fail ...

24th Jan 2016, 12:37 pm	#5
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: 100V audio transformer As Herald1360 says, most of these PA speaker transformers aren't designed for full level at 50Hz, if you have an audio oscillator, it's worth repeating the test again at a higher frequency - say 400Hz to avoid core saturation. Assuming you don't have a spare 100V PA amplifier around to produce a big enough test signal, you'd be best feeding into the secondary and measuring what comes out of the primary. That ought to give a more sensible result for the ratio calculation. If your speakers are actually marked with the Tannoy logo, they might well be collectable! Martin __________________ BVWS Member

24th Jan 2016, 12:44 pm	#6
ValvoStef Hexode Join Date: Mar 2014 Location: Fareham, Hampshire, UK. Posts: 482	Re: 100V audio transformer Yeah, OK, I did not put the frequency into consideration. I have a signal generator which has a 0 to 10 v output. With This connected to the secondary winding I'll repeat the test. __________________ Failing to prepare is preparing to fail ...

24th Jan 2016, 12:48 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,869	Re: 100V audio transformer Impedance ratio is the square of voltage ratio, so they will change differently. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Jan 2016, 12:51 pm	#8
ValvoStef Hexode Join Date: Mar 2014 Location: Fareham, Hampshire, UK. Posts: 482	Re: 100V audio transformer Damn, yes, of course. I did not put the square into consideration. __________________ Failing to prepare is preparing to fail ...

24th Jan 2016, 12:56 pm	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 100V audio transformer But you still had a non linear apparent winding ratio- 108 vs 125- that should have been constant. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

24th Jan 2016, 1:09 pm	#10
ValvoStef Hexode Join Date: Mar 2014 Location: Fareham, Hampshire, UK. Posts: 482	Re: 100V audio transformer Yes, I think this may be due to the fact that at 50V on the primary winding the secondary winding gave 0.4 V, this is where instrument accuracy may have an effect as well. I measured. This with a Fluke 114, some years old and never been recalibrated since purchase day. __________________ Failing to prepare is preparing to fail ...