Impedance converter for prewar loudspeakers - UK Vintage Radio Repair and Restoration Discussion Forum

Vakito227 · 20th Jun 2017, 10:42 pm

Hi all. I have recently bought a Marconi loudspeaker from the late 20s / early 30s and I understand these old speakers have a very high impedance of around 1500 ohms compared to the single digits of today. I remember seeing about an impedance converter that converted an 8 ohm signal into a 2000 ohm signal but I can't seem to find one anywhere.

Does anybody know where to find one of if anybody has any circuit diagrams so I could make my own?

Thanks
Peter

OscarFoxtrot · 20th Jun 2017, 11:08 pm

What are you driving it with (what sort of amplifier)?

For low power, audio signal transformers with 8 ohm secondaries are here (you'd run it backwards).

http://www.mouser.co.uk/Passive-Comp...mer+2K&FS=True

kalee20 · 20th Jun 2017, 11:39 pm

Horn loudspeakers are pretty efficient, so (depending what sort of amplifier you are using) you may get fairly decent results just connecting as-is!

Tractorfan · 21st Jun 2017, 9:02 am

Hi,
I'd be inclined to just use the output transformer from a scrap valve radio and connect it 'in reverse'.
In the far off days of my misspent youth I ran a modern low impedance speaker from a 1920s wireless using the frame transformer from an Ekco TV.

Cheers, Pete.

Vakito227 · 21st Jun 2017, 12:37 pm

Quote:

Originally Posted by kalee20

Horn loudspeakers are pretty efficient, so (depending what sort of amplifier you are using) you may get fairly decent results just connecting as-is!

Sorry should have stated it is a moving coil speaker very robust built to last!

Quote:

Originally Posted by Tractorfan

I'd be inclined to just use the output transformer from a scrap valve radio and connect it 'in reverse'.
In the far off days of my misspent youth I ran a modern low impedance speaker from a 1920s wireless using the frame transformer from an Ekco TV.

The speaker has an output transformer built in and has instructions to connect the input sockets to different terminals on the transformer depending on if the input is above or below 1500 ohms

21st Jun 2017, 2:44 pm

Bypass the transformer, i.e. simply connect the 'speaker proper to the source. It won't matter much if the original transformer is still in circuit.

Hartley118 · 21st Jun 2017, 2:47 pm

If it's a moving coil speaker unit, the voice coil itself is unlikely to be of high impedance, but is probably 3 or 4 ohms. Can you access the connections to it and connect it direct to your amplifier to save doubling up on transformers? The internal transformer will sit quite happily in parallel - just leave its high impedance primary floating.

Martin

Vakito227 · 21st Jun 2017, 5:14 pm

Yes, it's possible thanks

emeritus · 21st Jun 2017, 11:42 pm

I was once given a high impedance 1920's BTH horn loudspeaker. I think they were intended to be connected directly in series with the output valve. I found that I could get it to work by using a transformer from my odds and ends box that had 600Ω and 3Ω tappings, 600Ω to speaker, 3Ω to the loudspeaker output of my EL3302 cassette recorder. I still have the transformer, but not the speaker. If it is a high impedance speaker, I wonder if a transformer for a 100V line might work?

20th Jun 2017, 10:42 pm	#1
Vakito227 Hexode Join Date: Apr 2016 Location: Yeadon, West Yorkshire, UK. Posts: 251	Impedance converter for prewar loudspeakers Hi all. I have recently bought a Marconi loudspeaker from the late 20s / early 30s and I understand these old speakers have a very high impedance of around 1500 ohms compared to the single digits of today. I remember seeing about an impedance converter that converted an 8 ohm signal into a 2000 ohm signal but I can't seem to find one anywhere. Does anybody know where to find one of if anybody has any circuit diagrams so I could make my own? Thanks Peter

20th Jun 2017, 11:08 pm	#2
OscarFoxtrot Heptode Join Date: Apr 2011 Location: Edinburgh, UK. Posts: 805	Re: Impedance converter for prewar loudspeakers What are you driving it with (what sort of amplifier)? For low power, audio signal transformers with 8 ohm secondaries are here (you'd run it backwards). http://www.mouser.co.uk/Passive-Comp...mer+2K&FS=True

20th Jun 2017, 11:39 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Impedance converter for prewar loudspeakers Horn loudspeakers are pretty efficient, so (depending what sort of amplifier you are using) you may get fairly decent results just connecting as-is!

21st Jun 2017, 9:02 am	#4
Tractorfan Dekatron Join Date: Mar 2008 Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.) Posts: 3,183	Re: Impedance converter for prewar loudspeakers Hi, I'd be inclined to just use the output transformer from a scrap valve radio and connect it 'in reverse'. In the far off days of my misspent youth I ran a modern low impedance speaker from a 1920s wireless using the frame transformer from an Ekco TV. Cheers, Pete. __________________ "Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..."

21st Jun 2017, 2:44 pm	#6
Guest Guest Posts: n/a	Re: Impedance converter for prewar loudspeakers Bypass the transformer, i.e. simply connect the 'speaker proper to the source. It won't matter much if the original transformer is still in circuit.

21st Jun 2017, 2:47 pm	#7
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: Impedance converter for prewar loudspeakers If it's a moving coil speaker unit, the voice coil itself is unlikely to be of high impedance, but is probably 3 or 4 ohms. Can you access the connections to it and connect it direct to your amplifier to save doubling up on transformers? The internal transformer will sit quite happily in parallel - just leave its high impedance primary floating. Martin __________________ BVWS Member

21st Jun 2017, 5:14 pm	#8
Vakito227 Hexode Join Date: Apr 2016 Location: Yeadon, West Yorkshire, UK. Posts: 251	Re: Impedance converter for prewar loudspeakers Yes, it's possible thanks

21st Jun 2017, 11:42 pm	#9
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,339	Re: Impedance converter for prewar loudspeakers I was once given a high impedance 1920's BTH horn loudspeaker. I think they were intended to be connected directly in series with the output valve. I found that I could get it to work by using a transformer from my odds and ends box that had 600Ω and 3Ω tappings, 600Ω to speaker, 3Ω to the loudspeaker output of my EL3302 cassette recorder. I still have the transformer, but not the speaker. If it is a high impedance speaker, I wonder if a transformer for a 100V line might work?