Can't get this speaker to work properly

[boroman]

I have here vintage 8" speaker, that has... 4 wires. I measured impedance from pins that are going to coil and its 3,2 so speaker is 4 Ohm. Cone moves freely like any other speaker. It does do well with 9V battery test. But when I connect it to (any) amp it has very very low volume, like 1/10 of what it should be, or even less. I tried to bridge that green wires to the speaker connectors but nothing helped. Tried to push audio throught green ones - no sound. Is it sort of a choke?

What do I need to make it run well?

[Enzo]

You need power through its field coil. Look up "field coil speakers". This speaker has no magnet, it has a coil instead. DC running through that coil makes a magnetic field. The magnetic field is what the voice coil pushes against to make the cone move.

[TomCarlos]

You have a Sachsenwerk field coil speaker. It is intended to be used with a tube amplifier. You tried many amps? And what type of amps were those?

[J M Fahey]

Measure DC resistance of the field coil in the back.

It states 75 mA , knowing resistance we´ll know supply voltage needed.

Maybe you can feed it even in an SS amp, if coil voltage is same or less as rail voltage.

We await for your input.

[Helmholtz]

An electrodynamic speaker requires a magnet to produce a strong (static) magnetic field in the airgap around the voice coil.
Modern speakers carry a permanent magnet (ferrite, alnico or rare earth magnet).
A field coil speaker like yours uses an electromagnet, where the field is produced by a DC current in a coil.

From the coil data (75mA/6W) the DCR calculates as 1067R.
Means that it requires a voltage of 80VDC across the field coil terminals for the nominal current of 75mA.

Lower current than 75mA reduces speaker efficiency.
Higher current or voltage is likely to overheat the coil wire.

Edit: Measuring actual DCR as suggested by JMF might be more reliable.

[boroman]

Ahhh.... guys! Always good to be back here. THANK YOU
Never heard of field coil, great to learn something new

So, judging by those parametres I need to
- confirm 1Kohm DCR
- find a transformer which would provide set voltage 80V and SS rectifier to DC-it...

It will use 75mA of my tranfsormer power (so, I'd need to find more powerful one)?

[mozz]

Don't try to wire it up and make it work. Save it for the correct type of amplifier, one that uses a field coil speaker. Most likely a German tube radio speaker.

[Chuck H]

This^^^^^^^^^^^

But if you ARE going to do this...

It's likely there was a filter stage or stages before this choke in any design. I'm not sure what difference it would make to have really lumpy DC on the coil for speaker performance, but I can't imagine it's good. That said, I don't know where you live (Boro?) but if you have 120V wall power you could use any isolation transformer capable of 150mA. Just filter off a bridge rectifier with a 100u cap, then a 1k/15W resistor series with the voltage, then another 100u filter. That should give you about 77V with less than .05V of ripple. And you didn't have to find a transformer with a weird secondary.

[mozz]

Reason i said do not try to make it work is this. Since he does not even know what a field coil speaker is, looks like, or works, i do not think people should be trying to describe volts, ohms and amps to this person until he has a bit more electronic experience. Maybe i am a hard ass. It's a old speaker, probably only handles a few watts, could even be a open field coil which happens a lot on these. He still didn't measure the coil. Anyway, they sound like shit on old guitar amps if that was what he planned on using it for. They are usually placed in series with the B+ so i don't even see someone telling him to put a 80vdc across it. If the coil is approx 1000 ohms, chances are it is fine. No need to rig up a power supply.

[nosaj]

Here was my journey on making one similar to flextone's route.
nosaj
https://music-electronics-forum.com/...-speaker/page2

[boroman]

I confirm it's 1008 Ohms between the gren wires, I'll try to get simple AC/DC converter that will do 80 volts DC at the end.

Reason i said do not try to make it work is this. Since he does not even know what a field coil speaker is, looks like, or works, i do not think people should be trying to describe volts, ohms and amps to this person until he has a bit more electronic experience. Maybe i am a hard ass. It's a old speaker, probably only handles a few watts, could even be a open field coil which happens a lot on these. He still didn't measure the coil. Anyway, they sound like shit on old guitar amps if that was what he planned on using it for. They are usually placed in series with the B+ so i don't even see someone telling him to put a 80vdc across it. If the coil is approx 1000 ohms, chances are it is fine. No need to rig up a power supply.

Few things, brother - I really hate comments like these. Everyone learns something every day. I started to reapir my own collection of vintage Ampeg amps because no one can do it well (even 300W tube SVTs). All the so-called professionals made "imporvements" to original circuit and did the job messy and that was a thoughj, well it's not NASA stuff, I can do it by myself. With some help here and there, I learned A LOT. I even still can't build an amp using schematic only, I can read it to some degree, but not at all. I'm still learning. Like everybody. So if I dont know "what is field coil speaker", I should not touch anything electrical? I built few tube amps that I made from dusty, non working 70+-year old tube radios, trying to use as much I can - usually original speakers, output transformers, tubes, etc. I don't sell them. It's fun to sit in the cold winter evening, learn something new, recycle od non working radio and gif it to my friends.

Thanks to Helmholtz, Enzo and many, many others who helped me with the stuff. It's so inspiring.

Back to the topis. I really would like the original speaker to work with the rebuilt radio/guitar amp. even if the sound would be mediocre. I don't care. It's just fun

[nosaj]

Take a read over my thread I posted/ The crew here helped me make a variable voltage speaker from a field coil. So you could dime the amp but lower what came out the speaker.
nosaj

[boroman]

Take a read over my thread I posted/ The crew here helped me make a variable voltage speaker from a field coil. So you could dime the amp but lower what came out the speaker.
nosaj

Will do over next days, thank you!

I thought a simple cheap 80V DV transformer will do the job, as I have almost completed wiring the amp, along with transformer connections, and the last step was to hook up the speaker...

[nosaj]

So your transformer needs to output a 175vAC-265Vac to work.

On mine I found a Solidstate transformer a couple of caps , diodes and a 50watt rheostat. I put everything into a cabinet by itself. So you could hook it up to different amps that needed a 8ohm speaker. So it would work on solidstate and tube amps. I played with it for a bit then sold it cause thats what I do since I'm not a player but a tinkerer.
nosaj

[Greg Robinson]

Will do over next days, thank you!

I thought a simple cheap 80V DV transformer will do the job, as I have almost completed wiring the amp, along with transformer connections, and the last step was to hook up the speaker...

That power supply will not work correctly for this field-coil speaker.
It's a constant-current supply, it will try to put 280mA through whatever you connect it to, and has a compliance voltage rating of between 63 and 80V, if it's not able to deliver the rated current within that voltage range, it's an error condition and will shut down. Since your field coild speaker only draws 75mA, not 280mA, that power supply will try to put out 80V (probably a little more), see that only 75mA is being drawn, then shut down before restarting. It'll "hiccup" like this until turned off.

What you need is a constant VOLTAGE power supply.
Simplest rough-and-ready option is a 55VAC or 110VCT 20VA transformer with bridge or full wave rectifier (respectively) and a 220uF 100V cap will get you ~75VDC with less than 5% ripple, and be good enough for testing. Add further filtering (and higher voltage transformer to allow for voltage drop of smoothing elements) if desired.

EDIT: That type of supply is usually not galvanically isolated either, so the output needs to be treated as mains, as it presents a significant shock/electrocution hazard.