Hello,
Does anybody have experience driving very low resistance speakers? AP333 BL field coil driver has fallen into my hands, I expect that I will need max 1.5 W max to drive it. Is there standard OPT’s available or this can only be custom wound? Driver tube I have not decided yet, but it would be interesting to use something like D3A in a SPUD configuration..
Does anybody have experience driving very low resistance speakers? AP333 BL field coil driver has fallen into my hands, I expect that I will need max 1.5 W max to drive it. Is there standard OPT’s available or this can only be custom wound? Driver tube I have not decided yet, but it would be interesting to use something like D3A in a SPUD configuration..
Depending on power needed
Most manufactures dont even want to wind 2 ohm taps.
Because the current needed for a 1.5 ohm tap requires heavy wire.
Since it is low power you just need a transformer with the right
ratio to get the impedance the tube wants to see.
A " 4 ohm" output tap can be used as a 2 ohm
of course the reflected impedance on the primary will be different.
Keeping in mind the issue is current since 2 ohms will pull
twice the current. Hence why depending on power
usually a 2 ohm tap would use heavier gauge wire.
your case since only 1 to maybe 3 watts is needed
likely a larger 20 watt or higher 4 ohm tap could be used as 2 ohm.
Again you just need the right ratio to get the impedance the tube wants to see.
assume the bandwidth of a 1920s/30s speaker is rather limited.
Even Hammond transformer is more than enough.
Maybe mis understanding the field coil might be 1,5 ohms
I would expect the voice coil to be more like 16 to 32 ohms
Or could even be a small bucking coil for noise reduction with such low
impedance if the field coil was made for a radio which might have used
AC for the field coil. Of course DC operation is possible.
Many radio field coils just used AC and had a bucking coil to cancel
the noise ripple
Most manufactures dont even want to wind 2 ohm taps.
Because the current needed for a 1.5 ohm tap requires heavy wire.
Since it is low power you just need a transformer with the right
ratio to get the impedance the tube wants to see.
A " 4 ohm" output tap can be used as a 2 ohm
of course the reflected impedance on the primary will be different.
Keeping in mind the issue is current since 2 ohms will pull
twice the current. Hence why depending on power
usually a 2 ohm tap would use heavier gauge wire.
your case since only 1 to maybe 3 watts is needed
likely a larger 20 watt or higher 4 ohm tap could be used as 2 ohm.
Again you just need the right ratio to get the impedance the tube wants to see.
assume the bandwidth of a 1920s/30s speaker is rather limited.
Even Hammond transformer is more than enough.
Maybe mis understanding the field coil might be 1,5 ohms
I would expect the voice coil to be more like 16 to 32 ohms
Or could even be a small bucking coil for noise reduction with such low
impedance if the field coil was made for a radio which might have used
AC for the field coil. Of course DC operation is possible.
Many radio field coils just used AC and had a bucking coil to cancel
the noise ripple
Look to Sowter site there are some OT with multiple 1 ohm winding
https://www.sowter.co.uk/single-ended-output-transformers.php
There are plenty of 3 ohm transformers on e-bay, from reel-to-reel tape recorders, you may get lucky with them, they are cheap enough at least for an experiment. Probably not hi-fi though.
I got a couple of these, but have not tried them yet in a real circuit: https://www.ebay.co.uk/itm/256529179517
I got a couple of these, but have not tried them yet in a real circuit: https://www.ebay.co.uk/itm/256529179517
If a transformer is used in an "irregular" manner, you can get very low impedances. For example, a 4 ohm + 8 ohm secondary will have a much lower impedance between the 4 and 8 ohm taps.
The number of turns for 8 ohm is √2*4ohm. The difference between the two is thus √2-1=0.414 times the number of turns of the 4 ohm winding.
Since the impedance is proportional to the square of the number of turns, the resulting intertap impedance will be 0.414²*4=0.686ohm
The number of turns for 8 ohm is √2*4ohm. The difference between the two is thus √2-1=0.414 times the number of turns of the 4 ohm winding.
Since the impedance is proportional to the square of the number of turns, the resulting intertap impedance will be 0.414²*4=0.686ohm
UTC vintage output transformers in the HA and LS series can be configured for secondary impedance as low as 1.4 Ohms.
White Dragon,
PauliusT,
For 1 Watt rms . . .
A 2 Ohm driver requires 1.414 times the current, and 0.707 times the voltage; Versus the current and voltage of a 4 Ohm driver.
P = I x V
(The 2 Ohm driver does Not require 2 x the current of a 4 Ohm driver).
I have one Electra Print Audio TM3KB 3k single ended Output transformer (or two if I can find the second one).
The 3k primary is rated for 60mA. 5.9 Lbs of copper, laminations, and end bells. There are 4 each 1 Ohm secondaries.
With the correct secondary connections, you get 1 Ohm, 4 Ohm, 9 Ohm, and 16 Ohms.
So . . . 3K to 1 Ohm.
For me to ship to Lithuania is probably too expensive.
PauliusT,
For 1 Watt rms . . .
A 2 Ohm driver requires 1.414 times the current, and 0.707 times the voltage; Versus the current and voltage of a 4 Ohm driver.
P = I x V
(The 2 Ohm driver does Not require 2 x the current of a 4 Ohm driver).
I have one Electra Print Audio TM3KB 3k single ended Output transformer (or two if I can find the second one).
The 3k primary is rated for 60mA. 5.9 Lbs of copper, laminations, and end bells. There are 4 each 1 Ohm secondaries.
With the correct secondary connections, you get 1 Ohm, 4 Ohm, 9 Ohm, and 16 Ohms.
So . . . 3K to 1 Ohm.
For me to ship to Lithuania is probably too expensive.
Last edited:
It would be difficult to get an acceptably low leakage inductance with that turns ratio.
I looked at the driver.
Now that is a Real Spider!
You can see where the name 'Spider' comes from.
Loudspeakers today have voice coil position restoring springs, but they do Not look like a Spider.
Names die hard, even if they do not strictly apply anymore.
Now that is a Real Spider!
You can see where the name 'Spider' comes from.
Loudspeakers today have voice coil position restoring springs, but they do Not look like a Spider.
Names die hard, even if they do not strictly apply anymore.
efellings,
When wired for 1 Ohm, the Electra-Print Audio TM3KB high frequency roll off, caused by the leakage inductance and the distributed primary capacitance . . . Is probably much higher than the high frequency roll off of the driver in Post # 1.
The paper cone looks pretty light, I could be wrong about the high frequency roll off.
It is the total System Response that needs to be considered.
When wired for 1 Ohm, the Electra-Print Audio TM3KB high frequency roll off, caused by the leakage inductance and the distributed primary capacitance . . . Is probably much higher than the high frequency roll off of the driver in Post # 1.
The paper cone looks pretty light, I could be wrong about the high frequency roll off.
It is the total System Response that needs to be considered.
Was similar thought as mentioned.
Few issues with low impedance taps.
But since power is low and bandwidth is limited with the driver
not as big of deal.
Even though a 1.5 ohm coil still makes no sense for the era.
Few issues with low impedance taps.
But since power is low and bandwidth is limited with the driver
not as big of deal.
Even though a 1.5 ohm coil still makes no sense for the era.
Hammond 125xSE series have extra secondary taps. The lowest tap is 4 ohms:10K, 2 ohms:5K, or 1 ohm:2500. Hammond does not specify performance for less than 4 ohms, but the frequency response indicates -1dB at 40kHz or above at 4 ohms, so at 2 ohms worst-case would be -1sdB at 20kHz and -3dB at 40lHz. (I am assuming leakage inductance dominates capacitance, as is likely for a an inexpensive winding.)
At higher powers, I believe some guitar amps have an available 2 ohm tap, to drive four 8-ohm speaker drivers in parallel.
Two small 10K:4 ohm transformers can be paralleled to equal a larger 5K:2 ohms transformer with the same frequency response.
At higher powers, I believe some guitar amps have an available 2 ohm tap, to drive four 8-ohm speaker drivers in parallel.
Two small 10K:4 ohm transformers can be paralleled to equal a larger 5K:2 ohms transformer with the same frequency response.
I don't understand how a loudspeaker with only alternating current through the field coil could possibly work. Doesn't it just generate sidebands at the AC frequency above and below the signal frequency?
Probably confusing as written.
High Ripple DC
The whole point is their is often a bucking coil
And with some old speakers the voice coil , transformer, bucking coil
actually can get grounded using the case of the speaker.
So can get odd measurements or even shorts.
Was just confused why the impedance is so low at 1.5 ohms.
And just measurements can be misleading or wrong because
there is quite a few tap points many connected to case as mentioned
and could be bad coils or isolation issue to speaker case.
Or just the bucking coil was being measured not voice coil.
Maybe wrong assumption, just encouraging to be sure it is actually 1.5 ohms
before you figure out or argue a basic Hammond will run it
quick solution to actually hear the thing before you go wild
on spending money for a not needed expensive custom transformer.
Last edited:
Go look for an old radio schematic that has a field coil speaker.
I bet it uses the field coil as a choke in a CLC B+ filter, and that there is a hum bucking coil to make up for the moderate ripple in the field coil.
My old 1938 Philco Cathedral radio with a field coil loudspeaker sounded good, decent bass, and no noticeable hum.
I bet it uses the field coil as a choke in a CLC B+ filter, and that there is a hum bucking coil to make up for the moderate ripple in the field coil.
My old 1938 Philco Cathedral radio with a field coil loudspeaker sounded good, decent bass, and no noticeable hum.
Can you post a link? I could not find a picture on the net and it sound interesting. Thanks
RCA model 242 radio from 1929 also used the speaker field coil as the choke in the power supply after the rectifier tube. Not much hum from it either. Clever use of the coil for both field magnet and PSU smoothing. I would guess that the coil would have a low impedance for power supply use.
Bluesystems,
Donaldspace,
Post # 18
AP333 BL field coil driver:
https://www.ebay.com/itm/266541871087
Post # 19
A quality loudspeaker field coil would have a High Impedance @ AC to act as an effective choke;
And a low DCR to have less B+ voltage loss . . .
Just like any quality B+ choke.
Donaldspace,
Post # 18
AP333 BL field coil driver:
https://www.ebay.com/itm/266541871087
Post # 19
A quality loudspeaker field coil would have a High Impedance @ AC to act as an effective choke;
And a low DCR to have less B+ voltage loss . . .
Just like any quality B+ choke.