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Jensen F12N Theile-Small parameters
During the last couple of years, I've noticed several requests here and on other systems for the Theile/Small parameters for the venerable Jensen F15 and F12 series. It just so happens that I have a pristine original F12N. Having the equipment to measure such parameters, it was decided to do just that. First, a little info on the unit I have as there have been design changes made over the years.
In this unit, the field coil has a dc resistance of 1003 ohms @ 78 deg. F. The inductance is 2.654 hy @ 60hz and 0.159 hy @ 1 khz. Field coil resistances have been noted to range from as low as 700 ohms to as high as 3500 ohms. If memory serves me well, I've seen higher. I have a 1923 Western Electric FC speaker with a field coil resistance of 3647 ohms.
The voice coil measures 6.3 ohms and the demodulator (humbucking) coil measures 0.2 ohm.
Since this unit was measured with filtered dc applied to the field coil, the demodulator coil serves no purpose other than the cost of about 3% in power transfer from the amplifier to the voice coil. It can be bypassed (shorted) since it is wired in series with the voice coil. This, however, is not recommended unless one has a thorough knowledge of how the speaker is wired as shorting the wrong wires can be disastrous.
There has been much discussion on the actual power supplied to the field coil. The power ranged from 14 watts to about 25 with most recommendations being around 14 to15 watts. This unit was powered with 15w, which is about 123 vdc. The ambient temperature was noted at 78F. After an hour and 14 minutes, the field coil temperature was noted to be 121F and apparently stable. It was then decided to increase the power to 17w (130vdc); eighteen minutes later, the temperature of the field coil was noted to be 126F. Considering the age of the speaker, this level was abandoned.
Tests were made with 10w applied to the field coil and Theile-Small parameters determined. The biggest difference noted was less than a 1 dB drop in the calculated SPL at 1w1m. It is, therefore, my humble opinion that the power to the field coil be kept around 10-11 watts.
Common sense prevails. Keep in mind these speakers are 50 years old or older. The adhesives holding the voice coil wire to the coil form and the cone to voice coil are considerably aged. There is also the effect of time on the paper cone; it dries and becomes very brittle.
Anyway, here's the Theile-Small parameters for an 8 ohm Jensen F12N with 15w (dc) applied to the field coil. Included are piccies of normalised amplitide response curves in various size cabinets, closed and vented.
Since there are virtually an infinite number of alignments, internal volumes were kept to about 3 ft^3 or less. Higher low frequency output can be achieved with a vented enclosure of 7 to 10 cubic feet but that size is impractical. The old electric organs typically used such volumes and a lot were essentially open back or covered with a thin panel of about 0.25 inch or thinner. The one exception of which I can think is the Leslie.
As a side note, don't be concerned about the original sound of 50 years ago; it can't be reproduced knowingly. To do that, one would need a time machine to go back to the 40's or 50's and hear a new unit. Suffice it to say that any vintage speaker, especially one with a pristine original cone will sound different than it did back then and in all probability it'll sound different that anything else today. Let's face it, it's all about nostalgia and take it from a 71 year old, that's priceless,.
These next four were determined with 15w to the field coil. (15wFC)
Driver Properties Red curve
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 0.783 cu.ft
Qtc = 0.933
QL = 19.94
F3 = 111.4 Hz
Fill = heavy
Driver Properties Orange curve
Optimum closed box alignment (Qtc=0.707)
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 1.835 cu.ft
Qtc = 0.707
QL = 17.62
F3 = 96.44 Hz
Fill = heavy
Driver Properties Green curve
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 2.065 cu.ft
Fb = 63.57 Hz
QL = 6.772
F3 = 79.94 Hz
Fill = normal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 4.558 in
Lv = 0.75 in
Driver Properties Yellow curve
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 3.23 cu.ft
Fb = 55.93 Hz
QL = 6.772
F3 = 67.58 Hz
Fill = normal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 5.398 in
Lv = 0.75 in
These next two were determined with 10w to the field coil. (10wFC)
Driver Properties Pink curve
Name: F12N 10wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.63 Hz
Qms = 22.96
Vas = 201.4 liters
Cms = 552.3 µm/N
Mms = 18.62 g
Rms = 0.558 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.501
Re = 6.6 ohms
Le = 0.533 mH
Z = 7.92 ohms
BL = 8.746 Tm
Pe = 50 watts
Qts = 0.49
no = 4.698 %
1-W SPL = 98.92 dB
2.83-V SPL = 99.75 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square (optimum)
Vb = 2.389 cu.ft
Qtc = 0.707
QL = 16.41
F3 = 87.87 Hz
Fill = heavy
Driver Properties Blue curve
Name: F12N 10wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.63 Hz
Qms = 22.96
Vas = 201.4 liters
Cms = 552.3 µm/N
Mms = 18.62 g
Rms = 0.558 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.501
Re = 6.6 ohms
Le = 0.533 mH
Z = 7.92 ohms
BL = 8.746 Tm
Pe = 50 watts
Qts = 0.49
no = 4.698 %
1-W SPL = 98.92 dB
2.83-V SPL = 99.75 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square (optimum)
Vb = 2.661 cu.ft
Fb = 68.85 Hz
QL = 6.642
F3 = 72.83 Hz
Fill = normal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 6.59 in
Lv = 0.75 in
In the attached piccie, the trace colors correspond to the colors listed after Driver Properties
The piccy has been enlarged in PhotoShop to facilitate reading the traces.
The vents have been normalised for a tunnel depth of 0.75 inch and are consistent with maximum power tuning.
Another consideration worthy of mention; this is NOT a bass guitar speaker despite it's original use in an electric organ. Those old organ amps delivered about 15 watts and organ tones are not percussive. The transient of a percussive signal fed through a 100 watt or arc welder type amplifier will destroy this unit in short order.
This file can also be seen in my website. F12N
For some reason, the piccy of the alignments response curves won't upload. This attempt failed on two XP and one WIN 8 machine. If you go to the above link, this whole file can be viewed along with the alignment picky as well as 3 of the F12N.
In this unit, the field coil has a dc resistance of 1003 ohms @ 78 deg. F. The inductance is 2.654 hy @ 60hz and 0.159 hy @ 1 khz. Field coil resistances have been noted to range from as low as 700 ohms to as high as 3500 ohms. If memory serves me well, I've seen higher. I have a 1923 Western Electric FC speaker with a field coil resistance of 3647 ohms.
The voice coil measures 6.3 ohms and the demodulator (humbucking) coil measures 0.2 ohm.
Since this unit was measured with filtered dc applied to the field coil, the demodulator coil serves no purpose other than the cost of about 3% in power transfer from the amplifier to the voice coil. It can be bypassed (shorted) since it is wired in series with the voice coil. This, however, is not recommended unless one has a thorough knowledge of how the speaker is wired as shorting the wrong wires can be disastrous.
There has been much discussion on the actual power supplied to the field coil. The power ranged from 14 watts to about 25 with most recommendations being around 14 to15 watts. This unit was powered with 15w, which is about 123 vdc. The ambient temperature was noted at 78F. After an hour and 14 minutes, the field coil temperature was noted to be 121F and apparently stable. It was then decided to increase the power to 17w (130vdc); eighteen minutes later, the temperature of the field coil was noted to be 126F. Considering the age of the speaker, this level was abandoned.
Tests were made with 10w applied to the field coil and Theile-Small parameters determined. The biggest difference noted was less than a 1 dB drop in the calculated SPL at 1w1m. It is, therefore, my humble opinion that the power to the field coil be kept around 10-11 watts.
Common sense prevails. Keep in mind these speakers are 50 years old or older. The adhesives holding the voice coil wire to the coil form and the cone to voice coil are considerably aged. There is also the effect of time on the paper cone; it dries and becomes very brittle.
Anyway, here's the Theile-Small parameters for an 8 ohm Jensen F12N with 15w (dc) applied to the field coil. Included are piccies of normalised amplitide response curves in various size cabinets, closed and vented.
Since there are virtually an infinite number of alignments, internal volumes were kept to about 3 ft^3 or less. Higher low frequency output can be achieved with a vented enclosure of 7 to 10 cubic feet but that size is impractical. The old electric organs typically used such volumes and a lot were essentially open back or covered with a thin panel of about 0.25 inch or thinner. The one exception of which I can think is the Leslie.
As a side note, don't be concerned about the original sound of 50 years ago; it can't be reproduced knowingly. To do that, one would need a time machine to go back to the 40's or 50's and hear a new unit. Suffice it to say that any vintage speaker, especially one with a pristine original cone will sound different than it did back then and in all probability it'll sound different that anything else today. Let's face it, it's all about nostalgia and take it from a 71 year old, that's priceless,.
These next four were determined with 15w to the field coil. (15wFC)
Driver Properties Red curve
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 0.783 cu.ft
Qtc = 0.933
QL = 19.94
F3 = 111.4 Hz
Fill = heavy
Driver Properties Orange curve
Optimum closed box alignment (Qtc=0.707)
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square
Vb = 1.835 cu.ft
Qtc = 0.707
QL = 17.62
F3 = 96.44 Hz
Fill = heavy
Driver Properties Green curve
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 2.065 cu.ft
Fb = 63.57 Hz
QL = 6.772
F3 = 79.94 Hz
Fill = normal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 4.558 in
Lv = 0.75 in
Driver Properties Yellow curve
Name: F12N 15wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.77 Hz
Qms = 23.84
Vas = 200.1 liters
Cms = 548.9 µm/N
Mms = 18.63 g
Rms = 0.539 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.455
Re = 6.9 ohms
Le = 0.535 mH
Z = 8.28 ohms
BL = 9.4 Tm
Pe = 50 watts
Qts = 0.446
no = 5.184 %
1-W SPL = 99.35 dB
2.83-V SPL = 99.99 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square
Vb = 3.23 cu.ft
Fb = 55.93 Hz
QL = 6.772
F3 = 67.58 Hz
Fill = normal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 5.398 in
Lv = 0.75 in
These next two were determined with 10w to the field coil. (10wFC)
Driver Properties Pink curve
Name: F12N 10wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.63 Hz
Qms = 22.96
Vas = 201.4 liters
Cms = 552.3 µm/N
Mms = 18.62 g
Rms = 0.558 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.501
Re = 6.6 ohms
Le = 0.533 mH
Z = 7.92 ohms
BL = 8.746 Tm
Pe = 50 watts
Qts = 0.49
no = 4.698 %
1-W SPL = 98.92 dB
2.83-V SPL = 99.75 dB
-----------------------------------------
Box Properties
Name:
Type: Closed Box
Shape: Prism, square (optimum)
Vb = 2.389 cu.ft
Qtc = 0.707
QL = 16.41
F3 = 87.87 Hz
Fill = heavy
Driver Properties Blue curve
Name: F12N 10wFC
Type: Standard one-way driver
Company: JENSEN
No. of Drivers = 1
Fs = 49.63 Hz
Qms = 22.96
Vas = 201.4 liters
Cms = 552.3 µm/N
Mms = 18.62 g
Rms = 0.558 lb/s
Xmax = 2 mm
Xmech = 0.118 in
P-Dia = 10 in
Sd = 0.0507 sq.m
P-Vd = 0.00358 cu.ft
Qes = 0.501
Re = 6.6 ohms
Le = 0.533 mH
Z = 7.92 ohms
BL = 8.746 Tm
Pe = 50 watts
Qts = 0.49
no = 4.698 %
1-W SPL = 98.92 dB
2.83-V SPL = 99.75 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box
Shape: Prism, square (optimum)
Vb = 2.661 cu.ft
Fb = 68.85 Hz
QL = 6.642
F3 = 72.83 Hz
Fill = normal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 6.59 in
Lv = 0.75 in
In the attached piccie, the trace colors correspond to the colors listed after Driver Properties
The piccy has been enlarged in PhotoShop to facilitate reading the traces.
The vents have been normalised for a tunnel depth of 0.75 inch and are consistent with maximum power tuning.
Another consideration worthy of mention; this is NOT a bass guitar speaker despite it's original use in an electric organ. Those old organ amps delivered about 15 watts and organ tones are not percussive. The transient of a percussive signal fed through a 100 watt or arc welder type amplifier will destroy this unit in short order.
This file can also be seen in my website. F12N
For some reason, the piccy of the alignments response curves won't upload. This attempt failed on two XP and one WIN 8 machine. If you go to the above link, this whole file can be viewed along with the alignment picky as well as 3 of the F12N.
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