Hey, Hacknet
We are not speaking about the same jig test.
I suggested to feed the test bobin with a Variac from, says, 10V while measuring the current, increasing the voltage by 10V steps and noting the current at each step, void going over 50mA.
Now for each couple of U and I values you can obtain the Mu and the induction (B) at 50Hz:
Mu = U / I / 3.14/ N / N / A * MPL * 10e8
B = U / 4.44 / N / 50 / A * 10e4
N is turns number, A is core area, MPL is magnetic Path Len.(Respectively 6.8 and 13.2 for an EI66B)
You should observe a peak around 0.8 Tesla, for M6X it is around 25000
Regards, Yves.
We are not speaking about the same jig test.
I suggested to feed the test bobin with a Variac from, says, 10V while measuring the current, increasing the voltage by 10V steps and noting the current at each step, void going over 50mA.
Now for each couple of U and I values you can obtain the Mu and the induction (B) at 50Hz:
Mu = U / I / 3.14/ N / N / A * MPL * 10e8
B = U / 4.44 / N / 50 / A * 10e4
N is turns number, A is core area, MPL is magnetic Path Len.(Respectively 6.8 and 13.2 for an EI66B)
You should observe a peak around 0.8 Tesla, for M6X it is around 25000
Regards, Yves.
> the transformer needs to be around 1.7k p-p and around 15H to support the bass till ~25hz at 90ma bias. ....i only need a maxium of 8 watts.
Sanity-check: 1K7 p-p seems awful low for 6-8 Watts output. Are you really only running 150V-200V B+?
If so, and the available irons are made for power transformers....
I'll get laughed at, but: try a power transformer.
It won't work like a dream, but WILL work, and get you playing music 15 minutes after you open the box. Then you can test its performance and extrapolate a better design on a similar core.
120+120:18VAC gives the right ratio.
You only have 58VRMS on each "120V" winding, so your maximum Gauss (at 50-60Hz!) is half of what the tranny is made to stand. Note that power-tranny designers SOAK the cores, to 15,000-20,000 Gauss, and distortion is getting bad around 5,000-6,000 Gauss. You'd really like a 200+200VAC primary, but 120+120 is not far off and far more available.
Current: assuming 8Ω on the 18VAC secondary, a 2A rating will give low series resistance.
18V 2A is a 35 VA core; use 35VA or 50VA.
Power transformers are not interleaved, and often wound "badly" for HF response. Nevertheless they usually work OK past 5KHz. If held at full power at those frequencies, they would over-heat from core loss. But we have de-rated a LOT and also music isn't steady.
When you have it working, and have an idea how much better you want it to be, measure the transformer. Ohms are easy. Henries are almost impossible: may be shockingly low at very low level and rise with level. (In fact usually the inductance near rated power of a push-pull transformer is normally MUCH higher than you need for frequency response: the issue is to keep the low-level inductance tolerably high and not worry about the inductance at 8 Watts.) Turns are actually easy: slip a 1-turn "winding" around the core, drive the transformer near rated power, and measure the milliVolts on the 1-turn winding. Compare the primary and secondary volts with the 1-turn milliVolts, the turns-ratio pops right out. You can try to estimate wire gauge from turns and winding cross-section, but it would be more accurate to get calipers on a lead where it connects to a terminal, or tear the tranny apart if you can't find exposed winding wire.
If you think you need more inductance, and can stand more resistance (probably a necessary evil for 25Hz response), pick more turns of smaller wire. If you need more inductance and can't stand more resistance, you either need a bigger core or a "better" core-iron.
Note that the specs for power-transformer iron are not much use for audio. They are defining how bad you can abuse the iron, we treat our iron nice. Even if they do let you compute inductance, it will be for very high Gauss, much higher than we ever want in audio. So a prototype in circuit may tell you more than catalogs will.
Interleave your winding any standard way. 3-section (primary, secondary, primary) will be a lot better than typical power-tranny split-bobbin. 5-section or 7-section will usually be plenty good. Williamson split his windings much more (was it 11 sections?), until both leakage inductance and stray capacitance were equally troublesome. But then small variations in winding technique make large variations in response and stability. Simple tricks are often best.
Sanity-check: 1K7 p-p seems awful low for 6-8 Watts output. Are you really only running 150V-200V B+?
If so, and the available irons are made for power transformers....
I'll get laughed at, but: try a power transformer.
It won't work like a dream, but WILL work, and get you playing music 15 minutes after you open the box. Then you can test its performance and extrapolate a better design on a similar core.
120+120:18VAC gives the right ratio.
You only have 58VRMS on each "120V" winding, so your maximum Gauss (at 50-60Hz!) is half of what the tranny is made to stand. Note that power-tranny designers SOAK the cores, to 15,000-20,000 Gauss, and distortion is getting bad around 5,000-6,000 Gauss. You'd really like a 200+200VAC primary, but 120+120 is not far off and far more available.
Current: assuming 8Ω on the 18VAC secondary, a 2A rating will give low series resistance.
18V 2A is a 35 VA core; use 35VA or 50VA.
Power transformers are not interleaved, and often wound "badly" for HF response. Nevertheless they usually work OK past 5KHz. If held at full power at those frequencies, they would over-heat from core loss. But we have de-rated a LOT and also music isn't steady.
When you have it working, and have an idea how much better you want it to be, measure the transformer. Ohms are easy. Henries are almost impossible: may be shockingly low at very low level and rise with level. (In fact usually the inductance near rated power of a push-pull transformer is normally MUCH higher than you need for frequency response: the issue is to keep the low-level inductance tolerably high and not worry about the inductance at 8 Watts.) Turns are actually easy: slip a 1-turn "winding" around the core, drive the transformer near rated power, and measure the milliVolts on the 1-turn winding. Compare the primary and secondary volts with the 1-turn milliVolts, the turns-ratio pops right out. You can try to estimate wire gauge from turns and winding cross-section, but it would be more accurate to get calipers on a lead where it connects to a terminal, or tear the tranny apart if you can't find exposed winding wire.
If you think you need more inductance, and can stand more resistance (probably a necessary evil for 25Hz response), pick more turns of smaller wire. If you need more inductance and can't stand more resistance, you either need a bigger core or a "better" core-iron.
Note that the specs for power-transformer iron are not much use for audio. They are defining how bad you can abuse the iron, we treat our iron nice. Even if they do let you compute inductance, it will be for very high Gauss, much higher than we ever want in audio. So a prototype in circuit may tell you more than catalogs will.
Interleave your winding any standard way. 3-section (primary, secondary, primary) will be a lot better than typical power-tranny split-bobbin. 5-section or 7-section will usually be plenty good. Williamson split his windings much more (was it 11 sections?), until both leakage inductance and stray capacitance were equally troublesome. But then small variations in winding technique make large variations in response and stability. Simple tricks are often best.
Hey-Hey!!!,
Inductance is easy. requires care, a DMM, a variac and a resistor. Well maybe two resistors, but only one at a time...
Start with V=i*z...and z=omega*L....and omega=2*Pi*60( or perhaps 50 cps line voltae? )
you will series connect the sense resistor with coil in question. Apply ac voltage of known frequency and measure voltage across coil and resistor.
Apply algebra to above formula to isolate L. It is a series circuit so the i from the known voltage and resistance can be applied to the unknown inductance calculation...or:
V_inductor over V_resistor times R_value over 2*Pi*(line freq) equals L in Hy, if V is RMS, and R is in Ohms
Apply different voltages and see how L is changed by aplied voltage.
regards,
Douglas
Inductance is easy. requires care, a DMM, a variac and a resistor. Well maybe two resistors, but only one at a time...
Start with V=i*z...and z=omega*L....and omega=2*Pi*60( or perhaps 50 cps line voltae? )
you will series connect the sense resistor with coil in question. Apply ac voltage of known frequency and measure voltage across coil and resistor.
Apply algebra to above formula to isolate L. It is a series circuit so the i from the known voltage and resistance can be applied to the unknown inductance calculation...or:
V_inductor over V_resistor times R_value over 2*Pi*(line freq) equals L in Hy, if V is RMS, and R is in Ohms
Apply different voltages and see how L is changed by aplied voltage.
regards,
Douglas
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