At this point in time, I am lazy and do not wish to measure the "advanced specs" of an output transformer. I confirm the performance of an output transformer in circuit after the fact. I have to assume that companies like Edcor know what they are doing.
I bought an Audio Precision to do the work for me. Granted it cannot measure impedance ratio and inductance... it can measure other anomalies that indirectly reflect those other specs.
If my frequency response and distortion (and FFT) are what I expect, then I have to assume the inductance and impedance ratio (and other items) are good.
It is all great though, we all have our specialties and what we prefer to measure.
Below is a quick way almost anybody can measure an unknown transformer.
Output Transformer Impedance
I bought an Audio Precision to do the work for me. Granted it cannot measure impedance ratio and inductance... it can measure other anomalies that indirectly reflect those other specs.
If my frequency response and distortion (and FFT) are what I expect, then I have to assume the inductance and impedance ratio (and other items) are good.
It is all great though, we all have our specialties and what we prefer to measure.
Below is a quick way almost anybody can measure an unknown transformer.
Output Transformer Impedance
Nice trick. Do it again with a speaker attached to the secondary, preferably a nice high efficiency one.
Nice trick. Do it again with a speaker attached to the secondary, preferably a nice high efficiency one.
I did that when I was a kid. I liked smell of burned lacquer.
PS: we had 220V there.
5.04 volts into an 8 ohm load is just over 3 watts. I doubt too many speakers are hiding in fear. In fact most speakers are well over 8 ohms at 60 Hz. My "8 ohm" Yamaha's are 13 ohms at 60 Hz.
When I was younger and and a bit less respectful of electricity I had this habit of plugging things into the wall outlet. It started at age 5 with the paper clip. I worked in an Olsons Electronics store in the early 70's and every speaker we sent back as a warantee return met the wall outlet. That's 1800 watts, and no they don't live long. Usually there is just a pop, sometimes you might get a few cycles of buzz, but we had these 12 inch coaxials that put up a fight. The woofer would last about 1/2 second, but the tweeter would faithfully reproduce all of the HF hash and noise on the power line for several minutes until the crossover cap exploded.
Typical inductance measurements are made with just AC on the core. Keep in mind that applying DC current will reduce the material Mu even further. For standard SE OT operation, the max DC current spec. will be designed to bias at 1/2 the Max design flux, so 1/2 the magnetic domains are still available for AC operation. Hence the Mu of the steel material will drop to around 1/2 of its AC value. (actually somewhat less than since the highest Mu magnetic domains get pinned first)
The air gap dilutes this effect some, since it has already reduced the effective Mu (like something around 1/4). So the resulting effect will be around 0.75 of L AC with the design DC present.
The air gap dilutes this effect some, since it has already reduced the effective Mu (like something around 1/4). So the resulting effect will be around 0.75 of L AC with the design DC present.
Attachments
Another issue to check on SE OTs is the power rating at the lowest frequency (often dubious). (Meeting the inductance spec is not sufficient!) Magnetic saturation sets in and causes enormous spikey magnetizing current at some AC level. Using a Variac on the 8 Ohm winding, and a current probe (or 0.1 Ohm sense resistor) (and scope) will tell you the max peak current.
(Warning!!! Warning!! Dangerous High Voltage will be present on the primary side with this test, maximum caution is required, see below too)
The max peak current measured at AC (and 60 Hz) then gets x 1/2 for DC operation (resultant peak is DC + AC peak) and x LFspec/60Hz for frequency effect. Max power is then calculated as 1/2 (the reduced Ipeak squared) x Zpri. (the 1/2 factor from peak to RMS conv.)
Caution, Warning!!:
On some Hi Z primary OTs, testing for max AC current at 60 Hz on the secondary could over voltage the primary beyond the OT's spec. Check ratings first. You may have to use a SS power amplifier V source at 20 Hz or lower to test these Hi Z OTs. And in any case, dangerous High Voltage will be present on the primary winding for this type of testing. So be maximum careful!!
(Warning!!! Warning!! Dangerous High Voltage will be present on the primary side with this test, maximum caution is required, see below too)
The max peak current measured at AC (and 60 Hz) then gets x 1/2 for DC operation (resultant peak is DC + AC peak) and x LFspec/60Hz for frequency effect. Max power is then calculated as 1/2 (the reduced Ipeak squared) x Zpri. (the 1/2 factor from peak to RMS conv.)
Caution, Warning!!:
On some Hi Z primary OTs, testing for max AC current at 60 Hz on the secondary could over voltage the primary beyond the OT's spec. Check ratings first. You may have to use a SS power amplifier V source at 20 Hz or lower to test these Hi Z OTs. And in any case, dangerous High Voltage will be present on the primary winding for this type of testing. So be maximum careful!!
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"Do you load the primary with the target reflected impedance to do this test? "
No, just open circuit. It's just a test of the OT itself for magnetizing current draw. The voltage applied to the secondary should be kept to the range of expected output signal (times 3x for 60Hz used on a 20 Hz rated OT, but observe max HV rating of the OT!!) So do NOT apply 120 VAC!!! Start at low VAC, 0, and slowly ramp up while observing the current waveform on the scope. (and a DVM with adequate HV rating (1000 VAC or more), clip leaded to the primary, is useful for avoiding overvoltage of the OT. I blew a DVM fuse once too when I over voltaged the DVM.)
Keep away from the primary wires, and keep good clearance between them and any metal objects for safety!!)
Also, most Variac type units are NOT isolated from the power line, so using a 0.1 or 1 or 10 Ohm sense resistor in series to display current on a scope will generally REQUIRE an isolation transformer in front of the Variac to avoid sparks and flames! (Scope ground is usually real ground too.) Current probes (magnetic coupled) generally are isolated.
(Be prepared to be horrified by this measurement, by the way. Magnetizing current draw is something that SE OT vendors fear to mention, and SE Amp enthusiasts "apparently" are ignorant of. You will never refer to SE as HiFi again after seeing how badly they perform! The magnetizing current is the common cause of 3rd and higher odd harmonic distortion from transformers, and SE OTs have around 50X the magnetizing current of P-P OTs)
No, just open circuit. It's just a test of the OT itself for magnetizing current draw. The voltage applied to the secondary should be kept to the range of expected output signal (times 3x for 60Hz used on a 20 Hz rated OT, but observe max HV rating of the OT!!) So do NOT apply 120 VAC!!! Start at low VAC, 0, and slowly ramp up while observing the current waveform on the scope. (and a DVM with adequate HV rating (1000 VAC or more), clip leaded to the primary, is useful for avoiding overvoltage of the OT. I blew a DVM fuse once too when I over voltaged the DVM.)
Keep away from the primary wires, and keep good clearance between them and any metal objects for safety!!)
Also, most Variac type units are NOT isolated from the power line, so using a 0.1 or 1 or 10 Ohm sense resistor in series to display current on a scope will generally REQUIRE an isolation transformer in front of the Variac to avoid sparks and flames! (Scope ground is usually real ground too.) Current probes (magnetic coupled) generally are isolated.
(Be prepared to be horrified by this measurement, by the way. Magnetizing current draw is something that SE OT vendors fear to mention, and SE Amp enthusiasts "apparently" are ignorant of. You will never refer to SE as HiFi again after seeing how badly they perform! The magnetizing current is the common cause of 3rd and higher odd harmonic distortion from transformers, and SE OTs have around 50X the magnetizing current of P-P OTs)
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As long as you do the test without any DC current present in the transformer you should be fine. The positive swing of the AC will set up the magnetic flux, the negative swing will remove it. The core should be completely reset every cycle.
The voltage is tightly controlled across the winding. Unlike running an SE amp without load where the primary current is changed and there's nothing to clamp the voltage but the breakdown of the transformer insulation.
~Tom
The voltage is tightly controlled across the winding. Unlike running an SE amp without load where the primary current is changed and there's nothing to clamp the voltage but the breakdown of the transformer insulation.
~Tom
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