Alright, so I picked up this old receiver on ebay cheap for the torroid and chassis..nice buy btw...
Anyway, got a torroid question. The label says 40V 0V (0V) 40V and should be around 400VA. I want to try to use it for a LM3886 and get 20VAC out of it...read on...
Now here the tricky part...all four wires (blue black black blue) all have continuity and thus are the same winding. I get 40V between either of of the blue and black wires.
If I connect the 40V and 0V to the AC side of a single rectifier it should be the same as if I had 20V and 20V on the AC side of the rectifier, correct?
I tried it on a rectifier and it shows (after the smoothing caps) about 28VDC, the same as if I had 20V and 20V....am I missing something?
What would be my virtual Ground if wired this way for a dual 20V power supply? Or is this the reason why this torroid cannot be used?
Anyway, got a torroid question. The label says 40V 0V (0V) 40V and should be around 400VA. I want to try to use it for a LM3886 and get 20VAC out of it...read on...
Now here the tricky part...all four wires (blue black black blue) all have continuity and thus are the same winding. I get 40V between either of of the blue and black wires.
If I connect the 40V and 0V to the AC side of a single rectifier it should be the same as if I had 20V and 20V on the AC side of the rectifier, correct?
I tried it on a rectifier and it shows (after the smoothing caps) about 28VDC, the same as if I had 20V and 20V....am I missing something?
What would be my virtual Ground if wired this way for a dual 20V power supply? Or is this the reason why this torroid cannot be used?
I'm wondering if you're playing with a half-wave versus a full wave rectifier. Are you trying to reduce transformers output to please the 3886 requirements? F.W.I.W.... just got home(am a carpenter) and probably have little to offer here, none-the-less.., I'm also curious how you might use a salvaged transformer, with different specs than ones' specified (for instance) with the 3886 chip.
I am using a full wave rectifier. Yes, I would like to use this transformer if I can. At 40V secondaries, it is too much for the LM3886 straight up, but when connected as I have outlined...
I would like to dig up the rectifier and caps to try it again, but my basement is getting refinished and I can't get to it at the moment. I do have the torrid within arms reach...It came from a KLH KL2400. These receivers are quite ubiquitous on ebay - very cheap, and they are supposedly Chinese "fakes", as I read, KLH never made a model KL2400...
I took apart the receiver board and it was very cheap, but the torroid and soft start board looked reuseable...
Go figure...
I would like to dig up the rectifier and caps to try it again, but my basement is getting refinished and I can't get to it at the moment. I do have the torrid within arms reach...It came from a KLH KL2400. These receivers are quite ubiquitous on ebay - very cheap, and they are supposedly Chinese "fakes", as I read, KLH never made a model KL2400...
I took apart the receiver board and it was very cheap, but the torroid and soft start board looked reuseable...
Go figure...
Or how 'bout this...
Is there a way I can drop the 40V 0V 40V to a 24V 0V 24V using a couple of LM338 Voltage regulators BEFORE the smoothing caps? The torroid is 5A, so the LM338 is good for 5A.
Between the Rectifier and smoothing caps, the voltage should be lower than 40VAC rectifier input, correct? Putting it after the smoothing caps is no good, cause the 56VDC smoothing caps output is too high for the LM338, 40V is max... My guess is about 37VDC at this point, which would be perfect. Just wanna to drop it 10V - 12V.
I also could put the LM338 before the rectifier (on transformer output), as the LM338 regulates DC and AC.
But I remember some saying that you cannot use the dual LM338 regualated system on a center tap transformer, only a split winding. Is this true?
Don't see why I can't just use the centertap as grounds (0V) for both LM338 rails...
I have used the LM317's many times to regulate 20V laptop power supplies down to 13V for my Tripath amps, and think this should work here too...
Anyone?
Is there a way I can drop the 40V 0V 40V to a 24V 0V 24V using a couple of LM338 Voltage regulators BEFORE the smoothing caps? The torroid is 5A, so the LM338 is good for 5A.
Between the Rectifier and smoothing caps, the voltage should be lower than 40VAC rectifier input, correct? Putting it after the smoothing caps is no good, cause the 56VDC smoothing caps output is too high for the LM338, 40V is max... My guess is about 37VDC at this point, which would be perfect. Just wanna to drop it 10V - 12V.
I also could put the LM338 before the rectifier (on transformer output), as the LM338 regulates DC and AC.
But I remember some saying that you cannot use the dual LM338 regualated system on a center tap transformer, only a split winding. Is this true?
Don't see why I can't just use the centertap as grounds (0V) for both LM338 rails...
I have used the LM317's many times to regulate 20V laptop power supplies down to 13V for my Tripath amps, and think this should work here too...
Anyone?
I think those are only good for 1.5A, you will need a couple of times that....
I think wanting to build a gainclone is one thing, wasteing a perfectly good transformer that you could use for something more substantial is another... I mean it in the nicest way though... its just trannies are expensive....
I think wanting to build a gainclone is one thing, wasteing a perfectly good transformer that you could use for something more substantial is another... I mean it in the nicest way though... its just trannies are expensive....
The LM317 is good for 1.5A
The LM350 is good for 3A
The LM338 is good for 5A.
Correct me if I am wrong.
I have two of these 40V 0V 40V, and would like to use one of them on the "special" LM3886 I am building.
I planned on using one for the UCD180 I had, but was pushing it with 40V too, so I bought another toroid at 30V 0V 30V.
Just want to use what I have as opposed to buying another trannie...that's all...
The LM350 is good for 3A
The LM338 is good for 5A.
Correct me if I am wrong.
I have two of these 40V 0V 40V, and would like to use one of them on the "special" LM3886 I am building.
I planned on using one for the UCD180 I had, but was pushing it with 40V too, so I bought another toroid at 30V 0V 30V.
Just want to use what I have as opposed to buying another trannie...that's all...
Roger that....
Are those transformers 40v AC? If it is, you will have 56.6V - diode drop.
driving 8 ohm speakers at peak with 40V would require 5A before you start adding extra to deal with real world efficiency..
4 Ohm loads are almost not an option for you...
I'm not too familiar with the other chipamp brands, but still wonder if there isn't somthing that could use those rails with less compromises.
Are those transformers 40v AC? If it is, you will have 56.6V - diode drop.
driving 8 ohm speakers at peak with 40V would require 5A before you start adding extra to deal with real world efficiency..
4 Ohm loads are almost not an option for you...
I'm not too familiar with the other chipamp brands, but still wonder if there isn't somthing that could use those rails with less compromises.
Hi,
my short answer is buy the correct transformer.
The building of a high power regulator (that sounds good) will be more complicated than building a good sounding chipamp.
This extra complication and cost (and risk to the sound quality) completely negates the reason for going chipamp in the first place.
Either, build a discrete amp that suits 2*40Vac or buy 2*25Vac (or 25+25Vac).
BTW 40Vac can support 120W to 150W into 8r. It will need to be about 160VA to 250VA to do one mono channel. If you regulate it down to a suitable voltage for a chipamp, then the losses in the regulator and the amplifier result in less output power but still require the same VA rating for the transformer i.e 160VA to 250VA for 50W to 60W output.
Double these VA ratings for stereo and double again for 4ohm duty.
my short answer is buy the correct transformer.
The building of a high power regulator (that sounds good) will be more complicated than building a good sounding chipamp.
This extra complication and cost (and risk to the sound quality) completely negates the reason for going chipamp in the first place.
Either, build a discrete amp that suits 2*40Vac or buy 2*25Vac (or 25+25Vac).
BTW 40Vac can support 120W to 150W into 8r. It will need to be about 160VA to 250VA to do one mono channel. If you regulate it down to a suitable voltage for a chipamp, then the losses in the regulator and the amplifier result in less output power but still require the same VA rating for the transformer i.e 160VA to 250VA for 50W to 60W output.
Double these VA ratings for stereo and double again for 4ohm duty.
From John65b
A couple of points
One
You can (use the trafo you have) - If you are deterimed to run at +/- 20V, you just need to get hold of a couple of 8 - 10 mH chokes - one for each side of the PS - and use LC input filter instead of RC. THis will have 3 effects... 1) it will bring down the voltage of the trafo secondaries by about 25%, 2) it will do a way better job of filtering the ripple and 3) it will provide much better current stabilization.
Two
However, you don't really need to. I am running a LM4766 chipamp in native stereo mode (not bridged or paralleled) off a 38.5-0-38.5 MONSTER conventional trafo. Because of the really high PSRR of National's audio chips (God bless 'em) mine is a simple bridge>really big caps(12,000uF)>bypass caps(0.01uF) and that's all. DC Voltage to the chip is +/- 34V. And, it sounds wonderful! A couple of very knowledegable hifi buddies (both tube nuts) have said: 'that's the best sounding chipamp I've ever heard'. This compared to Peter D's 4780s and a host of 3886s.
Seems that most people don't read the National spec sheet very well - they specify power input range of 20 to 80 volts. That is
+/- 10 V to +/-40V. Since the basic technology for all of the National Overture chips is the same, the same electrical rules and parameters apply, except for the current needed to drive them to performance.
I should add that my LM4766 rig barely warms up the heatsink running at half-throttle for hours at a time.
So, I am a big fan of running these devices at far higher voltages than seems popular. Considering that the chip costs about $5, build one a try it. It won't blow up and it will sound much better.
Lucky you is what I say. If you have any spare 40-0-40 torroids, I'll take a few
A couple of points
One
You can (use the trafo you have) - If you are deterimed to run at +/- 20V, you just need to get hold of a couple of 8 - 10 mH chokes - one for each side of the PS - and use LC input filter instead of RC. THis will have 3 effects... 1) it will bring down the voltage of the trafo secondaries by about 25%, 2) it will do a way better job of filtering the ripple and 3) it will provide much better current stabilization.
Two
However, you don't really need to. I am running a LM4766 chipamp in native stereo mode (not bridged or paralleled) off a 38.5-0-38.5 MONSTER conventional trafo. Because of the really high PSRR of National's audio chips (God bless 'em) mine is a simple bridge>really big caps(12,000uF)>bypass caps(0.01uF) and that's all. DC Voltage to the chip is +/- 34V. And, it sounds wonderful! A couple of very knowledegable hifi buddies (both tube nuts) have said: 'that's the best sounding chipamp I've ever heard'. This compared to Peter D's 4780s and a host of 3886s.
Seems that most people don't read the National spec sheet very well - they specify power input range of 20 to 80 volts. That is
+/- 10 V to +/-40V. Since the basic technology for all of the National Overture chips is the same, the same electrical rules and parameters apply, except for the current needed to drive them to performance.
I should add that my LM4766 rig barely warms up the heatsink running at half-throttle for hours at a time.
So, I am a big fan of running these devices at far higher voltages than seems popular. Considering that the chip costs about $5, build one a try it. It won't blow up and it will sound much better.
Lucky you is what I say. If you have any spare 40-0-40 torroids, I'll take a few
JesseG -->
How can you get +/-34 at your chip with a 38.5V 0V 38.5V trannie? After your 12,000uF caps you should be at approx +/-54VDC, right?
Now what am I missing here? Do I need to go back to EE101 again?
And the LM4780 max voltage was 84V (+/-42V), which is a tad more than the LM3886...not that 2V matter much here...
I have thought of taking the 40V 0 40V Tranformer connections right after the rectifier (should be a bit under 40VDC here - no smoothing caps) direct to the two 2200uf caps right on the chip (Cs - just like how the real gaincard has it - no smoothing caps)...this supply voltage would be in compliance with the LM3886 datasheet...
But I really don't wanna blow the chip, tho...kinda gettin attached to it...its my first foray into soldering all components right on the chip (exact component values of the gaincard - except no electrolitic input cap - what were they thinking...I have a 2.2uF polypropolyne cap here)
Anyone?
How can you get +/-34 at your chip with a 38.5V 0V 38.5V trannie? After your 12,000uF caps you should be at approx +/-54VDC, right?
Now what am I missing here? Do I need to go back to EE101 again?
And the LM4780 max voltage was 84V (+/-42V), which is a tad more than the LM3886...not that 2V matter much here...
I have thought of taking the 40V 0 40V Tranformer connections right after the rectifier (should be a bit under 40VDC here - no smoothing caps) direct to the two 2200uf caps right on the chip (Cs - just like how the real gaincard has it - no smoothing caps)...this supply voltage would be in compliance with the LM3886 datasheet...
But I really don't wanna blow the chip, tho...kinda gettin attached to it...its my first foray into soldering all components right on the chip (exact component values of the gaincard - except no electrolitic input cap - what were they thinking...I have a 2.2uF polypropolyne cap here)
Anyone?
Hi,
the AC voltage from your transformer is rectified by the diode bridge.
The capacitors after the rectifier TRY to charge up to the peak voltage of the AC waveform. That's where the square root of 2 comes from.
40Vac has a peak voltage just about 58V. The bridge rectifier drops about 1.4v leaving about 56Vdc across the smoothing capacitors. As mains voltage fluctuates the transformer output will also fluctuate (in the UK our tolerance is -10% and +6%)
The transformer will also produce more voltage when on a light load. A small transformer is worse here, compare the regulation, it varies from 4% for 1000VA to about 30% for 10VA.
Now you need a transformer and rectifier and smoothing capacitor to make a PSU so you are lumbered with the root 2 times Vac rule.
The maximum voltage of the chipamps is relatively low but you would expect them to perform when the incoming voltage is normal and at the two extremes I quoted. You have to ensure that after taking regulation and maximum mains that you do not destroy your chipamp.
There is one other design decision to take account of. Load impedance.
The chipamp datasheet will specify the maximum voltage for each load and you will see that as load impedance falls that the chipamp maximum voltage also falls.
Many speakers are 8ohm but some are 6ohm and a few are 4 to 8ohm due to using a low sensitivity 4ohm bass unit in conjunction with an 8ohm treble unit. Be carefull to use a PSU supply voltage that suits the load YOU will be hanging on the end.
There is a very expensive alternative, the choke regulated PSU, but it has a very major drawback, namely, if the current draw drops below a design limit the output voltage rises up towards the waveform peak. It works very well with ClassA designs that have a very high quiescent current and tends to work better when the maximum current demand is very low, ideal for Tube designs. Forget it for solid state.
the AC voltage from your transformer is rectified by the diode bridge.
The capacitors after the rectifier TRY to charge up to the peak voltage of the AC waveform. That's where the square root of 2 comes from.
40Vac has a peak voltage just about 58V. The bridge rectifier drops about 1.4v leaving about 56Vdc across the smoothing capacitors. As mains voltage fluctuates the transformer output will also fluctuate (in the UK our tolerance is -10% and +6%)
The transformer will also produce more voltage when on a light load. A small transformer is worse here, compare the regulation, it varies from 4% for 1000VA to about 30% for 10VA.
Now you need a transformer and rectifier and smoothing capacitor to make a PSU so you are lumbered with the root 2 times Vac rule.
The maximum voltage of the chipamps is relatively low but you would expect them to perform when the incoming voltage is normal and at the two extremes I quoted. You have to ensure that after taking regulation and maximum mains that you do not destroy your chipamp.
There is one other design decision to take account of. Load impedance.
The chipamp datasheet will specify the maximum voltage for each load and you will see that as load impedance falls that the chipamp maximum voltage also falls.
Many speakers are 8ohm but some are 6ohm and a few are 4 to 8ohm due to using a low sensitivity 4ohm bass unit in conjunction with an 8ohm treble unit. Be carefull to use a PSU supply voltage that suits the load YOU will be hanging on the end.
There is a very expensive alternative, the choke regulated PSU, but it has a very major drawback, namely, if the current draw drops below a design limit the output voltage rises up towards the waveform peak. It works very well with ClassA designs that have a very high quiescent current and tends to work better when the maximum current demand is very low, ideal for Tube designs. Forget it for solid state.
Andrew any idea how to bring the dropout voltage of the actual chip into the equation too?
It seems to specify 2 and 3V, but I can't figure whicc applies when, or how to calculate mamimum supply ( not that I'd ever want to drive one at max volts).
the datasheet note says:
Note 12: The output dropout voltage is defined as the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the
Typical Performance Characteristics section.
It seems to specify 2 and 3V, but I can't figure whicc applies when, or how to calculate mamimum supply ( not that I'd ever want to drive one at max volts).
the datasheet note says:
Note 12: The output dropout voltage is defined as the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the
Typical Performance Characteristics section.
Hi,
if one were designing an amplifier for a customer and had to meet a particular power spec. then a first guess would be based on the various voltage drops that occur in the system from transformer to output.
Based on that one builds the prototype and tests it.
For your build the loss through the chip is unimportant. It simply makes power predictions more difficult. The other voltage drops will be much more significant.
Now, if you were to produce a model of maximum power vs all the other variables then it would be instructive, but is it any more useful when most of the design decisions are out of your hands and made by the chip designer?
if one were designing an amplifier for a customer and had to meet a particular power spec. then a first guess would be based on the various voltage drops that occur in the system from transformer to output.
Based on that one builds the prototype and tests it.
For your build the loss through the chip is unimportant. It simply makes power predictions more difficult. The other voltage drops will be much more significant.
Now, if you were to produce a model of maximum power vs all the other variables then it would be instructive, but is it any more useful when most of the design decisions are out of your hands and made by the chip designer?
Thank you Andrew.
You seem most knowledgable in power supplies, as this may be your bread and butter. I work with many EE's, but they do not seem to be able to remember and be able to help much of what I ask - they are mostly involved with control systems, COGEN Units, and high voltage power transformers.
Anyway, if your read the beggining of my post, you will see the first question I raised about hooking the 40V 0V to the rectifier to get the same result as the 20V 20V. Can you comment on if this is possible?
Also, as JesseG mentioned, is there truely a way to get a 38.5V 0 38.5V trannie rectified/smoothed with a 12,000uF caps to be able to get +/-34VDC with nothing fancy inbetween (LC filter or choke)? From your post, we are in agreement and should not be possible.
And the size of the caps on the downstream side of the rectifier does not matter as far as far as outputting peak voltage - in other words, post rectifier to 12000uF or 1000uf, they both will act as smoothing caps and should output peak voltage (x 1.414)...not less with smaller caps.
Sorry if these questions are too basic, but I am sure there are a few of us out here that could use the education.
Thanks
You seem most knowledgable in power supplies, as this may be your bread and butter. I work with many EE's, but they do not seem to be able to remember and be able to help much of what I ask - they are mostly involved with control systems, COGEN Units, and high voltage power transformers.
Anyway, if your read the beggining of my post, you will see the first question I raised about hooking the 40V 0V to the rectifier to get the same result as the 20V 20V. Can you comment on if this is possible?
Also, as JesseG mentioned, is there truely a way to get a 38.5V 0 38.5V trannie rectified/smoothed with a 12,000uF caps to be able to get +/-34VDC with nothing fancy inbetween (LC filter or choke)? From your post, we are in agreement and should not be possible.
And the size of the caps on the downstream side of the rectifier does not matter as far as far as outputting peak voltage - in other words, post rectifier to 12000uF or 1000uf, they both will act as smoothing caps and should output peak voltage (x 1.414)...not less with smaller caps.
Sorry if these questions are too basic, but I am sure there are a few of us out here that could use the education.
Thanks
The caps are charged at 100 or 120 times per second depending on whether you have 50 or 60Hz power...
The size and quality of the cap will influence how quickly they will charge. All that said, the moment the amp tries to draw more current than what the PSU is able to store and supply the voltage will drop...
The size and quality of the cap will influence how quickly they will charge. All that said, the moment the amp tries to draw more current than what the PSU is able to store and supply the voltage will drop...
Hi John,
I might be a techy teacher but my bread and butter was Civil Engineering.
This simple answer assumes no regulators.
+-34Vdc from 2*38.5Vac is not possible. Ignore that poster until he can prove his assertion.
Have you considered rewinding the secondary to get your lower voltage?
Nordic,
your simplistic view on voltage available from the PSU is misleading.
Think about the emf available from the source (transformer) and then the string of resistances from source to load and all the way back to the source.
Now apply ohm's law and calculate the voltage drops when no current is flowing. Increment the current and again calculate the voltage drops. Subtract the voltage drops from the emf and the remainder is the available voltage to drive the amplifier.
You could get very sophisicated and draw a graph of current vs voltage drops and (emf-voltage drops) to see how the available voltage varies with current draw.
You will see that voltage available starts to fall as soon as even a tiny current is drawn by the load (the load includes the amplifier quiescent current). There is no point at which the voltage holds steady and then suddenly starts to fall when a threshold is reached, unless your threshold is set at zero.
I just lost my addendum on ripple voltage and the charging pulse current voltage loss. But you can work those ones out.
I might be a techy teacher but my bread and butter was Civil Engineering.
This simple answer assumes no regulators.
+-34Vdc from 2*38.5Vac is not possible. Ignore that poster until he can prove his assertion.
Have you considered rewinding the secondary to get your lower voltage?
Nordic,
your simplistic view on voltage available from the PSU is misleading.
Think about the emf available from the source (transformer) and then the string of resistances from source to load and all the way back to the source.
Now apply ohm's law and calculate the voltage drops when no current is flowing. Increment the current and again calculate the voltage drops. Subtract the voltage drops from the emf and the remainder is the available voltage to drive the amplifier.
You could get very sophisicated and draw a graph of current vs voltage drops and (emf-voltage drops) to see how the available voltage varies with current draw.
You will see that voltage available starts to fall as soon as even a tiny current is drawn by the load (the load includes the amplifier quiescent current). There is no point at which the voltage holds steady and then suddenly starts to fall when a threshold is reached, unless your threshold is set at zero.
I just lost my addendum on ripple voltage and the charging pulse current voltage loss. But you can work those ones out.
Andrew,
That's what I thought about the previous post. Looks like I don't need to re-review my EE classes after all (I am an ME working 25 years in the Petroleum refining industry). Great hobby here BTW...
I should just get the correct transformer instead of try to get this one to work...
I thought about the Unwinding of the toroid, but I would need to remove the nice plastic wrapping...and I would also assume that unwinding the wraps on the toroid to reduce voltage would also reduce VA rating? If I had to derate (unwrap) the windings from 40V to 25V (is that almost 1/2 the winding?), that would mean I would have about 250VA? Not that it matters that much, since 400 is way more than I need, and 250 is fine...
As you can see from my questions, I have never unwound a torroid before, so a bit leary. Is it that cumbersome? Will I need to re-wrap it? If so, can I re-wrap with packaging tape? Common sense says just unwrap the 40V windings and leave the centertap windings alone?? Do I just unwind, cut wire, scrape off the coating and test the leads to the centertap until I get to 25V?
Thanks in advance for your help
That's what I thought about the previous post. Looks like I don't need to re-review my EE classes after all (I am an ME working 25 years in the Petroleum refining industry). Great hobby here BTW...
I should just get the correct transformer instead of try to get this one to work...
I thought about the Unwinding of the toroid, but I would need to remove the nice plastic wrapping...and I would also assume that unwinding the wraps on the toroid to reduce voltage would also reduce VA rating? If I had to derate (unwrap) the windings from 40V to 25V (is that almost 1/2 the winding?), that would mean I would have about 250VA? Not that it matters that much, since 400 is way more than I need, and 250 is fine...
As you can see from my questions, I have never unwound a torroid before, so a bit leary. Is it that cumbersome? Will I need to re-wrap it? If so, can I re-wrap with packaging tape? Common sense says just unwrap the 40V windings and leave the centertap windings alone?? Do I just unwind, cut wire, scrape off the coating and test the leads to the centertap until I get to 25V?
Thanks in advance for your help
John,
Unwinding is not that big of a deal.
Your VA rating will remain intact (in fact, it will increase slightly).
You do not need to replace the tape... if you do just buy some polyester (mylar) or kapton.
Ideally the remaining winding should be redistrbuted evenly. Forget about it... you will get a slight increase in leakage inductance and flux... neither of which will matter for a PSU.
Both halves of the secondary are generally wound at the same time... side by side... "bifilar". It is the connections that creates the center tap... you'll see this as you remove the tape. You will have to recreate the center tap when you're through... very simple... make a sketch BEFORE you take things apart. The center tap is created by joining the START of one winding with END of the other... pretty hard to screw up, but it is entirely possible to SHORT the a winding by connecting its own start to its own end. After you break connections, ohm through and paint or label one of the windings.
P.S. Don't unwind all the way to your desired voltage you should be about 5% high to allow for voltage drops that will occur when the trans is loaded.
Unwinding is not that big of a deal.
Your VA rating will remain intact (in fact, it will increase slightly).
You do not need to replace the tape... if you do just buy some polyester (mylar) or kapton.
Ideally the remaining winding should be redistrbuted evenly. Forget about it... you will get a slight increase in leakage inductance and flux... neither of which will matter for a PSU.
Both halves of the secondary are generally wound at the same time... side by side... "bifilar". It is the connections that creates the center tap... you'll see this as you remove the tape. You will have to recreate the center tap when you're through... very simple... make a sketch BEFORE you take things apart. The center tap is created by joining the START of one winding with END of the other... pretty hard to screw up, but it is entirely possible to SHORT the a winding by connecting its own start to its own end. After you break connections, ohm through and paint or label one of the windings.
P.S. Don't unwind all the way to your desired voltage you should be about 5% high to allow for voltage drops that will occur when the trans is loaded.
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