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Bucking transformers

john & Jake

john & Jake

Jake was smarter than me
Could more knowledgeable members please diagram how I would buck a torroidal transformer as I have a small one not doing anything. It is centre tapped.

I have looked at various diagrams on the net and they are as clear as mud to me.
 
Hi, thanks Mike, looked at that before as I'm making one of his kits, didn't make sense first time but is this right.
The centre tap becomes an AC out. Both 20v outs are connected to AC in and the earth is extended to case earth via transformer earth.
 
S l o w d o w n there.
What are you trying to do?
Are you trying to 'buck' the torroid or use the torroid to 'buck' another transformer? What do you mean transformer earth?
 
snowman_al said:
S l o w d o w n there.
What are you trying to do?
Are you trying to 'buck' the torroid or use the torroid to 'buck' another transformer? What do you mean transformer earth?
Click to expand...

Hi Sorry, buck another transformer, took it from another one of my threads and probably lost the context.
I need to reduce the DC voltage out of my Crimson boards and was told that a bucking transformer was the best way.
Transformer earth just the transformer mains earth.
 
If you are talking about a solid state amp then the difference between 220 and 240 is pretty modest. By the time you have transformed it down to 30-0-30 or 40-0-40 or whatever it will be 10% high. Hardly a big deal, 3 or 4 V on your rails. What's your reasoning on this being a problem?

@mikep, thanks for that link, it's really useful, I only partly understood buck trafos before and now I get it.
 
Hi Steve
My rails are on 59v and I have been advised that into a 4ohm load a slight drop in rail voltage could give a little more power.
I'm happy at 59 as is the designer of the boards but a doubt has been planted.
 
OK,
Understand that a bucking transformer is NOT ISOLATED from the mains. It carries FULL mains voltage on both the primary (240v) and secondary (20v) windings. You must make sure you cannot come into contact with any part of it at any time. It will kill you...
You will also need a multi-meter that can measure mains and output voltages.

Look at the left side Figure 4 on the Elliott Sound Products page above.
This is what you are trying to do. You have a transformer with a 240 volt mains (primary) winding. You use the 20 volt secondary winding and add that 'on top' of the primary.
If you look at the diagram you will see the AC IN (live) goes first to the 20 volt connection, through the 20 volt winding then to the top of the 240 volt winding, through that back to the other AC IN (neutral).
You have actually made a 260 volt transformer, but your mains is still only 240 volts, so where the 20 volt and 240 volt windings join (AC OUT on the diagram) you get 240 minus 20 = 220 volts.

BUT, only if you have the windings in the correct relationship. (That is what the small black dots on the diagrams signify) Unless you know the winding direction, you have a 50/50 chance of ending up with more than 240 volts output!
I doubt your transformer will be marked, so you have to measure your results BEFORE using it. If you measure about 220 volts between the AC OUT positions all is well. If you measure More than 240 volts there, swap the two 20 volt wires round and measure again.

As your transformer has a centre tap Only connect one half (if that is 20 volts) and carefully insulate the other connection. (Do Not join the 20 volt ends together, that will just short the transformer out...)
You do not say what current or VA rating your transformer is, or what current the Crimson amp uses, make sure it is actually large enough.
I still do not understand the transformer 'earth' connection, unless it is an internal screen inside the transformer. It might be marked Screen or SCN?

Again, if you have any doubts, please leave it alone.
 
john & Jake said:
Hi Steve
My rails are on 59v and I have been advised that into a 4ohm load a slight drop in rail voltage could give a little more power.
I'm happy at 59 as is the designer of the boards but a doubt has been planted.
Click to expand...
It's a long way round to this, with no small amount of expense, for "a bit more power into 4R". For a start I am not sure how this would work, lower rail voltages will give less power into all impedances, given the same circuit and output transformers. What's your reasoning that you will get more power? In addition you are going to have to find somehwre to put this trafo, it has to go in a case and it is as stated subject to full mains voltage so the safety aspects are very substantial. If I wanted to lower 59V DC I'd be using an LM317 or similar regulator on the critical bits of the board, it's a lot cheaper, a lot easier to house and not going to kill you if you get it wrong.
 
Hi, have no qualms about doing it now you've explained in better detail. Just two things bother me so I'll probably not bother.
I don't think it is necessary to do it, 59v is fine. and I don't have Jake anymore to warn me that I shouldn't really be plugging that in.
 
john & Jake said:
Hi Steve
My rails are on 59v and I have been advised that into a 4ohm load a slight drop in rail voltage could give a little more power.
I'm happy at 59 as is the designer of the boards but a doubt has been planted.
Click to expand...

Sorry, but this makes no sense as it stands. Is there more detail that we are missing?

Reducing a rail voltage cannot increase power. Anyway a slight increase in power is going to be undetectable. Do not forget that a whole doubling of power only adds a few db of headroom - and that at max volume, max loudness peaks only.
 
Hi, No that's it. I'm happy with the amp as is but another member reckoned that I may get more power into 4 ohms if I reduced the rail voltage.

Designer says 65v max 59 ok no need to reduce. The advice was regards protection reducing power I think.

I don't flog the amp so probably does not come into it.
 
john & Jake said:
Hi Sorry, buck another transformer, took it from another one of my threads and probably lost the context.
I need to reduce the DC voltage out of my Crimson boards and was told that a bucking transformer was the best way.
Transformer earth just the transformer mains earth.
Click to expand...

IF driving an easy 8 Ohm load your amps will be fine at the "pushing it a bit there" voltage you are using.... the problem is related to what's called Safe Operating Area (usually abbreviated to SOAR) of the output transistors. A typical output transistor may be rated at 150V, 15A but, and this is the bit in the fine print, not at the same time!! For such a transistor spec 80V and 2 Amp simultaneously would be a good spec (better than average). Any more and you reach "Secondary Breakdown" point for the transistor and the magic smoke comes out...
Use speakers that are a an awkward load, worse still 4 Ohm and awkward, and wind the wick up and I would expect it to be only a matter of time before it goes bang... Something that never drops below 7 Ohm and has a very simple crossover would be fine till the cows come home but you would be a brave man to turn the wick up driving Isobariks ;)
 
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Hi been there, I've used Bariks ( had two pairs over the years ) with numerous amps and never had a problem, Nytech, Quantum kit amp, latterly Kel84 valve amp. But best results with Exposure IV.
If you are sensible you can get satisfying results with most good designs.
 
Just finished boxing mine up, used a torroidal with two 9v out, as I didn't like the idea of having two unused wired floating about. Getting about 225v out, earthed the bolt as I'm using a plastic case, cos it was cheap :D
 
stevec67 said:
It's a long way round to this, with no small amount of expense, for "a bit more power into 4R". For a start I am not sure how this would work, lower rail voltages will give less power into all impedances, given the same circuit and output transformers. What's your reasoning that you will get more power?
Click to expand...

Hi Steve, Not my reasoning, theory is not my strong point. Another PFM members suggestion..
I have two Linn lk100 amps burning a hole in the cupboard and itching to get their hands on my Kabers, but they will have to wait until an appropriate time.
 
john & Jake said:
Hi been there, I've used Bariks ( had two pairs over the years ) with numerous amps and never had a problem, Nytech, Quantum kit amp, latterly Kel84 valve amp. But best results with Exposure IV.
If you are sensible you can get satisfying results with most good designs.
Click to expand...

Obviously you've not understood a word I've said...
 
Arkless Electronics said:
Obviously you've not understood a word I've said...
Click to expand...

That's correct, not all of us flog our amps and speakers with insane volume levels. Smaller amps may lack a little in dynamics but that's not the be all and end all in enjoying music.
I know Bariks inside out having restored a fire damaged pair and then using the measurements and internal design to build a pair of my own. They may go down to 4 ohms but you don't need high power amp to enjoy them, I've never destroyed an amp in over 40 years of HiFi. Specs are one thing, practice is another.
 
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