Hi,
I have a toroid with 2 double secondary windings. I do not know their output, but here's my guess:
2x20V~ 0.75A
2x30V~ 2.0A
The current is assumed from the thickness of the windings and the total power ~120 to 150VA.
I have a PCB with two TDA7294's. Can I switch between the 30V and the 20V windings for 8 ohm and 4 ohm operation?
This would mean I draw much more than the 0.75A from the 20V winding, BUT the 30V will not be in use then. I estimate the wire thickness of the 20V winding at 20AWG or 0.5mm^2. But it has been years since I actually saw it... The 30V is probably 17AWG or 1mm^2.
I understand the total power of the toroid (150VA max.) is small compared to the power of the PCB (150Wrms), but it is just for an as_small_as_possible test-amplifier to be build from scrap I have lying around, and I seem to have a severe lack of 18-22V toroids in my collection...
I have a toroid with 2 double secondary windings. I do not know their output, but here's my guess:
2x20V~ 0.75A
2x30V~ 2.0A
The current is assumed from the thickness of the windings and the total power ~120 to 150VA.
I have a PCB with two TDA7294's. Can I switch between the 30V and the 20V windings for 8 ohm and 4 ohm operation?
This would mean I draw much more than the 0.75A from the 20V winding, BUT the 30V will not be in use then. I estimate the wire thickness of the 20V winding at 20AWG or 0.5mm^2. But it has been years since I actually saw it... The 30V is probably 17AWG or 1mm^2.
I understand the total power of the toroid (150VA max.) is small compared to the power of the PCB (150Wrms), but it is just for an as_small_as_possible test-amplifier to be build from scrap I have lying around, and I seem to have a severe lack of 18-22V toroids in my collection...
Hi,
transformer can in general tolerate a massive short term overload.
The limiting factor is heat. Heat in the wire, heat in the core and the effect this heat has on the insulation.
An audio amplifier rarely runs at full power. Most high power delivery situations are short to medium term.
Transient current delivered to the amplifier load comes from the decoupling and smoothing capacitors, not from the transformer feeding the rectifier.
The transformer re-charges the smoothing capacitors. It does not supply transient current to the amplifier.
Now back to your question.
The generally accepted advice for powering a ClassAB amplifier is to provide a transformer where
VA ~ 1times to 2times the total maximum power that the amplifier/s can deliver to your load.
The amplifier will work when the transformer is outside that VA range. Just ensure a small transformer does not overheat. If it feels hotter than your hand on the surface then try to imagine how hot it is in the middle.
When you started to open your new thread, how many references to older threads did the Forum suggest you read?
Did you bother to read any of them?
All of the information I provided is repeatedly posted, by many Members, in various earlier Threads.
transformer can in general tolerate a massive short term overload.
The limiting factor is heat. Heat in the wire, heat in the core and the effect this heat has on the insulation.
An audio amplifier rarely runs at full power. Most high power delivery situations are short to medium term.
Transient current delivered to the amplifier load comes from the decoupling and smoothing capacitors, not from the transformer feeding the rectifier.
The transformer re-charges the smoothing capacitors. It does not supply transient current to the amplifier.
Now back to your question.
The generally accepted advice for powering a ClassAB amplifier is to provide a transformer where
VA ~ 1times to 2times the total maximum power that the amplifier/s can deliver to your load.
The amplifier will work when the transformer is outside that VA range. Just ensure a small transformer does not overheat. If it feels hotter than your hand on the surface then try to imagine how hot it is in the middle.
When you started to open your new thread, how many references to older threads did the Forum suggest you read?
Did you bother to read any of them?
All of the information I provided is repeatedly posted, by many Members, in various earlier Threads.
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4 or 5 iirc.
No, the thread titles of the suggestions did not seem to reflect my specific question.
If I was wrong in assuming this, I sincerely apologize.
I did do a Google search before posting, but it also did not turn up a clear-cut answer.
Obviously I know that it is unlikely that my questions have not been answered before somewhere on the net, and I always *try* to find out before posting here or on any other forum. It is very possible that I am simply not that good at searching because of e.g. not using the correct terms.
So despite apparently giving the impression that I am a lazy git that just posted here so I wouldn't have to search for myself
I am convinced that I am not that person.As for your answers: thanks! It has been made clear that I can do what I suggested, as long as I simply watch the temperature of the transformer somehow. If it gets to hot during normal conditions running 4 ohm speakers, I will have to stop using it at 4 ohm.
Whether I will actually go this route has yet to be determined. I have some time before I can complete the PCB's (parts not in yet), so I may find another solution before then.
That's a no-no. Those "backwards" turns will act like a short-circuit.
Yes, you can use the transformer as described, but not to the maximum power, only just to some 40%. You could burn up that 20V secondary.I understand the total power of the toroid (150VA max.) is small compared to the power of the PCB (150Wrms), but it is just for an as_small_as_possible test-amplifier to be build from scrap I have lying around, and I seem to have a severe lack of 18-22V toroids in my collection...
WRONG
It is like connecting a battery in reverse it will just subtract from the initial voltage.
My first thought was that a trafo like this is ideal for a class G project, but the higher voltage windings can't deliver the high current required.
Class-G Amplifiers
Class-G Amplifiers
Edit: the below is an answer to KatieandDad and SoNic_real_one. To ingenieus: I know (and hate) the class G concept... (it never worked for me and clipped at 25V with a supply of +/-20 and +/-40V... Never got to the bottom of it). I do have one working and one broken toroid (300VA, the broken one may have a break where the primary and the 10% primary are coupled) left over from that project if you're interested
I have considered this before and couldn't work out which theory would be correct...
Originally I had transformers wound for a project with 7294's.
It's the left-over TDA7294's that led me to this question in the first place.
But those transformers were wound for the class-G application with dual supply rails (+/-20 and +/-40 DC)
But since getting them custom wired anyway, I added an extra 10% primary winding (again for 4 or 8 ohm operation). If I could wire this in reverse, instead of 'series', it would LOWER my 2x30V~ outputs to 2x27V~, which would make it suitable for 8ohm operation...
So who is correct?
I have considered this before and couldn't work out which theory would be correct...
Originally I had transformers wound for a project with 7294's.
It's the left-over TDA7294's that led me to this question in the first place.
But those transformers were wound for the class-G application with dual supply rails (+/-20 and +/-40 DC)
But since getting them custom wired anyway, I added an extra 10% primary winding (again for 4 or 8 ohm operation). If I could wire this in reverse, instead of 'series', it would LOWER my 2x30V~ outputs to 2x27V~, which would make it suitable for 8ohm operation...
So who is correct?
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As you said - consider the extra winding to have two ends C & D.Consider the main winding to have two ends A & B.
If the two windings are wound in the same direction and you connect B to C you will get V1 + V2 out of A and D.
If on the other hand you connect B to D you will get V1 - V2 out of A and C.
Try it with the two current secondaries.
If you connect the two centre wires together you get twice the voltage out.
If you connect the second secondary the other way around you get nothing.
DO NOT CONNECT THE SECONDARIES IN REVERSE PARALLEL.
If the two windings are wound in the same direction and you connect B to C you will get V1 + V2 out of A and D.
If on the other hand you connect B to D you will get V1 - V2 out of A and C.
Try it with the two current secondaries.
If you connect the two centre wires together you get twice the voltage out.
If you connect the second secondary the other way around you get nothing.
DO NOT CONNECT THE SECONDARIES IN REVERSE PARALLEL.
I will measure the diameter of the 20V windings and re-***** (ok, let's make it: rethink) if I want to consider using it for this.
Maybe it's better to use a pair of 60VA 12V halogen transformers for this test amp. Or a pair of 14v~ 50A (!) transformers that are rotting away at work. No chance of heating up those 10Ga windings
Small world... Leaves the question whom you might be... Don't have a clue...
My standard name was not allowed or taken... Don't recall.
I mainly am trying to use up what I have lying around (like 500,000uF of 50V NOS caps).
But if you have something like a 2x20V 250-300VA for a stereo TDA7294 amp @ 4 ohm or a 500VA for two of those boards and want to swap, PM me!
Ideally this would be upped to 2x25V for when an 8ohm load is used by fiddling with an extra primary winding
Maybe it's better to use a pair of 60VA 12V halogen transformers for this test amp. Or a pair of 14v~ 50A (!) transformers that are rotting away at work.
No chance of heating up those 10Ga windings Small world...
Leaves the question whom you might be... Don't have a clue...My standard name was not allowed or taken... Don't recall.
I mainly am trying to use up what I have lying around (like 500,000uF of 50V NOS caps
).But if you have something like a 2x20V 250-300VA for a stereo TDA7294 amp @ 4 ohm or a 500VA for two of those boards and want to swap, PM me!
Ideally this would be upped to 2x25V for when an 8ohm load is used by fiddling with an extra primary winding
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4 or 5 iirc.
No, the thread titles of the suggestions did not seem to reflect my specific question.
If I was wrong in assuming this, I sincerely apologize.
I did do a Google search before posting, but it also did not turn up a clear-cut answer.
Obviously I know that it is unlikely that my questions have not been answered before somewhere on the net, and I always *try* to find out before posting here or on any other forum. It is very possible that I am simply not that good at searching because of e.g. not using the correct terms.
So despite apparently giving the impression that I am a lazy git that just posted here so I wouldn't have to search for myself
As for your answers: thanks! It has been made clear that I can do what I suggested, as long as I simply watch the temperature of the transformer somehow. If it gets to hot during normal conditions running 4 ohm speakers, I will have to stop using it at 4 ohm.
Whether I will actually go this route has yet to be determined. I have some time before I can complete the PCB's (parts not in yet), so I may find another solution before then.
Well, forums are meant to be a place to ask questions that you may have. I wouldnt feel bad about it.
I truly dont know why others would take offence to your post, it can simply be ignored by those who do not wish to answer.
Good luck, I hope you get your amp up and running soon.
Kirchoffs Law.
Kirchhoff's Voltage Law
It works just as well with AC as it does with DC as long as everything is in phase.
There are additional complications with AC when inductive components become involved but we are talking about simple AC here with two transformer windings.
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