twin bridge rectifiers on 50-0-50 toroidal?

My project is a monoblock amp based on a single “ljm” L25 pre-built amplifier board. My initial aim will be to run a 150W 12in 4ohm subwoofer.

The only part I have in my possession is a 300VA 50-0-50 toriodal transformer (bought cheap off someone who was selling due to buying it by mistake!). I have an eBay capacitor board ordered and I intend to fit eight 4700uF 100V “Lelon” caps on it. I will use a tunnel type heatsink with 80mm fan.

For the PSU I was planning to use twin 35A rectifier bridges and follow the TNT audio “Diagram 4” PSU schematic:

psu4.jpg



However, I have since realised that my transformer has only three output wires: a 50V a 0V and a 50V but this schematic show two separate wires for the middle 0V lines.

Question - can i still use the twin-bridge PSU topography if I only have a single centre wire?

If so - do i just share the transformer’s 0V wire to one AC leg of each rectifier? And the 50V lines to the other AC leg of each rectifier?

Or have I got the wrong transformer for a twin-bridge PSU and I have to use a single bridge rectifier?

A single rectifier design is not a problem, I had just read that twin rectifiers were better, but my last amp I built used single rectifiers per channel and it sounds fine!

thanks for any advice
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
I think I understand that this Voltage is right for your amp. Your transformer VA is low for the amp, but it's all you'll be using because the speaker doesn't need 500W. I encourage you consider this when designing the fusing arrangement. This is necessary.

To feed your speaker 150W your amp needs to provide ~25VRMS.
i thought power amplifiers should have a capacitor type PSU?
The amp wants DC rails that are a low impedance source. Anyway, I think we were at crossed purposes.
 
The amp wants DC rails that are a low impedance source. Anyway, I think we were at crossed purposes.

well I certainly am not picking up on what you are trying to get me to think about? :confused:

if the amp works and down the track i want to power a bigger driver - all i need is a bigger transformer - the caps & amp will take it? or is 37,600uF too much for the transformer?

or what am i missing?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
Hypothetically..

Using other than a capacitor input supply is one way to modify the output Voltage. Before you told me that you were trying to follow an existing design, I was trying to put Voltage and current into proportion for your needs.

Your ebay friend did what many of us have done and bought a higher Voltage transformer. These better suit a higher impedance speaker.
 
Hypothetically..

Using other than a capacitor input supply is one way to modify the output Voltage.
nearly everything i know about amps I've read from Rod Elliot - i kinda really only know of one way to set up a amp build.
Before you told me that you were trying to follow an existing design, I was trying to put Voltage and current into proportion for your needs.
the 300VA is low (in prop. to caps and amp), but was it was cheap.
Your ebay friend did what many of us have done and bought a higher Voltage transformer.
well, he said he needed 40-0-40 so i guess he wanted to make 57VDC rails?

so your bottom-line message is I should ditch my mistake of buying someone else's mistake and get perhaps a 500VA 40-0-0-40 transformer? to keep everything in proportion?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
I just want you to know the details so you can be in control. I think we've all used this part here and that part there. There is a right way to do these things. A 300VA transformer doesn't stop when it gets to 300VA, you have to ensure that in your design or it catches fire. If you modify an existing design, it becomes your own custom version.

Rod Elliot is a knowledgeable guy. He shows some good wiring diagrams on his site.
 
I just want you to know the details so you can be in control.


well, can you elucidate on these details?

A 300VA transformer doesn't stop when it gets to 300VA, you have to ensure that in your design or it catches fire.


like appropriately rated fuses? e.g. in this scenario (from the TNT website):


psu2.jpg



or will it actually catch fire if i put a 600W PA speaker on the output?


could I put a thermistor on it? what else?
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
Please be aware that speakers are not light bulbs - they don't draw their full rated power all the time. The rated power is the maximum that speaker can handle long term.

If you use a transformer that is undersized in terms of VA, what you will get is voltage sag at high output volumes (power). In other words, the small transformer's output voltage won't stay where is should under a heavy load, it will sag. But will you ever really be using that much power?

What you've shown above is a well protected circuit. You just need to figure out fuse sizes.
 
Please be aware that speakers are not light bulbs - they don't draw their full rated power all the time. The rated power is the maximum that speaker can handle long term.

yes indeed!

If you use a transformer that is undersized in terms of VA, what you will get is voltage sag at high output volumes (power). In other words, the small transformer's output voltage won't stay where is should under a heavy load, it will sag.
yes, with my current 2x 175W amp - i observed this monitoring the rail voltage while driving large 4ohm speakers at full volume with bass-heavy EDM - the rails 'sagged' to 55VDC from a nominal 60VDC.

but i put this down to the large cap size of 2x 15,000uF not being "fast" enough. the toriodals are 300VA on that set up.
But will you ever really be using that much power?
doubt it - i am not putting on backyard dance partys these days :D the amp is for HT sub woofer setup...
What you've shown above is a well protected circuit. You just need to figure out fuse sizes.
thanks- once the L25 amp board arrives i will test it out on my existing amp PSU which has a 7.5A main fuse and 2A slow-blo fuses between the mains switch and the 300VA transformers...
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi hifilutining,
The bigger problem is that the transformer will be heating up over time. It's bad enough that the allowable temperature rise at rated power is something the transformer maker decides. When you draw even more current from it, it runs hotter still. How hot? Depends on run time, ambient temperature and energy losses related to output power in your transformer.

These days transformers don't normally burn as commercially made units have what is called a thermal fuse inside. It is a chemical fuse that opens when it reaches a certain temperature. Once they open, that's it for the transformer. So it may get stinking hot and smelly, but it won't burn out like they used to do. If yours does open, don't touch the transformer. It will stay hot for hours.

-Chris
 
Hi hifilutining,
The bigger problem is the transformer will be heating up over time. It's bad enough that the allowable temperature rise at rated power is something the transformer maker decides. When you draw even more current from it, it runs hotter still. How hot? Depends on run time, ambient temperature and energy losses related to output power in your transformer.

OK, but just to get this 100% straight in my head - the transformer only "works" when it is helping to convert little signals into bigger ones, right?

it doesn't just heat up when mains power is applied? and the transformer would have to be doing some serious 'little-signal to big-signal' work at 300VA?

like pretty damn loud? how long would a 150W rated 4ohm subwoofer last if the amplifier was using all of the transformers rated power?

These days transformers don't normally burn as commercially made units have what is called a thermal fuse inside. It is a chemical fuse that opens when it reaches a certain temperature.
so what temperature should i set the transformers thermo cut-out circuit at to shut down the mains to avoid yukky smells?

60'C or 100'C? or what's the norm?

Once they open, that's it for the transformer. So it may get stinking hot and smelly, but it won't burn out like they used to do. If yours does open, don't touch the transformer. It will stay hot for hours.

-Chris
OK, thanks for the tips and advice Chris!
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
it doesn't just heat up when mains power is applied?
It does a little and the amp draws a standing current regardless of signal.
so what temperature should i set the transformers thermo cut-out
These should be built into the core of the transformer where they will be most likely to be effective.
 

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