So I need a 300VA transformer with 20V secondaries and either a single 230V primary or two 115V primaries? This is getting expensive!
Update: I found a 330VA toroid with 25V secondaries on Farnell for £27, which isn't too bad. Couldn't find anything suitable with 20V secondaries. URL follows...
http://uk.farnell.com/jsp/search/productdetail.jsp?sku=9530770
Oh, and I got the virtual ground thing after all. I just didn't know it by name.
Update: I found a 330VA toroid with 25V secondaries on Farnell for £27, which isn't too bad. Couldn't find anything suitable with 20V secondaries. URL follows...
http://uk.farnell.com/jsp/search/productdetail.jsp?sku=9530770
Oh, and I got the virtual ground thing after all. I just didn't know it by name.
Oh yeah, forgot about the 44V thing. It's bloody impossible to find the right transformer for this thing >_<
Hi,
You could use 2 smaller transformers in series (watch the phase and they should be identical) to give you the split supply you need. They don't need to be toroids - cheap EI core will do.
You could use 2 smaller transformers in series (watch the phase and they should be identical) to give you the split supply you need. They don't need to be toroids - cheap EI core will do.
Hi,
15Vac transformer.
add 6% mains tolerance.
add 7% transformer regulation.
Multiply by sqrt(2).
subtract one diode drop.
maximum voltage from a 240:15 transformer is 23.4Vdc.
a 240:15-0-15 will give +-23.4Vdc
a 230:15-0-15 will give +-24.4Vdc
Both of these are over the 44V absolute max chip voltage.
Go and check the datasheet.
15Vac transformer.
add 6% mains tolerance.
add 7% transformer regulation.
Multiply by sqrt(2).
subtract one diode drop.
maximum voltage from a 240:15 transformer is 23.4Vdc.
a 240:15-0-15 will give +-23.4Vdc
a 230:15-0-15 will give +-24.4Vdc
Both of these are over the 44V absolute max chip voltage.
Go and check the datasheet.
So what is it going to be? A single supply with virtual ground as in the schematic? Or a split power supply with a "real" ground?
The datasheet is very helpful. For single supply with transistors a 24 V 50 VA transformer is recommended. For a split power supply that leads to 2 x 12 V. The configuration with transistors (BD907/908) is given with 30 W @ 0,5 % THD into 4 Ohm. Your schematic is a bridged version of that, so you can expect about 120 W, if you use a 200VA transformer.
Yes, and don't forget the heatsink. 😉
The datasheet is very helpful. For single supply with transistors a 24 V 50 VA transformer is recommended. For a split power supply that leads to 2 x 12 V. The configuration with transistors (BD907/908) is given with 30 W @ 0,5 % THD into 4 Ohm. Your schematic is a bridged version of that, so you can expect about 120 W, if you use a 200VA transformer.
Yes, and don't forget the heatsink. 😉
So this is pretty much impossible to power without using a regulated supply? So far nobody has actually come up with a working solution - either there is too high a voltage over the amps or the transformer they post can't be sourced.
Can someone head over to http://uk.farnell.com/ and post me a link to something suitable, because so far I can't see anything.
Can someone head over to http://uk.farnell.com/ and post me a link to something suitable, because so far I can't see anything.
AndrewT said:
Very good Andrew, but surely the diode drop should come before the multiply by sqrt(2). And yes that's just sqrt(2) not 2* sqrt(2).
As for the transformers, part number 953-1947 if you wire the secondaries in series, part numbers 953-1971 or 953-1998 if you wire the secondaries in parallel. I would go for the series connection as parallel can be dodgy if the windings are not perfectly matched.
I have a dead 2x500W audio amplifier that my old house mate left behind, which I've opened up. Jackpot! There's a giant toroidal sat right in the middle. Specs are as follows:
Primary: 230V AC
Secondary: 71-0-71
Secondary: 71-0-71
Secondary: 12-0-12
Secondary: 15-0-15
No idea what the power rating is but it should be hugely sufficient considering where it came from. I'm assuming the two 15V rails should just about run this amp. This thing is a pain in the **** to deal with though, it weighs about 10kg!
Primary: 230V AC
Secondary: 71-0-71
Secondary: 71-0-71
Secondary: 12-0-12
Secondary: 15-0-15
No idea what the power rating is but it should be hugely sufficient considering where it came from. I'm assuming the two 15V rails should just about run this amp. This thing is a pain in the **** to deal with though, it weighs about 10kg!
Well there are no markings on the thing to tell me. Just a label saying the primary and secondary windings. The only identifying thing about it is a label saying PT160-21, but Google gives me nothing.
No, the whole thing is wrapped up in some sort of plastic sheath. The wires coming off the 15v aren't quite as high gauge as the ones coming off the 71v ones, but they're still reasonably beefy - a little thicker than your average PC PSU wires. Though that's to be expected.
What circuitry did the winding power in the old amp? I'm not convinced it will provide what you need.
This project can be made very easy, with some old school basic electronics, and knowing what can provide the power you want.
It's better in your case to go for a lower voltage to ensure the life of your chip. The music will still sound good with big capacitors and heavy transformer. It will be loud enough.
Regular everyday 12V automotive 6 or 10A battery chargers have a 12-0-12 (24V) transformer inside them. They normally use 2 diodes to get full wave at only 12V. Get an old charger's transformer, or buy a new charger for cheap. Use a bridge rectifier and ground the Center Tap of the transformer for your 24V. After the caps you will get over +/-18V (36V) to run your amplifier.
If you can't source those transformers, then get a Toroid that provides 2 x 12V or two 12V toroids if necessary. Don't go higher than 12V transformers, 15V is asking for trouble with your chip. If you must use a higher voltage, then build the WHOLE AMP discrete instead of driving transistors with a chip.
Have fun
It's better in your case to go for a lower voltage to ensure the life of your chip. The music will still sound good with big capacitors and heavy transformer. It will be loud enough.
Regular everyday 12V automotive 6 or 10A battery chargers have a 12-0-12 (24V) transformer inside them. They normally use 2 diodes to get full wave at only 12V. Get an old charger's transformer, or buy a new charger for cheap. Use a bridge rectifier and ground the Center Tap of the transformer for your 24V. After the caps you will get over +/-18V (36V) to run your amplifier.
If you can't source those transformers, then get a Toroid that provides 2 x 12V or two 12V toroids if necessary. Don't go higher than 12V transformers, 15V is asking for trouble with your chip. If you must use a higher voltage, then build the WHOLE AMP discrete instead of driving transistors with a chip.
Have fun
- Status
- Not open for further replies.