Hello everyone!
I'm a bit of a newb but in a mid of rebuilding my old high-school project. My old transformer died so I found this toroidal transformer that I got no idea if it will even work with this amp.. does it work? yup I checked.
here's the pic
and if it will work how should I connect it?
I'm a bit of a newb but in a mid of rebuilding my old high-school project. My old transformer died so I found this toroidal transformer that I got no idea if it will even work with this amp.. does it work? yup I checked.
here's the pic
An externally hosted image should be here but it was not working when we last tested it.
and if it will work how should I connect it?
It may not be suitable. The LM1875 has a maximum rating of 60 volts dc.
You need to know if your LM1875 circuit is configured as a "single rail" amplifier which means there is just one positive supply voltage. The amplifier would be AC coupled with a large capacitor to the speaker. If it were single rail you could use one half of the tranny to get 40 volts DC. You would still need to check the rest of the components (caps) were rated for this voltage.
Or, is the amp DC coupled with "split" supplies where there is a minus and a plus voltage rail. If it is then this tranny won't do.
You need to know if your LM1875 circuit is configured as a "single rail" amplifier which means there is just one positive supply voltage. The amplifier would be AC coupled with a large capacitor to the speaker. If it were single rail you could use one half of the tranny to get 40 volts DC. You would still need to check the rest of the components (caps) were rated for this voltage.
Or, is the amp DC coupled with "split" supplies where there is a minus and a plus voltage rail. If it is then this tranny won't do.
I used TI's dual "split supply" schematic.
4.7uF
47uF
1000uF all 50v
.22 cap is 63v
EDIT: old one got thrown away last year
it was huge and old.. center tap.. dont remember the specs at all
4.7uF
47uF
1000uF all 50v
.22 cap is 63v
An externally hosted image should be here but it was not working when we last tested it.
EDIT: old one got thrown away last year
it was huge and old.. center tap.. dont remember the specs at allAttachments
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OK, so spilt supply means you need an absolute max of -/+ 30 volts DC. Knock that down a little for a good safety margin to say -/+25 volts DC and that means you need either a centre tapped tranny or one with two separate windings of -/+18 volts AC.
Very strange that old tranny failed, particularly if it was a big 'un If it had a thermal fuse in it then could there be a short on your amp somewhere that overheated it ?
Very strange that old tranny failed, particularly if it was a big 'un
If it had a thermal fuse in it then could there be a short on your amp somewhere that overheated it ?
alright what about VA ratings?? 160VA is good?? or can I go 120??
n whats the smallest I can go?? my enclosure is about 45mm to 50mm so size does matter
http://www.rapidonline.com/electric...oroidal-transformer-120va-0-18v-0-18v-88-3816
???
n whats the smallest I can go?? my enclosure is about 45mm to 50mm so size does matter
http://www.rapidonline.com/electric...oroidal-transformer-120va-0-18v-0-18v-88-3816
???
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Should have asked. Is this a stereo or mono amp ?
The trouble with smaller transformers is that the regulation is poor. That figure (10% for the one in your link) means that at light loads (which is 95% of the time) the output voltage is 18 volts PLUS the regulation figure. So you have nearer a 20/0/20 vac tranny which gives -/+28 volts dc. The bigger the tranny and the lower the regulation figure.
I would say 120va was a good choice for the LM1875
The trouble with smaller transformers is that the regulation is poor. That figure (10% for the one in your link) means that at light loads (which is 95% of the time) the output voltage is 18 volts PLUS the regulation figure. So you have nearer a 20/0/20 vac tranny which gives -/+28 volts dc. The bigger the tranny and the lower the regulation figure.
I would say 120va was a good choice for the LM1875
Hi,
It depends on conflicting design philosophies what is
the ideal VA for the transformer and conflicting ideas
about the chips real output power into real speakers,
and that depends on the heatsinking arrangements.
For two chips 120VA is plenty enough, too much IMO.
You certainly would get away with quite a bit less. 60 -
80VA if squeezed for space. It depends on the application.
rgds, sreten.
It depends on conflicting design philosophies what is
the ideal VA for the transformer and conflicting ideas
about the chips real output power into real speakers,
and that depends on the heatsinking arrangements.
For two chips 120VA is plenty enough, too much IMO.
You certainly would get away with quite a bit less. 60 -
80VA if squeezed for space. It depends on the application.
rgds, sreten.
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Thank you for the info guys
Well as it goes for the size.. I might have to order custom tranny.. bit more money but its worth it. It's gotta go in U1 style rack mount case I got from work. [from a digital PSU]
SO here it is..
18v-0v-18v 120VA 4A
Also I had to down size on caps on power supply.. went from 4x4700uF to 4x1000.. just because of the size issue.. how wold that affect the functionality of the amp?
As for bride rectifier.. Can I use MURH840CT or GBU4G?? (trying to work with whats available at work)
Well as it goes for the size.. I might have to order custom tranny.. bit more money but its worth it. It's gotta go in U1 style rack mount case I got from work. [from a digital PSU]
SO here it is..
18v-0v-18v 120VA 4A
Also I had to down size on caps on power supply.. went from 4x4700uF to 4x1000.. just because of the size issue.. how wold that affect the functionality of the amp?
As for bride rectifier.. Can I use MURH840CT or GBU4G?? (trying to work with whats available at work)
Both those rectifiers are OK. The MUR would need four to make a bridge due to its commoned pinouts.
Caps. If you mean 2000uf per rail then its a bit on the low side, although dare I say it, perfectly adequate given correct construction (wiring) of the amp. Smaller value reservoir caps aren't all bad, they make life easier for the transformer and bridge, and they minimise really high current charging spikes. The downside is the overall ripple is higher at any given loading on the supply.
Caps. If you mean 2000uf per rail then its a bit on the low side, although dare I say it, perfectly adequate given correct construction (wiring) of the amp. Smaller value reservoir caps aren't all bad, they make life easier for the transformer and bridge, and they minimise really high current charging spikes. The downside is the overall ripple is higher at any given loading on the supply.
yup 2000uF per rail.
if you look at this kit Non-Inverting LM1875 Stereo Kit | Chipamp Electronics
that PSU using 1500uF per rail it seems..
if you look at this kit Non-Inverting LM1875 Stereo Kit | Chipamp Electronics
that PSU using 1500uF per rail it seems..
that can't be right.
VA comes from Vac times Amps AC, i.e. [18+18Vac] * 4Aac = 144VA
the minimum smoothing capacitance depends on the pass band required from the amplifier.It seems so
I think 2000uf per rail is fine tbh. Massive oversized caps have become as much a "fashion" thing as anything else over recent years. Not every in every case but often...
If the amplifier is expected to pass 20Hz to 20kHz into an 8ohms speaker then 20mF is usually enough for most builders/listeners. Many accept 10mF and a few settle for 4m7F
2mF would seriously restrict the low bass performance into 8ohms speaker and even more so into 4ohms speaker.
In what way ? The PSU caps are not part of the "output circuit", there is no high pass filter formed.
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