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Old 16th October 2011, 06:13 PM   #1
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Default Transformer recommendation

Hello,

I would like to build a LM1875 amp. Actually, I already built one from a design in MAKE magazine, but it was a failure because it was much quieter than I thought it would be, plus it overheats.

The design called for a RadioShack transformer, 12.6V per rail and only 0.5A. I really don't know what I'm doing, but I realize now that transformer sucks. I think I need more power. But I don't want to spend more than $50 on it. Is there anything you can recommend that would be more suitable?

Thanks!
Mossen
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Old 16th October 2011, 07:06 PM   #2
Mooly is offline Mooly  United Kingdom
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Checking the data sheet for the LM1875 shows a "recommended" supply of around -/+25 volts... this comes back to your earlier question ... and so an 18-0-18 Vac transformer such as this,
MULTICOMP|MCTA100/18|100VA TOROIDAL 2X18V | CPC

would allow for a a compact stereo power amplifier (two LM1875's) to be constructed.

The 18Vac would give approx 25 volts after rectification and smoothing (18 * root 2 to calculate) So that gives a -/+25 volt supply.
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Old 16th October 2011, 07:24 PM   #3
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Thanks Mooly. One more thing: As I am considering transformers, what other things should I be looking for, aside from whether it meets the power requirements? Is there any other specs that will tell me the quality or performance of it? Any brands you tend to go for over others? Any to avoid?

Thanks!
Mossen
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Old 16th October 2011, 08:17 PM   #4
gootee is offline gootee  United States
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A link to your schematic would be helpful.

Here it is, I think:

makezine.com: Squelette, the Bare-Bones Amplifier

And here is a discussion thread with the author/designer:

LM1875 integrated Chip Amp with external PSU - Page 3 - AudioKarma.org Home Audio Stereo Discussion Forums

Dowload the datasheet: http://www.national.com/ds/LM/LM1875.pdf

Here is the power supply schematic:

http://makezine.com/images/23/MAKE23...pPwrSupply.pdf

It looks like you used the wrong transformer. They show a 25.2VCT (center-tapped), 2 Amp transformer. For that, you could use a Hammond 185e24 or equivalent, which is shown at Mouser Electronics - Electronic Component Distributor 546-185 for $27.75 for qty 1. That one is rated at 3.3 Amps max, instead of 2A. Or you could use the 546-266L24 2-Amp model for $29.66. And there are others that would work. Since you don't need the dual secondaries, you could tie them together in the middle, to have a center-tapped transformer.

Here is another guy who built one:

LM1875 Chipamp

I would probably use a different schematic, and use a power supply that could produce +25v/-25v or so, as Mooly suggested, so you could get 25 Watts per channel instead of 10 Watts per channel, into 8 Ohms.

Last edited by gootee; 16th October 2011 at 08:19 PM.
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Old 17th October 2011, 04:51 AM   #5
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Default Power supply

Yes, that's the one I was trying to build. I had it working, but it had problems as I mentioned above. Plus I hard-wired the whole thing and for this next iteration I'm going to etch a board (something else I haven't tried doing yet ). So I'm trying to make it again, except more robustly. Also, if you look at the power supply schematic, you'll notice there's a 500mA fuse to the transformer. This is why I said the amp operated on 0.5A. Is this correct assumption? Why would the designer limit a 2A transformer to 0.5A? Also, (assuming the fuse wasn't there) what would prevent the circuit from drawing more current than 0.5A? The internal circuitry of the amplifier itself? Schematic is here: http://makezine.com/images/23/MAKE23...pSchematic.pdf

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Old 17th October 2011, 06:26 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mossen View Post
Thanks Mooly. One more thing: As I am considering transformers, what other things should I be looking for, aside from whether it meets the power requirements? Is there any other specs that will tell me the quality or performance of it? Any brands you tend to go for over others? Any to avoid?

Thanks!
Mossen
I would always buy from a reputable source. Can't really quote you any brands as I tend to use either Farnell (CPC) or RS for parts.

Look at the "regulation" figure of the transformer.
The rated voltage is always quoted under the transformers rated load. Off load (nothing drawing current) and the voltage rises by the "regulation" figure. So the 18 volts could be up 8% higher for the one I linked too. That means the -/+ 25 volts is nearer -/+ 28 volts.

Usually the larger the transformer (a bigger VA rating) and the better (lower % figure) the regulation.
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Old 17th October 2011, 12:19 PM   #7
AndrewT is offline AndrewT  Scotland
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Following on from Mooly.

If you find two transformers of the same type and same Vac and same VA but with different regulation, then generally the higher regulation transformer has used fewer materials and/or cheaper materials and this should reflect in the price.

If the price were the same I would be very tempted to go for the lower regulation. But there is hidden quality, that needs to be paid for.
A quiet transformer looks just like a noisy transformer. I can't tell them apart in a specification sheet.
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