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Amperes required by LM1875 Guitar Power Amp

Started by exztinct01, March 15, 2016, 07:01:29 AM

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exztinct01

Suppose I build an LM1875 power amp, how much current rating should my traffo have if I run it at +-25 volts? how bout at +-12v?
Let's say I'll be using the circuit in the ESP pages (project 72)
please don't give me VA, I don't fully understand it yet plus traffos sold here are rated in amperes not VA
~ Stephen

J M Fahey

For +/-25V rails and 8 ohms speaker, transformer rating is 18+18VAC , maybe you find it labelled as 35 or 36V CT (center tapped) , it's the same, and current rating must be 1A to 1.5A

VA is no mystery, it's simply V * A , here we are going from 36*1=36VA to 36*1.5=54VA
If your amp is stereo, double the current in A or the VA rating (V stays the same).

If you get a somewhat lower voltage for a *very*  good price, or pulled from a dead amplifier (old junked 80's HiFi amp or receiver) for free, go for it, you'll lose a little power , almost unnoticeable ... and nobody can argue the price ;)

FWIW when somebody brings for repair an old Sansui/Technics/Yamaha/whetever old faithful amp and balk at the price, (hey!!! I paid $89.99 for it, NEW!!!! ... in 1989 ... >:( ), they leave it.
I pull the transformer and sometimes the heatsinks, also the fancy knobs, and junk the rest, such tgransformers can always be used for projects similar to yours.
I build nothing with them, although might help me to replace a dead one, but donate them to enthusiastic Musicians who want to build "something".

As of powering it with 12+12V (I suppose DC) amp will work but it won't longer put out 20W RMS , think 5 or 10W depending on load, is that fine with you?

A very common 12+12VAC 1A transformer will give you raw 16+16V rails and around 15W power, not bad.

exztinct01

#2
 i do have transformers taken from an old amp and it has about 3 output windings. Is it okay to leave some windings unused? (since I'll only be using the 18+18 ct)
I already have these:
LM1875 chipamp
35A bridge rectifier
phenolic paper copper clad (is this ok or should i use fr4)
36vac ct traffo
large heatsink taken from a 300W amp connected to TIP41's and 42's
resistors (carbon film only)
caps (mylars and electrolytics)

please comment if these are enough

Edit: i don't know the fuses to use for the ps. And I already bought the Eminence Patriot Red White and Blues. I'm planning to use my amp with two speakers in parallel but impedance would not allow. Should I go for stereo?
~ Stephen

J M Fahey

Quote from: exztinct01 on March 15, 2016, 05:55:28 PM
i do have transformers taken from an old amp and it has about 3 output windings. Is it okay to leave some windings unused? (since I'll only be using the 18+18 ct)
IF the main windings were 18+18 (hint, check wire diameter, I mean copper wire not output plastic covered cable) meaning thickest wire, other secondaries are not that important and can be ignored.
Tell us: what was the original power amp rating?
if a 30/60W MI amp or, say, 25+25W Hifi, fine.

You *may* build 2 25W power amops inside the same chassis, if Power Supply and Heat sink agree.

You might build it on the table, un cased, just 1 channel first, and test it.

Then you decide whether to build it 25W or 25+25 .

QuoteI already have these:
LM1875 chipamp ...check
35A bridge rectifier ... check
phenolic paper copper clad (is this ok or should i use fr4) ...check
**I use nothing but fr4, so I can drill with regular 1mm bits, which last only 50 holes or so with glassn fiber; hard metal bits are incredible fragile and I'm sick of replacing them all the time**
36vac ct traffo ... check wire diameters
large heatsink taken from a 300W amp connected to TIP41's and 42's ... check
**parts: buy what you need, regular parts, no magic dust**
resistors (carbon film only)
caps (mylars and electrolytics)

please comment if these are enough

QuoteEdit: i don't know the fuses to use for the ps. And I already bought the Eminence Patriot Red White and Blues. I'm planning to use my amp with two speakers in parallel but impedance would not allow. Should I go for stereo?
measure transformer EI iron stack, please use mm if at all possible.

exztinct01

Quote from: J M Fahey on March 15, 2016, 07:50:18 PM
Tell us: what was the original power amp rating?
if a 30/60W MI amp or, say, 25+25W Hifi, fine.
I can't tell yet since I left it at home but the amp is a karaoke stereo amp using two TIP41's and two TIP42's
The traffo has 3 output winding, 18-0-18, 0-9, and 0-5-20. Looking at it, I think it can power at least two 100W channels (I doubt if it's RMS since the amp's cheap)

Quote
You might build it on the table, un cased, just 1 channel first, and test it.

Then you decide whether to build it 25W or 25+25 .
can i build it on a solderless breadboard (except the PS and the chipamp itself)

Quote
measure transformer EI iron stack, please use mm if at all possible.
I'll do it later at home

Should I go with panel mount fuse holder for the mains fuse and pcb mount for those between the ps and the powr amp?
~ Stephen

Bear

Timely thread for me.  I'm looking at some pt iron I got when my local RadioShack liquidated, and trying to figure out how to use it.

The big two for current are a 25.2v ct 2A and a 12.6v ct 3A.  The 25.2v should be plenty good for a home-use chip amp power amp. 

The lower-volt 12.6v doesn't obviously serve any project I can find, though the VA math is just fine for similar low power use thanks to the current. Ideas?

exztinct01

#6
To J.M.:
I can't measure that iron stack. Is that the part where the wire is being wound? I can't seem to see it. My traffo is this one

Also, besides my 35A panel mount bridge rectifier, I do have some PCB mount bridge rectifiers rated at 4A and 6A. Can I use them instead? How should I know what rating of bridge rectifier to use?

To Bear:
I think both can be used for home amps. Refer to J.M. Fahey's reply above
Quote from: J M Fahey on March 15, 2016, 12:32:33 PM

As of powering it with 12+12V (I suppose DC) amp will work but it won't longer put out 20W RMS , think 5 or 10W depending on load, is that fine with you?

A very common 12+12VAC 1A transformer will give you raw 16+16V rails and around 15W power, not bad.

Add:
Quote from: J M Fahey on March 15, 2016, 12:32:33 PM
FWIW when somebody brings for repair an old Sansui/Technics/Yamaha/whetever old faithful amp and balk at the price, (hey!!! I paid $89.99 for it, NEW!!!! ... in 1989 ... >:( ), they leave it.
I pull the transformer and sometimes the heatsinks, also the fancy knobs, and junk the rest, such transformers can always be used for projects similar to yours.
I build nothing with them, although might help me to replace a dead one, but donate them to enthusiastic Musicians who want to build "something".
I do love donations and I am an ENTHUSIASTIC MUSICIAN
~ Stephen

J M Fahey

Quote from: exztinct01 on March 16, 2016, 10:21:07 AM
To J.M.:
I can't measure that iron stack. Is that the part where the wire is being wound? I can't seem to see it. My traffo is this one
All I ask are external dimensions, I can get internal ones from an EI lamination chart , knowing 2 of them .

I need A , B , and the thickness of the piled EI sheets, you have a mounting strap around them, no big deal, I'll substract a couple mm from the thickness.
QuoteAlso, besides my 35A panel mount bridge rectifier, I do have some PCB mount bridge rectifiers rated at 4A and 6A. Can I use them instead? How should I know what rating of bridge rectifier to use?
The 4A and 6A ones should be ample, since the amp(s)  will pull 1A each.

Quote
FWIW when somebody brings for repair an old Sansui/Technics/Yamaha/whetever old faithful amp and balk at the price, (hey!!! I paid $89.99 for it, NEW!!!! ... in 1989 ... >:( ), they leave it.
I pull the transformer and sometimes the heatsinks, also the fancy knobs, and junk the rest, such transformers can always be used for projects similar to yours.
I build nothing with them, although might help me to replace a dead one, but donate them to enthusiastic Musicians who want to build "something".
QuoteI do love donations and I am an ENTHUSIASTIC MUSICIAN
Oh , no doubt, I can see that ;)
You're welcome, but remember I am in Buenos Aires, some 6000 miles away  :o

That said, maybe a local Tech junks similar unrepairable (for $$$ reasons) amps, or you can get them "untested" (might even actually work, but that's not the point) at garage sales or from a junk (unsellable)  pile at Salvation Army or even Aunt Emma's attic.
Of course, as is, you pull the useful bits yourself.

Any shipping is a deal killer, transformers are heavy, but search locally.

exztinct01

haha, I do realize that  :cheesy:

Okay, A is about 78 mm, B 64 mm, thickness is 35 mm
~ Stephen

J M Fahey

#9
Ok

1) let's deduct ~3mm from each because of strap metal thickness  plus it fits loosely, so we have:
A=75 mm ;  center leg C is 1/3A=25mm=1"

2) stack thickness is net 32mm=1 1/4"
In general, commercial EI sizes follow a definite scale, no random values in between (same as caps and resistors also follow a standard scale).

In general if American they grow in 1/4" steps, so popular values are, say, 3/4 " , 1" , 1 1/4" , 1 1/2" and so on so I'm reasonably certain about the 1 1/4" thickness value you found.

By the same token, the 1" center core width fits the standard table.

3) so your core is 1" * 1 1/4" , a very common size, but I'll calculate using mm or cm because it's way easier (when will you Infidel heathens convert to Metric?  :trouble )

4) Basic power handling for EI cores: Area (in sq. centimeters) squared.
Easy, huh?
Good iron will handle more, cheap one will handle less, but that's the general purpose estimate.

So for your core:
2.54*3.2=8.13 cm squared. Round down because of airspace between laminations: 8 cm^2 or 8 cm sq or sq cm , pick one.

basic VA (Power)  handling: 8*8=64VA .

You can easily pull 80% of that or some 50W .

A Stereo 25+25W amp, 50W total would need some 40% more, if it were subject to continuous tone testing, but on a Hi Fi amp, current demand is not continuous if you play regular Music.

And for Guitar amps?

Well, if you overdrive the amp, make the guitar squeal or feedback for an hour or two, non stop, you'll probably end with a quite hot transformer.

Under normal use, even if playing loud Rock?

No big deal.

I assume that being the highest rated, the 18+18V winding has most of the copper in that window, and other windings were low power, meant for preamp duty.

So it looks like you have a suitable PT for 2 x LM1875 amp  :dbtu:

exztinct01

Quote from: J M Fahey on March 16, 2016, 05:12:23 PM
So it looks like you have a suitable PT for 2 x LM1875 amp  :dbtu:
+1 ☺
Okay, now I can start building.
Oh wait, what size of wires should I use for the amp?
Can I use typical 22 gauge hookup wires with no shielding or is it very important to use shielded ones?
~ Stephen

phatt

#11
Just found some time to do a few pics;

This is one of those small LM1785 practice amps sold by the thousands.
The transformer sec is 12-0-12 VAC measures about 50mm x 40mm x 30mm

So the DC rails are around 16VDC.
The front panel says Casino 12watts and it's loud enough to annoy someone in the next room.
Even moreso through a large 12 inch speaker. :tu:

On most of these small amps there is only one fuse on the mains input.
I took the fuse off the PCB and mounted a separate holder on the back as I don't like the idea of mains potential on the audio PCB.
Hope it helps.

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The other pics are a couple of kits I picked up cheap but they never got used.
This might help you with layout of LM3886 power amp if you end up doing a larger build.
The PCB kit came with LM3876 chips but the supplier kindly swapped these for the LM3886. <3)
So hence the board reads 40watts.
Worth a mention; note how the heat sink has a thick base and the thinner fins radiate out from that mass. This is way better than a thin base with a lot of fancy fins.


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Phil.

exztinct01

wow, really big heatsink for the 3886
Guess I have to make some friends in the aluminum and glass supply industry here to have those for cheap
~ Stephen

phatt

The fuse for the LM1875 above is 500mA quick blow.
Don't over thunk it all,, this is a small amplifier and in rush is not really much of an issue fast blow is fine. :dbtu:
If a 500mA fuse keeps blowing then up it to 620mA,, if 800mA or more blows that tells you that you have a serious problem in the circuit,, time for deeper search. Then you use a limiter bulb tester otherwise you will destroy the whole unit. :'(
I've seen some with no fuse just a thermal cutout unit inside the transformer.

Yes that heat sink is over kill you don't need it that big but gives you a clue as to how a good heat sink is designed. BTW,, there are 2 power amps on that heat sink hence I used a larger one. 8|
Phil.

J M Fahey

Toroidals require slow fuses because of their much higher inrush current , even if unconnected to any load, it takes some power to magnetize all that iron.

A fast fuse blows on turn on.

EI are lossier so in a way somewhat self protected.

@ phatt, after originally liking that Jaycar PCB, it looks nice, I noticed input and output are within millimeters of each other, WTF?  :duh and also that output Zobel (2r7 + 100nF) are printed but not fitted.

Chipamps often work without it ... until you connect speakers with a long cable that is  8|