Trafo for 15W with an LM1875

[Rutger]

Hi,

I ordered an LM1875 kit to use in a guitaramp. I have a speaker which I want to use that rates 15 Watt max. To prevent the speaker from blowing out, I don't want the amp to put out more than 15W.

According to the datasheet the LM1875 poors out 20W with a 18V trafo. Is that in theory or is that the real output power? In that case, what trafo do I need to use to keep the power at max 15Watt? 15V?

[J M Fahey]

Hi Rutger.
Just build the regular 20W amp and tell us something about your speaker.
Size, impedance, what kind is it, maybe a couple pictures.
If you play clean, you won't blow it; if you play punk or thrash full tilt, a 15W amp will kill a 15W speaker, so let's sort that first.
Anyway I'll tell you how to safely limit an amp's power to match it to a smaller speaker.

[Rutger]

Thanks!

The speaker is a Celestion Super 8:

General specifications
Nominal diameter  8", 203mm
Power Rating 15W
Nominal impedance 8Ω  
Sensitivity  93dB
Chassis type Pressed steel
Voice coil diameter  1", 25.4mm
Voice coil material Round copper
Magnet type Ceramic
Magnet weight 6oz, 0.17kg
Frequency range  100-5000Hz
Resonance frequency, Fs  105Hz
DC resistance, Re  6.5Ω

I mostly play funk and bluesrock, some 70's rock at the most. I need this amp mainly for practice so there isn't much chance that I'll play this amp at max all the time. I just want to make it foolproof.

[J M Fahey]

Excellent speaker, I doubt you can kill it with your amp.
I had thought you were using a reclaimed "15W max" one, pulled from a stereo or found at the bargain bin .
Nothing wrong with that, but *this* one is meant specifically for guitar.
Build the regular kit, as specified.

[Rutger]

Great, thanks alot! That makes things alot easier! 

[Rutger]

Hi again,

It took a while but I've gathered all the components I need and started building the amp. I use a Talema toroidal 30VA 2x18V transformer. I measured the actual secundairy AC and that is ±20V, so I expected it to be like ±22V DC at max after the rectifier. A little higher than intended but that´s okay. Guess what... it is ±27,5V!

How is that possible? It´s like there are no losses in my powersupply. I use a standard diode rectifier, nothing fancy.

[Rutger]

Ouch... I see that I made a huge calculation mistake according to the losses in the rectifier. I calculated with a factor of 0,7 instead of a voltage of 0,7V. I thought I would get a railvoltage of 20x1,4x0,7=19,6V. I came up with 22V as a 'maximum' expected voltage.

So... do I have a real problem now?
The gainclone can handle it, and I just need a little correction on the preamp powersupply to make the railvoltage right for the preamp. As far as my knowledge goes, I just have a greater dynamic range and the max. output power is now limited by the power transformer.

I wonder what the max. output power will be in this situation? My power transformer can deliver 830mA per rail @ 18V. Speakerload is 8 Ohm.

I'm sorry that I ask you to do the math, but calculating electronics is some time ago for me, and the technical english on the web makes it very hard for me to understand. But I would really like to learn how!

[phatt]

Hello,
        With a Fullwave bridge? Then secondary ACV x 1.414 will give you the working DCV.
So 20 x 1.4 = 28VDC.
Save this and Print it out and paste it to your workbench,,, it saved me heaps many times over.
Phil.

[Rutger]

Ah, thanks alot! All the formula at one page, just what I needed!

Yes, it's a fullwave bridge rectifier. So that means that I have 0,6x830=500mA current per rail. When I look at the LM1875 datasheet that would be more than enough to get 30Watt output power at a +/- 27,5V rail. Am I right?

[J M Fahey]

Short answer, no math:
18+18VAC -> +/-25V DC rails -> 25W RMS into 8 ohms (just read LM1875 Datasheet)
That's it, in a nutshell.

[Enzo]

Assuming transformer current can keep up.

[Rutger]

@Fahey: thanks, but that's not exactly what I meant.
Maybe my question wasn't clear enough. My actual question is what Enzo says: can the transformer current keep up?
According to the formula it can. But then again it confuses me that I'm told that I need a 24VA for 15W output and now it seems that 30VA is more than enough for 25-30W output...? Aren't there any loses then?

[phatt]

Hi Rutger,
             Be carefull how you interpret OUTPUT POWER IN WATTS.
*Without any reference to the load it drives Watts means nothing*

Saying 20 Watts might at a glance seem quite stunning but unless a LOAD is clearly defined it can be very dissapointing in real world sound levels.

LM1875 data is not clear but my guess is that it means 20 Watts into 4 OHMS which in real world wattage terms converts to 11/12 Watts into 8Ohms.

As luck would have it I have a tiny LM1875 Cheap NoName Bass Amp on my bench right now and it's never going to be a loud Amp.
(guttless is the kindest thing I can say about it )

Its PSU developes 21-0-21VDC from a 30VAC C-tapped Transformer.
If you want to make it worth your while shoot for a DC of 60VDC.
30-0-30VDC rails to get max grunt.
(just check the MAX data though.. others here may know better)

Max va rating from tranny is proly not to much an issue unless you wish to use it for program music,, i.e. Disco Bass lines from a synth.

The LM3886 suffers from similar bad explanations in the datasheet.
Often advertised as a 60 WATT Chip Amp but in truth it's more like
35/40Watts into 8 Ohms.

That 40 watts quickly evaporates when the supply is lower.

To attain full power from most of these chip poweramps you need the MAX working Voltage otherwise you are often wasting money.
Cheers,,, Phil.

[Rutger]

Well, according to the datasheet the LM1875 does poor out 20 Watts @ 8 Ohms load.
But I can imagine that you need to get the max out of these gainclones to get it to work properly. So it's not a bad thing that my railvoltage turned out to be much higher than I wanted in the first place, it's now 50-55VDC The max rating for this chip is 60VDC.

Luckilly I don't need that much power anyway, because this amp is meant to be used by my students during guitarclasses. I'm just curious if the powertransformer will hold when I do set this amp at max volume. 

[Rutger]

New problem... I changed the powersupply for the preamp to get from +/-27,5V to +/-15V. In the original schematic there are a 1k Ohm/22 uF, I replaced them by 3k9 Ohm / 47 uF (had those lying around). According to my calculation with the Duncan tool 3k9 would be enough to get the voltage under 16V, but now I measure 27,3V, hardly any lower than the original voltage...

What did I do wrong?
I have a hard time dealing with the powersupply.