@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?

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?

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!

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.

Okay, I'll better be on the safe side then. I want to use this amp pretty much so no time for burned IC's!

Quote from: phatt on February 07, 2011, 06:48:47 PM
Just in case you missed it,, That Input trick is not an *Effects Loop* !
It's simply a high or low input setup.
Phil.

Didn't miss that

I actually only wanted to use the high-input, and instead of using a compressor stompbox, I wanted to put it in front of it right in the amp itself. I allways miss some compression in ss-amps, so I wonder if I can solve this issue by using a (optical) compression circuit.

But I gues I changed my mind, I will go for the lo/hi input annd place the compressor in a stompbox, so i can use the compressor in other setups as well.

Actually.... which input socket should i use with this switching option? On this guts shot it just looks ike a stereo input

Thanks Joe! Do you use a regulated powersupply/ zener diodes in your K20X, or an unregulated one like in the Lead 12? Actually I use 4x 4700uF elco's in my powersupply.
As you might noticed that's what makes me a little insecure about feeding it with it's max. supplyvoltage, I don't know what the chances are that 18V will be exceeded with such a unregulated powersupply...

But I wouldn't mind a little extra headroom, in my experience it makes things sound a little beefier.

Thanks!

Now I see, there is a little arrow in the schematic that says "hey, I'm switchable"!
Well that's a good thing because I want to add a compressor before the preamp, that means that I don't need to add this 'loop' to my build.

About the 1458, I simply forgot to order the components (2x 1k and 22u) that brings the voltage down to 16V...  :loco
I run the LM1875 on +/- 18V so I thought that maybe it wouldn't harm the 1458. But I guess it is, so now I need to order these components anyway.

Hi,

I'm planning on building my first small ss-guitaramp. I've bought a LM1875 kit on ebay and want to build a Marshall Lead 12 preamp myself. There is however one thing that confuses me in the Lead 12 schematic, probably due to my limited knowledge.

Here is the schematic: http://music-electronics-forum.com/attachments/2552d1212538675-3005.gif

There is a connection from pin 5 of IC1B back to the ring of the low-input. What does it do, is it a feedbackloop? What confuses me is that actually it's just connected to ground (via the ring). Does this mean that I can just make a connection to ground? Are does it work differently?

Second question is: according to the schematic the MC1458 opamp runs at +/-16V, will it be safe to run it at +/- 18V? That is it's maximum volt rating so I don't know if that's okay.

Great, thanks alot! That makes things alot easier! 

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.

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?

Thanks rowdy, I'll do some (alot) more reading

Great forum!

@phat: yeah, I already started reading this book. I like to understand things before I start to build it, so I know what to expect. But I might as well build and read at the same time and learn along the way

@teemuk: thank you very much for your clear explanation. I like those rules of thumb!  :tu:
But if I understand this right, I first need to know what the voltagegain of the whole LM1875-circuitry is, before I can do the math?

Thank you very much. I might as well read the whole manual first, instead of just scrolling through it  

About my first question, I really wonder how people can put a few mu-amps in front of the LM1875. A mu-amp can reach a gain of 60-80. Since the chip has a max.input of like 25V and a gain of 10, it seems to me that such a preamp will distort the LM1875 way too easily! I guess the preamp gain shouldn't be more than 20-25. Or do I overlook something?

Hi,

I'm new here and quite new to the diy world. I built myself a ROG Ruby amp with some personal adjustments, now I want to build a "bigger" amplifier based on the LM1875 with a nice and simple pre-amp. Actually all I want the LM1875 poweramp to do is increase (clean) volume, I don't want it to clip or alter my tone. All the toneshaping and clipping will take place in the pre-amp. I prefer this because then I have a basic poweramp, and I only need to mod or swap the preamp to shape my tone.

Now I wonder, how much inputsignal can the LM1875 handle before it starts to clip? Can I find this out with the datasheet?
I like to get the maximum clean power out of the circuit, so I wonder if I need to stick with fet-based preamps (like those ROG ampsims) or if I can mess around with MOSFET's and mu-followers?

Next question is: in some schematics of the LM1875 you'll find an inductor in parallel with a resistor right after the LM1875.
http://www.eleccircuit.com/wp-content/uploads/2007/11/amp20w_lm1875.gif
What does it do?

Thanks!