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Questions about LM1875 poweramp

Started by Rutger, June 09, 2010, 04:39:07 AM

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Rutger

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!

bry melvin

from the data sheet:

"Most power amplifiers do not drive highly capacitive loads
well, and the LM1875 is no exception. If the output of the
LM1875 is connected directly to a capacitor with no series
resistance, the square wave response will exhibit ringing if
the capacitance is greater than about 0.1 μF. The amplifier
can typically drive load capacitances up to 2 μF or so without
oscillating, but this is not recommended. If highly capacitive
loads are expected, a resistor (at least 1Ω) should be placed
in series with the output of the LM1875. A method commonly
employed to protect amplifiers from low impedances at high
frequencies is to couple to the load through a 10Ω resistor in
parallel with a 5 μH inductor."

Rutger

#2
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?

phatt

Answer to your Q's is best coverd by you doing 2 things.
Read *Teemu's Book* http://www.ssguitar.com/index.php?topic=711.0
And build stuff and don't panic about those sorts of things. :D

It will slowly all come together.
When I started out I just kept reading screads of stuff,,, and over time all the bits I missed came together by just experimenting with simple low voltage stuff.

Hard to kill yourself with a 9Volt battery and a few fets. 0:)
Phil.

teemuk

QuoteNow I wonder, how much inputsignal can the LM1875 handle before it starts to clip? Can I find this out with the datasheet?

In this case, I don't think so.

The voltage gain of the LM1875 stage is defined by external components and additionally the power supply voltage (which is a limit for the output signal's voltage swing, and thus output power as well) can be rather arbitrary too, depending on the design.

There most likely should be a graph illustrating how much output power to some specific load the chip can produce when using a specific power supply voltage (assuming the power supply is properly rated).

Using that graph, you can do a little math based on the basic Ohm's law and P=UI principles, to find out the maximum voltage swing (before clipping). Then divide that figure by the voltage gain of the circuit you're using and you'll have the "input sensitivity", the amplitude of the signal producing the maximum voltage swing before clipping. Then, if need, you can do some basic math to convert that from peak rating to RMS.

As a crude rule of thumb, and to skip math, I usually assume the maximum output voltage swing is about 1 - 5 volts less than the actual power supply rail voltage, depending on design, of course.

If yuo are assured the chip can produce it's maximum quoted output power just figure out how much voltage that will be across the load and divide by the voltage gain.

Rutger

@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?

rowdy_riemer

Learn how op-amps work and how you set voltage gain via negative feedback. This will help you understand how to look at the schematic(specifically the resistor ratios in the feedback path) to see what the voltage gain is.

Rutger

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

Great forum!

teemuk

#8
A quick link for particular theory:
http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

Two points worth mentioning:
The example circuit omits the AC coupling capacitor from the negative feedback voltage divider from output to inverting input. That cap is typically in series with R1 (refer to example circuit). That cap is an RC hi-pass filter and there to ensure that voltage gain at frequency of 0 Hz (DC voltage) is unity. Basically, in typical power amp designs that cap is large enough in value not to form a considerable filter at audio frequencies and you can simplify the gain calculation a lot by removing its effects from the equation. This helps to simplify things a lot.
The other feedback circuit resistor (R2 in the example) might have a parallel AC bypass cap that forms an RC low-pass circuit. It will usually be low enough in capacitance that it only affects frequencies above audio bandwidth. Again you can often remove its effects from the gain equation to simplify things.

phatt

#9
Hi Rutger,
            Try this for a quick run down.
http://www.electronics-tutorials.ws/opamp/opamp_1.html

Google is the go,, Try "basic op amp tutorial" in a search,, you will get heaps of links. Some stuff is not easy to grasp at first but you are bound to find one that's well written and makes sense for the novice.

It will of course help if you have a basic grasp of electricity, magnetics and basic physics.:tu:
Phil.
Edit ; LOL Arrh Teemus' beat me to it  ;D

DJPhil

Yet more reading material.

These are the first two sizable documents I found on opamps when I began working with them. TI puts out some great stuff, and they do an excellent job of keeping their pdf files small so they're easier to use on mobile devices.

Op Amps For Everyone - This 464 page monster covers everything opamp, and the second edition is available in print form. TI distributes the first edition for free for people just like us.

Handbook of Operational Amplifier Applications - This is a more historic document resurrected from old application notes from the 60's crafted by Thomas R. Brown (co-founder Burr-Brown). There are some very interesting insights here into the early limitations of opamps.

Hope you find those useful. :)