Tales From the Chip: LM1875 Audio Amplifier

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About: DIY audio, Arduino and whisky enthusiast

Intro: Tales From the Chip: LM1875 Audio Amplifier

I love me some chip amps - tiny packages of pure audio power. With just a few external components, a clean power supply and some hefty heatsinking you can get truly hi-fi quality sound that rivals complex, discrete transistor designs.

I went into a little more detail about the benefit of chip amps in my LM386 tribute - that might be a good place to start. Here, I'll dive right into what makes the LM1875 so great and how to build a simple circuit. Ride, Dobbin!

Step 1: Say Hello to the LM1875

The LM1875 ("eighteen-seventy-five") is a monster of a chip in a very unassuming package, and another much-loved chip in the DIY audio community. The official datasheet (PDF) claims the ability to drive 20W into 8Ω loads given +-25V, and up to 30W supplied with an extra +-5V of juice... and all at less than 1% THD. And rare as it may be, I can confirm the boasting in the datasheet is spot on - those figures can be reached quite comfortably in reality (given some healthy cooling).

Step 2: Pinout

The TO-220 package, with only 5 pins, is dead simple to wire up:

1 - Negative Input (-IN)

2 - Positive Input (+IN)

Standard op-amp inputs, with the positive input receiving the audio signal and the negative input tied to ground.

3 - Negative Supply (-Vee)

5 - Positive Supply (Vcc)

Here you feed the amplifier, ideally with a dual supply. It can also be driven by a single supply by tying pin 3 to ground, however performance may suffer.

4 - Output

Here's where you dine on some sweet, sweet amplified signal.

Step 3: Schematic and BOM

Here's a simple schematic for a single channel - for stereo you'll need two of these.

R1 and R2 are the gain resistors attached to the inverting input of the amplifier. The values of 22KΩ and 1KΩ work out to a gain of 23:

Gain = 1 + (R1 / R2)
= 1 + (22 / 1)
= 23 

To change the gain, simply swap R1 out with another resistor in the kohm range and plug it into the formula.

CIC1 to CIC4 are the decoupling capacitors for the LM1875. The smaller capacitor (100nF) filters out high frequency noise on the power rail, while the larger cap (220uF) provides a source of power to smooth out dips in the power supply. In a production circuit, these caps should be placed as close to the power input pins of the chip as possible. For more information, check out this surprisingly easy-to-understand article by Analog Devices on proper decoupling techniques.

Likewise C1, C2, R2 and R3 are there to filter out noise, while R5 acts as a pull-down resistor, allowing a path to ground if no signal is connected (hum reduction).

R6 and C3 form a RC circuit, a filter that removes radio frequencies from feeding back into the circuit and prevents oscillations from the speaker from returning to the amplifier.

____

BOM:

IC: LM1875

R1: 22kΩ

R2: 1kΩ

R3: 1kΩ

R4: 1MΩ

R5: 22kΩ

R6: 1Ω, 1W

C1: 10uF electrolytic (or preferably, polyester/polypropylene film)

C2: 47uF electrolytic

C3: 220nF X7R / film

CIC1, CIC3: 220uF electrolytic

CIC2, CIC4: 100nF X7R / film

___

You'll need a way to feed audio in - I harvested a 3.5mm jack from an old device and made a breakout which plugs straight into a breadboard, or you could chop the head off an old 3.5mm audio cable, stick some headers on the ends and connect it directly.

Also, you'll need the usual jumpers, wires, a speaker/dummy load and a power supply - a decent variable bench PSU that can provide +/- 30V will be useful.

Finally - a heatsink! Most class A/B chipamps require significant cooling, so get a bigger heatsink than you think you'll need and keep it around for prototyping purposes.

Step 4: Breadboard Build

So here's my breadboard...

...but DISCLAIMER!

This is not the most optimal layout - ideally, the components should be much closer together, and the decoupling caps in particular are too far from the IC pins. However, I spread it out to make it easier to understand in the photos, and to make my awkward heatsink fit. The results are fine for short periods of testing.

I put both of the power rail strips on one side of the breadboard, so I could keep space around the IC for the heatsink. This has the added benefit of making the dedicated positive, negative and ground rails easily accessible along the bottom of the board.

Step 5: Don't Forget the Heatsink!

To prepare a heatsink, first line it up on the board and mark where the hole should go to secure it to the IC. Then drill the hole, and sand the entire contact surface with very fine paper until the surface is smooth and glossy.

Next, apply a dot of thermal paste to the contact surface and position the insulating mica on top with some tweezers - try not to handle the mica with your fingers.

Lastly, use a top-hat (or "bush"), a nut and a bolt to secure the chip to the heatsink. It should be just tight enough that the IC can't be rotated around the bolt, and no tighter!

Lastly, double check that the tab of the chip is insulated from the heatsink by doing a continuity test with your multimeter - with one probe on the heatsink tab and the other on the heatsink itself. No beep = good job!

Step 6: Test It!

Check and double-check that all your connections are solid, and ensure you're sending + and - voltage into the correct rails. Set the power supply to around +-10V, stand back and switch on!

If no shocking eruption of smoke appears, you've probably succeeded. Play some music and listen to your test speaker. If your bench power supply has a built-in ammeter, you can see how much current your amplifier is drawing at any given moment - try turn up the volume to see the current draw increasing.

At low voltages, you'll likely run into clipping or other forms of distortion sooner rather than later, and at higher volumes your music will sound fairly awful. Slowly turn up the voltage - the LM1875 handles +-25V like a champ, so if you have a decent heatsink there shouldn't be anything to worry about.

Output Voltage

I ran the output into a gigantic dummy load (a 300W, 8Ω resistor) and scoped the output. With a 1kHz sinewave at 810mV peak, the LM1875 offered unto me a respectable, clean 20.15V peak (14.32V RMS) on the output - just a little over our gain setting.

Power

In terms of clean power, I make that...

Power RMS = Vrms^2 / R
= 14.32^2 / 8
= 25.63W

... just shy of 26W! Not bad at all.

At this point, I wanted to see if I could get to that mythical LM1875 30W mark, but first I needed to swap out the heatsink with something a little more reassuring...

Step 7: The Copper Monster

Check out this bad boy. It's a heatsink salvaged from an old graphics card - a pure copper monster with a high speed 12V fan built in. Perfect. I've never attached anything to it that could even make it break into a sweat. It's ugly. It's noisy. And it's very, very cool.

And bingo! At +/-30.5V with an input signal of 910mV, we get a clean 30.39 watts of power on the output. Lovely. What a chip.

Step 8: LM1875 Retail Kit

One more thing - once you've made an LM1875 on a breadboard, check out these awesome kits (sold everywhere - eBay, Ali, etc.). They're quite expertly laid out - a small, tightly designed PCB with a chunky star-grounding scheme and well placed components.

The resistors that come in the kits are a little dodgy - I'd replace them with decent resistors from Mouser or Digikey - but the chip seems to be completely legit. In fact, I was able to get not only my 30W target power out of two of these boards, but the noise level was utterly exceptional:

Using the Analog Discovery's spectrum analyser, that same 1kHz sinewave gave me a truly incredible -62.31dBc total harmonic distortion, which is around 0.07%! That's real hifi territory, although I'd need to measure that across the spectrum to be sure. However, this goes to show that the datasheet claims can definitely be achieved if you know what you're doing!

____

That's it! Next time, I'll dive right into the crown prince of the AB chip amps - the LM3886 itself.

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52 Discussions

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KumudM

6 months ago

I made this with 24 v single supply but there was too much distortion at low and mid range frequencies? what is the minimum DC voltage required to get a clean output?

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AlissonA6

9 months ago

This is awesome, I was looking for an Instructable like this for a long time (especially after burning 5 or 10 poor LM1875's haha).

You said that "It can also be driven by a single supply by tying pin 3 to ground, however performance may suffer." What do you mean with that?

Will I get some kind of distortion or the output power/volume will be reduced? Also can I use more than 25V with single supply too?

(I don't have any kind of dual supply, and it will be used with small speakers in my bedroom, so no need for a lot of power.)

*Also, I found some kits including tube pre-amps. Has anyone ever seen them? Are they fake? It seems way too cheap to be real valves...

3 replies
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abzzaAlissonA6

Reply 9 months ago

Yeah, powering an opamp (designed for a dual supply) with a single supply (tying the negative rail to ground) causes a few problems:

1. You get the potential for reduced frequency response, particularly at the low end.
2. You get increased threat of DC entering the speaker, which is normally resolved by using a large value capacitor on the output - not ideal
3. As the “zero” point is now one half of the supply voltage (say, 15V if you were feeding it 30V) you get this sudden inrush of current when the amplifier is powered on, with a huge “pop” on the speaker - this can be damaging if the voltage is high enough!

That being said, there are ways to get around all these issues - check out JohnAudioTech’s YT channel. I think he has a couple of videos on this :)

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AlissonA6abzza

Reply 9 months ago

Thank you! I've subscribed on his channel, and as I don't have any center-tapped transformer, I'll try splitting it in two "half-bridged" lines with good filtering to check whether it sounds good or not :-)

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GerhardusSabzza

Reply 9 months ago

I have two REVOX power amps out of an A 77 reel to reel recorder and the way they got around the "plop" is they connected two large 2200 microfarad caps in series across the power and ground with the centre tap as the negative of the speaker and the speaker output of the totempole to the plus of the speaker. These caps also formed the power supply smoothing circuit for every amp had their own rectifier on its P.C. board. J. Lindsley Hood when he published a Car battery P.A. amplifier said that he would rather trust a quality capacitor than a circuit where a transistor can turn sick and connecting the full supply to the poor speaker and cause its cone to become wall paper! The only audio loss would be at the lower side of the audio spectrum! Those were the days when those caps were bulky and expensive! I have found the 2200 uF is ample and the loss could not be heard.

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throbscottle

9 months ago

Thanks for the informative instructable - I knew nothing about these "chipamps" until today! Also thanks for the great link to the Analog .pdf on decoupling. Just what I needed!

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SpikePlayz.

9 months ago

So I took my old PlayStation ‘bluetooth’ (it has a USB dongle which you have to plug into the PS4 or computer) apart and I was wondering if it would be possible to make some Bluetooth speakers out of them. Obviously since they are from headphones the speakers aren’t very loud. Should I use the LM1875 to amplify the signal? I gather I wouldn't just be able to attach bigger speakers.

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throbscottleSpikePlayz.

Reply 9 months ago

You could but you will have to adjust R2 and R1 to get a fairly low gain. I don't know the power of headphones but I think it's in the order of a few hundred milliwatts, so I'd try a gain of 10 to start with and increase or decrease from there.

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w00blyn

9 months ago

Awesome instructable! I really appreciate your breakdown of the circuit and what everything is for. What does R4 (1Mohm) do? Does it allow higher impedance devices on the input (piezo pickup for example)?

Looking forward to the LM3886!

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throbscottlew00blyn

Reply 9 months ago

It looks like it's there to ensure C1 always gets discharged

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Mike-Mat

9 months ago

OK, I'm not an electronics guy. Probably a newbie question. How do you add volume and tone controls to something like this?

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abzzaMike-Mat

Reply 9 months ago

Great question! For volume you'd use a single gang (mono) or dual gang (stereo) potentiometer - a variable resistor. This is the classic volume knob, which you typically include just after the audio input (image attached). The more anti-clockwise you turn the potentiometer, the greater the resistance introduced to the incoming audio signal, and the less signal there is to amplify in the rest of the circuit - and thus less volume.

Volume.jpg
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abzzaabzza

Reply 9 months ago

Tone controls are similar in their use of mono or stereo potentiometers, however instead of applying resistance to the entire audio signal, they apply resistance only to low (bass) or high (treble) frequencies, or some other band of frequencies (mid range, upper mid range, etc.) These potentiometers are tied to the output of resistor/capacitor networks (filter networks) which isolate these frequencies from the audio signal to be manipulated. The outputs of each potentiometer are then all brought back together again in a complete, but now modified, signal to be amplified.

One problem is that passive (non-powered) adjustments to isolated frequencies - owing to the use of resistor networks - means a drop in voltage... and lower volume. The best tone controls will therefore need to be active (powered), using amplifiers/opamps to bring the signal back up to a healthy level to be amplified. The most famous basic design for a tone control is called the Baxandall Network - check it out here:
http://sound.whsites.net/dwopa2.htm

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ShannonW49abzza

Reply 9 months ago

Love that site (http://sound.whsites.net) Elliott Sound Productions. Lots of good information there about stuff like this. Very helpful. Got me through quite a few projects and problems ;)

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nikhilraghava

9 months ago

That was a very informative instructable! What software do you use for your circuit diagram?

1 reply
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abzzanikhilraghava

Reply 9 months ago

Thanks! I just make vector drawings in Inkscape/Adobe Illustrator!

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slimtender.

9 months ago

Interesting tutorial, by the way I'm just doing a gainclone based on LM3886. Maybe I will post me too here, but it seems a little articulated...

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GermanA2

9 months ago

Thanks for your posting. I have a doubt. Can you explain: "R6 and C3 form a RC circuit, a high-pass filter that prevents DC from from travelling along the output." How does it prevent DC current going to the output = speaker? perhaps it's cutting off high frequencies instead? At very low frequencies it's like an open circuit. At very high frequencies it's like 1 ohm in parallel with the speaker. I guess depending on the output impedance of the amplifier it's like a current divider and cuts some of the power to the speaker. Or is it different?

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ShannonW49GermanA2

Reply 9 months ago

These two components form what is called a Zobel network. It is use to prevent the chipamp from oscillating due to the inductive load of the speaker. It also prevents radio frequencies picked up by the speaker wires from getting back into the inverting input of the chipamp through the feedback loop (R1).

According to http://www.circuitbasics.com/design-hi-fi-audio-am... (which is about the LM3886 - it uses Rsn and Csn to refer to those two components):

"At high frequencies, the impedance of Csn is very low, so high frequency current is shorted to ground. Rsn limits the high frequency current so there isn’t a direct short to ground, which could exceed the current limit of the LM3886. Therefore, smaller values of Rsn make the Zobel network more efficient at filtering radio frequencies, but it also increases the cutoff frequency, which in turn reduces it’s effectiveness."

I've seen these on a lot of chipamp circuits with varying values of the two components, but usually very close to what is here (4.7 ohm + 220nf, 2.2 ohm + 100nf, etc). So I suspect there is a bit of a dance around a narrow line of efficiency and effectiveness.

BTW, nice article!