lm4766 Amplifier idea

[xxxxxxxxxxxxxx]

'ello 'ello 'ello, im back from hibernation!

well, I would like to make an audio amplifier that features:

• high quality, low noise sound
• dual mic inputs, plus normal stereo input as well
• the ability to mix the above chanels (with pots)
• high power (10W would be awesome)

and so I would like to try National's LM4766 Stereo amp. I want to combine a variety of different circuit ideas to get the result i desire. I have attached all that I feel is neccessary. It would be fantastic if someone could help me, review the circuit ideas (not mine, btw) or suggest a different system.

*note: will have two VU meters and tone control preamps (one per chanel)

THANKS!!

 

[Nigel Goodwin]

Sorry, but that preamp is really crappy! - an emitter follower input stage (to give a highish input impedance and no gain - WHY?), a passive tone control (to give a huge loss), followed by a little amplifier (gain of about 5 times). You're making a considerable loss through it, no gain at all.

What are you wanting to feed it from?, it looks fairly useless?.

I would suggest dumping the transistors, and using some opamps!.

 

[xxxxxxxxxxxxxx]

yeah, i was thinking op-amps, but dont they create a bit of noise? how can i get my tone controls?

thx

 

[Nigel Goodwin]

yeah, i was thinking op-amps, but dont they create a bit of noise? how can i get my tone controls?

No, audio quality opamps are nice and quiet, every CD in your collection has almost certainly been though hundreds (if not thousands) of them.

You use active tone controls, in the feedback loop of an opamp - it's the standard way to do it, and has been for many decades.

You appear to be wanting to make a small mixer/amp?, what's it for?.

 

[gootee]

Do a search for "Baxandall tone control schematic".

For audio amplifiers, most people use LM1875, LM3875, LM3886, or LM4780.

Do a search for "gainclone". Check out the "Chipamps" forum, at http://www.diyaudio.com .

- Tom Gootee

**broken link removed**

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[gootee]

Go to the following link and download Jung's "Op Amp Applications Handbook":

http://www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html

There are some excellent Mic Preamp designs, in there, plus almost everything else you'll need.

- Tom Gootee

**broken link removed**

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[xxxxxxxxxxxxxx]

i wanted to make like an i-pod (or other input) with like 2 karaoke mics that are tone controlled and mixed. with very low noise

 

[Nigel Goodwin]

So you need two mike inputs, and a line level input - presumably you are aware you can buy such a mixer at less cost than the knobs and pots for your own design?.

 

[audioguru]

The datasheet for the LM4766 power amp IC shows an output of only 3w per channel into 8 ohms with a 20V supply.

The VU meter will be a dim blur unless it has a Peak-Detector as shown in the datasheet for the LM3915. Use an LM3915 instead of an LM3916 then each LED is indicating double the power of the preceding LED.

 

[xxxxxxxxxxxxxx]

does it? where? with my calculations (hmm) it would be 'round about 13W per chanel for 24v (suply i'd like to use) and 11W per chanel for 20v. even +- inaccuracies, its still fairly ok.

Also, i intend to use LM3915, as its the only one readily availiable to me.

>Nigel well i'd rather build my own... but, where might i find such a cheap quality device?

 

[Nigel Goodwin]

>Nigel well i'd rather build my own... but, where might i find such a cheap quality device?

Try looking for Behringer mixers - excellent quality, and cheap prices.

I was going to build a mixer for my daughters band, I did one back in the 70's for a band I did the PA for - but when you price up the cost of just the knobs and the pots, you can buy them cheaper (and better!).

In the end I bought a Behringer mixer amp, it has eight mike inputs, 400W+400W, and two seperate digital internal effect units - it even has a button to cancel vocals so you can use it as a karaoke unit!. It's also small and light, important considerations for me.

 

[xxxxxxxxxxxxxx]

ok thanks. Ill have a look then, but i really did want to make my own.
Anyway, i did some research and it looks like I have 6 ohm impendance, so i think the amp should be run at around 21v giving about 48w total. I thin i better just use the datasheet circuit. (the one with single suply looks like what im after)

What'dya think about opamps for preamps? what would you reccomend? I have some standard ones, but i dont think thats what i need.. (easy to obtain, cheap plz)

Thanks


EDIT: Just found a toshiba TA7256P Audio Op-amp plus some SIP dual opamps too. im guessing thats what i'll be wanting.

 

[Nigel Goodwin]

What'dya think about opamps for preamps? what would you reccomend? I have some standard ones, but i dont think thats what i need.. (easy to obtain, cheap plz)

The Texas TL071/81 series are perfectly good for what you want, or you can use specific audio ones like the NE5534 or the OPA series. It's realy not that critical, opamps are amazingly good!.

My 1970's one used all 741's, and was perfectly fine for PA purposes.

 

[xxxxxxxxxxxxxx]

ok excellant! I honestly never thought they were that versitile and good! Thanks a heap to EVERYONE who helped me out! ill let you know how it goes!

 

[audioguru]

with my calculations (hmm) it would be 'round about 13W per chanel for 24v (suply i'd like to use) and 11W per chanel for 20v. even +- inaccuracies, its still fairly ok.

The datasheet has a graph of power output vs supply voltage. To an 8 ohm speaker the power is 25W with a dual 24V supply and is 17.5W with a dual 20V supply. Your schematic showed only a single +20V supply.

 

[xxxxxxxxxxxxxx]

oh ok, well I have a 6 ohm speaker, but when the graph says "24v", is it refering to a +12v, -12v supply or +24v, -24v supply?

Also, is there a ay I can change speaker impendance for better performance? (adding a resistor just doesnt seem right


EDIT: ... and do I need a split power suply? why can't it just be single supply. Also, there was a schematic in the datasheet for a single supply. Is that OK?

 

[audioguru]

when the graph says "24v", is it refering to a +12v, -12v supply or +24v, -24v supply?

The bottom of the graph is labelled "Supply Voltage (+/-)V". The graph shows that a +/-24V supply (48V total) allows 30W into a 6 ohm speaker when the distortion is low. You can calculate that the voltage across a 6 ohm load is 38V p-p. So the amplifier has a 10V loss.

Also, is there a way I can change speaker impendance for better performance? (adding a resistor just doesnt seem right

No.

do I need a split power supply? why can't it just be single supply. Also, there was a schematic in the datasheet for a single supply. Is that OK?

The single supply circuit uses many more parts. Its output capacitor reduces damping of the speaker at low frequencies and reduces bass output.
Use a split power supply.

 

[gootee]

oh ok, well I have a 6 ohm speaker, but when the graph says "24v", is it refering to a +12v, -12v supply or +24v, -24v supply?

Also, is there a ay I can change speaker impendance for better performance? (adding a resistor just doesnt seem right


EDIT: ... and do I need a split power suply? why can't it just be single supply. Also, there was a schematic in the datasheet for a single supply. Is that OK?

The posted graph has "(+/-V)", right after the words "SUPPLY VOLTAGE", in the horizontal axis label. So it does mean both + *AND* - 24 volts, i.e. a dual or "split" power supply. A +/-24v split supply is equivalent to a 48v single supply.

Also, your original 20v single supply would be worrisome. The datasheet shows 20v as the absolute minimum guaranteed-to-work difference between the supply rails. Plus, it would only give you about 3 Watts per channel.

See page 16 of the LM4766 datasheet, at **broken link removed** . It says "The typical application of the LM4766 is a split supply amplifier."

You CAN use it in single-supply mode. But it's a little more complicated, and might even compromise the sound quality, slightly, by necessitating electrolytic input and output AC-coupling/DC-blocking capacitors.

But yes, the single-supply schematic, in the datasheet, should be perfectly fine (especially if you have a 48v to 52v supply).

What makes you want to use a single supply, so badly? A split supply is easier. And making a split power supply is easy, too. Just get a transformer with a dual secondary, or a center-tapped secondary, with each secondary having the voltage needed, at a high-enough current. Or, get two separate transformers. Then all you need is a fuse, a bridge rectifier (or individual diodes), and two or more big electrolytic capacitors. That's it! These chipamps work just fine with a simple unregulated power supply. [ NOTE: Be safe. Working with AC mains voltages can kill you.]

If acquiring a transformer, remember that AC mains voltage transformers' "VAC" ratings are usually in terms of RMS, which means their peak voltage is actually 1.414 times that. Here is a very nice, concise page, about linear power supply design, that should have the necessary details: **broken link removed**

- Tom Gootee

**broken link removed**

 

[xxxxxxxxxxxxxx]

Ok, thanks. it's just that the only 56V transformers availiable to me are as (literally) as big and as heavy as a house brick... the datasheet does say min imum -9v for negative side, can I use a 24v transformer giving the pos side +24v and then an ICL7660 neg voltage creator to obtain -12vdc (with 12v regulator before 7660 as it is max 13v iput.)

 

[audioguru]

24VAC has a peak voltage of 34V. The full wave rectifier reduces it to 32VDC.
If you want 24VDC then use an 18VAC transformer and the DC will be 23.5VDC. Use a 36V center-tapped transformer to make positive and negative 23.5VDC.

An ICL7660A produces -12VDC at only 25ma. The LM4766 needs at least 2.5A which is 100 times higher. The transformer needs to be rated for 3.5A.