lm4766 Amplifier idea

[xxxxxxxxxxxxxx]

I can use a 30v (15-0-15) transformer- that should be fine, no? can i add a couple of extra wraps of wire around the transformer to boost the voltage a tad?


EDIT: I dont have access to a 3.5A transformer. i can use two 30v transformers (one for neg, one pos supply). each transformer is rated 150mA. they wont cut it. i suppose i can check e-bay (freight cost ), unless anyone knows a transformer supplier/distributer in aust (preferable brisbane)

Double Edit: No transformers suitable on ebay. No transformer distributers ... perhaps I could wind my own (preferably not) or SMPS? I didnt really want a SMPS- no isolation and I've never done it before (i dont want to die from a SMPS)

 

[xxxxxxxxxxxxxx]

I dont think the datasheet was very clear (for me) but isnt the curent of this only going to be 850mA or somewhere near there? not 3.5A (running at 35W, 60v(30-v-30))
??

 

[audioguru]

With a +-30VDC supply, the output power is 40W per channel into 8 ohms. The heating is 43W per channel. Therefore the power required from the transformer is 166W. A dual 24VAC transformer will supply 3.45A.

You can cheat a little since music is never full blast all the time (acid rock?) so a transformer rated for 82W (1.7A) will be OK.

Farnell are online in Australia, maybe their warehouse is in your city.

 

[xxxxxxxxxxxxxx]

I cant get one. Unless you know a farnell part number, i didnt see anything (at a reasonable price) Farnell seem to be pretty expensive anyway. Perhaps i might change the type of amp I use. Perhaps a 10W or so.

EDIT: Maybe just one of TI's new OPA opamps. they seem fairly good, plus i have some. That plus an opamp preamp, two for stereo, that could be ok.

 

[audioguru]

The minimum load for an ordinary opamp is 2k ohms. A few can drive 600 ohms. An opamp can drive two medium complimentary power transistors and produce a few watts into an 8 ohm speaker.

There are hundreds of amplifier ICs made for cars that have an outout of 14W into 4 ohms or 8W into 8 ohms.

 

[xxxxxxxxxxxxxx]

im sure this is annoying for people who know what they're doing D) but i have another question:

I can get a 30V, 2A transformer for the pos side, can I use a (significantly cheaper) 30v 150mA transformer for the neg side? what is the recomended amp rating for the neg side? (that datasheet i just cant make much sense of).

Heres the links for the transformers at Jaycar Aust.

**broken link removed**

**broken link removed**

**broken link removed**

Sorry about dragging price into my project, but I am on a tight students' budget

 

[Nigel Goodwin]

No, it takes equal current from BOTH sides.

 

[audioguru]

The Jaycar 30V/2A transformer is rated for only 60VA. It can power a stereo amplifier if it has an output power per channel of about 15W up to 30W per channel.
The LM4766 has a power per channel of 17W when its supply is + and - 20VDC which is made with the 0V, 15V and 30V wires on the transformer. The 15V wire becomes the grounded center-tap.

 

[xxxxxxxxxxxxxx]

and if I used two 30v/2A transformers, one for pos one for neg- thats ok?

 

[audioguru]

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?

The graph, "Output Power vs Supply Voltage" is labelled "plus and minus xxV". The 30VAC center-tapped transformer will make plus and minus 20V which will allow the amplifier to make 22W into 6 ohms per channel.

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

Adding resistors will reduce the power to each speaker, make them sound boomy and make the resistors get hot.

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?

It is designed to use a split supply. It needs many more parts to use a single supply. With a single supply it needs a huge capacitor in series with each speaker which reduces their bass response.

 

[audioguru]

The 30VAC output of the transformer makes 40VDC. A supply of plus and minus 40VDC is too high for the LM4766 IC. If you use the 24VAC output then the supply will be plus and minus 32VDC but the graph clearly shows the IC limits the current when its supply voltage is more than plus and minus 26VDC when it has a load of 6 ohms.

 

[xxxxxxxxxxxxxx]

ok- lowering the voltage is easy. making it bigger was the problem. is this preamp circuit any good? I want to modify it so it is 2 mic inputs, 2 normal inputs. The normal inputs will be mixed with a single, dual gang pot because it is in stereo. Feed that then to my LM4766 and I should be right, right?

 

[Hero999]

The maximum load for an ordinary opamp is 2k ohms.

Corrected.

 

[xxxxxxxxxxxxxx]

does that make it not suitable? are you referring to the schematic posted?

 

[audioguru]

Your preamp/mixer circuit is fine. But don't use lousy, noisy, narrowband old 741 opamps. Some of the other opamps you listed are also bad for audio.
Use TL072, TL074, OPA2134, OPA4134 opamps. National Semi have a new dual opamp, their LM4562 dual that has a distortion of only 0.00003% even with a 600 ohms load.

The [m]minimum[/b] allowed load resistance for most opamps is 2k ohms. A few can drive 600 ohms.

 

[xxxxxxxxxxxxxx]

thanks audioguru, I intended to use TI's OPA op amps anyway. Il see how it goes from here... fingers crossed!

 

[Hero999]

The [m]minimum[/b] allowed load resistance for most opamps is 2k ohms. A few can drive 600 ohms.

Sorry about that, you're right, the minimum impedance an op-amp can drive is normally 2k or 600R, I was thinking maximum load current.

 

[audioguru]

Most opamps have a max load current of at least 20mA into a short circuit. But they have an output voltage loss of up to 5V peak with a 2k ohms load (5mA peak) and a higher voltage loss with a lower load resistance.