# Which is the best amplifier : rate and comment

#### simeonz11

Joined Apr 29, 2008
98
poo poo poo

Last edited:

#### beenthere

Joined Apr 20, 2004
15,808
Not enough information. What is the frequency of the sine wave? What kind of load will it drive?

#### t06afre

Joined May 11, 2009
5,936
I think should go for an dedicated audio poweramp IC. How many watt do you need?

#### simeonz11

Joined Apr 29, 2008
98
Lol sorry for that obvious mistake , forgot to mention .

it will drive a 1:1 isolation transformer in the full audio ranges 300hz to 20000hz , so an inductive and resistive load .

A maximum of 5 amps .

Yes I do expect alot of heat and am prepared with big heatsinks , so about 100 watts .

Last edited:

#### t06afre

Joined May 11, 2009
5,936
You say max 5 A, are you sure you will be able to to create such a current with +/- 30 volt? Do you have some inforation about your transformer.

#### simeonz11

Joined Apr 29, 2008
98
You say max 5 A, are you sure you will be able to to create such a current with +/- 30 volt? Do you have some inforation about your transformer.

Unfortunately ,I still do not .

Probably a heavy duty 1:1 audio transformer that fits this frequency range , still looking around .

The components are just there as an example besides the op amp.

Last edited:

#### t06afre

Joined May 11, 2009
5,936
I see some problem here. First of all I think it would be best to find the transformer first. Then base your design around this transformer. Now it is to many loose ends. You may end up with a useless design.

#### simeonz11

Joined Apr 29, 2008
98
I see some problem here. First of all I think it would be best to find the transformer first. Then base your design around this transformer. Now it is to many loose ends. You may end up with a useless design.
What would those problems be so I can better inform myself ?

I am still undecided , most are expensive . I expect 5 amps with an inductive and resistive load with one of those center tapped heavy duty transformers.

My transistor gains and op amp rating looks good enough tho , the transistor pair I will choose have a gain of 50-70 rated for 15 amps

Last edited:

#### t06afre

Joined May 11, 2009
5,936
The problem is the transformer. You have a quite wide frequency range. So you might experience a phenomenon named transformer core saturation http://www.opamp-electronics.com/tutorials/ac_theory_ch_009.htm
On the bright there are is many small firms that have specialized in making custom transformers. You should call such a firm give them your specification. They will be able to advice you. Do this before you put to much effort into your project

#### Audioguru

Joined Dec 20, 2007
11,251
The output transistors must have typical gain or more or they will not be able to drive an output of 5A when driven from the max 200mA output of the opamp.

The single diode operates the output transistors in pure class-B which produces pretty bad crossover distortion.
Audio amplifiers operate in class-AB with two diodes or a Vbe multiplier transistor and the output transistors have emitter resistors. The output transistors conduct a small current all the time.
The opamp is fast so it will try to reduce the crossover distortion especially the voltage-follower where all the gain of the opamp is used as negative feedback to almost eliminate the crossover distortion.

#### simeonz11

Joined Apr 29, 2008
98
Hi Audioguru , thx for commenting .

So whats stopping me from just using a voltage follower with nothing but the 2 transistors and some feedback .

I heard it doesnt do so good and I also heard it eliminates distortion . So whats true ?

It sure would be the simplest and most efficient solution if it did work , whats your opinion ?

Last edited:

#### Audioguru

Joined Dec 20, 2007
11,251
Why don't you use a class-AB audio amplifier IC that is designed to produce extremely low distortion?

This simple class-B circuit has crossover distortion.

#### SgtWookie

Joined Jul 17, 2007
22,201
Why don't you just get something like a cheap pair of TDA7254 audio amps (about $5/ea), feed one a noninverted signal, feed the other an inverted signal, and connect the outputs to the primary of your transformer? It won't need to be center tapped that way. Avnet Express has them for under$3:
http://avnetexpress.avnet.com/store/em/EMController/Audio-Amplifier/STMicroelectronics/TDA7294V/_/R-1559185/A-1559185/An-0?action=part&catalogId=500201&langId=-1&storeId=500201&listIndex=-1
Mouser has them for about $4.50 or so. Digikey sells them for around$5.80.

You'll probably wind up needing a broadband toroidal transformer; a fairly large one. Saturation at the lower end of the frequency range will be a problem.

#### simeonz11

Joined Apr 29, 2008
98
Why don't you use a class-AB audio amplifier IC that is designed to produce extremely low distortion?

This simple class-B circuit has crossover distortion.

Can this be used @ +-24 volts ?

Can this drive 200mA , that is my absolute minimum ?

I need something simple and cheap , 5 dollars is too much , I need 6 of these 200 mA amps

#### simeonz11

Joined Apr 29, 2008
98
Looks like an efficient compromise

Last edited:

#### Audioguru

Joined Dec 20, 2007
11,251
Crossover distortion is at high audio frequencies. Opamps have extremely high gain at very low frequencies but not much gain at high audio frequencies so the negative feedback reduces but does not eliminate crossover distortion.

It is not difficault to properly bias the output transistors in class-AB so there is no cossover distortion.