Fairchild µA706 - TBA641B audio amplifier

Hey guys,

I just joined the forum.

I am new to electronics and I am trying to gather components and ICs for some small projects which are supposed to be relatively simple for beginners.

Basically, I am looking for the Fairchild µA706 or TBA641B audio amplifier. I did some research and the TBA641B is definitely obsolete. As for the µA706, it seems like the same story.

With what 14-pin IC can I replace either the µA706 or the TBA641B?

Or, where can I still find these parts without selling parts of my body?

Thanks,

Hi Zob,

Welcome to "The Lab"

Why would you want to use obsolete semiconductors for your project? Wouldn't it be easier to use some modern and readily-available (and much better quality) component like the NE5534 or TL071?

Jakob E.

The reason why I'm looking for those obsolete parts is because I have this book with simple electronic projects for beginners. It was written in 1979 (and revised in 1980, wow!).

I was able to find some other obsolete parts for a reasonable price. But, since I am not an expert at all, I decided to join the club and have the super-experts guide me.

My goal is really to assemble the G4000, the 1176, the G9, and the Pultec EQ. I thought that working in small projects would be a good start for building up soldering skills and parts hunting.

The Fairchild µA706 or the TBA641B would be used for a mini-amp that feeds signal to a speaker. The project is a talk box.

So, would you really recommend the NE5534 or the TL071 as a replacement for the obsolete one? Between the two, which one sounds the best?


P.S.: Big surprise indeed to have received the answer from the mighty Jakob at Gyraf.


Peace.

[quote author="zobomix"]So, would you really recommend the NE5534 or the TL071 as a replacement for the obsolete one? Between the two, which one sounds the best?
[/quote]

That depends entirely on the project in question. Could you post a schematic? You could use the free service at www.photobucket.com if you don't have your own space.

Jakob E.

Yes,

Welcome to "The Lab"!

Yes, if you can post a schematic (re-draw it in a graphics prog, or scan/photo it) it'd be handy. If you haven't got any space, send it to me via e-mail (my address below) and I'll host it while we have a look.

I'm sure the TBA641B is a power amp, so there are plenty of modern options here. A quick Google reveals the uA706 to be a voltage regulator- again, plenty of modern options here. Funny, I thought that'd be an op-amp, 3 numbers away from uA709!

Anyway, if you can get the schem for us to have a look at we may be able to help.

Mark

I'll make a scan of the schematics and parts (resistors, caps, etc.) in the next couple of days.

One concern of mine is that either one of the two obsolete parts has 14 pins. What if the newer IC replacement has only 8 pins? I am not really familiar with rewiring things.

I am going to use prototyping perfboards for the project. In this regard, I have one question. The book I have recommends to use a wiring pencil if using perfboards. I did a quick search and I found some that are battery operated while others that are manually operated. Which one would you recommend?

Also, what soldering iron size would you recommend for soldering CMOS parts?

Man, I haven't even started my journey into electronics and I am already in this mess. Whatever.

Thanks everybody.

Hey,

No worries, we've all got to start somewhere.

I reccomend that you post the schems, we have a look at what you've got vs. what you need, and then advise accordingly.

Wire-wrap is okay for most digital circuitry, but it isn't that good for audio equipment, and before you buy the (fairly) expensive wire wrap tools, I'd have a think about using Veroboard, or stripboard. Or even get a small "Breadboard" plugblock for experimenting.

Anyway, zip the scans over to me if you need them hosting.

Mark

I have sent the schematics scans to Mark for posting.

I tried the photbucket.com site but there are some problems.

I have tried photbucket.com and had problems with it too, even after registering 3 seperate accounts! :roll: It seems to me that they are just trying to collect email addresses and not really intended to host the pics for free, since you need to activate your photbucket account through your email.

Anyway, since then I have been using

http://www.villagephotos.com/

without any hassles and you don't even need to activate your account through email, which means you can make up any bullsh#t email address....:green:

Hi Zobomix,

Got the scans...I know this schem- it's from Craig Andertons excellent book "Electronic Projects For Musicians"!

Here's the schem:

Click Here

Okay, my copy of the book was published in 1980, but most of the IC's are still available- all the op-amps are available with the same pinouts (but better specs!) and the CMOS IC's are still used by industry, but the world of power-amp IC's have moved on rapidly.

The IC used in the Miniamp is a uA706 or a TBA641B. Now, the only way to do this project "as-is" would be to find a supplier of surplus semiconductors and buy one of these from them. This way you could still use CA's PCB layout. The problem here is that the IC may cost you a lot more, and I'd buy more than one (in case of trouble!)

I'm betting a better plan would be to build a similar amp (the output power is ~1W into 8R it says in the specs) but using a modern IC.

Lets have a quick look at the original Miniamp schem to see what's going on:

Click Here for Diagram!

The first thing to notice is all the components around the IC in the blue box. All these components (except the 100uF power-supply decoupling capacitor C9) are the "support" components to this particular power amp IC. They are connected to specific pins within the IC to help decouple internal stages, change the frequency response characteristics of stages (to help stability) etc.

Each type of power amp IC has different external components needed, so you can't just "plug and play" with another 14-pin IC. The nearest 14-pin power amp of this era which comes to mind is the LM380 (possibly slightly more output power, and less external components) but this isn't pin compatible with the uA706/TBA641B.

The pink circles highlight parts of the circuit which would be required no matter what type of power amp IC you used. These are all directly within the audio path, and are crucial to how the Miniamp operates. Lets go through each stage:

1. Input jacks and input level controls. The two inputs each feed a 1M Log pot. This allows the input signal to see a high-impedance input, as well as having the ability to vary the volume of each input.

2. Input mixing resistors and DC-blocking caps. Because there are two inputs, they need to be mixed resistively to stop one input affecting the second and vice-versa. The 100k resistors are the mixing network. The two capacitors are "DC Blocking" caps which allow the audio signal to pass through to the amp, but which block any DC from the IC input (the input of the IC may have a DC voltage sitting at its pin due to the internal bias of the IC) If the caps weren't there, a DC current would flow through the 1M pots and cause scratching and noise when you adjusted the pots, amongst other nasty gemlins!

3. Amp input-impedance resistor and roll-off capacitor. Many IC amps have a very high input impedance, which can be a problem because a high impedance input is easily effected by external voltages which you don't want amplified. The 470k resistor sets the input impedance to the amp to 470k. This also terminates the input-mixing network. The small value cap (20pF) rolls off (shorts to ground!) any high frequency signals above the audio range, which would otherwise be amplified too and use up amp power on signals you won't hear.

4. The 1000uF cap at the output of the IC is the output-decoupling cap. Because the amp IC works off a "single rail" supply of 0V and +12V, the output pin also sits at a DC voltage. This DC voltage has to be removed, as the speaker would just move in one direction and the cone would be rigidly fixed in one position (and usually have its voice-coil burnt out :roll: ) So this cap blocks the DC potential and allows the ac signal (the amplified audio) to pass.

5. A common thing to see at the output of a power amp is a "Zobel Network". This acts to damp any high frequency oscillations which can happen in a power amp. Usually it's a resistor in series with a capacitor, but just a cap is used here.

6. 1W is a lot of power into a set of headphones- it takes mW to get headphones loud, not Watts, so these resistors help pad the output level a bit. Two sets of headphones can be driven with the amp.

7. Speaker/phones switch. This can be a standard toggle switch on the panel or some headphone sockets have a switch feature (like on many hifi's) so when you plug the headphones in you switch off the speakers.

I've just run through these stages to help break it down a bit, and to show that most of the componentry is actually to support the power amp IC.

If you go somewhere like Natsems Power Amp IC page and have a look at the IC's available, you can see that there is quite a range of IC's available. Have a look at the PDF datasheets of something like the LM380 (PDF) and have a look at the "Application Circuits". If you get yourself a bit of Veroboard or a Plugblock, you could try one of these basic circuits, which would give you very similar performance to the Miniamp, but with a bit more work to get up and running.

Hope this is (some) help!

:thumb:

Mark