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Old 16th March 2013, 06:56 PM   #1
picbuck is offline picbuck  United States
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Default Mixer-Amp - TDA2050 - Critique?

Would any kind guru care to take a look at this schematic and, as you will, tear it to pieces? Explanations of how I don't know what I'm doing will be much appreciated.

As I understand, the input impedance of an inverting amplifier is the value of the input resistor (in this case R1a, R2a,...). It makes no difference what's added before the input resistor, the input impedance is always the value of the input resistor, period. Or...does anybody know different?
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Last edited by picbuck; 16th March 2013 at 07:05 PM. Reason: clarity
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Old 17th March 2013, 04:20 PM   #2
gootee is offline gootee  United States
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Do you have the appnotes an-20 and an-31 from national.com or ti.com? You basically need a summing amplifier. It looks like you are at least close but the material about them in an-20 especially might have some useful tidbits.
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Old 17th March 2013, 07:34 PM   #3
picbuck is offline picbuck  United States
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Thanks for your reply, gootee. Yes I do have those appnotes, much annotated.*

My intention from the first was to have a summing amp (called a virtual-earth mixer in the schematic) feeding master volume control Potvol. I intended that U1 and U2 (and others, indicated by MORE) would be summed by U3, then sent to Potvol.

But from what you say I've missed the mark? I'm glad to find that out.

But look as I may, and referring to the appnotes you mention, I can't find what I'm doing wrong. I've redrawn my schematic in this vain hunt (which I'm uploading), but still no luck. May I have a hint, please?

* For those who don't know, "Foxit Reader" is smaller, faster, and less intrusive than the huge Adobe Reader. Also it allows highlighting text, and saveing the highlights.
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Last edited by picbuck; 17th March 2013 at 07:37 PM. Reason: upload image
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Old 17th March 2013, 10:13 PM   #4
DUG is offline DUG  Canada
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Quote:
Originally Posted by picbuck View Post
Would any kind guru care to take a look at this schematic and, as you will, tear it to pieces? Explanations of how I don't know what I'm doing will be much appreciated.

As I understand, the input impedance of an inverting amplifier is the value of the input resistor (in this case R1a, R2a,...). It makes no difference what's added before the input resistor, the input impedance is always the value of the input resistor, period. Or...does anybody know different?
Save yourself some amps:
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Old 18th March 2013, 12:16 PM   #5
gootee is offline gootee  United States
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I wasn't actually implying that anything was wrong. Just wanted to be sure you had those appnotes. If you don't already have LT-Spice, now would probably be a good time to download it, from linear.com (where there are also many precious appnotes).
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Old 20th March 2013, 04:51 PM   #6
picbuck is offline picbuck  United States
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Some 200 people were interested in this thread, so it seems well to close things out on a high note, meaning situation resolved.

Thanks gootee, for your input. And thanks DUG for pointing out the cheap, simple, and equally effective way, although with no amplification.

Meanwhile, I ran across essentially the same situation resolved by real, grownup engineers with degrees, or so I confidently assume. This is from a company whose focus is the pro (not consumer) market. To avoid any teensy little irritations like copyright violation I redrew the schematic, and let the company name remain hidden in the mist, although I doubt very much that they'd mind. Heck, they'd probably come around and help.

Notice that the factory schematic (labeled as such) is direct- (D.C.) coupled. In an add-on version (also labeled) I added a blocking capacitor and its associated resistor. LTspice was OK with this change, but I don't know how it might work out with real parts.

Hope this might be of interest.
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Old 20th March 2013, 10:50 PM   #7
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dc coupling is .. well it can be bad, and it can be good.
it realy depends. if the source has otput capacitors (I.E. no DC at output) then capacitor-less coupling is not bad at all.
if the source has a verry minimal DC , even then it may still not be bad, it all depends on the headroom and the DC level. One might "just " block the DC at the last output of the whole device. Like... having a small 2 channel mixer and an EQ , and a chipamp as a single unit, You may - if it was built properly at least- just use de-coupling at the input of the channels. But not between the mixer-> EQ, EQ->Poweramp.

Other than that input decoupling capacitors are not a bad idea.
Most probably since it may or may not be needed -at least i think- it was not presented on the factory schematic.
But, i must agree, they might have included it markt with an axterix and make a notice, that it is optional, based on actualy circuitry enviroment.
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Old 21st March 2013, 03:03 AM   #8
picbuck is offline picbuck  United States
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Everything you said remaining true, it's a sad fact that the days of published schematics are largely behind us. Usually we shadetree techs (like a shadetree mechanic, but the parts are not as heavy) know nothing about the output circuitry of whatever might be our audio source.

Making things worse, our source tends to be consumer-grade equipment, and we don't know who had the last vote before production started: was it the engineers, or was it the bean counters?

We hear of headphone amps that won't drive headphones, and CD players that won't play CDs. These days it's astonishingly cheap and easy for a factory to turn out first-rate audio, and yet some factories don't do it.

I vote to err on the side of caution. Expect that $29 player to launch a nuclear attack against the amp you spent a week building, and take measures.
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Old 21st March 2013, 03:13 AM   #9
JMFahey is offline JMFahey  Argentina
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It's confusing, which one is the definitive schematic you want comments on?
Please attach only one.
Thanks.
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Old 21st March 2013, 03:23 AM   #10
picbuck is offline picbuck  United States
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<< It's confusing, which one is the definitive schematic you want comments on? >>

Very good question. This thread has kind of veered back and forth.

The posts are numbered in the upper-right corner of each post.

My request for comments involved only the schematic in Post #1. All others are discussion.

For clarity:

Post #1 was my original request, and the schematic with that post is the original.

Post #3 is the same schematic redrawn. I posted this in reply to another post.

Post #4 is the [valid] suggestion by DUG.

Post #6 is a final solution (in my opinion), involving a commercial circuit I ran across.

Last edited by picbuck; 21st March 2013 at 03:27 AM. Reason: clarity
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