Ham Radio Quiz

[kl7aj]

Brian Snertwelder, UN1NU, has built himself an external audio amplifier for his rather wimpy ricebox. He wants to run a nice Hi-fi speaker or two instead of the tinny communications speaker that most rigs have.
His amplifier is pretty straightforward, a couple of op-amps, with a complementary-symmetry output stage, and a more-than-adequate bi-polar power supply.
To Brian's consternation, the amplfiier just doesn't sound right, at normal listening levels it sounds just a tad "fuzzy." Oddly enough, if he really cranks up the volume, it sounds great. The results are the same with any of his numerous wide range speaker cabinets.

What is the most likely problem with Brian's amplifier?


Eric

[n2cfj]

His input impedance does not match the receiver's output.

[kl7aj]

Quote[/b] (n2cfj @ July 03 2007,11:01)]His input impedance does not match the receiver's output.
^^^^^^*****BUZZ******

WRong... Next please.


http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

eric

[N2RJ]

It isn't biased properly.

[wa4brl]

Alright! #An area in which I have little knowledge or exerience -- a "complimentary-symmetry output stage, whatever THAT is. #So, I'll take the plunge with a semi-instinctual approach.

At all volume levels, the crossover from one op-amp to the other produces a constant level of distortion. #At low volumes, this distortion is a significant percentage of the output signal. #As the volume increases, that distortion becomes a smaller and smaller percentage of the output. #Otherwise described as high THD at low output, low THD at high output.

Am I on the right track?

[kl7aj]

Quote[/b] (wa4brl @ July 03 2007,11:08)]Alright! #An area in which I have little knowledge or exerience -- a "complimentary-symmetry output stage, whatever THAT is. #So, I'll take the plunge with a semi-instinctual approach.

At all volume levels, the crossover from one op-amp to the other produces a constant level of distortion. #At low volumes, this distortion is a significant percentage of the output signal. #As the volume increases, that distortion becomes a smaller and smaller percentage of the output. #Otherwise described as high THD at low output, low THD at high output.

Am I on the right track?
bingo! Crossover distortion it is, indeed. You win!

Most lower power audio amps use a complementary symmetry output pair (one NPN, one PNP in an emitter follower configuration). You need to forward bias these a tad...into the AB region to avoid crossover distortion.

eric

[kl7aj]

Cackle-ator (http://www.ece.umd.edu/~neil/306/ckts/power.htm)

Here's a complementary-symmetry calculator. Pretty cool

[K7KBN]

I always thought "complementary symmetry" referred to a burial ground which magically offered phrases like "He sure looks good, doesn't he?" or "She looks like she's just asleep...". http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif

[ve2nsm]

Argh, I wish I saw this one before http://www.qrz.com/iB_html/non-cgi/emoticons/sad.gif

[kl7aj]

Quote[/b] (ve2nsm @ July 03 2007,11:17)]Argh, I wish I saw this one before http://www.qrz.com/iB_html/non-cgi/emoticons/sad.gif
I'm sure all is not lost. A couple of diodes and you're in!

[wa4brl]

Quote[/b] (kl7aj @ July 03 2007,13:12)]Quote[/b] (wa4brl @ July 03 2007,11:08)]Alright! #An area in which I have little knowledge or exerience -- a "complimentary-symmetry output stage, whatever THAT is. #So, I'll take the plunge with a semi-instinctual approach.

At all volume levels, the crossover from one op-amp to the other produces a constant level of distortion. #At low volumes, this distortion is a significant percentage of the output signal. #As the volume increases, that distortion becomes a smaller and smaller percentage of the output. #Otherwise described as high THD at low output, low THD at high output.

Am I on the right track?
bingo! # Crossover distortion it is, indeed. #You win!

Most lower power audio amps use a complementary symmetry output pair (one NPN, one PNP in an emitter follower configuration). #You need to forward bias these a tad...into the AB region to avoid crossover distortion.

eric
Sounds more like Ryan, N2RJ called it first, but thanks just the same. Perhaps I just described it better?

Ryan, I pass the crown to you.

[kl7aj]

Quote[/b] (wa4brl @ July 03 2007,11:26)]Quote[/b] (kl7aj @ July 03 2007,13:12)]Quote[/b] (wa4brl @ July 03 2007,11:08)]Alright! #An area in which I have little knowledge or exerience -- a "complimentary-symmetry output stage, whatever THAT is. #So, I'll take the plunge with a semi-instinctual approach.

At all volume levels, the crossover from one op-amp to the other produces a constant level of distortion. #At low volumes, this distortion is a significant percentage of the output signal. #As the volume increases, that distortion becomes a smaller and smaller percentage of the output. #Otherwise described as high THD at low output, low THD at high output.

Am I on the right track?
bingo! # Crossover distortion it is, indeed. #You win!

Most lower power audio amps use a complementary symmetry output pair (one NPN, one PNP in an emitter follower configuration). #You need to forward bias these a tad...into the AB region to avoid crossover distortion.

eric
Sounds more like Ryan, N2RJ called it first, but thanks just the same. #Perhaps I just described it better? #

Ryan, I pass the crown to you.
Indeed! # I somehow skipped over his post in my haste. #(I guess that makes it post-haste!)

Good job, RJ.

[ki4ned]

Quote[/b] ]His amplifier is pretty straightforward, a couple of op-amps, with a complementary-symmetry output stage, and a more-than-adequate bi-polar power supply


Methylphenidate ( Ritalin ) in bipolar depression http://www.qrz.com/iB_html/non-cgi/emoticons/laugh.gif