Note: When I first tested my OA3 op-amp in a gain of -10 configuration,
20V p-p out, I was getting close to ten times more distortion than theory predicted. After much darting down deserted blind alleys, I discovered the problem. Deposited carbon resistors have a non-negligeable voltage coefficient! Use metal film resistors in the feedback and any other gain determining areas if you are working in the
Was that the Philbrick one in a metal can with an edge connector out one of the long edges?
[ ... ]
Interesting. I did not realize that.
My preference, anyway.
Enjoy, DoN.
On Mon, 17 May 2004 21:31:21 GMT, Ted Edwards brought forth from the murky depths:
Bueno. Got a URL for that for me?
Heck, I'll be playing blues with overdrive, so it won't be noticeable most of the time.
Better hope she's not reading over your shoulder right about now...
-- Remember: Every silver lining has a cloud. ----
No. OA3 was my own designation for the third in a series I designed.
Ted
Would be nice but this was mid 60's. Maybe ARPA net was up then but I didn't have remote access until 1967. The article was in a British audio journal whose name I forgot. One can go through the math to look at the spectrum of various distortion generators but the perception does require experimental work. :-)
Is the "overdrive" at your end or in the source? Do you wear ear protection?
Ted
Use metal film resistors in the
That's similar to the method used to measure distortion in analog computer components. Feed a signal into the (inverting) DUT, pad the output back to unity, run the input signal and the output of the DUT into a summer with a gain of alot. Plug the input signal into the scope x-axis, the output of the summer to the y-axis and look at the pattern. What you see is phase shift plus distortion. The acronym was TIDE- total instantaneous dynamic error.
Kevin Gallimore
It might. It probably won't give much of a signal into a low impedance load. It would work out fine with an output buffer stage. For a single component amp capable of single supply (battery) operation and driving a
4 ohm load, you might look at the Linear Technology or Apex Micro stuff that has a bridge configuration.Kevin Gallimore
Interesting. I did that back in the fifties but didn't have a name for it. An important aspect of this is the perception of distortion in audio. i.e. If I want to build an amp that has a max distortion that will be well below the threshold of detectability by humans, I need a number. If that number depends greatly on the distortion mechanism, that makes life much harder. Also the TIDE method allows convenient measurement over a wide frequency range. When I designed my stereo amp, I was after less than 0.1% p-p distortion from DC to 100KHz. This is not easy to measure. :-)
Ted
Larry was looking for a low noise guitar pre-amp. I think the TL072 might work well up to but not including the output amp. For that I used a somewhat different discrete op-amp arrangement that could output 12.5 watts peak into 8 ohms with less than 0.05% distortion. A later version could do 50 watts peak. Note that I always spec peak output but by that I mean the peak independent of waveform, frequency and duration. Believe it or not, that is a lot of power. The problem with most very high output ratings is the method of specifying it - sorta like Sears horsepower.
BTW, RMS power is a meaningless concept. Peak power and average power are well defined.
Ted
The Danelectro box is a headphone amp with a line out. It also has a distortion effect built in (on purpose). Used for self-seranadeing.
In the US, the FTC requires audio ratings in a unit that doesn't exist (RMS Watts).
Clearly, it's time to start The Emperor's New Audio Manufacturing Company.
Kevin Gallimore
So how do they define it? As I understand it, RMS means Root Mean Square i.e. the square root of the mean (limit as T approaches infinity of 1/T Integral from 0 to T f(t)^2 dt) where f(t) is the function of time being measured. RMS voltage and current have well defined physical interpretations. What is the physical significance of RMS power?
Ted
What is the physical significance of RMS power?
It serves as proof that Poly Sci majors should not be making rules for the real world. At the time the rule was implemented (early '70s) Paul Klipsch put an article in the journal of the AES titled "Misnomer RMS Watts". Walked through the math. RMS Volts * RMS Amps = Average Watts. Apparently bureaucrats don't read technical journals.
Kevin Gallimore
Interesting. I have wondered who started the screw up and when.
Ted
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.