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In Reply to: RE: Long-term DBT example posted by rick_m on April 28, 2008 at 14:22:42
Ah but... the kink is only in the "distortion" channel that's attenuated 34dB if set to 2%.It's summed in with the main channel, though, so the combination must have the discontinuity in the derivative as well.
Edit: Update - I put the circuit into LTspice, using idealized op-amps (GBW=1GHz, DC OLG = 120 dB), and idealized diodes. I adjusted the pot to get the specified 2.5 percent THD at 1 kHz, assuming harmonics up to 20 (20 kHz). The pot value turned out to be 81.9k. Here is the spectrum.
Listed as harmonic number (1 = fundamental), relative dB
1 0.0
2 -32.2
4 -46.2
6 -53.6
8 -58.7
10 -62.6
12 -65.8
14 -68.5
16 -70.8
18 -72.9
20 -74.7You can see that there's plenty of high-order harmonics. Also, the discontinuity in slope at the origin can be seen with a DC sweep.
Oh, I did verify that the distortion stayed at 2.5 percent for input voltages of 1 Volt peak and 0.01 Volts peak.
One other data point from the DC sweep:
Slope K1 for positive voltages = 0.939885
Slope K2 for negative voltages = 1.05464
Edits: 04/28/08 04/28/08 04/28/08 04/28/08Follow Ups:
Hmmm, I didn't do a DC sweep. There, I just did, well a triangle...
I had had my pot set for 100K figuring it was close enough but I changed it to match yours. I get...
-32.4dB 2nd
Slopes=.948,1.054
Essentially the same thing. The models in Simetrix are ostensibly an 072 and 4148, but I didn't look at their parameters.
I'm probably missing your point. But it is sort of comforting that the simulations are close. If we do anything further with this and you send me your model I'll also use LTspice. Speaking of which, I've got an LT3837 based flyback switcher half designed and better get back to it if I want to eat.
Regards, Rick
I'm probably missing your point.
Well, from my perspective, we either had some disagreements as to the properties of the grunge circuit, or just a communication gap. I provided the data so that it might help to get us on the same page. I hope it helped to do that.
Let's go for the award: Rightest acrimoniousless thread for 2008.
I think we may be running out of steam unless we actually build the silly thing and listen to it. I reckon that we are on the same page now but that I'm inclined to believe that it's pretty benign and I think that you lean more towards believing that it's tougher than that at low levels. Regardless, it's an interesting critter and the design and test results provide a nice data point concerning distortion and the recognition thereof.
The mere fact that it's so "unnatural" limits my desire to build one and play with it. I think I'd rather spend the time time trying to learn better how to differentiate between common distortion mechanisms by ear. Think I'll finish the Geddes articles then decide if exploring distortion generating software would be a good thing as it seems interesting.
Regards, Rick
Download Keith Howard's software and play with it. If you are good at programming maybe you can improve it a bit. I was thinking about instead of applying the same distortion pattern over the whole spectrum as his software does to chop it into three regions, bass, mids and highs. The reason is this: Tube amps (with output transformers) tend to have increasing (and perhaps different components) distortion in the bass. Many SS amps (especially high negative feedback ones) tend to have increase distortion in the highs (one might fairly assume its harmonic content is different as well). If these regions were accounted for separately then we come a bit closer to replicating the amps "sound" onto a test recording.
Just an idea and maybe I will talk with some programer friends of mine how hard this would be to do.
Thanks, I'd very much like to play with that software. Where can I get it?
The SS amps have the characteristic you mention due to limited gain-bandwidth product (GBW). That decreases the loop gain at higher frequencies so the distortion and output impedance rise. Depending upon how they are implemented there may be a single dominant pole which would be relatively easy to model. The rub is that the overall GBW is rarely sufficient, especially in power amps, to completely cover the multitude of sins that can occur in the underlying circuitry. And some of those sins are only invoked dynamically and thus will not show up well on a sweep.
You might consider a more complete model. I think LTspice will accept WAV files as a PWL (piece-wise linear) input so you could potentially run music through it. I don't know if there are any absolute limits on file size, but I doubt it. I also don't know how thorough their models are for the linear parts and whether they pick up stuff like die heating. I've only used the SW for switchers and they've focused lots of attention on getting those good enough to be useful.
Regards, Rick
Thanks, I'd very much like to play with that software. Where can I get it?
Go here and look for adddistortion.zip.
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