[imackler]

Quote:

Low gain = more global feedback = less involving sounding. YMMV and depends on the implementation. Very noticeable on certain amps. 

 
What do you mean by "more global feedback"? I'm intrigued. I definitely have found the "less involving sound" part to be true! 

 

[stv014]

Quote:

Low gain = more global feedback = less involving sounding. YMMV and depends on the implementation. Very noticeable on certain amps. 

 
I find the volume mismatch explanation more likely, especially since people have reported the same effect on the perceived sound quality with digital gain. Does the sound improve if you put a 390 Ω resistor between the inverting and non-inverting input of the gain stage op amp (less feedback, but still only 2.5x gain) ?

 

[purrin]

Quote:

What do you mean by "more global feedback"? I'm intrigued. I definitely have found the "less involving sound" part to be true! 

 
Starting building some amps! http://www.tangentsoft.com/audio/
 
I stand corrected. I believe the feedback on the O2 is only local. Still likely to have an effect.

 

[purrin]

Does the sound improve if you put a 390 Ω resistor between the inverting and non-inverting input of the gain stage op amp (less feedback, but still only 2.5x gain) ?

 
Hmmm. It's worth a shot. The noise floor may rise from -232db to -228db though.

 

[MrMateoHead]

Quote:

Low gain = more global feedback = less involving sounding. YMMV and depends on the implementation. Very noticeable on certain amps. 

What the heck is "global feedback"???
 
What is "YMMV"?
 
Opps, someone beat me to the question. What is "Local Feedback"?

 

[purrin]

WARNING. THIS IS IN DIRECT OPPOSITION TO THE PHILOSOPHY OF NWAGUY AND HIS MASTER DOUG SELF 
 
http://www.passdiy.com/pdf/distortion_feedback.pdf

 

[agdr]

Quote:

What the heck is "global feedback"???
 
What is "YMMV"?
 
Opps, someone beat me to the question. What is "Local Feedback"?

 
Global feedback means feedback from the final output terminals of an amplifier back to the input, no matter how many amplifier stages (op amps or transistors or tube gain stages) are in the middle.  For example, in the schematic of AMB's b22 amplifier, R4 is a global feedback resistor.
 
YMMV = Your Mileage May Vary

 
Local feedback means feedback from the output back to the input of just one single stage in an amplifier.  For example, if an amplifier has 3 op amps in a row, each one setup for a voltage gain of 2 times (so 2 x 2 x 2 = 8 times voltage gain total from end-to-end), feedback from the output of the 1st op amp stage back to it's input is local feedback just for that 1st stage. Feedback from the output of the 3rd op amp stage back to the input of the 1st would be global feedback in this case.

 

[BleaK]

Quote:

Does anyone have experience with the O2+ODAC combo and a pair of HE-500s?

 
This is my main rig at the moment. With O2 in low gain. This is personally the best combo I have heard with the HE-500.
 
 
Again, personally I don't get the craze about having alot of power to drive orthoes. I have owned the NFB-10SE which puts out 6W at 50ohm, and I still thought the ODAC/O2 combo sounded better.
 
This my preference of course, and someone or many will surely don't agree with it, which is of course totally ok

 

[HamilcarBarca]

Quote:

WARNING. THIS IS IN DIRECT OPPOSITION TO THE PHILOSOPHY OF NWAGUY AND HIS MASTER DOUG SELF 
 
http://www.passdiy.com/pdf/distortion_feedback.pdf

 
Thanks for the warning. It's clear it simply marketing material to help sell Pass Labs equipment. It invents audiophile "extremes", and then uses the resulting strawmen to justify his $60,000+ equipment.
 
Does his equipment sound good? Perhaps.
 
Are his white papers any any more valid than AudioQuest's justification for its pure unobtainium wire? Not on its surface.

 

[purrin]

Actually the PassDIY stuff is his DIY stuff where you can get his schematics for free and build his designs. Lots of people on diyaudio.com have built his designs (or even tried to figure out the schematics of his new ones.) The Pass Labs is his commercial stuff which surprisingly isn't that expensive. No doubt he's very good at tooting his own horn. He's very well known after all. (I'm not endorsing his nor I do own any of his stuff.)
 
No matter who writes what, one still needs to think for themselves, and its best is to experiment to validate various claims. The negative feedback arguments have been around since the dawn of audio reproduction. Here is another article, this time from Stereophile (I know - not exactly the best source) http://www.stereophile.com/reference/70/index.html. My own experiences with building some simple amps and messing around with circuits of tube amps mirrored that of the Stereophile writers' experience, although I feel I'm more in the middle ground and not in the ZERO feedback camp.
 
Ultimately, I think you can take little bits from Doug Self and a little bits from Nelson Pass to use them for your one's own projects if one is so inclined to get his feet wet.

 

[HamilcarBarca]

Quote:

Actually the PassDIY stuff is his DIY stuff where you can get his schematics for free and build his designs. Lots of people on diyaudio.com have built his designs (or even tried to figure out the schematics of his new ones.)

 
I missed that reference.
 
While I'm not an EE, I find the technical discussions quite interesting. Thanks.

 

[MrMateoHead]

Quote:

 
Thanks for the warning. It's clear it simply marketing material to help sell Pass Labs equipment. It invents audiophile "extremes", and then uses the resulting strawmen to justify his $60,000+ equipment.
 
Does his equipment sound good? Perhaps.
 
Are his white papers any any more valid than AudioQuest's justification for its pure unobtainium wire? Not on its surface.

I looked at the paper. I got no idea why they are selling a 1-3 watt amp with upwards of 10% distortion and calling it clean.
 
Likewise, it doesn't seem rational that you could "add distortion" to an amp to achieve insane signal to noise levels.
 
I am no engineer but .. . . .

 

[stv014]

The error in that PDF file is that the "complex signals" are not normalized to a +/-1 Volt range, and they use a 3rd order polynomial for the distortion that gives increasing amount of distortion at higher input signal levels, so with nearly 5 Volts of peak voltage for the 7 tone signal (as opposed to just 1 Volt for the single sine wave), there is no surprise that the amount of distortion would be huge. Especially with the polynomial being applied four times in a row. For more realistic results the input signals should always have had a range of +/-1 Volt, and the distortion polynomial should have been scaled so that its output is also within the same range.
 
To illustrate the above point, here are a few graphs:
 
100 Hz sine wave with 1% 2nd order and 1% 3rd order distortion (y = (0.97*x + 0.02*x^2 + 0.04*x^3 - 0.01) / 1.02), the distortion is applied four times to simulate 4 amplifier stages (left channel is the input signal, right channel is the distortion residual):
   
101, 173, 307, 523, 919, 1607, and 2801 Hz sine waves, each at 1 / 6.0142 amplitude (peaks within +/- 1), the distortion is the same as above:
   
The same mix of sine waves, but with 1 / 2.5 amplitude for each tone (peak level = ~2.4):
   
 
With an out of range signal, like in the PDF, the distortion increases by orders of magnitude. However, once the peak level is normalized, the distortion peaks are not higher than with the simple sine wave input. Of course, that is to be expected, since the distortion is frequency independent, and can be described with a simple polynomial with no "memory" of any previous input. So, the article in the PDF file is somewhat misleading.
 
It is possible to test if an amplifier that uses negative feedback has unexpected high distortion when fed with a complex signal using null testing, and also double-blind listening tests. However, as far as I know, if the distortion+noise is low with any sine wave input (at any level and frequency within the range the amplifier is intended to be able to handle, and with a wide bandwidth measurement), then it is expected to be low also with music.
 
By the way, I recommend reading this article by Bruno Putzeys on negative feedback, if you have not already done so. It explains well the origin of some feedback myths, and also why they are wrong.

 

[BleaK]

Great post and article, thank you!
 
I am learning new things everyday. The more I read, the more I feel "drawn" to the objective camp.
 
There are so many things that can influence psychoacoustics, but your gear shouldn't be one of them.

 

[BleaK]

On another note, looks like the O2 measure quite well against more expensive amps: http://www.innerfidelity.com/images/121105_blog_update_FirstAmpTests.pdf