distortion characteristics

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guze

Well-known member
Joined
Mar 10, 2009
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85
Hi
I was checking the harmonics on some gear, and i was curious what is this characteristic that the tone and harmonics have "around" them

i was feeding a 1khz tone and all with 1-3db gain reduction

one is normal with the harmonics showing up but "clean" (overstayer fet)
another one has all these extra around the harmonics (collins 26j-1 vari mu)
and the other one, has no harmonics but a lot around the tone  (dbx 160x)

sorry if i am not clear with the language
 

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guze said:
Hi
I was checking the harmonics on some gear, and i was curious what is this characteristic that the tone and harmonics have "around" them

i was feeding a 1khz tone and all with 1-3db gain reduction

one is normal with the harmonics showing up but "clean" (overstayer fet)
another one has all these extra around the harmonics (collins 26j-1 vari mu)
and the other one, has no harmonics but a lot around the tone  (dbx 160x)

sorry if i am not clear with the language
Click to expand...
Each piece of gear has it's threashold at which it will start to generate higher harmonics. Especially FETs and tubes and VCAs. The harmonics "around" the peak in the DBX is Intermodulation Distorition.

So you should really study that threashold of each piece of gear by starting at -20dBu, watch the harmonics and then slowly increase the output level to see where the harmonics start to jump up.

It might be good to figure out exactly what the levels out / in from your audio interface are. What is -3dB? If 0dB is +4dBu, then is -3dB +1dBu?  That's pretty hot. In practice, you would not be feeding a pure tone through your gear. A vocal or drum track is not a pure tone. So if your plots look a lot better at -10dBu, then that would all be pretty normal.
 
On the second graph (Collins), the input signal is modulated by a strong hum component which you can clearly see at 50 and 100 Hz. It should be there even with no signal/no compression present, unless this unit only starts to hum when compressing which is highly unlikely...
 
squarewave said:
Each piece of gear has it's threashold at which it will start to generate higher harmonics. Especially FETs and tubes and VCAs. The harmonics "around" the peak in the DBX is Intermodulation Distorition.

So you should really study that threashold of each piece of gear by starting at -20dBu, watch the harmonics and then slowly increase the output level to see where the harmonics start to jump up.

It might be good to figure out exactly what the levels out / in from your audio interface are. What is -3dB? If 0dB is +4dBu, then is -3dB +1dBu?  That's pretty hot. In practice, you would not be feeding a pure tone through your gear. A vocal or drum track is not a pure tone. So if your plots look a lot better at -10dBu, then that would all be pretty normal.
Click to expand...
+1 look at lower signal levels

if the signal saturates a path into an unregulated PS rail the PS ripple will show up in the signal...

I can tell that you live in a country with 50Hz mains power.

JR
 
i was sending a -12dbfs(+6dbu) 1khz tone from the audio interface, compress 1-3dbs , makeup gain to be +- the same signal on the input again

I put the overstayer fet for comparison, nut i had other that behave "normal"

The 160x and the 26j-1 surprise and i don't know what is causing it.

the 160x and the 26j-1 have been recapped

is that intermodulation distortion or distortion from the power supply ripple?

i live in portugal, so , 50hz


 
guze said:
i was sending a -12dbfs(+6dbu) 1khz tone from the audio interface, compress 1-3dbs , makeup gain to be +- the same signal on the input again
Click to expand...

That's a pretty hot signal, especially for the 26J.  Consider that it's a tone, and we don't generally deal with steady state tones, but signals with varying dynamics, mostly much lower. 
 
Both the Collins and 160x shows a large dose of mains-derived hum, that probably permeates into the control voltage, thus producing these intermodulation fringes.
Apart from that, the harmonic distortion content is not surprizing.
I suggest you check the power rails on the Collins and the 160x, but it could also be magnetiaclly-induced hum. Have you made sure they are not submitted to undue magnetic field?
 
abbey road d enfer said:
I suggest you check the power rails on the Collins and the 160x, but it could also be magnetiaclly-induced hum. Have you made sure they are not submitted to undue magnetic field?
Click to expand...

The Collins at least, yes, it's probably somewhat induced hum from the power transformer, acceptable levels for the era in which it was made.    Tube quality and good push-pull balance also changes results easily. 

The Collins could definitely raise filter capacitance to see if anything changes.  Pair of odd value 60 mfd in the PSU section and one other 20mfd on the way to the 6386, that's it.  Elevated center tap on a filament AC winding. 

Also, it's very important with fixed gain amps like that to run tests under real world attenuator settings, even if you do also run with attenuators wide open for one benchmark. 
 
squarewave said:
Each piece of gear has it's threashold at which it will start to generate higher harmonics. Especially FETs and tubes and VCAs.
Click to expand...

I do not think  audio gear has a distortion threshold. A threshold would imply a discontinuity and audio systems, though they may be non-linear, are not usually discontinuous. However, the degree of non-linearity does vary with level (you can see this in the shape of the transfer function)  so you generally get more distortion at higher levels.

Cheers

Ian
 
I have the 160x and 26j1 in a separate rack because they interfere with the other outboard.
the 160x maybe because the transformer is in the right side, or maybe because it's not a toroidal
and the 26j1 because it really needs a distance from other gear

i am gonna do a test with a dual tone generator to check intermodulation distortion. And move the 160x far away from the 26j1

i don't want to lower/improve the harmonics present, i like them , i don't like the HUM and if that can get better is good. I am not trying to get "rid" of the harmonics, just understand them better
 
ruffrecords said:
I do not think  audio gear has a distortion threshold. A threshold would imply a discontinuity and audio systems, though they may be non-linear, are not usually discontinuous. However, the degree of non-linearity does vary with level (you can see this in the shape of the transfer function)  so you generally get more distortion at higher levels.

Cheers

Ian
Click to expand...
Most audio paths have an obvious threshold caused by power supply saturation. Some topologies become increasingly nonlinear as they approach rail saturation.

There are multiple different mechanisms involved in characterizing distortion.

JR
 
JohnRoberts said:
Most audio paths have an obvious threshold caused by power supply saturation. Some topologies become increasingly nonlinear as they approach rail saturation.
Click to expand...
IMHO  the power rails are a limit rather than a threshold and represent a region outside the normal operating parameters. Below that limit there are no thresholds.

I know guitar amps and fuzz boxes are an exception to this rule but the rule still holds.
There are multiple different mechanisms involved in characterizing distortion.

JR
Click to expand...

Indeed there are.

Cheers

Ian
 
ruffrecords said:
IMHO  the power rails are a limit rather than a threshold and represent a region outside the normal operating parameters.
Click to expand...
perhaps I've spent too much time close to the limits.

Tubes are notorious for beginning to distort well before saturations.

Even with solid state amps there are significant differences in power output between 3% distortion, 0.3% distortion and 0.03 %.  I was a product manager for power amps and knew all too well how they distort close to clipping.
Below that limit there are no thresholds.

I know guitar amps and fuzz boxes are an exception to this rule but the rule still holds.
Indeed there are.
Click to expand...
not talking about efx...

JR
Cheers

Ian
Click to expand...
 
JohnRoberts said:
perhaps I've spent too much time close to the limits.
Click to expand...
And I have spent little if any time there. My experience has mostly been at +4dBu with a good 20dB of headroom to play with. Not surprisingly our views reflect our personal experience
Tubes are notorious for beginning to distort well before saturations.
Click to expand...

I would say tube designs begin to distort well before saturation. This is principally because they lack the huge open loop gains and vast quantities of NFB that transistor designs have which linearizes the transfer function right up to the point of clipping.

Without NFB, tubes and transistors both exhibit distortion over a wide range of signal levels.

Cheers

Ian
 

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