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What do you look for in the distortion spectrum?
What do you look for in the distortion spectrum?
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Old 11th May 2009, 12:56 AM   #1
keantoken is offline keantoken  United States
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Default What do you look for in the distortion spectrum?

Hello all.

In my different threads regarding my designs, I have read "well I like this way better, because there's less 7th harmonics (or somesuch)".

This is not a thread to debate Distortion vs. Ear, I simply want to know what you prefer to see in the distortion spectrum so I have some other ideas to aim for when I get tired of simply trying for lowest THD. I expect differing views and encourage them.

Thank you,
- keantoken
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Old 11th May 2009, 05:21 AM   #2
unclejed613 is offline unclejed613  United States
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i usually look for lowest distortion in a spectrum display. some forms of distortion cause even harmonics and some cause odd harmonics, so if you have high odd harmonics, the causes are different from even harmonics, and the mechanisms that cause one type or the other is what i concentrate on correcting. it is also a good idea to check your signal source first to know what it looks like, and record the source's harmonic content by harmonic and relative amplitude. that way, you aren't chasing "ghosts" when you are "tweaking" your design. a spectrum analyzer will also show the hum and noise content as the amplifier is operating with signal. that's not something a noisemeter can do.

things like "high 7th harmonic" don't really happen. usually if one odd harmonic is high, so are the other odd harmonics, same goes for the even harmonics.
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Old 11th May 2009, 10:17 PM   #3
keantoken is offline keantoken  United States
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Hmm.

Surely we can hook the spectrum analyzer with a diff. amp and only look at the difference between the input and output.

BTW, as a side note for anyone interested, I ordered a 3A9 diff. amp plugin and a 3B4 timebase for my vintage Tek 561B, along with a Fluke 8800A, an S/D datapulse Versatester and an old HP signal generator.

So if and after I get done calibrating, I'll have a far more versatile test rig... Now I need to work on a power supply. O_O

Anyways, as I said, all opinions are welcome.

- keantoken
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Old 11th May 2009, 11:39 PM   #4
unclejed613 is offline unclejed613  United States
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Quote:
Originally posted by keantoken
Hmm.

Surely we can hook the spectrum analyzer with a diff. amp and only look at the difference between the input and output.
actually that's what a distortion analyzer does.
Quote:
Originally posted by keantoken


BTW, as a side note for anyone interested, I ordered a 3A9 diff. amp plugin and a 3B4 timebase for my vintage Tek 561B, along with a Fluke 8800A, an S/D datapulse Versatester and an old HP signal generator.

So if and after I get done calibrating, I'll have a far more versatile test rig... Now I need to work on a power supply. O_O

Anyways, as I said, all opinions are welcome.

- keantoken
i used to have a dual diff amp plugin for my 541. unfortunately it got damaged (bent switch shafts) during a move, so all i was left with is the 2 channel module. if you need to calibrate the horizontal timebase, let me know, there are a few ways of doing it correctly that don't require expensive timebases. the ramp linearity i think is the difficult one since there are a few adjustments that interact. (edit...) you live in TX, so you should have a stable WWV signal to zero-beat a 10Mhz oscillator against, which if i remember correctly should give you a 10Mhz timebase that's accurate enough for calibrating that vintage scope. in the Army, we had a timebase that was sync'ed to WWVB and was accurate to 1 part in 10E-11 (10E-12 on really good days), but for audio work that's not really necessary.

as far as the vertical alignment, that's usually simpler, requiring accurate voltage sources.
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Old 11th May 2009, 11:48 PM   #5
Wavebourn is offline Wavebourn  United States
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What do you look for in the distortion spectrum?
My rule about non-linear distortions:
The closer to zero power, the closer to zero non-linear distortions, the lower is their order. However, it does not mean that on higher currents they are allowed to be huge with wide specter, because they cause intermodulations.
Hence, the choice of topologies: less of active elements working linearly, plus feedback to sweep the garbage under the rug on higher powers.

And, of course, thinking of distortions I keep in mind everything I have the control of: from mics to speakers. The system made of optimal subsystems can never be optimal -- the main axiom I follow.
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Old 12th May 2009, 12:59 AM   #6
keantoken is offline keantoken  United States
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Quote:
Originally posted by unclejed613


actually that's what a distortion analyzer does.

i used to have a dual diff amp plugin for my 541. unfortunately it got damaged (bent switch shafts) during a move, so all i was left with is the 2 channel module. if you need to calibrate the horizontal timebase, let me know, there are a few ways of doing it correctly that don't require expensive timebases. the ramp linearity i think is the difficult one since there are a few adjustments that interact. (edit...) you live in TX, so you should have a stable WWV signal to zero-beat a 10Mhz oscillator against, which if i remember correctly should give you a 10Mhz timebase that's accurate enough for calibrating that vintage scope. in the Army, we had a timebase that was sync'ed to WWVB and was accurate to 1 part in 10E-11 (10E-12 on really good days), but for audio work that's not really necessary.

as far as the vertical alignment, that's usually simpler, requiring accurate voltage sources.
Hmm. I'll have to make a crystal radio or something so I can "tune in". As far as voltage sources, I might have a problem there. I think I have a TL431 sitting on the floor somewhere . I can use a pot to make it into a shunt reg. I'll have to see if that Fluke is accurate enough to adjust it. I can have my own poor-man's DC reference Shunt reg... I wonder if I can hook it up to the scope and see the noise? The 3A9 has 10uV/div sensitivity so it's possible...

And I've always been fascinated with the idea of making miniature ovens to keep important components temperature regulated. I would really love to make myself a bunch of calibration devices and references...

Anyways.

Quote:
Originally posted by Wavebourn
My rule about non-linear distortions:
The closer to zero power, the closer to zero non-linear distortions, the lower is their order. However, it does not mean that on higher currents they are allowed to be huge with wide specter, because they cause intermodulations.
Hence, the choice of topologies: less of active elements working linearly, plus feedback to sweep the garbage under the rug on higher powers.

And, of course, thinking of distortions I keep in mind everything I have the control of: from mics to speakers. The system made of optimal subsystems can never be optimal -- the main axiom I follow.
Interesting axiom. I will think about it. Theoretically, a perfect system must act as one. As far as speakers and Mics, I have pretty much nothing of value, except perhaps my Boston Acoustics A40 enclosures, which don't have the original speakers in them. This is one of the reasons I want to hurry up and make myself an amp. But until I get some kind of soldering rig, I think I'm stuck.

- keantoken
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Old 12th May 2009, 01:02 AM   #7
Wavebourn is offline Wavebourn  United States
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What do you look for in the distortion spectrum?
I assume speakers are the hardest part in the system to adjust.
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Old 12th May 2009, 01:05 AM   #8
keantoken is offline keantoken  United States
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That's what I get from what people say.

There's no .0002% THD speaker!

Or so they say.

- keantoken
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Old 12th May 2009, 01:12 AM   #9
Wavebourn is offline Wavebourn  United States
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What do you look for in the distortion spectrum?
Quote:
Originally posted by keantoken
That's what I get from what people say.

There's no .0002% THD speaker!

Or so they say.

There is a lot of wisdom in Passlabs forum, including about driving speakers. I strongly suggest you to go there and read, in order to train the proper approach from first steps.

http://www.diyaudio.com/forums/forum...p?s=&forumid=8

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Old 12th May 2009, 01:23 AM   #10
kenpeter is offline kenpeter  United States
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And if there was, then phase wouldn't be flat.
And if there was, then dispersion wouldn't be even.
And if there was, something else surely still be wrong...

Its mostly about musical errors vs non-musical errors.
Here, THD is usually your friend. IMD usually your enemy.
Assume things will occasionally go outside the envelope,
and the loudspeaker wasn't put in your way to help.

You got to look at the speaker and output stage as a
unified system that have to fail elegantly together, and
fake good like it was intentional. If they are trippin over
each other's toes, it can be a trainwreck.
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