Spectrum Analyzer Test - 6PS vs 715P Capacitors

[Littlewyan]

I'll try and do some reading up on it, thanks John.

[Merlinb]

Hi,
"One thing which hasn't been mentioned is injecting various different frequencies all at the same time just like a guitar would."
That is what intermodulation distortion is all about. In english.
It's a lot more complex to determine, so it seems possible that there are people who prefer to pretend it doesn't exist.

IMD is proportional to THD, so in general, measuring one is all that is necessary. Measuring both rarely gives you much new information.

[schaublin65]

Hi,

"IMD is proportional to THD, so in general, measuring one is all that is necessary. Measuring both rarely gives you much new information."

I believe the standard method of IMD measurement uses two tones.

Since he is quoted as an authority earlier in this thread I wonder what you think about Rod Elliotts conclusions on the subject of IMD measurement?

http://sound.westhost.com/articles/intermodulation.htm

The "short version" of his testing seems to lead him to believe that

"On the basis of this information, it is likely that the vast majority of intermodulation distortion tests are meaningless."

As he points out this problem has been understood since 1927 and yet it seems poorly covered in the available literature. The 1958 Crowhurst article certainly makes interesting reading.

http://www.vias.org/crowhurstba/crowhur ... 2_077.html

I feel that this is something often "handwaved" away without a scientific basis.

The "two tone" test is a long way short of determining the results of putting a minor 7 chord through the DUT. At least that's how it seems to my understanding.

Take care....

John

[Merlinb]

Since he is quoted as an authority earlier in this thread I wonder what you think about Rod Elliotts conclusions on the subject of IMD measurement?

I agree with Rod; IMD measurements are largely meaningless except as a way of comparing otherwise very similar circuits. There is an infinite number of ways to measure IMD (what frequencies to use, what relative amplitudes...) and each one will give a different answer depending on the 'type' of distortion in the circuit, making it virtually impossible to compare different measurements. Even if everyone agreed always to use the same test (and there is a couple of standards), critics of THD would simply shift their attention and say "yeah, but this IMD test doesn't reveal the true nature of the circuit. You need to use a different IMD test". Ad infinitum.

The great thing about THD is that it is a single number that everyone can understand and always means the same thing. Now, I agree that a THD number by itself is rather vague, but if you look at the harmonic spectrum at the same time as the total THD, you have all the info you need. The harmonics (by definition) tell you type of distortion you're dealing with. In principle, if you know the harmonic amplitudes, you can work out the IMD too, without actually measuring it. This is why an IMD test doesn't really tell you any more than a THD test does (provided you look at the spectrum, not just the THD number). But the THD test is easier to perform and a lot more 'visually intuitive' when looking at the spectrum.

[schaublin65]

Hi,

I am not so sure that IMD is a meaningless measurement.

http://www.rane.com/note145.html

"A more meaningful test than THD, intermodulation distortion gives a measure of distortion products not harmonically related to the pure signal. This is important since these artifacts make music sound harsh and unpleasant."

You say that the harmonics tell you the type of distortion.

http://www.aes-uk.org/forthcoming-meeti ... asurement/

"When more complicated signals are distorted by nonlinear functions, it is known that harmonic distortion is a very small part of the overall picture: Brockbank and Wass determined analytically that, for a signal containing thirty harmonic products, the intermodulation distortion generated by a nonlinearity in the system comprises 99% of the total distortion power2. Full measurement and analysis of intermodulation distortion requires at least as many components in the input signal as harmonics that are under scrutiny."

I agree that THD is the easiest to measure.

Take care...

John

[Littlewyan]

So there could still be something in changing capacitor types, I like where this discussion is going.

[Cantplay]

Are you able to plot waterfall charts that show decay of a note?

John

[schaublin65]

Hi

"So there could still be something in changing capacitor types"

I think we are just enquiring into the "a priori" assumptions that are sometimes made.

As you can see from the Rane link the cinema people were thinking about IMD before WW2.

The work cited in the AES link is an IEE publication from 1945.

The Crowhurst article from 1958 is concerned with IMD of amplitude.

The AES link is examining distortion of phase.

This is far from simple stuff. I am learning here myself.

Take care....

John

[Structo]

Something mentioned in passing here was the influence of the voltage rating of an electrolytic capacitor.

For example, in a Dumble ODS amp, the cathode bypass caps in the preamp are generally 50v.
5uF - 10uF caps are normal here.

It seems to get harder and harder to source these below 50v.

When not available, I'm sure that higher voltage rated caps are substituted out of necessity.

Can someone explain how the rated voltage of a, say a 4.7 or 5uF with a 100v rating can affect the tone of a preamp signal compared to a 50v or lower rated cap?

[schaublin65]

Hi,

Tone is hard stuff to get reliable information on.

A capacitor that is pure capacitance without any other electrical properties is equally difficult to put your hands on.

A classic analysis of circuit components is the Thevenin Theorem.

More complex methods exist but for an electro the Thevenin model is probably good.

The theory is that the cap acts as if it has a resistor in series with it.

Equivalant Series Resistance is a normal specified parameter for electros.

The ESR of an electro is probably its most significant property after its capacitance. If the resistance goes high so will the temperature. The current involved obviously influences this.

I'm sure you know how unpleasant the resulting mess can be to clean up.

So a cap with a higher voltage rating but the same ESR is likely to perform just as well or just as badly.

I frequently replace Surface Mount caps with low voltage ratings but they would look very strange in an amp.

Take care....

John

[Merlinb]

Can someone explain how the rated voltage of a, say a 4.7 or 5uF with a 100v rating can affect the tone of a preamp signal compared to a 50v or lower rated cap?

Electrolytic caps generate a good deal more distortion than film caps, but again, the same general rules apply:
1: Distortion is proportional to the signal voltage across the cap. This means the distortion will occur near to the -3dB frequency. Above or below this the signal voltage is either very small, or the output signal becomes so small that distortion becomes lost in the noise floor.
2: Distortion falls as the voltage rating is increased.
3: Distortion is less when the DC polarising voltage across the cap is just a few volts (more than zero but less than 10V, usually). Good news for cathode bypassing!
4: Non-polar aluminium electrolytics generate about an order of magnitude less distortion than polar aluminium types.
5: Tantalums are about an order of magnitude worse than polar aluminium electros.

Whether the distortion is detectable in a cathode bypass application, I don't know, but it is something I intend to investigate very soon.

[Reeltarded]

Man, if you think changing all the caps makes a difference you should try swapping the circuit out to my magic Marshall specs and stop messing with your amp for good.

[Littlewyan]

I might be changing out the bright cap silver mica tonight going from 470pF unknown brand to 500pF Cornell Dubilier. Are you guys interested in having before or after results from my Spectrum Analyzer?

[Littlewyan]

I did the same test as before except I was rushing this time and messed it up. I've got results but don't trust them so won't be posting them. Now this time the only capacitor I changed was the 470pF Silver Mica on the bright switch with a 500pF Silver Mica, again I heard a big difference in tone and also the noise level.

The tone is probably due to the old silver mica being absolute crap (I reckon this is what made a big difference with the last cap change), noise level could be for the same reason however I also redid my bright switch wiring at the same time. Although I still think it was the old silver mica that introduced a lot of noise as you even heard it without a guitar plugged in. There used to be a massive change in hiss going from 100pf and 470pf, now going from 100pf to 500pf doesn't make much difference to the noise level.

I've attached photos to show how I had the bright switch wired previously.

Unfortunately after I put the chassis back in the head cabinet I noticed that my volume pot is a bit scratchy

[Littlewyan]

Also I should add that I still think the Polyesters may have contributed to the change in tone. I should also point out that when I left my scope on the amp and was watching the harmonics I noticed that a lot of them were going up and down without any change in the input signal. So the differences I picked up with the scope originally may have been nothing. I guess the harmonics are going up and down due to the valves. Not all harmonics did this though, only some and they didn't seem to stabilize.

I still have two more coupling capacitors to change out in the output stage if anyone is interested in another test.