Measurements on amplifiers with speakers and cables connected

[March Audio]

The simple 1kHz test above showed no significant difference in distortion levels between the two cables. I will be doing more tests with intermodulation signals - I am also going to check out the low level signals either side of the 1kHz as they have disappeared on the 1m speaker cable plot.

However the frequency response test did show a small difference.

MDAC FR

NCORE FR 8ohm resistor

FR at speaker 1m cable

FR 10m cable (1m overlaid) Seems to peak at crossover.

 

[March Audio]

As a Hypex OEM, and you being a fellow member, maybe I could find some unit's for you for audio science experimentation applications.

View attachment 964

Now that would be very interesting. I would like to see if we can characterise the difference between the various input opamps.

 

[AJ Soundfield]

To my ears they just sound clean and neutral with no real "sound".

Exactly, the Hypex modules are amplifiers. Input voltage multiplication.
If you want special efx boxes, etc, look elsewhere for your real/imaginary "sound".

cheers,

AJ

 

[Purité Audio]

Very interesting thread ,thanks chaps, presumably valve amps with high output impedance may not enjoy driving loudspeakers which have wildly fluctuating impedance curves, and would prefer flat impedance characteristics?
Keith

 

[March Audio]

Very interesting thread ,thanks chaps, presumably valve amps with high output impedance may not enjoy driving loudspeakers which have wildly fluctuating impedance curves, and would prefer flat impedance characteristics?
Keith

That is my understanding, but never measured.

 

[AJ Soundfield]

Very interesting thread ,thanks chaps, presumably valve amps with high output impedance may not enjoy driving loudspeakers which have wildly fluctuating impedance curves, and would prefer flat impedance characteristics?

If you want relatively unchanged FR vs a real amplifier, yes. However, audiophiles often want "excitement" and "musicality", from throwing darts at the board over years to achieve that magical mystical "synergy", that a high school level science education could have found immediately.
It's trivially easy to design a passive speaker with relatively flat impedance. Heck, even I can do that.

cheers,

AJ

 

[Phelonious Ponk]

how is all of this affected by active designs? Any advantages, other than very short speaker cables?

Tim

 

[AJ Soundfield]

how is all of this affected by active designs? Any advantages, other than very short speaker cables?

Several.
Passive speaker crossovers are designed using small signal analysis of the components and simulation software. The problem is that everything changes under dynamic load, i.e large/varying signals. Measure a driver at small signal, get one set of T/S parameters. Large signal, get another. Which is correct for the passive filter design?
With active, the XO is before the amp. It drives the input of the amp and does no see such variations of the load.
With the amp directly coupled to the driver, variables like (passive) inductor saturation, etc are eliminated. There is also the possibility of driver control via motion feedback methods, aka servo. The amps face only the load they are connected to, so if the bass amp, which most likely has the toughest load and music power spectrum, clips, but the tweeter amp does not, then the distortion spectra is limited to the woofer, vs full bandwidth where it is far more likely to be audible.
Etc etc.
The downside is it gives audiophiles 2 less things to muck around/have illicit love relationships with - amps and speaker cables.

cheers

AJ

 

[March Audio]

how is all of this affected by active designs? Any advantages, other than very short speaker cables?

Tim

First impression is that the frequency response variation, which is pretty subtle in this example, is just the obvious voltage drop across the cable. Where the speaker impedance is lower, more current, more voltage drop. The very short cable, which is how I operate them, shows no FR variation. Nothing surprising there !

Active designs have two obvious advantages, the speaker cable is as short as is possible therefore has no significant impact. Second is the implementation of an active crossover, analogue or digital. The amp has a less complex load to deal with.

Im sure AJ will point out some further benefits.

He did seconds before me

 

[Phelonious Ponk]

First impression is that the frequency response variation, which is pretty subtle in this example, is just the obvious voltage drop across the cable. Where the speaker impedance is lower, more current, more voltage drop. The very short cable, which is how I operate them shows no FR variation. Nothing surprising there !

Active designs have two obvious advantages, the speaker cable is as short as is possible therefore has no significant impact. Second is the implementation of an active crossover, analogue or digital. The amp has a less complex load to deal with.

Im sure AJ will point out some further benefits.

He did seconds before me

He's a quick one.

tim

 

[Purité Audio]

If you want relatively unchanged FR vs a real amplifier, yes. However, audiophiles often want "excitement" and "musicality", from throwing darts at the board over years to achieve that magical mystical "synergy", that a high school level science education could have found immediately.
It's trivially easy to design a passive speaker with relatively flat impedance. Heck, even I can do that.

cheers,

AJ

Flat impedance is quite rare though isn't it?
Kef used to implement the design, then stopped because it was too expensive?
Keith

 

[DonH56]

Flat electrical impedance does not mean flat acoustic output, unfortunately. Usually goes the other way...

 

[Purité Audio]

I just remember reading about some KEF speakers that had been designed to have flat impedance 4ohm if I recollect ,can't rememeber the models or what exactly KEF did though
Keith

 

[March Audio]

Flat impedance is quite rare though isn't it?
Kef used to implement the design, then stopped because it was too expensive?
Keith

Dug out an old Stereophile review of my speakers. The tweeter has changed, so I suspect there may have been some XO changes since then, but you can see the impedance plot line up fairly with the Long cable FR plot above.

As I'm sure you already do Keith , the lesson is nothing esoteric on the cable front necessary, just short as possible, thick (low resistance) as possible.

 

[AJ Soundfield]

Flat electrical impedance does not mean flat acoustic output, unfortunately.

It means minimizing the EQ effect with high output impedance amps. Whatever the acoustic output of the speaker, the flat impedance will create minimal differences in the FR with a real amp vs one of those special efx SET, etc. boxes.

 

[Opus111]

Am I reading your FFTs correctly BE - I see slightly over 1% 3rd harmonic with the 1kHz test tone after 10m of cable? Try it again but use 200Hz rather than 1kHz to see if the distortion's different.

 

[Mivera]

Now that would be very interesting. I would like to see if we can characterise the difference between the various input opamps.

I've encouraged Colin from Nord to do this, but he doesn't have the gear. But he might wanna think about getting it as some of his competitors might

 

[DonH56]

REW plot of magnitude and phase, L+R+subs, Magnepan MG-IIIa's plus four Rythmik F12 subs crossed at 70 Hz. There is a significant slope from Dirac Live's target curve. There is a lot going on; I would not try to say what causes all the various peaks and valleys. It's a busy room despite all the treatments. The current response is a bit better but I didn't find a plot easily (may have used a different tool for the final measurements). This does show a generally falling phsae (unwrapped it is pretty linear) to about 200 Hz, at which point it rises for a while before falling again. Magnepan uses a pretty simple crossover.

 

[March Audio]

Am I reading your FFTs correctly BE - I see slightly over 1% 3rd harmonic with the 1kHz test tone after 10m of cable? Try it again but use 200Hz rather than 1kHz to see if the distortion's different.

You can see the 3rd increases from -150 to around -140dB on the plots. Thats around 100dB down on the fundamental so thats not 1% I mentioned earlier I should be able to get REW to plot distortion against frequency through out the range. Will try when I get time.

 

[March Audio]

REW plot of magnitude and phase, L+R+subs, Magnepan MG-IIIa's plus four Rythmik F12 subs crossed at 70 Hz. There is a significant slope from Dirac Live's target curve. There is a lot going on; I would not try to say what causes all the various peaks and valleys. It's a busy room despite all the treatments. The current response is a bit better but I didn't find a plot easily (may have used a different tool for the final measurements). This does show a generally falling phsae (unwrapped it is pretty linear) to about 200 Hz, at which point it rises for a while before falling again. Magnepan uses a pretty simple crossover.

View attachment 993

I presume thats an acoustic measurement? We are really looking at the electrical measurement at the speaker WRT amp and cable performance.