Do class D amps sound different?

[Bob Lee (QSC)]

High damping factor and slew rate are probably some of the least important specs in MI and pro audio, high numbers are nice for marketing, but do not make any meaningful difference as long as they are high enough. Both class D and linear AB amps are well beyond high enough.

If an amp slew-rate limits on bass, man, it would have do be almost deliberately bad.

 

[Son of Wobble]

Yup, primarily in the IR spectrum which we perceive as heat.

And thank you for not bringing up the dreaded TalkBass car analogy.

A car analogy and a lightbulb analogy both work equally well to deliver us to our point.



By the way -- great thread! More, please.

 

[Pilgrim]

Both of you guys are hitting on what I'm thinking as well. My brain says the the power amp topology (Class AB vs digital Class D) has basically nothing to do with the overall sound as they probably have sufficient damping factors, slew rates, etc to both be transparent. The preamp is where the sound is generated (or colored).

yes?

Well, maybe not. Everything matters, everything POTENTIALLY has an impact on sound. What about the cabinet and speaker inside it? The bass and its electronics? The strings? I think your overall sound is a kind of Mulligan Stew of factors. I suggest that you just tweak the settings until you like the sound regardless of which amp you use, then declare victory.

 

[seamonkey]

Both of you guys are hitting on what I'm thinking as well. My brain says the the power amp topology (Class AB vs digital Class D) has basically nothing to do with the overall sound as they probably have sufficient damping factors, slew rates, etc to both be transparent. The preamp is where the sound is generated (or colored).

yes?

BTW - class-d is analog

A "head" is a closed system of a pre-amp + "Amp" and in some cases some extras.

I'd agree "power amp" topology doesn't matter.

Your right in that the pre-amp is where most all the tone can be created. I'd call it signal processing. Rare heads may wrap the signal processing to include some interaction with the speaker. Many head makers don't. And you can't looking into a head, see a module, and assume it will sound like another head with the same module. Signal processing, and secrets around how the amp module is used are going to be different between heads.

When it comes down to "Power Amps" like those used in FOH they all can be used interchangeably. Many power amps now have DSPs which have EQ and Limiters that make a big difference in sound - DSP is "signal processing". A lot of power amps now just come with a DSP as it adds to the usability of the power amp in many situations. DSP can be adjusted to put out a little more bass, or be a little louder with a certain pre-amp.

When it comes down to comparing "Amps" it also takes a little gain matching. Two amps may sound different only because the actual volume levels are different. The ear/brain is fooled to think the amp with more volume sounds better.

 

[agedhorse]

For comparisons, when using the power amp in or effects return jack with many amps, there is still some processing present that may be unique to the power amp management. Direct comparisons of just the power amp may be impossible.

 

[sheltjo6]

I've had same amps sound different. Not a Class D thing.

Same here.
He friend's Mesa Walkabout Scout 15 has more of a mid-scooped tone than mine, even when the amps are set the same.
Go figure.

 

[kpc]

Same here.
He friend's Mesa Walkabout Scout 15 has more of a mid-scooped tone than mine, even when the amps are set the same.
Go figure.

I believe the Scout's midrange is flat when all the way up.

 

[Coolhandjjl]

Same here.
He friend's Mesa Walkabout Scout 15 has more of a mid-scooped tone than mine, even when the amps are set the same.
Go figure.

The 12 and the 15 sound different, and the silverface and blackface heads are a bit different as well.

 

[Al Kraft]

I believe the Scout's midrange is flat when all the way up.

That has always been a mystery to me. Here is a quote from the WA manual:
This is the only passive style control in the string of rotary tone controls. This scheme was chosen for its inherently musical blend and for the way the passive style midrange control, with its wide spread and smooth taper, fills in the holes. Unlike the BASS, this control is a boost only, and while it can competently remove this broad spectrum of midrange from the mix, it cannot provide the extreme attenuation of an active style control.

The words "boost only" followed by "it can competently remove" seem frustratingly at odds with each other to me. If it's boost only, then I would think all the way to the left (CCW) should be its most un-boosted (not necessarily flat???) position.

However, it is entirely possible that fully CCW is some degree of mid scoop and you can add mid-range back (reduce the degree to which it is attenuated - given it's a passive control) up to the point of having it flat when the knob is fully CW.

If anyone has the definitive answer, I'd love t hear it. Thanks!!

 

[agedhorse]

The passive mid control is cut only but is combined with a makeup gain stage that makes it function with boost.

 

[Al Kraft]

The passive mid control is cut only but is combined with a makeup gain stage that makes it function with boost.

Thanks for that insight Andy! Now comes the part where the student sheepishly walks up to the professor after class and admits he didn't completely understand the answer.

I think what you told me is that the mid filter circuit itself is a completely passive design and therefore can attenuate signals in it's designed frequency band, but that to work seamlessly with the active filter circuits on either side, as well as the multi band semi-parametric EQ, there is a gain makeup or balancing circuit afterwards that positions the passively created curve at the right place with the active control filters to not sound oddly out of place.

So that would mean fully CCW is mid scooped and fully CW is flat, or at least a version of flat that is properly placed via the makeup gain circuitry into the overall EQ curve so as to sound musical versus odd.

Am I even close? You how bad I suck at this electronics stuff!

 

[agedhorse]

Without getting to deep into a proprietary circuit, if you add gain to a cut circuit, it can appears to function as a boost circuit from which you subtract from. Circuits must be taken as a whole rather than dissected too literally. This is a good example of this.

 

[Passinwind]

Without getting to deep into a proprietary circuit, if you add gain to a cut circuit, it can appears to function as a boost circuit from which you subtract from. Circuits must be taken as a whole rather than dissected too literally. This is a good example of this.

Yep. You can leverage a passive mid cut against a baked in mid boost and end up with a flat response at noon and a boost/cut overall control response too.

 

[Al Kraft]

Without getting to deep into a proprietary circuit, if you add gain to a cut circuit, it can appears to function as a boost circuit from which you subtract from. Circuits must be taken as a whole rather than dissected too literally. This is a good example of this.

Thanks Andy...that I understand!! You know I almost always get it if someone is just willing to tell me twice.

Next lesson will be how alternating current reverses direction without running into itself regardless of where in the circuit it is and what device it's connected to...

 

[Al Kraft]

Yep. You can leverage a passive mid cut against a baked in mid boost and end up with a flat response at noon and a boost/cut overall control response too.

That makes perfect sense as well and I'm assuming there would be times when the passive circuit would provide response characteristics that would be more desirable/musical in the overall design than using an active filter circuit. Frankly I always thought active EQ was invented to get around the characteristics (I'll go so far as to say limitations) inherent in an all passive design like the FMV tone stack.

 

[Passinwind]

That makes perfect sense as well and I'm assuming there would be times when the passive circuit would provide response characteristics that would be more desirable/musical in the overall design than using an active filter circuit. Frankly I always thought active EQ was invented to get around the characteristics (I'll go so far as to say limitations) inherent in an all passive design like the FMV tone stack.

You're definitely on the right track, but like Andy I don't really want to get too specific.

 

[Al Kraft]

You're definitely on the right track, but like Andy I don't really want to get too specific. View attachment 2843434

...and you electronic wizards wonder why us mechanical engineers think that when an electronic component fails it's due to a bad seal in the part that let the magic smoke out!!

 

[Passinwind]

...and you electronic wizards wonder why us mechanical engineers think that when an electronic component fails it's due to a bad seal in the part that let the magic smoke out!!

Heh, you should see what happens when I have to do metalwork. Or as my favorite ME at my last day job used to say: "it's not a project until you're bleeding." And he meant that literally.

 

[sheltjo6]

The 12 and the 15 sound different, and the silverface and blackface heads are a bit different as well.

Both amps are 15s and silver faced. Different year models though.

 

[agedhorse]

Are they the same driver?