Class D "digital" amps. Opinions on performance

[fdeck]

Maybe the manufacturers that COULD make a standard SS class AB or class D dwarf SVT have no interest and have never tried. There's a large number of bass players (and designers) who play other amps specifically because they DON'T respond like an SVT.

It could be an engineering success and a marketing disaster. Also, success might be short term, if you spend all of your time and money convincing people that it can be done, so that everybody else can jump in with similar -- and cheaper -- products.

A better strategy is to form a solid understanding of what makes amps sound good, figure out how to design an amp that achieves your goals for tone and performance, and let bassists figure out that it sounds good while skirting the controversy.

It's likely that most class D amps out there are far closer to the definition of an ideal power amp than the SVT power amp, which is kind of what this thread is about. It's obvious that you like the coloration of the SVT. I like the coloration a VB-2 gives me in one of my bands, but I use a class D combo amp in another band because I'm playing a different style. I essentially designed the VB-2 for myself, and my design decisions were based on achieving maximum stage presence from only 225 watts, which involves a hell of a lot of coloration. If I'm designing a head with a 700 watt class D amp, I'm going to make different design choices throughout the whole design because I have more power to work with. I'm not going to try to make my 700 watt class D amp sound exactly like the VB-2. You can love the EQ on an a 300 watt amp, but put 1000 watts behind that same EQ and you may hate it.

It would have to put out the same power as the VB-2, to sound the same. And then you'd be competing on price with other ~ 200 Watt solid state amps, which is probably a pretty competitive field. But you can send me your prototype.

Finally, I not a believer in "transparent" power amps for all purposes. It's possible to make "colored" class D amps just like it's possible to make "transparent" tube amps. The Headliner power amp is class D and it was designed to feel like a tube power section.

One thing I've learned from the amp experts, including yourself, and also from my own bench testing, is that every amp has to make a transition from transparent to colored behavior when it approaches its hard clipping limit, and letting the amp depart from transparency to avoid hard clipping is a time honored way of addressing that issue.

Of the amps that I own, three can't be clipped. One is a GK Backline 600 head, which gently transitions into soft clipping when driving 8 Ohms (haven't tested it at 4 Ohms). The other two have built-in compressor limiters that seem to be pretty much transparent.

 

[David Jayne]

Don't designers of micros go to certain lengths to ensure they can't ever go into clipping? By using sophisticated compression/limiting schemes? Might this not explain the sag Jimmy says he hears/feels?
I know he said he perceived it even at moderate volumes. Just spitballing here.
My only 'micro' is a Markbass TA 501 and I absolutely don't hear this behavior from it. Maybe I would if I pushed it hard but I've never needed it to be that loud.

 

[fdeck]

If I knew, I wouldn't be having this conversation with you guys, would I? All I'm telling you is what I hear. If it makes you all feel better to pretend I'm nuts and have no clue what I'm talking about, that's fine, and quite honestly, there's evidence that I don't on a tech level. But I know what my ears tell me, and there's plenty of evidence to suggest that my ears are pretty good.

Nobody's pretending that you're nuts.

You're not a victim. Nobody's trying to offend you. If somebody says that you might be affected by factors such as expectation bias and crowd psychology, at worst they're just accusing you of being human like the rest of us.

 

[JimmyM]

Nobody's pretending that you're nuts.

You're not a victim. Nobody's trying to offend you. If somebody says that you might be affected by factors such as expectation bias and crowd psychology, at worst they're just accusing you of being human like the rest of us.

That would be accurate if I wasn't the first person on here to say that micros sounded tubey I have enough of a scarred childhood to not be the least bit afraid to go against the grain in my adulthood!

 

[BbbyBld]

Thank you for asking, Bobby I have two...a 69 with 6146b's, and a new Heritage SVT-CL. And yes they do sound identical. I consider the SVT one of the most consistent amps made. Sure, tiny little changes will exist, and I was able to make my Heritage sound a tiny bit less like my Blue Line with a pre tube change. But the basic character of tone is there in every single SVT I've played that works, from the very first ones like mine to the newest CL's and VR's and anything in between.

What you call "character" CAN be designed into a class D power amp, but not just by tossing a DSP or soft clip or compressor or whatever in front of any class D amp. You have to build it from the ground up specifically for bass guitar. Obviously, not everyone would like that because you'd be designing in coloration.

A fair point, but again, there have been some companies whose design goal was exactly that, and none have done it 100%. And Fender hasn't been able to make a SS guitar amp that sounds like the Twin, and Marshall hasn't been able to make a SS amp that sounds like a Plexi, and neither have the clone-meisters unless they use tubes and output transformers.

I think that Peavey Transtube guitar amps are pretty convincing, but a lot of the mojo (and patents) involves the power amplifier specifically. A lot of tube sim circuits are focused on the wrong areas in my opinion. I've said a lot on TB that when it comes to tube amps, the power amp is the magic part when people start asking about hybrids as an alternative to all tube, so I completely understand where you're coming from.

I've yet to play a micro that didn't resemble a tube power section in varying degrees But I haven't played a Headliner, so I'm not able to comment intelligently about it. Will look for one and see what happens, though.

If you get a chance, play it with an 810...and don't let anybody see you .

 

[fdeck]

Don't designers of micros go to certain lengths to ensure they can't ever go into clipping? By using sophisticated compression/limiting schemes? Might this not explain the sag Jimmy says he hears/feels?
I know he said he perceived it even at moderate volumes. Just spitballing here.
My only 'micro' is a Markbass TA 501 and I absolutely don't hear this behavior from it. Maybe I would if I pushed it hard but I've never needed it to be that loud.

Both of my micro heads have built-in compressors like you say. But as a DIY'er, I built a switchmode power amp from a kit, which had no such protections. So I was able to observe the inherent clipping behavior of the amp. It had a nice, gentle soft clipping response, and I ran it for a couple of hours at full clipping into a dummy load.

I can also run my two micro heads into hard clipping by feeding signals that overwhelm the gain range of the compressor, and I saw similar soft clipping behavior. On both of these heads, the compressor is not built into the power amp, but is on the preamp board. One uses a Class-D controller IC, and the other a IcePower module. So the power amp designers didn't see a need to include limiting.

Given that I've always liked having that compressor in my amps (starting with my first Peavey), I'm perfectly satisfied with the micro heads.

 

[grendle]

What I find intriguing about this conversation is in the "old days" no one really cared about how things were built as far as specs and accuracy went. They had a general idea and a plan, and then built and experimented by ear to get what they thought sounded good. Then cost choices and final specs were derived and implemented as required in a lot of cases. Some components were chosen simply because they sounded "better " for no apparent reason. When they had the magic mix though, they stuck with it. Some of those became legendary.

We don't experiment as much in the real world any more. We do some, but most is in a computer simulation or on paper and little thought is given to the fact that through unknown variables things may or may not actually sound different. In most things today, things are designed first then the build is changed as needed to fit the design specs or the intended final product. Something could be the best sounding most reliable "it" ever built but if the measured output is off vs the spec sheet it's relegated as trash regardless if it will blow your ear drums or not.

Audio is a funny thing. Specs are one thing but what actually sounds good to most is another. An Inefficient design could be the crown jewl but its not efficient so it wouldn't be built. Distortion is a dirty word yet the thing people like the most is what happens in that magic 10-20% sweet spot where components are pushed to their limits.

Ahh the old days......

 

[gurensan]

...But I'd say the character I hear in the micro bass amps (not just class D but AB as well) is reminiscent of a tube amp being pushed just a tiny bit too hard but without that (for lack of a better word) 3-dimensionality of tube amps. And I'm not the only one who hears it, as I've mentioned. Ken thought I was absolutely crazy for saying that about the LMII at one time, then others started saying it with zero prompting from me.

I can hear "the class D sound" as well. Live only; I can't tell from a recording and it's less obvious to me at high SPLs. I have heard it from the beginning when the first units shipped in the early 90s, and I have heard it every time I've tried one since.

I'm not sure I'd classify it the same way as Jimmy (in fact I wouldn't touch that description with a pole!), but it seems to me there is some high-order distortion there. It's.. "crispy." It's not like SS diode clipping, in fact, it doesn't sound like clipping at all to me. I can't tell what exactly it is but it's always there.

Whether I like it or not is immaterial, but my opinion is plastered in other places if someone wanted to look for it. I'm just glad I'm not the only one who hears this.

Due to lower internal impedances, a good SMPS would tend to be less prone to sag under heavy load than a conventional power supply, though the usual notions are that it's the other way around. Sag would show itself primarily as premature clipping.

This is a good thing to clear up. Regulated SMPS tend to be super stiff. Not all are as well regulated, but anything in an audio amp likely is. I don't think I'll be running into a DC/DC forward converter in a Markbass.

Audio is a funny thing. Specs are one thing but what actually sounds good to most is another. An Inefficient design could be the crown jewl but its not efficient so it wouldn't be built. Distortion is a dirty word yet the thing people like the most is what happens in that magic 10-20% sweet spot where components are pushed to their limits.

Ahh the old days......

Hear, hear! (haha pun intended) I'd say between 5-10% 2nd harmonic, but that's just me,

 

[Bob Lee (QSC)]

Regulated SMPS tend to be super stiff

Even unregulated SMPS can exhibit very low sag under heavy loads, if they're designed so.

 

[grendle]

Would we even be having this discussion if it were 6l6 tubes vs el34's vs kt66's? Or SS vs tube? Those all do the same thing, just different ways. There no question those all sound different, not better or worse, just different. Or the same goes for tubes. 12ax7's go as far as to be classified by the mfr. Seems kinda silly to think a class d power amp would sound anything like those and not it's own animal

The only power amps I haven't heard a major difference in except for volume levels were a std old transistor power amp vs a MOSFET power amp. The MOSFET amps have been stupid loud for the rated wattage ( Hartke 3500) Even though they shouldn't be.

 

[KJung]

Well I don't hear that "tube amp being pushed slightly hard" thing in any lead sled amps unless they're hybrids with the gain being pushed slightly hard. I don't even hear it in any tube amps until they're being pushed hard. But I hear it in every single micro I've ever played in varying degrees. For example, I thought the F1 didn't have as much of it going on as the LMII. But it still had it. And as Grendle just said, I hear it at very low volumes as well as very high, and gain setting is immaterial. So what else is it if not the topology?

God help us all. So, the LM is class AB and the F1 is class D. So I guess now you are saying that what you are hearing is due to micro amps that are small, but not too small.

 

[JimmyM]

So I guess now you are saying that you've got nothing to add constructively and you're just going to be a wisenheimer.

 

[KJung]

So I guess now you are saying that you've got nothing to add constructively and you're just going to be a wisenheimer.

I thought I was making a legitimate point. Your logic is killing me.

 

[Bob Lee (QSC)]

The only power amps I haven't heard a major difference in

You'll usually hear a major difference in any two power amps, even two of the same model.

Unless you actually match their gains with a reasonable amount of precision. Casual comparisons are usually done without good attention to gain, and thus they yield an overwhelming number of false positives on the matter of "sonic difference."

 

[JimmyM]

I thought I was making a legitimate point. Your logic is killing me.

And yet here you are still annoying me

Hey dude, like I said many many times, I can only tell you what I hear. You've been trying to tell me what I hear is wrong ever since I've known you. But what I hear is what I hear and I'm not about to lie about it or hold it in because it upsets your sensibilities.

 

[fdeck]

Would we even be having this discussion if it were 6l6 tubes vs el34's vs kt66's? Or SS vs tube? Those all do the same thing, just different ways. There no question those all sound different, not better or worse, just different. Or the same goes for tubes. 12ax7's go as far as to be classified by the mfr. Seems kinda silly to think a class d power amp would sound anything like those and not it's own animal

The only power amps I haven't heard a major difference in except for volume levels were a std old transistor power amp vs a MOSFET power amp. The MOSFET amps have been stupid loud for the rated wattage ( Hartke 3500) Even though they shouldn't be.

There are some reasons to think that amps might sound the same, and some reasons to think that they might sound different.

The reason to think that amps sound the same is if they are all relatively high fidelity designs with high open-loop gain and negative feedback, and are being operated at signal levels well below clipping, based on an understanding of how amps work. That's because the primary factor driving the tone of mainstream power amps is not output topology, but feedback topology. Virtually every power amp has the same basic feedback circuit: A voltage divider.

So these are all primarily voltage divider feedback power amps.

Now let's go down the list in order of importance. Among voltage divider feedback power amps, what's the next most important factor? Is it output topology? Given that we're mainly talking about heads, I'd put a couple items ahead in line: Overall output power capability (louder sounds different) and preamp voicing.

What's next? Output topology? Maybe power supply design -- how stiff the supply is. Maybe it's the clipping behavior of the amp. Now, output topology affects clipping behavior, but so do other things, and a designer may choose various tricks to tailor the clipping behavior of solid state amps.

By this point, we're left wondering whether it's possible to assign a "signature sound" to a particular topology. Further, whether a person like me has any hope of getting a straight answer to this question when the answers are virtually guaranteed to be clouded by expectation bias ("hearing with our eyes"), crowd psychology, and marketing hype because we are all human.

Faced with this dilemma, each person has to make their own choice about how to deal with the flood of information. My choice is twofold: When designing gear, I listen to all opinions, to learn what I can and hopefully develop a better understanding that includes both technical and musical knowledge. When choosing gear for my own use, I ignore most of it and have never regretted choosing based on largely technical factors.

 

[grendle]

You'll usually hear a major difference in any two power amps, even two of the same model.

Unless you actually match their gains with a reasonable amount of precision. Casual comparisons are usually done without good attention to gain, and thus they yield an overwhelming number of false positives on the matter of "sonic difference."

True. That precision is in the spec of the components and built into the tolerance of the unit. It's also taken into account in the final product and sometimes used as a guide to know where to measure the unit for the best specs.

 

[B-string]

Thank you Bob Lee, BbbyBld. fdeck and other CONSTRUCTIVE posters for your patience.

 

[gurensan]

Even unregulated SMPS can exhibit very low sag under heavy loads, if they're designed so.

That's why I was specific about it. Non-committally, of course, since I didn't finish my BSEE.

This is a good thing to clear up. Regulated SMPS tend to be super stiff. Not all are as well regulated, but anything in an audio amp likely is.

I designed a couple of these intended to run tube amps but never got all the way to prototype stage. I don't have the test equipment / related experience / money / smarts to pursue it, so I stick to conventional supplies when homebrewing.

 

[KJung]

And yet here you are still annoying me

Hey dude, like I said many many times, I can only tell you what I hear. You've been trying to tell me what I hear is wrong ever since I've known you. But what I hear is what I hear and I'm not about to lie about it or hold it in because it upsets your sensibilities.

It is not what you hear that is the issue, but what you are attributing what you hear as a causal driver. Your comments comparing the LM to the F1 make this painfully clear. All integrated bass amps sound different. For some reason, you seem to think that class of amp is the primary driver.