Comparison: A new Class D amplifier for an old Class AB player

[Snaxster]

Background

In years past I owned two Class D bass amplifiers: a ca. 1993 Walter Woods amp and a 2007 Acoustic Image Focus 1 Series III. It’s been a long time, but I remember really digging the Walter Woods amp. Though I still own the Acoustic Image amp, the last I played it regularly was over three years ago. In its day it was good, but the state of Class D advanced a lot since then. Even Acoustic Image’s own newer amps are much better than their older ones, I have heard. The Focus 1 Series III sounded pretty good to me, but it hissed a little and felt somewhat distant to play.

Also in the past I owned various Class AB or Class H amps: a big Carver for a biamped rig in the 80s, who knows what else, and currently a QSC PLX1104 and PLX1804. The PLX1804 is a recent arrival, meant to replace the PLX1104. The PLX1804 is Class H (or AB/H, as QSC describes it sometimes) to the PLX1104 being Class AB. At a given perceived volume they sound the same to me.

I won’t mention my past or present tube amps, since the comparison of their sonics is not relevant in my consideration of types of solid state amps.

Premise

Among solid state bass amps, the sound of high quality Class AB is my current standard. But reading players’ accounts of the latest crop of Class D bass amps, I felt it was time for me to try at least one, to give Class D another chance. (That plus I am a gear ho. Discriminating, but a ho nonetheless.)

Selection

My first attempt was an Eich Amplification T-900. That didn’t go well due to a very bad deal with the sole US dealer (you have been warned). I never even played it before returning it. My second try was a Glockenklang Blue Rock. That went perfectly; dealer, deal, amp and all (thank you, Luthier's Access Group!)

Inquiry

“At a given perceived volume they sound the same to me.”

That was a key part of my interest. How close could I come to being able to say that when comparing the QSC PLX1804 and the Glockenklang Blue Rock?

Today I did a first test to enable that comparison.

Harness

• 1973 Fender Precision Bass as P/J, ash and maple
• Kern IP-777 preamp, set clean and pretty flat, line out
• [sfx] micro-Thumpinator HPF
• QSC PLX1804, line in
• Glockenklang Blue Rock, return in, loop input volume maxed
• MA Soundworks MAS-112, Eminence woofer, 4 ohm, set flat

A few specs per the amp manufacturers:

[Invalid or Expired Link Removed]

• Power amp type: Class AB/H
• Output power: 1k Hz, 0.1% THD (EIA), 900 watts 4 ohms per channel
• Freq response: 20 - 20k Hz, ±0.5 dB
• Distortion: SMPTE-IM 0.02%
• Signal/noise: unweighted, 20 - 20k Hz -106 dB

Invalid Link Removed

• Power amp type: Class D
• Output power: 1000 watts 4 ohms
• Freq response: 20Hz to 20KHz, -1 dB
• Distortion: unspecified
• Signal/noise: unspecified

Using an external preamp and using the Blue Rock’s return input was the nearest I could get to comparing the two amps as being amplifiers only, without their own preamps. Among other factors, the Blue Rock loop’s variable input volume might suggest that the test harness wasn’t “pure” enough. But this was to be a qualitative test, about perception rather than quantified measurement, and that was the best arrangement possible.

Prep

First I warmed up all the electronics well. Then I chose a moderate, but spirited output volume on the Blue Rock.

The MAS-112 is quite sensitive. It doesn’t need much power to sing or to hit hard, and at 600 watts it was well outmatched by both amps. So at the chosen volume there was lots of headroom. Lots, I said.

Switching the input and output connections over to the PLX1804, then I matched its output volume to the Blue Rock’s as best I could, which after a couple of turns was pretty darn close.

Considering that the Blue Rock features a logarithmic output volume pot (smooth, linear-sounding transition — it doesn’t “come on the cam”, and full output is at about 3 or 4 o’clock), note that I set its output volume at just above 9 o’clock. To match that, I set the one PLX1804 channel's volume at five clicks up from shut, by chance also just above 9 o’clock.

Comparison

Next was the listening test proper. Playing with a variety of instrument sounds (pickup selection, tone pot setting, playing style, etc.), I stepped through several musical motifs, alternating between the two amps as often as time permitted.

That turned out to be a fairly long test. It was enough to let me feel confident that after all the variations, I understood each amplifier’s characteristic sound.

Conclusion

Did they sound the same? If not, how did they differ?

Allowing for weighting by primacy (first impression) and recency (latest impressions), I heard the following comparatively. The differences were subtle, but distinctive:

• the PLX1804 sounded thick, planted, even
• the Blue Rock sounded bright, spatial, edgy

• the PLX1804 felt neither introvert nor extrovert, warmly presenting what it was given
• the Blue Rock felt a bit extrovert, eager to present its power and beauty

• both the PLX1804 and the Blue Rock
  • seemed to have endless headroom for conveying clean, clear bass frequencies
  • were in their responsiveness to playing inputs immediate and tactile
  • presented great clarity across strings and in harmonic content.

In the week or so before this test, I played amplified using the Blue Rock exclusively. During that time, I knew that something was different; but the Blue Rock simply sounded and felt so good that I wasn't compelled to name it.

My guess is that this would be the same for most experienced players: after a while, they would not hear the difference unless they were either 1. really trying to or 2. were so strongly attached to the Class AB sound that they couldn’t help but hear the difference.

Though with only these two examples my preference still is for the sound of Class AB, after this test I accept that Class D has arrived; that when it is artfully integrated into an excellent design well implemented, it can sound basically as good as any other solid state power amp type (which point knowledgeable forum members often make here on TalkBass).

Afterword

In itself, especially with its own preamp and all, the Glockenklang Blue Rock is a fantastic bass amp and a beautiful-sounding instrument. As I wrote to the dealer after playing it for the first time,

“There is far to go before I learn all its pathways and features, but my intuition is that the Blue Rock is the Class D, all-solid state integrated bass amp I was waiting for. Barring a massive qualitative improvement in bass amplification in the near future, I should be happy with this amp for years to come, and not feel that I am missing out. It is a sweet spot device. Clearly its design pursues no extremes, and is based in a commitment to usefulness and quality rather. I even like the Blue Rock’s size and weight, but that could be my age talking.”

 

[funkytoe]

Nice review. I appreciate the scientific approach to your comparison. I too prefer class AB to class D amps. But the differences between the best versions of each are nearly undetectable these days.

 

[matti777]

From what I can tell the rigs you compared would be of a similar price (if Kern was still available) I'd be interested to hear the difference between some of the cheaper more popular Class D heads and a good Class A/B rig. Are we giving up anything in the tone department? Is it significant? My guess is that the bang for $$$ factor is way up with Class D especially considering the weight difference and that the sacrifice in tone is negligible.

 

[blubass]

Unless you're looking at 2 amps with identical preamp sections and different power sections, it starts to become very subjective as to what the power section is actually doing to the sound.

 

[Passinwind]

Very nice review. As a former PLX1804 owner, I know that gain settings around 9 o'clock are pushing pretty close to the "forbidden zone" though, and really won't get the best sounding results in many cases. I've been through this in reverse when A/B/X testing the 1804 and a Crest CA9, which were pretty tough to gain match properly. Also, the 1804 is actually a Class H amp, which to me sounded a little different than the straight Class A/B PLX amps I've owned. But all of those had different (lower) power ratings, so as usual, one has to be thoughtful about inferring cause and effect, IMO.

In any case, I was extremely impressed with the Blue Rock head I heard at NAMM this year.

 

[seamonkey]

Simple things like gain matching would help in a comparison
Or simple measurements, they are simple to do these days

But ... if you're convinced yourself Class-AB is the only class for you then have at it.
It looks like you've found a way to confirmed your beliefs that it is the "class". It justdoesn't turn out to be true for most people.
Plenty of used class-ab will be around for decades, maybe centuries. I see some manufacturers claiming some superiority of the class-ab amps they sell so they and their marketing will be around to confirm your beliefs. That niche will always be around

Amps with sliders are off my list for no good reason other than I don't like the looks. No one will ever get me to like the looks of them. That Blue Rock is a nice looking amp!

 

[Snaxster]

Very nice review. As a former PLX1804 owner, I know that gain settings around 9 o'clock are pushing pretty close to the "forbidden zone" though, and really won't get the best sounding results in many cases. I've been through this in reverse when A/B/X testing the 1804 and a Crest CA9, which were pretty tough to gain match properly. Also, the 1804 is actually a Class H amp, which to me sounded a little different than the straight Class A/B PLX amps I've owned. But all of those had different (lower) power ratings, so as usual, one has to be thoughtful about inferring cause and effect, IMO.

In any case, I was extremely impressed with the Blue Rock head I heard at NAMM this year.

Hi, Charlie. Thanks for weighing in. Yes, I knew that the QSC 1804 is Class H. To disclose fully, I did not A/B test it and the QSC 1104. I relied only my memory, having played the 1104 exclusively during the year prior to getting the 1804. And I agree with you about inference in this case.

About the output settings: I think that you and I have our "o'clock" values reversed.

Here is the clock face type I referred to:

In that representation, 7 o'clock is fully shut and 9 o'clock is only slightly open. That's what I meant by 9 o'clock.

 

[Snaxster]

Simple things like gain matching would help in a comparison
Or simple measurements, they are simple to do these days

But ... if you're convinced yourself Class-AB is the only class for you then have at it.
It looks like you've found a way to confirmed your beliefs that it is the "class". It justdoesn't turn out to be true for most people.
Plenty of used class-ab will be around for decades, maybe centuries. I see some manufacturers claiming some superiority of the class-ab amps they sell so they and their marketing will be around to confirm your beliefs. That niche will always be around

Amps with sliders are off my list for no good reason other than I don't like the looks. No one will ever get me to like the looks of them. That Blue Rock is a nice looking amp!

Well there was a pretty strident, presumptuous, and confrontational post. Good luck with that.

I've been thinking of ignoring you for a while. Done now.

 

[Passinwind]

About the output settings: I think that you and I have our "o'clock" values reversed.

Here is the clock face type I referred to:

View attachment 981692

In that representation, 7 o'clock is fully shut and 9 o'clock is only slightly open. That's what I meant by 9 o'clock.

Roger that. I was referring to this aspect of gain setting, quoted here from the manual:

That has seemed to be pretty non-intuitive for many users over the years, and @Bob Lee (QSC) has posted confirming its accuracy and the reasons it works that way quite a few times. The distinction between an input attenuator and a gain control in pro audio jargon can be less than obvious, for sure. And none of this necessarily means that what you were doing was creating that problem, just wanted to point out that it sometimes can.

 

[agedhorse]

Well there was a pretty strident, presumptuous, and confrontational post. Good luck with that.

I've been thinking of ignoring you for a while. Done now.

I didn't read this as a direct response to your post, but as general statement about comparisons and preferences, which I feel is pretty accurate about a lot of aspects of marketing anything to anybody.

 

[Snaxster]

I didn't read this as a direct response to your post, but as general statement about comparisons and preferences, which I feel is pretty accurate about a lot of aspects of marketing anything to anybody.

Andy, thanks, but it's done. Unlike you, I did read it that way exactly.

Concretely, and for the record, all I was doing was recounting my pursuit of my subjective assessment based on one instance of empirical comparison. I spoke only of my personal experience, impressions, etc.; not of how things are, but of how they seemed to me.

Yes, I am a human being and so am liable to be influenced by many sorts of things. Marketing is one; but I think it had no bearing on this assessment of mine.

 

[Snaxster]

Roger that. I was referring to this aspect of gain setting, quoted here from the manual:

That has seemed to be pretty non-intuitive for many users over the years, and @Bob Lee (QSC) has posted confirming its accuracy and the reasons it works that way quite a few times. The distinction between an input attenuator and a gain control in pro audio jargon can be less than obvious, for sure. And none of this necessarily means that what you were doing was creating that problem, just wanted to point out that it sometimes can.

Wow. Thanks... very interesting. Yes, it's an attenuator. Now I see it. Cool.

 

[Passinwind]

Wow. Thanks... very interesting. Yes, it's an attenuator. Now I see it. Cool.

Well, it's actually an attenuator working as a gain control with at least a few active elements before the pot. So if you turn the control way down and feed the input a hot enough signal you get problems that you might not intuitively expect. Here's a visualization taken from the CA9 manual:

The vast majority of pro sound amps are like this to one degree or another. Many QSCs have a filter stage or two before the attenuator as well, for example.

 

[Snaxster]

Well, it's actually an attenuator working as a gain control with at least a few active elements before the pot. So if you turn the control way down and feed the input a hot enough signal you get problems that you might not intuitively expect. Here's a visualization taken from the CA9 manual:

The vast majority of pro sound amps are like this to one degree or another. Many QSCs have a filter stage or two before the attenuator as well, for example.

Nothing is simple anymore, is it? Haha! Sigh... Thank goodness for those who do, because I don't. I try in my small way, but likely I never will understand anything serious to do with applied physics and so forth. My technical aptitudes very much lie elsewhere.

"So if you turn the control way down and feed the input a hot enough signal you get problems that you might not intuitively expect."

That I can handle, though. I thank you for the caveat. I will remember it.

 

[Gormsgade]

Hi guys.
I've been doing a little comparison myself between a cheap Crown XLS 1500 bridged to 4 ohm
driving my two Aguilar GS212 and a pair of recently serviced Crown Macrotech 600 in bridge mode
each driving its own cabinet. So in principle we're talking almost same power i.e. (theoretical) 1x1500 watt into
4 ohm or two times 600 into 8 ohm. And just to make things clear, I don't play particulary loud, more a matter of
not driving your equipment too hard.
Very much to my own surprise the cheap far east produced Crown class D is the winner. Quiet, requires less output
from my Aguilar 659 pre-amp and seems to have endless power and dynamic headroom. Hardly any difference in sound,
meaning both set-up sounds right but it seems the old Macrotech has to be driven harder.
I'm a tube loving guy having played SVT's, Mesa 400, Hiwatt's etc. but being able to carry your pre/power amp in a 3 units
rackbag over your shoulder without comprimizing too much on your preferred sound is hard to beat.
Regards
Thomas from Denmark

 

[agedhorse]

Nothing is simple anymore, is it? Haha! Sigh... Thank goodness for those who do, because I don't. I try in my small way, but likely I never will understand anything serious to do with applied physics and so forth. My technical aptitudes very much lie elsewhere.

"So if you turn the control way down and feed the input a hot enough signal you get problems that you might not intuitively expect."

That I can handle, though. I thank you for the caveat. I will remember it.

Correct, because of the need for high common mode rejection at the input, the attenuator will always be after the input differential amp.

Now the danger to this is that with some of the new amps with DSP front ends, they can overload with +8dBu or so, because the ADC operates on either 5V or 3V single ended rails. This means that the signal goes into a diff amp that may be operating at +/- 7 or 8 volts (the unregulated or pre-regulated +5 volts) and a created inverse of this which brings the input overload way down unless a balanced attenuator is included in the input circuit (generally not). The result is that it's possible to overdrive the input regardless of where the attenuator is set as long as it's lower than the level required to drive the amp itself to clipping.

 

[Snaxster]

Correct, because of the need for high common mode rejection at the input, the attenuator will always be after the input differential amp.

Now the danger to this is that with some of the new amps with DSP front ends, they can overload with +8dBu or so, because the ADC operates on either 5V or 3V single ended rails. This means that the signal goes into a diff amp that may be operating at +/- 7 or 8 volts (the unregulated or pre-regulated +5 volts) and a created inverse of this which brings the input overload way down unless a balanced attenuator is included in the input circuit (generally not). The result is that it's possible to overdrive the input regardless of where the attenuator is set as long as it's lower than the level required to drive the amp itself to clipping.

Thanks, Andy. I hope that none of my amps feature a DSP front end!

 

[fdeck]

Correct, because of the need for high common mode rejection at the input, the attenuator will always be after the input differential amp.

Now the danger to this is that with some of the new amps with DSP front ends, they can overload with +8dBu or so, because the ADC operates on either 5V or 3V single ended rails. This means that the signal goes into a diff amp that may be operating at +/- 7 or 8 volts (the unregulated or pre-regulated +5 volts) and a created inverse of this which brings the input overload way down unless a balanced attenuator is included in the input circuit (generally not). The result is that it's possible to overdrive the input regardless of where the attenuator is set as long as it's lower than the level required to drive the amp itself to clipping.

As an old-timer, I must admit that I'm still unnerved by 3-V analog rails, though I use them myself for day-job stuff. Of course it's just a matter of working out the dynamic budget of the system, but still, it doesn't seem right.

 

[Nic.]

I'm pretty impressed that you took the effort to account for experimenter bias and other effects!

 

[Snaxster]

I'm pretty impressed that you took the effort to account for experimenter bias and other effects!

Thank you. Since I have both personal and professional interest in such, it informs my avocational work.