Carvin DCM "Kick Drum Test"

[Metal Mitch]

The new Carvin catalog arrived in my mailbox yesterday. Previously, their DCM marketing had attacked "other lightweight" power amps, so I assumed they were talking about the QSC PLX series. Now they've changed their pitch to attack all Class H power amps, noting the switching distortion in addition to the lack of reserve current needed to reproduce peaks about 60ms apart. The "chart 3" graph looks pretty convincing.

After doing the math, it looks like you would need to be speed-picking 16ths at about 250bpm to fit that 60ms peak window. But at a more realistic 200bpm, 16th notes would be about 75ms apart and it looks like this could actually affect the attack sound.

So I wanted to put the question out to you guys. Is this just marketing voodoo, or a legitimate concern for speed players?

 

[BruceWane]

Uh....200 beats per minute is 3 1/3 beats per second....so 16th notes at that tempo means about 53 notes per second.....I don't think it'll be a problem.

What they are basing this on is the fact that the capacitors in a switching amp get "filled up" 60 times per second (in the US anyway, where the standard for ac power is 60Hz). Assuming they could also be depleted at 60Hz, I suppose it could theoretically be a problem, but that's a pretty extreme assumption.

 

[Drake]

200 bpm is 3.3 quarter notes per second and 14 sixteenths per second isn't it?

 

[BruceWane]

Ooops! Yeah that's true. But still, the "refresh rate" for the power supply capacitors is 60 times per second, so you'd need to deplete them at at least that rate to have a problem.

 

[Fuzzbass]

Originally posted by Metal Mitch
The new Carvin catalog arrived in my mailbox yesterday. Previously, their DCM marketing had attacked "other lightweight" power amps, so I assumed they were talking about the QSC PLX series. Now they've changed their pitch to attack all Class H power amps, noting the switching distortion in addition to the lack of reserve current needed to reproduce peaks about 60ms apart. The "chart 3" graph looks pretty convincing.

Wouldn't that only be true if the peaks were close to the limit of headroom? In other words, if I was able to churn out 60 notes per-second with my overpowered Stewart 2.1 (never lighting up the -3dB indicator), would I notice any distortion?

Seeing as how my musical goal is to use fewer notes I have no worries about this, but I am curious.

 

[throbbinnut]

Actually, the caps are refilled 120 times per second due to the rectification doubling the frequency of the power line.

It's probably not a concern.

Chris

 

[Metal Mitch]

Originally posted by Fuzzbass


Wouldn't that only be true if the peaks were close to the limit of headroom?

Yes, that's exactly what their chart is showing. The first peak is at full power with a wave about 20ms wide, the 2nd peak on their Class H chart is showing only about 60% power for the next peak approximately 50ms later.

Thanks Drake. Yep, 200bpm = 13.3 16th notes per second = 75ms per 16th note.

Fuzz, maybe you're right that it's not an issue at less than full power. But since I routinely have to pick 16ths at over 200 bpm I would still like to know if there's anything substantial to this...

 

[Steve]

There was a time "back in the day" when soundmen would say that the switching amps just didn't have what it took to get the job done on the low end. They were fine for the top of bi and tri amp systems but seeing the sublows powered by a rackful of Carvers was a badge of stupidity.

The new designs of switching amps may have addressed that issue although I have a friend who's opinion I trust implicitly and he swears that the Crown K2's blow chunks on the bottom end.

So...my response would be...I have no idea but it could very well be an issue for you. As has already been pointed out, adequate headroom is critical. Expect a switching amp to crap out on the low end when pushed to the limit.

 

[Phat Ham]

Originally posted by BruceWane
What they are basing this on is the fact that the capacitors in a switching amp get "filled up" 60 times per second (in the US anyway, where the standard for ac power is 60Hz). Assuming they could also be depleted at 60Hz, I suppose it could theoretically be a problem, but that's a pretty extreme assumption.

If you're talking about a switching power supply, then the frequency is much higher. The PLX amps have a power supply that operates at 240khz.

 

[throbbinnut]

Originally posted by Phat Ham


If you're talking about a switching power supply, then the frequency is much higher. The PLX amps have a power supply that operates at 240khz.

Is that the power supply switching frequency, or the switching frequency of the output devices?

Chris

 

[rickbass]

Marketing is a wonderous thing;

- Here's the conclusion......now let's cook up the data to support it."

 

[Steve]

Could someone post a link?
I stumbled around Cravins site for a while and couldn't find it.

I mean, what's Carvin trying to say? "Our heavy weights are better than the other light weights?"

or "Our light weights are better than their light weights"

I can almost get my mind around the former but the latter...that's a friggan' joke. IMHO

But then, I am an unrepentant gear snob...

 

[Fuzzbass]

Hmmm, this is Stewart's side of the story:

The Switch Mode Power Supply

The World 2.1 features a radical departure from conventional power supply design: the High Frequency Switch Mode Power Supply. Because of this pioneering use of advanced technology, some explanation is in order. Conventional supplies operating at 60Hz (Standard line frequency) recharge only 120 times per second, requiring their storage capacitors and transformer to be quite large to supply the energy needed between cycles, when power is not available from the wall. The power supply must act as a local reservoir of power from which the amplifier circuits draw. Their storage function is responsible for much of the bulk, weight and cost of the traditional power amplifiers. The Stewart-High Frequency Switch Mode Power Supply fully recharges 120,000 times per second [1,000 times faster than conventional supplies], requiring far less capacitance for filtering and storage. This high speed recharging reduces power supply "sagging" common with other designs. in a Stewart Amplifier, it is almost as though power is fed directly from the wall to the speaker, with the amplifier acting only as the "valve" in between to control its flow. This power supply's remarkable efficiency at converting the energy from the AC line to a usable form for loudspeaker applications allows many of the major components to be downsized, saving weight, and cost.

 

[chucko58]

The "rail-switching" amp that Carvin refers to in their catalog is a Class H amp with a traditional power supply, not a lightweight amp with a switch mode power supply.

I was informed by a Carvin staffer that the "brand X" trace was from full-power testing of a popular, low-priced Class H power amp (sorry, I'm not naming names). "Full power" means driving a bridged load at the amp's rated minimum impedance at clipping, which was a couple of kilowatts in this case.

Carvin also used a drum machine programmed to generate the double kicks in quick succession. Anyone here know a drummer who can do two identical kicks in 200 ms? I didn't think so...

This is a valid test if you're driving PA subwoofers hard with heavily compressed beats. I don't think it has a whole lot to do with the way most of us use our bass amps. The whole reason we buy mega-powered amps is for headroom to handle the occasional peaks.

Class AB vs. Class H is an engineering tradeoff. Carvin's class AB amps may handle this contrived test better, but they will use more power and generate more heat than a comparably rated Class H amp. You pays your money, and you takes your choice...

 

[Metal Mitch]

Originally posted by chucko58
Anyone here know a drummer who can do two identical kicks in 200 ms? I didn't think so...

Sure, I play with two of them in two different bands that can pump 2 kicks in about the 120-150ms range. Each kick may not be 100% identical, but there are plenty of guys out there using MIDI triggers that are. Guess you've never been to a death metal / goregrind show and listened to those machine-gun kicks all night.

Originally posted by chucko58
This is a valid test if you're driving PA subwoofers hard with heavily compressed beats. I don't think it has a whole lot to do with the way most of us use our bass amps. The whole reason we buy mega-powered amps is for headroom to handle the occasional peaks.

"Most of us" being the keyword there, and probably true - but doesn't include me. I match my picking to those machine-gun kicks with heavy compression, so my peaks aren't "occasional". More like an artillery barrage.

I respect your tact in not naming names, but I recently purchased a "popular, low-priced Class H power amp" made by a manufacturer who is well-respected on these TB forums. So I am concerned I may have made a bad decision. If you wouldn't mind PM'ing me with that information, I'll respect your wish not to publicize it.

Luckily, I still have an old Carvin cab with impedance selector switch I can use to run out the headroom at 16 ohms and see if the new amp mushes out on the attack. I may even go as far as to use Carvin's return policy to do an A/B comparison... and I'll let you guys know if I have a used "popular name brand" amp for sale.

BT82> I looked for it on the Carvin website before I made my first post, so I could include a link. It's not there. It's only in the catalog, which is free by the way.

 

[notanaggie]

OK, couple points.....

first, it doesn't matter much what your switching frequency is, because the power comes OUT OF THE WALL at 60 Hz, (120Hz rectified). In other words, the amp is on its own running on stored energy for quite a bit of the time during the 60 (or 50) Hz power cycle when the input voltage is below the power supply voltage.
If the amp has a "PFC" circuit (Power Factor Correction), that is improved a bit, so you folks in EC countries (where it is often required) may do better. But, I understand that very few amps have that PFC circuit yet.
So a lot of that stuff like Stewart says about a constant flow of power etc is kinda BS unless the amp runs on three phase industrial power. Might be a fine amp even so....not dissing the amp...



Second, most of the rail switching amps have a glitch at the switch point. I have yet to hear one, they are usually very high frequency. But they ARE there to see, and someone might complain about them representing distortion, I suppose.
The rail boosting amps, (class-G vs class-H?) tend not to have that as much if at all. Carver made a number of that type at one time.
I don't think that can be heard, at least not directly. Might be for an audiophile at home listening carefully, but no way with bass at the gig....fuggettaboutit.



Third, There is a reason for switching power supply amps sounding weaker.
Most if not all have regulated power supplies, always producing the same voltage at any load. (a few run partly unregulated and may be different)

Most conventional amps have "power supply sag" under load, so that to produce 500W at full load, the amp may be capable of as much as 800 watts on transients before the supply is drained down. If the input is sustained, the voltage falls to the 500W level.

A regulated switching power supply doesn't have that sag, so a 500W amp is maxed at 500W on transients or continuous. That means it might easily sound weaker with a musical input than an equal power conventional amp.
Particularly bass...with string popping, its gonna be all about peak power....If you do industrial punk, you may want rms.........

 

[Phat Ham]

Originally posted by throbbinnut
Is that the power supply switching frequency, or the switching frequency of the output devices?

Chris

it's the power supply switching frequency, and I was wrong about the frequency. it's actually 230,000 times a second according to the QSC website.

 

[Phat Ham]

Originally posted by notanaggie
first, it doesn't matter much what your switching frequency is, because the power comes OUT OF THE WALL at 60 Hz, (120Hz rectified). In other words, the amp is on its own running on stored energy for quite a bit of the time during the 60 (or 50) Hz power cycle when the input voltage is below the power supply voltage.
If the amp has a "PFC" circuit (Power Factor Correction), that is improved a bit, so you folks in EC countries (where it is often required) may do better. But, I understand that very few amps have that PFC circuit yet.
So a lot of that stuff like Stewart says about a constant flow of power etc is kinda BS unless the amp runs on three phase industrial power. Might be a fine amp even so....not dissing the amp...

Here's a boost converter:

Invalid Link Removed

Basically, when the transistor is "on" (consider it a closed switch), current flows into the inductor. When the switch turns "off" (open switch), the current stored in the inductor has to go somewhere, so it flows across the diode and into the capacitor. Since the diode has to be forward biased for the current to flow into the cap, the voltage coming out of the inductor increases to match the voltage across the cap.

So you can see that current will still be drawn even if the supply voltage is lower than the rail voltage. The inductor, diode, and capacitor act together to boost the supply voltage up to the rail voltage.

 

[notanaggie]

PhatHam..

What you posted would be a PFC front end (less control circuits) if it didn't have the input capacitor....

In any case, you are discussing "stored energy" in the inductor, which is just as "stored" as in a capacitor.

The typical capacitor power supply as on a plain old traditional amplifier is only re-charged with energy when the line voltage is at its peak. The rest of the time it is running on stored energy.

A standard switching power supply is also like that, but often can use input power at lower voltages.

Even so, there is a good bit of time when it is running on stored energy. As the stored energy is used, it "runs down" and internal voltages may drop reducing power output capability. That means it may run out of power and clip/distort during part of the 60Hz cycle. You hear that as 'fuzzy" sound in subs.

The PFC is a boost converter, and is designed to use nearly all of the input power cycle, since it can boost low voltages up to usable ones. There are some details, and even the PFC has a fluctuating power flow, but it's better. It also tends to even out mains power flow, so it has been required by law in europe.

Anyhow....switching power supply amps can sound weaker than equal power regular ones, but they generally will run at full power even when the mains voltage drops due to the load your truckload of lighting puts on the mains......so its a tradeoff.

Since they cost a bit more usually, the marketing folks come up with ways to make them worth it....like the hype quoted somewhere in a previous post.

 

[uglybassplayer]

Originally posted by Metal Mitch
I respect your tact in not naming names, but I recently purchased a "popular, low-priced Class H power amp" made by a manufacturer who is well-respected on these TB forums. So I am concerned I may have made a bad decision...

Seems like Carvin's marketing hype is working . Specs aside, Carvin doesn't have the greatest reputation out there in the amplification & electronics department. I'd take (and I have) a "PLX" series amp over any "DCM" amp any day.

Listen with your ears, not with your eyes


Peace,

Frank.