TC Electronics RebelHead 450 - Rebel with a Cause

[agedhorse]

You are exactly wrong. The opposite is the case. Class D amps rely on inrush current to replenish their supply rails at a MUCH higher rate than any A/B amp ever could. Far from being less susceptible to AC power fluctuations, class D amps are much prone to damage or shut down due to bad power. While they are smaller and more effiecient in utilizing the power they do get, when faced with a need for a large dynamic swing, they are far more likely to either trip a breaker, shut down, or otherwise malfunction.

As these comments are being made towards switchmode power supplies and class D in general, I would like to make some corrections so as to lend some support towards the SMPS/class D platform...

1. There is no more inrush current for a class D amplifier than for any other amplifier class (normalizing for efficiency of course), the current is averaged out by all the low pass filtering in place that use both inductors and capacitors. In fact, it would be a plausable argument that class AB amplifiers may exhibit higher inrush currents because they are limited by the inductance of the mains transformer and the charging peak currents will be just as high. There are current risetime limiting chokes in most SMPS designs to limit harmonics (due to both the low and high frequency current pulses that show up as distortion on the AC waveform).

2. Power factor correction, present on many SMPS designs these days, reduce current pulses and make the current draw look more linear. This is not common on "linear" (sorry Bob) power supplies.

3. Some SMPS designs incorporate current monitoring foldback to reduce current to a lower value specifically to allow a larger amp to work under dynamic conditions with a given power input capability. This is not practical with a "linear" supply.

4. A power supply will only deliver what is demanded of it by the amplifier and the load. Since the speaker load is almost always inductive, there are not the current spikes present that would exist with a capacitive load. When driving a capacitive load on a test bench (do not try this on your own amp, many amps will fail under these condition) large current spikes will indeed result as the amplifier attempts to chage and discharge the load. This is not a usual condition.

5. The amount of power delivered by the SMPS/class D amplifier per ampere of AC line current will always be greater than for a "linear" power supply and a class AB (or B or G or H) amplifier. Perhaps as much as 50% more.

6. Branch circuit breakers do not trip on small current spikes anyway, the time constants are very long even for magnetic types. We are talking about time constants 1000x longer than the typical current pulses.

When evaluating amplifier types, it's important to seperate fact from fiction in order to avoid overlooking a perfectly good solution on the basis of inaccurate information. I have designed with both technologies and like it or not the future will be migrating towards the SMPS/class D platform. This is a good thing for most bass players, and there will be some amazing options by many manufacturers available in the coming years. It's a good time to be a bass player!

 

[newbold]

Hear Hear!

it sure is a good time to be a bass player.

Until I can pay a guy to lug my rig for me, I'll only be putting food in my fridges.

 

[psychotiger]

As these comments are being made towards switchmode power supplies and class D in general, I would like to make some corrections so as to lend some support towards the SMPS/class D platform...

1. There is no more inrush current for a class D amplifier than for any other amplifier class (normalizing for efficiency of course), the current is averaged out by all the low pass filtering in place that use both inductors and capacitors. In fact, it would be a plausable argument that class AB amplifiers may exhibit higher inrush currents because they are limited by the inductance of the mains transformer and the charging peak currents will be just as high. There are current risetime limiting chokes in most SMPS designs to limit harmonics (due to both the low and high frequency current pulses that show up as distortion on the AC waveform).

2. Power factor correction, present on many SMPS designs these days, reduce current pulses and make the current draw look more linear. This is not common on "linear" (sorry Bob) power supplies.

3. Some SMPS designs incorporate current monitoring foldback to reduce current to a lower value specifically to allow a larger amp to work under dynamic conditions with a given power input capability. This is not practical with a "linear" supply.

4. A power supply will only deliver what is demanded of it by the amplifier and the load. Since the speaker load is almost always inductive, there are not the current spikes present that would exist with a capacitive load. When driving a capacitive load on a test bench (do not try this on your own amp, many amps will fail under these condition) large current spikes will indeed result as the amplifier attempts to chage and discharge the load. This is not a usual condition.

5. The amount of power delivered by the SMPS/class D amplifier per ampere of AC line current will always be greater than for a "linear" power supply and a class AB (or B or G or H) amplifier. Perhaps as much as 50% more.

6. Branch circuit breakers do not trip on small current spikes anyway, the time constants are very long even for magnetic types. We are talking about time constants 1000x longer than the typical current pulses.

When evaluating amplifier types, it's important to seperate fact from fiction in order to avoid overlooking a perfectly good solution on the basis of inaccurate information. I have designed with both technologies and like it or not the future will be migrating towards the SMPS/class D platform. This is a good thing for most bass players, and there will be some amazing options by many manufacturers available in the coming years. It's a good time to be a bass player!

+1

 

[seamonkey]

As these comments are being made towards switchmode power supplies and class D in general, I would like to make some corrections so as to lend some support towards the SMPS/class D platform...

Wow - great information. This post needs to be on a sticky FAQ. "What Class-D and SMPS means for bass players"

This is a good thing for most bass players, and there will be some amazing options by many manufacturers available in the coming years. It's a good time to be a bass player!

That is a big +1

To be a suck-up. I know this is a TC Electronics thread, but honestly I don't think there'd be a TC Electronic RH450 unless there was first a Genz Benz. Genz Benz set the water mark for this technolgy in bass heads, and grew the market enough for other manufacturers to provide options. It has to have the quality and customer service GB has or it's kaputs. We'll see.

 

[newbold]

This is the closest thing I've seen to Yamaha's BBT500 head. parametric EQs, settings can be saved, classy discrete look.

I look forward to playing the rest, but so far this TC head looks to be an amazing product. There's lots of GB and MB love on the board, but as time goes on we're gonna see everyone bring little amps onto the market and the differences will be in ergonomy, design, and marketing.

the 'tube tone' looks like an amazing feature, as well as the multi-band compressor.

When it's time to get a new rig I bet I'll be able to get one of these for a much better price than the $1200 CDN it's opening at.

My Yamaha will probably last me for a very long time. What a great amp IT is.

 

[bongomania]

I would be grateful if someone with hands-on access to both could compare the compressor in the RebelHead with the Nova Dynamics pedal. I had some significant complaints about the pedal even though it looked brilliant on paper... so I will be very keenly interested to know if the multiband comp in the head is identical to the multiband comp in the pedal, along with any other comparisons. Thanx!

 

[Psycho Bass Guy]

As these comments are being made towards switchmode power supplies and class D in general, I would like to make some corrections so as to lend some support towards the SMPS/class D platform...

Respectfully, you are comparing apple and oranges.

1. There is no more inrush current for a class D amplifier than for any other amplifier class (normalizing for efficiency of course), the current is averaged out by all the low pass filtering in place that use both inductors and capacitors.

If you average current draw, of course not. The better relative efficiency of the class D amp is such that an average inrush will be lower, but I'm not talking about average, I'm talking about dynamic surges and a Class D does NOT have the stored energy on tap that a class AB amp does.

In fact, it would be a plausable argument that class AB amplifiers may exhibit higher inrush currents because they are limited by the inductance of the mains transformer and the charging peak currents will be just as high.

ONLY at powering up, NOT under use.

2. Power factor correction, present on many SMPS designs these days, reduce current pulses and make the current draw look more linear. This is not common on "linear" (sorry Bob) power supplies.

No, because it's not needed to start with. A linear supply "acts" like a linear supply. It doesn't require extra regulation to do so.

3. Some SMPS designs incorporate current monitoring foldback to reduce current to a lower value specifically to allow a larger amp to work under dynamic conditions with a given power input capability. This is not practical with a "linear" supply.

Again, in most cases, this is NOT needed with a linear supply. Reducing output power to keep the amp running may be a good real-world design, but the fact remains that the needs for it at all only reinforces the point I made.

4. A power supply will only deliver what is demanded of it by the amplifier and the load. Since the speaker load is almost always inductive, there are not the current spikes present that would exist with a capacitive load. When driving a capacitive load on a test bench (do not try this on your own amp, many amps will fail under these condition) large current spikes will indeed result as the amplifier attempts to chage and discharge the load. This is not a usual condition.

I never specified a capacitive load and was, in fact, talking about an inductive load, just as you have been for the past three "points" as well.

5. The amount of power delivered by the SMPS/class D amplifier per ampere of AC line current will always be greater than for a "linear" power supply and a class AB (or B or G or H) amplifier. Perhaps as much as 50% more.

...and I said as much. However, that figure is for average use and not large dynamic swings, which is the very specific situation I described. As I said, you're mixing just enough facts to obscure the truth: Class D amps have higher transient current draw, period.

6. Branch circuit breakers do not trip on small current spikes anyway, the time constants are very long even for magnetic types. We are talking about time constants 1000x longer than the typical current pulses.

The quote I used specifically related to circuits already skating on the edge of their limits as are common in most substandard venues where the PA and backline probably share a circuit with refrigerators, stage lights and/ or any other number of things. No, the amp itself won't trip the breaker in most cases, but when you add its increased current need in a transient situation, everything I said will occur.

When evaluating amplifier types, it's important to seperate fact from fiction in order to avoid overlooking a perfectly good solution on the basis of inaccurate information.

...and you are being borderline dishonest. ALL of your points address the average operational condition, which I said myself was more efficient with Class D. The situation I correctly refuted referred to substandard AC conditions and real-world dynamic current draw, not overall efficiency.

...and for the record, I have nothing against Class D amps at all, but they have shortcomings, compromises, and caveats for use, just like any design. The scenario which I quoted and correctly refuted is one of them. Don't misinterpret my words to say something they do not. I was simply correcting technical misinformation which was purposefully posted.

Matter of fact, I'm especially curious about this amp. TC stuff is first rate.
__________________

 

[agedhorse]

The point I was trying to make is that you HAVE to average out all the pulses of an SMPS (and the attached class D amplifier) because the switching of both occurs 10x faster than the highest audio signal. The most basic operation of SMPS/class D averages due to the integration of the HF switching pulses over the AF signal via the LC elements. I suggest that you measure some of these products and you will see that the pulses are not present on the AC line... they can't be or else they wouldn't meet EMC conducted emissions standards.

Also, there is generally similar amounts of energy storage in an SMPS versus "linear" supply, the input bulk capacitors are typically similar in energy storage size when the higher voltage of the rectified line is taken into account on the SMPS input section. The energy stored increases as the square of the voltage, so the capacitor "value" can decrease by a factor of 4 for each doubling of voltage. Add this to the HF capacitance and it's pretty darned close. Dynamic current draw is proportional to energy stored and the impedance of the power supply. In general, the impedances of an SMPS and a "linear' supply are pretty close, and in fact I recently designed an SMPS that matched the impedance curve of a "linear" power supply. As far as the AC line was concerned, there was no difference between the two supplies other than the "linear" supply had a lower power factor, hence MORE current for the same real power.

Class D amps do NOT have inherently higher transient current draw. It will be identical to a linear amplifier with identical output impedance (damping factor). The dynamic current will follow ohm's law in both cases, it's purely a function of the amplifier's transfer function.

Another interesting and unique aspect of class D that you may not have considered... for a full bridge output stage class D topology, driving a reactive load will REDUCE current draw over a linear amplifier because energy stored in the speaker's "spring" will be returned to the power supply and used to power the opposite side of the bridge returning the driver to it's midpoint. This is unique to class D amplifiers by the way.

Current foldback and PFC are used to improve an already more efficeint process. A "linear" supply is anything but linear. It's full of pulses as the bridge rectifier conducts over a very small peak duration of the AC waveform. Looking at an input current trace of a linear supply reveals that it's a series of very high current spikes, necessary for charging the filter capacitors. PFC on an SMPS really decreases these current pulses. Current foldback mamagement is used to allow some of the really big amps to operate on smaller than ideal AC power sources. Bob's PL380 is a great example of how this works in the real world.

I object to your characterization that I am being borderline anything.

 

[Brad Johnson]

I got my non-selection email a couple of days ago and since I'm sure my wrists will eventually heal, my interest in the rig remains the same: I'm still curious about it.

I'm in the demo that I think this is aimed at... gigging (or not) musician who will use lightweight gear and has the finances and disposition to actually buy it.


I'd have to agree that the offer of a free with purchase footpedal is not a consolation prize. You actually get consolation prizes. A year's supply of Turtle wax would've been nice.

 

[Psycho Bass Guy]

The point I was trying to make is that you HAVE to average out all the pulses of an SMPS (and the attached class D amplifier) because the switching of both occurs 10x faster than the highest audio signal. The most basic operation of SMPS/class D averages due to the integration of the HF switching pulses over the AF signal via the LC elements. I suggest that you measure some of these products and you will see that the pulses are not present on the AC line... they can't be or else they wouldn't meet EMC conducted emissions standards.

...then measure the "average" current draw over the limited timeframe of a transient signal in both types. Class D is still more.

Highlighted for clarity:

Also, there is generally similar amounts of energy storage in an SMPS versus "linear" supply, the input bulk capacitors are typically similar in energy storage size when the higher voltage of the rectified line is taken into account on the SMPS input section. The energy stored increases as the square of the voltage, so the capacitor "value" can decrease by a factor of 4 for each doubling of voltage. Add this to the HF capacitance and it's pretty darned close. Dynamic current draw is proportional to energy stored and the impedance of the power supply. In general, the impedances of an SMPS and a "linear' supply are pretty close, and in fact I recently designed an SMPS that matched the impedance curve of a "linear" power supply. As far as the AC line was concerned, there was no difference between the two supplies other than the "linear" supply had a lower power factor, hence MORE current for the same real power.

...the "general" state of being is average duty cycle, not heavy, as was specified, where the differences in supply topology become more pronounced. Under stress, the impedance of switching supply rises much faster than that of a linear supply, making it less efficient in the limited situation where a linear supply is actually more efficient.

Class D amps do NOT have inherently higher transient current draw. It will be identical to a linear amplifier with identical output impedance (damping factor). The dynamic current will follow ohm's law in both cases, it's purely a function of the amplifier's transfer function.

That is only true if both devices have theoretically perfect power supplies, which would render the discussion moot from either perspective. Given the real world constraints of heat, impedance and component limitations, a linear supply has less parts to "lose" energy and edges out SMPS for heavy duty loads.

Another interesting and unique aspect of class D that you may not have considered... for a full bridge output stage class D topology, driving a reactive load will REDUCE current draw over a linear amplifier because energy stored in the speaker's "spring" will be returned to the power supply and used to power the opposite side of the bridge returning the driver to it's midpoint. This is unique to class D amplifiers by the way.

The inductive energy of a speaker coil's motion is far less than the inductive energy of a linear amp's supply. Most of the power disappated through the speaker is lost as heat, so even a "return" on that will be neglible compared to actual supply draw. It's nice to have, but not a "deal-maker or breaker."

A "linear" supply is anything but linear. It's full of pulses as the bridge rectifier conducts over a very small peak duration of the AC waveform. Looking at an input current trace of a linear supply reveals that it's a series of very high current spikes, necessary for charging the filter capacitors. PFC on an SMPS really decreases these current pulses.

That's exactly the advantage of SMPS, HOWEVER, that fails to account for the difference in rise in supply impedance in an SMPS versus linear. You're quoting ideal lab conditions in regards to a situation that is anything BUT ideal.

Current foldback mamagement is used to allow some of the really big amps to operate on smaller than ideal AC power sources. Bob's PL380 is a great example of how this works in the real world.

...and how much is the output power reduced? Again, apples and oranges.

I object to your characterization that I am being borderline anything.

Why do you find it necessary to list the design attributes of an SMPS amp that are advantageous to its operation when they DO NOT APPLY to the situation specified?

 

[bongomania]

PBG, I know you have established the strength of your grasp on tube amp technology here many times, but I have to ask what is the basis for your statements about SMPS/Class D? Do you design or repair them, or have you studied them in school recently?

It's clear what AH's qualifications are, as he does in fact design these types of amp for a living.

 

[Psycho Bass Guy]

Do you design or repair them,

Yes. That should be clear to anyone who has observed this discussion.

It's clear what AH's qualifications are, as he does in fact design these types of amp for a living.

...not that that would give him a motive to be overly defensive of them.

It's baffling how I can point out one of the few caveats of using a SMPS design and suddenly I'm being painted as on some sort of crusade against them when I've published many of the drawbacks of conventional linear designs here for years and nobody seems to have noticed. I do not dispute that SMPS amps are the way of the future and that they have made vast leaps in the past ten years, but the simple facts of physics remain that they are not always going to be the superior design in every situation and seamonkey provided exactly such a situation.

 

[psychotiger]

It's baffling how I can point out one of the few caveats of using a SMPS design and suddenly I'm being painted as on some sort of crusade against them....

Uh, maybe because you admit you are a psycho from the "hills of Tennessee"? Didn't they make a movie about that in the early 70's?

Woot! Electrical Engineer fight! Electrical Engineer fight!

It's the only kind of fight where the audience's heads hurt a lot worse than the combatants.

 

[Bob Lee (QSC)]

You are exactly wrong. The opposite is the case. Class D amps rely on inrush current to replenish their supply rails at a MUCH higher rate than any A/B amp ever could. Far from being less susceptible to AC power fluctuations, class D amps are much prone to damage or shut down due to bad power. While they are smaller and more effiecient in utilizing the power they do get, when faced with a need for a large dynamic swing, they are far more likely to either trip a breaker, shut down, or otherwise malfunction.

Well, that's not true.

 

[Bob Lee (QSC)]

As these comments are being made towards switchmode power supplies and class D in general, I would like to make some corrections so as to lend some support towards the SMPS/class D platform...

1. There is no more inrush current for a class D amplifier than for any other amplifier class (normalizing for efficiency of course), the current is averaged out by all the low pass filtering in place that use both inductors and capacitors. In fact, it would be a plausable argument that class AB amplifiers may exhibit higher inrush currents because they are limited by the inductance of the mains transformer and the charging peak currents will be just as high. There are current risetime limiting chokes in most SMPS designs to limit harmonics (due to both the low and high frequency current pulses that show up as distortion on the AC waveform).

2. Power factor correction, present on many SMPS designs these days, reduce current pulses and make the current draw look more linear. This is not common on "linear" (sorry Bob) power supplies.

3. Some SMPS designs incorporate current monitoring foldback to reduce current to a lower value specifically to allow a larger amp to work under dynamic conditions with a given power input capability. This is not practical with a "linear" supply.

4. A power supply will only deliver what is demanded of it by the amplifier and the load. Since the speaker load is almost always inductive, there are not the current spikes present that would exist with a capacitive load. When driving a capacitive load on a test bench (do not try this on your own amp, many amps will fail under these condition) large current spikes will indeed result as the amplifier attempts to chage and discharge the load. This is not a usual condition.

5. The amount of power delivered by the SMPS/class D amplifier per ampere of AC line current will always be greater than for a "linear" power supply and a class AB (or B or G or H) amplifier. Perhaps as much as 50% more.

6. Branch circuit breakers do not trip on small current spikes anyway, the time constants are very long even for magnetic types. We are talking about time constants 1000x longer than the typical current pulses.

When evaluating amplifier types, it's important to seperate fact from fiction in order to avoid overlooking a perfectly good solution on the basis of inaccurate information. I have designed with both technologies and like it or not the future will be migrating towards the SMPS/class D platform. This is a good thing for most bass players, and there will be some amazing options by many manufacturers available in the coming years. It's a good time to be a bass player!

+1 (including the caveats about non-linear "linear" power supplies)

 

[Bob Lee (QSC)]

but I'm not talking about average, I'm talking about dynamic surges and a Class D does NOT have the stored energy on tap that a class AB amp does.

What makes you believe that?

No, because it's not needed to start with. A linear supply "acts" like a linear supply. It doesn't require extra regulation to do so.

Actually, so-called "linear" supplies are not linear at all, and behave just as agedhorse described.

Again, in most cases, this is NOT needed with a linear (sic) supply. Reducing output power to keep the amp running may be a good real-world design, but the fact remains that the needs for it at all only reinforces the point I made.

Line-frequency power supplies may not have current foldback built in, but they usually do have fuses--and often also thermal switches on the power transformers.

...and I said as much. However, that figure is for average use and not large dynamic swings, which is the very specific situation I described. As I said, you're mixing just enough facts to obscure the truth: Class D amps have higher transient current draw, period.

That is not truth.

 

[Bob Lee (QSC)]

...the "general" state of being is average duty cycle, not heavy, as was specified, where the differences in supply topology become more pronounced. Under stress, the impedance of switching supply rises much faster than that of a linear (sic) supply,

If that were true, then line-frequency conventional power supplies would show less voltage rail sag under heavy load than SMPS do. Usually, SMPS demonstrate less sag than conventional supplies, due to much their smaller internal impedances.

a linear (sic) supply has less parts to "lose" energy and edges out SMPS for heavy duty loads.

Parts count is not a measure of efficiency.

 

[agedhorse]

Usually, SMPA demonstrate less sag than conventional supplies, due to much their smaller internal impedances.

And better (higher) power factor, especially those w/ PFC.

 

[seamonkey]

And better (higher) power factor, especially those w/ PFC.

OK guys - you need to commit. Only SMPS with PFC from now on. I think you'll get an Energy Star rating and a green sticker if you do. Oh and an AMA no aching back approval rating.

 

[meekee73_2008]

You're right. The form factor is cool, and if workable, I could ditch my tuner and compressor, but if the price is true to TC Electronics form, The head would most likely be in the $1,500 range and the cabs would probably be at least another $1,500 for two.

Not fiscally feasible at this juncture.

i'm an owner of both the head and the rs212 cabinet

they simply kick a**es (mark bass who?)


Michele