Common amp myths

[fdeck]

fdeck are u sure?? we are talkin watts. not current draw from the wall... I know for a fact the QSCs have limit lights that will come on before a crown.... I thin Bob lee knows that too... This is not meant to diss QSCs, just different flavors for different people

Here is my rationale: I am thinking about the achievable output power when the amp is plugged into a normal electrical circuit that could even include an extension cord. "All other things being equal," the more efficient amp will run at a higher line voltage, and will reach a higher output power as a result.

It's like a drag race between two Corvettes, but one of them has a trunk full of bricks.

 

[seamonkey]

fdeck are u sure?? we are talkin watts. not current draw from the wall... I know for a fact the QSCs have limit lights that will come on before a crown.... I thin Bob lee knows that too... This is not meant to diss QSCs, just different flavors for different people

Isn't current draw from the wall the important here? Given a 15A wall circuit what's the most volume you can get out of it. That's your real world limit.

 

[billfitzmaurice]

and basically end up sending a DC voltage straight to your speakers.

Oh please, not that chestnut again.

It's been a few terms but from what I learned, clipping is when an AC signal is clipped, in essence cutting a portion of the waveform, correct or not?

Not. A clipped musical waveform simply has a higher than normal harmonic content, that's all.

 

[NKUSigEp]

Oh please, not that chestnut again.

Oh please, not Mr. Know-it-all again!

 

[Rick Auricchio]

Let's not get into current draw and circuit efficiency. That brings in the entire amplifier circuit design, which is beyond the scope of this discussion.

As Bill F said, the same waveform at the same power output will be identical on the Real Time Analyzer, no matter what amplifier you have. If, however, the amp changes the waveform, then the RTA will sense something different. Your ears do the same.

George, nobody is arguing that we may hear a tube amp's signal louder than a SS one. We're just saying that with the same waveform, they'll sound the same.

 

[JimmyM]

I'm going to mod my own thread, and ask everyone to please be civil. This has turned out to be an awesome thread, and I don't want it closed because someone gets a hair up their ass that their favorite myth is being debunked.

 

[Loudthud]

I connected an A/B footswitch to switch between an 8 ohm load and two 16 ohm cabinets connected in parallel. One cab has a single JBL E-140-16 and the second four Knight 16 ohm 10's connected like an old Marshall cab. The top waveform is the output when connected to the speaker, the bottom is when connected to the resistor. The first amp is a tube amp with a pair of 6550's the second is a Peavey Standard (80's) with a 260 type power amp. The signal source was a bass guitar. Scope is a Tektronix 7854. You can see that the top waveform in the tube amp picture is bigger than the bottom but in the second picture the waveforms are nearly identical.

 

[seamonkey]

What's the current?

[Invalid or Expired Link Removed]

"You must use a non-reactive load, however, when testing an amplifier for power output, because the power measurement is only accurate into a purely resistive load. If you are using a reactive load, the actual output power is equal to the voltage multiplied by the current, multiplied by the cosine of the phase angle between them. Highly inductive or capacitive loads can fool you into thinking you have a higher output power than you really do, because, even though the voltage is higher, the current is not in phase with it, so the real power output cannot be accurately determined by measuring the voltage, squaring it, and dividing by the specified impedance, you must multiply by the cosine of the phase angle. A purely resistive load will have the voltage and current in phase, so the cosine of the phase angle is equal to 1, and can be ignored."

 

[bongomania]

The first amp is a tube amp with a pair of 6550's the second is a Peavey Standard (80's) with a 260 type power amp.

That's interesting, for sure, but did you verify that the input signals in fact had the exact same amount of gain applied?

 

[black_labb]

seems like the way a tube output deals with variable loads vs. the solidstate's version where lower impedance means more output changes things significantly when there is a complex impedance ( a speaker).
really thats showing that the tube amp is putting out more watts when connected to the complex impedance, so the watts arent even. very interesting though.

 

[bongomania]

I give :::::::::::::u guys are right....I have no clue what u are bassin ur "facts on" ....I'm not gonna even offer an opinion...the phone thing was an issue of how u can present bass...

Bill, the other guys were presenting facts that can be found in any textbook on the subject, while you were presenting things that may "seem related" to you but are not really relevant.

Also, look at your syntax. Is a thinking person going to believe someone who writes correctly, or somebody who can barely write a legible sentence and who can't explain the science behind their own examples?

 

[spode master]

Isn't current draw from the wall the important here? Given a 15A wall circuit what's the most volume you can get out of it. That's your real world limit.

Current is on Demand.

If you had a Dead Short

The 15 amp limit has to do with what will trip your breakers not a brick wall in current supply.

If you tapped straight into PG&E mains your current limit would be what the fuse is on the Transformer is set to.

After that what ever the transformer could supply before it melted.

 

[bongomania]

You show up for a gig.
Drummers got the biggest set of vistalites you've ever seen, plays with marching band sticks. He's about 7 feet tall, played offensive line for Ohio State in college.
Each of the three guitarists have Marshall stacks, all knobs dimed.
You are presented with an 8x10 cabinet and your choice of an SVT or a B2RE. ...Watt for watt you can count on tubes providing more usable volume in the real world. Period.

I know, use the BR2E, but post a chart next to it explaining how it's just as loud as any tube amp so the crowd is sure to hear it.

You guys both miss the point completely. The B2RE is not a valid comparison, because it is incapable of using its watts efficiently in terms of "loudness". But that is not strictly because it's solid state.

If the choice given was between an SVT with its EQ set flat, and an equivalent-wattage solid state amp plus a good compressor plus a good overdrive pedal plus adjusting the EQ of the ss amp to match the preset EQ voicing of the "flat" SVT, then you might have a fair comparison.

Sure, you can argue that the SVT's inherent compression, distortion, and EQ shaping make it an easier choice over a rack of complicated SS gear, but the fact is all the gear I mentioned in the second example can be 100% solid state. And using it in a manner that carefully recreates the described qualities of the SVT will get you equal "loudness".

 

[Gearhead43]

I'm going to mod my own thread, and ask everyone to please be civil. This has turned out to be an awesome thread, and I don't want it closed because someone gets a hair up their ass that their favorite myth is being debunked.

+1

If I hear the words "DC voltage to speakers" again, I am gonna blow chunks.

By the way, what's the difference in a "dead short" and just simply a "short"?

 

[seamonkey]

Current is on Demand.

If you had a Dead Short

The 15 amp limit has to do with what will trip your breakers not a brick wall in current supply.

If you tapped straight into PG&E mains your current limit would be what the fuse is on the Transformer is set to.

After that what ever the transformer could supply before it melted.

My point was given a X-Amp circuit, I can get a lot more volume out of it using efficient amplifier than an inefficient one. i.e. a 90% efficient class-d is going to be louder than a 50% efficient class-ab. Less than 50% if you have to run heaters.

 

[seamonkey]

seems like the way a tube output deals with variable loads vs. the solidstate's version where lower impedance means more output changes things significantly when there is a complex impedance ( a speaker).
really thats showing that the tube amp is putting out more watts when connected to the complex impedance, so the watts arent even. very interesting though.

Really - No
See the Aiken's link in my previous post. He's a famous tube amp guru.

You have to account for the current, phase, etc into a reactive load. It's not a simple algebraic equation. You just can't measure the voltage like with a resistive load.

 

[spode master]

Well thats a different story.

I think a Perfect Class AB should be ~75%.

15% Better for Clean output with digital, potentially.

Unless your running lots of RMS Sinewaves all night you can push the limit of your breaker with Audio amps a little.

But its recommended to keep your constant load on a breaker under 80% Capacity.

I think with Electric Bass the Demand on the breaker won't be too bad unless your Playing with excessive Sub bass rigs with lots of EQ like me

 

[Geddy Claypool]

LOTS of good info Jimmy. Thanks a ton. Now, when anybody tries to tell me one of these, I'll know what to say.

 

[shakerattleroll]

You guys both miss the point completely. The B2RE is not a valid comparison, because it is incapable of using its watts efficiently in terms of "loudness". But that is not strictly because it's solid state.

If the choice given was between an SVT with its EQ set flat, and an equivalent-wattage solid state amp plus a good compressor plus a good overdrive pedal plus adjusting the EQ of the ss amp to match the preset EQ voicing of the "flat" SVT, then you might have a fair comparison.

Sure, you can argue that the SVT's inherent compression, distortion, and EQ shaping make it an easier choice over a rack of complicated SS gear, but the fact is all the gear I mentioned in the second example can be 100% solid state. And using it in a manner that carefully recreates the described qualities of the SVT will get you equal "loudness".

Bongo, a voice of reason.

 

[fdeck]

I connected an A/B footswitch to switch between an 8 ohm load and two 16 ohm cabinets connected in parallel. One cab has a single JBL E-140-16 and the second four Knight 16 ohm 10's connected like an old Marshall cab. The top waveform is the output when connected to the speaker, the bottom is when connected to the resistor. The first amp is a tube amp with a pair of 6550's the second is a Peavey Standard (80's) with a 260 type power amp. The signal source was a bass guitar. Scope is a Tektronix 7854. You can see that the top waveform in the tube amp picture is bigger than the bottom but in the second picture the waveforms are nearly identical.

At roughly 100 Hz, the speakers are probably close to their impedance peaks. Also, the hard clipping thresholds are an indication of the power supply rail voltages.

I owned a 260C. It has fairly stiff power supply rails, so the clipping levels are not affected by load impedance.

My interpretation of the tube amp is that its headroom comes from having a relatively "soft" power supply, so it has effectively higher rail voltages when pushing the speaker load at 100 Hz.

It would be interesting to repeat the experiment an octave up.