Quilter SuperBlock US: why is the power output lower at 4 ohms than at 8?

[Jon S.]

From the SuperBlock US manual at page 4:

"Provides 25W into an 8-ohm speaker. 4 and 16 ohm speakers may be used, with slightly reduced power."

I understand why the power would be reduced using a 16 ohm load. But why the same at 4 ohms?

 

[Swirling Snow]

Lack of power supply.

 

[Ringo]

I don't know, Quilter has amazing customer service, contact them and ask them. Someone on one of the Quilter FB pages said that according to Pat Quilter even though the SuperBlock US / UK amps are rated 25 watt they really are 50 watt.
I don't know but I have gigged and rehearsed regularly using a SBUS into a semi open back 1-12 cab with a WGS ET65, and it can get really loud, I never have to crank it up.
I had a Interblock 45, supposedly it was less wattage at 8 ohm, I got a 4 ohm speaker and I couldn't tell any difference between the 4ohm / 8ohm speakers as far as volume.

And I'd say my SBUS seems as loud as the Interblock that I had and it seems volume wise similar to the 101 Mini Reverb head that I had too.

 

[Henry Mars]

Max Pwr transfer happens when the input Z of the speaker matched the output Z of the amp. Simply put at 8 ohms the match is closer than at 4 or 16 ohms ... so you get more useable power out of the amp The Quilter SuperBlock is designed so that it can work with 4 8 or 16 ohms. It just happens that 8 ohms is the best match.

 

[Dostradamas]

I don't know, Quilter has amazing customer service, contact them and ask them. Someone on one of the Quilter FB pages said that according to Pat Quilter even though the SuperBlock US / UK amps are rated 25 watt they really are 50 watt.
I don't know but I have gigged and rehearsed regularly using a SBUS into a semi open back 1-12 cab with a WGS ET65, and it can get really loud, I never have to crank it up.
I had a Interblock 45, supposedly it was less wattage at 8 ohm, I got a 4 ohm speaker and I couldn't tell any difference between the 4ohm / 8ohm speakers as far as volume.

And I'd say my SBUS seems as loud as the Interblock that I had and it seems volume wise similar to the 101 Mini Reverb head that I had too.

Interesting news to me on the actual power output of my SBUS

Looking at speaker upgrades in my cabs and it seems prudent for me to not look at the 40W ones

 

[telemnemonics]

You might need an electronics engineering degree to really undertand but each power amp design has what it has and most SS power amps "wattage" varies according to speaker load impedance.
Pretty sure the Superblock is class D which has a switching PS design so it is on off on off continuously.
Not that I understand but I know a little about what it is I don't understand.
Worth just accepting it!

 

[AAT65]

You might need an electronics engineering degree to really undertand

You don’t need to actually take your degree — it’s EE101. I think we learned this (Maximum Power Transfer) on our first week at Uni on an Electronic & Electrical Engineering course.

Here’s a simple thought experiment: replace your speaker with a short circuit (0 ohms). How much sound (= output power) do you hear? Not a thing. Now replace it with an open circuit (unplug the speaker - that gives infinity ohms). What do you hear now? Also nothing.
However somewhere in the middle between 0 ohms and infinity ohms you did hear something!
So now if you imagine a graph with increasing speaker impedance as one axis and output power as the other it must be a curve which is 0 at each end and rises to some value somewhere between. The top of that curve is the point of Maximum Power Transfer — and if you do some maths (high school / first week at Uni level) you can prove that, as @Henry Mars said above, that occurs when amplifier output impedance is the same as the load (speaker) impedance.

 

[chezdeluxe]

You don’t need to actually take your degree — it’s EE101. I think we learned this (Maximum Power Transfer) on our first week at Uni on an Electronic & Electrical Engineering course.

Here’s a simple thought experiment: replace your speaker with a short circuit (0 ohms). How much sound (= output power) do you hear? Not a thing. Now replace it with an open circuit (unplug the speaker - that gives infinity ohms). What do you hear now? Also nothing.
However somewhere in the middle between 0 ohms and infinity ohms you did hear something!
So now if you imagine a graph with increasing speaker impedance as one axis and output power as the other it must be a curve which is 0 at each end and rises to some value somewhere between. The top of that curve is the point of Maximum Power Transfer — and if you do some maths (high school / first week at Uni level) you can prove that, as @Henry Mars said above, that occurs when amplifier output impedance is the same as the load (speaker) impedance.

Your simple thought experiment is not simple it is dumb.
Why would you hear anything in those situations where no transducer is extant.

 

[2L man]

I think the output audio voltage and current are measured (sampled) and they are used for simulation algorithm. In electronics it is called closed loop.

If it is possible to use guitar loudspeaker there should be a mechanism to increase audio voltage when frequency is in speaker "resonance window" where speaker impedance come typically 5X...10X. If output signal voltage is not increased the volume/power would drop when frequencys resonate with speaker. Guitar loudspeaker impedance also increase when frequency comes higher and can be 2X or 3X at 6kHz and there come another need to increase output voltage so that highs do not sound attenuated.

 

[Peegoo]

most SS power amps "wattage" varies according to speaker load impedance.

Same goes for tube amplifiers.

@Jon S.

A good thought experiment is to consider volume as horsepower or torque: the amp's power stage is the engine, the output transformer is the transmission, and the speaker is the tire on the road.

The tire's traction presents a 'load' to the engine, which must produce the power to rotate the tire and move the mass of the vehicle. When the engine makes power, the power drives the wheel and as the vehicle begins to accelerate, the load reflects backward through the drive train from the tire through the transmission to the engine. This load is impedance.

If the transmission's gear ratio is too low, the engine will overspeed and run hotter than it should for a given speed on the road and all moving parts wear faster than they should. This overspeeding is caused by too little of a load reflected back through the drive train, greatly reducing the system's efficiency. The vehicle will never achieve a top speed that would be possible with proper gearing.

If the gear ratio is too high, the vehicle will be sluggish on acceleration and take a long time to reach top speed. This also makes the engine run hot and increases wear on moving parts because the load reflected back through the drive train is too high. The vehicle will never have sufficient power to pull a trailer or climb a hill.

However, if the transmission is thoughtfully designed with proper gear ratio to accommodate the power curve of the engine and uses a properly-sized tire, the entire system operates as efficiently as practically possible and we get the best performance.

Back to amplification: the speaker's impedance is critical because when it matches what the output transformer is designed to "see" as a load, the amplifier's power is most efficiently used to generate maximum volume from the speaker.

 

[Peegoo]

Remember also that a ~3 dB increase equals a perceived doubling of volume, and a 3 db decrease creates a perceived halving of volume. Three is a small number, but in audio science 3 is huge.

This article explains all this fairly well, but it does get into the weeds because weeds are necessary to explain what's going on. And there is some math involved

How Does Increasing Speaker Impedance Affect dB & Power? - Sound Certified

Confused about speaker impedance and how it relates to volume and power? This article explains everything you need to know, in easy-to-understand terms.

soundcertified.com

 

[AAT65]

Your simple thought experiment is not simple it is dumb.
Why would you hear anything in those situations where no transducer is extant.

Well that isn’t very polite is it??
If you can conceptualise something that you still recognise as a speaker which has 0 ohms impedance, and another that has infinite impedance, please feel free to share so that we can make a better explanation.

If you like then you can replace the speaker in the middle of the curve with a resistor and measure the power it dissipates. You will find some power dissipated with a load which is greater than zero and less than infinity and the curve will be as I described it. But it doesn’t change anything: the purpose of the amplifier is to produce sound, and if you don’t hear sound then the output power is zero.

 

[BigDaddyLH]

I had no idea!

For SS amps, all the example ratings I've seen go strictly up in wattage as the speaker impedance goes down, until they hit the minimum value the amp can handle.

 

[Jon S.]

Thank you, everyone. Great responses and I understand the electronics now.

And yes, I've communicated with the good folks at Quilter before about the SuperBlock US. It is indeed a 50W-capable amp that Quilter rates conservatively at 25W. Honestly, I love mine. It is way more than powerful enough for me to use with my full band.

 

[Peegoo]

Well that isn’t very polite is it??

Your reply showed restraint and maturity. I probably would not have been that nice.

BRAVO for being a gentleman, sir.

 

[PhredE]

If you want to take your inquiry to the 'next level' (I know, overused buzzword-- sorry..), look up the datasheet for any of the commonly used Class D audio amp chips (TDA3116, TDA3118, etc). About halfway through the document, you'll see a series of x/y type graphs where amp output power is plotted against output load (~speaker impedance). Those usually give a really good indication of the power vs output impedance relationship.

 

[JDB2]

Thank you, everyone. Great responses and I understand the electronics now.

And yes, I've communicated with the good folks at Quilter before about the SuperBlock US. It is indeed a 50W-capable amp that Quilter rates conservatively at 25W. Honestly, I love mine. It is way more than powerful enough for me to use with my full band.

Quilter says the same about their Tone Blocks rated at 200 watts - they say that they use a 400 watt amp in them. As a result, they can stay clean up to deafening levels. As a TB202 owner I can attest that the headroom seems almost infinite.

 

[Jon S.]

A couple of months ago, I emailed Quilter asking when we can expect a "Mach 4" with built in Neunaber boost and reverb. The rep who responded to me replied that is not currently in the works but to look forward to other new products soon.

 

[chezdeluxe]

Well that isn’t very polite is it??
If you can conceptualise something that you still recognise as a speaker which has 0 ohms impedance, and another that has infinite impedance, please feel free to share so that we can make a better explanation.

If you like then you can replace the speaker in the middle of the curve with a resistor and measure the power it dissipates. You will find some power dissipated with a load which is greater than zero and less than infinity and the curve will be as I described it. But it doesn’t change anything: the purpose of the amplifier is to produce sound, and if you don’t hear sound then the output power is zero.

Keep digging
No sound means there is no power? ……Seriously?

 

[Swirling Snow]

I was wrong. I thought the Quilter was A/B solid state.

As others have mentioned, the power output depends on a match between the speaker and amp's output impedance. Class D amplifiers have a filter as a last stage to reduce the noise they make, and it's a mismatch between this filter and various speaker impedances that cause one impedance to be preferred.

We used multiple transformer taps to solve this with tubes. Expect multiple output filters on Class D amps when they stop trying to make as much money as possible.