Pass Labs F7 Class A Stereo Power Amplifier Preview

[gene]

Pass Labs unveiled their new F7 power amplifier with emphasis on sonic purity and getting the first watt right. Unlike other models in their line up, the F7 is a relatively low power (25W/channel into 8-ohms) simple Class A circuit, that sells for a whopping $3k.

Pass Labs touts the usage of minimal negative feedback in this design, but is that a good thing? Is Class A operation really necessary in this day and age where electronics have evolved so much and we are more conscious about energy conservation?

Read: Pass Labs F7 Class A Stereo Power Amplifier Preview

Tell us what you think about this type of product.

 

[PENG]

Let me start by saying it is my right to guess that Mr. Pass could not tell it apart from his other SS amps using the first watt in a DBT. Another thing, +ve feedback? Wow, I wouldn't want to spend 3K experimenting. Still, for 3K it seems like an affordable toy for die hard audiophiles.

 

[tyhjaarpa]

Haven't played with power amps as I can get really nice receiver / 2 channel integrated amplifier for 500$. Paying 3k for such power amplifier seems pointless to me. Yes, I haven't experienced with power amps, but I'm pretty sure it won't give me 2500$ worth of upgrade if any. People have to make money from something and apparently there is demand for such amplifiers as he keeps making them.

 

[highfigh]

How is the power consumption of this amp, when used at normal average listening levels, different from most other Class AB amplifiers when paired with high sensitivity speakers (that's the MO of the low power crowd, right?) and why would this be a bad amp WRT power consumption when a Class AB amplifier's output transistors are conducting at the same time, anyway? If someone has solar panels, 160W power consumption won't make a difference to anyone outside of their building.

 

[slipperybidness]

Come have a drink of this KoolAid and have a listen to my new esoteric amp

 

[slipperybidness]

How is the power consumption of this amp, when used at normal average listening levels, different from most other Class AB amplifiers when paired with high sensitivity speakers (that's the MO of the low power crowd, right?) and why would this be a bad amp WRT power consumption when a Class AB amplifier's output transistors are conducting at the same time, anyway? If someone has solar panels, 160W power consumption won't make a difference to anyone outside of their building.

I would think that you know about quiescent current, biasing, and the efficiency of Class A vs. Class AB vs. Class D, right?

If you are biased into class A, you are dumping a lot of power straight to heat.

 

[gene]

How is the power consumption of this amp, when used at normal average listening levels, different from most other Class AB amplifiers when paired with high sensitivity speakers (that's the MO of the low power crowd, right?) and why would this be a bad amp WRT power consumption when a Class AB amplifier's output transistors are conducting at the same time, anyway? If someone has solar panels, 160W power consumption won't make a difference to anyone outside of their building.

Power consumption of Class A amps are very inefficent at EVERY power level since the transistors are always conducting. Best case scenario is 25% efficiency.

see: http://www.audioholics.com/audio-amplifier/amplifier-classes

 

[shadyJ]

seems to me that class A amps are like Rolex watches; it's not what they do that intrigues their users, its how they do it. Rolex are terribly inaccurate compared to ordinary quartz watches, but the clockwork mechanism of their functioning is a neat trick in design. For practicality, I will take a class D or A/B, but for fun (and also to save on heating bills) I would like to try a class A amp.

 

[mtrycrafts]

... (and also to save on heating bills) ...

Well, the heating bill should be the same, no? More heat from the amp, less heat from the furnace.
But then, which fuel is more expensive per BTU, electricity or the fuel in the furnace?

 

[TLS Guy]

Power consumption of Class A amps are very inefficent at EVERY power level since the transistors are always conducting. Best case scenario is 25% efficiency.

see: http://www.audioholics.com/audio-amplifier/amplifier-classes

Unless it is a Quad current dumper. They run very cool. The class A amp corrects the AB dumpers in a feed forward arrangement. The spec is entirely class A and not AB. So you get class A with no downsides, and a very simple circuit to boot, with no internal adjustments. I have used these amps for forty years now and never regretted it. I have 9 Quad 909s in service now and also 3 Quad 405-2, to be exactly correct 2 and half 405-2s in service.

 

[mtrycrafts]

...

Tell us what you think about this type of product.

Insanity at work.

Would be nice to have a bench test of that unit.

 

[shadyJ]

Well, the heating bill should be the same, no? More heat from the amp, less heat from the furnace.
But then, which fuel is more expensive per BTU, electricity or the fuel in the furnace?

OK, but can your furnace power your speakers and make music?

 

[TLS Guy]

Insanity at work.

Would be nice to have a bench test of that unit.

May be and may be not.

50 Years ago I built a pair of labyrinth bookshelf speakers driven by a pair of 4" JW full range drivers, for my oldest sister.

They were, and still are powered by an AR Sugden a21 10 watt per channel class A transistor integrated amp.

Every time I visit her I'm amazed how good this rig sounds. It has not had any service in 50 years.

Good examples of that amp fetch the equivalent of $1500 to $2000 US on eBay. I note they are among the most sought after amps on eBay. I suspect their reputation is more deserved than not.

 

[PENG]

How is the power consumption of this amp, when used at normal average listening levels, different from most other Class AB amplifiers when paired with high sensitivity speakers (that's the MO of the low power crowd, right?) and why would this be a bad amp WRT power consumption when a Class AB amplifier's output transistors are conducting at the same time, anyway? If someone has solar panels, 160W power consumption won't make a difference to anyone outside of their building.

Good point, at low level output the difference won't be much for power amps such as Parsound Halo and Bryston's that are supposed to operate like class A (the bias). The level could be a few watts (e.g. Halo A21), or as much as 20W (e.g. Bryston 4B SST/SST2). At rated output the efficiency difference between class A and AB amps will obviously be huge.

 

[ski2xblack]

This seems like a commercial version of his First Watt DIY kits. Without the diy savings, this amp seems a bit ridiculous.

Oh, regarding the kool-aid comments and ability to tell them from others in blind tests, this whole realm is a side branch of the single ended tube amp part of the audiophile rabbit warren. All of the First Watt amps have differing distortion profiles, some low enough to pass an a/b test, some with predominant 2nd harmonics, some with predominant 3rd but little 2nd... These amps do in fact sound a bit different from each other. They forego the complexity that would result in more power such as additional gain stages or more complex output topology, or that would produce better specs (many use absolutely no global negative feedback, or very little). How else would one get those chocolaty mids?

If it's not obvious, I've dabbled in DIY and find it quite enjoyable, even with (particularly with?) such designs. It's definitely off the reservation, as far as conforming to prevailing engineering norms go. It can be done inexpensively. Why anyone would spend $3k on a commercial version is beyond me. This is an amp seems limited to the more well to do old Klipsch farts to use on their Khorns, and as a conversation piece. And to keep the place warm in the winter months.

 

[mtrycrafts]

OK, but can your furnace power your speakers and make music?

I need to give that a test.

 

[mtrycrafts]

May be and may be not.

50 Years ago I built a pair of labyrinth bookshelf speakers driven by a pair of 4" JW full range drivers, for my oldest sister.

....

Yes, all well and good. But that didn't cost $3k then.
I am very curious how well that tests compared to their other amps and one with feedback. Or, for that matter, your amp.

 

[highfigh]

I would think that you know about quiescent current, biasing, and the efficiency of Class A vs. Class AB vs. Class D, right?

If you are biased into class A, you are dumping a lot of power straight to heat.

Class A is often around 25% efficient, AB is roughly 60% and Class D is sometimes as high as 90%, but it's not as if any of these will make the lights dim at high output unless the output power is also extremely high. 160W of consumption is less than 1.5A and I have a floor lamp that draws over three times as much with its Halogen bulb.

I looked at the specs (power consumption) of the P7, as well as others. The 160W number is for the P7 that's the subject of this thread and another one that came to mind at the time was a Classe amp that's more powerful and draws more, but they're still not as inefficient as a tube amp. One of my tube guitar amps is rated for about 45W in a 5881/6L6 Push-Pull configuration. The consumption rating is about 180W, exactly 25%.

While it's nice to use higher efficiency equipment, is that really the main design criterion for audio? I would say it's not- sound quality and durability would be higher on my list.

 

[TLS Guy]

Yes, all well and good. But that didn't cost $3k then.
I am very curious how well that tests compared to their other amps and one with feedback. Or, for that matter, your amp.

I have dug up some history. It is a fascinating read. The A21 was introduced by Sugden at the end of 1968. It was also made by Richard Allan. My sisters amp is actually the Richard Allan, but they are the same amp.

Here is a history of that amp, including J.E. Sugden's hand written circuits.

Here is the data on the Richard Allan.

How J.E. Sugden solved the class A reliability issue is interesting. The fact is that my sister's amp has worked perfectly for nearly half a century now.

By the way here are the specs for the Quad 909. It behaves exactly like a class A amp.

Maximum Power Output 140 WRMS at 8 ohms (0.5% THD)
250 WRMS at 4 ohms (0.5% THD)
Maximum Output Current 11 Amps peak each channel
Total Harmonic Distortion <0.02% (100 watts into 8 ohms; 20 Hz to 20 kHz)
Input Sensitivity (phono) 775 mV
Output Impedance 1.5 µH parallel with 0.05 ohms
DC offset voltage less than 10 mV
Frequency Response 13 Hz to 40 kHz (+0dB/-1dB)
Crosstalk at 1 kHz -90dB
Signal-to-noise ratio (70w)>108 dB (20 Hz to 20 kHz)

 

[PENG]

Class A is often around 25% efficient, AB is roughly 60% and Class D is sometimes as high as 90%, but it's not as if any of these will make the lights dim at high output unless the output power is also extremely high. 160W of consumption is less than 1.5A and I have a floor lamp that draws over three times as much with its Halogen bulb.

I looked at the specs (power consumption) of the P7, as well as others. The 160W number is for the P7 that's the subject of this thread and another one that came to mind at the time was a Classe amp that's more powerful and draws more, but they're still not as inefficient as a tube amp. One of my tube guitar amps is rated for about 45W in a 5881/6L6 Push-Pull configuration. The consumption rating is about 180W, exactly 25%.

While it's nice to use higher efficiency equipment, is that really the main design criterion for audio? I would say it's not- sound quality and durability would be higher on my list.

You cannot just use the power consumption and output specs to calculate or even estimate efficiency to compare products made by different manufacturers because they don't necessarily use the same methodologies, terms of reference etc. Class A or not, efficiency also varies with load (though less so with A than AB) among other things. Practically speaking, only the manufacturers themselves know the basis of their power consumption specs, assuming they always bench test and specify this spec for their products under the same conditions.

I have previously listed many examples to show how one would be misled using power consumption figures for such purposes. Depending on the specific purposes, power consumption specs can still be useful, such as when comparing power outputs of products in the same line, made by the same manufacturer but only when comparing in relative sense.