Pass Labs F7 Class A Stereo Power Amplifier Preview

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, designed with an unusually low parts count. It is the 19^th product from their First Watt concept, since its founding in 1998 as Nelson Pass’ “personal laboratory”.

Explaining First Watt, Nelson Pass says:

“I spend a lot of time building and evaluating basic amplifier concepts with an eye toward producing the highest quality sound with elegantly simple circuits. There is no such thing as a perfect amplifier, but there is a best amplifier for each type of speaker, music, and listener. Each First Watt model is a unique design and is ‘best’ in some particular way. They look alike because they all use the same basic chassis and power transformer. On the up side, it makes it easy for me to test ideas and compare different designs with the hardware being a fixed constant."

“Looking for tomorrow's faceplate with yesterday's circuits? You're in the wrong place.”

The desire for a simpler circuit is self-explanatory – apart from the aesthetic, Nelson Pass believes that that simpler circuits tend to sound better:

“In such a simple circuit, there are opportunities for improving performance by careful choice of transistors, resistor values, voltage and current values and precise matching of parts.”

The F7 has most of what was on his Wish List:

• Very wide bandwidth
• Low distortion and noise
• Large Class A operating region
• Less feedback
• No degeneration in the output stage
• Very low thermal distortion and drift
• No capacitors or transformers (apart from the power supply)

According to Pass Labs, the F7 incorporates an innovative balance of very low Negative Voltage Feedback and a small amount of Positive Current Feedback to give an astonishing measure of control over reactive loudspeaker loads. Modest amounts of Negative Feedback are balanced in counterpoint to a small amount of Positive Current Feedback, creating an equilibrium where the output impedance approaches zero, improving transient and frequency response.

From Nelson Pass:

“Of course you can achieve a similar effect with tons of negative feedback, but I think this is more elegant and sounds better. For brevity, I call it ‘PCF’,”

“Also, I put more capacitance in the power supply and found a clever way to further reduce the effect of high frequency DAC noise and environmental RF. This is a different amplifier. The diversity of audio taste being what it is, not everyone will prefer it. I presume that a certain segment of audiophiles will like it.”

Why Does Feedback Get a Bad Reputation?

Quite frankly, I never understood this.  Only in the audio world can the word amplifier "feedback" be a dirty word, especially in the high end community.  Feedback not only provides stability in an amplifier circuit, it greatly reduces distortion and also increases usable bandwidth and lowers output impedance to make the amplifier output less dependent on the load it is driving.  A couple of years ago we tested the X350.5 Pass Labs amp to see just how much "better" or different it could be than the competition based on their core philosophy of "first watt" and minimalist design. While we felt the amp sounded great, it's measured performance fell a bit short, especially for its hefty $11k asking price.

Pass Labs X350.5 FFT Distortion Analysis @ 1 watt 

Notice the high DC offset at 1 watt.  In our opinion this is unacceptable for SOTA amplifier design.  We have to wonder why a DC servo-feedback scheme wasn't used to keep the DC offset at the output as near to zero as possible?  By comparison look at the low DC offset of this much less expensive amplifier from Classe, namely the CT-2300.

 

Classe CT-2300 FFT Distortion Analysis @ 1 watt

Based on these measurements, the "first watt" of the Classe amp bench tests significantly better so we can't help but scratch our heads about the claim.

Pass Labs X350.5 FFT Distortion Analysis @ Full Power

Notice the high distortion residuals at full rated power. In our opinion this could have been improved with a more aggressive feedback circuit scheme.  We've tested numerous high power amps that didn't distort as badly at rated output, so again we remain puzzled by the desire to "minimize" feedback in amplifier design.

The reader is encouraged to check out this great article by Bruno Putzeys on Amplifier Negative Feedback for a more in-depth discussion on the topic.

The Final Word

Although we haven't been blown away by how Pass Labs amplifiers have tested on our bench, we did thoroughly enjoy our listening sessions with the X350.5 unit we reviewed.  This new F7 amplifier does look intriguing. But, it's rather low power spec and high inefficency (25%) makes it quite impractical in this day and age where we live in a more energy conscious, "green" friendly world.  In our opinion, modern advances in electronics have rendered the need for pure Class A operation useless and a relic of the past.  The question is, will Pass Labs find enough of a fan base that hangs on to the myth of Class A superiority that would be willing to fork over $3k for a 25 watt/ch amplifier?  Only time will tell.  Let us know your thoughts in the related forum thread below.

Unless otherwise indicated, this is a preview article for the featured product. A formal review may or may not follow in the future.