Have I mismatched my preamp/amp?

Hey all; I recently purchased a Cary SLP-50B which I will be mating to a Rawson clone of the Pass Aleph 2. When I started doing the math, I know I'm slow, I looked up the output impedance of the Cary and its Aleph 2 from the Pass site and it's 2.2 K Ohms I checked the Pass site for the Aleph 2, being I don't have the exact spec on the Rawson, and the input impedance is 10 K Ohms unbalanced and 25 K Ohms balanced differential. The Cary only has unbalanced outputs. So, how mismatched is this? I understand the 20:1 rule and I'm way off. My point was to use the Cary to warm up the sound a bit of the Rawson, its brutally sharp and I thought warming it up was a good idea. Advice/Comments are welcome. Jack
I've always heard it as a 10:1 rule and it looks like a mismatch although your ears should be the final guide. But it sure sounds like you don't like your amp.
Thanks for the response, but I did not mean I did not like my amp. They sound excellent. I was looking to warm them up a bit. When I said they were brutally sharp, I should of been more expressive, they reproduce the music on the sharper side, I like a tad of warmth, I figured I would be able to warm them up with a nice tube preamp.
So, I gotta say this is a great site. I send tim rawson an email and he responded,and it was a good response. He said typically it should be either 100k ohms or minimum 47k ohms input impedence on his amp, all nicely above the 10:1 or 20:1 ratio. I suppose I'm in good shape.
Nothing to worry about...if it sounds good to you, that's the only criteria.
I would be surprised that Aleph series would have such low input impedance. If that's the case you'll have dynamic headroom limitations. Since your sound is sound than enjoy the music!
I have too much respect for Nelson Pass to contribute advice to those that buy Pass "clones".
I composed the following before seeing your last post, about 100K/47K. I think you would have probably been ok even if it were 10K:

The usual rule of thumb guideline is that power amp input impedance should be at least 10x greater than the output impedance of the preamp, at the frequency within the audible range for which preamp output impedance is highest. If only a nominal output impedance is known, which presumably occurs at a mid-range frequency such as 1 kHz, a much larger factor should generally be used.

With a tube preamp, the worst case output impedance commonly occurs at 20Hz, due to the impedance rise that occurs at low frequencies as a result of the coupling capacitor that is usually (but not always) used at the output. If the 10x ratio is not satisfied at that frequency, perceptible rolloff and/or phase shifts can occur in the bottom octave.

In this case, however, as indicated on page 2 of the manual, the output coupling capacitor is 5uf, which is a relatively large value, and which I suspect is large enough to reduce those effects to insignificance with a 10K load.

Assuming that the output impedance doesn't rise to a major degree at any higher frequencies within the audible range (which would be unusual although not unheard of), I think you're fine.

Do try to avoid long interconnect cable lengths, though, and/or use cables having low capacitance per unit length. Otherwise a small but perceptible degree of upper treble rolloff could result from interaction of the high output impedance with cable capacitance.

-- Al
Just to make you aware that Nelson Pass publishes his obsolete designs so the followers would still build components based on his design. Consumers will purchase the "cloned" products that match their budget. So having respect to Nelson Pass implies respecting his products and clones as well.
Did the Cary warm things up?
I can only comment that a Cary SLP 05 Preamp and my Pass Labs X350.5 Amp was definatly a mismatch. The sound had weak bass and treble and unimpressive dynamics.
I sold the Cary Preamp and bought a Pass Labs XP10 Preamp.
My rule of thumb is to look for 40Kohm input impedance minimum or higher as a general indicator to expect good performance with a tube pre-amp. I view it as an insurance policy to help achieve the kind of sound that I like, ie tight, clean, detailed, with good dynamics.

Based on that the ops gear does not appear to be a mismatch.
Well, thanks for all comments! I have not received the Cary yet, so I can't share my listening results, I will when I receive the pre of course. I know my ears are the ultimate judge.
I looked up the Pass Aleph 2 specs online and the low impedance was correct. However, Tim corrected me for his clone. Almarg; I thank you personally for that detailed explanation. I somewhat understand what your saying, basically where it needs the "headroom" it has it and should be fine. The cable runs are < 1 meter, so pretty short.
As for the "respect" comment; I was a) under the same understanding of what Marakanetz wrote and b) believing in a) allows me to enjoy the Pass sound within what I can afford.
Unsound. Most of the designs by Pass are not that unique as some people think they are. I have spoken with some electronic engineers and according to them most of his designs are just text book. Like 99.9% of all amp designs.
If that were the case, why are they marketed as "Pass clones"?
Because Pass publishes the designs he uses. So people can copy/clone them.I'm not saying Pass amps are not good. In fact they are one of the best I ever heard. But that doesn't make them unique in design.
Thought Pass was providing designs for the DIY community, not for commercial purposes (i.e., selling clones).

If Pass' designs are text book, he must be using a different text book, his amps consistently sound better than most other test book SS amps.
My understanding is that Nelson Pass publishes his designs on the condition that they are not used for commercial purposes. This is much discussed at diyaudio.com
...and for commercial purposes there are permissions exist and need to be acquired.
This interview from Stereophile seems to suggest that perhaps Nelson Pass was
somewhat inventive and unique - I don't think they give out patents absent
that. Of course, he has done a bit of tinkering and experimenting since the
interview with some First Watt products - they too seem to reflect some
inventiveness and uniqueness. If you spend sometime in the DIY forums, it
sounds as if most of those folks seem to think Pass isn't copying a textbook,
but writing it.

Nelson Pass On The Patents Of Pass
By Brian Damkroger • Posted: May 23, 2009
If high-end audio were to carve its own Mt. Rushmore, whose faces would
appear there—besides that of Stereophile founder J. Gordon Holt, of course? It's
likely that no two audiophiles would ever come up with identical lists of
subjects, but I wouldn't be surprised if they could agree on at least one name:
Nelson Pass.
Pass's influence has now spanned three decades and shows no signs of
stopping. Card-carrying audio junkies and the more-power-is-always-better
crowd have long lusted after his megawatt beauties. The flea-power amp and
open-baffle types drool over the Zen models of his First Watt line, and the true
hard core, the DIY crowd, refer to him as "Papa" as they eagerly
absorb his donated designs, wisdom, and general good humor.

Nelson Pass is many things, but most of all he's an engineer. He views the world
as a puzzle to be solved, and immediately after figuring out how something
works, he begins thinking about how to make it work better. He's approached
audio design with this combination of curiosity and pragmatism, and the result
has been a string of innovative, often brilliant designs, many of which have
been based on technologies that Pass has patented. I tried to follow the
evolution of his designs through these patents but was quickly buried in
legalese, so I asked Pass to walk me through them chronologically. The result
turned out to be a mini-history of his career as an audio designer.—Brian

"1976—US Patent 3995228: Active bias circuit for operating push-pull
amplifiers in class-A mode: [Pass's first patent describes what's often referred
to as "sliding bias," where the bias on the output devices varies with
the signal to prevent them from shutting off, ergo avoiding crossover
distortion. ]

There was a lot of interest in class-A operation, but there were practical issues,
mainly their size and inefficiency. The maximum power output was theoretically
limited to twice the bias current, and really more like half that. I modulated the
bias to stretch the bend in the operating curve. Technically, this kept it
operating in class-A, but at a lower bias level.

The design was successful; the amps sold like gangbusters and were copied
immediately. Unfortunately, the approach ended up getting a bad reputation.
Our first amplifier, the Threshold 800A, used about a 1:1 ratio, so that it idled
at 150W for 150W output instead of 300W. That seemed reasonable and worked
well, but some people took it to an extreme. Instead of idling at 150W, there
were 200Wpc amps that idled at 10W, ran cool, and didn't have any heatsinks—
all claiming to be class-A. In retrospect, we could have sued the copycats, but
we were young and foolish, and besides, I had another design waiting in the

1978—US Patent 4107619: Constant-voltage/constant-current high-fidelity
amplifier: The patent application actually shows the circuit of the [Threshold]
Stasis 1 as the example circuit. This is a kissing cousin to the Quad 405,
injecting current into the output for error correction, but with way more current.
We had a massive power supply and a huge bank of power devices that we used
to support a smaller output-voltage source.

All distortions are variations in a device's characteristic with changes in voltage
and current. Ideally, you want to lock the operating point of your gain device,
the voltage and current, at a constant value, thereby eliminating distortion.
Cascoding covers locking the voltage, so for the current, I devised a
"current bootstrap," an external current source in parallel with the
gain stage. It responded to the current going out and sourced current to the
load, outside of the loop gain path. The result was to keep the output current
very nearly constant, which dramatically lowered distortion, so we could operate
without feedback. These were great amps. They sold for years and years. We
licensed it to Nakamichi. They're still great amps.

1988—US Patent 4752745: Opto-isolated bias circuit for operating push-pull
amplifiers in class-A and class-AB modes: This one was optically coupling
current sensing to the bias circuitry, which locked it nicely. Typically, bias drifts
over time, and with changes in temperature. Even if it's perfect, the amp heats
up and things shift. This was a way to look at the current and couple it back to
the bias circuit for a constant bias. Nowadays, we just let the amp warm up and
settle in. One hour is pretty good, two hours is better.

1990—US Patent 4899387: Active low-frequency acoustic resonance
suppressor: [This patent isn't for an amplifier circuit, but for a floorstanding
device that canceled out problematic room resonances. You'd set it where a
peak was occurring, usually in a corner. It would measure the sound, amplify it,
and play it back out of phase, canceling out the resonance—kind of like Bose
noise-canceling headphones for your room.]

Oh! The [Phantom Acoustics] Shadow (footnote 1). You probably never heard of
it—it was an active acoustic absorber. It worked, but dealers didn't really know
what to do with it. Plus, it cost a fortune. By the time they got done with the
manufacturing [by this time Pass had left Threshold], I think it cost something
like $2000 just to build it. I think to be successful, it would have had to retail
for less than $1000.

1994—US Patent 5343166: Efficient high-fidelity audio power amplifier: One of
my favorite tricks is to look at what other people do and think creatively. For
example, see what happens if you just swap the words voltage and current.
There's a Panasonic patent, by Sano, Hirota, et al, where a little class-A amp has
its power-supply ground driven by the output of a bigger amp that can swing a
lot of voltage. If you switch voltage for current, you have an altogether different

With this circuit, you can run a cascoded gain stage at low voltage, but at high
current through a separate power supply. Then, you can bias the gain stages at
enormous currents. Bipolar outputs usually have a sweet spot around 100mA or
so, but MOSFETs just keep getting more linear with higher-bias currents.
MOSFETs love current. For example, a 100W class-A amp would need a 5A peak
output into 8 ohms, so you'd normally bias it at 2.5A. Here, you could bias it at
10 or 15A. The distortion is inversely proportional to bias, so 10 times the bias
gives roughly 1/10 the distortion. There's no need for feedback—you get great
performance without any. It's a cute idea. Technics made a few [of their version]
but it didn't seem to go very far. This is one that I've got, still waiting to be
turned into product.

1994—US Patent 5376899: Amplifier with gain stages coupled for differential
error correction: This is Supersymmetry, which was the basis for the X series of
amplifiers. It's a way of connecting two matched, balanced amplifiers to more
effectively cancel out noise and distortion. One of the advantages of balanced
circuits is that when the two halves are summed, noise and distortion that are in
phase cancel out. This only works to the extent that the noise and distortion are
identical in both halves. Rather than trying to totally eliminate noise and
distortion, Supersymmetry works to make the noise and distortion in the two
legs identical, which is comparatively easy. The two inputs of the balanced
circuit are cross-coupled; the noise and distortion created in each half is fed
through the other, so that they're more closely matched at the output, and
effectively canceled. It's another way of making a very simple circuit work well
enough to operate with less feedback—although it's a kind of feedback itself.
You may notice that this amplifier was invented before the Aleph 1998 patent,
but came out years later.

1998—US Patent 5710522: Amplifier having an active current source: This the
Aleph circuit, where the output device is biased to run in single-ended class-A
mode. It draws current from the negative rail to the output. One of the reasons I
started Pass Labs was because I wanted to go a new direction. When a company
is successful, as Threshold was, it limits what you can do, how far you can
diverge from successful products. If you do something different every year, no
one will buy your products. Pass Labs was a clean slate for me.

There was a lot going on with single-ended designs, but it was mostly in the 1–
2W region. I discovered that, with the Aleph circuit, I could get a reasonable
amount of power before it transitioned into push-pull. The Aleph 0, which
actually preceded this patent, was biased to produce 75W in single-ended
class-A mode. It was a simple circuit with only three stages. The Aleph 3 was
different. It had only two gain stages and was purely a single-ended design,
with no transition to push-pull. There were no adjustments of any kind and it
was impossible to break.... "

Well, you get the idea - no?
Holy mackerel! What a wonderful education for me and I'm sure others regarding Nelson Pass's contribution to audio. Not being an electrical engineer I honestly have a tough time following a lot of the technical discussion. I've read a lot of what Nelson wrote, but again, most of it goes right over my head. I know I heard a Pass Aleph 0 years ago and loved the sound. When I had an opportunity to purchase his design, I did, at a cost I could afford. I felt and still do, the clones are more a labor of love, just by the cost being so low. More towards, pay for the parts, labor, etc. and here you go. No one has started a Pass Clone Company, its a few that basically we here pass around to each other. I thank everyone for contributing.

Nelson Pass technically is everywhere. His patents are in vast majority of the solid state commercial and home amplifiers. He's like King David and we're all related to him. Anybody who starts building electronic circuits or studies an amplifier basics goes through the NP patents in the study books.

The idea is that respecting Nelson Pass as an engineer don't have to be associated with criticizing those who purchased clones or built clones.
The Quicksilver Full preamp has very low output impedance. The older version is 12.5 ohms and the new version is 2.5 ohms. These Quicksilvers match well and always produce great sound with any amplifier and with their low output impedance they are insensitive to cables and cable length.

It is my understanding Pass had some involvement in the design of some Adcom products and I am not impressed.
There is nothing wrong with making and selling clones in accordance with a
licensing agreement; misappropriating intellectual property and selling for a
profit is worthy of criticism - as is theft in general. I'm not saying Rawson did
this (I don't know what arrangements exist between him and NP).

Now if you made a DIY project for yourself from NPs recipe book, than decided
to sell it to recover your costs and time, that would seem fair and ok in most
anyone's book. However, a production line of "F5" clones brought to
market, absent a license to do so, would not be ok. And it is also not Ok to buy
a product if you have full knowledge that it is the result of the "theft" of
intellectual property (which for some reason some seem to find
unobjectionable) -- intellectual property is just as real and worthy of
protection as tangible goods IMHO.

Now if there is no inventiveness in the Pass designs, and simply an application
of a public domain circuit design, then I suppose anyone can use them, but
even then I'm not sure it would be ok to sell it and brand it as a clone of a
better know company with credibility and brand equity. If you want to serve the
needs of budget segment anyone is free to do so, but not by copying someone
else's work without a licensing agreement (the Robin Hood model).

Again, I'm not saying one way or the other if Rawson falls into one camp or the
other here as I am not privy to any information on arrangements made with NP
- I understand he is a hobbyist and does this stuff for fun - though there does
seem to be an awful lot of Rawson stuff out there for a mere DIY hobby - so
maybe he has some licensing rights to copy and sell Pass/First Watt clones.
I've always been under impression that Cary is good but for each good Cary there's a better Quicksilver as a matter of fact.
Those are pretty amazing output impedances for any preamp, let alone a tube one, how does he do it? I enjoyed everything I ever owned by Quciksilver.

I believe NP designed (but did not build - designed to a price point specificed by Adcom) the Adcom 555 (not the mkII which was altered by Adcom) and no other Adcom amp. Is that the one you were not impressed with? Pretty cheap amp if I remember correctly - but touted as one of them giant killers at the time.
Those are pretty amazing output impedances for any preamp, let alone a tube one, how does he do it?

Lots of feedback. But if you check the 20Hz output impedance you will find it to be higher.
Isn't it more important to have minimized feedback on power amp vs. preamp?
As far as I know, the differential preamp stages have 100% feedback... Why do engineers need to avoid it technically?
OK, received the Cary and took a long long listen. As the ears are the true test, hear goes (pun intended); did what I wanted it to do. Before the sound was what I considered sharp and I did not hear the space between sound. When I qued up Cowboy Junkies "Trinity Revisited" well honestly I've listened to it since it came out 4 or so years ago and never heard it before! The space between Margo's voice is palpable, the harmony, and Natalie's voice is breath taking. The sharpness has been replaced with rich thick sound, clear, but smoother. I have telsa tubes in and have ordered some more, just to play. I'm very happy with the outcome, so thanks for the opinions, comments and slight moral dilemma.
Now that is a nice way to settle into the holidays. Cheers!
Pubul57; Nicely said my friend!