Welcome to SAS Audio Labs. SAS Audio Labs came about in December 1996 (after 16 years of research), and continues to play a leadership role in the evolution towards audio perfection. I hope the knowledge presented over the years in my "White Papers" has helped you in understanding what makes a great sounding audio design and helps you when evaluating a component.
Afterall, this hobby is about you and your journey toward more satisfying and realistic music. I am beholden to neither recent technology nor old, but to my own innovative designs.
I start each topic with general easy to understand information and then get more technical. If you wish to bypass the technical information, just scroll down past the technical portion in each section.
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SAS Audio Labs believes that the entire design is important, which includes the power supply, the active circuitry, individual parts incorporated, and even the layout. For instance, although a particular design may look good on the surface and have excellent specifications, and a lot of support, digging deeper may reveal inherent weaknesses that renders it inferior.
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I start with the layout as virtually no one ever mentions this aspect of design. Does it have hot parts or tubes (especially octal tubes) in close proximity to temperature sensitive parts, such as capacitors? A 250-490 degree octal tube within two inches of a 170 to 220 degree maximum rated capacitor risks power supply failure sometime in the near future rather than later. Some electrolytic capacitor's life span is halved with every 20 F degree rise in temperature. To understand what I mean, put your fingers close to a light bulb and feel the warmth.
A blown power supply could cause an expensive down stream component to fail which both you and I do not want. One indication of how knowledgeable and experienced a designer is by the layout.
Another issue involving layout is "crosstalk" between channels, especially at higher frequencies. Parts placement, lead placement can easily degrade "crosstalk" rejection ratio. For instance the channel separation may be 90db at 1khz but only 60db at 10khz. Suppose we have a cymbal crash in the left channel, with harmonics rising out of sight. As the frequency rises, more higher frequency harmonics appear in the right channel, which should not be. So although single channel measurements may indicate flat response as the frequency rises, such is not the case. Problems with imaging, brightness etc occur.
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SAS Audio Labs was the leader in the concept and use of "lead to lead tm" connecting (December, 1996 white paper). "Lead to lead" connecting refers to connecting part leads together, thus reducing the number of connecting wires and solder connections. Again layout is key to maximize performance.
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In December, 1996 I published a "White Paper" listing the positive and negative virtues of both hardwiring and PC boards. The trick to using PC boards is, of course, avoiding the negatives while incorporating the positves. So one exploits the positives of pc boards and hardwiring while avoiding the negatives of pc boards and hardwiring.
Two big pluses of incorporating PC boards are component consistency and higher frequency response due to less stray capacitance, both between adjacent parts and from parts to ground.
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In October, 1997, I presented a "White Paper" that discussed the issue of frequency dependent sonic feedback from stage to stage, through the power supply. This type of feedback is prominent in almost every design, even if not stated. (Actually, I was prompted to write this paper by what I read in the "RCA Radiotron Designers Handbook," 1960, 4th edition, written by 26 engineers, so this issue is not new, just brought back to life.)
De-coupling capacitors are touted as eliminating this problem. However, capacitors increase in reactance as the frequency lowers. Using large de-coupling capacitors reduce reactance, but unfortunately raise the ESR/DF and DA problems. ESR is equivalent series resistance and DA is dielectric absorption. Both degrade the signal/music, so we simply exchange one set of problems with another set.
Musical feedback extends from near zero hertz through at least midrange frequencies and its consequences are huge phase shifts within the audio band; unlike conventional global feedback which generally has phase shifts well outside the audio band (although with time delays). A smaller feedback problem may occur at the high frequencies as well due to the capacitor becoming more inductive, but generally with less phase shifting.
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Another philosophy is the absence of any buffer stages, cathode followers, or Mu stage (which uses a cathode follower), with their inherent ultra high negative feedback. For future reference, I shall simply use the term "buffer stage" to mean any ultra high feedback stage. They are usually mentioned as simple impedance matching stages, producing low output impedances. Unfortunately, the inherent high feedback is rarely mentioned. In the below quotes, notice the phrase "common-plate"/cathode follower.
- From "Semiconductor and Tube Electronics, An Introduction," by James
G. Brazee:
"The common-plate or cathode- follower circuit configuration is an extreme example of degenerative (negative) feedback, since all output signal is fed back in such a way that it appears in the input (grid to cathode) voltage loop in phase opposition to the signal voltage." - From the "Radiotron Designer's Handbook," page 316-7:
"It is therefore frequently referred to as "cathode loading" in distinction to the conventional "plate loading". As a result of 100% negative voltage feedback inherent in the cathode follower...."
The feedback is so high that the gain is less than 1. (The gain a Mu follower stage creates is from the lower tube while the upper tube/FET is the high feedback cathode follower stage.)
One may ask, what about feedback in a regulator stage. Luckily its affects can be small due to its location in the circuit, depending upon the type of feedback. But even then one must be careful with the design.
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This section deals with impedance "mismatch" distortion, high frequency response vs IC capacitance etc.
What is the effect of amplifier input impedance VS preamplifier/source output impedance. I use the 10:1 ratio to be safe. That is the input impedance (Z) of the amplifier is 10 times that of the output Z of the preamplifier, or source. The RCA Radiotron Designers Handbook recommends a minimum of 5 times, or 5:1. Using 10:1, the amplifier input impedance should be 20,000 ohms (20k ohms) with the preamplifier output impedance of 2000 ohms (2k ohms). (The preamplifiers output capacitor should be large enough to insure accurate bass response with 20k amplifier input impedance. Almost all output capacitors used are way too small.)
Some teach a 100:1 ratio, which actually creates problems because one needs to add an additional buffer output stage to obtain this very low output impedance. However, the additional stage itself deteriorates the musical quality in and of itself.
Understand that teaching the 100:1 ratio attempts to legitimize the use of a buffer stage and unfortunately infers that those who don't use it are inferior designs. Not only does adding a buffer stage deteriorate the musical quality, it increases frequency dependent feedback, increases the complexity, increase "crosstalk" problems between channels, and adds to the cost (which increases the profit margin). And there is another obvious question.
That obvious question is why not just design a single stage, low output impedance, wide bandwidth, and low distortion design to begin with and forget the additional buffer stage with its problems? One reduces the signal degradation, almost eliminates frequency dependent feedback, decreases complexity, decreases "crosstalk" problems between channels, and reduces the cost, price to you.
Proponents claim a lower output impedance of, say 100 ohms, is better and thus higher frequency response, lower distortion, and less interference/hum occur. Let's investigate if this is true or simply marketing.
Decreasing the ratio from 100:1 to 10:1 results in virtually no increase in harmonic distorion. For example if the total harmonic distortion of a JJ E88cc tube, at 2v rms output measures .01% (-80db) using the 100:1 ratio, changing to 10:1 raises the distortion by approximately .012% to -79db, a rise of approx 1db. The point is the distortion rises very little.
Now let's check for any high frequency response advantages. If one uses a high capacitance interconnect cable (IC) (and include amplifier input capacitance), say 250pf of total capacitance, and the preamplifier/source output impedance rises from 100 ohms to 2000 ohms, how much does the high frequency response suffer? Well, the high frequency response drops approx 0.4db at 100 khz, and approximately .015db at 20 khz.
Using a 50pf interconnect cable results in less than 0.02 db drop at 100 khz, and virtually zero at 20 khz. By the way, everyone knows that capacitances should be minimized with interconnect cables. (However, rarely, a longer IC with higher capacitance is neccessary as there is no choice.) Actually a major portion of the loss in high frequency response is due to the volume control resistance/input tube capacitance relationship.
As far as hum/interference, it all has to do with proper grounding/shielding scheme. I live in a second floor apartment approximately 5-6 miles from two, 2.2 million watt visual and 200,000 watt aural producing television stations. I hear no interference with my ear on the speaker cones.
As one can see, the added buffer stage does not do much except contribute artificial flavors/distortion. In conclusion I will not add anything that is harmful to the music. Enough is enough, and more is simply too much.
Acknowledgements
Each page is designed to help educate the consumer when purchasing gear. From time to time, other articles and products will be added; so stay in touch.
I would like to thank God for inspiring the designs and parts used. It has been a real adventure, with His inspiration leading the way.
The articles covering theory require a special thanks to Walter G. Jung for discussions and his articles entitled "Picking Capacitors". Without these articles and discussions, these pages would be of lesser quality.
Thanks to Svetlana and JJ for allowing us to use their vacuum tube graphics on our site.
SAS Audio Labs
503 W Jefferson St. Suite #2
Morton, Illinois USA 61550 (10 miles east of Peoria)
TEL: (309) 263-0736
1:00 PM - 6:00 PM CST Monday - Friday
Auditions on weekdays, nights and weekends available
Email, sasaudio@omnilec.com
Totally Confidential. No email will ever be sold or given away.
*sas audio labs, SAS Audio Labs, SAS AUDIO LABS, and the SAS AUDIO LABS banner are trademarks of SAS Audio Labs."
*copyright©: 05-17-2008 Updated 08-29-2011. All contents of this page article (except Sovtek, JJ, and Svetlana Tubes) are copyrighted. Any and all
designs and schematics, layouts of all our components, term "lead to lead wiring", "lead to lead connecting", "we make music come alive", "we make music truly come alive", and "the last watt is as important as the first watt" are copyrighted. All rights reserved. No portion of this article may be reproduced without written permission from Steve Sammet at SAS Audio Labs.