SAS Audio LabsTM
28 years in Audio Design
Where Music Comes Alive TM
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Engineering and Design
- Design and Layout
- Synergy Absolute?
- Frequency Dependent Feedback Through the Power Supply Filtering System
- Impedance "Mismatching", Distortion and Frequency Response Changes
- Hardwiring and PC boards
- What about global negative feedback?
Welcome to SAS Audio Labs. SAS Audio Labs came about in December 1996 (after 16+ years of research and still counting), and continues to play a leadership role in the evolution towards audio perfection. I hope the knowledge presented will help you in understanding the difficulties in designing a great sounding audio system, and helps you when evaluating a component.
For the record, I am beholden to neither recent technology nor old, do not belong to any audio organizations, nor do I belong to any buddy buddy groups, or financially attached to any particular forums.
I start each topic with general easy to understand information. My conclusion is as non technical as possible. Almost all of the information presented is first semester, first year electronics. Some information was addressed some 50+ years ago in the RCA Radiotron Designers Handbook, written by 26 engineers. Yet very few, if any designers/engineers/companies address all the issues now or in the past. As one reads through, notice the variety of topics and difficulties that an engineer must contend with, but usually do not.
There is alot more to designing a component than solving a few equations, running a computer program, using a certain tube, or reading a few "scientific" journals with conclusions that contain comments such as "we believe" or "it appears". The problem is that a few equations, a computer program model are not complete models. An example later, under global negative feedback.
Let us continue on, we have alot to cover.
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. Although a particular design or layout may look good on the surface and have excellent specifications, and a lot of support; digging deeper may reveal inherent weaknesses that renders the design or layout used inferior. For example:
Concerning layout, 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 failure sooner than later. For electrolytic capacitors, the life span is approximately halved with every 20 F degree rise in temperature. Blowing a power supply could cause an expensive down stream component to fail which both you and I do not want.
It has been found that a 5khz (5,000 cycles per second) or lower signal will couple to other wires or parts. Thus "crosstalk" between channels, especially at higher frequencies is a concern. For instance the channel separation may be 90db at 1khz but only 60db at 10khz. Now suppose we have a cymbal crash in the left channel, with harmonics rising from a few hundred cycles to 20khz (and beyond). As a result, some harmonics to 20khz will also appear in the right channel, which causes a couple of problems. First we will have imaging problems. Secondly, the frequency response (FR) is changed due to extra output from the unwanted right channel. So although single channel measurements may indicate perfection in the lab, such is not the case with both channels operating.
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 wires and solder connections. One now sees variations of the concept all over the net. (I know, a little advertising, but true.)
As one can see, layout is very important indeed.
One of the mistaken "facts" in audio is that simply matching components maximizes synergy, and is absolute. Unfortunately, there are at least four problems with that line of reasoning.
- First, synergy is not an absolute, but a variable. As an example, if one has average components and a better component match comes along, the synergy can be improved. So "synergy" is variable. ( Now I am not saying a $10,000 preamplifier is better than a $3,000.000. However, a $500.00 preamplifier is certainly not better.)
- Second, one can not perfectly cancel a fault. I recently read where Nelson Pass stated (paraphrase), once pristine is gone, it is gone. That is very true. How does one recover lost dynamics, separation of instruments, proper tone? In order to have perfect reproduction, perfect synergy, the components must be the best, most natural, and accurate possible.
- Third, very very rarely does a component have only one flaw, sonic signature. As such, how does one combine two or three components and cancel all the flaws? It is better to remove as many flaws as possible to begin with from each component.
- Fourth, how does one compare amplifier to speaker combinations when the "volume control" is flawed? One cannot. If the volume control is absolutely perfect when auditioning, the result is fewer flaws to deal with, thus better synergy.
In October, 1997, I presented a "White Paper" that discussed the issue of frequency dependent musical feedback from stage to stage, through the common power supply itself. This type of feedback is problematic in almost every single positive voltage power supply type designs, although swept under the rug. (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 particular issue is not new, just brought back to life.)
Larger de-coupling capacitors are sometimes mentioned as solving this problem. However, as my white paper explains, the problem can be lessened while other problems, such as ESR and inductance problems become more prominent. So it becomes trading one problem for another.
Musical feedback extends from near zero hertz through at least midrange frequencies, thus its consequences are wide spread within the audio band.
This section deals with impedance "mismatching", distortion and frequency response changes.
What is the effect of amplifier input impedance VS active preamplifier/source output impedance. To be more precise, the input impedance (Z) of the amplifier verses the output Z of the preamplifier or source. (The specifications of both impedances can be found in the owner's manuals.) Most recomment a 10:1 ratio.
I also recommend a 10:1 ratio to be safe. (The RCA Radiotron Designers Handbook recommends a 5:1 ratio.) Using a 10:1 ratio, the amplifier input impedance should be 20,000 ohms (20k ohms) with the preamplifier output impedance of 2000 ohms (2k ohms).
However, some claim/market a 100:1 ratio to "reduce distortion". For an amplifier input impedance of 20k ohms, one would need the preamplifier/source to have an output impedance of only 200 ohms. So does adding a low output impedance buffer stage lower distortion?
Understand that teaching the 100:1 ratio attempts to legitimize the use of a buffer stage while inferring that those who use a 10:1 ratio are inferior. Not only does adding a buffer stage not significantly reduce distortion, but increases the complexity, deteriorates the musical quality, increases "crosstalk" problems between channels, and adds to the cost (which increases the profit margin). Let's check out an example.
We have an amplifier with 20k ohms input impedance (Z). Let's compare a preamplifier with a 100 ohm output Z to a 2000 (2K) ohm output Z. As such, we are decreasing the ratio from 200:1 to 10:1.
The total harmonic distortion of a JJ E88cc tube, at 2v rms output measures approximately 0,01% (-80db) using the 200:1 ratio. changing the ratio to 10:1 raises the distortion by approximately 0,0012% to -79db. So the distortion rises from -80db to -79db. The extra buffer stage, itself, would add more distortion than the savings. Other types of stages may give different results, but then other problems are introduced.
How about frequency response changes.
This section deals with the high frequency response of an active preamplifier with and without a buffer stage. We will use a 50pf IC vs 250pf IC. The output impedance with buffer stage is 100 ohms. Without a buffer is 2,000 (2k) ohms.
First, the high capacitance 250pf interconnect cable and the buffer stage, 100 ohms. The high frequency response drops approx 100udb at 100 khz, and approximately 6udb at 20 khz. With output Z of 2khz, the drop is 0,045db at 100khz and 0,002db at 20khz. Not much is it.
Now we use the 50pf interconnect cable. The result is less than 0,150udb drop at 100 khz, and 0,5udb at 20 khz. With output Z of 2khz, the drop is 0,002db at 100khz and 75udb at 20khz. (Rarely, a longer IC with higher capacitance is neccessary as there is no choice.)
As one can see, the added buffer stage not only does not lower the distortion, but also does not appreciably extend the high frequency response. Yet the additional stage adds cost while degrading the music. If it sounds better adding the buffer stage, then either the IC capacitance is very large, or the previouis stage(s) have problems.
So the 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 additional distortion, increased crosstalk and other associated problems and cost to you?
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.
Two big pluses of incorporating PC boards are unit to unit specification consistency and higher frequency response due to less stray capacitance, both between adjacent parts and from parts to ground. At the same time, exploiting the virtues of hardwiring such as very low resistance and fewer solder connections. In fact, "lead to lead" beats hardwiring at its own game.
What about global negative feedback? Very very carefully.
Ok, an example from above. Both a computer model, and/or a few mathematical calculations clearly demonstrate that global negative feedback, in an amplifier, reduces harmonic and IM distortion. What some do not directly tell you is the amplifier overall time constant delaying the signal from intput to output (frequency dependent). Some accept these incomplete and misleading models as "all in total". However, a simple oscilloscope measurement with, say, a 10khz signal (open loop amplifier bandwidth 70khz), and one will discover that global feedback is delayed by many microseconds (us), not nano-seconds (ns) or "instantaneous" as some claim. At least two major studies, one with three mainstream national medical organizations and universities, demonstrate the ear is sensitive to 2us and 5us changes. Computer models and mathematical calculations are not complete. (Watch out for the fireworks, reactions of all sorts by global feedback proponents.)Page 2
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
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*copyright©: 05-17-2008 Updated 10-26-2013. 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.