Table Of ContentElliott Sound Products - DIY Audio Articles
The Audio Pages
Elliott Sound Products Audio Articles
Page Last Updated - 11 May 2006
Feature Articles Updated
Active Vs. Passive Crossovers - An explanation for the profound audible
11 Jan 04
differences
Benefits of Bi-Amplification - Not quite magic, but close (Part I) 02 May 05
Benefits of Bi-Amplification - Crossovers, tri-amping, etc. (Part II) 02 May 05
Counterfeit (Fake!) Semiconductors - Don't you be caught with fake, below
21 Feb 06
spec transistors when you build a project!
Education - Customised projects for schools, technical colleges and universities 30 Apr 03
The State of Manufacturing - This article should be a mandatory read for all
03 Sep 02
businessmen and politicians, a real wake-up call !
Phase Angle Vs. Transistor Dissipation - A simplified approach to
19 Mar 05
understanding Safe Operating Area
Sound Impairment Monitor - Is This The Answer? - A new method to
23 Sep 00
monitor distortion in real time
Transistor Safe Operating Area - What is SOA, and how does it affect amplifier
14 Aug 03
designs
Cables
The Truth About Cables, Interconnects and Audio in General - A "must
20 Sep 02
read" article
More Truth About Cables - A white paper discussing speaker cables and
22 Jul 01
interconnects *
Cable Impedance - Characteristic impedance, high capacitance designs and
17 Oct 03
amp stability
Amplification
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Elliott Sound Products - DIY Audio Articles
Amplifier Sound - What causes the perceived differences between amps? Is it
09 Apr 00
real or imaginary?
Class- A - a discussion of the benefits of Class-A amplifiers 02 Apr 05
Clipping Behaviour - One of the lesser known effects of amplifier clipping, and
15 Oct 04
the damage it can do.
Distortion & Negative Feedback - A detailed look at the way negative feedback
06 May 06
reduces distortion
Musical Instrument Amplifiers - Guitar, Bass and Keyboard 04 Feb 01
Short Circuit Protection - Testing amplifiers to the limits * 22 Aug 00
Soft Clipping - Is adding a soft-clip circuit worth the effort? 15 Apr 06
The Need For Speed - Very high speed opamps from Texas Instruments 17 Aug 01
PWM Amplifiers - Theory and design concepts behind these 'new' amplifiers 25 Jun 05
Valve Amplifiers - do they really sound different - Includes a review of one of my
28 Nov 99
valve amps
General Information
A-B Switch Box for Amplifier Comparisons - Something to offend everyone,
15 Aug 00
or a great tool? *
ABX Comparator - A true ABX system that does not need a PC or
11 Aug 02
microcontroller *
AM Radio - Approaches to high fidelity AM broadcast band reception * 18 Feb 06
Capacitors - What are the influences that determine the so-called 'sound' of
25 Oct 05
capacitors. Covers dielectrics, dissipation factor, etc.
CD, SACD and DVD-A -A Comparison of the formats in a discussion between
28 Aug 02
Niklas Ladberg and Ing. Öhman (very interesting) *
Compression In Audio - Ever wondered why some music sounds flat and
21 Dec 01
lifeless, even though it's loud?
Earthing Your Hi-Fi - Tricks and Techniques - How to eliminate hum without
30 Dec 99
affecting safety
Build Your Own Heatsink? - Yes, it can be done. If you have the tools, you can
11 Feb 06
make a heatsink as big as you'll ever want.
Impedance - What does it really mean? A general discussion on impedance,
24 Jan 05
damping, etc.
Microphones - An introduction to the different types, polar response and
12 Jan 06
behaviour of microphones
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Elliott Sound Products - DIY Audio Articles
Why Do Tweeters Blow? - The real reason that tweeters (and other drivers) are
28 Jul 01
destroyed by distortion
What is Hi-Fi - A contributed article on some of the subtle aspects of Hi-Fi * 05 May 01
What is Hi-Fi (Part II) - Continuation of 'What is Hi-Fi' explaining the idea of
29 Mar 04
accuracy (amongst other things)
Beginners' Luck - The beginners' Guide to ...
Amplifier Basics - How Amps Work - How amplifiers do what they do 06 Apr 05
Amplifier Efficiency - Power Loss, DC voltages and current explained 29 Jan 00
Amplifier Troubleshooting and Repair Guide If it's broken, you may be able
27 Apr 03
to fix it. This article is designed for those new to repairs
Bridging - How the "Trimode" bridging system works in car power amplifiers 20 Oct 02
Bulletin Boards - A guide to etiquette when posting at your favourite forum * 09 Apr 01
Designing With Opamps I - What they are, what they do, and how they do it 03 May 00
Designing With Opamps II - Audio applications, filters, crossovers, etc. 08 May 00
Designing With Opamps III - More Filters, Specifications, Comparators, etc.
11 May 06
Electronics - Part 1 Passive components - resistors, capacitors and inductors
13 Mar 01
explained
Electronics - Part 2 The resistor 'E' series (e.g. E12, E24, etc), Zener Diodes 05 Mar 04
Meters, Multipliers & Shunts - How to use analogue and digital panel meters
06 May 06
Potentiometers. - what they are and how to use them in practical circuits. 22 Jan 03
Power - An explanation of the derivation of PMPO in amplifiers. Light reading,
19 Jun 00
really
Power Supply Wiring There are a lot of considerations for the wiring of a PSU -
07 Jun 03
read about it here
Tools - The first in a series on mechanical aspects of your hobby * 21 Mar 01
Transformers - Part 1 The basics of transformers, what they do, and how they
05 May 01
do it.
Transformers - Part 2 The details of magnetic circuits, windings, core types and
03 Apr 01
losses
Transformers - Part 3 Designing and analysing transformers - not at all for the
16 Jan 06
faint-hearted, but all very useful information.
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Elliott Sound Products - DIY Audio Articles
Voltage Dividers & Attenuators - A short article describing how to design these
21 Dec 02
essential circuits
Design Guidelines
Amplifier Design - ESP's contribution to the everlasting debate 09 Jun 01
Amplifier Design With HEXFETs - Getting good performance from vertical
13 Mar 06
MOSFETs is possible (Updated)
Balanced Driver with Floating Output - An in depth discussion of the design
14 Apr 02
process *
Current Sources, Sinks & Mirrors - A discussion on these much
19 Feb 01
misunderstood circuits
Heatsinks - An article on heatsink selection, transistor mounting and thermal
17 Sep 03
transfer principles
Noise in Audio Amplifiers - An explanation of noise figures, and how circuit
29 Jan 00
noise is calculated
Opamp Alternatives - A sample of the circuits that were in common use before
28 Apr 05
opamps 'took over'
Power Supply Design - How to design linear power supplies - transformers,
03 Mar 01
rectifiers and filters
Transistor Matching Tests - Matching Bipolars or MOSFETs for critical amplifier
08 Dec 01
applications
Variable Impedance Amplifiers - How you can control the output impedance of
22 Apr 00
a power amp
Loudspeaker and Crossover Design
Baffle Step Compensation - What it is and why we need to compensate for it 08 Dec 01
Compliance Scaling - Fitting just about any driver to just about any alignment 30 Jun 05
Derived (Subtractive) Crossovers - An examination of these networks, and
21 Sep 05
why they are unsuitable for serious use
Doppler Distortion in Loudspeakers - Is the effect real or imaginary when
23 Aug 04
applied to loudspeakers? The results may surprise you
Impedance - Effects Of Source Impedance on Loudspeaker Drivers 22 Jul 01
The Linkwitz Transform Circuit - What it is, what it does and how it does it * 16 Sep 02
NTM™ Crossovers - This is a topic that has virtually zero technical information
15 Sep 05
on the Net ... until now.
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Elliott Sound Products - DIY Audio Articles
Loudspeakers (Part I) - see the production of my latest triamped speaker boxes
14 Jul 01
unfold
Loudspeakers (Part II) - The electronics that drive the 3-way system are
18 Nov 01
described
Passive Crossovers - The Design of High Quality Passive Crossover Networks 05 Nov 01
Phase, Time and Distortion in Loudspeakers - Is "Time Alignment" worth the
30 Jul 02
effort?
Serial vs. Parallel Crossover Networks - Are there any real differences? 14 Aug 03
Phase Correction - Myth or Magic? - Do all-pass filters really work to correct
12 Mar 04
loudspeaker phase anomalies?
Thiele Small Parameters - Determining the Thiele/Small parameters for a
12 Jan 06
loudspeaker driver (Updated)
Satellites & Subwoofers - Description of the little known QB5 loudspeaker
28 Aug 04
alignment. Maximum SPL from 2-way bookshelf speakers. *
Subwoofer Conundrum - Some tips and hints on subwoofer selection, efficiency
12 Mar 04
and placement
Explanation of Symbols ...
The article is new - that must come as a surprise Less than 2 months old (typically).
RED Dates in the update column indicate a new or updated (within 2 months) article.
Indicates an article that may include humour - caution is advised
* Contributed article (see copyright notice on article page)
Main Index
Please refer to Copyright information in the individual articles. All articles are Copyright © 1998 ...
2006 Rod Elliott, unless otherwise stated.
My Thanks
I must express my great appreciation to Geoff Moss from the UK, who has provided me
with information about spelling errors, inconsistencies, and general errors in many of
these articles. Geoff has done the thing that I cannot do - looked at the articles with
detachment, and has spotted and brought to my attention several howling blunders as
well as the general editorial help.
Geoff, my sincere thanks and appreciation for what has been many hours of work. This
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Elliott Sound Products - DIY Audio Articles
short acknowledgement is scant reward, but I really appreciate the help and assistance.
In addition, I would like to thank the many readers who have pointed out various
additional errors and omissions - Your help in ensuring that the information on these
pages is as accurate as possible is appreciated, always!
Although it goes without saying (but I will say it anyway :-) the contributed articles and
projects from all over the world have added considerably to the overall value and range
of topics covered in these pages, and on behalf of all the readers of The Audio Pages, I
thank you for you efforts. Writing a clear and descriptive technical article is not an easy
task, but your perseverance is acknowledged, and I am grateful that you have taken the
time to share your knowledge.
May the flow of input continue - for anyone who would like to contribute, please do not
hesitate to contact me.
Upcoming Articles
At any given time, there are usually articles in production either by myself or
contributors. It is no longer possible (or sensible) to leave hints as to what may or may
not be coming, since it is so easy to get sidetracked by something new that comes
along. Feel free to offer suggestions, or offer your own contribution.
Total Visitors since Jan 2001 - 5322194
Last significant page update 11 Feb 2006 - rearranged each section into alphabetical order, added
DIY heatsinks
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Active Crossovers Vs. Passive Crossovers
The Audio Pages
Elliott Sound Products Active Vs. Passive Crossovers
Copyright © 2004 - Rod Elliott (ESP)
Page Created 11 Jan 2004
Articles Index
Main Index
Contents
l 1.0 Introduction
l 2.0 How Many Ohms?
m 2.1 The Crossover - a Different View
m 2.2 The Active Solution
m 2.3 Other Network Orders
m 2.3 Zobel (and Other) Networks
l 3.0 Conclusion
1.0 - Introduction
In the article Benefits of Biamping, I discussed the many advantages that are to be had by using
separate amplifiers for bass and mid+high. There is also a section devoted to tri-amping (for a typical 3-
way system). Essentially, the ideal arrangement is to use a separate amplifier for each loudspeaker
driver in the system. Although there are still many who consider this to be overkill, the advantages are so
compelling that there is no reason not to adopt this approach as a matter of course.
Of course, if the speaker arrangement uses two drivers in parallel (for example the well known MTM or
D'Appolito topology), a single amp may drive both mid-woofers - dual amplifiers will usually not give any
major benefit in this setup.
One area of the original article was not covered in sufficient detail - driver control. While I firmly believe
that the ideal situation is to damp a resonant body at the source, this is not always feasible or even
possible. There is also the occasional driver that simply cannot be controlled from "an ohm away" - i.e. it
may require that the source (amplifier) is hard-wired to the driver, with an absolute minimum of
resistance or impedance between the two. Some compression drivers (for horn speakers) are an
example, where even a few hundred milliohms may allow the driver to do "its own thing" rather than
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faithfully reproduce the applied signal.
Driver control (AKA "damping factor" - somewhat erroneously IMO) is a much touted parameter, and is
considered important by the majority of hi-fi enthusiasts/ audiophiles. Indeed, even where a defined
amplifier output impedance is used (such as 4 ohms, as used in my own system), this is done to provide
a specific loading to the voice coil motor to control the back-emf that is developed in any electromagnetic
loudspeaker driver. The most commonly sought after figure is zero ohms, implying an infinite "damping
factor", but the laws of physics conspire to make this unrealistic.
However, a damping factor of (say) 100 or more is easily achieved, even with typical loudspeaker cables
and amplifiers ... or is it?
Figure 1.1 - Typical 2nd Order Crossover
There is no attempt on my part to add impedance compensation networks, notch filters, or any of the
other typical additions to the circuit, and for convenience I have used purely resistive "speaker drivers".
W
My one concession to a conventional design here is that I included 100m resistance in the inductors.
Any additional circuitry will affect the impedance seen by the driver - in some cases it will introduce an
advantage, in others a disadvantage. I shall leave it to the reader to determine the specific differences
(with a little guidance, of course).
One may rest assured though, that the performance changes due to extra circuitry will only modify the
performance to a marginal degree - the primary issues remain unchanged.
2.0 - How Many Ohms?
For the sake of this discussion, we will assume a perfect amplifier, with an output impedance of zero
ohms, and zero ohm speaker cables. I know this is unrealistic, but it shows the real situation very clearly
- "real" components will be worse - always!
The exact parameter we will examine is the impedance "seen" by the loudspeaker driver, over a range of
frequencies from well below the crossover frequency, to well above. Conventional logic indicates that
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this should be as low as possible over the entire frequency range. There has been a concerted effort by
amp makers to ensure that their products output impedances are as low as possible to satisfy this
requirement. Valve (tube) amps are naturally different in this respect, although that is not part of this
discussion.
Figure 1.1 shows the crossover connections used, and the circuit is a conventional 2-way, 2nd order
(12dB/octave) Butterworth type. Note that all versions of crossover will have similar response
characteristics, although there are significant differences that will be looked at a little later in this article.
2.1 - The Crossover - a Different View
To see exactly what happens (and why), we need to redraw the crossover network so that it can be
examined from the loudspeaker driver's perspective. As shown below, we see each section of the
crossover (high and low pass). This is exactly the same crossover as shown in Figure 1.1, but redrawn.
Since we are looking only at the damping, the amplifier is irrelevant and has been removed from the
picture. It is assumed (along with speaker leads) to have zero impedance.
Remember that for this exercise, we are looking at the impedance seen by the loudspeaker, as this has
a direct effect on the ability of the amplifier to damp the back-EMF (Electro-Motive Force) from the motor
assembly. The back-EMF is produced whenever the cone is moved by a current, or the current is
removed or changes direction. Inertia of the cone and suspension means that it cannot move or stop
instantly, so there will be "overshoot" and "undershoot" caused by the cone continuing to move after the
applied current has stopped.
A simple demonstration can be done to show that the speaker does indeed "generate" a voltage and
current. Take a small speaker (not a tweeter), and connect it to an unused input on your preamp.
Advance the gain of the amp slowly, whilst gently tapping on the cone. "Thump, thump" says your hi-fi.
You can even speak into the loudspeaker, and it will act as a microphone.
Be careful - if you increase the gain too far, you may get acoustic feedback - potentially at very high
volume levels. This will do little for your hearing, and may also damage loudspeakers. Make sure
you keep the "microphone" as far from the speakers as possible to minimise the likelihood of
feedback.
This simple test shows that loudspeakers do indeed generate a signal, and it is this signal that the
amplifier is meant to absorb, by means of "damping factor". A back-EMF signal is generated every time
your amplifier sends a signal that causes the cone to move - namely, all the time when you are listening
to music (or home theatre). It is this signal that we will investigate in this article, and no other
parameters. All dynamic (electro-magnetic) loudspeaker drivers do this - bar none. It should be obvious
that if you short circuit the speaker that you used as a microphone, then you will hear no sound from it -
this is maximum damping factor, and is what is meant to happen with your loudspeakers.
The phenomenon that you have experienced by using a loudspeaker as a microphone also happens with
your real speakers! The woofer will produce a signal that is picked up by the midrange or tweeter, which
in turn will generate a signal. This signal should be dissipated entirely by the amplifier to prevent (as far
as is possible) the cone moving in sympathy with the soundwaves. As we shall see, this cannot happen
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as it should with a passive crossover!
Even electrostatic drivers will do the same thing (although by a different mechanism entirely), but their
mode of operation is such that the generated signal is of extremely small amplitude (perhaps a few
millivolts at the very most). We shall not concern ourselves with this.
Figure 2.1 - 2nd Order Crossover Redrawn
The generators in series with each "driver" in the above diagram are to simulate the back-EMF from real-
world drivers, and this is exactly the equivalent circuit that exists in reality. The only difference is that I
W
used 8 resistors rather than the complex impedance of real drivers. This changes nothing, but makes
the following graphs more comprehensible, without the wild fluctuations that would only confuse the
issue.
Figure 2.1 shows the crossover network as it is seen by the loudspeaker. The amplifier and speaker
leads no longer exist, as they were assumed from the beginning to have zero impedance. The crossover
now appears as a simple parallel LC network, with resonance tuned to the crossover frequency. For
those who know what this means, the implication is obvious. For the remainder, we have a parallel tuned
circuit, and with ideal components (no losses), its impedance is infinite at resonance! That means that at
resonance, there is no damping whatsoever, and the "damping factor" is ... zero!
But wait - there's more! Let's look at the impedance of this network over a couple of octaves below and
above the crossover frequency. This gives a more balanced perspective, and we can determine the
effective damping over a sensible range. The damping within the stop band (i.e. the band of frequencies
the crossover network section rejects) is not so important, as the signal applied to the driver is minimal
anyway. There is still the potential for considerable energy within the first octave above the crossover
frequency (X) for a low pass section, and an octave below for a high pass section, so this is still of some
f
importance.
Figure 2.2 shows the impedance curve of a 2kHz XO, looking backwards from the loudspeaker into its
crossover section. I have shown the low and high pass sections here, but they cannot be separated
because they are identical (as one would expect, since the inductance and capacitance are in parallel in
both cases). There may be small differences with real components having some tolerance, but they do
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