anatech said:
Zobel Network Filter
is to be seen as a part of LOUDSPEAKER or Woofer.
This filter is in loudspeaker design known as
Impedance Equalizer Link or Impedance Correction Filter
It is used to achieve a flat impedance across major frequency span.
If a load does not have a rise in Inductance at higher frequency
there is no need for Amplifier to have a Zobel network at output.
Because most loudspeakers does not include
an impedance equalizing filter,
most sensible amplifier designers add this at amplifier output
just for protection.
Some brands of speakers have an almost flat impedance
across the upper audio frequency
because manufacturer as a standard correct impedance curve.
Even if your amplifier works good without Zobel now,
if you or somebody else at some time change loudspeakers
you can get your trouble.
The good MR. ZOBEL
Ah yes. Metalic facilities... What the phone company called their copper. Since some of the wires were longish without repeaters (like 40 miles), compensating networks were designed to deal with the resistance, capacitance, and even the inductance of those long lines.
Regarding matching networks... The good Mr. Zobel is reported to have said something to the effect... "If there's a frog in the line there better be a frog in the network."
Ah yes. Metalic facilities... What the phone company called their copper. Since some of the wires were longish without repeaters (like 40 miles), compensating networks were designed to deal with the resistance, capacitance, and even the inductance of those long lines.
Regarding matching networks... The good Mr. Zobel is reported to have said something to the effect... "If there's a frog in the line there better be a frog in the network."
thanh said:
Very unlikely, especially if you are using op amps, which are defined stable in the gain you are using.
There is a small possibility that if it is an exotic design, it may require a predictable impedance at high frequencies. If a long and inductive cable is used, this may remove this impedance at the open loop frequency, and allow oscillation or ringing. However, if it did this I would be concerned about its stability in general.
lucpes said:
Amplifiers likes it having Resistive Loads
Bipolar output stage prefers capacitive or inductive load?
There are 3 types components:
R - resistive
C - capacitive
L - inductive
At AC signals only a pure resistive behaves totally Linear.
Transfer is like straight line.
The other 2 have transfers that are curves - they bend and have non linear looks.
Now any component or load have a at least a tiny tiny bit of the other 2.
A resistor can have some pF C and some nH L. It is just that when it is very little, it wont notice.
Loudspeakers are INDUCTIVE because that is what a COIL is.
And speakers have large VOICE COILS transforming electricity to mechanical movement of membrane.
A Zobel filter is the cure for loads that have a lot of Inductance.
That is why we add them to Power Amplifiers.
Preamplifiers see mostly another type of loads,
at least very few hang a SPEAKER to the output of an Op-Amp.
Preamplifer output look into an input resistor and/or signal cable.
Resistor, R - have mostly as little C and L, that it is not to be regarded as an inductive or capacitive load.
We can say they are at audio frequencies almost pure resistance = linear.
They are in fact very much linear than any semiconductor, transistor FET, Bipolar or any Tube invented.
Signal cables has got very low amount of anything.
Low resistance, low inductance and only a little capacitance.
BUT and here it comes:
Even the very much expensive interconnect cable
has got at least 50 pF/ meter or more often 250 pF/meter cable.
You go read the technical specification of signal cables offered
and you see there are 3 insulator plastics used:
PFTE, PE, PVC.
Like in a plastic film cap some plastic material has to be used to separate one pole from the other.
In a stereo ' coaxial ' cable this is + positive and - ground.
In caps are used mostly polyester or polypropylene.
The typical capacitance of signal cables, depending on most used insulation material is like this:
( figures from Lineup Research Lab, which needs to get preamplifiers to be stable in CAPACITIVE LOADS )
---------------
Capacitance for acceptable quality cables
can be 42.5 - 240 pF per meter
------------------------------------------------------------------------------------
Material ... C per meter:
Air insulated ... 42.5 pF
PTFE ... 50 pF -
PE, foamed ... 50 - 80 pF
PE ..... 70 - 160 pF
PVC ..... 180 - 240 pF
------------------------------------------------------------------------------------
Typically PVC and FE is used for audio
and we can expect from a signal cable 0.5 - 2.0 meter:
40 - 480 pF of Capacitance, C
The inductance for such signal cables is so low we have not to worry,
not for audio signals, 20 - 100.000 Hertz.
Loudspeaker cables are different as is the load itself, The Speaker Coil.
There are those that say inductance of a speaker cable matters ( those selling high quality cables ).
But I do not worry too much,
because the dominant inductance at power amplifier output
comes from crossover inductors and/or the voice coil.
Source of information used by Lineup Audio Lab Research:
www.elfa.se
.... as so many times before provide good info and knowledge!
http://www.elfa.se/elfa-bin/set_country.pl?lang=en ( UK english )
http://www.elfa.se/elfa-bin/set_country.pl?lang=pl ( polish )
http://www.elfa.se/elfa-bin/set_country.pl?lang=fi ( finnish )
http://www.elfa.se/elfa-bin/set_country.pl?lang=dk ( danish )
http://www.elfa.se/elfa-bin/set_country.pl?lang=no ( norwegian )
http://www.elfa.se/elfa-bin/set_country.pl?lang=se ( swedish )
AUDIO CABLES from ELFA with product data:
Audio Cables - product images and links
lineup
King of Cables & Amplifiers
thanh said:
Hi thanh
you should not need a Zobel network for preamp to power amp connection.
Seems that there is some misunderstanding about a Zobel network in an amp. Zobel networks are supposed to turn a reactive load into a resistive load. So, if you have an inductive load, like a loudspeaker, a capacitor correction can make the load appear resistive. But most RC circuits attached to amps aren't true Zobels - the capacitance is too small - but they are intended to provide a resistive load at high frequencies, usually well above audio, to help the amplifier to remain stable.
John
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