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 What is the difference between a receiver and an amplifier?
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A receiver is basically an amplifier that contains a radio or other audio visual facilities. As the
receiver has so many sections sharing the same limited power supply, sound quality cannot be better than an integrated or seperate amplifier of the same league.
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What is push pull? What is single ended? Which is better? |
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The amplifier power supply can be set at, say, +100volts or set at +50 volts and -50 volts. The total
magnitude is the same. In the cause of setting at +50 and -50, or differential voltage, then it requires two different electronics device (transistor or tube) to handle the range of voltage. The top half
works in the positive region and the bottom half works at the negative region. Therefore, to the speaker, the two devices are like pushing and pulling.
If the voltage range is only in the positive region, then only one device is used. However, there is a
need to use coupling capacitor, or transformer so that there will be no direct DC sent to the speakers.
Push pull type amplifiers are very efficient, and constitutes to most amplifiers made today. However, they
are more complex. Also, due to cross over distortion (the distortion that happens when one transistor turns off and the other transistor turn on to take over), push pull amplifier can sound awful or less
refined.
Single ended amplifiers do not suffer from cross over distortion and are therefore more linear and
precise. But they have very poor efficiency.
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Is switched mode power supply really good for audio? |
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Switched mode power supplies are designed for PC mainly, where the load is consistent and almost like an
ideal resistor. It has the advantage of eliminating the large transformer and capacitor bank. Due to the switching strategy, it can also use the input power from the mains more efficiently by using unity
power factor.
However, switched mode power is not suitable for amplifiers that need to drive a heavy and varying load.
The high frequency switching also cause the amplifier to have a false sound of being ‘fast’ and ‘airy’. When driving demanding loads, the sound stage is messy and easily collapsed, even with an over
rated current capability.
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What is Biamping? Biwiring? |
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Most speakers are connected to an amplifier by one pair of terminals on each speaker. Within these
speakers, a crossover distributes the signal (modified appropriately) to each of the drivers in the speaker.
Some speakers are set up to be either biwired or biamped. A much
smaller number allows triwiring and triamping. The same principles apply but use three sets of wires or three amplifiers instead of two. Most speakers that support biamping/biwiring have two
pairs of terminals and some mechanism for shorting the two pairs together when used in the normal way. This mechanism is most likely a switch or a bus bar. To help the descriptions
below, I will refer to these two pairs as LO and HI (because normally one pair connects to the woofer and the other pair connects to the tweeter/midrange).
Biwiring
means that a speaker is driven by two pairs of wires from the same amplifier output. One cable pair connects HI to the amp, and the other cable pair connects LO to the same amp output that you connected the HI cable to. Biwiring is controversial; some folks hear a difference, some do not. The most plausible explanation involves magnetic induction of noise in the relatively low current HI cable from the relatively high current signal in the LO cable. Accordingly, Vandersteen recommends the two cable pairs for a channel be separated by at least a few inches. In any case, the effect appears to be small.
Biamping
means that the two pairs of terminals on a speaker are connected to distinct amplifier outputs. Assuming you have two stereo amplifiers, you have two choices: either an amp per channel, or an amp per driver. For the amp per channel, you connect each terminal pair to a different channel on the amp (for example, the left output connects to HI and the right side to LO). In the other configuration, one amp connects to the LO terminals, and the other amp is connected to the HI terminals.
The point of biamping is that most of the power required to drive the speakers is used for low frequencies. Biamping allows you to use amps specialized for each of these uses, such as
a big solid-state amplifier for the LO drivers and higher quality (but lower power) amp for the higher
frequencies. When you have two identical stereo amps, some folks recommend distributing the low-frequency load by using an amp per channel. In any case, whenever you use two different amplifiers, be careful to match levels between them.
Biamping also allows you to use high-quality electronic crossovers and drive the speaker's drivers (the voice coils) directly, without the series resistance and nonlinear inductance
of a passive crossover. Biamping which uses the speaker's crossover is therefore much less desirable. Replacing a good speaker's crossover with an electronic crossover has
advantages, but involves some very critical tradeoffs and tuning which is best left to those well-equipped or experienced.
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Can any amplifier drive 2 ohm or 4 ohm speakers? |
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Almost any amplifier can drive almost any load if you don't turn the volume up too high. Tube amplifiers
are one exception. Some amps clip if you play them too loud. This is bad and damages speakers. Other amplifiers shutdown if they are asked to play too loud. Many will overheat, with bad
consequences. However, in
almost all cases, it takes seriously loud sound or low speaker resistance (less than 4 ohms) to do damage. Running two sets of 8 ohm speakers at once with common amplifiers represents a 4 ohm load. Four sets of 8 ohm speakers makes a 2 ohm load. Two sets of 4 ohm speakers also makes a 2 ohm load. If you stay sober and don't turn it up past the point where it distorts, you are PROBABLY safe with most amplifiers and most loads.
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How do I drive more than two speakers with one stereo amplifier? |
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One amp can drive many speakers. However, there are two limits to this practice. The first is that you can
overheat or damage an
amplifier if you drive too low of an impedance to loud listening levels. Avoid loading any amplifier with a lower impedance than recommended. Adding two speakers to one amp output loads that output with half the impedance of one speaker.
The second is that with tube amplifiers, which are uncommon in today's common system, it is important that the speaker impedance and the amplifier output impedance be well matched.
When
driving two or more
speakers from one amp output, always wire them in parallel, rather than series. Series connection, while safe in terms of impedance levels, can hurt sound quality by raising the impedance that the speakers themselves see. Also, when different speakers are wired in series, amplifier voltage will divide between the speakers unevenly, because different speakers have different impedance-versus-frequency characteristics.
Many amplifiers have
connectors for two pairs of speakers. In general, these amplifiers also have a speaker selector switch. Most amplifiers connect speakers in parallel when both are selected, although some less expensive ones will wire the speakers in series. It is common for these amplifiers to require 8 ohm speakers only, because the amplifier is built to drive either 4 or 8 ohms, and two sets of 8 ohm speakers in parallel loads the amplifier like one set of 4 ohm speakers. It is almost always safe to connect one set of 4 ohm speakers to an amplifier with two sets of outputs, provided that you NEVER use the second terminals for any other speakers.
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How big an amplifier do I need? |
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Amplifier power ratings and speaker power ratings are almost always misleading. Sometimes,
they are factually wrong. Speaker ratings are almost useless.
To start with, sound pressure, measured in dB, often stated as
dB SPL, is a function of the log of the acoustic "sound" power. Further, human hearing is less sensitive to differences in power than the log transfer function would imply. This means that the perceived difference between a 50 watt amplifier and a 100 watt amplifier, all else equal, is very small! One columnist said that a 250 watt amplifier puts out twice the perceived loudness of a 25 watt amplifier, but quantitative statements about perception should always be treated with caution. That statement came from Electronics Now Magazine, Jan 1994, Page 87, Larry Klein's "Audio Update" Column, which is also good reading on the subject of required amplifier power.
There is
a wide variation in the "efficiency" and "sensitivity" of the various speakers available. I have seen good speakers with under 80 dB per watt efficiency and have also seen good speakers with over 96 dB per watt efficiency, measured one meter from the speaker. This difference of 16 dB represents a factor of 40 difference in power requirement!
So the first step in determining
amplifier requirements is to estimate relative speaker efficiency. Other factors include how loud you will want to listen, how large your room is, and how many speakers you will drive with one amplifier. This information will give you a rough starting point. For an example, a typical home speaker will produce 88 dB at 1 watt. In an average room, a person with average tastes will be happy with this speaker and a good 20 watt per channel amplifier. Someone who listens to loud music or wants very clean reproduction of the dynamics of music will want more power. Someone with less efficient speakers or a large room will also want more power.
Past that point, you will have to use your ears. As with all other decisions, your best bet is to get some
candidates, borrow them from a friendly dealer, take them home, and listen to them at your normal and loudest listening level. See if they play cleanly when cranked up as loud as you will ever go, into your speakers in your room. Of course, it is also important to be sure that the amp sounds clean at lower listening levels.
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Do all amplifiers with the same specifications sound alike? |
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No. Some demonstrated that many amplifier differences can be traced to very slight frequency response
difference. Let your own ears guide you. If you want to compare amplifiers, you can do it best in a controlled environment, such as your home, with your music and your speakers. Also be very careful to match levels precisely. All you need to match levels of amplifiers is a high input-impedance digital voltmeter set to AC volts and a test recording or signal generator. For best accuracy, set levels with the speakers wired to the amplifier.
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Is this amplifier too big for that set of speakers? |
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There is no such thing as an amplifier that is too big. In fact the bigger the better for sound quality.
Small amplifiers are more likely to damage speakers than large ones, because small amplifiers are more likely to clip than larger ones, at the same listening level. I have never heard of speakers
being damaged by an overly large amplifier. I have heard of 100 watt
speakers being damaged by a 20 watt amplifier, however, in really abusive hands. This will happen because when an amplifier clips, it will generate much more energy at high frequencies than normal music would contain. This high energy at high frequencies may be less than the continuous power rating of the speaker, but higher than the actual energy rating of the tweeter. Tweeters tend to be very fragile components.
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Where can I get a cheap power amplifier? |
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In Sim Lim Towers there are many Japanese amplifiers selling for about S$200 or lower and yet can deliver
more than 50w. Many simple kits are not good in quality and are far too troublesome to make. For the high quality May Sing kits, buy from Byeston Pte Ltd situated at level 4 Sim Lim Square in Singapore.
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Are American amplifiers really awesome? |
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Many American amplifiers are of high quality and a big power supply.
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What is a preamplifier? |
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A preamplifier is an amplifying electronic
circuit which can be connected to a low output level device such as a phono cartridge or a microphone, and produce a larger electrical voltage at a lower impedance, with the correct frequency response. Phono cartridges need both amplification and frequency response equalization. Microphones only need amplification.
In most audio applications, the term 'preamplifier' is
actually a misnomer and refers to a device more properly called a 'control amplifier'. Its purpose is to provide features such as input selection, level control, tape loops, and sometimes, a minimal amount of line-stage gain. These units are not preamplifiers in the most technical sense of the word, yet everyone calls them that.
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What is a passive preamplifier? |
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A passive preamplifier is a control unit without any amplification at all. It is a classic oxymoron,
because it has no capability to increase the gain of the signal. It is only used with line level sources that need no gain beyond unity.
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Do I REALLY need a preamp? |
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The tasks of a preamp are to:
Switch between various input signals,
Amplify any phono inputs to line level,
Adjust the volume,
Adjust the treble and bass if necessary,
* Present the right load impedance for the inputs, &
* Present a low source impedance for the outputs.
If you have a turntable, you NEED a preamp with a phono input. This is
because the turntable has an output which is too small for driving amplifiers and because the output of the turntable requires frequency response equalization. You can't connect any other source to a phono input other than a turntable (phono cartridge). Also, you can't connect a phono cartridge or turntable to any input other than a phono input.
Microphones also
require special preamplifiers. Some microphones also require "phantom power". Phantom power is operating power for the microphone which comes from the preamp. Microphone preamps are often built into tape decks and microphone mixers.
If you only have high level inputs, such as the output of a CD
player and the output of a tape deck, the main value of a preamp is selecting between inputs and providing a master volume control. If you only listen to CDs, it is tempting to skip the preamp entirely by getting a CD player with variable level outputs and connecting them directly to a power amplifier. But the trade off is lower sound quality and inconvenience.
The variable outputs on a CD player are often lower sound quality than fixed outputs. Two, some sources have high
or nonlinear output impedance which are not ideal for driving an amplifier directly. Likewise, some amplifiers have an unusually low or nonlinear input impedance such that common sources can't drive the input cleanly. A good preamplifier allows use of such devices without sacrificing sound quality.
Unfortunately, the only way to be sure that a preamplifier is of value with your sources and your amplifier is to try one.
Almost all receivers contain a phono preamp, a volume
control, and input switching. Therefore, if you have a receiver, you may never need a preamp.
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Should I leave equipment on all of the time or turn it on and off? |
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Some gear draws significant electricity, so you will waste money
and fossil fuel if you leave it on all of the time. As an example, a common amplifier consumes 100 watts at idle. High-end gear uses far more electricity, but ignoring that, 100 watts x 168 hours x 52 weeks x S $0.20 per K watt hour (rough estimate) is $35/year. Now add a CD player, a preamp, and a tuner, and it really adds up.
High-end enthusiasts claim that equipment needs to
warm up to sound its best. If you care about the best sound, give your equipment at least 20 minutes to warm up before serious listening. Warm up will allow the inside temperature to stabilize, minimizing offsets, bring bias currents up to their proper values, and bringing gain up to operating level.
Either way, good gear will last a very long time. Tubes are known to have a finite life, but good tube designs run tubes very conservatively, giving them life exceeding 10 years of
continuous service. Some amplifiers run tubes harder to get more power out, and thereby may be more economical to turn off between use.
Filter capacitors will fail after enough time at
temperature with voltage applied. They will last longer if turned off between use. However, like tubes, filter caps can last tens of years of continuous use, as can power transformers, semiconductors,
and the like.
Filter capacitors have a funny problem that need a simple break-in or reforming when they are restarted after many years of rest. It involves bringing up the power line
voltage slowly with a variable transformer.
Semiconductors in equipment made pre 1995 are more prone to failure due to bad surges and abuse than age. Leaving gear off may be best for
semiconductors and other surge-sensitive gear if you expect power line surges, as come from an electrical storm or operation of large motors.
Fuses seem to age with temperature and
get noisy, but they are so inexpensive that it should not bias your decision. However, some are inconvenient to change, and may require opening the case and even voiding the warranty.
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Do tube amps sound better than transistor amps? FETs? |
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Lets first list some commonly used active electronic components and their good and bad attributes.
TUBE: (Valve, Vacuum Tube, Triode, Pentode, etc.)
Tubes operate by thermionic emission of electrons from a hot filament or cathode, gating from a grid, and collection on a plate. Some tubes have more than one grid. Some tubes contain two separate amplifying elements in one glass envelope. These dual tubes tend to match poorly.
The characteristics of
tubes varies widely depending on the model selected. In general, tubes are large, fragile, pretty, run hot, and take many seconds to warm up before they operate at all. Tubes have relatively low gain, high input resistance, low input capacitance, and the ability to withstand momentary abuse. Tubes overload (clip) gently and recover from overload quickly and gracefully.
Circuits that DO NOT use tubes are called solid state, because they do not use devices containing gas (or liquid).
Tubes tend
to change in characteristic with use (age). Tubes are more susceptible to vibration (called "microphonics") than solid state devices. Tubes also suffer from hum when used with AC filaments.
Tubes are capable of higher voltage operation than any other device, but high-current tubes are rare and
expensive. This means that most tube amp use an output transformer. Although not specifically a tube characteristic, output transformers add second harmonic distortion and give gradual high-frequency roll-off hard to duplicate with solid state circuits.
TRANSISTOR: (BJT, Bipolar Transistor, PNP, NPN, Darlington, etc.)
Transistors operate by minority carriers injected from emitter to the base that are swept across the base into the collector, under control of base current. Transistors are available as PNP and NPN devices, allowing one to "push" and the other to "pull". Transistors are also available packaged as matched pairs, emitter follower pairs, multiple transistor arrays, and even as complex "integrated circuits", where they are combined with resistors and capacitors to achieve complex circuit functions.
Like
tubes, many kinds of BJTs are available. Some have high current gain, while others have lower gain. Some are fast, while others are slow. Some handle high current while others have lower input capacitances. Some have lower noise than others. In general, transistors are stable, last nearly indefinitely, have high gain, require some input current, have low input resistance, have higher input capacitance, clip sharply, and are slow to recover from overdrive (saturation). Transistors also have wide swing before saturation.
Transistors are subject to a failure mode called second breakdown, which occurs when the device is operated at both high voltage and high current. Second breakdown can be avoided by
conservative design, but gave early transistor amps a bad reputation for reliability. Transistors are also
uniquely susceptible to thermal runaway when used incorrectly. However, careful design avoids second breakdown and thermal runaway.
MOSFET: (VMOS, TMOS, DMOS, NMOS, PMOS, IGFET, etc.)
Metal-Oxide Semiconductor Field Effect Transistors use an insulated gate to modulate the flow of majority carrier current from drain to source with the electric field created by a gate. Like bipolar transistors, MOSFETs are available in both P and N devices. Also like transistors, MOSFETs are available as pairs and integrated circuits. MOSFET matched pairs do not match as well as bipolar transistor pairs, but match better than tubes.
MOSFETs are also available in many types. However, all have very low input current and fairly low input capacitance.
MOSFETs have lower gain, clip moderately, and are fast to recover from clipping. Although power MOSFETs have no DC gate current, finite input capacitance means that power MOSFETs have finite AC gate current. MOSFETs are stable and rugged. They are not susceptible to thermal runaway or second breakdown. However, MOSFETs can't withstand abuse as well as tubes.
JFET:
Junction Field Effect Transistors operate exactly the same way that MOSFETs do, but have a non-insulated gate. JFETs share most of the characteristics of MOSFETs,
including available pairs, P and N types, and integrated circuits.
JFETs are not commonly available as power devices.
They make excellent low-noise preamps. The gate junction gives JFETs higher input capacitance than MOSFETs and also prevents them from being used in enhancement mode. JFETs are only available as depletion devices. JFETs are also available as matched pairs and match almost as well as bipolar transistors.
IGBT: (or IGT)
Insulated-Gate Bipolar Transistors are devices that use a MOSFET to drive the bipolar transistor. The MOSFET part of the
device serves as the input device and the bipolar as the output. IGBTs are only available today as N-type devices, but P-type devices are theoretically possible. IGBTs are slower than other devices but offer the low cost, high current capacity of bipolar transistors with the low input current and low input capacitance of MOSFETs. IGBTs suffer from saturation as much as, if not more than bipolar transistors, and also suffer from second breakdown. IGBTs are rarely used in high-end audio, but are sometimes used for extremely high power amps.
Now to the real question. You might assume that if these
various devices are so different from each other, one must be best. In practice, each has strengths and weaknesses. Also, because each type of device is available in so many different forms, most types can be successfully used in most places.
Tubes are prohibitively expensive for very high power amps. Most tube amps deliver less than 50 watts per channel.
JFETs are sometimes an ideal input device because they have
low noise, low input capacitance, and good matching. However, bipolar transistors have even better matching and higher gain, so for low-impedance sources, bipolar devices are even better. Yet
tubes and MOSFETs have even lower input capacitance, so for very high source resistance, they can be better.
Bipolar transistors have the lowest output resistance, so they make great output
devices. However, second breakdown and high stored charge weigh against them when compared to
MOSFETs. A good BJT design needs to take the weaknesses of BJTs into account while a good MOSFET design needs to address the weaknesses of MOSFETs.
Bipolar output transistors require
protection from second breakdown and thermal runaway and this protection requires additional circuitry and design effort. In some amps, the sound quality is hurt by the protection.
All said, there is much more difference between individual
designs, whether tube or transistor, than there is between tube and transistor designs generically. You can make a fine amp from either, and you can also make a lousy amp from either.
Although tubes and transistors clip differently, a good amp will keep all devices from ever clipping, so this difference should be moot.
Some people claim that tubes require less or
no feedback while transistor amps require significant feedback. In practice, all amps require some feedback, be it overall, local, or just "degeneration". Feedback is essential in amps because it makes the amp stable with temperature variations and manufacturable despite component variations.
Feedback
has a bad reputation because a badly designed feedback system can dramatically overshoot or oscillate. Some older designs used excessive feedback to compensate for the nonlinearities of lousy circuits. Well designed feedback amps are stable and have minimal overshoot.
When transistor amps were first produced, they were inferior to the better tube amps of the day. Designers made lots of mistakes with the new technologies as they learned. Today, designers
are far more sophisticated and experienced than those of 1960.
Because of low internal capacitances, tube amps have very linear
input characteristics. This makes tube amps easy to drive and tolerant of higher output-impedance sources, such as other tube circuits and high-impedance volume controls. Transistor amps may have higher coupling from input to output and may have lower input impedance. However, some circuit techniques reduce these effects. Also, some transistor amps avoid these problems completely by using good JFET input circuits.
There is lots of
hype out on the subject as well as folklore and misconceptions. In fact, a good FET designer can make a great FET amp. A good tube designer can make a great tube amp, and a good transistor designer can make a great transistor amp. Many designers mix components to use them as they are best.
As
with any other engineering discipline, good amp design requires a deep understanding of the characteristics of components, the pitfalls of amp design, the characteristics of the signal source, the characteristics of the loads, and the characteristics of the signal itself.
As a side issue, we lack a perfect set of measurements to grade the quality of an amp. Frequency response, distortion, and signal-to-noise ratio give hints, but by
themselves are insufficient to rate sound.
Many swear that tubes sound more "tube like" and transistors sound more "transistor like". Some people add a tube
circuit to their transistor circuits to give some "tube" sound.
Some claim that they have measured a distinct difference between
the distortion characteristics of tube amps and transistor amps. This may be caused by the output transformer, the transfer function of the tubes, or the choice of amp topology. Tube amps rarely have frequency response as flat as the flattest transistor amps, due to the output transformer. However, the frequency response of good tube amps is amazingly good.
For more information on tubes, get one of the following old reference books, or check out Glass Audio Magazine (see the magazine section of the FAQ for more info on Glass Audio).
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What about swapping op-amps? |
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Many components use ICs called op amps as audio amplifiers. Earlier op amps have very poor sound quality,
especially if misused. Some engineers with a strong background in ICs and op amps learned
that they could improve sound if they replaced slow, noisy, low slew-rate, or otherwise bad op amps with better ones. Some less informed people tried doing the same thing and made the sound worse.
One pitfall with op amp swapping is that some op amps are more prone to unwanted oscillation than
others. The faster the op amp, the more likely it will cause an unwanted oscillation, which will really damage the sound. For that reason, Joe may succeed in replacing 741 op amps with 5534 op amps in his gear, and you may fail. It is dependent on design, layout, etc.With the new video type op amps, the power supply requirements may be different.
As technology and design
expertise improves, audio op amps get better and swapping is getting less and less useful. Newer op amps are displacing yesterday's best, and sound surprisingly similar to straight wire.
Still, there are different op amps for different purposes. Bipolar op amps are ideal for preamplifiers where noise is critical. The OP-27, OP-37, LT1028, and LT1115 are very well received
for phono preamps, head amplifiers, and microphone preamplifiers. Bipolar op amps are also more practical for signals with low source impedance.
FET devices like the OPA604 and OPA2604
have higher slew rate, higher bandwidth, and lower input current. These op amps are better for line-level inputs and high source-resistance signals. Some amplifiers, like the OP-37 and LT1115
achieve higher bandwidth by using less internal compensation. These amplifiers are not unity gain stable, and should not be used in circuits with low closed loop gain or large feedback capacitors.
Some of the better op amps for audio as of today include (* means highly recommended):
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Dual
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AD845*
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AD842
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AD847
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AD827
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AD797*
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NE5535
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NE5534
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NE5532
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OP-27
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AD712
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LT1115*
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LM833
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AD811
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OPA2604*
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AD841
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OP249*
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HA5112*
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LT1057
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LT1028
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AD744
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SSM2016
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With op amp part numbers, there is a lot of room for confusion. Here is a guide to the numbers
that is often accurate:
Op amp part numbers start with a manufacturer's prefix:
Analog Devices uses AD
Burr Brown uses OPA
Linear Technology uses LT
Motorola uses MC
National uses LF and LM
PMI uses OP
Signetics uses NE and SE
TI uses TL
This can be confused because if TI copies a Signetics op amp, they may assume the
Signetics prefix, or they may use their own. Fortunately, if the part numbers are the same, circuitry is almost exactly the same, as is the performance.
The next thing in the part number is two, three, four or five digits. This is invariably the key to
the part. If the numbers are the same, the parts are almost surely the same. For example, an LM357N and an LM357J are electrically identical and sound the same.
Next is a letter or two indicating the op amp package and possibly how it has been tested and what tests it passed. Unfortunately, manufacturers haven't standardized these letters.
Fortunately, you almost never care. If it is a dual-inline (DIP) package and you are replacing a DIP, you shouldn't have to | | |
