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From: Peter Larsen on 10 Jan 2007 10:53
Eeyore wrote:

>> The Behringer input op-amps are JRC4580, and very cheap.
>> The OP275 is a drop-in replacement and worked perfectly.

One of the possible good choices.

> The 4580 is actually a pretty good op-amp designed specifically
> for audio and with a better spec than the ubiquitous 5532.

It is not harsh, but it is slightly dull and boring when used in large
numbers, I replaced all opamps in a NAD Surround preamp some years ago,
it was well worth doing, the first set was indeed 275's, I socketed them
for easy replacement, so now it runs on something else that I wanted to
give a test listen and I did not want to put the 275's back in for that
application.

> I'm sure you must have an idea what
> the mixer would cost if it had OP275s instead !

That is not the good question, the good question is how it would sell!

> Graham


Regards

Peter Larsen
From: Justice on 15 Jan 2007 16:42
On Wed, 10 Jan 2007 16:53:27 +0100, Peter Larsen
<SPAMSHIELD_plarsen(a)mail.tele.dk> wrote:

>Eeyore wrote:
>
>>> The Behringer input op-amps are JRC4580, and very cheap.
>>> The OP275 is a drop-in replacement and worked perfectly.
>
>One of the possible good choices.
>
>> The 4580 is actually a pretty good op-amp designed specifically
>> for audio and with a better spec than the ubiquitous 5532.
>
>It is not harsh, but it is slightly dull and boring when used in large
>numbers, I replaced all opamps in a NAD Surround preamp some years ago,
>it was well worth doing, the first set was indeed 275's, I socketed them
>for easy replacement, so now it runs on something else that I wanted to
>give a test listen and I did not want to put the 275's back in for that
>application.
>
>> I'm sure you must have an idea what
>> the mixer would cost if it had OP275s instead !

Ok, now in english, please explain the difference between a JRC4850
and an OP275, and what difference they would make in a mixer.

From: jonas_me on 15 Jan 2007 23:05
> Ok, now in english, please explain the difference between a JRC4850
> and an OP275, and what difference they would make in a mixer.

I pulled this general op-amp explanation from another site, but it is
generally accepted in the pro-audio industry that the OP275 is a
quality op-amp:

Electronic News, March 2, 1998 by Troy Murphy
The single most important factor in choosing the audio op amp needed
for the best sounding result is THD+N, or the total harmonic distortion
plus noise specification. By looking at both the distortion of the
amplifier and its noise contribution, this parameter is truly the
closest to how we actually hear things.

The number is usually given in terms of percentage of the original
input signal amplitude: A sine wave input into an op amp at a given
frequency, which is usually 1 kHz. The output of the amplifier is then
put into a notch filter, removing the original signal, and the leftover
energy is measured. This will include the background noise of the
amplifier as well as any harmonic distortion artifacts, which appear at
integer multiples of the original input signal (two kHz, three kHz,
etc.). For example, in an amplifier that has one percent THD+N, the
leftover energy is 1/100th of the original input signal energy.

To achieve CD quality, an audio amplifier should have a THD+N
specification of no more than 0.001 percent, or -100dB. For even higher
quality applications, such as audio for DVD, an even lower THD+N
specification should be required.


The harmonic distortion of an amplifier will also vary with the gain of
the amplifier. In most cases, the THD+N specification in a datasheet is
given with the op amp at unity gain. A high quality audio amplifier
will have a relatively flat THD+N over all audio frequencies.

In addition to distortion, the noise of an op amp should be closely
considered. Much can be said on the subject of noise itself, but simply
put there are two types of noise specs given: voltage noise and current
noise. The total noise contribution from the amplifier will generally
be dominated by the input voltage noise. Current noise will only begin
to become significant if the resistors connected to the amplifier are
very large, for instance, over 100 kW, so a well designed audio circuit
should only use resistors in the range of several kilo-ohms or less.

Similar to distortion, the noise contribution of an amplifier will
increase with gain. An op amp with 1 microV of noise at unity gain will
have 100 microV of noise in a gain of 100, or 40 dB. Therefore, in an
application that requires high gain, like a microphone pre-amplifier,
the input voltage noise should be closely regarded. For most audio
applications, an op amp with less than 10 nV /Hz of input voltage noise
should suffice. For higher performance audio applications, lower noise
figures, even down to 900 pV /Hz may be necessary.

Another important consideration for an audio amplifier is its dynamic
range. This range is the difference between the highest signal the
amplifier can output to the amplifiers output noise. The upper end of
the dynamic range depends on the output stage of the amplifier, as well
as the supply voltage. A rail-to-rail output amplifier will be able to
output a larger signal than a non-rail-to-rail amplifier on the same
supply voltage. For single supply applications that run from lower
supply voltages, it may be necessary to use a rail-to-rail output
amplifier to help improve the dynamic range. However, both noise floor
and maximum output voltage together must be considered.

Lastly, the slew rate of an audio amplifier should be fast enough to
allow frequencies through 20 kHz to swing the maximum required output
voltage without distortion. This minimum slew rate requirement is
proportional to the required output swing. For a maximum output signal
of 1 V peak, the minimum slew rate of the op amp should be at least 0.5
V/microsecond; for a 10 V output peak, the op amp should have at least
5 V /microsecond of slew rate.

From: jonas_me on 15 Jan 2007 23:05
> Ok, now in english, please explain the difference between a JRC4850
> and an OP275, and what difference they would make in a mixer.

I pulled this general op-amp explanation from another site, but it is
generally accepted in the pro-audio industry that the OP275 is a
quality op-amp:

Electronic News, March 2, 1998 by Troy Murphy
The single most important factor in choosing the audio op amp needed
for the best sounding result is THD+N, or the total harmonic distortion
plus noise specification. By looking at both the distortion of the
amplifier and its noise contribution, this parameter is truly the
closest to how we actually hear things.

The number is usually given in terms of percentage of the original
input signal amplitude: A sine wave input into an op amp at a given
frequency, which is usually 1 kHz. The output of the amplifier is then
put into a notch filter, removing the original signal, and the leftover
energy is measured. This will include the background noise of the
amplifier as well as any harmonic distortion artifacts, which appear at
integer multiples of the original input signal (two kHz, three kHz,
etc.). For example, in an amplifier that has one percent THD+N, the
leftover energy is 1/100th of the original input signal energy.

To achieve CD quality, an audio amplifier should have a THD+N
specification of no more than 0.001 percent, or -100dB. For even higher
quality applications, such as audio for DVD, an even lower THD+N
specification should be required.


The harmonic distortion of an amplifier will also vary with the gain of
the amplifier. In most cases, the THD+N specification in a datasheet is
given with the op amp at unity gain. A high quality audio amplifier
will have a relatively flat THD+N over all audio frequencies.

In addition to distortion, the noise of an op amp should be closely
considered. Much can be said on the subject of noise itself, but simply
put there are two types of noise specs given: voltage noise and current
noise. The total noise contribution from the amplifier will generally
be dominated by the input voltage noise. Current noise will only begin
to become significant if the resistors connected to the amplifier are
very large, for instance, over 100 kW, so a well designed audio circuit
should only use resistors in the range of several kilo-ohms or less.

Similar to distortion, the noise contribution of an amplifier will
increase with gain. An op amp with 1 microV of noise at unity gain will
have 100 microV of noise in a gain of 100, or 40 dB. Therefore, in an
application that requires high gain, like a microphone pre-amplifier,
the input voltage noise should be closely regarded. For most audio
applications, an op amp with less than 10 nV /Hz of input voltage noise
should suffice. For higher performance audio applications, lower noise
figures, even down to 900 pV /Hz may be necessary.

Another important consideration for an audio amplifier is its dynamic
range. This range is the difference between the highest signal the
amplifier can output to the amplifiers output noise. The upper end of
the dynamic range depends on the output stage of the amplifier, as well
as the supply voltage. A rail-to-rail output amplifier will be able to
output a larger signal than a non-rail-to-rail amplifier on the same
supply voltage. For single supply applications that run from lower
supply voltages, it may be necessary to use a rail-to-rail output
amplifier to help improve the dynamic range. However, both noise floor
and maximum output voltage together must be considered.

Lastly, the slew rate of an audio amplifier should be fast enough to
allow frequencies through 20 kHz to swing the maximum required output
voltage without distortion. This minimum slew rate requirement is
proportional to the required output swing. For a maximum output signal
of 1 V peak, the minimum slew rate of the op amp should be at least 0.5
V/microsecond; for a 10 V output peak, the op amp should have at least
5 V /microsecond of slew rate.

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