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  #1   Report Post  
jnorman
 
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Default Preamp Design Fundamentals

we are lucky on this forum to have folks like scott dorsey, john
hardy, dan kennedy, john lagrou, and others who are top pros in their
field. i would like to take advantage of this, and request that you
guys please give the rest of us a "primer" on preamp electronics
design basics.

1. what is the simplest preamp design? ie, what exactly is required
in the circuit and why?

2. what are the basic problems with the simplest possible design?
(noise, fidelity, whatever)

3. what are the approaches to turning the basic design into a
reasonably clean and useable mic preamp? (better parts, additions to
the basic circuit, etc.)

4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3 when they are
both designed to be clean and transparent, or why tubes vs
transformers for more "colored" preamp dsigns?
  #2   Report Post  
Kurt Albershardt
 
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Default Preamp Design Fundamentals

jnorman wrote:

4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3


You can read some of Michael Grace's thoughts here
http://www.gracedesign.com/support/tech3.pdf




  #4   Report Post  
Arny Krueger
 
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"Kurt Albershardt" wrote in message


jnorman wrote:


4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3


You can read some of Michael Grace's thoughts here
http://www.gracedesign.com/support/tech3.pdf


I'm not saying it's all snake oil, but there's some really hilarious stuff
the

"By terminating the destination end of the output cable with a 300 Ohm
resistor, the load resistance matches the output source resistance to create
a 600 Ohm balanced transmission line. Matching the impedance at or near the
characteristic impedance of the cable eliminates signal reflections in the
wire which greatly improves transient performance and preserves harmonic
integrity. This type of termination is very advantageous for driving very
long lines because it provides maximum power transfer through the cable and
virtually eliminates the effects of cable capacitance and inductance."




  #5   Report Post  
Arny Krueger
 
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Default Preamp Design Fundamentals

"John Penovich" wrote in message
om

Harvey Gerst has an excellent discussion on just that topic over at
PSW, with some of the designers you mentioned participating. 14 pages
of it:


http://recpit.prosoundweb.com/viewtopic.php?t=7774



Most significantly it lays the groundwork for this excellent link:

http://www.mil-media.com/docs/articles/preamps.shtml




  #6   Report Post  
Scott Dorsey
 
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Default Preamp Design Fundamentals

jnorman wrote:
we are lucky on this forum to have folks like scott dorsey, john
hardy, dan kennedy, john lagrou, and others who are top pros in their
field. i would like to take advantage of this, and request that you
guys please give the rest of us a "primer" on preamp electronics
design basics.

1. what is the simplest preamp design? ie, what exactly is required
in the circuit and why?


It has to have a differential input and it has to have gain. I think you
can do that with a single long-tailed pair, or with a step-up transformer
and a single transistor stage.

2. what are the basic problems with the simplest possible design?
(noise, fidelity, whatever)


You get noise, you get poor linearity, you don't get enough gain.

3. what are the approaches to turning the basic design into a
reasonably clean and useable mic preamp? (better parts, additions to
the basic circuit, etc.)


Constant current sources, for one thing. Trying to get less gain per stage
and getting more stages. Using higher voltage rails for more headroom.
Using follower outputs for more current drive.

Take a look at a typical tube preamp, though. Two, sometimes three gain
stages. A lot of the gain is from the input transformer. It's pretty
simple, and you can do the same thing with FETs if you want.

4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3 when they are
both designed to be clean and transparent, or why tubes vs
transformers for more "colored" preamp dsigns?


You can make very transparent tube preamps too. The problem is that there
are widely varying philosophies. Since you cannot make anything completely
transparent (especially given the widely varying loading requirements of
different mikes), you are stuck with making tradeoffs between different kinds
of coloration. And different people make different tradeoffs.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #7   Report Post  
Harvey Gerst
 
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(John Penovich) wrote:

Harvey Gerst has an excellent discussion on just that topic over at
PSW, with some of the designers you mentioned participating. 14 pages
of it:

http://recpit.prosoundweb.com/viewtopic.php?t=7774

Some of the participants in that discussion of preamp design include Geoff
Tanner, Dan Kennedy, Mark McQuilken, John Le Grou, Tim Ferrant, and John Hardy.

Harvey Gerst
Indian Trail Recording Studio
http://www.ITRstudio.com/
  #8   Report Post  
Kurt Albershardt
 
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Arny Krueger wrote:

http://www.mil-media.com/docs/articles/preamps.shtml


Thanks, couldn't find that one earlier.




  #9   Report Post  
Justin Ulysses Morse
 
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jnorman wrote:

1. what is the simplest preamp design? ie, what exactly is required
in the circuit and why?


I think the simplest possible preamp would be an input transformer
followed by a single unity-gain active stage.

The transformer would have to have a high turns ratio in order to
provide all the gain necessary.

The active device would have to have a nearly infinite input impedance
and an output impedance near zero.


2. what are the basic problems with the simplest possible design?
(noise, fidelity, whatever)


The transformer will have a limit to its operating level and bandwidth,
will cause distortion, and will be bulky and expensive. The high ratio
required to obtain the necessary gain will compound every one of these
limitations.

The active device will require peripheral components including a power
source, and will have limits to its linearity and bandwidth, in
addition to adding noise.

There is no control over the gain, and the output signal is unbalanced.
There are no utility features such as input pad to prevent overload,
polarity switching, phantom power, etc.

The preamp will be susceptible to interference from environmental
noise, be it sound pressure, electrical, or magnetic.

3. what are the approaches to turning the basic design into a
reasonably clean and useable mic preamp? (better parts, additions to
the basic circuit, etc.)


Adding more active stages to allow for a lower-ratio transformer.
Adding a gain control or an input attenuator. Adding still more gain
stages to compensate for losses in the attenuator or gain control.
Adding a differential amplifier or transformer for balanced output.
Adding contol over polarity, an input pad, and phantom power. Housing
the whole thing in an enclosure to protect it from damage and noise.
Adding a control surface to allow the user to easily and effectively
manipulate all the features you just added. Some connectors to allow
quick and easy interfacing with different microphones and destinations.

4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3 when they are
both designed to be clean and transparent, or why tubes vs
transformers for more "colored" preamp dsigns?


Dunno.

ulysses
  #10   Report Post  
normanstrong
 
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I've decided that, for me at least, as long as the mike preamp is
considerably better than the microphone, there's little to gain from
using an expensive one. The preamps that come with a Mackie mixer
are better than my mikes. Are yours?

Cheers,

Norm Strong


"Harvey Gerst" wrote in message
...
(John Penovich) wrote:

Harvey Gerst has an excellent discussion on just that topic over at
PSW, with some of the designers you mentioned participating. 14

pages
of it:

http://recpit.prosoundweb.com/viewtopic.php?t=7774

Some of the participants in that discussion of preamp design include

Geoff
Tanner, Dan Kennedy, Mark McQuilken, John Le Grou, Tim Ferrant, and

John Hardy.

Harvey Gerst
Indian Trail Recording Studio
http://www.ITRstudio.com/





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Harvey Gerst
 
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"normanstrong" wrote:

I've decided that, for me at least, as long as the mike preamp is
considerably better than the microphone, there's little to gain from
using an expensive one. The preamps that come with a Mackie mixer
are better than my mikes. Are yours?

Cheers,


Norm,

Generally speaking, we use the board's preamps for most things. For certain
mics and/or certain sounds, we use outboard preamps to get what we want. I'm
not a big fan of Mackie preamps. When we were looking around for our first
decent board for the small studio, we passed on the Mackie, Behringer,
Soundcraft, and several other boards, in favor of the Soundtrac Topaz. Not only
because the Topaz had more features and a better EQ (IMHO)than the other boards,
but because it had better sounding preamps, to my tired ears, anyway.

As far as your statement that "The preamps that come with a Mackie mixer
are better than my mikes", I don't understand that at all. The Shure SM57 comes
alive thru a really nice preamp. The Mackie does nothing for the SM57. Are you
saying that you don't even have an SM57 that would benefit from a better preamp?
Or are you saying you don't even have a mic as good as an SM57 in your mic
collection, so it doesn't matter to you?

Harvey Gerst
Indian Trail Recording Studio
http://www.ITRstudio.com/
  #12   Report Post  
Scott Dorsey
 
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Default Preamp Design Fundamentals

normanstrong wrote:
I've decided that, for me at least, as long as the mike preamp is
considerably better than the microphone, there's little to gain from
using an expensive one. The preamps that come with a Mackie mixer
are better than my mikes. Are yours?


Compare the sound of an SM-57 into a Mackie and then into the Great River.
Very substantial difference, mostly due to loading it's true, but a very
substantial difference.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #14   Report Post  
normanstrong
 
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Default Preamp Design Fundamentals


"Harvey Gerst" wrote in message
...
"normanstrong" wrote:

I've decided that, for me at least, as long as the mike preamp is
considerably better than the microphone, there's little to gain

from
using an expensive one. The preamps that come with a Mackie mixer
are better than my mikes. Are yours?

Cheers,


Norm,

Generally speaking, we use the board's preamps for most things. For

certain
mics and/or certain sounds, we use outboard preamps to get what we

want. I'm
not a big fan of Mackie preamps. When we were looking around for

our first
decent board for the small studio, we passed on the Mackie,

Behringer,
Soundcraft, and several other boards, in favor of the Soundtrac

Topaz. Not only
because the Topaz had more features and a better EQ (IMHO)than the

other boards,
but because it had better sounding preamps, to my tired ears,

anyway.

As far as your statement that "The preamps that come with a Mackie

mixer
are better than my mikes", I don't understand that at all. The

Shure SM57 comes
alive thru a really nice preamp. The Mackie does nothing for the

SM57. Are you
saying that you don't even have an SM57 that would benefit from a

better preamp?
Or are you saying you don't even have a mic as good as an SM57 in

your mic
collection, so it doesn't matter to you?


I have 3 AKG C451's, 2 Nak CM-300's and 3 RadShack PZM's. No Shure
mikes.

So far as I can tell, mikes vary in noise level, frequency response
and output impedance. As long as the preamp has the right impedance
level for the mike used, has a smoother and more extended frequency
response than the mike, and adds little noise to the self noise of the
mike, it should be good enough. Anything else falls into the
category of mystery; I don't do mystery. If you feel I've missed
something pertinent, please feel free to correct me.

Norm Strong



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Carey Carlan
 
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"normanstrong" wrote in
news:gg9tb.151521$ao4.482861@attbi_s51:

I have 3 AKG C451's, 2 Nak CM-300's and 3 RadShack PZM's. No Shure
mikes.

So far as I can tell, mikes vary in noise level, frequency response
and output impedance. As long as the preamp has the right impedance
level for the mike used, has a smoother and more extended frequency
response than the mike, and adds little noise to the self noise of the
mike, it should be good enough. Anything else falls into the
category of mystery; I don't do mystery. If you feel I've missed
something pertinent, please feel free to correct me.


I have 451's and traded up from Mackie to Great River preamps. There is an
audible difference, particularly at quiet levels.

Things that better preamps do better (that you can easily measure):
* More headroom. Lets you stretch beyond 20 dB gain without breaking up
* Better loading. Get more out of dynamic and ribbon mics.
* Better phantom power. Produces more gain and runs more mics.

If you're recording a loud, noisy source on a single condenser mic close
up, the Mackie will sound almost as good as a Great River.

What you can't easily measure is the lower distortion and noise levels that
let you pull more detail out of the microphone. Anything with subtle
nuances, like a single voice in a quiet studio, benefits from the clarity a
better preamp produces. The difference is more noticeable when the source
being recorded is familiar to the listener (again, like the sound of a
human voice). Accuracy counts.


  #16   Report Post  
Scott Dorsey
 
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Default Preamp Design Fundamentals

Carey Carlan wrote:
"normanstrong" wrote in
news:gg9tb.151521$ao4.482861@attbi_s51:

I have 3 AKG C451's, 2 Nak CM-300's and 3 RadShack PZM's. No Shure
mikes.

So far as I can tell, mikes vary in noise level, frequency response
and output impedance. As long as the preamp has the right impedance
level for the mike used, has a smoother and more extended frequency
response than the mike, and adds little noise to the self noise of the
mike, it should be good enough. Anything else falls into the
category of mystery; I don't do mystery. If you feel I've missed
something pertinent, please feel free to correct me.


I have 451's and traded up from Mackie to Great River preamps. There is an
audible difference, particularly at quiet levels.


In part this is because of the output transformer in the 451. It needs
to see just the right input impedance to be happy.

Things that better preamps do better (that you can easily measure):
* More headroom. Lets you stretch beyond 20 dB gain without breaking up
* Better loading. Get more out of dynamic and ribbon mics.
* Better phantom power. Produces more gain and runs more mics.


I agree that if a preamp has the right impedance for the mike use, a
smooth and extended frequency response, and adds little noise, AND has
low distortion (and therefore little coloration), it should be good
enough.

The problem is that very few mike preamps fall into that category, if any.,

What you can't easily measure is the lower distortion and noise levels that
let you pull more detail out of the microphone. Anything with subtle
nuances, like a single voice in a quiet studio, benefits from the clarity a
better preamp produces. The difference is more noticeable when the source
being recorded is familiar to the listener (again, like the sound of a
human voice). Accuracy counts.


Yup, that stuff CAN be measured, and it will show up very easily on a
straightwire test. It is AMAZING to see the difference between a Mackie
and a Great River in terms of distortion spectra. The Great River has a
few big peaks, the Mackie has lots of little ones going way up the chart.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #17   Report Post  
Pooh Bear
 
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Default Preamp Design Fundamentals

jnorman wrote:

we are lucky on this forum to have folks like scott dorsey, john
hardy, dan kennedy, john lagrou, and others who are top pros in their
field. i would like to take advantage of this, and request that you
guys please give the rest of us a "primer" on preamp electronics
design basics.

1. what is the simplest preamp design? ie, what exactly is required
in the circuit and why?


I'm going to rule out transformers because of cost. The circuitry may be
simple but it won't find its way into a practical 'simple preamp' design.

You need a differential input, typically a long tailed pair and a means
of converting the differential output at the collector loads to a single
ended signal - usually an op-amp suitably configured.

2. what are the basic problems with the simplest possible design?
(noise, fidelity, whatever)


Common general purpose transistors are noisy w.r.t. 200 ohm sources. Best
choice is typically pnp ( higher carrier mobility in the base region
results in lower flicker noise ) and large geometry ( lower intrinsic
resistance has lower thermal noise ). Devices originally designed for
moving coil pre-amp designs work very well.

The input stage also needs to be run at several milliamps to get the
benefit of the noise improvements because of the emitter dynamic
impedance ( 27 / Ie ( mA ) Ohms, true up to a few milliamps ).

Single transistor gain stages have poor linearity and generate plenty of
THD esp at high signal levels. This is overcome by 'compound'
arrangements or indeed using the op-amp stage to close a feedback loop to
the input transistor emitters. There are more elaborate methods too.

3. what are the approaches to turning the basic design into a
reasonably clean and useable mic preamp? (better parts, additions to
the basic circuit, etc.)


Better parts for the input transistors especially for noise.
More advanced circuit design ( more bits ).

4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3 when they are
both designed to be clean and transparent,


Don't know them so can't comment.

or why tubes vs transformers for more "colored" preamp dsigns?


That's easy. Some ppl like technically inaccurate designs since they
produce a 'warm sound'.

Valves produce little gain compared to transistors per stage, and rarely
have much feedback applied around them, so a valve design will produce
plenty of nice 2nd harmonic distortion.

Transformers do all sorts of stuff to the signal. Kinda where do you want
me to begin ?


Regds, Graham


  #18   Report Post  
Pooh Bear
 
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Arny Krueger wrote:

"Kurt Albershardt" wrote in message


jnorman wrote:


4. what are the primary philosophies of top-end preamp design? ie,
why does a grace preamp sound so different from an HV3


You can read some of Michael Grace's thoughts here
http://www.gracedesign.com/support/tech3.pdf


I'm not saying it's all snake oil, but there's some really hilarious stuff
the

"By terminating the destination end of the output cable with a 300 Ohm
resistor, the load resistance matches the output source resistance to create
a 600 Ohm balanced transmission line. Matching the impedance at or near the
characteristic impedance of the cable eliminates signal reflections in the
wire which greatly improves transient performance and preserves harmonic
integrity. This type of termination is very advantageous for driving very
long lines because it provides maximum power transfer through the cable and
virtually eliminates the effects of cable capacitance and inductance."


As classicly used by telephone companies ;-)

Graham


  #20   Report Post  
Scott Dorsey
 
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Carey Carlan wrote:
(Scott Dorsey) wrote in
:

What you can't easily measure is the lower distortion and noise levels
that let you pull more detail out of the microphone. Anything with
subtle nuances, like a single voice in a quiet studio, benefits from
the clarity a better preamp produces. The difference is more
noticeable when the source being recorded is familiar to the listener
(again, like the sound of a human voice). Accuracy counts.


Yup, that stuff CAN be measured, and it will show up very easily on a
straightwire test. It is AMAZING to see the difference between a
Mackie and a Great River in terms of distortion spectra. The Great
River has a few big peaks, the Mackie has lots of little ones going
way up the chart. --scott


Do you have a preference on the few big peaks vs many small peaks?


Yes. High order even harmonic distortion is what gave transistor amps
the reputation for harsh and glassy sound back in the seventies.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


  #21   Report Post  
P Stamler
 
Posts: n/a
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The Great
River has a few big peaks, the Mackie has lots of little ones going
way up the chart. --scott


Do you have a preference on the few big peaks vs many small peaks?


Yes. High order even harmonic distortion is what gave transistor amps
the reputation for harsh and glassy sound back in the seventies.


High-order harmonic distortion is bad, period, but in general the high-order
odd harmonics are worse than the evens.

Ideally, you'd have a little bit of 2nd harmonic distortion, somewhat less 3rd
harmonic, and nothing else measurable, at all levels up to clipping.

(Well, semi-ideally; ideally, of course, there'd be none at all. But that
doesn't happen in the real world, and in practice, what outlined above produces
a nice clean-sounding preamp. You can do this with tubes, or discrete
transistors, or -- somewhat more difficultly -- with ICs.)

Peace,
Paul
  #22   Report Post  
Dave Collins
 
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In article ,
Pooh Bear wrote:

As classicly used by telephone companies ;-)

Graham



Yeah, over thousands of feet where it actually is a transmission line
for audio frequencies. All audio is lumped parameters. Certainly
anything to do with mic preamps. But it's a pretty common mistake.

DC
  #23   Report Post  
S O'Neill
 
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Dave Collins wrote:
In article ,
Pooh Bear wrote:


As classicly used by telephone companies ;-)

Graham




Yeah, over thousands of feet where it actually is a transmission line
for audio frequencies. All audio is lumped parameters. Certainly
anything to do with mic preamps. But it's a pretty common mistake.


And if it *was* a transmission line, it sure looks like a 300-ohm (not
600) line to me.

  #24   Report Post  
Pooh Bear
 
Posts: n/a
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S O'Neill wrote:

Dave Collins wrote:
In article ,
Pooh Bear wrote:

As classicly used by telephone companies ;-)

Graham


Yeah, over thousands of feet where it actually is a transmission line
for audio frequencies. All audio is lumped parameters. Certainly
anything to do with mic preamps. But it's a pretty common mistake.


And if it *was* a transmission line, it sure looks like a 300-ohm (not
600) line to me.


Uhuh, that too !


Graham


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Arny Krueger
 
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"Scott Dorsey" wrote in message


Yes. High order even harmonic distortion is what gave transistor amps
the reputation for harsh and glassy sound back in the seventies.


Two recent relevant AES papers are

and Audio Engineering Society Convention Paper 5890 Auditory Perception of
Nonlinear Distortion - Theory Earl R. Geddes 1 and Lidia W. Lee

Audio Engineering Society Convention Paper 5891 Auditory Perception of
Nonlinear Distortion Lidia W. Lee 1 and Earl R. Geddes

Basically, they propose weighting nonlinear coefficients (e.g., harmonics)
based on the cosine of the amplitude times order squared.

The claim is made that the audibility of nonlinear distortion is inversely
related to amplitude (nonlinearities at low levels are more audible) and
also related to order squared (nonlinearities that generate spurious
responses that are more broadly dispersed from the original signal are far
more audible). They back their new metric up with listening test results
showing that this criteria does a better job of fitting a variety of
synthetic distortion sources to perceptions of degraded sound quality.

In short, the Geddes/Lee papers show that the way most people formally
characterize audio gear nonlinear distortion today, which focuses on high
outputs and unweighted harmonics, is about as close to irrelevant as one
could imagine. The papers report experimental studies of current metrics
that support the idea that they are irrelevant or at least uncorrelated with
human perceptions of sound quality.

My own simplistic research into the subject and the scientific literature
was highly supportive of Geddes/Lee general thinking before they got the far
more polished and complete results that are published in the articles. My
point here is that it seems reasonable to view the Geddes/Lee results as
being orthodox and reasonable, as far as they go. To their credit they seem
to have found a far better metric, but maybe not the best metric. I'll take
better as long as it is the best we have!

Today, most audible nonlinear distortion comes from loudspeakers and other
electromechanical/electroacoustic transducers. Nonlinear analysis of
speakers is in its infancy, but two methods for characterizing speaker
nonlinear distortion are currently being used pretty widely. One is due to
Klippel, and one is due to Clark and Geddes. AFAIK the Klippel methodology
only produces results for the second and third order, which would appear to
be a significant failing. The Clark & Geddes methodology (AKA Dumax)
produces results for higher orders as well which would appear to be a
significant strength.




  #26   Report Post  
Terry King
 
Posts: n/a
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Two recent relevant AES papers are
and Audio Engineering Society Convention Paper 5890 Auditory Perception of
Nonlinear Distortion - Theory Earl R. Geddes 1 and Lidia W. Lee

Arne says:
My own simplistic research into the subject and the scientific literature
was highly supportive of Geddes/Lee general thinking before they got the far
more polished and complete results that are published in the articles.

Today, most audible nonlinear distortion comes from loudspeakers and other
electromechanical/electroacoustic transducers.


Arne, this is very interesting, and I like it because it agrees :-) with
my feeling/observation of 'distortion' effects in lower-level signals.
Some systems seem to have artifacts discernable in the low-level passages
of material with high dynamic range, while the 'loud' parts sound fine,
and typical distortion measurements give good numbers.

How do you think this may relate to preamps?? What can we learn about
testing preamps that is relevant to these findings? Can you suggest
testing approaches that we little guys could use in evaluating our own
preamp designs??

Hey, good new information always generates LOTS of questions!

--
Regards, Terry King ...In The Woods In Vermont

The one who Dies With The Most Parts LOSES!! What do you need?
  #27   Report Post  
Scott Dorsey
 
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Terry King wrote:
Two recent relevant AES papers are
and Audio Engineering Society Convention Paper 5890 Auditory Perception of
Nonlinear Distortion - Theory Earl R. Geddes 1 and Lidia W. Lee

Arne says:
My own simplistic research into the subject and the scientific literature
was highly supportive of Geddes/Lee general thinking before they got the far
more polished and complete results that are published in the articles.

Today, most audible nonlinear distortion comes from loudspeakers and other
electromechanical/electroacoustic transducers.


Arne, this is very interesting, and I like it because it agrees :-) with
my feeling/observation of 'distortion' effects in lower-level signals.
Some systems seem to have artifacts discernable in the low-level passages
of material with high dynamic range, while the 'loud' parts sound fine,
and typical distortion measurements give good numbers.


Crossover distortion in class B amplifiers is this way. It produces a
"dead band" at the zero-crossing point which is very large in comparison
with a low-level signal, but rather small in comparison with a large one.

How do you think this may relate to preamps?? What can we learn about
testing preamps that is relevant to these findings? Can you suggest
testing approaches that we little guys could use in evaluating our own
preamp designs??


Preamps are hard because they have two problems: first you have whatever
distortion the system has in isolation (which is easy to detect with a
straight wire test), and secondly you have whatever distortion is produced
due the microphone/preamp interaction and loading (which is hard to
quantify easily).
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #28   Report Post  
Arny Krueger
 
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"Terry King" wrote in message


Two recent relevant AES papers are
and Audio Engineering Society Convention Paper 5890 Auditory
Perception of Nonlinear Distortion - Theory Earl R. Geddes 1 and
Lidia W. Lee


Arne says:
My own simplistic research into the subject and the scientific
literature was highly supportive of Geddes/Lee general thinking
before they got the far more polished and complete results that are
published in the articles.


Today, most audible nonlinear distortion comes from loudspeakers and
other electromechanical/electroacoustic transducers.


Arne, this is very interesting, and I like it because it agrees :-)
with my feeling/observation of 'distortion' effects in lower-level
signals. Some systems seem to have artifacts discernable in the
low-level passages of material with high dynamic range, while the
'loud' parts sound fine, and typical distortion measurements give
good numbers.


How do you think this may relate to preamps??


I have to regrettably plead ignorance. Right now I class myself as a mic
preamp newbie. I have a working theory of sorts about why mic pres sound
different that revolves around:

Noise at low levels
Clipping at high levels
Mic loading effects
EMI/RFI

Not on the list is any kind of low-level nonlinear distortion, but that
might just be ignorance on my part. I've had my nose rubbed in most of the
other items on the list. I await further practical experience and time on
the test bench.

What can we learn about
testing preamps that is relevant to these findings?


In all seriousness, show me some current online technical tests of mic
preamps that have levels of completeness that PCAVTech provides for sound
cards and power amps. I don't even see a starting point... So I don't see a
lot that is readily available to even learn from. Learning on your own is a
slow process, whether its you, me, or somebody else.

Can you suggest
testing approaches that we little guys could use in evaluating our own
preamp designs??


Well one thing I'm trying to say is that w/r/t mic preamps I'm very much a
little guy. But I don't see anybody who really seems to be running with the
ball. This is the sort of thing that got me into PCAVTech in the first
place...

Hey, good new information always generates LOTS of questions!


I think the Geddes/Lee results are going to make a lot of people feel good
about their past reservations about current means of characterizing
nonlinear distortion. However, I don't see a final answer or even a really
workable interim answer. I see the beginning of a potentially productive
line of questioning.


  #30   Report Post  
P Stamler
 
Posts: n/a
Default Preamp Design Fundamentals

Two good starting points:

1) Harmonic spectra at a variety of levels (near-maximum, nominal, and, say, 50
dB below nominal), done using bass, midrange and high-frequency tones. One
looks for higher-order harmonics, and also for odd changes in spectrum from one
level to another.

2) Intermodulation tests, again at a variety of levels. These can be 2-tone
(19kHz and 19.5kHz, for example) or multitone.

Most of these tests can be done, assuming adequately clean source and
converters (big assumption!), using the spectrum-analyzer features of popular
audio editing programs such as CoolEdit. The hard one is high-frequency
harmonic spectra, since the higher harmonics will be chopped off by
anti-aliasing filters.

I've found that HF intermod. measurements help separate sheep from goats in the
case of ICs. They can also show up quirks, such as the typical rise in
distortion of FET-input circuits when the impedance feeding a non-inverting
input is high.

Peace,
Paul


  #32   Report Post  
Arny Krueger
 
Posts: n/a
Default Preamp Design Fundamentals

"Terry King" wrote in message


In article ,
says...


Most of these tests can be done, assuming adequately clean source and
converters (big assumption!), using the spectrum-analyzer features
of popular audio editing programs such as CoolEdit. The hard one is
high-frequency harmonic spectra, since the higher harmonics will be
chopped off by anti-aliasing filters.


Paul, This is a good approach. This reminds me that I shouldn't mind
no longer having the HP Distortion Analyzer I had at IBM.. It's more
important to know what the (Not MY Fundamental!) residuals ARE, and
that means decent spectrum Analysis.


I can do 96Khz at 24 bits with my 1010LT's; Can I assume I can SEE
higher-order products, such as from the high-frequency IM tests?


You betcha. If you want to see a whole lot of equipment tests based on FFT
analysis, please visit
www.pcavtech.com. Some of the more complete tests
include:

http://www.pcavtech.com/pwramp/macrot-5000VZ/index.htm

http://www.pcavtech.com/soundcards/LynxTWO/index.htm


Please note that all of the numeric test results are hyperlinks. Click them
to see the supporting FFT-based graphical analysis.

Maybe we can figure out what guys without a lot of gear can do. I have
an HP function generator, but I THINK I could put together a 2-tone
signal in Cool Edit or N-Track at 24Bits/96Khz that would be a
respectable test source for IM tests.


This is exactly how the tests at www.pcavtech.com were done. The FFT
analysis tool used was Spectra Lab, which can be had free for 30 days by
downloading from http://www.soundtechnology.com/download-center.htm

Has anyone done what they consider 'good' tests like this using
computer-based 24bit/96Khz equipment? Sorry, I've been away for a
while. I feel a little like Rip Van Winkle meets Robert Pease...


There is also a handy freebie FFT-based tool that is highly automated, You
can download it free from

http://audio.rightmark.org/

Here are results from it that you can compare to the ones mentioned above:

http://audio.rightmark.org/test/lynx-two-b-32192.htm

http://audio.egregious.net/lynx_righ..._96/24_96.html

http://audio.egregious.net/lynx_righ...4_48/24_48.htm


And here are some delta 1010lt results from audio rightmark:

http://www.alfainfocom.ru/delta-rmaa/Comparison.htm


  #33   Report Post  
Terry King
 
Posts: n/a
Default Preamp Design Fundamentals

I can do 96Khz at 24 bits with my 1010LT's; Can I assume I can SEE
higher-order products, such as from the high-frequency IM tests?


You betcha. If you want to see a whole lot of equipment tests based on FFT
analysis, please visit www.pcavtech.com. Some of the more complete tests
include:

Arny, That is a great bunch of info; I'm very encouraged and digging into
that stuff. Thanks for saving us a lot of WWW research time.

I'm amazed at the SNR that both M-Audio and SB have achieved on a card
physically inside a PC, with Unbal RCA connectors. I worked on the IBM
Mwave stuff (DSP Based sound/modem cards), and we were fighting like heck

to get to -80. Fortunately, back then (1994 or so) SB was a lot worse!!

Now if I could just read Russian....

--
Regards, Terry King ...In The Woods In Vermont

The one who Dies With The Most Parts LOSES!! What do you need?
  #34   Report Post  
P Stamler
 
Posts: n/a
Default Preamp Design Fundamentals

I can do 96Khz at 24 bits with my 1010LT's; Can I assume I can SEE
higher-order products, such as from the high-frequency IM tests?


Yes. In fact, you can see many of the products with a 44.1kHz A/D, since
they're down in the audio band.

Maybe we can figure out what guys without a lot of gear can do. I have
an HP function generator, but I THINK I could put together a 2-tone
signal in Cool Edit or N-Track at 24Bits/96Khz that would be a
respectable test source for IM tests. What do you and the other guys
here think??


Certainly. I've generated 2- and 3-tone signals in CoolEdit.

Has anyone done what they consider 'good' tests like this using
computer-based 24bit/96Khz equipment?


I've done quite a few using 16bit 44.1kHz equipment. I burned a CD of the
aforementioned test signals, played it on a decent-quality but not spectacular
CD player (Philips/Magnavox), recorded the results using a decent-quality DAT
recorder (Sony 2000ES) and loaded it into my computer via S/PDIF. I then used
DC-ART to lowpass filter the signal at 2400Hz and raise the level of the
filtered signal by 20dB, and used their spectrum analyzer to look at the
results. Very clean, almost entirely noise with perhaps just a hint of a
distortion product around -96dB (that's *after* raising by 20dB). I tested a
lot of IC's that way, before I got a 24/96 setup, and it's good enough to at
least see some of the differences, such as the IM increase with higher source
impedance on FET-input circuits I mentioned earlier. Must see what the fancier
setup can measure now that I've got it. But with my quite modest former setup I
got much usable information.

Such as what the output section of a Mackie does to the signal. Trust me, you
don't want to know.

Peace,
Paul
  #35   Report Post  
Justin Ulysses Morse
 
Posts: n/a
Default Preamp Design Fundamentals

P Stamler wrote:

I've done quite a few using 16bit 44.1kHz equipment. I burned a CD of the
aforementioned test signals, played it on a decent-quality but not spectacular
CD player (Philips/Magnavox), recorded the results using a decent-quality DAT
recorder (Sony 2000ES) and loaded it into my computer via S/PDIF. I then used
DC-ART to lowpass filter the signal at 2400Hz and raise the level of the
filtered signal by 20dB, and used their spectrum analyzer to look at the
results. Very clean, almost entirely noise with perhaps just a hint of a
distortion product around -96dB (that's *after* raising by 20dB).


I was thinking...If you were to do this again with modern equipment,
you could play the test tone out through a channel of a DAW interface
and simultaneously record the result back into an input on said same
DAW. The resulting recording would be sample-synchronized to the
original test tone file due to both converters running off the same
clock. You could therefore use a mathematical subtraction of the
original test tone so that the removal of the original test tones would
be complete, and without artifacts.

Is this what everybody else already does?

I suppose you could do this with the equipment you described IF you
synchronize the DAT machine and the CD player.

ulysses


  #38   Report Post  
P Stamler
 
Posts: n/a
Default Preamp Design Fundamentals

I was thinking...If you were to do this again with modern equipment,
you could play the test tone out through a channel of a DAW interface
and simultaneously record the result back into an input on said same
DAW. The resulting recording would be sample-synchronized to the
original test tone file due to both converters running off the same
clock. You could therefore use a mathematical subtraction of the
original test tone so that the removal of the original test tones would
be complete, and without artifacts.

Is this what everybody else already does?


Probably not; old devil latency would get in the way. However, you can filter
digitally with few artifacts generated, depending of course on what you're
looking for.

I suppose you could do this with the equipment you described IF you
synchronize the DAT machine and the CD player.


Which'd probably be almost impossible. But filtering's easy enough.

Peace,
Paul

  #39   Report Post  
Arny Krueger
 
Posts: n/a
Default Preamp Design Fundamentals

"Justin Ulysses Morse" wrote in message
m
P Stamler wrote:

I've done quite a few using 16bit 44.1kHz equipment. I burned a CD
of the aforementioned test signals, played it on a decent-quality
but not spectacular CD player (Philips/Magnavox), recorded the
results using a decent-quality DAT recorder (Sony 2000ES) and loaded
it into my computer via S/PDIF. I then used DC-ART to lowpass filter
the signal at 2400Hz and raise the level of the filtered signal by
20dB, and used their spectrum analyzer to look at the results. Very
clean, almost entirely noise with perhaps just a hint of a
distortion product around -96dB (that's *after* raising by 20dB).


I was thinking...If you were to do this again with modern equipment,
you could play the test tone out through a channel of a DAW interface
and simultaneously record the result back into an input on said same
DAW. The resulting recording would be sample-synchronized to the
original test tone file due to both converters running off the same
clock. You could therefore use a mathematical subtraction of the
original test tone so that the removal of the original test tones
would be complete, and without artifacts.


Is this what everybody else already does?


I've done it, and it ain't a bed of roses. One serious problem is that the
granularity of the alignment is one sample due to its basic crudity. The
next step of finesse is to upsample and do the alignment in a higher sample
rate domain where the sample size is far smaller. Been there done that. In
fact that's how plots like
http://www.pcavtech.com/soundcards/L...192-xfus10.gif were
made. Note that I used upsampling to do a more precise alignment, and then I
turned around and did the analysis by means of FFTs, not subtraction or
nulling.

I suppose you could do this with the equipment you described IF you
synchronize the DAT machine and the CD player.


The problem with nulling or subtraction approaches is that the difference
signal is very sensitive to variations in both the frequency domain and
amplitude domain.

FFT analysis is a lot more straight-forward IMO.



  #40   Report Post  
Justin Ulysses Morse
 
Posts: n/a
Default Preamp Design Fundamentals

P Stamler wrote:


Is this what everybody else already does?


Probably not; old devil latency would get in the way. However, you can filter
digitally with few artifacts generated, depending of course on what you're
looking for.


But you can compensate for latency after you get your signal back into
the computer, and before you add the inverted original test tone.

I suppose you could do this with the equipment you described IF you
synchronize the DAT machine and the CD player.


Which'd probably be almost impossible. But filtering's easy enough.


I think it would be pretty easy IF your DAT machine allows clocking to
SPDIF input while recording the analog input. I know I can do this
with my AudioMedia II card, which costs less than a DAT machine, but I
don't recall if my DA30 does this or not. I think yes.

ulysses
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