How to determine a good preamp? - AVS Forum
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post #1 of 17 Old 12-20-2009, 08:40 PM - Thread Starter
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What's important in a preamp? What sets apart the various preamps when it comes to sound quality? Is it the THD? The frequency response? The signal to noise ratio? What must one measure to determine a preamps' "worth?" Can I spend $500 and get the same sound quality as a $5000 preamp?

Besides the various connectors/decoders/modes/room correction/etc. is a preamp nothing more than a DAC? And if so, how would one determine the 'sound quality' (electrical signal quality) empirically to set apart the various preamps?

Sorry for so many questions and a big thanks in advance!

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post #2 of 17 Old 12-20-2009, 09:02 PM
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They all measure really well. They all have flat response. They all have very low THD numbers.

DACs also measure very well.

Some do measure better. Like the Denon pre processor. It's fully balanced and measures better than some other pre amps.

Of course one may sound better than another measurements aside, but I have never seen anyone prove that in a blind listening test.

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post #3 of 17 Old 12-21-2009, 05:09 AM
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Different pre/pros do have different amounts of distortion, noise levels, inter-channel crosstalk, linearity, etc. How much or even if the various types of distortion are audible depends on many factors and has generated thousands of heated discussions.

More expensive preamps tend to have more of the design features which are intended to minimize noise and distortion, like better RFI shielding, cleaner power supplies, minimal wire lengths, better cooling, etc. You'll have to decide how valuable those features are to you.

As an individual, if those details matter to you and if you don't have access to the appropriate audio test equipment, you have to go by the reviews that have been published which include the results of bench testing. Some have been published online and some are available only in hardcopy magazines like Stereophile.

Your options vary from using the least expensive receiver which has preamp outputs to an audiophile grade pre/pro with plugin circuit boards. Smaller companies tend to have a lot of trouble incorporating recent digital processing features into their products. Some larger companies tend to lag behind, too, justifying this by spending more effort on the analog aspects of their pre/pros.

There is no one right answer to what matters the most. You'll find strong proponents for every model that's available.

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post #4 of 17 Old 12-21-2009, 09:57 AM
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Strange, I was asking myself the same question, since I am shopping for a new pre-pro.
I am an engineer, so I do believe in measurements. If you can hear it, then it must be measurable. But, as noted two posts above, all or almost all modern pre-pros measure very well. So is there a real world difference between them , and can it be revealed by measurements?
First thing I noticed reading various reviews is that measurements are usually performed only for analog inputs. I wonder if "digital" performance can also be measured? Perhaps by playing pure single frequency of sweep test tones recorded on CD?
Second is that measurements are typically restricted to frequency response linearity and signal-to-noise. Are there other parameters that might be also relevant to sound quality? Inter-modulation distortion, slew rate, phase linearity, resistance to jitter? Why are those almost never measured or reported? How much do they differ between different pre-pros and how does it correlate with subjective sound quality perception?
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post #5 of 17 Old 12-21-2009, 09:58 AM
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Correction - I meant to say "pure single frequency OR sweep tones".
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post #6 of 17 Old 12-21-2009, 11:17 AM
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Quote:
Originally Posted by Kreator View Post

Strange, I was asking myself the same question, since I am shopping for a new pre-pro.
I am an engineer, so I do believe in measurements. If you can hear it, then it must be measurable. But, as noted two posts above, all or almost all modern pre-pros measure very well. So is there a real world difference between them , and can it be revealed by measurements?
First thing I noticed reading various reviews is that measurements are usually performed only for analog inputs. I wonder if "digital" performance can also be measured? Perhaps by playing pure single frequency of sweep test tones recorded on CD?
Second is that measurements are typically restricted to frequency response linearity and signal-to-noise. Are there other parameters that might be also relevant to sound quality? Inter-modulation distortion, slew rate, phase linearity, resistance to jitter? Why are those almost never measured or reported? How much do they differ between different pre-pros and how does it correlate with subjective sound quality perception?

Well, the DACs themselves are measured, or at least a sample is measured in the data sheets. When they are integrated into the receiver, they have associated circuitry such as op amps which could effect the sound (but I have my doubts that they would.)

There are other parameters. Some benchmark tests cover some things the mfg's specs don't provide. Sometimes you see slew rate for amps. But pre processors are likely faster than amps as they a smaller voltage range to swing, I am not sure slew rate matters, but I could be wrong.

Seeing how you are interested in the topic, I suggest this reading - (which is about power amps, but some of the points apply; crossover distortion won't apply as pre processors don't use push pull topologies as far as I know)

http://sound.westhost.com/amp-sound.htm

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post #7 of 17 Old 12-21-2009, 11:54 AM
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My guess would be that part of the problem of measuring distortion introduced by digital processing is the wide variations in the kinds of processing performed. What kind do you select? Maybe 60 Hz and 1KHz tones that get sent to the center channel (for IM measurements) -- but what encoding or compression would be used? And how can you reliably determine how much distortion was in the digital source and how much was added by the circuit being tested? Ideally you want to do the test in exactly the same way for all systems being tested.

In contrast, it's relatively easy to see when a 1KHz analog sine or square wave is clean on input and distorted on output.


Stereophile
magazine and the Secrets of Home Theater web site each have published reviews which include test bench measurements. Many of the reviews published by both of them consist only of subjective comments, however, and neither tend to do detailed reviews of comparable equipment from several manufacturers at the same time.

Some years ago, I finally decided to get a pre/pro which had an excellent test-bench review, even though I couldn't find any equivalent reviews of comparably priced pre/pros from other manufacturers. I've been happy with that purchase for quite a few years. Just recently, though, I decided to upgrade to its current generation, which had an equally good review. Again, though, there haven't been test results published for its direct competition.

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post #8 of 17 Old 12-21-2009, 12:21 PM
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Thanks. I have seen this article before, but never explored the rest of the site - a lot of interesting reading!
So one of the possible culprits is "transient intermodulation distortion", if I understand correctly, a change in the amount of feedback and output impedance of the operational amplifier with frequency, which does not show up in "standard" one-frequency measurements but can lead to quite audible effects.
It is even more disappointing then that so called "professional" reviewers still resort to frequency response and signal-to-noise tests that tell us nothing since results are the same for all (pre)amplifiers tested.
From my own professional experience with RF amplifiers, I suspect that all that is needed is to measure the (pre)amplifier response to a square wave and short pulse. If both come out undistorted in shape and with low "ringing", then the (pre)amplifier is designed well. I just wish somebody would adopt the same approach in the audio world.
In the digital domain, the square wave and short pulse test signals would have to be recorded on a CD, DVD or BD. Not so hard to do, I believe.
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post #9 of 17 Old 12-21-2009, 12:22 PM
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Not too start an argument, but it's hard for me to understand how pre amps are less than perfect at least when it comes to the basics.

The analog audio path from input to output, when measured always seems to be nearly perfect. No bumps in the audio band response curve of any importance. Levels of noise and distortion well below .01% THD (I think .00n% THD is the standard?)

The digital path seems simple enough, excluding room correction. Take the input, process it in the DSP (using what should be perfect decoders,) and dump it out to the DACs (which I guess are about as good as they can make them in most pre pros.)

But audiophles obsess about things. I remember reading reviews about the distortion from volume controls. So some pre pros were using very expensive volume controls. Nowadays, volume controls seem to be commodity items; they seem to be mostly electronic volume controls using op amps. If such a minor thing is important, you may as well give up hope achieving perfection.

If some audiophiles stopped at worrying about DACs (which are rarely tested under any scientific conditions by people claiming this and that,) I would be ok with them. But many of these people are the same people who are sucked into thinking power cords matter.

At some point, you have to take one of these stances -

* Any reputable electronics are really good, especially something specialized like a pre amp, and worrying about their audio quality is (mostly) a waste of time
* That you have golden ears, and you will simply have to buy the gear and try it out to determine whether the gear has objectionable audio; a reasonable if time consuming and possibly expensive proposition
* Obsess about it so much your life is ruined; After a few months of comparing specs, reading biased reviews, and pushing into the crazy high end world of audiophile power cords, we will find you curled into a little ball muttering just a few words like jitter and skin effect (ok, I exaggerate, but you get my point maybe)

"But this one goes up to 11"
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post #10 of 17 Old 12-21-2009, 12:27 PM
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Selden - that is exactly what i am talking about! Why waste time determining if signal-to-noise is 95 or 110 dB or deviation from flatness at 20 kHz is 0.1 or 0.2 dB? Who is going to claim he/she hears the difference?
And test signals I suggested (pulse and square wave) would not be hard to put in any form of digital encoding (wav file or cd-da) using only a simple audio software.
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post #11 of 17 Old 12-21-2009, 12:52 PM
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The preamp and receiver reviews I pay attention to include plots of both harmonic distortion and intermodulation distortion, not just signal to noise. I've only seen one recently that included a square wave test, although those used to be quite common.

Fpr example, Secrets recently reviewed a mid-level Denon receiver and the Marantz AV8003 pre/pro, with spectrum analyzer outputs for both. To me, at least, the Denon IM plot was quite distressing.

Denon 3310CI:
http://www.hometheaterhifi.com/recei...r.html?start=3

Marantz AV8003:
http://www.hometheaterhifi.com/surro...3.html?start=4

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post #12 of 17 Old 12-21-2009, 01:18 PM
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I have done so many blind tests in my room. I have many processors and amps. What have I concluded, there are differences but more similarities. The most obvious test to me is when I used a very highly regarded amp(Sim audio and McIntosh) and compared it to a Samson S1000 pro amp for center channel duties. There were differences between the 2 but not information. All the details were there with both. One amp had some things louder or more clear(like certain background effects) than the other but that was it. One amp cost $300 for 2 channels and the other cost $3500 for 6 channels. After that happened I focused on power and the ability to drive low impedance speakers. For processors I look for bandwidth. Some have rolloff at 10 hz and others at 5hz. I prefer the full bandwidth as I like bass and want it all. I notice difference between processors mostly with bass. Another test I use is to play a favorite scene at reference and whatever piece does not sound harsh at that level is a keeper. You will be surprised on how many pieces fail that test. Now harsh to me will be different for others. Make sure the speakers can do this to begin with. My Ada amp has both excellent bass and can be played at reference levels without sounding harsh or aweful. The Marantz 8003 I had also sound great at reference but lacked in the bass in comparison. I maybe crazy but I test these with a group of 6-7 that have no idea what I changed. Everyone picked my behringer ep-2500's over the Mac amp I was using. It is fun to fool people. I am not sure what was wrong with the Marantz but it uses Audyssey where the Ada does not. maybe there is a cutoff or filter with audyssey? I also had to turn the gain up 10 db's for the LFE with the Marantz. People tell me that it is how it is supposed to sound but I had 8 18's at the time and I could not feel anything anymore until I turned it up 10 db's. The cinema has bass I can feel so I knew it was not how it was supposed to sound. Anyway, I purchased used gear and tried everything I could to hear differences and just picked the one I liked.
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post #13 of 17 Old 12-22-2009, 09:16 AM
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The question of what is audible and what is not involves subjectivity and psychoacoustics, and we will never have a final answer to that. What I am interested in is strictly objective - I want to know if my processor is able to reproduce the encoded waveform truthfully. And my frustration is with the fact that in most cases, nobody seems to care to test that. Perhaps one day I will bring the box to my lab at work and run a few measurements myself.
I am not aware of any pre-pro reviews that even report IM distortion (receivers are different beasts alltogether), can you point me to one?
And the reason I suggested to test with a short pulse is that I have seen amplifiers (RF) that are perfectly linear in frequency response and have 100+ dB signal to noise but produce garbage at the output when supplied a short pulse, due to very nonlinear phase response.
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post #14 of 17 Old 12-22-2009, 09:18 AM
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Quote:
Originally Posted by Kreator View Post

I am not aware of any pre-pro reviews that even report IM distortion (receivers are different beasts alltogether), can you point me to one?

Marantz AV8003
http://www.hometheaterhifi.com/surro...3.html?start=4

Denon AVP-A1HDCI
http://www.hometheaterhifi.com/surro...w.html?start=3

Integra 9.8
http://www.hometheaterhifi.com/surro...w.html?start=2

Audio Control Maestro M2
http://www.hometheaterhifi.com/volum...06-part-3.html

Mark Levinson No 40 HD Media Console
http://www.hometheaterhifi.com/volum...07-part-8.html

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post #15 of 17 Old 12-22-2009, 12:51 PM
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Selden - thanks!
I am happy to see that there is actually a Philps test SACD that can be used to measure THD+N and IMD for a digital signal chain.
And even more happy to see that Integra measures better than $7000 Denon! I am so glad now that I just ordered the new 5507!
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post #16 of 17 Old 12-22-2009, 01:05 PM
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Is IM a problem with some pre amps?

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post #17 of 17 Old 12-23-2009, 03:00 PM - Thread Starter
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I guess I'm late to my own party

Thanks for all the replies. I guess I was hoping to hear which measurements determine the difference in sound quality between the preamps, but it sounds like any modern preamp will vary only in the lab and will not make a difference in terms of measured sound quality. Given the relatively high SNR, super low THD numbers and frequency responses which vary by factions of fraction of a decibel for even the cheapest preamps, I'm guessing the real differences between them aren't in the sound quality department.

I wanted to find out what to measure, because I wanted to do a test using RightMark Audio with a PC-based solution. People constantly talk about interference but I've seen testing for sound cards which rival "audiophile" grade components. This post at DIY Audio is what got me thinking:

Quote:
Try a loopback test with RightMark Audio software and see how it specs. As far as actual sound, listen to these 2 clips and tell me which was recorded with a $25.00 SB card and which one with a $8000.00 Apogee 8000.


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