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    On the use of a preamp to improve sound quality with VTA70

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    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Thu Jul 19, 2018 12:08 pm

    corndog71 wrote:Instead of looking at how the preamp "improves" the sound, consider exploring how a fixed signal going into passive attenuator negatively affects the sound.

    I can't tell if you are joking. Is this a serious question?

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Thu Jul 19, 2018 12:27 pm

    Dogstar wrote:...
    I think that for anyone that thinks a preamp improves sound quality what it really is is that they like the way the tubes installed color the sound which is fine if you like that sort of thing.

    I think you are correct, but I am trying to keep an open mind. I realize there are many people a lot more experienced in audio than me that are paying huge amounts of money for these things, so this makes me think they must be providing some significant benefit. But so far I have not been able to find any sound technical reason why a preamp should improve the sound, and I've found a few reasons why it should degrade the sound (i.e., any additional stage can only degrease SNR and increase THD for example).

    I am starting to think that perhaps the primary benefit is indeed control, integration of multiple sources, and in some cases pleasant sonic coloration as you mentioned.

    EDIT: I'm wondering if they were more perhaps important in older days when input source signals were more fragile?


    Last edited by rjpjnk on Thu Jul 19, 2018 12:29 pm; edited 1 time in total
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    corndog71

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by corndog71 on Thu Jul 19, 2018 12:28 pm

    No, I'm serious. In my experience, going from a fixed source to an attenuator and immediately into an amp has consistently left me with two impressions:

    1. Slightly better clarity and detail compared to the active preamp I was using at the time.

    2. Dulled dynamics and somewhat less defined bass.

    Now I admit these were just casual observations but I've tried this numerous times with different sources and amps and observed the same effects. So take it for what it's worth.

    For me the improved dynamic swings in the music that came with the active preamp won the contest every time. I appreciated the clarity of the passive attenuator but if the music doesn't move me then what's the point?
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    solderblob

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by solderblob on Thu Jul 19, 2018 12:36 pm

    I found a dB gain calculator on line.  My Cornet phono pre has 44 dB gain.  Add 10 dB for the step up transformer.  54 dB.  My Denon DL103 has an output of 0.3 mV.  

    Using the calculator (which does dB = 20 × log (V2 / V1)) 54 dB results in a voltage multiplier of 501.  So 0.0003V x 501 = 0.15V.

    What am I doing wrong???

    dave

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Thu Jul 19, 2018 12:40 pm

    corndog71 wrote:No, I'm serious.  In my experience, going from a fixed source to an attenuator and immediately into an amp has consistently left me with two impressions:  

    1. Slightly better clarity and detail compared to the active preamp I was using at the time.

    2. Dulled dynamics and somewhat less defined bass.

    Now I admit these were just casual observations but I've tried this numerous times with different sources and amps and observed the same effects.  So take it for what it's worth.  

    For me the improved dynamic swings in the music that came with the active preamp won the contest every time.  I appreciated the clarity of the passive attenuator but if the music doesn't move me then what's the point?

    If the dynamics are duller without the preamp, I would assume the preamp is adding dynamics rather than a passive attenuator is removing them.

    The passive attenuator can do nothing but provide a lower voltage copy of the original input signal. It cannot possibly degrade the signal or remove dynamics. At least I can't think of any reason why it could. The preamp is probably enhancing the signal. Which is not a bad thing as long as you're not a purist. I'm with you. Ail that matters to me is the music sounds good.

    Wish I could borrow a good preamp to test out these less tangible qualities.

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Thu Jul 19, 2018 1:10 pm

    solderblob wrote:I found a dB gain calculator on line.  My Cornet phono pre has 44 dB gain.  Add 10 dB for the step up transformer.  54 dB.  My Denon DL103 has an output of 0.3 mV.  

    Using the calculator (which does dB = 20 × log (V2 / V1)) 54 dB results in a voltage multiplier of 501.  So 0.0003V x 501 = 0.15V.

    What am I doing wrong???

    dave

    Looks correct to me. Why do you think it is wrong?
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    corndog71

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by corndog71 on Thu Jul 19, 2018 1:13 pm

    rjpjnk wrote:It cannot possibly degrade the signal or remove dynamics.

    That is absolutely not true. Try comparing a cheap Alpha pot to a PEC pot or stepped attenuator with individual resistors. All 3 will sound different.
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    Peter W.

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by Peter W. on Thu Jul 19, 2018 1:19 pm

    corndog71 wrote:
    rjpjnk wrote:It cannot possibly degrade the signal or remove dynamics.

    That is absolutely not true.  Try comparing a cheap Alpha pot to a PEC pot or stepped attenuator with individual resistors.  All 3 will sound different.

    Even the skeptic in me agrees entirely with the above. Not to mention that a passive attenuator makes it harder to get to "just right" from too loud to too soft, which, in turn leads to the absolute fact in listening psychology that *Louder* sound better 100% of the time - until listener fatigue sets in - which is a whole 'nuther' thing.
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    solderblob

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by solderblob on Thu Jul 19, 2018 1:39 pm

    rjpjnk wrote:
    solderblob wrote:I found a dB gain calculator on line.  My Cornet phono pre has 44 dB gain.  Add 10 dB for the step up transformer.  54 dB.  My Denon DL103 has an output of 0.3 mV.  

    Using the calculator (which does dB = 20 × log (V2 / V1)) 54 dB results in a voltage multiplier of 501.  So 0.0003V x 501 = 0.15V.

    What am I doing wrong???

    dave

    Looks correct to me. Why do you think it is wrong?

    OK, I think I know what I did wrong.  The SUT has a turns ratio of 15:1.  That's not gain, it's voltage multiplication.  That would nominally multiply the voltage by 15, not 15 dB gain (or 10 as I used before).  So starting over:

    0.0003V x 15 = 0.0045V.  Putting 44dB gain for the phono pre into the dB gain calculator gives a 158 voltage multiplier.  So 158 x .0045 = 0.71V.

    My SUT also has a center tap which you can select to get 30:1 ratio.  That would work out to 1.42V to the amp.

    I've been listening with the SUT in the low 15:1 mode, but now after reading this thread, I think I use the 30:1 setting and maybe have a bit more headroom/dynamics.

    dave

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Thu Jul 19, 2018 2:20 pm

    corndog71 wrote:
    rjpjnk wrote:It cannot possibly degrade the signal or remove dynamics.

    That is absolutely not true.  Try comparing a cheap Alpha pot to a PEC pot or stepped attenuator with individual resistors.  All 3 will sound different.

    Sorry if I was not clear. I was talking conceptually. The point I was trying to make is that reducing the signal with an (ideal) voltage divider cannot decrease the quality or dynamic range. Of course one could degrade a signal with a poorly designed and noisy mechanical pot or stepped device, but that's another story.

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    Peter W.

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by Peter W. on Thu Jul 19, 2018 2:39 pm

    I think I might take this off-group. For all our sakes.

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Thu Jul 19, 2018 2:39 pm

    solderblob wrote:

    OK, I think I know what I did wrong.  The SUT has a turns ratio of 15:1.  That's not gain, it's voltage multiplication.  That would nominally multiply the voltage by 15, not 15 dB gain (or 10 as I used before).  So starting over:

    0.0003V x 15 = 0.0045V.  Putting 44dB gain for the phono pre into the dB gain calculator gives a 158 voltage multiplier.  So 158 x .0045 = 0.71V.

    My SUT also has a center tap which you can select to get 30:1 ratio.  That would work out to 1.42V to the amp.

    I've been listening with the SUT in the low 15:1 mode, but now after reading this thread, I think I use the 30:1 setting and maybe have a bit more headroom/dynamics.

    dave

    I just meant the math was right.

    As far as which tap to use on the SUT it all depends on what range on input signal your particular amplifier requires. If it is a VTA amp I think the 0.71 is too low and the 1.42 is too high. If you are using a simple passive attenuator on the input to the power amp I'd go with the 1.42 and cut it down with the attenuator so at least you can reach full volume if you want to.

    These are RMS voltages, right?
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    solderblob

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by solderblob on Thu Jul 19, 2018 3:39 pm

    rjpjnk wrote:I just meant the math was right.

    As far as which tap to use on the SUT it all depends on what range on input signal your particular amplifier requires. If it is a VTA amp I think the 0.71 is too low and the 1.42 is too high. If you are using a simple passive attenuator on the input to the power amp I'd go with the 1.42 and cut it down with the attenuator so at least you can reach full volume if you want to.

    These are RMS voltages, right?

    Yes, I'm using a stepped attenuator.  And it's a VTA Mk3 octal amp. Very Happy

    Normally they would be RMS voltages, but the phono cartridge specifications only state 0.3mV at something like 50 mm/sec.  I used to have a test record whereupon could measure the output.  But I think I'll just listen to some Beethoven instead!

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Fri Jul 20, 2018 11:47 am

    I've done a lot of research on this over the past 24 hours, and pretty much all technical explanations of the role of preamps indicate that they are necessary only when connecting a small fragile signal such as from an acoustic transducer like a microphone, guitar pickup, or phono cartridge to a power amp. These are all devices that supply relatively low voltages (10mV range) and have high output impedances (meaning they cannot supply any significant current without effecting their output voltage. A fragile signal like this cannot be connected to a power amp directly because (1) the signal level is too low to drive the power amp to full scale, and (2) the load of the power amp itself would try to draw more current from the transducer than it could supply without dropping its voltage, which introduces distortion.

    In contrast, if a device is capable of outputting a line level signal (i.e., 1V rms a.k.a. 0dBV) and has a low enough output impedance to maintain this voltage when fed to a power amp, than there is no technical reason to use a preamp. From what I gather, all modern CD players, iPhones, iPads, and stand-alone DACs, etc are easily capable of supplying such a signal. For example, the iPhone 6S output is 1V rms and its output impedance is 3.18 ohms. The input impedance of a VTA70 is on the order of 500KOhms, and its input voltage range is 1V rms. This is a perfect match for direct connection to the iPhone audio output jack and many other modern line level devices. I suppose that's why it was designed that way in the first place. The only thing missing is a way to control the volume. There are two obvious options for this: (1) install a high quality pot or stepped attenuator, and (2) just use the volume control on your iPhone. It is extremely well balanced L to R and moves in nicely calibrated 3.5dB steps with each click. (Yes, more research). To maximize SNR, however, it is best to leave the iPhone at or near full volume and use the attenuator in the power amp to cut the signal down to size.

    So my opinion so far based on a few days of research, is that the only really useful purpose for a preamp today in HiFi stereo (other than phono) is if you like the sound of the preamp's circuits better than the sound of the first stage voltage amplifier in your power amp. This is a perfectly valid reason to use a preamp, and I do not mean to discount it. This is the stage responsible for the overall signature of the sound you ultimately hear, and is perhaps the most influential component in the chain other than speakers. But, there is no longer any relevant technical need for it with modern line level devices.

    Well, that's what I learned. And for now, at least, that is my opinion. I still may buy a preamp. I realize they can make the music sound much better. But I understand now this is due to favorable coloration of the sound rather than some issue of degradation by impedance mismatch or input level.

    I realize this puts me at odds with the millions of serious audiophiles paying thousands of dollars for extremely high end preamplifiers, (well, I'm already at odds. I bought Klipsch speakers) but until I hear any sound reasons otherwise, what else can I conclude? I will definitely keep an open mind and learn more about this special device as time goes on.
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    Kentley

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by Kentley on Fri Jul 20, 2018 1:34 pm

    Good job, rjpjnk. Your research and your conclusions are, er, sound.
    And welcome to the forum. I sense you will have much of value to share.
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    solderblob

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by solderblob on Fri Jul 20, 2018 2:10 pm

    rjpjnk wrote:
    I realize this puts me at odds with the millions of serious audiophiles paying thousands of dollars for extremely high end preamplifiers, (well, I'm already at odds. I bought Klipsch speakers) but until I hear any sound reasons otherwise, what else can I conclude? I will definitely keep an open mind and learn more about this special device as time goes on.

    I'm sure there'll be those who disagree, but I'm not one of them.  But I will admit that I've never used a high quality preamp in my system.  What I experienced a few years ago when I first replaced my Adcom GFP-555 preamp with a stand-alone stepped attenuator was a huge improvement in transparency and detail with no loss of, or even better, dynamics.  So I've never seen a need for a line preamp.

    A follow up on switching my SUT to the 30:1 winding setting -- For a few days I had been listening with the 15:1 setting and while it could play at very loud levels (knob at 2:00 o'clock), I noticed occasional distortion and a dryer sound, less bass, overall bleh...  Last night I switched it back to the 30:1 setting and everything was back to wonderfulness.  I'm puzzled because with the lower voltage with the 15:1 windings I still wasn't even close to the end of the attenuator which goes to 6:00 o'clock for zero Ohms.

    Anyway, thanks for the research!

    dave
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    Bob Latino
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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by Bob Latino on Fri Jul 20, 2018 2:12 pm

    rjpjnk wrote: The input impedance of a VTA70 is on the order of 500KOhms, and its input voltage range is 1V rms.

    The input impedance of all VTA amps is 270,000 ohms. This is suitably high enough to work well with any preamp. You need about 1.2 to 1.3 volts OUT from your device to get full output from all VTA amps. Nearly all audio based upstream input gear (CD player, all preamps, outputs from computers, DAC's, phono preamp out etc.) will have an output of at least 2 volts. This 2 volt output is more than enough to get full output from VTA amps.

    Bob


    Last edited by Bob Latino on Sat Jul 21, 2018 10:28 am; edited 1 time in total

    audiobill

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by audiobill on Sat Jul 21, 2018 7:09 am

    Just weighing in -

    In my system, using the SP14 adds a level of dynamic headroom, clarity and transparency beyond not using it.  Not to speak of switching convenience.

    And that's on top of using very high end input sources with superb analog stages of their own.

    I was previously a skeptic, but this is my experience.


    Last edited by audiobill on Sat Jul 21, 2018 10:28 am; edited 1 time in total
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    corndog71

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by corndog71 on Sat Jul 21, 2018 10:26 am

    I’m with audiobill on this one.

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by monkuboy on Sat Jul 21, 2018 11:53 am

    I've tried source direct to my ST-120 using its attenuator, an Emotiva XMC-1, a Schiit Saga preamp, and the SP14.  Logically I shouldn't need a preamp but I get the best results to my ears using the SP14. For me that was a totally worthwhile purchase.
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    solderblob

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by solderblob on Sat Jul 21, 2018 10:32 pm

    audiobill wrote:Just weighing in -

    In my system, using the SP14 adds a level of dynamic headroom, clarity and transparency beyond not using it.  Not to speak of switching convenience.

    And that's on top of using very high end input sources with superb analog stages of their own.

    I was previously a skeptic, but this is my experience.

    Maybe Roy can weigh in here...

    My setup is strictly phono so I'm wondering how my phono preamp enters into the equation.  I'm currently using a Hagerman Cornet phono preamp into a stepped attenuator into my amp.

    Question -- Is it truly a preamp?  Does it provide the same output as a line preamp?

    I hope to scrape up the cash for building Roy's PH-16 soon.  The $64 question is do I need to also build a SP14?  Maybe you can loan me yours!

    dave

    rjpjnk

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Sat Jul 21, 2018 11:25 pm

    Haha, That question will be more than $64.00 Wink They do look nice though...

    You know what's really funny? An active preamp being fed a line level signal actually spends most of its time reducing the signal level! That is, unless you are listening near full volume.
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    Peter W.

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by Peter W. on Mon Jul 23, 2018 10:23 am

    rjpjnk wrote:Haha, That question will be more than $64.00 Wink They do look nice though...

    You know what's really funny? An active preamp being fed a line level signal actually spends most of its time reducing the signal level! That is, unless you are listening near full volume.

    Not really. Well... Yes. Um, Not so much. Uh... Maybe. For a start, go look at the output curve of the typical Audio-Taper pot. http://mathscinotes.files.wordpress.com/2011/12/compensated1.png

    a) Assume a line-level source with a fixed output. For a simple example - a tuner.
    b) Assume the tuner is on a typical 'modern' FM station that uses AVC, compression and high levels of modulation.
    c) The result will be a +/- Peak-to-Average of about 10 dB, with a dynamic range of <20 dB on a good day.

    Yes, the pre-amp _will_ be attenuating, OR the amp will be clipping.

    Or:

    a) As above - only this time a Phono Pre.
    b) Assume a very good cartridge playing a very dynamic source with a P/A of 20 dB.
    c) Resulting in a dynamic range approaching 30 dB.
    d) Assume that one wants the most quiet passages to be well above the noise floor.
    e) Assume well-designed components end-to-end and moderately efficient speakers.

    The pre-amp will be allowing/creating greater-than-line-voltage at the peaks, and so be 'amplifying' most of the time. even though the actual voltage output as seen by the amp will be lower than (generic) 2V for 99-44/100% of the time. That figure chosen in memory of Marilyn Chambers, the original Ivory Snow model, and meant to be illustrative. The reality is that the peaks are for vanishingly brief moments, and typically not more than 5% of the overall signal.

    So, it depends. And much of that is in how one defines the terms.

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by rjpjnk on Tue Jul 24, 2018 7:37 pm

    monkuboy wrote:I've tried source direct to my ST-120 using its attenuator, an Emotiva XMC-1, a Schiit Saga preamp, and the SP14.  Logically I shouldn't need a preamp but I get the best results to my ears using the SP14. For me that was a totally worthwhile purchase.

    I believe it. Though I wish I could have uncovered at least one technical reason *why* it should sound better with a preamp, I have come up empty.

    My latest theory is that some of these preamps simply sound better than the first gain stage built into the power amps. And I suppose the first amplifier to touch the signal has the greatest effect on its tone as far as imparting a certain sonic signature and nuances of dynamics.

    This seems to imply that a perfectly transparent "ideal" preamp (i.e., one that imparts no sonic change whatsoever to the input signal) would not provide this benefit, and since the power amp already has sufficient gain to be driven to full output from the line level input, would be a complete waste.

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    Peter W.

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    Re: On the use of a preamp to improve sound quality with VTA70

    Post by Peter W. on Wed Jul 25, 2018 7:14 am

    [quote="rjpjnk"]
    monkuboy wrote:I've tried source direct to my ST-120 using its attenuator, an Emotiva XMC-1, a Schiit Saga preamp, and the SP14.  Logically I shouldn't need a preamp but I get the best results to my ears using the SP14. For me that was a totally worthwhile purchase.

    I believe it. Though I wish I could have uncovered at least one technical reason *why* it should sound better with a preamp, I have come up empty.

    By having the capacity to overdrive the amp, you now have the capacity to utilize it to its fullest extent. Without this capacity, you will not realize transients and/or the lowest signal may be below the noise floor. It is this last issue that contributes to the belief that pre-amps add color. Good ones do not.

    Technical enough?


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