The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


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

    Share

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Wed Jul 18, 2018 1:17 pm

    Greetings all. I see that many people are running active preamps before their power amplifiers, and I would like to understand if a preamp could be of any use to me.

    For the sake of discussion, let's say I don't need phono, don't need input selection, and I don't need tone controls.

    If I have only a single input source (say, a CD player), what possible advantage could a preamp (such as a PAS 3x or SP14) offer in terms of sound quality compared to just running directly into the VTA70? I already have a stepped attenuator for volume control.

    I have read a lot of discussion already and posted on some less technical foums, but the only answers I got were along the lines of "Of course a preamp makes things sound better, you must be an idiot for asking". Even if this is correct, it is wholly unsatisfactory technically. So I am hoping some of the more educated folks here could help me understand if in fact there are some sound technical reasons why an active preamp between my CD player (or iPhone) and my VTA70 could improve the sound quality.

    I am an EE and have extensive electronics and signal processing background but limited audio experience, hence the uncertainty. From what I can see and measure, the first stage amplifier in the VTA70 already seems to be very high quality, and the input impedance and signal level appear to be a perfect match for many modern input sources. So no preamp gain is required, only attenuation, which a stepped attenuator seems to accomplish nicely.

    Based on traditional noise figure analysis, it would seem that the addition of an active preamp in this case could only degrade the sound when compared to the direct path. But so many very smart people are using them, and sometimes paying huge amounts of money for them, so I am wondering if I am missing something.

    Thank you in advance for any inputs.




    avatar
    peterh

    Posts : 985
    Join date : 2012-12-25
    Location : gothenburg, sweden

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

    Post by peterh on Wed Jul 18, 2018 1:58 pm

    rjpjnk wrote:Greetings all. I see that many people are running active preamps before their power amplifiers, and I would like to understand if a preamp could be of any use to me.

    For the sake of discussion, let's say I don't need phono, don't need input selection, and I don't need tone controls.

    If I have only a single input source (say, a CD player), what possible advantage could a preamp (such as a PAS 3x or SP14) offer in terms of sound quality compared to just running directly into the VTA70? I already have a stepped attenuator for volume control.

    I have read a lot of discussion already and posted on some less technical foums, but the only answers I got were along the lines of "Of course a preamp makes things sound better, you must be an idiot for asking". Even if this is correct, it is wholly unsatisfactory technically.  So I am hoping some of the more educated folks here could help me understand if in fact there are some sound technical reasons why an active preamp between my CD player (or iPhone) and my VTA70 could improve the sound quality.

    I am an EE and have extensive electronics and signal processing background but limited audio experience, hence the uncertainty. From what I can see and measure, the first stage amplifier in the VTA70 already seems to be very high quality, and the input impedance and signal level appear to be a perfect match for many modern input sources. So no preamp gain is required, only attenuation, which a stepped attenuator seems to accomplish nicely.

    Based on traditional noise figure analysis, it would seem that the addition of an active preamp in this case could only degrade the sound when compared to the direct path. But so many very smart people are using them, and sometimes paying huge amounts of money for them, so I am wondering if I am missing something.

    Thank you in advance for any inputs.





    The main benefit of a preamp is - tonecontrols and possibly rumble and/or scratch filters.
    There is also a possibility to adjust levels from various sources giving similar volumes ( this is not generally implemented but may be done by soldering fixed resistor networks )

    tape output ( not that tapes are in widespread use but rather for connecting a typically
    low-impedance computer ) might be of value if one wants to digitize any material.

    Buffered line output will give the opportunity to locate the poweramp further away, closer
    to the speakers.

    To summarize , a preamp will give more functions, if they are needed is up to You!

    avatar
    corndog71

    Posts : 661
    Join date : 2013-03-19
    Location : It can get windy here

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

    Post by corndog71 on Wed Jul 18, 2018 2:42 pm

    I've tried passive attenuators vs active preamps and the latter always come out on top. With a passive pot you might gain some clarity but lose dynamics. A good active preamp like Roy's SP13/SP14 bring the music to life in a way that no passive attenuator can.

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Wed Jul 18, 2018 3:05 pm

    corndog71 wrote:I've tried passive attenuators vs active preamps and the latter always come out on top.  With a passive pot you might gain some clarity but lose dynamics.  A good active preamp like Roy's SP13/SP14 bring the music to life in a way that no passive attenuator can.  

    Ok, I believe that it sounds better. But why? What is the preamp doing to make the signal sound different than a direct connection?

    Is the preamp enhancing the signal or just preserving it?

    I assume in this case it is adding something to the signal that was not there in the original source since you say it "brings the music to life". Was it originally more flat sounding? Is it altering the dynamic is some way? Changing the tone spectrum to something more musical sounding than the original?

    I am not a purist, and am ok with adding something to the source to make it more enjoyable. That's one reason I like tubes so much after all!


    avatar
    corndog71

    Posts : 661
    Join date : 2013-03-19
    Location : It can get windy here

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

    Post by corndog71 on Wed Jul 18, 2018 3:09 pm

    I think it's a better impedance match between the source and the amp. The source may have enough signal to drive the amp but when you throw an attenuator in there it messes up the impedance. At least that's my take on it. Maybe someone smarter can explain it better.

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Wed Jul 18, 2018 3:14 pm

    peterh wrote:

    The main benefit of a preamp is - tonecontrols and possibly rumble and/or scratch filters.
    There is also a possibility to adjust levels from various sources giving similar volumes ( this is not generally implemented but may be done by soldering fixed resistor networks )

    tape output ( not that tapes are in widespread use but rather for connecting a typically
    low-impedance computer ) might be of value if one wants to digitize any material.

    Buffered line output will give the opportunity to locate the poweramp further away, closer
    to the speakers.

    To summarize , a preamp will give more functions, if they are needed is up to You!


    This is my current understanding as well, but so many people say the preamps make music sound so much better I am reevaluating my position and want to learn more.

    So would you say you're of the opinion that for a single source input to an ST70 the best possible sound is achieved with no added preamp?
    avatar
    Peter W.

    Posts : 831
    Join date : 2016-08-07
    Location : Melrose Park, PA

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

    Post by Peter W. on Wed Jul 18, 2018 4:16 pm

    [quote="rjpjnk"]Greetings all. I see that many people are running active preamps before their power amplifiers, and I would like to understand if a preamp could be of any use to me.

    For the sake of discussion, let's say I don't need phono, don't need input selection, and I don't need tone controls.

    If I have only a single input source (say, a CD player), what possible advantage could a preamp (such as a PAS 3x or SP14) offer in terms of sound quality compared to just running directly into the VTA70? I already have a stepped attenuator for volume control.

    I have read a lot of discussion already and posted on some less technical foums, but the only answers I got were along the lines of "Of course a preamp makes things sound better, you must be an idiot for asking". Even if this is correct, it is wholly unsatisfactory technically.  So I am hoping some of the more educated folks here could help me understand if in fact there are some sound technical reasons why an active preamp between my CD player (or iPhone) and my VTA70 could improve the sound quality.

    I am an EE and have extensive electronics and signal processing background but limited audio experience, hence the uncertainty. From what I can see and measure, the first stage amplifier in the VTA70 already seems to be very high quality, and the input impedance and signal level appear to be a perfect match for many modern input sources. So no preamp gain is required, only attenuation, which a stepped attenuator seems to accomplish nicely.

    Based on traditional noise figure analysis, it would seem that the addition of an active preamp in this case could only degrade the sound when compared to the direct path. But so many very smart people are using them, and sometimes paying huge amounts of money for them, so I am wondering if I am missing something.

    Thank you in advance for any inputs.

    The answer is almost so basic as to be very nearly disguised by its simplicity. Let's compare a:

    Passive Attenuator:  A passive attenuator will allow only the line-out voltage from the source to reach the amp. That is the given:

    a) Most modern power-amps will produce full output when fed +/- 2V or so, and most sources produce 2V or so of Line-Out voltage.
    b) Meaning that most passive attenuators will deliver sufficient output to the amp for it to produce full output.
    c) If most modern amps are fed greater than 2V for any sustained period of time, they will clip. The amount and result of said clipping depends on the nature of the amp - keeping it simple for now.

    Active Pre-Amp:  An active pre-amp will send higher-than-line-level voltage to the amp, if required. I have several that will deliver up to 13V.
    a) If I send a sustained 13V through my most powerful (solid-state) amps for any sustained period into my most tolerant speakers, I will watch the voice-coils fly across the room as melted slag.
    b) Meaning that this is clearly not a good idea.
    c) The same logarithmic scale applies to pre-amps as it does to power amps.

    NOW, comes the BUT:

    https://www.discogs.com/Saint-Sa%C3%ABns-Michael-Murray-Eugene-OrmandyPhiladelphia-Orchestra-Symphony-No-3-Organ-Encores-La-Fran/release/11954946  

    Obtain a very good recording of the Saint-Saens Organ Symphony - VERY good. The Peak-to-average on the recording linked is very nearly 30 dB.

    You will NOT be able to play this comfortably with only 2V  of headroom. The quietest passages will be inaudible. The loudest, not very. Being just a bit didactic, I am NOT stating that the dynamic range of the recording is 30 dB, but that the PEAK output is, in a few instances, 30 dB Louder than the AVERAGE output. Meaning that the dynamic range approaches 40 dB. Note that typical heavy-metal type recording has a dynamic range of less than 20 dB, with a P/A typically around 10 dB.

    A well-designed power-amp will easily deliver up to 10X its RMS rating for enough time to handle a music peak without clipping. And any (repeat, any) well designed speaker will handle such without damage.

    Cutting to the chase, a pre-amp allows the full dynamic range of a recording to be delivered to the listener in a usable form at an adequate volume to encompass a listenable quiet passage and an un-distorted peak.

    A Passive Attenuator will not do so.


    Last edited by Peter W. on Wed Jul 18, 2018 4:17 pm; edited 1 time in total (Reason for editing : Punctuation)

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Wed Jul 18, 2018 8:09 pm

    Peter W. wrote:
    A well-designed power-amp will easily deliver up to 10X its RMS rating for enough time to handle a music peak without clipping.

    I'm not sure about this part. If this is correct, then it pretty clearly makes the case for a preamp just to get the voltage swings. I'd like to get some confirmation.

    I have been under the impression that the VTA70 power amp was basically just a voltage amplifier that produced a bigger copy of whatever you put into it up to the point that it clipped. Maybe the transient response needs to be considered separately.

    A while back when I had my VTA70 on the bench I tested the maximum power output as follows.

    (I don't have my notes in front of me now, but I believe these numbers are about right from memory)

    I put an 8 ohm resistive load on each channel, and connected a signal generator set to 1KHz sine wave to the input of both channels. I connected an oscilloscope across one output channel at a time and measured the output as I increased the input level. The maximum output level I could obtain before I saw clipping was about 25V peak. (Which equates to 38WRMS.) The input signal at this point was at about 1.5V peak. (i.e., 1.06 VRMS)

    So I have been under the impression that any input voltage greater than 1.5 peak would cause the output to clip. Is this not correct?



    avatar
    j beede

    Posts : 458
    Join date : 2011-02-07
    Location : California

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

    Post by j beede on Wed Jul 18, 2018 9:58 pm

    rjpjnk wrote:
    So I have been under the impression that any input voltage greater than 1.5 peak would cause the output to clip. Is this not correct?

    Correct... for a continuous sinusoidal input. If you have access to a programmable signal generator and an oscilloscope you should be able to compare the onset of clipping for a continuous output versus the onset for a single event.

    I believe you said that you saw 50 Vrms(p-p) at the output before continuous clipping was apparent with a 1 kHz, 1.5 Vrms(p-p) sinusoidal input.

    The sine of zero is zero so if you program a single r*sine*theta pulse for theta from 0 to π(pi) and trigger the oscilloscope on that same pulse you should be able to look at single event clipping. Sweeping the value of "r" from 1 Vrms to 5 or 10 Vrms in 100 mV increments might produce interesting results.

    The test should be repeated for theta from π to 2*π in case the clipping is happening below zero.
    avatar
    Kentley

    Posts : 496
    Join date : 2015-03-06
    Age : 65
    Location : Worcester, MA

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

    Post by Kentley on Wed Jul 18, 2018 11:09 pm

    I know you're looking for solid technical reasons for the use of a preamp, Sir. I'm not the guy for that. I believe that Peter W. may be on a rational path here.
    Being a firm believer in the "some things can be measured but not heard, and some things cannot be measured, yet heard" camp, I can vouch for the across-the-board improvement of an SP-14 in the path between my DAC and my ST-120. So does our esteemed audiobill.
    Our most sophisticated test instruments remain our ears - providing, of course, that they are well-connected, by patient experience and experiment, to a coherent brain.
    And I sense that your ears are well-connected.

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Wed Jul 18, 2018 11:29 pm

    j beede wrote:

    I believe you said that you saw 50 Vrms(p-p) at the output before continuous clipping was apparent with a 1 kHz, 1.5 Vrms(p-p) sinusoidal input.


    No. It was actually 50V p-p, (or 25 V peak), which for a sine wave is 17.5V RMS.

    I will try a test like you mentioned to explore the transient response and see what voltages I can achieve. Thanks.

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Wed Jul 18, 2018 11:35 pm

    Kentley wrote:I know you're looking for solid technical reasons for the use of a preamp, Sir. I'm not the guy for that. I believe that Peter W. may be on a rational path here.
    Being a firm believer in the "some things can be measured but not heard, and some things cannot be measured, yet heard" camp, I can vouch for the across-the-board improvement of an SP-14 in the path between my DAC and my ST-120. So does our esteemed audiobill.
    Our most sophisticated test instruments remain our ears - providing, of course, that they are well-connected, by patient experience and experiment, to a coherent brain.
    And I sense that your ears are well-connected.

    I agree, the ears are the only instrument that ultimately matters. But even if my ears say it sounds better with the preamp, I still want to understand why.
    avatar
    Kentley

    Posts : 496
    Join date : 2015-03-06
    Age : 65
    Location : Worcester, MA

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

    Post by Kentley on Wed Jul 18, 2018 11:54 pm

    So do I. But there are times when we either are measuring the wrong thing, or, unfortunately, have not even developed the instrument to measure what matters at any given juncture.
    Look at the history of particle physics for a model.
    avatar
    bluemeanies

    Posts : 254
    Join date : 2015-02-09
    Age : 68
    Location : Folsom Pa.

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

    Post by bluemeanies on Thu Jul 19, 2018 4:55 am

    I am by no means qualified to answer technical issues to a EE.
    Simply put my SP14 makes my system sing with an external dac. I have the m125 mono-blocks.
    I am not a person who pays particular attention to specifications,graphs or oscillators
    avatar
    Peter W.

    Posts : 831
    Join date : 2016-08-07
    Location : Melrose Park, PA

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

    Post by Peter W. on Thu Jul 19, 2018 7:05 am

    Please note the interpolations:

    rjpjnk wrote:
    Peter W. wrote:
    A well-designed power-amp will easily deliver up to 10X its RMS rating for enough time to handle a music peak without clipping.

    I'm not sure about this part. If this is correct, then it pretty clearly makes the case for a preamp just to get the voltage swings. I'd like to get some confirmation.

    I have been under the impression that the VTA70 power amp was basically just a voltage amplifier that produced a bigger copy of whatever you put into it up to the point that it clipped. Maybe the transient response needs to be considered separately.

    OK - let me give you a close analogy: Imagine the typical small town out in the flatlands. What do you very nearly always see there? What is the tallest structure around but-for the grain elevators? The water tower. Keep that in mind as we move on.

    A while back when I had my VTA70 on the bench I tested the maximum power output as follows.

    (I don't have my notes in front of me now, but I believe these numbers are about right from memory)

    I put an 8 ohm resistive load on each channel, and connected a signal generator set to 1KHz sine wave to the input of both channels. I connected an oscilloscope across one output channel at a time and measured the output as I increased the input level. The maximum output level I could obtain before I saw clipping was about 25V peak. (Which equates to 38WRMS.) The input signal at this point was at about 1.5V peak. (i.e., 1.06 VRMS)

    Sure. Feeding it a sine-wave (single-frequency signal). All good so far.


    So I have been under the impression that any input voltage greater than 1.5 peak would cause the output to clip. Is this not correct?

    No, and yes. Remember the water tower.  The pumps that fill the tower = the RMS power of your amp. Half-time of the Superbowl, those pumps could not begin to meet the demand when very nearly every toilet in town flushes, and every sink is turned on. BUT - the reserve water in the tower maintains the peak flow and nobody runs dry. THAT is the PEAK demand against the average demand - and that is what the power-supply of a well-designed amp will handle for such peaks. But, yes, should the water in the tower become exhausted, and the demand continue, there will be problems - in an amp, that is clipping. So, as you ramp up power on your bench test into a dummy load using a single-frequency sine-wave signal, you are not allowing the tower to fill. And the amp will clip.




    As I noted previously, the reality-as-applied is so simple that it gets hidden by actual facts not fully explained or understood.

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Thu Jul 19, 2018 9:18 am

    Ok, so it sounds like you are saying that the transient response may be significantly higher than the max level obtainable as a steady state response. JBeede mentioned this as well. I will attempt to measure the transient response to see "how tall that water tower is". Thanks,

    But in addition to this, we need the input signal to also contain these transients. You mention that having a preamp that can swing 10v allows this. But the preamp cannot produce these transients unless they were also present in the original source signal. The fact that the preamp is taking a signal in the 0-2 volt range and amplifying it to the 0-10 volt range does not create anything new. It just makes everything louder.

    In other words, the dynamic range is still exactly the same. If Eugene Ormandy's crescendo was not in the original CD/iPhone signal it won't be in the preamp output either.

    So I don't see how having this additional stage of amplification changes anything.
    avatar
    Peter W.

    Posts : 831
    Join date : 2016-08-07
    Location : Melrose Park, PA

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

    Post by Peter W. on Thu Jul 19, 2018 9:35 am

    Once again, please note the interpolations.

    rjpjnk wrote:Ok, so it sounds like you are saying that the transient response may be significantly higher than the max level obtainable is a steady state response. JBeede mentioned this as well. I will attempt to measure the transient response to see "how tall that water tower is". Thanks,

    But in addition to this, we need the input signal to also contain these transients. You mention that having a preamp that can swing 10v allows this. But the preamp cannot produce these transients unless they were also present in the original source signal. The fact that the preamp is taking a signal in the 0-2 volt range and amplifying it to the 0-10 volt range does not create anything new.

    Do you understand the energy involved in sound?

    a) To double the volume, you need 10 times the energy.
    b) Every 10 dB = double the volume.
    c) If your P/A = 30 Db, and you are operating at 1-watt on average, you will need (theoretically) 1,000 watts for a peak.
    d) Your lowest volume sound may be only 1/10 watt (76 dB)


    So I don't see how having this additional stage of amplification changes anything.

    If your passive attenuator  can give only 2 V into the amp, the amp will never clip, and no reserves will ever be needed. So, if your  speakers deliver 86 dB at 1 watt, and you have a 38 watt amp, the loudest peak you can achieve will be 102 dB. And the softest passages will be close to unlistenable. Remember, you are covering a dynamic range of 40 dB, spanning the loudest passage requiring 10,000 times the energy of the softest.

    If you want to be sitting on the 10th row and 30 feet from the Bombard Pipes 2 V is not going to cut it. Your peaks will want to be 116 dB - and, no, that will not damage your hearing.  


    avatar
    Bob Latino
    Admin

    Posts : 2673
    Join date : 2008-11-26
    Location : Massachusetts

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

    Post by Bob Latino on Thu Jul 19, 2018 10:56 am

    Peter,

    Re: > "If your passive attenuator can give only 2 V into the amp, the amp will never clip"

    The VTA amps all need about 1.3 volts in for full output. At 2.0 volts IN all the VTA amps will be overdriven and be clipping at least to some extent ... See the photo below of the sinusoidal output on an oscilloscope showing how the output on the top and bottom of the signal is "clipped" off.



    Bob

    Dogstar

    Posts : 340
    Join date : 2014-06-23

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

    Post by Dogstar on Thu Jul 19, 2018 11:02 am

    I had an SP12 built by Roy and still have a Glassware AF-2 tube buffer and a Cary SL-100. When I first got them I wanted to believe the made the music coming from sources sound better. But through informal experimentation and being truly unbiased the sound quality really was no better.

    With the AF-2 Tube Buffer I did experiment with tubes. With 12AX7’s installed I thought there was more dynamic range compared to using 12AU7’s but when I adjusted the volume level so it was the same for both the dynamics were the same.

    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.

    All this aside I still use both the Cary and AF-2 because I do I have multiple sources for my VTA ST-120 and my Tubelab SSE.


    Last edited by Dogstar on Thu Jul 19, 2018 11:10 am; edited 2 times in total
    avatar
    solderblob

    Posts : 29
    Join date : 2018-05-20
    Location : Placerville, California

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

    Post by solderblob on Thu Jul 19, 2018 11:02 am

    So far this discussion has been about a CD player or dac directly driving an amp vs inserting a line preamp before the amp.  How about when a phono preamp is used as a single input to the amp using a stepped attenuator?  

    Is the phono preamp essentially equivalent to a dac in the sense of this discussion?

    dave
    avatar
    Peter W.

    Posts : 831
    Join date : 2016-08-07
    Location : Melrose Park, PA

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

    Post by Peter W. on Thu Jul 19, 2018 11:20 am

    solderblob wrote:So far this discussion has been about a CD player or dac directly driving an amp vs inserting a line preamp before the amp.  How about when a phono preamp is used as a single input to the amp using a stepped attenuator?  

    Is the phono preamp essentially equivalent to a dac in the sense of this discussion?

    dave

    Dave:

    What all this comes down to is providing enough voltage to the amp such that it will produce transients at the proper (relative) volume. This entire discussion also requires that the amps in question do have sufficient transient headroom to do it in the first place. I keep two (solid-state) amps capable of making clean transients up to 1,000 watts, in one case about 2,200 watts. DO NOT ask them to keep that up, but for the attack of a cymbal or the Bombard Pipe 'start' of an organ, they will do.

    If the source is limited to a *generic* 2 volts or so, it will not make those transients at a reasonable volume.

    So, I will summarize and give the assumptions around this discussion - at least as I apply(ied) them:

    1. We are listening at a reasonable volume - possibly approaching concert hall levels from about the 10th row or so. But enough to suppress conversation.
    2. We are listening on good, well-designed speakers that are also well made. With specific reference, I am "assuming" something like the AR3a for conventional designs or Maggies for planar speakers.
    3. We are listening to a highly dynamic source - I linked a recording of the Saint-Saens Organ Symphony.
    4. We want the peak transients to be realistically loud.
    5. We are using a well-designed (tube or solid-state) amplifier with a well-designed power-supply capable of making significant transients (relative to its RMS output) without clipping.

    Pretty much everything follows from that.

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Thu Jul 19, 2018 11:37 am

    Bob Latino wrote:Peter,

    Re: > "If your passive attenuator can give only 2 V into the amp, the amp will never clip"

    The VTA amps all need about 1.3 volts in for full output. At 2.0 volts IN all the VTA amps will be overdriven and be clipping at least to some extent ... See the photo below of the sinusoidal output on an oscilloscope showing how the output on the top and bottom of the signal is "clipped" off.



    Bob

    I was hoping you would chime in Bob. This agrees with my observations.  For steady state sinusoidal inputs I see the VTA clip when input voltage hits about 1.5V (one sided peak), which is equivalent to 1.06V RMS.

    What happens during transients? Can the amplifier handle input voltages much higher than this for short durations without clipping? i.e., still produce an output signal that is in proportion to the input even though the input is in the, say, 5V range? I did not think it was possible, but I may be mistaken.

    EDIT: I think the 2V figure Peter mentioned was just reuse of the numbers from my example question to him. I don't believe it was meant to be taken literally. The point being, if an amp has some maximum input level before clipping (call it 2V), what good would it do to amplify a 0-2V input signal to the range 0-10V with a preamp?


    Last edited by rjpjnk on Thu Jul 19, 2018 11:55 am; edited 1 time in total

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

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

    Post by rjpjnk on Thu Jul 19, 2018 11:48 am

    Peter W. wrote:Once again, please note the interpolations.

    rjpjnk wrote:Ok, so it sounds like you are saying that the transient response may be significantly higher than the max level obtainable is a steady state response. JBeede mentioned this as well. I will attempt to measure the transient response to see "how tall that water tower is". Thanks,

    But in addition to this, we need the input signal to also contain these transients. You mention that having a preamp that can swing 10v allows this. But the preamp cannot produce these transients unless they were also present in the original source signal. The fact that the preamp is taking a signal in the 0-2 volt range and amplifying it to the 0-10 volt range does not create anything new.

    Do you understand the energy involved in sound?

    a) To double the volume, you need 10 times the energy.
    b) Every 10 dB = double the volume.
    c) If your P/A = 30 Db, and you are operating at 1-watt on average, you will need (theoretically) 1,000 watts for a peak.
    d) Your lowest volume sound may be only 1/10 watt (76 dB)


    So I don't see how having this additional stage of amplification changes anything.

    If your passive attenuator  can give only 2 V into the amp, the amp will never clip, and no reserves will ever be needed. So, if your  speakers deliver 86 dB at 1 watt, and you have a 38 watt amp, the loudest peak you can achieve will be 102 dB. And the softest passages will be close to unlistenable. Remember, you are covering a dynamic range of 40 dB, spanning the loudest passage requiring 10,000 times the energy of the softest.

    If you want to be sitting on the 10th row and 30 feet from the Bombard Pipes 2 V is not going to cut it. Your peaks will want to be 116 dB - and, no, that will not damage your hearing.  



    Peter, I understand what you are saying about needing enough power so the softest passages are not so quite they are unable to be heard, but the proposed daisy- chaining of preamp and power amp to achieve this is technically no different than just buying a more powerful single amp in the first place. (Assuming, for the sake of discussion, that the preamp in not over driving the power amp)

    There must be something else beneficial about preamps than just making things louder. Or maybe there isn't? This is what I am trying to explore.
    avatar
    Peter W.

    Posts : 831
    Join date : 2016-08-07
    Location : Melrose Park, PA

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

    Post by Peter W. on Thu Jul 19, 2018 12:04 pm


    [/quote]

    Peter, I understand what you are saying about needing enough power so the softest passages are not so quite they are unable to be heard, but the proposed daisy- chaining of preamp and power amp to achieve this is technically no different than just buying a more powerful single amp in the first place. (Assuming, for the sake of discussion, that the preamp in not over driving the power amp)

    There must be something else beneficial about preamps than just making things louder. Or maybe there isn't? This is what I am trying to explore.[/quote]

    I am doing what I can to avoid pushing the 'Snarky Rant" button.

    Another analogy:

    Obtain four coins each: Dimes, Nickles, Quarters.

    Line each type up, touching, in a row, first three, then one about 2" away.

    Slide-flick the separate coin into the other three - and you will note that only one (1) coin moves on the other end.
    Try the same flicking two coins. Two will slide at the other end.
    Now, slide a nickle into the dimes. Observe what happens.
    Now, a dime into the nickles. Observe what happens.

    Now, for giggles, try the various permutations with the quarters and the other coins.

    The size of the three coins is the functional equivalent of the power of the amplifier.
    The size of the first coin - the one flicked - is the equivalent of the voltage delivered to the amp.

    A very powerful amplifier will make good volume without the need for transient input voltages in some cases. So, my 225-watt RMS/Channel amp won't start to clip until the very loudest passages, and will not clip even whebh fed massive transients. Flicking a dime into the quarters. My little 35-watt tube amp is an entirely different story. Flicking a quarter into the dimes.

    I will stop here, or, I will get snarky.
    avatar
    corndog71

    Posts : 661
    Join date : 2013-03-19
    Location : It can get windy here

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

    Post by corndog71 on Thu Jul 19, 2018 12:07 pm

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

    Sponsored content

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

    Post by Sponsored content


      Current date/time is Thu Aug 16, 2018 3:04 am