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Tubes and Digital Audio


twk123

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I noticed a discussion pop up in some other threads like the Vintage Tube Amp Restoration and instead of derailing anything I figured to start a new topic. The discussion is that of the role of tube amplification in regards to digital music and mixing tube power with SS power. I personally have a tube in my DAC and like how it sounds and definitely could tell the difference when I upgraded the tube to my current Amperex 7308. That being said, this is my first DAC so I dont know if I could achieve just as good sound with a 'normal' DAC and SS amps as I use my Denon receiver for 2 Channel as well as HT. 

 

So the questions I have are:

 

Do tubes 'color' the signal in a way that makes digital music warmer/smoother or more like analog and is this just distracting from good source material and chipset?

 

Is there a significant difference between the sound of tube amps vs SS and is is difference under linear operation or only in clipping? In that case would a single tube buffer make any difference if SS amps are used vs all tube gear?

 

Can tube amplification work well with a digital signal vs analog? Im my opinion as long as the digital files are lossless or near lossless there should not be a difference as vinyl is an imperfect medium anyway and electrical info from a DAC or a turntable are both 'analog' signals.

Edited by twk123
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Did you see Maynard's and the new guy's responses in the other thread?  They were pretty spot on.

 

Do tubes 'color' the signal in a way that makes digital music warmer/smoother or more like analog and is this just distracting from good source material and chipset?
 

 

The digital storage medium is not a limiting factor.  The quality of the production all too often leaves a lot to be desired.  Warm to me is spectral balance, smooth is a bit harder to quantify, but I do tend to prefer tubes for this.

 

would a single tube buffer make any difference if SS amps are used vs all tube gear?

 

The tube buffer may or may not make an audible difference, it depends.  A tube amp coupled to a speaker, however, will more than likely result in an audible difference.

 

My own general use:

Boring digital front end ---> SE tubes ---> Klipsch  :emotion-21:   

Boring digital front end ---> boring ss amp ---> Klipsch  :emotion-21:    

vinyl --->   well, I just fail to see the charms of vinyl.

Edited by Ski Bum
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Except for the depth of and breadth of sonic information from vinyl that even my 34 year old nephew can hear yes vinyl has no charm.  Yes tubes color the sound.  everything colors the sound and its all slightly different.  If  you like what you have be happy with it. 

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Some answers here: https://community.klipsch.com/index.php?/topic/131163-corner-horn-imaging-faq/?p=1464501

 

Do tubes 'color' the signal in a way that makes digital music warmer/smoother or more like analog and is this just distracting from good source material and chipset?

 

They CAN add a reverb effect if the output impedance of the tube amplifier is too high relative to the input impedance of the loudspeakers.  They can also "color" the sound if they don't have enough output power to keep from clipping--and there are a lot of very low power tube amps that have this issue when connected to Khorns or Reference loudspeakers when playing at or above concert volume.

 

Is there a significant difference between the sound of tube amps vs SS and is is difference under linear operation or only in clipping? In that case would a single tube buffer make any difference if SS amps are used vs all tube gear?

 

SET-type tubes will clip producing even-order harmonics (and a LOT of modulation distortion because of clipping).  Push-pull tube amps will clip producing mostly odd-order harmonics, and so will SS amps, but junction-transistor-type SS amps will usually produce higher-order odd harmonics due to use of moderate amounts of feedback (if you want to know more about this, I can provide a link).  Tube amps with high output impedance will exhibit "tube magic" sound, otherwise known as the reverb effect--adding an artificial depth of soundstage to the loudspeaker presentation in smaller rooms, just like planar dipole loudspeakers produce--emphasis on "artificial depth of soundstage".

 

FET SS amps can mimic tube amplifiers in terms of lower harmonic distortion production (see Nelson Pass' First Watt amplifiers). Clipping isn't at all desirable at any time.  Harmonics are produced by all amplifiers, it's the % amplitude of harmonics (vs. output power) and the order of the harmonics that sound different.

 

Can tube amplification work well with a digital signal vs analog?

 

Absolutely, but you must have enough power to prevent clipping, and you don't want the output impedance of the amplifier to be higher than about 0.5-1 ohm if you want to avoid the reverb effect.

 

YMMV.

 

Chris

Edited by Chris A
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Tube amps with high output impedance will exhibit "tube magic" sound, otherwise known as the reverb effect--adding an artificial depth of soundstage to the loudspeaker presentation in smaller rooms, just like planar dipole loudspeakers produce--emphasis on "artificial depth of soundstage".

 

 

 

They CAN add a reverb effect if the output impedance of the tube amplifier is too high relative to the input impedance of the loudspeakers.

 

Note: Chris and I have had some discussion on this topic before.

 

 

I have to state that I have very serious doubts about the claims of the "reverb effect" being audible and to date I haven't seen any actual Test Data that would make me believe it reaches audible levels..!!! 

 

I would also point out based on my understanding of the proposed "reverb effect" that Bob Carver claims to occur with high output impedance amplifiers the amplifier would also have to be designed with feedback regardless of output impedance before the amplifier could produce this effect and since Single Ended Tube amplifiers often are designed with no global feedback I don't see how the claimed "Reverb Effect" could be used to explain their sound qualities.

 

 

If anyone has actual test data showing the reverb effect I would very much appreciate them showing or linking to it please.

 

Thanks,

miketn

Edited by mikebse2a3
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See https://community.klipsch.com/index.php?/topic/131163-corner-horn-imaging-faq/page-5#entry1674485

 

The following is from http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm

 

 

Speaker-Microphone Effect

It is well known that a loudspeaker can work backwards, converting sound pressure into an electrical signal at its input terminals. Many years ago, it was Roger West who mentioned having used a Sound Lab ESL as a microphone, though I'm not sure with what degree of success. Carver believes that his amps can take advantage of this effect and says that:

 

"The extra cool thing about this amp is that it has the ability to listen to the room via the principal of reciprocity. The loudspeaker the amp is connected to behaves like a microphone and "listens to the room." That signal is then sent back to the input to be mixed with the forward signal to help generate a more delicious sense of acoustic space by including the room in the final sound in a way other amps can't."

 

I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows:

 

"You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts rms. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it."

 

"As low as it is. All tubes amps do this a little bit by their nature, transistor amps don't do it at all, and this amp does it a lot by intentional design. It's easy to hear the room signal if we hook up a small speaker on the end of about 150 feet of wire and go outside. Hook it across the main speaker in your living room with the amp turned on, but no input. Then have a friend come into your living room and clap his or her hands, stomp around, make noise and sing happy birthday. While you are 150 feet away outdoors with the door closed so you can't hear your friend from the room, hold the small test speaker in your hand and listen to it. You will hear the sound of the room. With a solid state amp, you will not hear a thing, just silence. All tube amps that have a non-zero output source impedance do it, at least a little bit as you surmised. Transistor amps don't do it at all because the almost zero source impedance shorts out any "speaker microphone" signal. Yes, my amps do it a lot by virtue of the current feedback loop. With this amp, lower idle current (as read on the meter), makes the amp do it more. Higher idle current makes it do it less."

 

Horn-loaded loudspeakers--like Klipsch that are more than 100x more efficient than "89 dB sensitivity" loudspeakers--so the speaker-microphone effect quoted above will be 20 dB higher...in the -20 dB range rather than the -40 dB range as quoted. 

 

This is very audible and is a reason why many people like tube amplifiers driving extremely efficient loudspeakers in small listening rooms. 

 

The problem is: this "reverb" isn't in the source input signal, so it is artificially injected into the sound in the room.

 

Chris

Edited by Chris A
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Did you see Maynard's and the new guy's responses in the other thread? They were pretty spot on.

 

Thanks for the heads up I went back through the thread and found them, definitely some good info. It sounds like the tube has its best advantages when amplifying a signal to a large extent such as a turntable. I really like the way my system currently sounds but I am always interested in the mechanics behind sound reproduction.

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So Chris based on what you just quoted do you agree that this "microphone/reverb effect" requires that the amplifier be a design using feedback..?

Chris if you agree then isn't it a fact that a high output impedance Single Ended Tube amplifier without global feedback could not have this distortion by the "microphone/reverb effect"....and thus Carver's claim that all tube amplifiers exhibit this to some degree is false..?

Miketn

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See https://community.klipsch.com/index.php?/topic/131163-corner-horn-imaging-faq/page-5#entry1674485

 

The following is from http://www.enjoythemusic.com/superioraudio/equipment/0512/bob_carver_cherry_180_tube_monoblock_amplifier.htm

 

 

Speaker-Microphone Effect

It is well known that a loudspeaker can work backwards, converting sound pressure into an electrical signal at its input terminals. Many years ago, it was Roger West who mentioned having used a Sound Lab ESL as a microphone, though I'm not sure with what degree of success. Carver believes that his amps can take advantage of this effect and says that:

 

"The extra cool thing about this amp is that it has the ability to listen to the room via the principal of reciprocity. The loudspeaker the amp is connected to behaves like a microphone and "listens to the room." That signal is then sent back to the input to be mixed with the forward signal to help generate a more delicious sense of acoustic space by including the room in the final sound in a way other amps can't."

 

I expressed my doubts to Carver about the audibility of this effect, noting that the speaker microphone effect couldn't amount to more than a few millivolts. Carver's response was as follows:

 

"You are right - about 3.1 mV at 90 dB SPL at our listening position, then multiplied by the gain of the amp, 32 X = 0.0992 volts rms. With a high quality speaker about 89dB or so sensitive at 14 feet away we have about 8.5 forward volts at the speaker, and 0.0992 "speaker microphone" volts. Finally, 0.00992/8.5 = -38.6 dB, just barely noticeable, but quite significant after we get used to listening to it."

 

"As low as it is. All tubes amps do this a little bit by their nature, transistor amps don't do it at all, and this amp does it a lot by intentional design. It's easy to hear the room signal if we hook up a small speaker on the end of about 150 feet of wire and go outside. Hook it across the main speaker in your living room with the amp turned on, but no input. Then have a friend come into your living room and clap his or her hands, stomp around, make noise and sing happy birthday. While you are 150 feet away outdoors with the door closed so you can't hear your friend from the room, hold the small test speaker in your hand and listen to it. You will hear the sound of the room. With a solid state amp, you will not hear a thing, just silence. All tube amps that have a non-zero output source impedance do it, at least a little bit as you surmised. Transistor amps don't do it at all because the almost zero source impedance shorts out any "speaker microphone" signal. Yes, my amps do it a lot by virtue of the current feedback loop. With this amp, lower idle current (as read on the meter), makes the amp do it more. Higher idle current makes it do it less."

 

Horn-loaded loudspeakers--like Klipsch that are more than 100x more efficient than "89 dB sensitivity" loudspeakers--so the speaker-microphone effect quoted above will be 20 dB higher...in the -20 dB range rather than the -40 dB range as quoted. 

 

This is very audible and is a reason why many people like tube amplifiers driving extremely efficient loudspeakers in small listening rooms. 

 

The problem is: this "reverb" isn't in the source input signal, so it is artificially injected into the sound in the room.

 

Chris

 

So at the end of the day, would it be better to put more diffusion in your listening room and run SS with enough headroom to avoid clipping? That way you get more acoustic liveliness but still a more pure source material?  Awesome write-up btw on the corner horn thread.

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Very interesting about tube amp feedback, there. Even though I was aware that speakers (at least typical moving-diaphragm speakers) and microphones (at least typical moving-diaphragm microphones!) work more or less the same way, I'd never considered how that could affect playback. It makes sense once it's put like that; very cool explanation.

Edited by chriswhotakesphotos
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So at the end of the day, would it be better to put more diffusion in your listening room and run SS with enough headroom to avoid clipping? That way you get more acoustic liveliness but still a more pure source material?  Awesome write-up btw on the corner horn thread.

 

Almost any SS amplifier will have enough power if it has at least 10 w/channel, RMS.  40 W/channel is needed for the lower efficiency Klipsch models such as the Reference line and the Heresy.  This is due to passive crossover use and the amount of power dissipation that goes into those passive networks in order to level the much higher efficiency midrange and tweeter horns/drivers with the inefficient woofers/bass bins.

 

So the issue is with low power tubes and chip amps, understanding that the measure of merit is playing source material at or slightly above concert level (assuming acoustic instrumentation) --we're talking 90 dB at the listening position or above for typical rooms and for the musical peaks. 

 

One thing that you didn't ask about: why do some SS amps sound flat and opaque?  The answer is higher-order harmonic distortion caused by moderate feedback levels in the amplifier design.  Either find a SS amp with little feedback  (a dB or so of forward loop gain reduction) or a whole of of feedback (greater than 30 dB of reduction).  Tube amplifiers (and the FET amps from Nelson Pass) that have low levels of feedback will not have higher order harmonic issues.  The audibility of higher order harmonics (e.g., 5th harmonic and above) is due to the human listening system and a phenomenon known as "masking".  It's the higher-order harmonics that can be heard most clearly and that cause the most dissatisfaction in power amplifier performance.

 

Chris

Edited by Chris A
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So at the end of the day, would it be better to put more diffusion in your listening room and run SS with enough headroom to avoid clipping? That way you get more acoustic liveliness but still a more pure source material?  Awesome write-up btw on the corner horn thread.

 

Almost any SS amplifier will have enough power if it has at least 10 w/channel, RMS.  40 W/channel is needed for the lower efficiency Klipsch models such as the Reference line and the Heresy.  This is due to passive crossover use and the amount of power dissipation that goes into those passive networks in order to level the much higher efficiency midrange and tweeter horns/drivers with the inefficient woofers/bass bins.

 

So the issue is with low power tubes and chip amps, understanding that the measure of merit is playing source material at or slightly above concert level (assuming acoustic instrumentation) --we're talking 90 dB at the listening position or above for typical rooms and for the musical peaks. 

 

One thing that you didn't ask about: why do some SS amps sound flat and opaque?  The answer is higher-order harmonic distortion caused by moderate feedback levels in the amplifier design.  Either find a SS amp with little feedback  (a dB or so of forward loop gain reduction) or a whole of of feedback (greater than 30 dB of reduction).  Tube amplifiers (and the FET amps from Nelson Pass) that have low levels of feedback will not have higher order harmonic issues.  The audibility of higher order harmonics (e.g., 5th harmonic and above) is due to the human listening system and a phenomenon known as "masking".  It's the higher-order harmonics that can be heard most clearly and that cause the most dissatisfaction in power amplifier performance.

 

Chris

 

 

So is there a way to look this up in the specs of an SS amp or does it have to be measured by the end user to determine the harmonics? For example, here are the specs on my Denon 3801, can you calculate the feedback based on the total harmonic distortion and the dynamic range etc?

 

Audio quality
  • Signal-to-noise ratio 74 dB
  • Dynamic range 96 dB
  • Frequency response 20 - 20 Hz
  • Total harmonic distortion 0.05%
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Unfortunately, you'll need an independent testing lab to give you that data.

 

Industry practices lump all the harmonics into "THD" so that they don't have to tell you how much of that is due to the individual harmonics, of which the higher order ones are at a much lower level than the low-order (2nd-4th) harmonics. 

 

Here is one such testing lab that posts its results of its client's equipment tests: http://milleraudioresearch.com/avtech/index.html  (You have to request a login to get a password to see their test database results.)

 

Chris

Edited by Chris A
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Here is a link to the Nelson Pass article describing feedback levels and relative levels of higher-order harmonic distortion, which is an excellent article:  https://community.klipsch.com/index.php?app=core&module=attach&section=attach&attach_id=73904

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Other topics include:

 

-Feed forward amplifier design

-Hysterisis, linearity, and the class A / capacitor bias connection

-A better input gain stage, none at all

-Digital audio, the logisitcs of sound reproduction

-Holograms and clocks, temporal symmetry between channels

-EQ and the air we breathe

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I would guess that Tom's point is that there are many other topics to consider just on the subject of amplifiers (...although he does seem to run off the subject toward the end of his list :huh:).  Seemingly, there are an unending number of sub-subjects to consider in any piece of gear. 

 

However, my focus has always been on system-level performance.  When you do that...and you have to first decide what is important in a sound reproduction system--for me it's the sound, followed closely by reliability and stability of the hardware performance...then it becomes a lot clearer what subjects to pay attention to. 

 

Loudspeaker and room acoustics performance almost always drift to the top of the list, followed closely by the quality of the source music that you're trying to play on your sound reproduction system. 

 

Only then, after those weak link topics are satisfactorily addressed--that amplifiers, preamps, DACs, and other source electronic devices seem to gain in relative importance, IMHO...and usually in that order of importance.  It's the big hitters that count and a lot of the time, it's the performance parameters that you can't find in the list of specifications at the back of the owner's manual that you're looking for ;)

 

Otherwise, you'll wind up buying cable elevators,  loudspeaker spikes, $10,000 turntables/cartridges, and magic rocks to tape to your cables--all in the pursuit of sonic perfection. 

Edited by Chris A
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-EQ and the air we breathe

 

Im glad you mentioned that. A while back a guy sold me a $600 wireless altimeter that automatically connects to my receiver and compensates the EQ for how thin/thick the atmosphere is is when running Audyssey, I cant tell the difference but he said the calibration really adds to the spaciousness of the music and the highs are much more airy... :unsure:

Edited by twk123
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vinyl ---> well, I just fail to see the charms of vinyl.

 

Don't see them either.  But damnsure hear them.  Charms of shellac and wax as well.  No original medium benefits from transcoding.  But more so than any other factor the original engineering is what matters. 

 

 

I prefer tube amplification for horns for a number of reasons that have been rehashed here for endless for over a decade.  But you can't fix crap.

 

Dave

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vinyl ---> well, I just fail to see the charms of vinyl.

 

Don't see them either.  But damnsure hear them.  Charms of shellac and wax as well.  No original medium benefits from transcoding.  But more so than any other factor the original engineering is what matters. 

 

 

I prefer tube amplification for horns for a number of reasons that have been rehashed here for endless for over a decade.  But you can't fix crap.

 

Dave

 

 

I have wondered this about buying new Vinyl. Most of my collection is handed down and old enough to be pre-digital and mastered from tape. However, new pressings usually come from a digital master so is there any advantage of a vinyl pressed from digital vs a high quality digital file from a digital master?

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