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Tube preamp distortion - mountain or molehill? An example

Chazz6

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A tube preamp feeds my Class A power amplifier. Does the preamp add audible distortion? It's a Van Alstine Transcendence RB 10. I haven't found distortion measurements for it, but I did for their FET Valve CF preamp, which came out about five years earlier than the RB 10. A review reported what appear to be careful measurements:
  • For a 1 kHz sine wave at 2V RMS, THD+N measured 0.00381% with the first harmonic being 90 dB below the test tone. Bumping the output to 5 Volts (14 dBV) RMS, THD+N rose to 0.00862% and the first harmonic was 83 dB below the fundamental.
  • The Intermodulation Distortion test was done with signals at 60 Hz and 7 kHz. The measured IMD at 6 dBV (2 Volts) came to 0.0028%.
  • The response is within 0.25 dB of flat from DC to 20 kHz. From there to 90 kHz the response is 1.4 dB below the 1 kHz level.
  • https://hometheaterhifi.com/reviews...fet-valve-cf-vacuum-tube-preamplifier-review/
It is only one preamp, but a single counterexample rebuts blanket statements that distortion reduces the fidelity of the sound. Audibly? Nope.
 
If distortion is at the levels of those measurements, not a problem. I don't know what blanket statements you have in mind. Sounds like a straw man to me. I don't know of anyone saying distortion always audibly reduced fidelity. At some level obviously it does. At a measurement level it does though it may not be audible. Then again why a tube preamp if it sounds same as any other high fidelity preamp? Looks, appreciation for the circuitry or whatever which is fine.
 
...I don't know what blanket statements you have in mind. Sounds like a straw man to me. I don't know of anyone saying distortion always audibly reduced fidelity.

Slightly paraphrasing a typical remark on ASR: "You add distortion back into the signal chain with a tube pre? Seems a waste of money to spend on the fidelity of a fine DAC only to effectively then negate said fidelity."
 
Slightly paraphrasing a typical remark on ASR: "You add distortion back into the signal chain with a tube pre? Seems a waste of money to spend on the fidelity of a fine DAC only to effectively then negate said fidelity."
It's a rational thought about tubes if all you look at is fidelity. Do they add audible distortion? Then it's lower fidelity and runs against the ethos of the site. Does it NOT add audible distortion? Then tubes are just an inefficient / expensive way to achieve good fidelity.

If you like tubes because of how they look or some other subjective reason, more power to you, and it's nice that they don't degrade the signal.
 
... Does it NOT add audible distortion? Then tubes are just an inefficient / expensive way to achieve good fidelity.
Why ignore various audio-related reasons for a preamp, which might be tube or solid state so long as it does not add audible distortion? For example, to have a volume control, or to have extra inputs for the occasional moment one wants to play vinyl even though 99% of the time the input is from a DAC, or to have a balance control.
 
Why ignore various audio-related reasons for a preamp, which might be tube or solid state so long as it does not add audible distortion? For example, to have a volume control, or to have extra inputs for the occasional moment one wants to play vinyl even though 99% of the time the input is from a DAC, or to have a balance control.
Well, in general I think people are encouraged to consider I/O and usability, build quality, etc. However, all else held equal, a tube amp is usually not the top price/performance option for a given set of features.
 
If you use a tube (pre)amp today, it's because the harmonic distortion they create. Otherwise there is no point in using them, and you better use more modern (opamp or transistor) designs that are much more neutral in sound, cheaper, easier to manage/maintain and more efficient with power.

Both choices are legit for me, as long as you know why and how. I have several setups, and some are set up to sound clean low distortion (dsp, class D amps, ...), but an other is full tube powered, just for the harmonic distortion that colours the sound that i love. Both have their merits, but claiming that tubes are the ultimate "hifi" sound is bulls**t. They are not, they are a very coloured and inefficient way of amping speakers, and should only be used for their harmonic distortion i think.
 
...modern (opamp or transistor) designs that are much more neutral in sound, cheaper, easier to manage/maintain and more efficient with power. (emphasis added)
Three of the four differences you list are not at issue. But given the distortion measurements I quoted, it seems a tube preamp is not necessarily much less neutral in sound than a solid state preamp.
 
Slightly paraphrasing a typical remark on ASR: "You add distortion back into the signal chain with a tube pre? Seems a waste of money to spend on the fidelity of a fine DAC only to effectively then negate said fidelity."
But you have to understand. It's not about fidelity; it's about BIG TONE! Some listeners want that slathered onto everything they listen to. I say if you like that, then fine. However, you don't get get to brag about fidelity when discussing it.
 
That's a well measuring valve pre amp, while it's 20-25db off the best solid state can do the distortions it adds will be low in level and should be barely noticeable.

So on that basis it should be a lot less valve sounding than other amps.

No reason why it shouldn't sound fine
 
It is only one preamp, but a single counterexample rebuts blanket statements that distortion reduces the fidelity of the sound. Audibly? Nope.

Distortion reduces signal fidelity. Even the best of the best adds distortion there is no way around that simply because of non linearity.

Whether or not distortion becomes audible depends on many factors and circuit dependent (regardless what devices are used for amplification)
In some cases 0.01% could be audible in other cases well over several percent.
There isn't a single number that can tell when 'distortion' becomes audible and to whom under what circumstances.

Even when it becomes audible it may not even be sound quality degrading.

A pre-amp can be handy as input/output selector, quality volume control, balance or tone control, phono pre-amp (with additional functionality), to allow low output voltage devices to be used (tape/cassette/tuner etc.), to drive mono blocks or power amps close to the speakers etc.

When the functionality is great and looks as well as price point all is fine.

Only when owners or reviewers start to declare it sounds 'better' then all the alarm-bells go off on ASR and DBT and measurements are required to prove that.
 
A tube preamp feeds my Class A power amplifier. Does the preamp add audible distortion? It's a Van Alstine Transcendence RB 10. I haven't found distortion measurements for it, but I did for their FET Valve CF preamp, which came out about five years earlier than the RB 10. A review reported what appear to be careful measurements:
  • For a 1 kHz sine wave at 2V RMS, THD+N measured 0.00381% with the first harmonic being 90 dB below the test tone. Bumping the output to 5 Volts (14 dBV) RMS, THD+N rose to 0.00862% and the first harmonic was 83 dB below the fundamental.
  • The Intermodulation Distortion test was done with signals at 60 Hz and 7 kHz. The measured IMD at 6 dBV (2 Volts) came to 0.0028%.
  • The response is within 0.25 dB of flat from DC to 20 kHz. From there to 90 kHz the response is 1.4 dB below the 1 kHz level.
  • https://hometheaterhifi.com/reviews...fet-valve-cf-vacuum-tube-preamplifier-review/
It is only one preamp, but a single counterexample rebuts blanket statements that distortion reduces the fidelity of the sound. Audibly? Nope.
I've looked at the technical part of the review. Nothing specific on noise (which is more important than distortion) other than THD+N at 1kHz. I'd like to see the noise spectrum and multitone since those are more challenging tests.

BUT, I've seen multiple discussions on ASR that concluded a well-designed tube PREAMPLIFIER, could be indistinguishable from a well-designed solid state preamplifier on level-matched blind tests. Where tube designs struggle is in power amplifiers, where transformers and output impedance can cause audible issues.
 
That's how an AR tube pre measures at about unity gain with input signal at -3dB and at -15dB (about where music bangs) :

1735728608807.png

-3dB


1735728639578.png

-15dB

Good luck if you can listen to distortion down there.
 
Interesting. Probably inaudible, but the -3dB result is surprisingly uncompetitive.
Probably cause input sensitivity is at the 300mV -ish range,the whole Volt E-MU feeds it is 3 times as much at -3dB.
I'm surprised that is even at seventies,others don't tolerate as much without severe results.Luckily music bangs at 50-150mV at line level range.
 
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