Blind Test Results: Benchmark LA4 vs Conrad Johnson Tube Preamp

[MattHooper]

Hey folks, I thought I'd post about a little blind test I did between my two current preamplifiers:

Benchmark LA4

vs.

Conrad Johnson Premier 16LS2 tube preamplifier.


People are familiar with the Benchmark LA4 I'm sure:

Benchmark LA4 Line Amplifier

The award-winning Benchmark LA4 may be the most transparent line-level preamplifier available at any price. The 100% analog signal path features a pair of fully-balanced relay-controlled attenuators with 256 steps and 0.5 dB resolution. Designed, assembled, and tested in the USA using USA and...

benchmarkmedia.com

A brief bit of info on the CJ Premier 16LS:

https://conradjohnson.com/owners-manuals/prem16man.pdf

I'm certainly not presenting this as worthy for publishing scientifically; just giving a little report of my own modest attempt to do a blind shoot-out of the two pre-amps. All comments welcome of course! It arises out of my own subjective impressions. I've always felt I preferred tube amplification, tube preamps as well, but after borrowing a friend's Bryston 4B3 power amp for a couple of months, comparing it (non-blinded) to my CJ Premier 12 tube monoblocks, I felt I was hearing some things I liked about the solid state amplification, even though I preferred the tube amps overall. So I thought I'd try my luck with some solid state preamplification and purchased a Benchmark LA4. I have been going back and forth between the LA4 and CJ preamp in my system and have had great results with the LA4 (details below), though when I put the CJ preamp in...dang!...hard to part with the qualities I perceive there.

On to the test.

I'd talked to Benchmark about running the CJ preamp through one of the LA4 inputs to be able to do quick switching between the LA4 driving my amps directly, vs the signal going through the CJ tubed preamp in the chain. I was told it would work fine: on their suggestion I ran my Benchmark DAC2L outputs to both the LA4 preamp and my CJ preamp, and then ran the outputs of the CJ preamp in to one of the inputs on the LA4. Then I set that LA4 input at unity gain ("0" fixed). Once I've selected the CJ input on the LA4, I adjust the volume via the CJ remote control to match the output of the LA4 on it's own (switching between inputs).

For quite a while I've used this just to switch between each preamp to listen for differences. The characteristic differences I heard seemed to mirror when I'd run the CJ directly in to the amp vs the LA4 directly in to the amp. That's been made for very easy comparisons over the past little while. But I wanted to finally do it "blinded."

I first watched Amirm's video on blind testing again, just to refresh my memory and get tips I may have forgotten.

METHOD:

Music streamed from Raspberry Pi Squeezebox server to Benchmark DAC2L.
Benchmark DAC2L XLR outputs to LA4 XLR input 2.
Benchmark DAC2L RCA outputs in to CJ Preamp.
CJ preamp output run to RCA analog input "4" of LA4.
Benchmark LA4 preamp run directly in to my Conrad Johnson Premier 12 tube monoblock amps (out to Thiel 2.7 speakers).

LEVEL MATCHING: I had borrowed a voltmeter from a friend to match the signal at the speaker terminals but it didn't seem to work right (I'll put that in a separate post). Therefore, I ended up first using both the dB pro app and the NIOSH SLM decibel measuring app, on my iPhone 13, positioned at the listening seat, using a 110Hz test tone. Using the LA4 and CJ separate volume controls I was able to get exactly the same decibel reading for both signals. Afterward I checked this again using a SCOSCHE spl1000 Decibel meter. It turned out the SCHOSCHE read the signals as louder than the iPhone reading, but nonetheless the relative decibel reading on the SCOSHE between the Benchmark and CJ signals was bang on, just like with the iphone reading. Subjectively the levels seemed indistinguishable. (As someone who adjusts volume levels minutely all day for my job in sound editing I have a good ear for minute differences...not that it makes me infallible of course).

My son was doing the switching between the LA4 inputs. We were doing 2 test runs of 15 trials. "Forced choice" method, so I was to write my guess as to whether it was the LA4 playing or the CJ. We had two pages with columns of numbers 1 - 15, one for my son for switching, one for me to write my guesses. My son used an on-line random number generator to create the switching pattern for the two trials, writing down "L" for when he was to switch to the LA4 direct, "C" for the CJ preamp input. I would write "L" or "C" for my guesses, on my page, and we'd compare after.

I deliberately put the LA4 direct signal on input 2, the CJ signal on input 4, to leave input 3 blank. That allowed my son to always first switch to the "blank/silent" input first before selecting the next input, so that there would be no "tell" in the switching for me. All the source/amplification is in a separate room down the hall from my listening room, so I couldn't see or hear anything he was doing. The only communication, once the tests began, was me yelling "switch" when I wanted him to switch.

Whew! Sorry, boring stuff but this is the type of forum where people may care about such details.

So the test was to see if I could discern which preamp I was listening to - the solid state LA4 or the CJ tube preamp in the chain.

RESULTS:

Trial 1: 15/15 correct.
Trial 2: 14/15 correct.

(On trial two I'd initially guessed wrong, but immediately recognized the differences again between the preamps once the switching continued).

 

[MattHooper]

Discussion:

Essentially I seemed to perceive the same sonic differences between the LA4 and the CJ preamp as I did during my sighted tests.
The LA4 more transparent, precise and dynamic; the CJ preamp richer, more relaxed, more "body" to the sound.

An example: A piece of music with an acoustic guitar picking and strumming, bass player, simple drum part, electric guitar doing occasional chords in the background.

The LA4 sounded more "transparent" on such material. The exact tonal differences in the drum cymbals, snare, timbre of the acoustic and electric guitar were separated and untangled, more easy to hear, as were even the slightest differences in reverb applied to each instrument. The LA4 also seemed to have greater dynamic impressions, the differences in force from the drummer, even the dynamics of the acoustic guitar strumming and picking seemed more obvious.
Also, the bass seems both more tight and dynamic - easier to hear the precision with which a bass player is playing for instance.


In comparison, sound through the CJ tube preamp sounded a bit more homogonized, a bit more 'blended together.' However it sounded richer, more full bodied, more "relaxed" both dynamically (a bit like the drummer was more laid back) and in terms of transients and upper frequencies, which felt a bit more thickened, slightly softer and 'easy on the ears.' There was also that "golden glow," this sort of slight altering of the entire timbre of the presentation "lightening" it up, making it feel more "airy" with an upper midrange/lower treble frequency texture that made instruments sound a bit more 'present' in the room. It's a weird thing because it simultaneously gives the impression of heightened upper frequencies for more "it's there" vividness yet at the same time more relaxed and easy on the ears! The bass takes a small hit in tightness, getting a bit bigger and rounder, but depending on the source material, the Benchmark's bass can sound "too much" and the CJs more laid back, or visa versa.

I used Mark Morrison’s R&B track Return Of The Mac for the tests, and it displayed those essential differences
between the preamps.


I really like both presentations for their strengths. Ultimately I have to say the tube sound still grabs me more. "Density" is something I'm always searching for. The impression of something solid and moving air, as much as can be asked for in the stereo illusion. I get more of that with the tube preamp. With the Benchmark LA4 preamp the sonic impression was like "seeing through musical objects with great clarity, almost like X-rays." With the CJ it was a bit more like like "solid objects, snares, voices, guitars, dense and occupying space, not see-through."


That combined with the way the CJ tube preamp seems to both increase the sense of sparkle and presence and texture, a more vivid and slightly to my ears "more real" presentation tonally, while at the same time relaxing and enriching the sound that makes it so easy to crank the volume...it's a hard combination of characteristics that check my boxes for me to give up.

 

[dougi]

Nice work. I see the CJ inverts polarity. Was that corrected or left as is? The CJ has a fair bit of harmonic distortion (at high voltage levels) so I assume that it must be pleasant in this case. What the input level into the CJ? About+23dBu for 0dBFS as per DAC2 bypass which is about 10V less? I see from the Stereophile measurements of the CJ that the HF rolls off at high volume settings. What was the CJ approx setting?

 

[MattHooper]

Thanks dougi. Not sure I'll give the the answers you need but:

I don't bother about the CJ amp switching polarity. As I understand it, that's rarely audible, and "polarity switches" can often be embedded in different source material, so I wouldn't even know "which polarity" to use for any particular song including the one I used.

Not sure if this helps for your other questions but:

For an 82 dB reading of the 110Hz test tone measured on the SCOSCHE spl1000 Decibel meter from each source, the LA4 preamp volume was set at -30.

When switched to the CJ's input signal, the CJ preamp's volume was at 21 (to match levels).

 

[Shazb0t]

Why weren't you able to level match properly with a volt meter?

 

[MattHooper]

Thanks for reminding me Shazb0t!

As suggested by Amir in the video (and by my pal showing me how to work his voltmeter - I'm frankly a moron when it comes to electricity) I set the meter to measure AC volts. On this meter there was only a selection between "750" and "200," my friend suggesting the 200 setting. Well...it just didn't seem to work very well. Using both music and the test tone, at the comfortable listening level I wanted for the test, I tried measuring at the speaker terminals. I seemed to get barely any reading! First I'd just get 00.0

Then if I turned the volume up further I'd get 00.1 then at best 00.2 and that was pretty loud (no matter where I placed the measuring wires on the speaker terminals or on the open speaker wire ends themselves). That just didn't seem to be registering the voltage in a fine enough manner to feel confident I was matching audible levels. So...please treat me like the electronic moron I am and suggest what the problem could have been.
(My friend who owns it thought maybe this particular meter wasn't sensitive enough for such low voltage, perhaps designed more for measuring
AC outlets etc?)

 

[Blumlein 88]

I'd like to hear more about the problems matching with a voltmeter as well.

Thanks for reminding me Shazb0t!

As suggested by Amir in the video (and by my pal showing me how to work his voltmeter - I'm frankly a moron when it comes to electricity) I set the meter to measure AC volts. On this meter there was only a selection between "750" and "200," my friend suggesting the 200 setting. Well...it just didn't seem to work very well. Using both music and the test tone, at the comfortable listening level I wanted for the test, I tried measuring at the speaker terminals. I seemed to get barely any reading! First I'd just get 00.0

Then if I turned it up further I'd get 00.1 then at best 00.2 and that was pretty loud (no matter where I placed the measuring wires on the speaker terminals or on the open speaker wire ends themselves). That just didn't seem to be registering the voltage in a fine enough manner to feel confident I was matching audible levels. So...please treat me like the electronic moron I am and suggest what the problem could have been.
(My friend who owns it thought maybe this particular meter wasn't sensitive enough for such low voltage, perhaps designed more for measuring
AC outlets etc?)

Well yeah if it was a 200 volt scale it might not have read much. You needed a voltage setting with a smaller scale.

As to the rest if I follow your test then when listening to the CJ it was actually the CJ going thru the Benchmark preamp right? So any differences you hear would be colorations added by the CJ. Also what was the level setting for the DAC feeding the C-J? I'd worry it might have been enough to somewhat overload the CJ by having too much input voltage. So even though you reduced it with the volume of the CJ output it may have altered distortion levels prior to reaching the CJ volume control.

 

[MattHooper]

Well yeah if it was a 200 volt scale it might not have read much. You needed a voltage setting with a smaller scale.

Thanks. It seems I'm constrained via this particular meter to those two options - 750 or 200 - when set to measure AC voltage.

As to the rest if I follow your test then when listening to the CJ it was actually the CJ going thru the Benchmark preamp right? So any differences you hear would be colorations added by the CJ.

Exactly.

Also what was the level setting for the DAC feeding the C-J? I'd worry it might have been enough to somewhat overload the CJ by having too much input voltage. So even though you reduced it with the volume of the CJ output it may have altered distortion levels prior to reaching the CJ volume control.

I had gone through the set-up of the DAC2L in my system (both for use with the CJ preamp and the LA4) in detail with Rory of Benchmark, to the point of opening up the DAC2L to ensure all the jumpers were in the right position for output levels. It's all set up correctly for the outputs to the CJ preamp and LA4 (for both XLRs and RCAs).

 

[HarmonicTHD]

Just looking at it from remotely the level matching by SPL meter might be the biggest uncertainty. I use a voltmeter which can do also higher frequencies (up to 10kHz if I remember correctly) and was about 150USD from Mouser or even my oscilloscope (ca 400USD). You might consider such an invest to completely eliminate any uncertainty in your test setup if you spent the time anyhow. Also not too much money given what we usually invest in our gear, me thinks.

If I haven’t screwed up my math here on the fly. A difference from about 0.05V can already lead to 0.3dB in SPL change (depending on speaker sensitivity etc). So yes it requires accurate level matching.

Great job though and thanks for sharing.

 

[DSJR]

What is it about some audiophiles wanting a more homogenised, nicer and thicker-textured sound presentation?

Live music doesn't do this to me at least - it's immediate, strings sound 'rosinous,' trumpets can hurt, saxes make you jump with the intensity and drums will pin you to the wall behind (this on music genres I don't always listen to). To me if nobody else, the CJ is NOT a 'high fidelity' component if it's making the signals passing through sound 'nice.' Some passive speakers do enough of this as it is I found some ARC preamps did this too - wonderful tonal 'textures' and a wide deep soundtsage which was actually altered from the source going in, so thereby a multi-thousand pound/dollar 'processor.'

Anyway, many thanks for the trouble taken to do the comparison and report. It confirms I wasn't wrong to go in the direction I made thirty years back and deeply regret falling off that particular path in recent times as it's going to cost to get back 'on the wagon' so to speak...

 

[charleski]

Stereophile's measurements of the 16LS from 2001 can be found here:

Conrad-Johnson Premier 16LS preamplifier Measurements

Sidebar 3: Measurements Measurement-wise, Conrad-Johnson's Premier 16LS was very similar to C-J's Premier 17LS, which Brian Damkroger reviewed for Stereophile in May, in that many of its "static" parameters varied with the setting of the volume control. Fortunately, because of the very high...

www.stereophile.com

The distortion in the 16LS output varies a lot depending on the input impedance it's seeing, but I'd assume the LA4 has a fairly high impedance, though this isn't listed on their website.

 

[whazzup]

Question:
Has it been established elsewhere that both preamps have a linear frequency response? Is there one with a earlier hf roll off etc?

I'm just curious whether level matching at only one frequency is sufficient.

 

[Cote Dazur]

Hey folks, I thought I'd post about a little blind test I did between my two current preamplifiers:

Thank you for taking the time and efforts to do this test, this is the kind of post I thought would happen often here instead of hardly ever. rant: Instead most I get is constant purely subjective opinions vaguely based on some measurements graphic interpretations rant off.
What would still be interesting in this thread, is a post from one of the measurements specialist, with measurements graphics explanation on the performance difference from this 2 units.
Of course that you can spot "blindly" which is which between a tube and a solid state preamp is not a surprise, but still nice that it is confirmed.
Here is to hope more members, will emulate you, as without blind testing, subjective impression are pretty much useless and measuring alone, without a controlled subjective evaluation, is not much better.

 

[pkane]

Discussion:

Essentially I seemed to perceive the same sonic differences between the LA4 and the CJ preamp as I did during my sighted tests.
The LA4 more transparent, precise and dynamic; the CJ preamp richer, more relaxed, more "body" to the sound.

20+ years ago I tried a few high-end CJ preamps that a local dealer let me borrow against no preamp (a DAC with a digital volume control). I could also tell the difference, including in a blind test with an audiophile buddy helping me out. In the audiophile parlance, CJ preamps sounded veiled to me, while no-preamp sounded natural and transparent. I felt like the sound became muddier, thicker, denser with CJ. I've not used a preamp since.

 

[Gorgonzola]

Nice work indeed, @MattHooper, and thanks.

That the sonic difference between these two preamp should be audible is, of course, what any dyed-in-the-wool subjectivist audiophile would expect. The nature of the differences would also be predicted by audiophiles. The "richer, more full bodied, more 'relaxed'" quality of the CJ would be fully predictable; likewise the greater transparency of the Benchmark. (On another forum I've been hearing from a subjectivist person who has had both CJ and Benchmark preamps whose impression are exactly these.)

For my part the the differences are predicted by measurements: thanks, @charleski, for the link to JA's CJ measurements. There the salient point is the very high 2nd order harmonic. What is also comment-worthy are the relatively low higher order harmonics beginning, maybe surprisingly, with the 3rd order.

 

[raindance]

Question: why would you use 110Hz for level matching? I'd use pink noise. Just curious.

I also don't understand how a sound can be laid back and easy on the ears and, perhaps, veiled, but also be sparkly at the same time. Once again, simply curious as to the descriptions.

Many years ago I did some detailed analysis on the PV-12 preamp and found that it was designed for 12AX7 tubes and basically CJ charged the tubes to 12AU7 without changing the operating parameters (probably to obtain a lower gain because a 12AX7 would require tons and tons of negative feedback and possibly a voltage divider to bring the gain down enough to use it with a 2 volt source). This resulted in the 12AU7 tubes running in a very non linear portion of their transconductance curve (significantly under-biased) and was very likely the reason the preamp sounded so ...... tubey. I'd love to see one of those measured to see the spray of harmonics resulting.

 

[pma]

Question: why would you use 110Hz for level matching? I'd use pink noise. Just curious.

You are right. 110Hz is not a good choice, pink noise would be much better.

 

[Blumlein 88]

Actually I think pink noise would be a good signal to do the blind test with.

 

[pma]

Actually I think pink noise would be a good signal to do the blind test with.

Definitely more revealing than music, but this would be hardly admitted by audiophiles. I like white noise even more to find the differences.

 

[rebbiputzmaker]

Cool project. Might be a bit difficult for some to discuss because they might not want to try and understand that tubes can sound nice and can be enjoyed by audiophiles without guilt. The CJ stuff is not necessarily the most revealing but it can be pleasant sounding. I think an issue is possibly the more one has the opportunity to listen to live music the more one might appreciate what the tubes are doing. It is difficult to impossible to necessarily accurately record the true harmonic presentation of music, versus the sound in a live space. every sound has accompanying harmonics, which are natural and familiar to the ears. Any alterations can affect what we hear but the tendency of tubes to add more lower and often more even harmonics is just a bit more pleasingly natural. Doesn’t mean it’s more accurate but just more of something we are accustomed to, and hence some may gravitate towards it. It can even get more complicated during the recording process. Allot to go into but just maybe a not perfect but simple example. If you take say a Gibson L5 and listen to it, depending on the amp of course, live versus recorded. Do you close mic the amp do you close mic the guitar do you plug directly into a board? These are at the end of the day all going to affect the outcome of the sound, and up to the engineer to make it sound the way he would like. This is the reason some of the most natural sounding classic recordings were done with very simple microphone configurations. Probably the cleanest playback system will give you the most accurate presentation of the recording , but if a tube amp preamp etc. adds a little more naturalness to the individual listeners ear it can sometimes be pleasing. And that can be an attraction.

No matter how good a recording is it is never going to be the same as live. As a kid I used to love going to Astoria Queens and the Steinway showroom. Pianos definitely sound different but when recorded it is often impossible to tell.