• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Blind Test Results: Benchmark LA4 vs Conrad Johnson Tube Preamp

xaxxon

Active Member
Joined
Apr 3, 2022
Messages
244
Likes
277
New to this thread. I find it interesting that we assume that the LA4 is essentially a wire and a switch. It is such a low distortion device, that’s probably correct. This was not a comparison of the Benchmark to the CJ, it was a test to see if one could hear the sound of the CJ being inserted into the chain, one preamp vs two in series. If the LA4 has any “sound” the other preamp’s sound was added to it. I doubt that using another switch would have changed the preference but that would have made the test more “pure”.

I used a tube preamp for about a decade. I sold it and bought an AVR. Soon after, I decided I wanted a dedicated two channel preamp again and bought a solid state one. No more need to have spares in the house and no more fooling around chasing line frequency hum. I know that it’s not a universal problem but an awful lot of tube gear has slightly audible 60hz noise that grates after a while.
LA4 goes to +15db. It's not a wire and a switch.

(at least my hpa4 does and I've been told la4 is the same without headphone amp)
 

rwortman

Addicted to Fun and Learning
Forum Donor
Joined
Jan 29, 2019
Messages
740
Likes
683
LA4 goes to +15db. It's not a wire and a switch.

(at least my hpa4 does and I've been told la4 is the same without headphone amp)
I know it isn’t. My point is that for the OP‘s experiment to be a comparison of the LA4 and the CJ preamp, whenever the CJ was the device under test, the LA4 would to be considered a wire and a switch because it was in the circuit too.
 
OP
MattHooper

MattHooper

Master Contributor
Forum Donor
Joined
Jan 27, 2019
Messages
7,194
Likes
11,806
why people would want to "hear" their volume knob and source selector has always been beyond me. If you want to introduce distortion somewhere then I guess that's up to you, but why would you want your volume knob to do that?

Ha! That's a question I've answered numerous times, not surprisingly given the general inclinations that attract folks to this forum.

Answering for myself: I like the aesthetics of tube amplification. I find my tube amps and preamp much more visually appealing than most solid state amplification. I also like the connection with the history of audio gear that comes with a tube amp. And I love the fact that the glowing tubes represent the actual musical signal being amplified. It is aesthetically and conceptually very satisfying, in a way yet another enclosed black or grey box doesn't satisfy.

As to adding coloration, to answer your question presumes one may find the tube preamp adds a pleasing distortion (as I seem to perceive).

The answer: If I like the sound, why not?

Why would it matter if the sound alteration was coming from the preamp, or amp, or speakers, or source, or an equalizer, or whatever?

The common reply is "well at least if you start off with neutral equipment, like a neutral amp/preamp, you can just add something else to change the sound to taste, be it some distortion-producing add on or an EQ to your taste. Then at least you have the option of turning off that coloration when you want to!"

Well...what if you don't care to turn it off? Then you are in the same situation whether you've arrived at an EQ setting you like or wherever else you put the "I Like This" coloration in your chain.

One reason why most people aren't adding some tube distortion thingy (device or plug-in) rather than an actual tube amp is because....where do you go for that?
There don't seem to be many easy to integrate ways of doing this. And I had a digital parametric EQ in my system for almost 20 years. For one thing I was never able, to my ears, to perfectly mimic the sound of my tube amplification using the EQ. For another, I just found I never needed it. I just liked the sound of my system. The tube coloration was a "set and forget" solution so I didn't have to bother fiddling with EQs or any other solution. I like pretty much everything I play on my system. No feeling of "needing EQ handy" like it seems some others need.

So if a tube pre-amp offers someone both aesthetic and conceptual pleasure a SS device doesn't, and offers a set-and-forget sound one likes...why fuss with other solutions that don't in the end offer all the things an actual tube amp does for an owner?
 

pkane

Master Contributor
Forum Donor
Joined
Aug 18, 2017
Messages
5,626
Likes
10,202
Location
North-East
One reason why most people aren't adding some tube distortion thingy (device or plug-in) rather than an actual tube amp is because....where do you go for that?

Where do I connect a tube preamp when listening to music on my smartphone? ;)

1669939949165.png
 
OP
MattHooper

MattHooper

Master Contributor
Forum Donor
Joined
Jan 27, 2019
Messages
7,194
Likes
11,806
Where do I connect a tube preamp when listening to music on my smartphone? ;)

View attachment 247229

Someone must have tried to provide such a product somewhere!

I can enjoy music on practically anything. Including on my meager smart-phone speakers. (In fact, I like the sound of my smart-phone speakers!). But I don't make the same demands. I can listen while doing other things. In order for me to be compelled to sit my butt down in front of a system and devote all my attention to just the music, I prefer that the system offer more than what I'll hear from my iphone or smart speaker or whatever. I want it to offer not just the music, but a greater sensuousness to the sound itself. Which can also enhance the listening experience.
 

Martini

Active Member
Forum Donor
Joined
Jun 13, 2020
Messages
288
Likes
292
Probably have to use Nuvistors. I think they never took off because they aren't pretty like big glass enclosed tubes. Had a preamp that used them at one time.

View attachment 247232

I recall Musical Fidelity used to produce a line of equipment that incorporated the Nuvistors.
 

rwortman

Addicted to Fun and Learning
Forum Donor
Joined
Jan 29, 2019
Messages
740
Likes
683
So if a tube pre-amp offers someone both aesthetic and conceptual pleasure a SS device doesn't, and offers a set-and-forget sound one likes...why fuss with other solutions that don't in the end offer all the things an actual tube amp does for an owner?
Bob Carver proved he could duplicate the transfer function of a particular tube amp with a solid state device. What the tube amp did to the signal was a lot more than EQ. Today you can probably get pretty much the same thing with digital emulation. The DAW world is rife with tube this and tube that plugins. Some people claim the actual tube equipment somehow sounds better but no one is doing ABX testing because no one cares that much. Use the tools you like.

If I like the looks of tube equipment (I do) and the sound, why bother with solid state emulation? I don’t run tube amps now because powerful ones are expensive and ultra high sensitivity speakers are too compromised. I also don’t like sitting down to hear some music and having something not work. The MTBF of my tube preamp was about 6 months. All tube replacements but still a hassle when I wanted to relax. My job was as a Mr. Fixit engineer in high tech manufacturing. I do all the work on my cars, bikes, motorcycles, tractor, power equipment etc. I build my own computers and fix my own electronic gear. At the end of the day, when I sit down to unwind with some music, I just want it to work. I have a tube integrated in a dedicated mono system but never had a big powerful tube stereo amp. I might try one someday just for fun. Fun, isn’t that what we are doing this for?
 

Blumlein 88

Grand Contributor
Forum Donor
Joined
Feb 23, 2016
Messages
20,521
Likes
37,049
Bob Carver proved he could duplicate the transfer function of a particular tube amp with a solid state device. What the tube amp did to the signal was a lot more than EQ. Today you can probably get pretty much the same thing with digital emulation. The DAW world is rife with tube this and tube that plugins. Some people claim the actual tube equipment somehow sounds better but no one is doing ABX testing because no one cares that much. Use the tools you like.

If I like the looks of tube equipment (I do) and the sound, why bother with solid state emulation? I don’t run tube amps now because powerful ones are expensive and ultra high sensitivity speakers are too compromised. I also don’t like sitting down to hear some music and having something not work. The MTBF of my tube preamp was about 6 months. All tube replacements but still a hassle when I wanted to relax. My job was as a Mr. Fixit engineer in high tech manufacturing. I do all the work on my cars, bikes, motorcycles, tractor, power equipment etc. I build my own computers and fix my own electronic gear. At the end of the day, when I sit down to unwind with some music, I just want it to work. I have a tube integrated in a dedicated mono system but never had a big powerful tube stereo amp. I might try one someday just for fun. Fun, isn’t that what we are doing this for?
You can always get a low powered tube amp, load the outputs, and feed that to a SS high powered amp to get your power and your tube sound. If you go with one of those SETs you won't even have to attenuate the output prior to sending it to the SS power amp (at least with some of them).

I think the transformers are where most of the tube sound is anyway. I've wondered about getting some hardware clones of the old Pultec EQ units to see if they give most of that sound. A couple of small signal tubes and transformers.
 

kongwee

Major Contributor
Joined
Jan 22, 2022
Messages
1,024
Likes
276
Basically what you describe about tube and sold-state are quite universal. 3 dB in SPL won't make affect what you describe.
 

solderdude

Grand Contributor
Joined
Jul 21, 2018
Messages
15,891
Likes
35,912
Location
The Neitherlands
Bob Carver proved he could duplicate the transfer function of a particular tube amp with a solid state device.

Not exactly, he altered some aspects (probably FR and probably output R) so these amps could not be told apart in a level matched blind test. For that there is no need to duplicate a transfer function.
It is rather easy to get a similar transfer function as tubes though. Just use a J-FET with local feedback to reduce the gain.
It was never disclosed what he did exactly though. Just rumors.
 

kongwee

Major Contributor
Joined
Jan 22, 2022
Messages
1,024
Likes
276
Not exactly, he altered some aspects (probably FR and probably output R) so these amps could not be told apart in a level matched blind test. For that there is no need to duplicate a transfer function.
It is rather easy to get a similar transfer function as tubes though. Just use a J-FET with local feedback to reduce the gain.
It was never disclosed what he did exactly though. Just rumors.
It is not just FR respond, it is the relation of input to output signal. It is not as linear as solid state. As best as I describe is a kind of very very soft compression. Developer of Airwindows plug in like to call it saturation.
 

solderdude

Grand Contributor
Joined
Jul 21, 2018
Messages
15,891
Likes
35,912
Location
The Neitherlands
For that to happen you need a lot of distortion to be added.
Plugins can do that but are designed to work for a single instrument, not the entire signal.
Saturation is basically soft clipping and adds a lot more distortion than any audiophile wants, not only 2nd (and 3rd) but also IM products which can make an instrument sound a bit 'dirtier'.
Nah... the amp that the Carver was up against was not a huge distortion machine so there was no need to alter the transfer function to make it pass a blind test.
Some roll-off at the extremes and adding a small resistor would have been enough. It was just a blind test. People aren't as discriminative when something is not compared sighted.
 

Blumlein 88

Grand Contributor
Forum Donor
Joined
Feb 23, 2016
Messages
20,521
Likes
37,049
For that to happen you need a lot of distortion to be added.
Plugins can do that but are designed to work for a single instrument, not the entire signal.
Saturation is basically soft clipping and adds a lot more distortion than any audiophile wants, not only 2nd (and 3rd) but also IM products which can make an instrument sound a bit 'dirtier'.
Nah... the amp that the Carver was up against was not a huge distortion machine so there was no need to alter the transfer function to make it pass a blind test.
Some roll-off at the extremes and adding a small resistor would have been enough. It was just a blind test. People aren't as discriminative when something is not compared sighted.
We know Carver added output resistance. We know he increased distortion. Seems he added some small hand wound coils to some of the circuitry for that. We know he reduced power output only into lower frequencies. He may have done other things.

Seems some tube power amps do act a bit like very soft compressors. The FR can change a bit at extremes as power level changes also (transformer effects I think). I forget whether the claimed null by Carver was in the 60 db range or maybe he said he achieved 70 db nulls. The later production amps with the tube transfer function mod he made were claimed to be a 38 db null. Which seems odd. Looks like he would have made it same as his test amp modifications.
 

kongwee

Major Contributor
Joined
Jan 22, 2022
Messages
1,024
Likes
276
Harmonic distortion, you can't do it in EQ it is base on fundamental frequency and start to peak a little on 2nd, 3rd or so on. One way it is "livelier" or "brighter".
 

anmpr1

Major Contributor
Forum Donor
Joined
Oct 11, 2018
Messages
3,721
Likes
6,405
Not exactly, he altered some aspects (probably FR and probably output R) so these amps could not be told apart in a level matched blind test.
Not exactly. At least not exactly as far as Bob's methodology went. And at least not as far as a level-matched blind testing goes. Of course, in the 'final analysis' what you suggest is certainly the case.

I'm referencing Bob's original (first) 'Carver Challenge' (not the subsequent Stereophile fiasco) conducted in 1982 at Peter Aczel's lab. In this first setup, Bob's stated goal was to match the transfer function of his (then) new M1.5 amplifier to Aczel's ML-2 class A mono unit. The idea was to 'null' the two amplifiers so their output signals would cancel each other, thus proving that two amplifiers of dissimilar topology could effectively have the same input to output transfer function. I'm not sure that FR was ever measured as a working or deciding criteria. The test report (Audio Critic 10) indicates that an oscilloscope and voltmeter were used. However it was, the mod took two days of work.

According to Aczel:

In the process, the open loop of the (Carver) was retailored, the negative feedback was readjusted several times, various transistors were rebiased, the input impedance characteristics were changed, lower-resistance wiring was substituted in crucial places, but no active devices were replaced and no changes in basic topology was made. German polypropylene capacitors were used whenever new values had to be inserted.

Aczel reported that except for a 'miniscule' ground loop hum, the amps nulled at -74dB (Bob's actual production 't-mod' amp was claimed to be nulling in the mid 50s against the ML-2). One thing: during listening sessions program material and playback level had to be carefully selected because of the Levinson's limited power output (35 watts v 350 watts), which could easily lead to clipping on loud passages.

Secondly, it is not clear that levels were 'matched' rigorously, if at all (see below for an interesting and telling sequel). Aczel did not own an ABX comparator until some years later (1988 or so). Nevertheless, the Carver listening evaluation was done 'blind' in that during test panelists did not know which amp was connected.

Panelists were first given time to listen 'sighted' to each amp in order to determine what they thought were sonic attributes of each. In this, panelists were convinced they could identify unique 'sonic signatures' exclusive to each amp. Then they left the room and formal testing began. Bob hooked up one of the amps, and left the room (that is, he was not present during the actual listening tests). Listener had no idea which amp was connected, but was allowed to listen as desired. Once a panelist made his determination as to what amp was playing, he left the room. Bob then switched (or didn't switch out) the amp--based upon the predetermined toss of a coin. 15 times panelists would come in and listen, then decide.

Tallied scores indicated randomness. No one correctly identified anything. The conclusion was that previously certain 'differences' between the two amplifiers had been 'nulled out' (again, see below). Bob was said to have made both amps 'sound the same' as a result of his tweaking. But did he really do that?

What is telling is that after Peter got his ABX device and proved to himself that level matching was the key, he refined his thinking. In fact, he told me that the 'secret' to the Carver Challenge was probably that the Carver and Levinson sounded same all along, even before the tweaking. But because he never matched levels, he had been fooling himself, imagining that differences were there in the first place, when they likely were not. Because of the intensity of the two day 'workout', and the suspected 'belief' that the amps were now sounding the same when at first they 'sounded different', and because participants couldn't reliably tell anything when the brands were hidden, they all believed that Carver had actually worked some magic, when in fact he was just doodling.

Peter told me that he came to realize that all of his pre-ABX 'tweako' amp reviews were worthless (he didn't use that word, but it was clear to me what he meant). The only thing I could tell him was that if he was fooled, the rest of us who 'followed him' were bigger and derivative fools, so he shouldn't take it too hard. Live and learn. That's what it's all about, anyway.

I think what at first bothered him was that after he changed his thinking, the tweako crowd piled on him pretty hard, calling him a sellout, a Carver shill, and a lot worse. It was all pretty ugly. Later, however, after the Stereophile Conrad Johnson/Carver affair, I think he felt vindicated to a degree, but was mostly simply over it, and could not have cared less.
 

RichB

Major Contributor
Forum Donor
Joined
May 24, 2019
Messages
1,946
Likes
2,611
Location
Massachusetts
Not exactly. At least not exactly as far as Bob's methodology went. And at least not as far as a level-matched blind testing goes. Of course, in the 'final analysis' what you suggest is certainly the case.

I'm referencing Bob's original (first) 'Carver Challenge' (not the subsequent Stereophile fiasco) conducted in 1982 at Peter Aczel's lab. In this first setup, Bob's stated goal was to match the transfer function of his (then) new M1.5 amplifier to Aczel's ML-2 class A mono unit. The idea was to 'null' the two amplifiers so their output signals would cancel each other, thus proving that two amplifiers of dissimilar topology could effectively have the same input to output transfer function. I'm not sure that FR was ever measured as a working or deciding criteria. The test report (Audio Critic 10) indicates that an oscilloscope and voltmeter were used. However it was, the mod took two days of work.

According to Aczel:

In the process, the open loop of the (Carver) was retailored, the negative feedback was readjusted several times, various transistors were rebiased, the input impedance characteristics were changed, lower-resistance wiring was substituted in crucial places, but no active devices were replaced and no changes in basic topology was made. German polypropylene capacitors were used whenever new values had to be inserted.

Aczel reported that except for a 'miniscule' ground loop hum, the amps nulled at -74dB (Bob's actual production 't-mod' amp was claimed to be nulling in the mid 50s against the ML-2). One thing: during listening sessions program material and playback level had to be carefully selected because of the Levinson's limited power output (35 watts v 350 watts), which could easily lead to clipping on loud passages.

Secondly, it is not clear that levels were 'matched' rigorously, if at all (see below for an interesting and telling sequel). Aczel did not own an ABX comparator until some years later (1988 or so). Nevertheless, the Carver listening evaluation was done 'blind' in that during test panelists did not know which amp was connected.

Panelists were first given time to listen 'sighted' to each amp in order to determine what they thought were sonic attributes of each. In this, panelists were convinced they could identify unique 'sonic signatures' exclusive to each amp. Then they left the room and formal testing began. Bob hooked up one of the amps, and left the room (that is, he was not present during the actual listening tests). Listener had no idea which amp was connected, but was allowed to listen as desired. Once a panelist made his determination as to what amp was playing, he left the room. Bob then switched (or didn't switch out) the amp--based upon the predetermined toss of a coin. 15 times panelists would come in and listen, then decide.

Tallied scores indicated randomness. No one correctly identified anything. The conclusion was that previously certain 'differences' between the two amplifiers had been 'nulled out' (again, see below). Bob was said to have made both amps 'sound the same' as a result of his tweaking. But did he really do that?

What is telling is that after Peter got his ABX device and proved to himself that level matching was the key, he refined his thinking. In fact, he told me that the 'secret' to the Carver Challenge was probably that the Carver and Levinson sounded same all along, even before the tweaking. But because he never matched levels, he had been fooling himself, imagining that differences were there in the first place, when they likely were not. Because of the intensity of the two day 'workout', and the suspected 'belief' that the amps were now sounding the same when at first they 'sounded different', and because participants couldn't reliably tell anything when the brands were hidden, they all believed that Carver had actually worked some magic, when in fact he was just doodling.

Peter told me that he came to realize that all of his pre-ABX 'tweako' amp reviews were worthless (he didn't use that word, but it was clear to me what he meant). The only thing I could tell him was that if he was fooled, the rest of us who 'followed him' were bigger and derivative fools, so he shouldn't take it too hard. Live and learn. That's what it's all about, anyway.

I think what at first bothered him was that after he changed his thinking, the tweako crowd piled on him pretty hard, calling him a sellout, a Carver shill, and a lot worse. It was all pretty ugly. Later, however, after the Stereophile Conrad Johnson/Carver affair, I think he felt vindicated to a degree, but was mostly simply over it, and could not have cared less.

Very interesting, wasn't there also a challenge where Carver made his amp sound like a tube amp or is that internet lore?

- Rich
 

RichB

Major Contributor
Forum Donor
Joined
May 24, 2019
Messages
1,946
Likes
2,611
Location
Massachusetts
We know Carver added output resistance. We know he increased distortion. Seems he added some small hand wound coils to some of the circuitry for that. We know he reduced power output only into lower frequencies. He may have done other things.

Seems some tube power amps do act a bit like very soft compressors. The FR can change a bit at extremes as power level changes also (transformer effects I think). I forget whether the claimed null by Carver was in the 60 db range or maybe he said he achieved 70 db nulls. The later production amps with the tube transfer function mod he made were claimed to be a 38 db null. Which seems odd. Looks like he would have made it same as his test amp modifications.

I have read that this test was not level matched and did not include fast switching.
This makes it a very poor-quality test, IMO of course.

It is fashionable to cite Carver sessions to prove that the golden ear reviewers have no clothes, and I suspect that is true for some, but I don't think it has much relevance other than embarrassing and perhaps humbling some.

- Rich
 

Els

Active Member
Joined
Sep 30, 2022
Messages
123
Likes
51
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.
Any difference in decay time, for exemple on cymbals?
 

anmpr1

Major Contributor
Forum Donor
Joined
Oct 11, 2018
Messages
3,721
Likes
6,405
Very interesting, wasn't there also a challenge where Carver made his amp sound like a tube amp or is that internet lore?

- Rich
That was the later Stereophile test. Then, a Conrad Johnson tube amp was used. That led to some very nasty allegations and comments directed toward everyone involved, on both sides of the fence.

The only takeaway from that little episode was to demonstrate how ugly and deranged the 'high end tweako' scene can get. The two sides will probably never come to any meetings or greetings, and it's best they stay apart, for the most part.
 
Top Bottom