Short List of Amps I prefer over the Pass Labs XA25 or INT 25

"CONRAD-JOHNSON preamps are designed for optimal sonic performance , yet this is not always reflected in the specifications of distortion."

@niodari What this means in a nutshell is the THD is a bit higher but not really audible as it is mostly lower ordered harmonics.

Actually no. I've been advocating for that for years, but the industry doesn't like the idea of a spec sheet that would actually tell you how the amp or preamp **sounds**.

Then you need a good DSP unit I guess.

The DSP is built-in to such amps that have a direct digital input.

Conrad Johnson preamplifiers have notably high thd I saw in specs of all the recent models et6 et7 and gat it is 0.15 (I was surprised to see this). I didn't have a chance to audition them but I assume they sound good.
Do they (do the higher order harmonics help)?

There is a huge difference  between high THD and higher ordered harmonics! Higher ordered harmonics as distortion is bad and highly audible, but high THD might not be so bad, if the harmonic content is mostly lower orders (2nd, 3rd and 4th) as the ear is far less sensitive to them.

Eric, i thought feedback was something to create the gain. I am too far from this field, but will try to have a look on some books (e.g. why the feedback is unavoidable).

Could something similar to switching waveform be used to create a correct and accurate digital signal from the source analog signal and "amply it digitally" without any distortion? Is there no way to amplify a digital signal directly from the digital sources (e.g., CD player), what this would really mean? Then you will again need a DAC just to output the amplified digital signal to the speakers (but i understand that "amplified digital signal" cannot exist, a binary number cannot be amplified and that that the conventional amplification is analog). 

You can have gain without feedback. There are class A, AB and D amplifiers that use no feedback.


The problem with what you propose in the 2nd paragraph above is that digital audio switches at speeds far too low to be practical to amplify directly, and at some point you do have to create an analog signal to drive your loudspeakers. So what some products do is they accept the digital input and then convert the switching of the digital to a much higher frequency that is practical for a switching power amp.  Some amount of DSP is required for this approach. But a switching amplifier itself is an analog process plain and simple.

Feedback is not the only way amps can give the gane?
Somehow class d amps use double feedback to get to the switching waveform to transform analog sound to digital one? But how digital amps transform that digital sound back to analog (do they have some integrated DACs)?

You have to have gain to be able to apply feedback to an amplifier design. The feedback kills some of that gain and how much is measured in dB (decibels). But you still need gain for the amp to amplify the signal, so overall the circuit needs enough gain for that and the feedback.

Class D amps are analog, not digital. There is confusion because of the 'D'; it does not stand for 'digital'. Class A, B, and C were already taken when the idea was proposed in the 1950s.

It is! But still audible as brightness.

Two class D Cherry amplifiers that i use now (a small integrated Maraschino and a larger Megamaraschino) do reproduce with little distortion, clear and detailed as far as i can see.

Class D might be the one way out of this. The issue is something called 'gain bandwidth product' and has to do with how much gain you have combined with bandwidth. That's easy enough to understand; where it gets tricky is when you apply feedback. If the bandwidth is limited, you can't apply much feedback because the amplifier will have phase shift at its bandwidth limits and this phase shift can result in the negative feedback becoming positive if something isn't done to limit it.

But the problem is that the application of feedback introduces distortions of its own. An amplifier with good linearity prior to feedback, and having a fairly benign distortion signature (meaning a predominant 2nd or 3rd with not much above that) will have many more harmonics and intermodulations (which form at the feedback point), while doing a pretty good job of suppressing the lower orders.

Since the ear converts all distortions into tonality, the addition of the higher orders and the IMD makes the amp sound brighter and harder. To get rid of this issue, you have to have enough feedback so that the feedback is able to compensate for the distortions it adds. Generally speaking this value is in excess of 35dB; if the amp has less the feedback has detrimental effects although it might look 'good' on paper.


(This is why feedback has a bad reputation and why there are a good number of products that use none.)


So to get 30dB of gain out of the amp, you need 75dB as a minimum if the amp is operated open loop (no feedback). That's a lot! That's why gain bandwidth product becomes a useful phrase because getting the gain and the bandwidth at the same time gets tricky to say the least.

Now a class D amp can get around this problem through a fairly simple mechanism called self-oscillation. The idea here is to add so much feedback that the phase margin of the amp (where negative feedback becomes positive due to phase shift as I mentioned earlier) causes the amp to go into oscillation.  You simply use that oscillation as the switching frequency. Class D amps make gain through the comparison of the switching waveform (usually a triangle wave; its fairly easy to convert a squarewave to triangle, so with a small bit of circuitry we use the switching of the amp itself for this) to the input waveform. So all you have to do is set the ratio between the two to set the gain, and then run as much feedback as you need. This can result in extremely low distortion and no phase shift, even if the output filter frequency is set fairly low. Time will tell if this proves out to be a viable path.

@atmasphere, interesting and beautiful theory /hypothesis on harmonic orders. it is not easy to imagine how it works in practice. how different harmonic orders are related in phase/time, why their combination should lead to a soft pleasing sound? how many amps with 0 distortion did you see, what you mean when you refer to an amp without any distortion? and what if the distortion is too small, say .001? would then still a several harmonic order sound with distortion be better, by the way, how many harmonic orders give tube amps (all of them the same number)?

i was a tube fun for 15 year used only tube amps. once i tried a class ab/a amp i was indeed pleased and even released with a clean clear sound that it produced. if your auditioning experience is based on a sound with distortion, you are merely in a wrong musical world.

There are no amps with '0 distortion'. Such is impossible at the current time! 0.001% is audible if there are no lower orders to mask their presence. This is due to the fact that the ear uses the higher orders to sense sound pressure, and so is keenly sensitive to their presence.


All amps make distortion, and since the ear converts all forms of distortion into tonality, this is the primary difference you hear between amps. IOW **everyone** is basing their auditioning experience based on a sound with distortion.

Have any of you compare the pass amp with Bedini 25/25 100/100 ? For 1/3 prices of pass amp , I’m happy with my Bedini

@atl4love  Although we've not done any comparisons, we have had the Bedini in our shop (one was recently in here for repair) and it is one of the better solid state amps we've heard. If you like the amp though, seriously consider getting the filter capacitors in the power supply replaced- that was part of the problem with the one we had here.

I was confused because earlier you mentioned that tubes have more detail, but the masking of distortion is unrelated to the perception of detail.

Thank you for the clarification.

Masking though, AFAIK has to do with closely spaced frequencies. Hard to mask higher order distortion, isn't it?

BTW, I like tubes a great deal. :) I'm not attacking them I just want to make sure I understand the arguments.

@erik_squires To be clear I didn't feel you were making any sort of attack! Asking for clarification is not an attack, and I welcome it.

Masking is a tricky thing but AFAIK doesn't have to involve closely spaced frequencies; if they are not closely spaced the ability to pick out the lower level sound(s) is better. This is fairly easy to experience. One example of this is our ability to pick out information when there is something like white noise in the foreground; analog signals can have noise but we can pick out signals that are a good 10dB below the noise floor! (I suspect this has something to do with the fact that wind and water sounds are very similar; so this could be a survival issue)


It appears that the ability to pick out signals in the noise floor relates directly to amplifier design. Amps with zero feedback tend to have a noise floor that involves a fairly natural 'white noise' hiss; amps that run feedback tend to have a noise floor that consists of inharmonic information and intermodulations, into which it **seems** that the ear cannot penetrate as it can a natural noise floor. This might explain why amps without feedback can often seem as if they have more low level detail. This is an area that I also feel needs more research (one exception to this is if the amp with feedback has sufficient gain bandwidth product such that it is stable with over 35dB feedback; such amps are quite rare). 


At any rate, I did say that tubes have more detail, and that they manage this without being bright. However I wasn't suggesting that masking distortion was the reason. I was instead suggesting that the presence in the correct quantity of 2nd and 3rd harmonic content somehow contributes to the ear's ability to pick out natural detail in the recording (including soundstage information). While this is documented, exactly how this occurs is an unknown (AFAIK) and something which I find very interesting.

Right, and this is the basis of MP3 compression, among other things. Why do you think the masking principle benefits tubes??

I thought I had explained that enough, but nutshell explanations can be tricky. Essentially though the tubes have enough 2nd and 3rd harmonic distortion that those harmonics are able to mask the presence of the higher orders. So they **sound** to the human ear as if the higher orders are absent altogether. That is why they sound smoother.

Pass Labs admittedly and intentionally adds some even order harmonic distortion to all their amp designs.   This is commonly known to be perceived by humans as a euphonic overall sound quality similar to tube amps, which many devoted Pass fans and others desire and enjoy.  
    There are many other amp designers of amps in this expensive price range who restrain themselves from doing this for the sake of accuracy.

I should point something out here. The big difference between tubes and transistors is that tubes tend to sound smoother, with greater actual detail rather than brightness masquerading as detail.


But tubes have measurably greater distortion- so how is this possible?

The answer has something to do with the ear's masking principle; that is where a louder sound can mask the presence of a quieter sound.

It also has to do with how the ear perceives distortion, and in this case I'm talking about distortion where the amplifier is not being overloaded. The ear interprets the 2nd and 3rd harmonics as 'body', 'warmth', 'fullness' and the like, all terms to describe their presence. If these harmonics are present in sufficient quantity, they will mask the presence of the higher orders. Since tube amps generally have more of the lower orders (2nd, 3rd and 4th) than solid state amps, the result is that the higher orders are masked and so tubes sound smoother even though they have greater higher ordered harmonic content than solid state.


So solid state tends to sound brighter and harsher (harder) than tubes because the ear interprets the higher orders in this manner, simply on account of the fact that the higher ordered harmonics (to which the ear is keenly sensitive as it uses them to gauge sound pressure) are exposed. 


But there is more, and this bit is really fascinating. The lower orders, if in sufficient quantity, also contribute to soundstage width and depth, as well as lower level detail! This is why tube amps tend to have a wider and deeper soundstage than solid state. Oddly, this is not an exaggeration, but in order to understand that this is so it is helpful to have master tapes or files and also to have been present at the recording to know how it is supposed to sound.

The bottom line is this: unless the amplifier has **no** distortion and by that I really mean no distortion at all and not just vanishingly low, the inclusion of a bit of 2nd and 3rd in sufficient quantity can actually result in the amplifier sounding more 'neutral' to the human ear. Again, this is a topic that deserves more research, but this phenomena has been known for quite some time.


So its going a bit out on a limb to use the word 'accuracy' as in the comment below.

There are many other amp designers of amps in this expensive price range who restrain themselves from doing this for the sake of accuracy

Put another way, if the amplifier designer is aware of how the human hearing perceptual rules work, and is pragmatic about the simple fact that building an amplifier that truly has no distortion is impossible, then the next best thing to do is to include some of the lower harmonics for the perceived benefit they bring, even though it might look bad on the spec sheet. In this way its easy to show that the spec sheets are a good example of the Emperor's New Clothes, as for the most part they ignore human hearing perceptual rules as they have been ascertained in the last 40 years.


Nelson Pass seems to have sorted this out. He is one of the few solid state designers to have done so, and no surprise, his amps sound more 'natural', 'neutral' and musical (IMO) than his competition with lower distortion as a result.