Neurochrome HP-1

True. Fig. 28 has a major flaw, though. Using the LME49720 as a DC servo is not the best idea due to the rather large input bias current of the LME49720. You're better off without the servo in that case.

There are also designs out there that combine the OPA1611/12 and the LME49600. I wonder if the designers noticed that if you simply replace the LME49720 in Fig. 28 with an OPA1612, the amp will oscillate when the load capacitance is in the range of 100-500 pF (but not outside this range). I noticed and I applied compensation to address this. Without the compensation, you could end up in the situation where a long headphone cable (3-5 meter) would cause the amp to oscillate. Not a good scenario! The HP-1 is stable with 0-22 nF cable capacitance. That's about 1 km of cable. That should be enough...

Re. DC servo: You can get rid of the servo and use an AC coupling cap, but that causes high 1/f noise as the input noise current from the opamp "sees" the high impedance of the input impedance at that point. You can address this in various ways.
1) Use an opamp with MOS input for lower Ib. That usually gives you higher noise and higher THD as MOS tends to be noisier and distort more (due to their lower gain).
2) Use a lower input impedance and higher capacitance. Now the source has to drive a lower impedance and not all sources can do this cleanly.
3) Add a buffer to the input of the AC coupling cap. Same drawback as 2) but now at least we can select an opamp that can drive a low impedance. However, this opamp needs to be low Ib (same drawback as 1)) to avoid moving the 1/f problem to the output driver of the source.

High performance analog design. It's always a multidimensional optimization problem. What separates the good designers from the wannabes is that the good designers are better at finding the global optimum.

Tom

 

Tom, could you qualify amps that provide a "less than precise experience"? Does that include most or all tube amps? How about the DG300B, which has some very positive feedback around here? I'm trying to get at the perception of compression because it's a common impression with solid state in general. Doug's DHSA-2 had it a bit for me in its transistor version and less so in its transformer version. Both well engineered and unoffensive/elegant sounding SS amps, but "polite" if you will. None of the "good" tube amps I've heard have it to anywhere near the same degree.

That is my preference as well. Assume the perception of muted or truncated sustain and decay is a valid observation (with respect to another reference), then let's explain it in a scientifically meaningful way.

But not this.

 

What is it about solid state that creates dynamic compression? Doesn't seem to e distortion if amps with little of it also sound a bit compressed. Or is it that some kinds of distortion add something pleasant or desirable? Just want to,like, learn stuff, and always happy to indicate the limits of my knowledge for others to help fill in the gaps.

 

For sure, I don't doubt that psalm hears what he hears, his descriptions are both vivid and hilarious. I'll see if my hearing matches up. I'm mostly curious what causes dynamic compression, but maybe that's like asking why does the sun rise or some other broad thing. And also maybe better for a different thread, but yeah I'd like to learn more about this.

 

I've been following this thread closely since the beginning, mostly because I'm always interested to see Canadian audio products. Since I now consider myself an expert at deciphering Psalmanzar's memes I wanted to comment on a few things.

Unfortunately no one notified you of the caveat that Psalm does the majority of his testing with metal music. I believe Psalm is referring to the trail of distortion (noise) left by over-driven guitars and the lack of interruption of this noise by a new note being strummed which is the backbone of most metal and hard rock passages. If an amp or any other gear doesn't exhibit this sustain the guitars appear as if they are being continually choked each time a new note is picked and it sounds more like a banjo than a Fender.

This is different from unpleasant distortion that you want to minimize, but not everyone listens to audiophile classics like Jazz at the Pawn Shop and Rebecca Pidgeon (Bob Katz Re-master Gone Sexual). Different tastes have different needs.

I haven't heard the HP-1. Somehow I doubt it's a machine that defies the laws of acoustics and what guitars sound like in reality. The only thing I would like to be more scientific in his description is what amplifiers he had on hand to compare it against, since he recommends the Jotunheim and others over it.

I am perplexed as to why you keep bringing up the concept of an effects box. You appear to be either confusing or conflating the idea of what people consider pleasant coloration's (a normal thing to rebel against) and what an instrument should sound like in reality (a big no-no to me). If something that makes music sound good is an effects box then you could easily argue that designs like the HP-1 are cutting boxes that kill the music.

It's also human to want to defend something you've put a lot of work into designing and manufacturing to bring to market. Psalm's tastes are weird. The way he describes a lot of things are weirder. Compression seems to be the buzzword of December for him, last month it was steel and the month before that it was bloom. Despite this he brings up a lot of salient points other people gloss over or don't mention at all and because of that it's naive to disregard his opinion because it's [just one guy/cognitive dissonance/he probably can't afford my amp anyway].

 

That's all noble of you and I believe you that the amp performs per your measurements.
The measurements you take are one of the most comprehensive I've seen on the internet, but these are still not enough to be proof that your amplifier beats best SET-s in sound.
It shouldn't have to anyways, but I'm open to anything if I can get the chance to listen to one.

Many folks here would have no problem spending that $$ on an amp, it's just that people have little faith in solid state here.
Your talk of time invariance and -130dB thd will not help in believing, that is not all there is to the signal and are in my understanding at best secondary characteristics to sweat about, because as you say, an amplifier is time invariant anyways, if it's indeed an amplifier.
Scientifically meaningful criticism is lacking when talking about truly top amplifiers, we have yet to develop the language. At least I have not heard anyone speak it yet.
We can pass pseudo objective descriptions like '' this solid state amp lacks in microdyamics '' and '' this SET has wonderful expansive stage '' but no one have proper scientific proof to show.

 

I'm going to let him fill in the blanks here, but you may find this link helpful since I genuinely don't know how much you know about e-guitars: http://entertainment.howstuffworks.com/electric-guitar2.htm

Chiefly:

This is where I think at least part of the confusion on page 9 came from. Tom is insisting that the HP1 is transparent as a headamp, Psalm is insisting that it isn't because it fails to properly render the desired distortion produced by the guitar amp (which I assume he knows is in the recording because he's heard it on other amps. This is where comparisons and context would come in handy). Again, this is totally different from something like the pleasant distortions you would expect from a gooey tube amp or a JFET input stage designed to mimic tubes like that in the G1217 Polaris.