John Curl's Blowtorch preamplifier

[john curl]

No, I did NOT publish the exact schematic. It is my intellectual property. However, the essence of the design has been derived, both by visual inspection of the circuit boards, and some good guesses.
We are now going forward with discussion of more intrinsic design principles that are usually more interesting to experienced engineers in the audio field.

[GRollins]

Originally posted by PMA
Grey,

parallel capacitor divider took place only in one simulation, in my reply to you, i.e. here:

http://www.diyaudio.com/forums/showt...27#post1493527

Following simulations were without caps, according to John request and directions.

Thanks. It's not a problem either way...I'm just trying to match your simulations to my thought experiments/experience. I do a lot of this by taking things to extremes and seeing what happens--somewhat like your 50000uF cap a couple of pages back as you attempted to get rid of phase shift. I do this while driving to work or on the way home because that's the quietest time I get all day, and no, I haven't caused any accidents...yet.
It helps that everyone scatters when they see me coming.

Originally posted by john curl
Grey, actually a larger value resistor can be quieter, all else being equal, because more first stage gain is developed. See what I mean? It is important to learn the subject thoroughly through immersion into practical noise theory and its tradeoffs. Sometimes it is not as obvious as it first might seem.

Again, agreed...assuming that I'm following you properly (honestly, last night was a booger at work and I was only able to give this matter a fraction of the attention it deserves--worse yet, I can hardly remember my train of thought at the time--rats), this is an avenue I've already been exploring for my discrete phono stage. I'm following this 75% from a perspective of noise and 25% from a gut feeling that I'd rather let the front end JFETs dominate in terms of distortion components, rather than the cascode. Percentages subject to change with further experiments.
The idea of a unity gain first stage (not all that uncommon in amps) showed me that way was folly...you add noise (and distortion, for that matter), but get nothing in return. Well, obviously that's stupid. After that it was a hop, skip, and a jump to the realization that there was going to be a optimum load value that gave positive gain, submerged the noise, and provided a decent Zout leading into the next stage.
While it's nice to have preamps and amps be as quiet as possible, it's absolutely essential for phono stages to be silent. Got any hints, tips, rules of thumb, suggested (available) books, or work-arounds that you'd care to share?

Grey

[john curl]

Grey, phono first stages are really tough. The most important thing is to not use mosfets in the gain path, or even the power supply buffer. Mosfets are just too inherently noisy, to keep them from adding noise, especially low frequency noise. However, in many cases, jfets can be used for 2'd stages, without too much trouble.

[scott wurcer]

Originally posted by john curl
That's because you have a mid-fi phono playback system, Scott. I like FM too, but I also know its limitations.

Thank you John, Actually I include a system of a Goldmund Reference table, $7000 Koetsu cartridge, and the best tube pre available at the time in that comparison. As well as John Dennison's demo of the JC-80, Lynn, Dennon 103(?) those two systems almost matched the master tapes over FM.

[GRollins]

John,
The MOSFETs were just for the GR-25 and there only because I wanted the current. I tried JFETs in the cascode position in the amp but was unable to get a reasonable amount of current so the MOSFETs went in. The phono stage is all JFET at present, although I can see a couple of places where I might consider low noise bipolars. Maybe.

Grey

[john curl]

You don't own any of this stuff, do you?

[scott wurcer]

Originally posted by john curl
You don't own any of this stuff, do you?

Of course not, I got to listen to both at length though I have to admit the source material on the second was poorly chosen.

As I have pointed out on numerous occasions many very hightly regarded cartridges (last I checked the Denon DL-103 still has a spherical stylus) are physically incapable of playing many recordings without HUGE amounts of distortion. This is easily demonsrtrated with a 1/3 octave noise band on any good test record, the IM distortion sprays everywhere. I have only owned a Monster Alpha One (years ago it died) and various Grado References. I like the Grado's because you can get down to a small fraction of the coil's thermal noise pretty easily. They all 'crackle' on a few KHz and up 1/3 octave band. As they say on the Telarc Omni Disk "this is normal".

[john curl]

Dennon 103C and D, I had both 25 years ago. It is not A level, anymore.

[scott wurcer]

Originally posted by john curl
Dennon 103C and D, I had both 25 years ago. It is not A level, anymore.

Pick one, any one, I picked the Denon because there are some people here who seem to have taste and an opinion worth respecting who still like this cartridge. I have never owned one and I find all the fiddling involved with a totally new cartridge to be just tedious. The Grados always have 10Hz resonance in my arm and behave the same as 35yr ago.

[john curl]

Grado? Who uses a Grado in hi end?