Lowering the value of the lead resistor

[Nietzschelover]

I am a newbie. I had a tech add an external, regulated PSU with tube rectifcation (5V4) to my Jadis DPL pre. The pre has two 12au7s, "half" of each used as cathode followers. In addition to adding the PSU, the tech raised the value of their lead resistors (among other changes, like bypassing caps). Their value was, supposedly, "way too low". Now, I am told, for some French reason, Jadis deliberately does it this way, though. Although the sound has been improved drastically in many ways, microdynamics have been reduced and musicality has been sacrificed for detail. Natural decay, especially of cymbals has been traded for speed and tightness, it seems. I don't want to just undo everything. What about starting with putting those "way too low resistors back in? What'll happen? This is a very skilled tech, by the way.
Here is link to the schematic that's in an Audio Asylum post:
http://www.audioasylum.com/audio/tub...es/174282.html

[SY]

That's not a preamp, it's an effects box. You may indeed enjoy the effects, so turning it into a preamp will not please you. Return it to stock values.

[kevinkr]

I have worked on a number of Jadis Defy 7's and a few other items in years now long past.

To put it nicely I was very puzzled by some of the design choices made in this amplifier, the front end tube was a 12AU7 running at a couple of mA, and the tube driving the output tubes (a quartet per channel) was a 12AX7 running at even lower currents. (No this isn't a typo or an inadvertant juxtaposition of tube types..) Without referring to the schematics I don't remember all of the details, but as I recall the input and driver stages were long tailed pairs, global feedback was used and the outputs operated in UL mode. The few I have seen all had myriad workmanship issues and the above mentioned design issues. Repairing them was a nuisance due to the huge pcb with many very short wires running to the power and output transformers.

From an engineering standpoint the design of that amplifier was probably as bad as anything I have ever seen. It's a testament to the linearity of vacuum tubes that it did not sound totally awful.

They might have done better to plagiarize one of the old heath or eico designs.

IMHO Older Jadis products are attractive looking musical instruments not musical reproducers. By that I mean that by nature of their design they interprete the music, they don't just reproduce it - not an attribute incidentally that I want in the devices I use to listen to music. (I am assuming that it was deliberate and not just design incompetence??)



edit to umm.. fix my incompetent spelling errors.. LOL

[Nietzschelover]

I'm not the only one to enjoy and admire the Jadis DPL--preamp, if you'll pardon the expression. Michael Fremer, of Stereophile and, formerly, The Absolute Sound, owned one, himself. He even wrote a positive reveiw of it, which never made it to print. His favorable review of the model above , the Jadis RC JP80 MC Mk.II preamplifier, did, though. http://www.stereophile.com/amplifica...ews/1298jadis/
(Note: the DPL is line stage, only.)
Their circuitry may be counter-intuitive and unorthodox, but Jadis' produce good results. I apologize if my preamp's circuit is offensive to some, as that seems to be the case. Although, it does appear that WHY it works as well as it does is something beyond many people's level of sophistication. Perhaps someone can explain what is RIGHT about the circuit and elucidate others. I would find that more productive than the previous post. Thank you for your time, though.

[SY]

I guess I wasn't clear enough- it's a circuit that is not designed to have the output replicate the input. There's no design secret or mystery, effects boxes are well-known. This particular one, if you simulate or measure it, has limited bandwidth and high amounts of second and third harmonic distortion. It will sound different than a preamp designed to have the output replicate the input. Presumably, that is what the designer had in mind- it's well-known in the musical instrument world that adding a lot of second and third will fatten up the tone of an instrument. If you like that difference, why should you try and change it?

BTW, if you don't have H.L. Mencken's book on Nietzsche, run right out...

[Nietzschelover]

Thanks Sy and Kevin. You have both told me, essentially, the same thing as the tech and other posters at audio asylum. I guess I like a bit of pleasing distortion, sometimes, especially on poor recordings (which seem to be mostly what I have). Great recordings sure sound great, now, though.

[ivegotmono]

Hi Paul,
I think you may be right about helping the poor recordings. The first tube amp I purchased to learn on ( a year ago) was an Eico HF-12 mono EL84 integrated. It had a 12au7 for the first gain stage with the tone controlls in between sections. It also had "weird" resistor values. In addition once I got more amps to work on and use I started playing around whith the HF12 and ended up with a 12aY7 in that slot. It sounds amazing. Every one said I was crazy and that everything was all wrong and they're probably correct but I love the way that amp sounds, especially old 80s cds which sound like crap on other amps.
BTW Eico changed the design after one or two years. Most HF-12s have a 12ax7 there.


Steve

[Brian Beck]

It's a testament to the linearity of vacuum tubes that it did not sound totally awful.

How true. Try whipping up a poorly-conceived transistor amp design and you'll be covering your ears when music plays. Someone once said that it's hard to make tubes sound bad. I think that may be overstating it, but the essential truth is that even when tubes circuits are not optimized for desirable technical parameters such as low distortion and wide bandwidth, they can still sound quite musical, if colored, like the idiosyncratic Jadis line. For example, that old mono tube record player that your parents (grandparents?) may have had probably sounded mellow and smooth - not grating. Colored, technically imperfect, but listenable and true to essence of music. As we work to make both tube circuits and transistor circuits more perfect, there is a gradual convergence, but not yet a meeting at perfection. What imperfections remain in good tube designs seem to offend music less than what imperfections remain in good transistor designs, as far as musical expression is concerned, in the opinion of many people, myself included.

If stranded on a desert island and offered this insane choice, I would pick a Jadis-based system to listen to, over a Krell-, or Mark Levinson- based system. Having said that, and not being stranded on a desert island, I think there are many better tube designs, commercial, vintage and DIY, that are more faithful to the music than the Jadis line.

[ivegotmono]

For example, that old mono tube record player that your parents (grandparents?) may have had probably sounded mellow and smooth - not grating. Colored, technically imperfect, but listenable and true to essence of music.

I have an old 6bq5 SE amp that came out of a Sears Console. I hooked it up to a CD palyer and some small 2 way Infinity speakers and while it's quality (or lack thereof) is very obvious after having much better amps , I love listening to it. It has a certain character and sounds quite good!!

[kevinkr]

I guess my perspective on audio design is just about diametrically opposite to that represented by the Jadis philosophy. I didn't mean to offend you Paul. I spent quite a few years trying to design audio equipment that aimed at what I perceived at the time to be musical truthfulness, I will admit that my perceptions have evolved over time and experience - not to mention the effects of inevitable changes in my hearing due to aging. (I don't like things nearly as loud for example, even to the point of not being able to tolerate it.)

I started with pushpull power amplifiers based on 6550 and EL34 and there are quite a few of them still lurking out here in the north east. I tried to make them as musically accurate as I knew or learned how.

I am an EE as well as an audiophile and applied the disciplines of electrical engineering to my design process. I wanted musicality, but respectable measured performance as well, as SY put it I wanted something that was an accurate representation at the output of the signal applied.

Later with a more open mind I moved on to designing dht based SE amplifiers and eschewed the use of global feedback - but appropriate measurements are still important to me.

Of course all electronics interpretes the original signal, at some level there are always going to be significant deviations from the original, philosophically I have always tried to minimize those deviations based on my knowledge and experience at the time.

I have always preferred the colorations of good tube equipment to those of the best solid state gear, so obviously it is a matter of preference.

I wanted to believe Julian Hirsch and his mantra, invested heavily in the gear he recommended and was disappointed. My epiphany came with a pair of derelict MC-30's about 25yrs ago.. I thought I heard more detail and a more musical presentation as well, and that was with rock music/new wave not symphonic music. It was an interesting experience..