John Curl's Blowtorch preamplifier

[Steve Eddy]

Originally posted by MikeBettinger
This is the basis for another circular conversation so I'll bracket my comments with a couple of thoughts: First this is DIY audio and I'm not being forced to use any safety ground at all.

No, but then just because someone DIYs doesn't mean they've DIYed everything in their system.

Second, I do allow one such ground in my system and that is in my CD player. I see no need to disconnect it, as it by itself is doing no sonic harm and does provide the whole system with the intended path to the neutral in case Heck does freeze over and the worst occurs.

Actually you'd probably be more safe if you did disconnect it.

The ground conductors in interconnects and the ground wiring and circuit board traces in the various components were never designed to handle AC mains fault currents. Expecting them to do so not only creates a potential shock hazard, but a potential fire hazard as well.

NOT a good idea.

That said, a more realistic problem is noise induced into the ground networks (chassis, etc.) of the various system components (preamp, cd, etc.) that are allowed to circulate between components on the cable shields as well as interference that is picked up by these cables. These are more to the point of my comments. These have a defined loop destination and through thoughtful layout can be routed to bypass the signal path.

Interference picked up by the cables isn't a "grounding" issue and "thoughtful layout" won't bypass it. Either you sufficiently shield the cable so it doesn't pick up any interference, or you have an input stage with good common-mode rejection to keep from passing the noise along.

By the way, I'm not taking a side in the transformer debate, I'm just commenting on the need for galvanic isolation.

And I never said or intended to imply that there was any inherent, universal need for galvanic isolation. Only that it comes along with the transformer territory. If it's not needed, it doesn't hurt. But it's there for those who don't expect circuit board traces and their interconnects to handle AC mains fault currents.

se

[PMA]

Here you can see devastating effect (simulated) of bias reduction from 12mA to 800uA on my preamp. Low feedback does not handle rise of crossover distortion. Even if higher feedback did, it would sound bad. 4Vp/600R.

[john curl]

Thanks, PMA, you have done me a great service. I am poised to measure the distortion, but I can't find any 797's around here to test. I have been originally set-up with a 5534, just for getting the bugs out. Now I am ready to finalize the design. No extra buffer stage for this design, just the minimum that I can get away with, WITHOUT adding 7th harmonic to the mix, if I can help it. I can't even find a LT1028 at the moment. Darn, I will have to order some.
I had hoped to bypass actual measurements, but to no avail. Thanks everyone.

[Charles Hansen]

Originally posted by x-pro
a friend of mine has designed and produced a small series of IC's for a measurement application. These chips would not be cheap thought, about $20-25 each, I suppose.

The main reason to use an IC is to save money. If the IC costs $25, it is probably less expensive to build a discrete circuit. So in this case the IC would only save space and potentially some assembly labor (one part instead of 10 or 20 parts).

[Charles Hansen]

Originally posted by scott wurcer
I though maybe you commercial high-end guys could justify an all out IC op-amp and maybe someone like THAT would be interested. I would gladly donate my time gratis to the project, Les Tyler is an old friend. That's not an idle offer, I'm serious.

Scott, could you please clarify what you are offering?

There is no high-end company in the world that can afford a $1,000,000 NRE for a custom IC.

On the other hand, there are many designs that would only need a small amount of modifications to make them much more usable for true high-performance audio.

I think the onus has to be on the IC manufacturer to make the part that is attractive enough to sell to the audio market. Obviously this is what National is working on right now, but in my opinion they are taking the wrong approach. The audio market does not need another "zero" in front of the THD spec. We need something that is actually innovative and will sound better.

The opportunity is there to "own" that market segment. For example, Burr-Brown pretty much "owns" the audio DAC segment, Cirrus pretty much "owns" the S/PDIF receiver/transmitter segment (AKM is making a go at this one), and ADI "owns" the market for video DACs (usually called encoders).

If you want, I would be glad to consult with you on this. I have done this before with the video division of ADI in Scotland. Our DVD player was using 14-bit ADI DAC chips designed for satellite communications and such. The design group called us up and asked us what was needed in terms of features and performance to make true high performance video DAC. It took them three generations before they finally hit the targets, but now the level of performance we established in 1999 is (kind of) available in mass-market DVD players from Denon, Pioneer, et cetera.

I figure that the only companies that could do this properly would be ADI, National, and Burr-Brown (TI), as this part should have FET inputs. It would be trivial to make this so it could be used either with or without feedback, and also as a single-ended or balanced design, so that you could reach as broad a market segment as possible.

But I can pretty much guarantee that we could come up with a chip that would be universally recognized as the best sounding IC on the market. As long as the price was somewhere between $5 and $10, there wouldn't really be any reason for people not to use it. If the marketing was done properly, whoever made this chip could pretty much "own" the audio IC market segment.

[Charles Hansen]

Scott, this is a follow-up to my previous posting. I finally realized that you meant "That Corp." when you said "THAT". Obviously they are focused on the audio segment, which is good. But the big question I see with regards to them is whether or not they have the expertise to make low-noise complementary JFETs.

I kind of doubt it, but would love to be proven wrong. Right now even we discrete guys are in a world of hurt because Toshiba recently discontinued the last audio-grade, low-noise P-channel JFET in the world. It appears that it is quite possible that *nobody* has the expertise build these. We placed a large EOL order that should last us for at least 50 years, but I don't know what other designers are going to do. I guess they will have to either use bipolars and input coupling caps, or else non-complementary designs with their higher distortion. (Remember Pavel's simulations a couple of dozen pages ago of JC's complementary input stage?)

So to make an IC with low-noise JFET inputs, I figure that there are only a handful of companies on the planet that even stand a chance of being successful....

[john curl]

Good input, Charles. We will someday succeed in making an 'adequate' p ch j-fet , but it will cost real money. So what?
High feedback and high quality just don't go hand in hand, I'm afraid. How about those transformers, anyway?

[Steve Eddy]

Originally posted by john curl
High feedback and high quality just don't go hand in hand, I'm afraid. How about those transformers, anyway?

Zero feedback. Not even emitter degeneration.

se

[john curl]

Yes, just added distortion.

[MikeBettinger]

Originally posted by Steve Eddy


1.) No, but then just because someone DIYs doesn't mean they've DIYed everything in their system.

2.)Actually you'd probably be more safe if you did disconnect it.

The ground conductors in interconnects and the ground wiring and circuit board traces in the various components were never designed to handle AC mains fault currents. Expecting them to do so not only creates a potential shock hazard, but a potential fire hazard as well.

NOT a good idea.

3.) Interference picked up by the cables isn't a "grounding" issue and "thoughtful layout" won't bypass it. Either you sufficiently shield the cable so it doesn't pick up any interference, or you have an input stage with good common-mode rejection to keep from passing the noise along.
se

Innaresting response. My posts were nothing more than a lighthearted, end of workweek ""here's my take on this based on my experience" sort of comments. No part of it was dogmatic or judgmental. Every part of it is legitimate and based on working components or many experiments; nothing is based on hear-say. Just trying to spark some discussion. Once again I must be speaking in another lanquage.

So in response, I guess:

1.) I thought I touched on this.
For your concerns: Run as many safety grounds as you have wire for.
For the real world problem of noise circulating on the interconnect shields between components and ending up as part of the signal, this can be dealt with by proper grounding. (What is he talking about? Don’t bother asking!)

2.) If you want to live in fear of that transformer shorting to the core or case then by all means, ground the Cr#p out of everything. I don't run any power ground through PCB traces nor do I use lightweight shields/connectors on my cables. I work around high power RF generators and electronics with power cords that rival fire hoses in size and understand the reasons for the way safety grounding is implemented. Translating this down to 110v in the US is more based on stupid people and lawyers than necessity.

3.) I wasn't talking about poorly shielded cables bringing interference in through the front door, I was talking about (see 1. above) noise on the grounds (shields) finding a path back through the signal path to the supply and becoming, for the very reasons you quoted in one of your posts, part of the ground reference. This, by the way, is my understanding of the need for galvanic isolation. You don't have to look very hard to see many examples of this proudly posted. Grounding and layout I do understand.

Poor interpretation on my part of what was being discussed.

Back to the background for me.