John Curl's Blowtorch preamplifier part II

[john curl]

I am 1 of those 10, Scott, and it is people like you giving info to people like me, that made me famous, because I took it seriously, and used the results.

[Joachim Gerhard]

That´s true. Only when a voltage is dropped over the resistor it can generate excess noise.

[stinius]

Originally Posted by simon7000

If any one wants to they have my permission to re-post the attachment to this thread in a PDF format. ES

Cheers
Stein

[john curl]

Joachim, please get up to speed, we know this more than 40 years ago. There is more to it than just that, if you think about it.

[john curl]

Thanks Ed Simon, for the very informative input. I doubt that I could duplicate it, easily.

[Joachim Gerhard]

What do you think about that? It is 4 potmeters. 2 are wound one way around, two are wound the other way around and they are turned clockwise and anticlockwise. The switch allows switching between 47kOhm and 500 Ohm aerea. That way you avoid end stop resistance problems.

[simon7000]

Originally Posted by stinius

Cheers
Stein

Thanks

BTY I have an article on PC layout that just came out in the digital version only @

http://www.audioamateurinc.com/digital/ax/issue/410/

Free views for now.

(Although I think they may have mislabeled a figure or 2! Need to check.)

[1audio]

Interesting article on layout. I have used expresspcb for years, just to be quick and cheap. One thing you didn't touch on is inductive coupling between star ground returns. if traces are close enough and carrying currents they will couple. That may cause some unanticipated effects, especially when handling high currents at high di/dt.

[john curl]

Well, back to resistors.

[john curl]

I might do a quick historical review of why we concern ourselves about resistors and specifically EXCESS NOISE.
In the old days, like Marantz, Ampex, and original McIntosh, through the 1960's, carbon resistors were used in almost every audio circuit, even feedback resistors.
In those days, precision 1% metal film resistors cost about $1/ea. For your own computation, multiply your European currency by 4.
In the USA, that was the cost of lunch for one day. Therefore every precision resistor cost us 1 lunch/resistor. This is why we kept them to a minimum, if we used them at all. Did we know about EXCESS NOISE in those days? You bet! We generally found that some brands of carbon resistors were better than others in this way, and we used them.
The first time I ever saw 1% metal film resistors used throughout in a circuit design, was with Mark Levinson's LMP-2 preamplifier. Mark told me that Dick Burwin suggested it. It was relatively expensive, but Mark worked at it to keep it affordable.
Today, everybody has access to 1% metal film resistors, as they are so cheap. However, some metal film resistors are better made than other metal film resistors, and this remains the problem between resistor choices.
Excess noise, itself is measured by putting DC across a resistor and measuring its noise with the DC voltage across it. IF it gets noisier than its noise value, then the extra noise is called EXCESS NOISE. Many metal film resistors have very little EXCESS NOISE, others have more EXCESS NOISE than others. We have test equipment that can measure it, if necessary.