John Curl's Blowtorch preamplifier part II

[SY]

Here's a handy calculator that will illustrate DF96's point quite nicely:

KEMET Software

This really is pretty simple stuff.

[simon7000]

Originally Posted by SY

Here's a handy calculator that will illustrate DF96's point quite nicely:

KEMET Software

This really is pretty simple stuff.

SY,

The issue was inserting a .01 uf leaded capacitor at the base of an RCA connector to case to produce RF shielding.

Pavel said it gives up by 10 Mhz.

Please let me know if the Kemet software shows otherwise.

ES

[simon7000]

Originally Posted by DF96

Yes, for decoupling caps self-resonance can be a good thing and can be usefully exploited. Decoupling falls off above the resonance but still useful for a few octaves. A resonance at 94MHz would be perfect for getting rid of Band 2 FM interference.

In the issue here the self resonance due to lead inductance is not as clear. The capacitor has one side connected to a shield cable of unknown length. The other end may be modeled as a ground since it is going hopefully to a chassis.

The result is that the self resonance frequency can be much lower than in a more typical application such as mounted on a circuit board.

[SY]

Originally Posted by simon7000

Please let me know if the Kemet software shows otherwise.

It shows otherwise. Even adding in 3nF for the leads, the impedance at 150MHz is a whopping 2.5 ohms.

[ThorstenL]

Originally Posted by SY

adding in 3nF for the leads

[SY]

Argue with Ed- that came from his example.

edit: typo, make that nH

[scott wurcer]

Anyone ever tried their MC stage? I have uncovered something very special in my junk box, possibly the only two diffpairs left in existance of the transistors mentioned in the paper Low Base Resistance Integrated Circuit Transistor, Willemsen and Bel, 1979. They were a gift from the authors to Barrie Gilbert and he gave them to me (>30yr ago). There are also two op-amps in a fixed gain of 50 that I was told were made for Ortofon, but I have never seen any evidence of that.

The transistors have rbb' of 1.5 Ohms, so paralleling a pair could ge me down to sub .2nV (sort of a bipolar version of the crazy Interfet JFET). They are in nice old fashion military gold/ceramic packages so 50-100mA Ic is no problem. The authors claim "significant" improvement in the Nordholt & van Vierzen circuit, but offer no details.

[bcarso]

Anecdote. Back when designing multimedia/computer-oriented powered speakers, the assignment was to provide a synthetic soundfield out of two satellite speakers from a multichannel output soundcard. The particular ultra-simple scheme of Norris was selected (a patent that Harman abandoned, despite it already getting office action --- but Gina had deposed Druckrey, and was appalled at the money spent on patent lawyers, so a whole host of patents languished, including some decent utility ones).

I suggested that it would be nice if, contrary to Norris' scheme, which had a means of transitioning from synthetic surround to mono, we provided a way to go from synthetic surround to stereo. I then had to figure out how to do it.

It turned out that a version of the "phase splitter", in this case using complementary feedback pairs of bipolars, was part of a felicitous solution. Early in the signal chain the two polarites were produced and thereafter processed. The board carrying the processor and potentiometers for adjustment, accessible to the customer on the top of the "subwoofer" was a simple double-sided one with ground planes put down wherever possible. But the customer (Dell) did not place any particularly stringent conditions on RF immunity, so we just hoped for the best. As usual the schedule was absurdly tight.

At a trade show the system was set up in a hotel room. At one point one of the sales guys was using his cellphone in proximity to the speaker. I was stunned --- there was no trace of audible demodulation, which otherwise was always present in earlier powered speakers.

And yet bipolars in the front end? And a plastic case, with no shielding whatsoever? At least in terms of that path for pickup, the likely reason was that, for purposes of achieving low noise, the transistors were being run at a rather high bias current and consequently had appreciable gain-bandwidth, and as well high base currents, particularly as compared to, say, opamps commonly used for audio. So the rectification/envelope detection propensities of the transistors were mitigated. There may have been some luck with the layout too.

The particular variable surround synthesis scheme and variations on it are patented, although frankly I never cared much for how it sounded


Brad

[bcarso]

Originally Posted by scott wurcer

Anyone ever tried their MC stage? I have uncovered something very special in my junk box, possibly the only two diffpairs left in existance of the transistors mentioned in the paper Low Base Resistance Integrated Circuit Transistor, Willemsen and Bel, 1979. They were a gift from the authors to Barrie Gilbert and he gave them to me (>30yr ago). There are also two op-amps in a fixed gain of 50 that I was told were made for Ortofon, but I have never seen any evidence of that.

The transistors have rbb' of 1.5 Ohms, so paralleling a pair could ge me down to sub .2nV (sort of a bipolar version of the crazy Interfet JFET). They are in nice old fashion military gold/ceramic packages so 50-100mA Ic is no problem. The authors claim "significant" improvement in the Nordholt & van Vierzen circuit, but offer no details.

Yow 1.5! That's low. As I may have mentioned I exhorted a friend to buy some Toshiba 2 ohm parts, and along the way got 200-piece bags of each polarity from him for myself. They will come in handy someday, if I live long enough (my nearly 1k pieces of 2SK389V parts are also awaiting use, somewhere).

Do you have links or other specifics on Willemsen and Bel?


Brad

[scott wurcer]

Originally Posted by bcarso

Yow 1.5! That's low. As I may have mentioned I exhorted a friend to buy some Toshiba 2 ohm parts, and along the way got 200-piece bags of each polarity from him for myself. They will come in handy someday, if I live long enough (my nearly 1k pieces of 2SK389V parts are also awaiting use, somewhere).

Do you have links or other specifics on Willemsen and Bel?


Brad

P5

There was an Elector article using BF*** parts, I guess this was best for 1979 and integrable but not hard to do with only 4 or so paralleled modern devices. The link got tiny, P5 above. Not a very special circuit, giant electrolytic at input yuk.