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Old 7th February 2011, 05:04 PM   #21
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Quote:
Originally Posted by P.Lacombe View Post
Yes, 100kohm. W is very frequent mistake for (omega) see :

http://www.datasheetcatalog.org/data...nec/2SA992.pdf
That explains the mystery noise specification. I prefer a graph of noise voltage or noise current vs frequency for various collector currents. That is much easier to understand and you can estimate the base spreading resistance from the noise voltage.
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Old 8th February 2011, 06:46 AM   #22
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I had some evenings listening to this MC stage. I started with 4 Volt rails, instead 12V, and gradully increased the voltage. At all voltages, I measured the voltage offset at the emitters of the two input trannies. Unfortunately, at rails > 8V, there was an increasing offset > 50mV, so did not dare to use this with my delicate Ortofon MC7500 cartridge.

Soundwise, this MC stage is way more transparent than the original Hiraga LePrepre. Though, when varying rail voltages, there were no significant effects.

This was with 2SC2546/2SA1084 trannies.

Next steps: either using a THAT's transistor array, or modifying the circuit to virtual zero impedance.

Hartmut
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Old 10th February 2011, 08:18 PM   #23
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One really really does not want a current greater than a small proportion of a uA flowing through the cartridge ( Rs 3 to 5R typical). I have seen this statement from leading authorities and I have seen and played with common base MC preamps. And I have wondered where a current "preloads" the cartridge and what it does for distortion.

There are a limited number of options:-

1.) Always use an input coupling capacitor...several millifarad electrolytic
2.) Use an incredibly clever bias current cancelling scheme, I have tried several and they always have a fatal flaw. But some close to being cunning.
3.) use Mr D Self's servo integrator to cancel offset in a single ended amplifier.
4.) Use a differential amplifier and either a.) accept that you have just added 3dB to the noise component or b.) you have a cunning scheme to reduce noise.

I have finally arrived at No. 4 option b.) and it uses op-amps,


2SA1085 & 2SC2547 etc....with all due respect to others, I have never ever been aware of a "sound" to individual basic semiconductor devices. I understand the ancient phenomena of 1970's "transistor sound" and it's cause and why it might even be an issue with certain op-amps. One transistor amplifies just as well as another of similar parameters. Many parameters are never quoted by manufacturers, eg. if a certain transistor is manufactured with a high level of contamination by heavy metals it may have a high level of flicker noise, and I do not think any semi manufacturer has ever admitted that flicker noise exists, at least not in their own products . (Actually YES they do when they publish 100Hz and 10 Hz noise figures I am unfair to them) Some transistor parameters are poorly understood by designers and devices are used innappropriately. Nevertheless it seems unfortunate to me that a perfectly good device might acquire a bad name without a clear engineering analysis of why so, and this can then become a reputation and urban myth, which is unfortunate.
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Old 10th February 2011, 08:39 PM   #24
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I should also add to my previous post..

I do not include 100% symmetrical discrete amplifier schemes as a solution per se, because 1.) Input transistor currents necessarily should and must run at Ic of 1 to 3mA for minimal noise, implying several uA of input bias current. 2.) Matching NPN and PNP parameters eg. hfe over temperature etc within a fraction of a uA bias... not even in your dreams. Vbe is dependant upon Is which varies unpredictably between NPN and PNP.
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Old 11th February 2011, 08:51 AM   #25
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Quote:
Originally Posted by Blackjack8635 View Post
One really really does not want a current greater than a small proportion of a uA flowing through the cartridge ( Rs 3 to 5R typical). I have seen this statement from leading authorities and I have seen and played with common base MC preamps. And I have wondered where a current "preloads" the cartridge and what it does for distortion.
...
Blackjack,

I intend to take some distortion figures for an MC cartridge using the circuit above and compare it to the same cartridge using other circuits free of DC input currents.

Hartmut
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Old 11th February 2011, 09:27 AM   #26
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by rayfutrell View Post
graph of noise voltage or noise current vs frequency for various collector currents.............. and you can estimate the base spreading resistance from the noise voltage.
Could you explain the procedure?

Rbb came up in another thread but the link to measuring it was not available to read.
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Old 11th February 2011, 02:07 PM   #27
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Quote:
Originally Posted by AndrewT View Post
Could you explain the procedure?

Rbb came up in another thread but the link to measuring it was not available to read.
The voltage noise of a transistor, ( or an opamp that generally has 2 input transistors), comes principally from 3 sources. 1/f or flicker noise at very low frequency, say 100Hz down to below 10Hz and which we will ignore for now, so instead we will use a 1KHz frequency as useful. The other two are....

Johnson noise.. Vnoise=sqrt(4kTRB) and R in this case is rbb' which is the base spreading resistance. It varies from 4 or 5R in a 2SB737, 6R in each input transistor in a AD797, 30 or 40R in each of the pair in a LM394 (SSM2210). and much greater again in common or garden transistors. For obvious reasons it is lower in power transistors.

Shot noise....is fundamental just like Johnson noise. Inoise=sqrt(2qIB) where I in this case is the steady dc collector current of the transistor. But we also know Ebers-Moll that re the intrinsic emitter resistance...
re=25/Ic ( in milliamps ). At 1mA collector current the re is 25R and at 3mA it is 8R. The Shot noise current runs through this impedance and so adds as a voltage noise ......

....to the Johnson noise as an RMS sum (re is not a real resistance and so does not generate Johnson noise). This is en as sometimes quoted on data sheets. Now. As Ic increases so does the shot noise current, BUT only as a square root and the re reduces linearly, so the overall shot noise drops and en drops as a result as the current increases.

To all of this we must add in the current noise which is simply the Shot noise of the base current. This increases ( shot noise formula above) as Ic increases ( a high Beta/HFE is good for reducing base current).

The overall noise of an amplifier is therefore the Johnson noise of Rs the source impedance summated ( not simply added) with en and with in/Rs.

Noise figures and noise figure curves are an excellent guide to "how much worse" all of the noises of a less than perfect amplifier affect the amplification performance of a simple signal from known source impedance. Of course this signal source has it's own Johnson noise and hence noise figures are a ratio of perfect/real.

(Close to the end now!)
Noise figure curves demonstrate the optimal operating point in terms of Ic for a given frequency ( 1Khz typically ) and resistive source impedance, where the contribution of en is minimal and so is the contribution of in. The curves cover a range of Rs, usually several decades up from 100R.

If you draw a line through the lowest optimal point of the curves, slanting back up left, you can make a rough estimate of in, and from that estimate en and calculate back to rbb'.

The quick and dirty answer is....if the noise curves optimise at 10mA or above for an Rs of 100R then is rbb' is a "few ohms". Optimal at 1 to 3mA then rbb' is a dozen or two ohms. Optimal at 1mA or less the rbb' likely to be a few tens of ohms.

And above is a clue that while power transistors have a lower rbb' they will have a higher in and quite probably worse flicker noise. The End
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Old 11th February 2011, 04:04 PM   #28
AndrewT is offline AndrewT  Scotland
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Thank you.

I was aware of many of the parts of the story.
You have pulled it all together into a coherent whole.
I shall look at a few examples and using some typical LTP current data, see how that comes out.
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Old 11th February 2011, 05:17 PM   #29
AuroraB is offline AuroraB  Norway
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How does the monolithic duals MAT02/03 fit in this picture.....?
Seen them used in some very low noise instrumentation preamps.
Quite expensive, but still made, AFAIK..........
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Old 11th February 2011, 06:41 PM   #30
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Burkhard Vogel
The Sound of Silence
Lowest-Noise RIAA Phono-Amps:
Designer’s Guide

Mr. Vogel's book contains a formula for calculating Rbb, but you have to enter lots of constants and a noise voltage from the graph of noise voltage vs frequency in the transistor specification then solve a quadratic equation. What I do is just look at the graph of noise voltage Vs frequency and calculate the value of resistor that produces the same Johnson (thermal) noise. A 1k resistor produces 4Nv-rt-Hz thermal noise, a 250 ohm resistor 2Nv-rt-Hz, a 62 ohm resistor 1 Nv-rt-Hz. Typically a low noise transistor will have a noise voltage on the order of 1 Nv-rt-Hz around 1ma collector current. The noise voltage is usually a minimum around 1 ma of Ic. Calculate the resistor value that generates the same noise voltage and this will give you an estimate of Rbb. I compared this method with the value calculated by Mr. Vogel's formula and it's typically within 10%, and it's a lot simpler to calculate.
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