MPP

[mediatechnology]

Here's a copy of the AD797 documentation showing 1.5nV/rtHz@20Hz and a copy of a measurement done with the ZTX851 showing less then 0.3nV/rtHz@20Hz.
Two ZTX trannies in parallel per input amp. giving a noise improvement of 3dB per amp Two amps in a balanced config causing 3dB additional noise.
So, the remaining noise just that of one single ZTX 851.
I will reproduce Dick's measuring circuit diagram in LTSpice and see what it brings.
One thing is already obvious, measuring transistor noise with a Cap in series with the base will result in wrong results at LF.

Hans

Thank you Hans.
You further the argument for direct-coupling of phono inputs.
A well-known author here at DIY scoffed at the idea a capacitor could introduce noise.
The reactance in series with the source certainly does.

FWIW the PNP ZTX951 is even quieter, by a small margin, than the ZTX851.
The ZTX951 is considerably quieter than the 2SB737.
Pardon the hum: https://proaudiodesignforum.com/forum/php/viewtopic.php?p=14790#p14790

As to the consistency of the ZTX851 I've used hundreds and never found a noisy one.
I've only thrown away one device: It was born with a base emitter short.

 

[Joachim Gerhard]

Jan, you have to diffenciate Current and Voltage noise. For an MM cartridge, BC550 and 560 work well. You can make a halve decent MC Stage too by paralleling say 8. In the old Time we called that Faulkner circuit. I think He came up with that idea in the 60th of 70th.

 

[as_audio]

Faulkner : "Optimum Design of Low-Noise Amplifiers"
https://www.keith-snook.info/Stuff-you-should-read/Faulkner - The design of Low-noise audio frequency amplifiers.pdf

 

[Joachim Gerhard]

Many thanks for that link.

 

[as_audio]

I remember in the old company we used 4 parallel BC550C and 4 560C in the complementary
input of the MM stage (with emitter resistors 47R). Other models only had a single pair.

 

[Joachim Gerhard]

Interesting new topology. And what does he build : an MM phono stage. Seems to be a kind of virus.
sorry, wrong picture

 

[Joachim Gerhard]

That is my SACD player.

 

[Joachim Gerhard]

Here comes the paper.

 

[as_audio]

I am not sure why anybody would want to write an essay about the simple folded cascode.

 

[as_audio]

.. but I take my hat off to the fact that he received funds from a "National Sustainability Programme project" for this.

 

[Joachim Gerhard]

This was the first time i realised that this is a folded cascode back in the 80th.

 

[as_audio]

.. with emitter input.

You should always show your picture source, one is
https://www.bonavolta.ch/hobby/en/audio/ats.htm

 

[Joachim Gerhard]

I have drawn the circuit a little bit different to understand it better.
The datasheet of the 2N5515 shows that the parameters of that Fet are not very well defined.
For example Gm can vary with a factor of 4. Interpolalating I get the approximate idle current.
After modern standards this is not a particular low voltage noise circuit . On the other hand my friends in Paris in the 80th usually used the DENON DL103 that has a rather high impedance of 40 Ohm and only 0,3mV output.
In comparison my Lyra Atlas Lambda has 0.56mV and 4,2 Ohm, pushing out approx 20 times the current.
I have an idea how this circuit can be modified for cartridges like that.
Also I am not sure if the 120 Ohm resistor is the noise budget or if it appears parallel to the rest of the circuit.

 

[Joachim Gerhard]

Here is an idea……

 

[as_audio]

According to the original schematic total current for a single channel is 3 mA.

 

[Joachim Gerhard]

So I was not totally wrong with 2,4mA.
A few more parts and we have a total transimpedance phono stage.

 

[jan.didden]

Here comes the paper.

Interesting circuit. But I note that while they say it is to solve a large output swing with low distortion, they only quote distortion figures with 1V output.
Suspicious.

Jan

 

[Joachim Gerhard]

What circuit ?

 

[Joachim Gerhard]

By the way :
When will the Silent Switcher be availlable again ?

 

[jan.didden]

What circuit ?

The study on the special Vas/Cascode you posted above: Cascode-based voltage-amplifier stage

Silentswitcher is passé. Manufacturing cost have almost doubled, as have the chips, if you can get them. It´s a mess.

Jan