Phono Preamps with "balls" ?

Ralph, although I completely agree with you regarding the overriding importance of designing sufficient overload margin and circuit stability into phono stages, while rectification of RF energy (and subsequent intermodulation) is indeed an issue with bipolar transistors due to the base-emitter connection functioning as a diode, JFETs are free of this effect.

A phono stage with JFET front-end devices can therefore be run with input resistors in the megohm range without problem.

The RF rectification issue of bipolar transistors can be reduced if local feedback (emitter resistors) are added, but doing so will worsen the noise, which is the main reason for using a bipolar transistor front end in the first place.

A further problem with a bipolar transistor front-end is the base current, which will go through the cartridge coils (gradually magnetizing the coil former, which is definitely undesirable). And if the input resistor is of high value, the base currents will cause DC offsets, unless the input is capacitor-coupled (a band-aid that wouldn't be necessary with a JFET front-end).

The main difficulties with JFETs are their comparatively low maximum voltage rating, and significant device-to-device variation.

Unlike the situation with bipolar transistor front-ends, both JFET issues are solvable cleanly; the voltage rating with cascoding, and the device variation by measuring and sorting prior to assembly (although this does mean extra work).

kind regards, jonathan

PS. Nor do I accept that there is a need to insert the extra Neumann constant, as listening tests (LP vs. master tape) have not necessarily shown it to be an improvement. And with half-speed LPs, the target frequency will be an octave wrong.

Hi @rauliruegas : bipolars are better for LOMCs... how? Please substantiate why.

I specified two key areas in which a JFET front-end is superior to bipolar transistors for use with LOMCs, and also spelled out two areas in which JFETs are weaker (but described the countermeasures).

If you want others to accept your claim that bipolars are better for LOMCs, surely you are capable of describing the issues at a sufficient level of detail for EEs such as Ralph and myself to either concede to your superior wisdom, or refute it.

Regarding the extra Neumann constant, yes I have tried it, and have listened to various other phono stages that incorporated it. While it does change the sound, I don't recall ever feeling that it improved the sound.

Let us keep in mind that not all LPs were cut with Neumann cutting heads; Westrex, Ortofon, Haeco are some well-known alternatives.

By the way, what do you feel of the interaction between the Neumann playback constant and DMM-cut LPs?

kind regards, jonathan

>There is a real bipolar " problem " against FETs and is that you need perfect matched devices<

As I wrote earlier, the JFET matching issue can easily be addressed by measuring and sorting. This requires extra work and organization on the part of the manufacturer, but it is completely doable, and the countermeasures will not bring any disadvantageous side-effects (unlike the base current and internal diode issues of a bipolar transistor front-end).

>the Neumann constant/pole makes that come back the " spark " in the high frequencies that is totally losted with out it and this is what that panel tell us in a different kind of tests.<

What implementing the Neumann constant will unequivocally do is force the phono EQ amplifier response to rise at HF, which will boost the high-frequency energy in the pops and clicks on your LPs, making them noisier.

The issue with DMM is a bit similar - the cutter operates with a high-frequency bias signal (of around 70kHz) to make it easier to cut the amorphous copper blank. This bias signal is sufficiently large enough that you may be able to discern it as a distinctive pattern if you look at a DMM LP with a microscope.

Implementing the Neumann constant in a phono stage again will give extra amplification to the 70kHz bias signal. This doesn't stand out as being the most optimal approach.

>You are a LOMC designer so I don't need to enligth you why bipolars are better electrical match than FETs.<

Your arguments haven't been very convincing so far.

>This is not about wiisdom.<

At least we can agree on one thing!

>That phono stage originally was designed by P-mares and a contribution from you latter on. Is not even balanced and used Jfets at the input with an AD829m overall topology and I was thinking was your own design an a unique one but it's not.<

@rauliruegas, wrong again. My present phono equalizer circuit can be loosely regarded as sharing a similar conceptual approach as the HPS 5.1, shown at the following page, but done with completely discrete devices.

http://www.synaesthesia.ca/LNschematics.html

In any case, I hope that you will become able to technically substantiate your opinions.

kind regards, jonathan