Hello all and Question about MOSFETs

[Magz]

Hello all, newbie here but frequent contributor to Audiokarma and Audio Asylum under the name Maxamillion (someone already had it here).

I have a question regarding the MOSFET outputs in a low-frequency equalizer that is used with Infinity RSIIb speakers (early 1980s vintage). The equalizer was sold with the speakers and is used to boost the very low frequencies and to help the transition from the two 10" poly woofers to the planar EMIM midranges.

I have extensively modded the equalizer, replacing all the resistors with 1/2W PRP and TX2575 resistors, replacing and bypassing all the old electrolytic caps, upgrading the output coupling caps and wiring, and installing Cree SiC rectifier diodes. The equalizer now sounds MUCH more transparent than previously, but I can still tell when it is in line between my Supratek tube preamp and Gilmore Raptor class D monoblocks. I'm wondering if the original MOSFETS, VN40AD designation, have a more modern equivalent that might sound better. The VN40AD is no longer made, so I'll eventually need to find a replacement if they ever die. I have a few VN40AF which are also obsolete but I suspect they will sound pretty close to the ADs.

Any help from the MOSFET experts is greatly appreciated!

[Magz]

Hmm, must have been a tough question. Let me re-phrase:

Is there a possibility of improving the sound of the equalizer by replacing the old vertical N-channel enhancement MOSFETs with newer technology? Has there been a leap forward in this technology over the past 25 years, or are the old MOSFETs just as good as the newest ones?

[peranders]

Those mosfets are rather rare these days because the have low voltage rating and rather low current also.

VN40AD
40 Vds
5 Ohms Rdson
1.5 A
50 pF input
10 pF reverse
TO220 package

Do you know where the VN40AD is located in your circuit?

[Magz]

The VN40AD is located right before the output capacitor. Here is a schematic of the unit (it shows a VN40AF but my unit has a VN40AD).

[jacco vermeulen]

AD and AF versions of the VN40 are one and the same chippendale in a different package.
The T0220 AD is good for 20W, the AF is a TO202 top hat type and only good for 15Watts.
Just use a (large) replacement heatsink, the originals run at 1/2W dissipation each.

[Magz]

I figured that the difference was only the package from looking at the datasheets. They don't run anywhere near their limits in this application, in fact they don't even have heatsinks on them, so the AF should work fine as a replacement if I need one.

What I'm wondering is whether there are newer n-channel enhancement MOSFETs available which will have improved audio-relevant properties in this application. The entire signal passes through them, so any improvements I can make should be beneficial.

I already replaced the opamps with OPA2107s, and noticed a bit fuller and tighter bass.

[Magz]

I asked International Rectifier this question, and their tech guy replied that the IRF4104PBF HexFET would be a good modern replacement for the VN40AD. Looking at the specs it has the same breakdown voltage but the Rds is 1000X lower at the same Vgs.

I'm thinking that lower Rds will be a good thing for performance, correct?

The capacitances are quite a bit higher for the HexFET, however. The input cpacitance is 3000pF for the HexFET vs. 50pF for the VN40AD, the output capacitance is 660pF for the HexFET vs. 50pF, and the reverse transfer capacitance is 380pF for the HexFET vs 10pF, all measured at Vds = 25V, Vgs = 0 and f = 1MHz.

What does the higher capacitance mean in terms of performance as an audio amplifier?

[jacco vermeulen]

Originally Posted by Magz

lower Rds

Rds(on) is the resistance between drain and source when the device is fully opened, important for switching purposes, Rds(On) pretty much determines how many amps the drain of the device can handle.
Rds(On) is not relevant for audio purposes, as in linear operation.

An IRF610 has stats which are more comparable with your VN40 parts.
An option would be to raise the bias level to 50mA and higher, hence the heatsink comment
Other alternative would be the use of Lateral MOSFETs, Hitachi 2SK213 or higher.

Datasheet of the VN40AD=> http://www.datasheet.org.uk/pdf-data...ok203-1791.pdf

[Magz]

Thanks jacco, those Hitachi mosfets look interesting. I think I'll look into them further as they seem to get excellent reviews for audio quality. Different pinout, but that's easy to fix with a little judicious lead-bending.

[peranders]

The IRF4104PBF would probably not work as intended in this design due to the high input capacitance. The lower mosfet, it is the same type as the one above? The lower one is a depletion mode mosfet.