PROTECTION DIODE on HexFET Amp, HELP!

[de Ocampo]

HI all,

I am showing here for the second time the HexFET amp from Elektor for the following reasons-

Quoted from the previous help thread on this amp;

*connect a 1N4004 or similar and a 12V zener protection diodes to Gate - Source terminals

*A of zener1 to G of T12
K of zener1 to S of T12
A of zener2 to S of T13
K of zener2 to G of T13

Looking further at the circuit, both channels means S to VAS rail and D to mid-v (out). If the symbol used for the output was precise then to my understanding the suggested placement for the protection diodes would become -

A of zener1 to D of T12
K of zener1 to G of T12
A of zener2 to G of T13
K of zener2 to D of T13

Is this correct?

From the previous help thread there was a mention of using 6.8v, and 12v for the zeners. Was there a difference?

One constructor mentioned of replacing P2 I wonder what rating would that be... Sorry some links no longer exist from the previous thread can't view your modified versions

Your guidance will surely clear things out for me....


Thanking you in advance



(sorry mooly, no can do, I guess your design will have to wait, lateral mosfets will only become available upon ordering in this place. I don't do this..too risky and most likely you'll get fake ones...always happen.)

 

[Junm]

hi abeeter, where you located in PH?

refer to the attached schemetics on how to connect the protection diodes. You can also try this schematic which uses N channel mosfets which are abundant here in PH.

you can purchase lateral Mosfet in RAON in manila but in higher price compared to IRF's.

 

[de Ocampo]

Easternmost in Region V, I work here in a geothermal facility, fact is Kapampangan runs my blood Mekeni ku naman...surprised? I live here na

I'm still confused 'bout the pin assignment of the drain & source. Upon encountering various mosFET circuit, to most of them the source was tapped to mid-voltage(out) and the drain to voltage supply. The gate is always fixed. This one here was different though... I had the parts, well at least for one channel (no turning back eh). I'm currently doing a revision of the pcb, that is why I need to figure-out the proper circuit for the protection diodes.


Thanks & Regards

 

[de Ocampo]

Here's a quick draft for you to see if I doing it the right way

 

[meanman1964]

Why would you install those zener and diodes?

 

[djk]

So you don't blow the gates when clipping into a low impedance load.

A small resistor needs to be inserted between the collectors of the driver transistors and the diode strings

 

[meanman1964]

The main problem of this amp is oscillation.

 

[bigpanda]

I am not trying to discourage you but I hope you are not expecting it to give you 120w (not even 60w) with a power rail of +-35v and a pair of outputs like that.

 

[de Ocampo]

So you don't blow the gates when clipping into a low impedance load.

A small resistor needs to be inserted between the collectors of the driver transistors and the diode strings

You're quite right both collectors of the driver transistors were tied to VAS rail, will it do if I move the diode string and place it after the resistors at the gates?

 

[WuYit]

Hi,

will it do if I move the diode string and place it after the resistors at the gates?

Yes. You don`t need the external diodes across drain and source.

 

[WuYit]

meanman1964,
that's certainly true. Have you built it?

 

[de Ocampo]

to meanman,

from the previous help thread on this amp a forum member named ricknl posted the following-

In the July-Aug 2002 Elektor mag, a revision to get better stability from both versions (mentioned the next higher power brother) was published.

Place 27nF parallel to R31
Place a resistor of 20K between collector of T8 & ground
Place a resistor of 20K between collector of T9 & ground
Replace R20 by 1K8
Replace R17 & R18 by 390 ohms
Replace R3 & R4 by 33 ohms

According to Elektor this should improve the stability & the performance of the amp.

If someone though can confirm, that these mod work perfectly right, then it would be a relief


to bigpanda,

Yep I am aware of it, so long that it sounds alright though, I only had 150watts pair of loudspeakers for this amp.

 

[meanman1964]

Before you do all the mods try by adding 220p caps between the collector and base of T10-T11 and T8-T9 use a Mica a tip from Hugh Dean of the Famous Aksa amps(Aspen)http://www.diyaudio.com/forums/solid-state/27200-hexfet-power-amplifier.htmlAlso look here http://www.diyaudio.com/forums/solid-state/112426-hexfet-amp-please-help-me-fix-6.html post56

 

[bigpanda]

Hi Junm,

Can you kindly show us the link to your 'sample schematic'. Just want to have a closer look at it.

Tks

 

[meanman1964]

meanman1964,
that's certainly true. Have you built it?

I've one original Hexfet with nothing changed but planning to add those caps.I also have 4 pairs of the IGBT versions only one(1 channel) is playing stable without any mods.By one IGBT stereo version I did the Eddelarue mod http://www.diyaudio.com/forums/solid-state/112426-hexfet-amp-please-help-me-fix-2.html but still need to be tested.
There are many people saying take another amp design with fets but I can't explane but they have me in a grip I need them to play stable and they will be no matter what.

 

[WuYit]

meanman1964,
in order to reduce trouble select the devices more carefully. The driver needs to be very fast.

 

[de Ocampo]

With all of the complications that this amp had gone, perhaps the suggested caps (220pf/100pf) might do the trick It looks like the suggested mod from Elektor was not helping at all eh

Well, I'll take my chances then build the thing and see what will happen


Thank you guys for the help


-Pls. disregard my own interpretation of the protection diode in post #1, my failings upon seeing the schematic of the quasi (posted by JunM) I get the idea.

 

[meanman1964]

With all of the complications that this amp had gone, perhaps the suggested caps (220pf/100pf) might do the trick It looks like the suggested mod from Elektor was not helping at all eh

Well, I'll take my chances then build the thing and see what will happen


Thank you guys for the help


-Pls. disregard my own interpretation of the protection diode in post #1, my failings upon seeing the schematic of the quasi (posted by JunM) I get the idea.

Soon gonna try it myself keep you all informed

 

[de Ocampo]

meanman,

How to Build a 100 Watt MOSFET Amplifier Circuit? Simple Design Explored

check this link, perhaps the designer of this one has seen the shortcomings of the Elektor hexfet, take notice of the caps at the drivers and the source resistors only, that this one uses lateral mosfet instead, similar design same power requirements

 

[djk]

That schematic can't work, no drive for the N-Channel FET, bias circuit isn't right either.