Hexfet power amplifier

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Mindaugas

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Joined 2004
#1
I'm planning to build a medium power(60-100W for 8ohms) amp.
I found this project at
http://users.otenet.gr/~athsam/power_amplifier_65w_hexfet_eng.htm

Now I wonder how this thing sounds. Is it worth to build it? Or it's just a waste of time and I must search for another better power amp schematics.

Any Comments would be appreciated
 

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#2
I tend to think that if the the mosfets are to be connected common source, then a circuit using only N-channel devices benefitting from their superior performance should do better. If an amp uses a P-channel device, it may as well use source follower configuration. The main hitch with source followers is that the gate drive circuit needs to operate about 15 volts beyond each drain power supply.
 
#4
This volt thing is true but you must also consider how amperes at 15 V. With a nornmal load (speaker) you only have to have 5-7 volts gate-source voltage. With this design you loses a coulple of watts but in decibells it's almost nothing (1 dB?). Otherwise the design is pretty normal, nothing wierd. 🙂

The amp will very much likely work since it's already built. It seems that the output stage is having local current feedback.
 
#5
It is impossible to say beforehand if this amp will work.

This is an amp from Elektor magazine called hexfet amp. The problem with this amp is stability. It may, or may not be stable. The designers never figured out why it does work in one case and not in another.

I know because I e-mailed about it with the designer (t.giesberts@segment.nl).

use mica insulators for Q12,13
use ceramic insulators for Q10,11,8
very close matching of Q1-4, Q7,9, Q10,11

....and hope for the best. The sound is alright though.


I have its higher power brother with IGBT's instead of hexfets. This amp has the same stabitlity problems!

hope this helps
 
#6
ABO said:
It is impossible to say beforehand if this amp will work.
This is an amp from Elektor magazine called hexfet amp. The problem with this amp is stability. It may, or may not be stable. The designers never figured out why it does work in one case and not in another.
Click to expand...
Not only difficult to get stable in the sense of not oscillating, but also in DC offset. I built one (channel) of these and couldn't get the offset stable; it would 'jump' up and down and personally I don't like an amplifier that does that :cannotbe:.
At least it didn't oscillate in my case.

If you like this 'kind' of amplifier (symmetric, mosfet output) maybe you should take a look at the cresendo ME
http://www.diyaudio.com/forums/showthread.php?s=&threadid=12885

A more thoughtful design, also from elektor, with a servo to keep DC offset within limits (some people don't like that either though).
Sound is supposedly OK, but I can't really comment on that since I never built/heard one.

I have its higher power brother with IGBT's instead of hexfets. This amp has the same stabitlity problems!
Click to expand...

Probably because it is the exact same design with IGBTs instead of mosfets 😉

This amp was mentioned earlier on this forum as well, see
http://www.diyaudio.com/forums/showthread.php?threadid=25384

And the IGBT version too:

http://www.diyaudio.com/forums/showthread.php?threadid=23766


Remco
 
#7
depends, the basic topology could work ok with some changes for high frequency compensation

the pcb layout you point to sucks

so if you have a >100MHz 'scope and enough knowledge to cure the high frequency problems you may encounter with this design and you are capable of doing your own layout/prototype with good power, ground and RF practice - you wouldn't have to ask

bottom line, look elsewhere for a less problematic design
 
#8
ojg said:
I have a feeling this amplifier will not work... The drain and source of Q12 need to be switched around.
Click to expand...

I see no reason for the amplifier do not work...the output stage is a regular compound complementar feedback pair where the output devices are Mos-Fets. (nothing new under the sun!!) 😉
 
#9
Tube_Dude said:


I see no reason for the amplifier do not work...the output stage is a regular compound complementar feedback pair where the output devices are Mos-Fets. (nothing new under the sun!!) 😉
Click to expand...

Hmmm, you may be right. I was confused by the drawing of Q12, usually the gate "lead" is drawn towards the source, like in Q13. As long as Vgs is 2V for both Q12 and Q13 it should be fine.
 
#10
11.

Hi Mindaugas,

Nice to see you here! How the hell are you? Finished your new home?

I recognize this as the Elektor circuit too. I have built a couple of Elektors, notably the Crescendo and one other from Giesberts. All these amps had small, niggling problems.

First, it is over complex. Much more than it needs to be. (Per-Anders, I'm looking at you.......... grin!) Redolent with CCS and complementary input and VAS stages.

Second, instability problems are likely coming from the CFP output stage. My advice would be small mica caps, around 100pF, across the base/collector of Q10 and Q11. You might also lower gain of the driver by inserting a 0R47 source resistor in line with F1 and F2; this reduces feedback factor across the CFP.

Generally, CFPs don't work well in Class AB - they really prefer Class A.

Cheers,

Hugh
 
#12
Sir
I have assembled giesberts HEXFET power amplifier with MOSFETS IRF 530 and IRF9530 and it was working fine since 12 years without any problem. But there is a power limitation and it should be used with 8 Ohm speakers only. There was no oscillation and no MOSFET blowing off as in IRF 540 and IRF 9540. I recently assembled one more amplifier with IRF 540N and IRF 9540N,but the amplifier is not stable and mosfets are blowing off. Can we use two IRF 530s and two IRF 9530s are in parallel to get more power instead of using IRF 540 and IRF 9540. Please help in this regard.
 
#13
Well most of Giesberts circuits suffer from the same issues ...Bad designed circuits based on poor simulations only

Matching of the semis in the input is extremely critical but also general implementation together with room temperature will effect the stability of the amplifier when it comes to local bias and offset ...poorly ventilated box will have A behavior while good ventilated box will have B behavior ...Under the rool of the average temperature of the country that the amp will operate ...

@Hugh
bad bad boy!!!!!😀😀
Saying things like that for the CFP amplifiers ....
The P3A for example is the king of stability but you already know that ...Truth is that multiple output CFP is not stable ( cannot be made to my opinion also no point to make one ) and as about mosfets I really don't have an idea still i grab the opportunity to make a joke every now and then

My best wishes to you and family Hugh hope that everything runs smooth for you .

Kind regards
Sakis Athens
 
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#14
Hi Sakis,

Thank you for your postl, amazing, my comment was made in 2014, almost TEN years back!

I would never argue you Sir; you fix amps, and you know just about everything that can go wrong, and corresponding, you know what works too.......

I will PM you..... and I return your greetings to YOUR family too, thank you.

Ciao,

Hugh
 
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