Amp design for subwoofer and wideband duty

[CBS240]

Originally Posted by AndrewT

The output impedance of the cascoded VAS is comparatively high cf. a Follower.

Most Designers that suggest the cascode also say you must lower that output impedance by using a Follwer after it.

Since there is no separate driver stage. The Crecendo is flouting those design recomendations.
Even D.Self who adopts the double EF as a VAS says there are attractions to using the cascoded VAS + EF.

I agree. The cascode VAS creates a transimpedance amplifier, changing the input current X current gain into the output voltage. There are great advantages to this but variations in cascode VAS current create voltage related distortions, which must go back through the loop and so on. Less distortion is better.

[AndrewT]

Originally Posted by AndrewT

I believe the current mirror only becomes effective, if both sides of the LTP are fed and loaded symmetrically.
I further believe that this may be the reason that resistor loading is preferred to mirror loading when sound quality is uppermost in the priority list.

Originally Posted by homemodder

Andrew how do you conclude this,

which part? :
I believe
mirror becomes effective if
LTP symmetrically loaded
LTP symmetrically fed
sound quality difference

[formula 22]

Hello!
Why amplifier for the subwoofer is so expensive when you have a mosfet IRFP240/IRFP9240 or IRF250 etc,.... rather than those expensive 2sk/2sa!???
It does not have the subwoofer amplifier to be a High-End?
It should be the simplest amlifier to about 200 .. 300W(rms)!
(See topic APEX Audio of Mr. Mile Slavkovic)
Cheers

[AndrewT]

Or go to our resident Member's Quasi threads.

[homemodder]

Originally Posted by AndrewT

which part? :
I believe
mirror becomes effective if
LTP symmetrically loaded
LTP symmetrically fed
sound quality difference

I agree that mirrors will be more effective objectively when there is symmetry, yet it also works in designs, objectively and subjectively where there isnt symmetry involved, a good example is Lavardin which uses highly tweaked Lin circuits and huge amounts of feedback which of course is another big no no but yet their amps sound superb.

[AndrewT]

Are you agreeing that I am allowed to reach all those conclusions?

BTW,
I have never owned any HiEnd equipment. Not even connectors. I am way below that target market.

[formula 22]

Originally Posted by homemodder

I agree that mirrors will be more effective objectively when there is symmetry, yet it also works in designs, objectively and subjectively where there isnt symmetry involved, a good example is Lavardin which uses highly tweaked Lin circuits and huge amounts of feedback which of course is another big no no but yet their amps sound superb.

And all this for the amplifier for the subwoofer-please let!!??

[Bensen]

Originally Posted by AndrewT

The output impedance of the cascoded VAS is comparatively high cf. a Follower.

Indeed, I was struggling with that also the last days, that's wy I'm gonna use a driver stage (probably FET, 2SK2013 + 2SJ313)

Regarding the quasi topology; don't no wy, but it'n not attractive to me, maybe it's not symmetrical enough
I really love those symmetrical designs (it's something in my head)

Originally Posted by formula 22

And all this for the amplifier for the subwoofer-please let!!??

I had to weight my words better when choosing a good thread name.
Yes, this amp will be used for a sub, BUT, I also wan't to try a new topology for me, this to compare and to learn, for off course the next project.
And this, will for all of us DIY'ers a never ending story.

[Bensen]

I've choosen for the standard VAS and a driver stage. The cascoded VAS doesn't bring any advantage at the low frequencies for a sub amp.

Please have a look at the attachments:
1 Schematic
2 Phase margin plot, phase margin is +-84°
3 PCB layout (orange = ground)
4 PCB in 3D (done with Target 3001, max 250pins, that's wy only 2 output devices are drawn)
5 FFT plot 1Khz at 150Wrms in to a 4Ohm Load

All comments are welcome.

[Bensen]

I'm doubting which FET to choose for the driver stage.
I've both the 2SK216/2SJ79 as the 2SK2013/2SJ313 in my collection.

Driver stage Parameters:
1: load is 3Pair of 2SK1530/2SJ201 FET's
2: Diver stage idle current = +-25mA
3: Power dissiplation in one FET = +-2.2Wpeak

For schematic: see previous post.

Can anyone give me some help choosing the best of the above two pairs?
The 216/79 pair has very low capacitances and could be used as VAS imo, the 2013/313 look more like real drivers.
To bad that spice models for he 2013/313 are not excisting.

Thanks in advance