Max fet capacitance before follower circuit required?

[Apogee]

Is there a rule of thumb regarding the maximum number of fets that can be paralleled before a circuit begins to suffer high freq roll-off? Is there a total capacitance figure that we should try to stay below? How does one estimate that value for a given circuit?

I know the Aleph 1.2's used 24 outputs. How noticeable is the difference if you keep hanging additional fets on a circuit? Does it affect the overall tonal balance or just the "air" at the very top end?

What about simply paralleling drivers or adding additional heatsink capacity to a single pair and upping the circuit bias? Will this "overcome" the increase in capacitance of the additional fets as long as the heat is kept under control?

Finally, does the use of a follower circuit help with bass control? Does its use change/color the sound?

Thoughts?

Thanks,

 

[Zappa]

To my humble opinion, experimentation is the key here.

You could get more speed and bandwidth from a healthy driver stage but also the distortion associated with it. In the other hand, an underfeeded output mosfet bank can sound horrible and be instable. More, different parts = different input capacitance = variable answer.

I think a good old try could solve the question more rapidly than we think...

And, is that not the aim for DIYing???

Anyway, good luck.

 

[Nelson Pass]

The Aleph 1.2 represents about as much capacitance
as you can do in an Aleph before you start seriously
compromising the high end. It's a little darker than an
Aleph 2, but some people like it that way.

Of course you can always drive the banks of devices
with a lower impedance, and that works just fine.

 

[nar]

re banks(not hold-up)

That means a very low impedance transmission line between 1st stage and follower.Perhaps we would need to bias the 9610 a little higher,to get a good gain figure even at such low impedances
I guess for me a 6 + 6 config.like in the Aleph 4 is enough for me.I wouldn't like a darker sound.On my Alephs I find sounds very sweet and subtle as we go towards high frequencies,like a no end road but which would be lower level as we climb the hill.
This is a nice and so honnest sound character.I realized recently for a friend a power amp using some tda 7294 power packs.(he is not fortunate,or he doesn't want to put more money inside).The sound is no comparaison in high frequencies with Alephs.The 7294 character is less clean,more agressive and even a bit accentuated,sounded "vulgar"to me!!!!....

Regards

 

[Apogee]

Thanks all for the replies!!!!

A couple of other novice questions if I may:

What determines whether or not a circuit will adequately drive a low impedance load? For example, why is a follower more suited to driving the output stage rather than another gain stage?

Why does the 3 gain stage Aleph drive low impedance loads better than the 2 stage circuit without using a follower?

Lastly, can anyone recommend a good sounding follower design that I should look at to learn from?

Thanks very much,

 

[Nelson Pass]

There is no intrinsic reason why followers will drive a low
impedance load better than a common Source stage.

Remember that the semiconductor itself doesn't know the
difference. It sees the same voltages and currents either
way.

If you want to drive low impedance loads, you simply get
enough hardware together for the occasion. There is no
magic technique.

The 3 gain stage Alephs did a better job on low impedances
for 2 reasons: 1) 3 stages does give more feedback and
more drive 2) More importantly, the Aleph 0 and Os were
capable of operating push-pull beyond the single-ended
Class A bias point, so they could easily deliver much more
than the 2X bias current rating of the 2 stage design.

 

[Apogee]

Thanks Nelson!

Can anyone recommend a circuit example using a follower that I can use as a starting point to add to either an Aleph or the Aleph-X to drive additional outputs?

Thanks,

 

[GRollins]

Adding a follower isn't that hard. The problem will be that you'll have to completely redesign the front end if you want to remain direct coupled. The DC offset on the load resistors for the front end is what sets the bias for the back end...you'll need that 4V or so offset to get the back end of the amp to light up.

Grey