Will this work as preamp?

[lumanauw]

Hi, Mr.Pass,

Zen's spirit is about simplicity. From your tutorial about differential pair, I come with this idea.
Path A gives inverted signal, while path B gives non-inverted signal compared to input.
If one is inverted, and one is non-inverted, they can be made somekind of "feedback".
My idea is to put R and VR in Drains before R1 and R2 to make "Adjustable Gain" Preamp.

What is the pro and cons about this idea?

[Tazzy]

Without any technical backing, I must say that it's looks cool

[tschrama]

I think you might need some source resistor... else the max input voltage is limmited... besides that, I dunno if it's a practical solution...

just try it and tell us!


cheers,
Thijs

[PMA]

You must define Vgs of the 1st FET (the left one). Put a resistor from its gate to ground. After that you will obtain working amp, but with horrible distortion.

In case you want an usable amp, you have to use resistors in sources of FETs, provided the gate resistor has already been connected.

[lumanauw]

Hi, Tazzy,
Thanks !

Hi, Thjis,
The reason I dont put RE is that the transistors are mosfets, already got low OL gain. If I use bipolar, RE will lower the gain, offcourse.
I never tought of input voltage capabilities. Thanks.

Hi, PMA,
Thanks for the tip.

but with horrible distortion

Why is that? Because no feedback is being fed to gates?

But it will work as adjustable gain, or wont it?

[PMA]

Without source resistors you have high voltage gain, but high nonlinearity.

[MikeB]

Hi !

I would say there are 2 problems:

1, As already said, without Sourceresistors the amp drives immediately
into "saturation", giving extreme high 3rd harmonics. These resistors
are a must, they enlarge the drivingrange and reduce distortion.
Diffamps typically doesn't normal clipping, they compress.
With adding cfp and these resistors, you get rid of most distortions.
Are these jfets or mosfets ? jfets have a leakagecurrent through
the gate, so you must have a resistor to ground from gate, or the
diffamp does not work at all.

2, it has a very high outputimpedance.

Yes the resistors between legs are an easy way to reduce gain and
speed up the amp, but they don't create feedback.

Mike

[azira]

Hmm,well you could likely control the gain with R and VR but they have to be somewhat smallish in value and your actualy load impedance from whatever this is connected to has to be largish in value. You're going to have a large output impedance from this stage to top if off. I would firstly suggest that you buffer your output with a Source-Follower and then it would work, possibly pretty well, atleast in theory.

The gain won't be controlled by feedback though. Which I guess would add appeal to this for the Zen crowd. Basically R and VR combined appear as a load for your diff pair. So you can tune your load value to set your gain because gain is set by: -gm(Rc||RL) where Rc = collector resistance and Rl = load resistance or in this case, 1/2 ( R+Vr ). Assuming Rl is small enough compared to Rc (or for those with karma, Rc is large enough compared to Rl), you can ignore Rc and then your gain is -1/2(R+Vr)*gm. Adjust Vr and your stage gain gets adjusted, fine tune for your desired gain...
--
Danny

[lumanauw]

Hi, Mike,

What do you think of Son of Zen, or Zen ver6 or Firstwatt designs? They dont use RE, but it is power amp. What is the pro-cons about these designs? Mosfets have already got "internal" RE, dont they?
The devices are mosfets (IRF610), running at 20mA ccs.

but they don't create feedback.

I don't understand. If a device is running with output less than its OL gain, it must have feedback, don't they?

Hi, Azira,

I get your explenation. So this arrangement that I made is the same as if I built the same cct, without adjusting R+VR, but with very low R1 and R2 running at non-feedback (like Mike said)?

[DougL]

What you describe is called a Long tail Pair.
Tube Cad Jurnal

Works well with 6SN7's.

Doug