Preamp concept

[PMA]

I am thinking about building a new preamp. The concept is shown in the attached image.

[bocka]

Hi PMA,

have you thought once about a folded cascode? Your preamp can modified simply: Connect the base of your third bjt to a constant voltage. Remove the resistor from the collector of the second transitor to ground and connect the collector directly to the negative supply.

The benefits of a folded cascode are:

1. Only one amplifying stage
2. Very high cutoff frequency

I like the output stage you made. But it can be made more simple: Remove the output transistors at the right side and connect two resistors (lets say about 470 ohms) from the drivers to the output. I made something similar and found the results very promising.

[darkfenriz]

Hi Pavel
Before I can comment anything I have some questions:
1. What's the RC near diamond buffer? Does it make anthing useful?
2. How do you set low offset voltage?
3. I assume that you are aware that you need very clean rails for your LTP-VAS arrangement and CCSs too, right?

cheers and regards

[PMA]

Hi bocka and darkfenriz,

B1) The folded cascode is great, I am thinking about comparing more solutions, cutoff frequency of the circuit shown is about 6MHz
B2) I want to keep the lowest output impedance, as I have excellent sound results with it (even if compared to 50 Ohm output impedance)

D1) The RC is a kind of frequency compensation
D2) I do not take into account low DC output offset in this step, this can be solved by standard way (BTW output offset depends on currents through differential pair)
D3) Yes, I am aware of it, again it can be solved by several ways, this is just a concept, which is just able to work

[darkfenriz]

The RC is a kind of frequency compensation

Applied along the section of voltage gain of 1....?
Is it class AB output?
On this RC there is a buffer error voltage drop, dividing by RC impedance you get parasitic error current drawn from VAS.
I think you are trying to be mysterious about the point of this arrangement... This seems to generate more distortion at higher frequencies. Or was it just a cure for non-stability on 6Mhz when strange things do happen?

I think I don't have to say that the rest of the circuit is perfect?

regards

P.S. For some extend your VAS is also folded cascode.

[anli]

I'd connect pre-out stage transistors collectors to out stage transistors emiters. Such step will make output buffer input impedance significantly more linear.

[darkfenriz]

Originally posted by anli
I'd connect pre-out stage transistors collectors to out stage transistors emiters. Such step will make output buffer input impedance significantly more linear.

isn't it simply an effect of higher bias?

[anli]

It is effect of constant Uce for pre-out transistors.

[anli]

BTW, Pavel, where are emiter resistors for pre-out pair?

[PMA]

Originally posted by darkfenriz

Is it class AB output?

No, it is class A.

Originally posted by anli
It is effect of constant Uce for pre-out transistors.

Yes, this is greatly desirable.

Originally posted by anli
BTW, Pavel, where are emiter resistors for pre-out pair?

Andrej, where would you like the output transistors to travel?