F5X -- the EUVL Approach - The Build Thread

[EUVL]

> Can someone explain the purpose of R21 and R22 which connect between the outputs and ground? Is it for DC stability ?

In a proper F5X with floating X, the feedback resistor network is not referenced to Gnd, unlike the single ended F5X.
Even if the speaker is connected, it is wise to have a reference path to ground to keep absolute DC (as opposed to differential DC) under control.
You will find the same resistor at the output of the Aleph-X.


Patrick

[EUVL]

The DIL sockets I buy from a local shop. So no part number I am afraid.
But the PRP 1/4 resistors has lead diameter of 0.6mm. This will fit most, if not all DIL sockets.

Failing that you can always solder some 0.8mm solder pins on the PCB at those positions, and then solder the resistor onto the pins.


Patrick

[lhquam]

Originally Posted by EUVL

> Can someone explain the purpose of R21 and R22 which connect between the outputs and ground? Is it for DC stability ?

In a proper F5X with floating X, the feedback resistor network is not referenced to Gnd, unlike the single ended F5X.
Even if the speaker is connected, it is wise to have a reference path to ground to keep absolute DC (as opposed to differential DC) under control.
You will find the same resistor at the output of the Aleph-X.


Patrick

I have been analysing equations for the X circuit and cannot see any evidence that the reference path to ground is ill-defined. The ground reference is established by the JFET input inputs and is the common-mode voltage of the inputs. The voltages on the X wires will be nearly identical and the same as the common-mode input voltage minus the difference of the P and N JFET Vgs values. There will also be minor voltage differences due to the different distortion currents running through the 2 connections.

I still do not know if there is a significant difference in distortion cancellation ability between the X and the ungrounded balanced topology (ie. shorting the to X wires).

Maybe I am confused or missing something?

[EUVL]

> Maybe I am confused or missing something?

Yes.

Think what is in the circuit that will stop the outputs from drifting relative to Gnd together (common mode DC) with floating X.
Differential DC is taken care of by the feedback network.

> I still do not know if there is a significant difference in distortion cancellation ability between the X and the ungrounded balanced topology.

In terms of distortion no.
Nelson did published another balanced frontend in his F5Turbo article, which I would call F5H.
By all means try them all. The PCB will support all these configurations.


Patrick

[lhquam]

Originally Posted by EUVL

> Maybe I am confused or missing something?

Yes.

Think what is in the circuit that will stop the outputs from drifting relative to Gnd together (common mode DC) with floating X.
Differential DC is taken care of by the feedback network.

> I still do not know if there is a significant difference in distortion cancellation ability between the X and the ungrounded balanced topology.

In terms of distortion no.
Nelson did published another balanced frontend in his F5Turbo article, which I would call F5H.
By all means try them all. The PCB will support all these configurations.


Patrick

I see the problem now.

The problem is if the DC voltages of the outputs drift equally inopposite directions, keeping the voltages at the X wires the same. Bad news: the DC bias current increases on one side and decreases on the other side.

[EUVL]

The feedback loop works when they drift in opposite direction.

Actually you may be right. Maybe we do not need those output resistors.
Need to think about it more.


Patrick

[lhquam]

I retract my previous post, and now totally agree with you about the problem.

The output voltages can drift together causing the X voltages to also drift by the same amount. This is equivalent to the common mode DC input voltage drifting by that value with the X wires grounded. Not a good situation w.r.t. output bias currrents.

[lhquam]

Originally Posted by lhquam

I retract my previous post, and now totally agree with you about the problem.

The output voltages can drift together causing the X voltages to also drift by the same amount. This is equivalent to the common mode DC input voltage drifting by that value with the X wires grounded. Not a good situation w.r.t. output bias currrents.

I now retract my retraction. Here is a pictorial DC analysis of the X circuit
assuming transconductance amplifiers, and only common mode input.

[NYCOne]

Originally Posted by lhquam

I now retract my retraction. Here is a pictorial DC analysis of the X circuit
assuming transconductance amplifiers, and only common mode input.

Is your final conclusion that R21 and R22 are useless?

[lhquam]

Originally Posted by NYCOne

Is your final conclusion that R21 and R22 are useless?

Yes.