Bottlehead C4S as a differential pair constant current sink?

[Paul Joppa]

The LED drops about 1.55v each, and the smaller transistor shows about 0.7v base to emitter, leaving 0.855v for the current-set resistor R1.

On the schemo, you have reversed the transistor orientation but have not reversed the rest of the circuit. The base of the 2N2222 goes to the junction of the two LEDs, and the MJE340 base goes to the junction of R2 and the top LED.

There should be no cap across the C4S (from ground to cathodes), and as PB said, R2 should go to the high voltage (I get 70K ohms). Both of these changes are to maintain a high AC impedance at audio frequencies, which is the reason for using a C4S in the first place.

In class A, where both triodes are conducting all the time, the most meaningful inductance at signal frequencies is that of the whole winding as driven by the two triodes in series. The two halves of the primary winding are very tightly coupled and the diff-amp cannot be analyzed as two independent halves.

[Horns Forever]

Ok, I think I've got it.  Please verify the attached schematic.  Here are my responses:

In class A, where both triodes are conducting all the time, the most meaningful inductance at signal frequencies is that of the whole winding as driven by the two triodes in series. The two halves of the primary winding are very tightly coupled and the diff-amp cannot be analyzed as two independent halves.

The combined plate resistance of the two triodes in series is about 14.8k.  Based on my understanding of your comment, I would want an output transformer with a total primary inductance of about 300 Henrys.  This would be just over 2.5 times the plate resistance at 20 Hz.  Does that sound about right?

There isn't anything about a balanced cable interface that both the hot and cold legs are actually driven by signal.  This is a huge assumption that a lot of modern HiFi manufacturers would like their customers to make, but it simply isn't true.

In some cases, I would agree; it depends on the circuit.  A lot of pro audio equipment is balanced and does have signal on both legs.  The signal coming off a phono cartridge is balanced and has signal on both legs.  If my phono preamp uses fully balanced gain stages throughout the circuit, the output (into this preamp) will have signal on both legs.  On the other hand, I've seen many balanced connections that are just cosmetic. 

[Paul Birkeland]

The combined plate resistance of the two triodes in series is about 14.8k.  Based on my understanding of your comment, I would want an output transformer with a total primary inductance of about 300 Henrys.  This would be just over 2.5 times the plate resistance at 20 Hz.  Does that sound about right?

That's a good target to shoot for.

In some cases, I would agree; it depends on the circuit.  A lot of pro audio equipment is balanced and does have signal on both legs. 

A lot of pro audio equipment (especially a lot of the high dollar vintage gear and copies) have transformer coupled inputs and outputs to provide balanced inputs and outputs.

The signal coming off a phono cartridge is balanced and has signal on both legs.  If my phono preamp uses fully balanced gain stages throughout the circuit, the output (into this preamp) will have signal on both legs.  On the other hand, I've seen many balanced connections that are just cosmetic.

This is only true if the + and - connections to the cartridge for each channel see equal impedances to ground.  If you connect -L and -R to hard ground at the input of your phono preamp, then you no longer have a balanced cable interface between your cartridge and phono preamp.  If you put a step-up transformer between your MC cartridge and your phono preamp, you can actually add a few switches and resistors to listen to both a balanced and unbalanced cable interface.  I would suggest trying and evaluating this setup.

[Paul Joppa]

You have understood my erratic comments on the circuit correctly.  :^)

I'll just add that the notion of primary inductance is pretty fuzzy when you have no airgap in the core. The effective inductance depends on signal level and frequency, and can vary over a very wide range. Usually the manufacturer's spec will indicate a suitable impedance level, i.e. the inductance will be acceptable at that impedance.