Pre amp therapy / therapy pre

[abbey road d enfer]

Can somebody please explain the two 10k resistors at the output transformer primary? Are they there to load the transformer?

Yes. In order to linearize the impedance, which, if these resistors were not there, would vary considerably with frequency, in particular when the inductance and parasitic capacitance resonate.
In othere words it's called damping.
It is necessary because the circuit does not use global NFB.

The aforementioned Rohde & Schwarz output transformer have a rather high dcr compared to the Lundahl. In my simulations, lowering these 10k resistors resulted in lower output but also flatter frequency response, especially the low end extended. With no resistors at all the low frequency response dropped considerably but only in pentode mode.

That's all normal and predictable. In pentode mode, the internal resistance of the tubes is very high, so the circuit is undamped. In triode mode, the internal resistance of the tubes is much lower, which provides the necessary damping.

I’m also asking because I’m contemplating about where these transformers are better suited. Am I right in assuming that the high primary DCR makes them less ideal for push pull circuits? The older single ended circuits like BA-2A had output transformers with rather high dcr but all the push pull transformers I looked at have lower dcr. I suspect that the gap in the single ended transformers account for the higher dcr because it needs more windings to get similar inductance?

Your conclusions are correct.

 

[Murdock]

One more question about these dubious Rohde & Schwarz Transformers. How does the current flow between these two resistances (transformer dcr and load resistors)?
Is the current through the transformer reduced because some can flow through the load resistors? In my simulations they act as it were two resistors in parallel (20k and 9k4 is 6k4). I’m asking because I’m concerned with the heat of the transformer. 12mA through 6k4 is 0.9W but 12mA through 9k4 is 1.4W which is on the high side for this transformer.

 

[abbey road d enfer]

One more question about these dubious Rohde & Schwarz Transformers. How does the current flow between these two resistances (transformer dcr and load resistors)?

Most of the current goes through the transformer. Don't worry about the resistors.

Current is split according to the inverse of resistance.
I don't know the DCR of the windings, but they cannot be more than 1 kohm (probably much less).
That would result in 10.9mA in the winding, for a dissipation of 118mW, and
1.09mA in the 10k resistor, for a dissipation of 11.8mW.

 

[Murdock]

Yes. In order to linearize the impedance, which, if these resistors were not there, would vary considerably with frequency, in particular when the inductance and parasitic capacitance resonate.
In othere words it's called damping.
It is necessary because the circuit does not use global NFB.

That's all normal and predictable. In pentode mode, the internal resistance of the tubes is very high, so the circuit is undamped. In triode mode, the internal resistance of the tubes is much lower, which provides the necessary damping.

Your conclusions are correct.

Most of the current goes through the transformer. Don't worry about the resistors.

Current is split according to the inverse of resistance.
I don't know the DCR of the windings, but they cannot be more than 1 kohm (probably much less).
That would result in 10.9mA in the winding, for a dissipation of 118mW, and
1.09mA in the 10k resistor, for a dissipation of 11.8mW.

The dcr of the transformer is 9k4 ohm.

 

[abbey road d enfer]

The dcr of the transformer is 9k4 ohm.

That's very high. Inductance must be very high, as well as resonance.
It means current splits about equally betwen winding and resistor, for about 6mA each and 360mW.

EDIT: I've checked the datasheet, Windings are supposed to measure 156 ohms, not 9.4k.

 

[Murdock]

That's very high. Inductance must be very high, as well as resonance.
It means current splits about equally betwen winding and resistor, for about 6mA each and 360mW.

EDIT: I've checked the datasheet, Windings are supposed to measure 156 ohms, not 9.4k.

What datasheet? These are old transformers from the German company Rohde & Schwarz. I doubt that there is any datasheet. Maybe you confuse them with another transformer.

Wouldn’t the currents split equal only when the resistors and windings are equal in resistance?
What if the resistors are higher than transformer dcr?

I’m also puzzled by these really high winding resistance. And yes, inductance is higher than other gapped transformers I’ve seen. But high frequency response is also pretty good. I think it was good up to around 60khz or more.
They are wound on a split bobbin with primary-screen-secondary-screen-primary as far as I can recognize it.

 

[abbey road d enfer]

What datasheet?

The schematic in post #7 shows a LL1571.
There's a whole lot of confusion in this thread.

Wouldn’t the currents split equal only when the resistors and windings are equal in resistance?

As I wrote earlier, current splits according to the inverse of resistance.

What if the resistors are higher than transformer dcr?

Then they receive less current.