Transformer secondary resistors?
« on: January 13, 2021, 01:00:51 PM »
Hi all,
 Likely an easy question for some of you. In many schematics with input and output transformers, sometimes we see a resistor across the secondary windings. I somewhat understand their purpose in a zobel network but what do they really do when there is no capacitor involved?

For example, the original EQP1A schematic has a 600ohm resistor across the secondary of the input transformer before the passive filter stage. Jakobs schematic has a 10k.


CurtZHP

Re: Transformer secondary resistors?
« Reply #1 on: January 13, 2021, 01:07:59 PM »
I could be mistaken (and I usually am...), but I think the 10K across the output secondary on Jakob's G9 is to keep the polarity switch from going bump in the night when you switch it.

From the circuit description on www.gyraf.dk...
"The output of the transformer is taken to SW4, the phase reverse switch, that has 10K resistors across
it to control transformer load when switching. And then to the output XLR....."
Electrons don't read schematics.

squarewave

Re: Transformer secondary resistors?
« Reply #2 on: January 13, 2021, 01:28:08 PM »
It sets the impedance. The inductance of each coil in the transformer has a certain inductance / reactance that defines the characteristic impedance that it can work with before you get low frequency loss or distortion or high frequency peaking and such. Meaning a transformer doesn't have a specific impedance like a resistor. It just reflects the impedance that it "sees" on the other side. So there should be a well defined impedance such as in the form of a resistor on at least one side to set a well defined impedance of the network surrounding the transformer. If the characteristic impedance of the transformer secondary is 600 ohms, it is customary to have a 600 ohm resistor or maybe 680 ohms would be a standard resistor value in range. A 10K transformer would use 10K.

A Zobel network is somewhat unrelated. It is very common to get "peaking" at the high end of the frequency response when using transformers.  At least the good ones usually do. To correct for this, a low pass filter is applied in the form of a capacitor and resistor to bring the peaking down just enough so that the peaking is damped.

EmRR

Re: Transformer secondary resistors?
« Reply #3 on: January 13, 2021, 01:53:03 PM »
You see the occasional old schematic with multiple x2 loadings, like 1200Ω on each side of a 600:600, or even (4) 600Ω, one on each split winding of a center tapped 600:600.    Old Altec tube preamps are a rare US example of matched loading also, 82K secondaries with 82K resistors on them, as opposed to the usual open grid arrangement. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

abbey road d enfer

Re: Transformer secondary resistors?
« Reply #4 on: January 13, 2021, 02:13:17 PM »
In many schematics with input and output transformers, sometimes we see a resistor across the secondary windings.
A transformer without a load is a variable impedance, that increases with frequency. Electronic ciorcuits that don't rely on heavy NFB are sensitive to the impedances they are connected to.
Tube circuits in particular, but also many SS need to see controlled impedances. The stage the xfmr is connected to may have very large gain variations, that may not only alter the frequency response, but also result in unstability/oscillation.

Quote
I somewhat understand their purpose in a zobel network but what do they really do when there is no capacitor involved?
  But here are always capacitors involved!  Particularly in xfmrs. There is capacitance between layers of wire, that appear as in parallels with the windings, and there is capacitance between primary and secondary, and between windings and frame/core.
Thesecapacitance react with the leakage inductance to produce a resonant Low Pass filter

Quote
For example, the original EQP1A schematic has a 600ohm resistor across the secondary of the input transformer before the passive filter stage. Jakobs schematic has a 10k.
The EQP1A is a passive EQ. Its correct operation implies perfectly controlled input and output impedances. If the impedances were not correct, the response would be skewed.


A Zobel circuit is not unrelated, on the contrary; it's a way of compensating the increase of impedance, by applying progressively more load (damping), without unnecessarily loading the circuit at low and mid frequencies, which results in improved distortion and headroom compared to simple loading.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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