Series Resistors on Outputs of Transformer?

[Magz]

I am familiar with loading a transformer by placing a resistor across the + and - leads of the secondary, but I have a preamp in which there are 22ohm resistors in series with each of the + and - output leads, in other words they are between the + out and RCA jack center pin and the - out and RCA ground connection.

What function would these resistors serve? I never see them in any of the schematics I've looked at, and I'm trying to learn why someone would use them? To suppress ringing?

What would happen if these resistors were removed and replaced with wire?



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[coldcathode]

post a diagram or schematic?

 

[Magz]

I don't have a schematic I can post right now, but it is a parafeed-type single ended output stage from a 2C22 tube with a DC blocking cap on the cathode. The stock transformer is a small toroidal power transformer (115V+115V, 22V+22V, 0.8VA+0.8VA) with primary DC resistance of 640ohm and secondary DC resistance of 42ohm, 4:1 stepdown.

I'm in the process of replacing the output transformer with a Lundahl LL1674, and I was wondering if I should keep the 22ohm resistors on the outputs, or if I could get rid of them.

Also, I'd like to learn a little about what their function is.


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[Arnulf]

Their function is to waste some power.

 

[Magz]

Their function is to waste some power.

Hi Arnulf,

I always appreciate your insightful replies, but can you elaborate a bit, please? Are you saying that they are useless and could be replaced with straight wire? Why would it be necessary to "waste power" at the output of a preamp? To avoid having too much gain?


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[Arnulf]

Sorry, I'm not a native English speaker so I can't explain things very well. Resistors waste power (= "drop voltage") based on their resistance and current passing through them. This is their only electrical function.

There is no reason to waste power at the output of well-designed preamplifier. Then again there is no reason for step-down transformer at the output of well-designed preamplifier (with cathode follower output, no less) either.

2x 22R in series with 10+K isn't much. You really should put together schematic of your preamplifier so we can see what you're dealing with.

 

[Magz]

Thanks, Arnulf.

Since there is no obvious reason why they are there, I'll just remove them and see what happens. Probably nothing.

 

[Tubelab_com]

I'm trying to learn why someone would use them? To suppress ringing? ....The stock transformer is a small toroidal power transformer

Yes, ringing or overshoot on square waves are both possibilities when running audio through a non audio quality transformer. The resistors were probably added after some scope time. If you are swapping is a transformer made for audio use, you probably don't need them. Use a loading resistor if called for in the Lundahl spec.

 

[bob91343]

Yes, something like parasitic suppression. Or EMI considerations. It's more a precaution than a necessity.

 

[Magz]

Well, I went without the series resistors, and here is a link to the scope results with the new LL1674 transformers:

http://www.diyaudio.com/forums/tubes-valves/170453-grid-stopper-advice-graphs-included-3.html

 

[wa2ise]

I have a preamp in which there are 22ohm resistors in series with each of the + and - output leads, in other words they are between the + out and RCA jack center pin and the - out and RCA ground connection.

What function would these resistors serve? .

These resistors might serve as source termination resistors. To match the impedance of the shielded twisted pair cable. If your piece of equipment was designed for use in say a radio station studio, the designers may have figured that the output might have to drive a long length of cable. You sometimes see similar things done for digital lines like Ethernet cable drivers and such, though the frequencies involved there are way much higher.