Input Impedance question

[jonasnoble]

Hello all, I have been working on a bass DI for a few weeks, following many of the old threads in this forum. I have a lot to learn about load lines and impedance matching.

When spec'ing/buying output transformers, they are almost always assumed to be run to a 600 ohm input. I just ordered several 10k:600 transformers, and out of curiosity looked at the specs to my audio interface. The input impedance is 3k ohms. My limited understanding is that this would look like 50k ohm load, reflected to the tube.

Is there a limit to what a preamp tube can drive? (I'm assuming yes.)
What happens as you approach that limit? (I'm assuming loss of power and increased distortion)
How do you know that the impedance is going to be too much, is it by looking at the load lines?
And if it's a problem, should I order some 2:1 transformers rather than the 4:1?

Thanks, hope everybody's day is going as well as mine.

Moderators, please re home this question in the proper category, if I got it wrong.

 

[ccaudle]

Is there a limit to what a preamp tube can drive?

The limit is usually a lower limit, driving a higher impedance just means the device has to supply less current to the load, which will (speaking in very broad generalities) mean less power dissipated and less distortion.
The use of 600 Ohm input impedance is an antiquated practice used when analog telephone lines were used to convey audio between remote locations (like from a theater to a radio station) and these days is primarily found in reproductions of antique equipment (i.e. direct copies of 1950's or 1960's designs).

The short of it is don't worry too much about it, just be sure to check the transformer datasheet to see if it needs a particular load for proper frequency response, it may require a resistor or resistor in series with capacitor across the secondary winding to use with modern (i.e. designed within the last 50 years) equipment.

 

[jonasnoble]

Thank you so much for the answer; that's good news. I did some YouTube research while I was awaiting a reply, but couldn't find exactly what you explained. I did learn something about the way impedance affects the signal (voltage with respect to current), that makes your response make sense to me.

I guess I thought that the 10K in the 10k:600 output transformer was selected by the tube's load line.

 

[scott2000]

the specs to my audio interface. The input impedance is 3k ohms

Seems a bit low....Sure that's not recommended max at the input?

 

[jonasnoble]

Seems a bit low....Sure that's not recommended max at the input?

Fair question. I had always unconsciously thought that because preamp output transformers are x:600 that means they go into the mic input. But I would probably put this into the line input, which has a much higher impedance. 15k if I remember correctly.

 

[ccaudle]

because preamp output transformers are x:600 that means they go into the mic input

Depends on the output level of your DI. Often it is the case that a DI will reduce the output level in the course of reducing the output impedance, but with a tube DI you may have enough swing at the tube plate that even after the transformer reduction the level is still high enough to use with line input rather than mic. That also depends on whether you are using the term "DI" in the tradional sense (which typically means no gain, just impedance transformation and possibly buffering), or whether your tube-thing is really more of a small pre-amp with a transformer coupled output.

 

[scott2000]

the mic input.

oh gotcha...makes sense