General opamp question

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soundguy

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Jun 4, 2004
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when looking at that api output transformer thread, got me thinking about a general idea that is likely too general to have an answer, but figured Id ask just to remind everyone here how green my grass is...

When looking at a 2520, designed to push a 75 ohm load, but able to handle a 600 ohm load with decreased headroom, can this state of affairs (driving lower the imedance the better) for some opamps? Is there some very basic and general theory that can be layed out in kindergarden terms for me? I had thought that the higher the bridging impedance, the better, what are the circumstances upon which a 2520 likes driving a 75 ohm load better than a 10K load?

this is very general I know, but...

dave
 
I want to be in your kindergarden class, cuz I was just thinking along similar lines.

I'm gonna pull up a nap mat, and wait for the teacher to start "Head, shoulders, knees, and toes"

ju
 
In a solid-state circuit, which has a limited maximum supply voltage, but is capable of high currents and low output impedance, one way to increase voltage swing is to use a step-up output transformer.

Whether the op-amp is performing "better" with a stepup transformer, as opposed to driving the load directly, depends on your definition of "better." With the transformer, the output voltage swing is increased, but the op-amp has to work harder to drive the low impedance primary.

Driving the load directly, the op-amp would tend to dissipate less power and have lower distortion, but the maximum output voltage swing would be limited by the supply rail(s).

As PRR has pointed out elsewhere, using a stepup transformer was common in the days of fairly low transistor supply voltages and expected maximum output levels of +28 or +30dBM into 600-ohm loads, but is quite obsolete now.
 
> designed to push a 75 ohm load, but able to handle a 600 ohm load with decreased headroom

Same voltage either way (actually a hair higher in 600Ω).

But at the same voltage, the power is greater the lower the impedance.

And if you use a transformer to lower the impedance the amp sees for a given load, the tranny voltage step-up means higher voltage in the load. If the amp is fed +/-15V and can swing 8V RMS, then after a 75:600 transformer you will get 32V RMS.

In days when good transistors were not rated more than 40V, and an amp could barely make 28V peak-peak or 10V RMS, this was the obvious way to get more than +22dBm in 600Ω. With 75:600 transformer you can get +32dBm (assume 2dB transformer loss) in 600Ω (87V p-p) from your 40V transistors.
 
If you all don't mind sticking my nose into this :wink:, this reminds me of a while back. A circuit + 1:2.5 output-TX I found in an old Ampex machine and for which I got help from various people here w.r.t. pinning & other details and not in the least of PRR with more circuit info.

from NYD:
In a solid-state circuit, which has a limited maximum supply voltage, but is capable of high currents and low output impedance, one way to increase voltage swing is to use a step-up output transformer.


So maybe as one of the many possible illustrations of the topic:

http://www.groupdiy.com/index.php?topic=186&highlight=se540

I wasn't sure what the imped. of the output-TX was but from the above I figure it'd be safe to say it's something like 100:600.

BTW, TX is here:
TX-out_pic07_400.jpg



Peter
 
trying to follow...

by driving the load directly, you get lower distortion but the trade off would be lower head room from less of a voltage swing, right?

discrete electronics design really is some kind of crazy balancing act of compromise, isnt it?

dave
 
[quote author="soundguy"]discrete electronics design really is some kind of crazy balancing act of compromise, isnt it?[/quote] Let me make it clearer for you Dave, ALL electronic design is a balancing act of compromise!

HTH!
Charlie
 
There is always something in a design that seems to be the first step or first prerequisite.

It might be a 15 volt rail ... 32dB into 600ohm ... 40 volt transistor.

If we could re-write or throw away history then many of our old staples would be thrown away too. Many in the Broadcast Industry would love to throw away 30dB 600ohm requirements.

Even the Speaker Biz has some awkward stuff. Why do we end up down at 4 and 2 ohms for speaker rigs ? There was a time when 32 ohm speakers were normal and Horns were rated at Volts and not Watts.

Much of this may be irrelavent as we are here now and have to deal with what we have created.
Even though a couple of specific specifications can tell much of the story. always remember that it is only part of the story and 2 or 3 specs can not spell out the whole story of what a unit might be.

Yes this is a world of compromise.
 

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