Understanding & Calculating In/Out Impedance

[ERRP55]

Apologies if I leave information out or if my question is confusing. I don’t know if I’m asking or searching for the right answers as I am still fairly new to audio electronics theory and design.

I am struggling to understand or find information that makes sense for calculating the input and output impedance of an audio circuit. I’m interested in understanding these specific topics because I’d like to build a triode based preamp with input and output transformers. I am also interested in adding balanced line output transformers to a quantum audio mixer I acquired a while back.

I’ve found a lot of videos and blog post online that throw equations at you but don’t use examples of actual circuits to apply them to.

I’ll attach a schematic of v72 which is a triode based preamp and the schematic for a QM-8A channel schematic. How would i calculate the input and output impedances of these circuits. Thanks!

 

[GussyLoveridge]

Hi ERRP55 -

I would suggest picking up a copy of Merlin Blencowe's Designing High-Fidelity Valve Preamps. I have spent a really good amount of time with this book and it covers in depth input and output impedence. You should also be able to find a copy the RCA Receiving Tube Manual that should cover this stuff in detail as well. I've found most of it to be fairly straightforward expect for figuring out some of the transformer models and how they fit in.

I've been going over and over both texts and it gets little more clear each time. Seems like this E stuff is more of a marathon than a sprint.

 

[user 37518]

It is not straightforward in most cases, I am afraid. You need to know about different amplifier configurations, you need to know about transformers, op-amps, tubes, you need to be able to simplify resistive and capacitive/inductive circuits, etc... Unless you are looking at a simple circuit, many times you need a lot of experience to do it for a more complicated schematic. Those are the sort of questions that I ask my EE students, I will draw a circuit with some transistors and ask for the input/output resistances.

That being said, in the V72 circuit you showed there is a 400k resistor in the secondary of the trafo, and, it appears that the transformer is 1:20; transformers reflect impedances as the square of the turns ratio, so, between the primary terminals you would have a resistance of 400k/(20^2) = 1K (Edit: I saw it says 2X1:20, so that means that it is effectively a 1:10 transformer if the primaries are in series and the input impedance will be 4k; it will be 1k if they are connected in parallel). It depends how you connect the primaries of the transformer, but that is what will be seen at the primary if you were to measure between the external pins of the primary. Same thing for your other circuit, R3 is essentially an open circuit at audio frequencies due to the series 100pF cap, and R5 is also considered an open (to compute the input impedance) for audio freqs, since it is directly connected to the input of the op-amp, which draws practically zero current. Hence, the only resistance that will come into play to compute the input impedance is R4, that divided by the square of the turns ratio of that trafo will give you the input impedance. However, notice that the primary coils are in parallel, so that will change the turns ratio, and, therefore, the reflected impedance to the primary will be lowered. You would also have to consider those Phantom bias resistors.

As you can see, it is not straightforward if you don't know what you are doing. It is not like we can give you a "plug numbers here" formula and that is it; you require a good deal of electric circuit analysis. Usually, people study EE to do this, or, you can delve into books and study. If you really want to know, I can suggest you to look at this thread Analog Theory & Design reference books and start reading. Start with the Electric Circuits books; I recommended a ton of books in that thread. If you have a question of where to start, I can help you out.

 

[sodderboy]

The quick (less than 4-year at uni), fairly accurate approach is with a 10K pot and an accurate DDM. Feed it an unbalanced osc signal, say 1K at -30dBv and insert a 10K linear pot in series, set CCW 0 ohms. Measure the voltage at the load(V72 input) make sure it is .032 volts. Increase the pot CW until you get a reading of .016 volts (-36 dBv, 6dB drop). Measure the value of the pot and you have your answer for INPUT impedance. This measurement kinda ignores the osc source impedance, but for a present day osc it should be below 30 ohms.
You can do a similar ohms law measurement for the output impedance, but paper is required. Feed a known level, 1K at +4 dBv out of the pre into a measured known load, say 600 ohms, with the 10K pot in series, CCW 0 ohms. Increase the pot until you get a 6 dB drop. From there you have to do the math, but you are dealing with three resistive loads in series with an AC source so it is quite simple math, No EE degree required. You can make a simple excel spreadsheet to do the calculations for future tests.
It is not perfect, but the results are probably within 10%. I never tried it balanced with a dual pot.
If you are trying to impedance match pairs or something, I would quote Les Paul's answer for many of my "this or that" questions- "Who cares, you are going into an EQ anyway! Make 'em sound the same".
Mike