What could account for severe loss of LF response of mic input xfmr?

[nevetslab]

The Amber 3501a serves as a Balance Front End for the PCSU2000, giving a wide variety of gain/input sensitivity. As my Laptop is still dead, I have NO Velleman system. That task is still ahead of me. Without it, if I want plots, my options are to 'ink up' and break out either the B & K 2308 XY Plotter, or bring in the HP 7090A Plotter along with a Wavetek 98 Sweep Oscillator to drive the system and plotter.



In getting the input impedance measured, at each spot frequency, the decade box was adjusted to achieve 50% of the level across that selected resistance (-6dB). When I've done this using the compressor loop, such as measuring loud speaker impedance, you have a measuring amp across the output to maintain a constant current driving the impedance that varies with frequency. Using the decade box and the dB meter lets me do the same thing, only its a manual adjustment, rather than a regulated swept response.

How would YOU measure the input impedance of a device, be it a Transformer, or loudspeaker? For that matter, the output impedance of a tube amplifier?

[Helmholtz]

The Amber 3501a serves as a Balance Front End for the PCSU2000, giving a wide variety of gain/input sensitivity. As my Laptop is still dead, I have NO Velleman system. That task is still ahead of me. Without it, if I want plots, my options are to 'ink up' and break out either the B & K 2308 XY Plotter, or bring in the HP 7090A Plotter along with a Wavetek 98 Sweep Oscillator to drive the system and plotter.

Wow, that really looks sophisticated.

To measure speaker impedance I just use the Velleman generator feeding a power amp and resistor of 5k in series with the speaker to get current drive.
The error compared to a real current source is low, max. error being around 2% at high resonances and really low outside resonances.
Considering that speaker impedance at the resonance(s) is influenced by the size of the room (I noticed that even opening a door can change the height of a resonance peak),
the error above seems negligible.
The voltage at the speaker terminals is proportional to its impedance and gets fed to the Velleman scope input.
No need for balanced measuring, as one side can be grounded.

In getting the input impedance measured, at each spot frequency, the decade box was adjusted to achieve 50% of the level across that selected resistance (-6dB). When I've done this using the compressor loop, such as measuring loud speaker impedance, you have a measuring amp across the output to maintain a constant current driving the impedance that varies with frequency. Using the decade box and the dB meter lets me do the same thing, only its a manual adjustment, rather than a regulated swept response.

How would YOU measure the input impedance of a device, be it a Transformer, or loudspeaker? For that matter, the output impedance of a tube amplifier?

A transformer doesn't have a real own input impedance without a load.
So its secondary should be connected to its designated load resistance.
The primary is fed by a voltage source in series with a resistor that equals the source resistance in the application

This way I measured the frequency response of a number of OTs with the Velleman. No power amp required here.
Source resistance is tube plate resistance (see tube data), I used a 22k series resistor for KT66 OTs.

With PP OTs I ground the primary CT and the 0R secondary wire.
I feed only one side of the primary (corresponding to class B operation).
Secondary voltage is recorded by the Velleman.

[Helmholtz]

I edited my post above because it wasn't clear that I described the setup for frequency response measurement.

To get OT primary impedance I simply connect the designated load resistor and measure AC resistance at the primary with my LCR meter at 1kHz. That's the primary impedance.
Of course you could use your 50% method as well.

Regarding tube amp output impedance measurement I don't have a good idea yet.
It could be roughly calculated from tube and OT data.
From Zollner's book I know that it's often in the 100 Ohm range without NFB.
The impedance lowering effect of NFB can be calculated from the feedback ratio.