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amplifier output impedance measurement

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This project could be of interest: it is an impedance meter, well suited to amplifier measurements and it can be made to float easily.

Although it is a DIY-grade instrument, the measurement accuracy will be much better than any indirect method based on variable loading.

In order to make proper measurements, it is essential to work on a completely silent amplifier (and it should also have a low noise of its own, but that is not difficult to achieve in a PA)

http://www.diyaudio.com/forums/equipment-tools/229629-impedance-meter-add-simplistic-thd-meter.html
 
elvee...thank you for your intresting reply, however my circlotron impedance falls somewhat out of the range of the intended target of your device's scope, being in the 200 ohm area. However a quick cobbling together of parts outlined in your discription has led to an intresting new form of distortion meter formatting, of which I will soon begin a rudimentary study and report back results here when assembled and calibrated. midmoe
 
elvee...thank you for your intresting reply, however my circlotron impedance falls somewhat out of the range of the intended target of your device's scope, being in the 200 ohm area.
You can easily add other ranges as required: for a 1K full scale range, R13 would become 1.5K, the calibration trim 1K, and R15 should preferably become 31K (or be paralleled with a 560K)
 
How about these methods.

I think one post mentioned the second one.
 

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Wake up an sleeping thread

Hello All,

I have been reading this thread and chapter 8 in the Loudspeaker Design Cookbook 7th edition by Vance Dickeason.

In post #7 Dr DF96 is right on with the theory and in post #3 CounterCulture is right on with the maths.

Reading the Loudspeaker Design Cookbook it is apparent that not only is the Amplifier output impedance important so is the internal impedance of the crossover network. Throw in the amplifier output impedance, 10 meters of zip cord and the series resistance of the chokes in the crossover and all our assumptions about system Q are completely wrong.

DT
 
If your still looking for an easy way to find output impedence why not give this a try:-

Use a one ohm resistor as the load. Put an AC voltmeter accross this resistor. Use a sine wave to drive the amplifier. Secondly place a series low range variable resistor in series with the load and amp. This pot should be adjustable from zero to more than your expected impedence. Now with some power running from the amp tweak the pot and look at the voltmeter. You are trying to peak it. Because your load is a fixed resistance rarther than a variable resistance of the pot on its own you should find a peak when the total resistance of both load and pot matches the amplifiers output impedance and maximum power transfer is achived.

This should work and should be a quick usefull way of measuring output impedance ( adjust resistor values if you expect impedance to be lower than 1R )

Would this work guys?? :confused:
 
Yes, peak power in the whole load resistance, not peak voltage across the small resistor. Peak voltage just comes from the variable one being set at zero.

Much better to vary the resistance, and measure the voltage across it. Then use a little algebra or some graph paper or a spreadsheet to find the answer. If you don't know how to find the answer, then you don't know why you are seeking the answer so it is not worth doing anyway!
 
Adjustable output impedance

Not sure if this is the best place to ask the following, but it is related to amp output impedance.

I have an amp system which allows for the adjustment of the output impedance. According to the manual, to zero out the impedance of the amp, do the following using the output impedance pot.

!. Send a (I assume a sine wave) signal to the load and measure the voltage at the output, I assume at the speaker terminals in this case. I use CAT 5 DIY 4ft long cables.
2. disconnect the load and measure the voltage again.
3. Adjust the pot until the measured voltage at the terminals is the same with the load connected and disconnected, thus a zero amp output impedance.

Now that this is possible(I have done this), what should be the freq of this adjustment
.
1. speaker lowest or highest impedance?
2. nominal 1k hz?
3. 60hz where the DVM is the most accurate?
4. Using white noise?

I assume that the maximum damping occurs at this point and would be best used in the low freq zone. This is where the biggest stored energy in the speaker is transfered back to the amp, correct? Any thoughts?
 
Other things being equal, you should get the most accurate setting at the frequency of the lowest loudspeaker impedance. However, I doubt the usefulness of this adjustment - I suspect it is just there for cosmetic purposes. I will explain:

You can, with some care, probably adjust the control until the two voltage readings are within about 1% of each other. That means a damping factor somewhere around the 100 region, or maybe better (or maybe negative).

If you are worried about such things you could easily have an amp with a low frequency damping factor of 1000. What I am saying is that you can design (via feedback) a higher damping factor than you can realistically adjust, therefore there is no point in having an adjustment - except for the odd case of unusually high speaker cable resistance.
 
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What I am saying is that you can design (via feedback) a higher damping factor than you can realistically adjust, therefore there is no point in having an adjustment - except for the odd case of unusually high speaker cable resistance.

Yes, to do much better than a moderate DF in practice, remote sensing from the speaker terminals is necessary. Or, locating the amp in (or next to) the speaker with very short connections.
 
I have been reading up on damping factor and the output impedance adjustment in my amps the last few days. I have read that damping factors greater that 20 are "not much" needed in a well damped(mechanically) woofer driver(some opinions). Also, that valve amps normally don't have damping factor above this point very often, thus "wooly" bass response is expected. This was with a 8 ohm speaker. If you use a 4 ohm speaker it is a case of greater woolyness, in other words less damping factor thus less control of the bass speaker.

In my case, valve amps/4 ohm speakers, it appears that anything that I can do to help decrease the output impedance of the amp is going to improve the low freq sounds. The amp manual indicates that the "positive feedback adjustment" reduces the output impedance by decreasing the negative feedback as the load increases. Thus increasing output that offset the losses in the output transformer as load increases. Make since to offset the resistive losses as the load(current) increases.

I guess the adjustment should be made at the bass unit mechanical resonance, or at the most likely highest current load which would be at its lowest freq responce point(max current)?

I have 45 inch speaker cables and they are soldered connected to the speakers.
 
just some ideas

Didn't want to start a debate about ss vs tube bass perfomance. There are people that think that SS give a better lower freq sounds. Some do not. I do not have any experience with SS amps. I am just going thru the motions to optimize the performance of the amps/ speaker combination. I did go thru the motions of previously mentioned steps, adjusting the output voltage(at 40hz) with and without speaker, AT THE SPEAKER TERMINAL END(the manual indicates this to be done at the amp terminals). There was a voltage difference( don't remember how much) between the zero setting and the "optimized" setting. This optimized setting did result in exact voltage output with and without speaker load at 40hz output. . What the real setting is at the zero position of this control is unknown, it may not be at a non influential position(?). What this adjustment did was dialed out not only the cable losses(as little as it might be) but also the losses in the output transformer. Customized for the load at that freq, real zero system output impedance at that freq. There was a small change in the bass and I perceived some changes however small in the mids.

With so many variables, freq, speaker impedance, amp output impedance, at what freq would you adjust the zero output impedance? Just looking for some opinions, guesstimate, swag Ideas.
 
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