Testing Frequency Response

[Blair]

How do you guys test FR? Like in my other post, where I am tinkering with feedback.

I usually go over to a friends home and use his calibrated distortion analyzer to test, but getting bench time isn't easy because he makes his living doing audio repair.

Do most of you use a scope and signal generator and just test the amp for voltage output at varying frequencies into a load?

Thank you!

Blair

[20to20]

How do you guys test FR? Like in my other post, where I am tinkering with feedback.

I usually go over to a friends home and use his calibrated distortion analyzer to test, but getting bench time isn't easy because he makes his living doing audio repair.

Do most of you use a scope and signal generator and just test the amp for voltage output at varying frequencies into a load?

Thank you!

Blair

That's the way I do it. Not just the output but all other points, too. And when you do it with speakers at a normal listening level you can see how the speaker load FR is very different from a dummy load. MCM has a neet little sig.gen for @$50. Flat and accurate. Gives sine and square waves.

[battradio]

I use a signal generator , two HP wide band volt meters , dummy loads and a scope . One wideband voltmeter to monitor the voltage from the signal generator to make sure it is constant with frequincy and the other to monitor the out put of the amp to make sure it stays constant with frequincy , and the scope to make sure that the waveforms are clean .

[Geek]

Function generator and oscilloscope.

I kill the horiz. sweep so that it`s just a vertical line and calibrate to amplitude at 1KHz. Then I sweep looking for the 0.707 points

Cheers!

[battradio]

Hi Gregg ,
Thats a good idea , but i use a 3" scope and and 60 year old near sighted eyes , I can read the meters a lot easer .

Mark

[Geek]

I have a 5" scope and still have to squint with 44 year old eyes

[Blair]

Hi Gregg,

I tried, my understanding, of your suggestion yesterday. I hit the "Horiz" button to kill the constant line, but when I inject signal, the sine waves appear anyway. Just lighter.

Is there a writeup about this method?

When you say .707 points. Is that the -3db point in voltage/amplitude? Say I have 1V out, and drop the freq a few db, and measure .293V. Is that where my "knee" is?

Thanks!
Blair

[Shannon Parks]

I've tested several free Function Generator apps for my smartphone. The Sine output is perfectly flat from 20Hz to 20kHz. I haven't tested the square wave. BK Precision 20MHz oscopes are plentiful and cheap on eBay. Watch for a week or two and get one for $60 shipped or less.

Shannon

[20to20]

I tried, my understanding, of your suggestion yesterday. I hit the "Horiz" button to kill the constant line, but when I inject signal, the sine waves appear anyway. Just lighter.

Is there a writeup about this method?

When you say .707 points. Is that the -3db point in voltage/amplitude? Say I have 1V out, and drop the freq a few db, and measure .293V. Is that where my "knee" is?

Thanks!
Blair

My scope doesn't have a seperate button for Horiz. In/Out. It has a [X(-)Y] setting on the sweep time control at the end.

It's .707v only for a 1v wave. Otherwise it's a percentage of the reference voltage in. You can have anything on the scope and if it drops to .707% of the reference, then that is the 3dB down, (half power) point. I like to use 4 squares up. If it only drops to 3 squares, it's 75%. Otherwise, you got do all the danged "times this and times that, .1 x 3 squares and 2 ticks, whooeey.."