I measured the gain of several amplifiers and came up with some
numbers that just don't seem to add up. Shouldn't the gain be
close to the same at all frequencies? Is it possible that my
DVM doesn't read the correct voltages at higher frequencies?
Or is it due to the fact that some of the amps have zobels and
others have inductor/resistor circuit in the output?
Rotel RB-850
26.40dB @ 60Hz
26.81dB @ 1,001Hz
31.25dB @ 9,999Hz
36.34dB @ 9,999Hz
44.91dB @ 15,9999Hz
Yamaha M-45
27.99dB @ 60Hz
28.31dB @ 1,001Hz
40.30dB @ 9,999Hz
52.40dB @ 15,999Hz
Accurian Amp w/o preamp
14.30dB @ 60Hz
14.58dB @ 1,001Hz
22.78dB @ 9,999Hz
17.25dB @ 15,999Hz
Radio Shack wireless amp
27.1dB @ 60Hz
23.9dB @ 1,001Hz
39.1dB @ 9,999Hz
50.8dB @ 15,999Hz
numbers that just don't seem to add up. Shouldn't the gain be
close to the same at all frequencies? Is it possible that my
DVM doesn't read the correct voltages at higher frequencies?
Or is it due to the fact that some of the amps have zobels and
others have inductor/resistor circuit in the output?
Rotel RB-850
26.40dB @ 60Hz
26.81dB @ 1,001Hz
31.25dB @ 9,999Hz
36.34dB @ 9,999Hz
44.91dB @ 15,9999Hz
Yamaha M-45
27.99dB @ 60Hz
28.31dB @ 1,001Hz
40.30dB @ 9,999Hz
52.40dB @ 15,999Hz
Accurian Amp w/o preamp
14.30dB @ 60Hz
14.58dB @ 1,001Hz
22.78dB @ 9,999Hz
17.25dB @ 15,999Hz
Radio Shack wireless amp
27.1dB @ 60Hz
23.9dB @ 1,001Hz
39.1dB @ 9,999Hz
50.8dB @ 15,999Hz
Hi,
you cannot use an ordinary DMM to measure frequency reponse.
I use a switched attenuator between the source signal generator and the amplifier. I set the attenuator loss to exactly equal the amplifier gain.
This makes the input signal to the attenuator exactly equal to the output of the amplifier.
Now set the attenuator to X+1db. i.e. from -30db to -29db.
adjust the generator frequency until the amp output = attenuator input.
at this frquency the amp response is 1db down from it's nominal gain.
This can be done at many different attenuations eg -0.1db, -0.5db, -1db, -3db etc).
Due to comparing voltages that are identical at each end of the testing set-up the accuracy of the DMM is eliminated from the measurement.
My switched attenuator does -0.0db to -60.05db in steps of 0.05db using 12 DPDT switches and allows frequency response measurements from 0.6Hz to 500kHz. The lower limit is set by my frequency counter and the upper limit is set by my DMM, but I generally never go above 200kHz.
you cannot use an ordinary DMM to measure frequency reponse.
I use a switched attenuator between the source signal generator and the amplifier. I set the attenuator loss to exactly equal the amplifier gain.
This makes the input signal to the attenuator exactly equal to the output of the amplifier.
Now set the attenuator to X+1db. i.e. from -30db to -29db.
adjust the generator frequency until the amp output = attenuator input.
at this frquency the amp response is 1db down from it's nominal gain.
This can be done at many different attenuations eg -0.1db, -0.5db, -1db, -3db etc).
Due to comparing voltages that are identical at each end of the testing set-up the accuracy of the DMM is eliminated from the measurement.
My switched attenuator does -0.0db to -60.05db in steps of 0.05db using 12 DPDT switches and allows frequency response measurements from 0.6Hz to 500kHz. The lower limit is set by my frequency counter and the upper limit is set by my DMM, but I generally never go above 200kHz.
In a couple of minutes I would like to follow up on AndrewT's talk about
frequency response. I think my question has been answered to the
point that the gain measurements are inaccurate when I used the
DVM above its frequency limit. Could I say then that the gain results
at 60Hz are fairly accurate?
I would like to get a better voltmeter. . . .maybe Santa will drop one
down the chimney this year.
I really don't understand AndrewT's circuit for measurement of
frequency response.
frequency response. I think my question has been answered to the
point that the gain measurements are inaccurate when I used the
DVM above its frequency limit. Could I say then that the gain results
at 60Hz are fairly accurate?
I would like to get a better voltmeter. . . .maybe Santa will drop one
down the chimney this year.
I really don't understand AndrewT's circuit for measurement of
frequency response.
Most DVMs are very limited in frequency response. Since you seem to have a signal source to feed the amp, why not feed the meter directly and see how good (or bad) it is?
Andrews method is almost foolproof as long as the meter has linear amplitude response at the frequency extremes, which is very likely. Using null meters and comparison techniques, combined with accurate voltage dividers and ratio transformers, was the mainstay of calibration labs and people who needed super accurate measurements for decades. These days test equipment is direct reading and, if you buy the right stuff, will cover any reasonable frequency range, but the divider technique is still the gold standard.
Andrews method is almost foolproof as long as the meter has linear amplitude response at the frequency extremes, which is very likely. Using null meters and comparison techniques, combined with accurate voltage dividers and ratio transformers, was the mainstay of calibration labs and people who needed super accurate measurements for decades. These days test equipment is direct reading and, if you buy the right stuff, will cover any reasonable frequency range, but the divider technique is still the gold standard.
Let's assume your test amp has a gain of exactly 31.3db.
attach an attenuator to the front end of the amp which reduces the test voltage by exactly 31.3db.
The signal at the input to the attenuator is exactly the same as the signal at the output of the amplifier.
Your inaccurate DMM will read the same voltage at both locations.
Now reset the attenuator to -28.3db. At the same frequency the amplifier will now read too high by about 40%. As you increase the test frequency the test voltage will hold steady and then start to fall.
When the input voltage =output voltage the gain of the amplifier has fallen to match the attenuation of the attenuator. ie. the gain =28.3db.
This is -3db compared to the start voltage. Read off the frequency and you have the F-3db frequency. The inaccuracy of the DMM does not affect this reading.
The accuracy of the switched attenuator is critical to this method.
Conrad,
the generator frequency reponse errors are also cancelled by this method.
attach an attenuator to the front end of the amp which reduces the test voltage by exactly 31.3db.
The signal at the input to the attenuator is exactly the same as the signal at the output of the amplifier.
Your inaccurate DMM will read the same voltage at both locations.
Now reset the attenuator to -28.3db. At the same frequency the amplifier will now read too high by about 40%. As you increase the test frequency the test voltage will hold steady and then start to fall.
When the input voltage =output voltage the gain of the amplifier has fallen to match the attenuation of the attenuator. ie. the gain =28.3db.
This is -3db compared to the start voltage. Read off the frequency and you have the F-3db frequency. The inaccuracy of the DMM does not affect this reading.
The accuracy of the switched attenuator is critical to this method.
Conrad,
the generator frequency reponse errors are also cancelled by this method.
I wouldn't bet on cheap a DVM even being linear at higher AC frequencies
If you've got a pc you've likely got a built-in audio codec with way better frequency flatness than a cheap meter
make a few atenuators to reduce the power amp output V to safe levels and use the free download of RMAA for a quick look
http://audio.rightmark.org/products/rmaa.shtml
If you've got a pc you've likely got a built-in audio codec with way better frequency flatness than a cheap meter
make a few atenuators to reduce the power amp output V to safe levels and use the free download of RMAA for a quick look
http://audio.rightmark.org/products/rmaa.shtml
jcx said:
Hi jcx,
I'm building an amp here:
http://www.diyaudio.com/forums/showthread.php?s=&postid=1342905#post1342905
I lack a distortion analyzer. I see the RMAA performs a distortion test (among others). Would this be accurate enough for an indication of my amps performance?
I have a fairly decent sound card in my lab computer.
Thanks.
loop-back testing gives an indication of the soundcard's limits
you should be able to make basic measurements with the free RMAA, down to the noise/distortion limits of the soundcard
but you would be limited to the functionality of RMAA - only a few test options, also sampling frequency/Nyquist prevents soundcards from being useful for amp stability testing
you should be able to make basic measurements with the free RMAA, down to the noise/distortion limits of the soundcard
but you would be limited to the functionality of RMAA - only a few test options, also sampling frequency/Nyquist prevents soundcards from being useful for amp stability testing
I think I understand the attenuator circuit for testing amplifier gain
at different frequencies. If I understand correctly, the meter is going
to measure "a" voltage however incorrect, but the ratio will be the
same.
The cheap DVM/DMM inaccurate readings are used but taken as
a relative value against the accurate attenuator.
If my DMM is accurate at 60Hz. . .then do the values listed at the
beginning of the post seem close to what standard amps should
produce:
Rotel RB-850
26.40dB @ 60Hz
Yamaha M-45
27.99dB @ 60Hz
Accurian Amp w/o preamp
14.30dB @ 60Hz
Radio Shack wireless amp
27.1dB @ 60Hz
All of this is leading up to a tri-amplified system and the amps need
to have very close to the same gain to begin with (I can make
attenuators to match them) otherwise the electronic crossover
will set the crossover point, but do to one amp having higher gain
than the other. . the crossover point will shift.
at different frequencies. If I understand correctly, the meter is going
to measure "a" voltage however incorrect, but the ratio will be the
same.
The cheap DVM/DMM inaccurate readings are used but taken as
a relative value against the accurate attenuator.
If my DMM is accurate at 60Hz. . .then do the values listed at the
beginning of the post seem close to what standard amps should
produce:
Rotel RB-850
26.40dB @ 60Hz
Yamaha M-45
27.99dB @ 60Hz
Accurian Amp w/o preamp
14.30dB @ 60Hz
Radio Shack wireless amp
27.1dB @ 60Hz
All of this is leading up to a tri-amplified system and the amps need
to have very close to the same gain to begin with (I can make
attenuators to match them) otherwise the electronic crossover
will set the crossover point, but do to one amp having higher gain
than the other. . the crossover point will shift.
you can use your scope for frequency response measurements. most scopes have a set of dotted lines on the graticule that correspond to 0.707 of the full screen deflection. attach your scope to monitor the load, input a 1khz sine wave, and adjust your scope sensitivity for full screen deflection. then change your frequency to whatever other frequencies you want to test (20khz, 20hz are usually the most important). assuming a flat frequency response from your test generator, you are at your -3db point when the peak of the signal drops to the dotted lines.
also if your scope has an "ADD" mode for CH1 and CH2, and CH2 has an invert switch, you can use your oscope as a basic distortion analyzer. for instance, you would set CH1 for 500mV/DIV, CH2 for 20V/DIV. then push (or pull as the case may be for your scope) the CH2 INVERT switch, and set VERT MODE to ADD (this is for the majority of amps that DON'T invert the signal. for an inverting power amp, you don't need CH2 inverted). CH1 is connected to the input signal, and CH2 is connected to the load. drive the amp to full power, then adjust CH2's V/DIV switch to get as close to a signal null as possible, and do a fine null with CH2's VARIABLE knob. what you see will be your distortion residual, but since this setup is so basic, you don't have phase nulling like a real distortion analyzer would. this setup shpuld be good enough, however to set your output bias correctly, but you want to do that at 2 watts/20khz, not at full power, and increase the sensitivity of the scope channels accordingly
also if your scope has an "ADD" mode for CH1 and CH2, and CH2 has an invert switch, you can use your oscope as a basic distortion analyzer. for instance, you would set CH1 for 500mV/DIV, CH2 for 20V/DIV. then push (or pull as the case may be for your scope) the CH2 INVERT switch, and set VERT MODE to ADD (this is for the majority of amps that DON'T invert the signal. for an inverting power amp, you don't need CH2 inverted). CH1 is connected to the input signal, and CH2 is connected to the load. drive the amp to full power, then adjust CH2's V/DIV switch to get as close to a signal null as possible, and do a fine null with CH2's VARIABLE knob. what you see will be your distortion residual, but since this setup is so basic, you don't have phase nulling like a real distortion analyzer would. this setup shpuld be good enough, however to set your output bias correctly, but you want to do that at 2 watts/20khz, not at full power, and increase the sensitivity of the scope channels accordingly
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