# 10 KHz Square wave output?

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#### mura

Square wave output at 2.83Vrms at 10Khz into 8 ohms - Why is this parameter mesured?

What do you mean by "output at 2.83Vrms " ,

Is this the Vrms at the output or the output with 2.83Vrms input?

Why is getting a perfect Square wave important , what effect does it have audiably?

Thanks

Murarie.

#### ashok

Paid Member
2.83 volts rms is generally used to specify speaker sensitivity ( not using square waves). 2.83 volts gives 1 watt into 8 ohms ( 2.83x2.83/8). But you will also see spl ratings specified with a 1 watt input. The difference is because 2.83 volts need not produce 1 watt into the speaker because the impedance need not be 8 ohms. So to get 1 watt into the speaker the voltage will be something else , if the speaker is not 8 ohms at the measuring frequency or band of frequencies.

This 2.83Volts has nothing to do with amplifier ratings.

Square wave testing is generally done with amplifiers as the square wave consists of the fundamental sine wave and its harmonics extending to very high frequencies . You can see the amplifiers low frequency and high frequency performance by looking at it's square wave response. Additionally some amps are unstable with high frequencies and with a square wave signal you can see that easily - often not seen with a simple sine wave.
That is a simple explanation. Hope it is clear enough.

#### AndrewT

Hi,
you would also look at the 1kHz square wave and 100Hz square wave outputs at various output voltages checking to ensure you do not exceed the max output voltage.
These three square waves tell the designer a lot about frequency response and phase linearity as well as stability.

Q. is there any advantage driving the amp into clipping when displaying a square wave output? i.e. trying to exceed maximum output voltage.

#### ashok

Paid Member
You should be careful while using square waves to test the amp.
Generally the amplitudes used are not very large. Full amplitude square waves can cause excessive heating of some devices especially the resistor in the Zobel network ( if it is there) and burn it . You could blow some amps running full amplitude square waves. Blow it pretty fast ! Especially if it has HF instability problems.
Be careful.

#### fab

Paid Member
Hi Mura

Of course when you test a new amp you need to be careful not to blow the components but the final goal - once the design is completed - would be to have a high voltage (close to sin wave max rating and considering the power amp capability)10 KHz (or more) square wave not showing HF instability. However, when you test 10 KHz square wave into capacitive load (parallel to 8 ohms) the current driven from the amp will become excessive and that is why no one test at higher voltage than let say about 10V (depending on the current capability and ability of amp to drive high current).

#### fab

Paid Member
AndrewT said:
...Q. is there any advantage driving the amp into clipping when displaying a square wave output? i.e. trying to exceed maximum output voltage.

Hi AndrewT

Any amp can possibly be exposed to exceed max output voltage because input voltage is too high and/or the amp current limit has been reached. When you listen to high level of sound it is hard to know if the max level is reached or not due to the constantly changing level of sound in music. Thus, the amp can clip and when it does can sound worse or better than another amp or maybe have problems (maybe latch-up of active devices). On my side I test it to ensure the design has no flaw.

#### mura

Thanks all for your valuble inputs,

So Sq wave testing is to find out the hi frequency stability.

I use Cooledit Pro2.0 to generate sine waves and Sq waves,is this a reliable method?

In my Amp i can get a 1kHz Sq wave perfectly but a 10K Sq wave looks almost like a sine.

Any ideas why?

It is a Leach with a few Mods.....

110 V rails,Choke Regulated , Dual Mono with 2 X 1KVA transformers,

Output stage MJ21195 and MJ21196 8 pairs per channel.

Driver stage MJE15034 NPN,MJE15035 PNP.

#### EchoWars

In my Amp i can get a 1kHz Sq wave perfectly but a 10K Sq wave looks almost like a sine.
What's the input wave look like? PC sound cards aren't known for their ability to generate square waves...the amp is likely amplifying an already slew-distorted signal.

#### Duo

Interesting, 10K square should be pretty square in a good amplifier.

I was able to push my design to 200K without much degradation (certainly didn't look like a sine.)

Perhaps the computer is not outputting a good square? Try putting your scope right on the computer and see what you get, it could just be that.

Otherwise, I'd imagine something's up with the amplifier.

#### paologatto

mura said:
I use Cooledit Pro2.0 to generate sine waves and Sq waves,is this a reliable method?

In my Amp i can get a 1kHz Sq wave perfectly but a 10K Sq wave looks almost like a sine.

Any ideas why?
[/B]

If you use for example a sample rate of 48 kHz, it's quite normal that a 10 KHz Sq wave looks almost like a sine because you don't have any harmonic inside the Nyquist frequency (half of sample rate).
The first odd harmonic would be at 30 kHz that is greater than 24 kHz.
So a soundcard, also a pro model, cannot generate a Sq wave of that frequency. You should use a frequency generator with some MHz bandwith for this task.

For example there are a lot of projects on the net that use the IC MAX038 for generating signals sine/tri/sq from 0.1 Hz to 20 MHz. I builded one and is perfect for testing audio gear..

#### boraomega

Many people are using square wave signal for testing and estimation of amps performances. But factor that is seldom taken into account or stated is rise and fall times of the used square wave signal, which is of outmost importance. Many amps will looks pretty good with, say, 1uS rise and fall time square, with minimal or even completely absent ringing, but try it with 20nS rise time square and you will se a hell of a different picture. Slew rate of such 20nS signal is somewhere about 300V/uS allowing you to test all but fastest amps. Try adding this shaper after your sine signal generator, for it will produce square wave from sine with right about 20nS of rising and falling time.

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#### fab

Paid Member
Hi boraomega

I do not understand how a real audio input signal could produce such a square wave that fast in the amp. Could you explain.

thanks

#### AndrewT

Hi,
I'm no expert and still learning but you are right. No conventional audio amp will ever reproduce that fast an input signal.
The real point is that you are injecting a test signal to help you determine that the amp is operating correctly.
Those very fast edges really stress the electronics and if they are not quite right, odd looking traces appear on the oscilloscope.
Finally your RF filter on the input will round off the the corners of the square wave by quite effectively slowing down the rise time of the test signal. In fact this could be an argument for easing the RF filter for testing & then returning to the longer time constant for ordinary (audio) use.

#### boraomega

FAB, you are partially right. Average real audio input signal is never that fast and you will find that somewhere about 8V/uS is all the speed you need to be more than "fast enough" with usual average output power (say... 10-30W) used in room listening (higher power amps needs higher Slew Rate figure to maintain quality and capabilities). But don't forget that in a good number of high quality recordings very fast transients exists which "requests" much better Slew Rate figure than 8V/uS to be reproduced faithfully. If amp's Slew Rate figure is not "up to the job", it will cause something known as "slew induced distortion".

AndrewT already gave you second part of explanation.

#### Duo

This has become a very interesting thread about slew rate. I like what's being said.

Definitely, I'm glad someone mentioned the nyquist theory on sound cards. That's such a true thing.

As far as slew induced distortion, I've listened to a great many amplifiers in which that occurs at high power level.

I should do some more serious research on this.

#### amplifierguru

Square wave testing gives much information about an amplifier's bandwidth and stability in one simple visual test as has been said, but the test gear needs to have a capability beyond the amp unity gain bandwidth of NFB closure.

You need to disable any input filtering when tuning so the full bandwidth of the amplifier is stimulated by the harmonics of the sq wave. Then you can tune compensation C's and evaluate reactive load behaviour. Then the input filter (usually 100KHz +) should be reconnected to remove unwanted HF garbage and RF pickup.

The slewing rate of an amplifier only needs to exceed that of the highest frequency expected at full power - usually 20KHz which would be 2u7S for a 30W amp and double for a 4 times, 120W. Anything above this band should be filtered out but a simple 6dB passive input filter below about 100KHz will start to rollof the 20KHz. Having said that the distortion of an amp generally rises dramatically approaching the slewing rate limit with high level modulation of current usually at the Vas, so for low THD through to 20KHz it a good measure to have a higher slew rate.

I have designed amplifiers which have an infinite slew factor where the input (and feedback) filters cut in at 12 dB/octave before the amp slew rate is reached - so it cannot be slewed with normal signal levels. Rather acadaemic with 22KHz CDs.

Greg

#### mura

Hello and thanks again for all inputs,

So if my amplifier has 110V rail voltage to what extent should i drive the amplifier with a 10K Sq wave?

#### amplifierguru

Hi Mura,

If you are testing the amp for the first time, start off with a small square wave with a , say, 1 MHz BW - a generator with sq wave output to 100KHz should do this. 2.83Vrms is OK. Look for square and smooth rising and falling then try 1uF. See if ringing is well damped.

Then crank it up to say 10Vrms do the same tests. It's still only 12.5W. Should handle that easily and well damped.

Nows the time to adjust any Ccomps for smoothest rise time i.e. quickest rise/fall without overshoots. Then try 2uF.

It should instill confidence at this stage, before trying full power SINEWAVE test at 10KHz. Ensure no triangulation or fuzzy waveform indicating oscillations AT ANY POINT on waveform. Check no zero crossing notches.

Good luck.
Greg

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