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Rise Time / Slew Rate

V

vintagestereo

Addicted Member
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OK....all joking aside....what exactly do these specs mean in the context of SOUND reproduction?

The Specs page for my Kenwood KR-1000 "Galaxy Commander" Receiver mentions a Slew Rate of +/- 150 V/sec and a Rise Time of 1.2 usec. What do these specs mean, since I've never seen them mentioned on a Spec page before?
 
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vintagestereo said:
OK....all joking aside....what exactly do these specs mean in the context of SOUND reproduction?

The Specs page for my Kenwood KR-1000 "Galaxy Commander" Receiver mentions a Slew Rate of +/- 150 V/sec and a Rise Time of 1.2 usec. What do these specs mean, since I've never seen them mentioned on a Spec page before?
Click to expand...

The 'slew rate' of an amplifier is simply it's ability to follow the input signal. An amp with a 'fast enough' slew rate will mimic exactly at the ourput, the input signal. Remember, the audio signal is composed of two elements; signal level and time.

The 'rise time' of the amplifier is the measure of it's speed when changing from, for example, zero volts to the maximum volts permitted. The time it takes for the amplifier to reach that maximum voltage is the 'rise time'.

Clearly, these two specification are related. An amp with a poor 'rise time' is unlikely to achieve a satisfactory 'slew rate' measurement.

An amp with spectacular 'slew rate' measurements can amplify tricky wave forms (cymbal crash + the harmonics) better than one with slow 'rise time' figures. The resulting output of the amp with poor 'rise time' figures is likely some distortion as the wave shape is altered by the amp's circuits.

Paul
 
So what would be considered a satisfactory rise time and slew rate?
A few usec sounds quick to me for rise time.
 
If you consider that a 100kHz signal has a cycle time of 10us and you want to play a sinus signal of this frequency, I would estimate that a risetime of around 1 us is good for that. So this should be prettey good for HiFi listening.
 
Frequency response at X distortion is the key. It defines all of this at once. On an amp 100K half power is good. Preamps can go this high but not necessary since interference may get in them with bad results..

Ron-C
 
I agree that 1 microsecond is a good figure provided you also specify the transition value; for example 0.5V/microsecond. Otherwise you need to consider that the device will achieve the full maximum voltage in the time base you specified.

I think (but can't verify as I can't locate a suitable amp spec :no: ) that somewhere in the region of 50 microseconds to reach a maximum of 44 V (a typical 100 watts/channel amp) is, I think, passable for 20 KhZ signal.

Has anyone got some sample specs to prove or disprove this?

Paul
 
Sansui Au-719
90 Watts/channel
10-20000 Hz
.015 THD at rated power
Rise time = .5 micro sec
Slew rate = + or - 170v/micro sec.
Freq response at 1 watt = DC to 400,000 Hz
 
Specs for Krell Evolution One amp:

Output power

450 W RMS at 8 Ohms

900 W RMS at 4 Ohms

1800 W RMS at 2 Ohms



Output voltage

170 V peak-to-peak

60 V RMS



Output current

57 A peak



Slew rate


100 V/us



Power consumption

Standby: 225 W

Idle: 300 W

Maximum: 3600 W
 
Morden2004 said:
I agree that 1 microsecond is a good figure provided you also specify the transition value; for example 0.5V/microsecond. Otherwise you need to consider that the device will achieve the full maximum voltage in the time base you specified.

I think (but can't verify as I can't locate a suitable amp spec :no: ) that somewhere in the region of 50 microseconds to reach a maximum of 44 V (a typical 100 watts/channel amp) is, I think, passable for 20 KhZ signal.

Has anyone got some sample specs to prove or disprove this?

Paul
Click to expand...
50µS to rise to 44V is pretty slow...less than 1V/µs. But lets call it 1V/µS for simplicity sake.

Here's a visual of slew rate distortion...as well as a formula for the maximum frequency that can be amplified given a slew rate and a desired output amplitude:

slewratepic0kf.jpg


For your amplifier with a slew rate of 1V/µS, the maximum undistorted frequency for a peak voltage of 44V:
1V/µS/(2(3.14159)*44V = 3.62KHz
So the theoritical amp here cannot amplify a signal any faster than 3.6KHz to a 44V peak without slew-induced distortion.

(Note: When doing calculations, you have to consider that slew rate is expressed as a fraction itself, i.e., 1V/µS, or 1V/.000001S. Therefore when using the above equasion to solve for Fmax, you have to multiply your answer by 1,000,000. Similarly, when solving for slew rate [Sr = Fmax * (2 * pi * Vp)], your answer is in volts per second. You will need to divide by 1,000,000 for the answer to make sense)

Frequency Fmax is also sometimes called the Power Bandwidth of an amplifier. If you know the voltage rails and the power bandwidth, you can calculate slew rate.

Clearer now?
 
OK...for your 100W amplifier to reproduce a 20KHz sine wave with no slew-induced distortion at full output (+/-40V)...

Sr = Fmax * (2 * pi * Vp)
= 20KHz * (2 * 3.14159 * 40V)
=5V/µS

Lets try working backwards some more... The Kenwood Model 600 is given a power bandwidth of 50KHz. The rated output power is 135WPC, but we'll assume that it can slew undistorted into it's actual output power of 160WPC, which translates to a peak voltage of about 50V.

Sr = 50KHz * (2 * 3.14159 * 50V)
=15.7V/µS

Specs like power bandwidth are tossed about lightly by manufacturers. In reality, these are generally a worst-case scenerio, and I'd be suprised to see any halfway decent amplifier fail to beat the given specs by a fair margin.
 
vintagestereo said:
OK....all joking aside....what exactly do these specs mean in the context of SOUND reproduction?

The Specs page for my Kenwood KR-1000 "Galaxy Commander" Receiver mentions a Slew Rate of +/- 150 V/sec and a Rise Time of 1.2 usec. What do these specs mean, since I've never seen them mentioned on a Spec page before?
Click to expand...
So the KR-1000 is about factor 10 better than needed in EW´s calculation (related to the same output power), so there is still some nice reserve.
 
Thanks for putting that in black and white, so it's not as complicated as I thought.
Now I need more coffee to absorb it..
Terry
 
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