Amplifier Soft Starts

7/23/2019 Amplifier Soft Starts

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Amplifier soft-starts

Designs with larger transformers may require soft-startcircuits to prevent the mains fuse blowing on switch on,especially with toroidal types which use less copper wire(reducing winding impedance) and a high-grade corematerial. With no magnetic field established and

discharged smoothers the fuse often sees a near short-circuit when an amp is turned on, and does it's ob. !orthis reason a toroidal type's mains fuse will usually havea higher rating than that of an equivalent "-# type,which may not be desirable. !or e$ample, the%a&amichi - paralleled * $ + /W resistors (eachwith it's own fuse) in series with the (//0) primary, arelay then shorting these after a short delay.lternatively, thermistors can be wired in series with theprimary windings (eg1 23+/ for domestic types).

4witch-on surges can be quite profoundand can bemaor contributors to component failure. educing suchsurges can then prolong operating life. #n-rush currentlimiters, similar to varistors, are available, but discretesolutions can be very effective. lthough earlier designs,li&e that used in the 56W 7/can be employed, someother tested e$amples are given below (8*/0 mainsassumed)9
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:riacs can cause problems by 'dropping-out' on lowloads, say ;//W. :#28/+D can suit loads of up to7/W, whilst a :#288+ will suit heavier ones. %oiseless(


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#ntended for a 8 $ 7//W = $ &W amplifier, this circuitcombines a soft-start, output D2 protection and visualindication of condition. n au$illary supply gave >80for ancillary circuits and the 7-/-70 2 feed.

2ombining the amplifier inputs and spea&er switchingvia a single circuit shuts down bothchannels when onlyone malfunctions.

:wo simpler arrangements, the :itan 8/// version for a
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&0 load, capable of switching 8&0. !or this, aseparate more comple$ circuit provided D2 protection,temperature sensing, visual indication and sequentialswitching of multiple (?) relays for the output andanother shorting the amplifier input to ground.

#n these &inds of circuits the potential across the relayscan be used to drive opto-isolators or smaller relaysthat are then used to mute inputs.

neat arrangement used the switched mains outlet of apreamp to trigger a single heavy-duty soft-start circuit.n array of power amps and associated equipment canthus be protected and powered up by a single switch,but ensure a manual by-pass e$ists, ust in case.

standard ? (@A) mains outlet equates with someB&W usable audio output (rms), and more with WC.

:o prevent a spea&er being damaged as an outputelectrolytic charges through the load on switch-on, asimple supply arrangement will bring the supply upslowly allowing the cap to charge at a less precipitousrate. llow an operating input=output differential of 7-+0. dditional benefits include hum filtering.

s an alternative, an output stage can be ramped toachieve the same result.


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nother approach ramps the input pair and class driver on power up.


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4ometimes long storage times can affect electrolytic

capacitors adversely although a simple precaution canresolve this. :he l o$ide layer may deteriorate whenstored without an e$ternally applied voltage, especiallyat higher temperatures. 4ince there is no lea&agecurrent to transport o$ygen ions to the anode in thiscase, the o$ide layer is not regenerated. :he result isthat a higher than normal lea&age current will flowwhen a voltage is applied after prolonged storage. sthe o$ide layer is regenerated in use, however, thelea&age current will gradually decrease to its normal

level. uality l electrolytic capacitors can be storedvoltage-free for at least 8 years, and long-life types foras long as 7 years without any loss of reliability.rovided that these storage periods have not beene$ceeded, the capacitors can be operated at ratedvoltage directly after being ta&en out of storage. #n thiscase, reforming of the o$ide layer is not required (this isnormally achieved by running the capacitor in serieswith a // resistor for an hour). When designingapplication circuits, attention must be paid to the factthat the lea&age current may be up to // times higherthan normal during the first minutes following the


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application of power. When the capacitors have beenstored for more than two years, it is decisive whetherthe circuit will tolerate high initial lea&age currents. circuit that has been stored for more than two yearswith the capacitors incorporated, should be operated

trouble-free for one hour. :his will usually regeneratethe capacitors so far that storage can be continued.When applying power to a unit stored for some time,running it in series with a +/W light bulb for an hour ormore can prevent failures.

DistortionE 3ayouts and Feat-sin&ingE rotectionEDimensioning a designE 4etting up an audio amplifier

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Amplifier Soft Starts