Quad 306 switch on thump

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01-Sep-2017 08:32 PM

chesterdavid

chesterdavid

Posts: 17

I have a Quad33/306 combination, both having been updated with the dada electronics upgrade kits.  The 306 is powered from the 33 and the whole lot is switched on via the 33 volume/on/off control.  Sometimes on switch on I get quite a loud thump from the loudspeakers.  I am considering a loudspeaker protection kit but vaguely remember reading somewhere that the 306 should not have a switch on thump.  Is that the case?  If so any clues as to what may be causing the thump?

Many thanks.

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01-Sep-2017 10:33 PM

Fabian

Fabian

Posts: 54

Both of my 306 do perform a lovely switch on thump. :)

I found it to be slightly reduced after applying the Dada upgrade kit, but it's still clearly audible.

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02-Sep-2017 07:51 AM

Joost Plugge

Joost Plugge

Posts: 943

Normal behauviour of 306/606/909 amps should be a mild switch on thump, silent switch off. The 33 has no signal delay at switch on, like a 34 or 44, so the switch on thump, could be louder in some circumstances.

Joost Plugge

DaDa Engineering

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02-Sep-2017 08:40 PM

chesterdavid

chesterdavid

Posts: 17

Fabian, Joost,

Many thanks for your swift comments.  Very reassuring.  I can stop panicking now.

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05-Sep-2017 03:05 PM

chesterdavid

chesterdavid

Posts: 17

Following on from the earlier advice I decided to investigate the Velleman K4700 loudspeaker protection kit.  However on another hifi enthusiasts site there were recommendations to move the relays off the pcb, use relays with more contacts and put contacts in series to increase the fault current breaking capacity, use a relay contact to short out the loudspeakers when the relays are de-energised state and lastly put the psu off the pcb.  Some of these recommendations seem practically contradictory.  Anyone have experience of this kit and recommendations for these mods?

Also there is a resistive load applied to the amplifier output until the relays energise.  The instructions say the resistors should be included for an asymmetrical power supply but excluded for a symmetrical one.  I would assume the 306 has a symmetrical power supply although the positive and negative voltages are not quite the same.  Would it be classed as symmetrical or asymmetrical. 


Many thanks again

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06-Sep-2017 04:03 AM

EJP

EJP

Posts: 1412

That all sounds like gilding the lily to me. I would ignore it all. Shorting out the speakers is particularly pointless.

The 306 has symmetrical rails in the sense that the rails are + and -. The 303 by constrast has an asymmetrical PSU in that the rails are + and ground, which implies that there are output coupling capacitors which need a discharge path to ground, hence the recommendation for bleed resistors; however the 303 already has bleed resistors.

NB My 306s have had a gentle turn-on thump for 30 years, both before and after renovation. It is nothing to worry about.
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30-Sep-2017 08:25 PM

chesterdavid

chesterdavid

Posts: 17

Thanks for all the input.

I have now built the kit but have one, hopefully, last query.  The Velleman connection diagram shows one return (common) connection from the amplifier (306 in my case) which is routed to both speakers and the protection unit.  The 306 has one return connection for each channel although from the circuit diagram the two returns are tied together inside the 306.  My question being is there any problem/disadvantage in connecting the 306 as per the Velleman instructions.


Many thanks

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