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Quad 303 power amplifier hissing n pop sound.

lwong2's picture

I am a novice in electronic. Can some one kindly help me with this problem

My quad 303 when power up , 1 of the speakers ( right channel) has a hissing sound then a pop sound when power cuts in.It takes a few seconds to normalise.The left speaker is OK

I have

a) replaced the 2 power supply capacitors from 2000 uf 200 V to Elna 3300 uf 100V and

b)replaced a single core to a multi cores cable from the rectifier to the + ve end of the capacitors and

c) connected in parallel .33 uf 1000vdc West capacitors to the out-put capacitors.

Prior to this change the 2 speakers were normal no pop sound etc.

My questions are :

1. What can likely gone wrong.
2. Can this phenomenon hurt the speaker.
3. Can the biasing cause this problem.
4. Does the polarity of the west caps matter . how to identify their polarity
1 end is silver and the other side is black in color.

Thank you and appreciate for any advice.

best regards,


Re: Quad 303 power amplifier hissing n pop sound.

Dear Wong,

This QUAD 303 amplifier need maintenance.
Modifying will not solve the problem.
Replace all small electrolytic capacitors by the same value ( use e.g. Panasonic EB series ). ( use for C104 10µF, 12µF is hard to find ).
The input capacitor ( tantalum type ) can be replaced by a small 680nF MKT capacitor.
Replace all resistor trimmer and check all the fixed resistors for their value and replace the out of tolerance ones, mainly the resistors over 18kΩ have the tendency to increase their value at this age.
Sometimes some transistors became unstable and generate rumble and noise as well, mostly the drivers Tr105 and Tr106 but the other transistors can be affected as well. The original transistor type are obsolete, you can use the following type's:
1) Power transistors Tr1,Tr2 and Tr3 ( power regulator ) Motorola or On-Semiconductor MJ15003.
2) Tr105 ( driver PNP ) 2N5322 ( fit with the original heatsink) or BD140 ( doesn't fit, smart bending of the pins and use of a spacer the same thickness as the BD 140 make use of the original heatsink possible ).
3) Tr106, Tr201 ( driver NPN ) 2N5320 ( fit with the original heatsink) or BD139 ( doesn't fit, smart bending of the pins and use of a spacer the same thickness as the BD 139 make use of the original heatsink possible ).
4) Tr102,103 BC546B.
5) Tr104,200 BC556B.
6) Tr101,107 BC550B ( low noise ).
7) Tr100 BC560B ( low noise ).
8) MR201 BZX79C12 or other 12V 400 mW zener diode.
9) All other small diodes can be replaced by 1N4148.

Adjust the amplifier following the service manual ( can be found on hifi-engine ).

Altough this amplifier is designed in 1967 it is in good shape a very good amplifier. I repaired hundreds of these and all left the workshop producing 2 times 50Watt with a THD+N @ 400Hz full power less than 0.02% some even as close as 0.005%. S/N 96 dB(a) respected 50Watt//8Ω.

The only drawback this amplifier have are the 2200µF output capacitors, with some loudspeaker systems it is lacking control in the lowest frequency range ( below 50Hz ). Then I will swapped the output capacitors ( C1 ) for 4700µF or even 6800 µF.

Best regards

lwong2's picture

Re: Quad 303 power amplifier hissing n pop sound.

Dear Paul,
Thank you for your valuable input. This seems a major task to me.
I may need to look for our local agent if any to do this for me in Singapore .
Meanwhile i was wrong to say that the pop sound came from 1 speaker. In fact it is from both the speakers.

So far I have made the following changes to this amp.

1. Replaced a broken C200 in the regulator board.( this was made before the pop sound problem).

2. Replaced the 2 pcs of power supply caps with 2 pcs of Elna 3300 uf 100V
( poping sound occurred after this change. But when i changed back to the original caps -- 2000 uf x 200V . This pop sound persists)
3. Replace 2 pcs of output caps with 2 pcs of Elna 3300uf x 100V
4. Connect in parallel 2 pcs of 4.0 uf x400v Solen caps to the 2pcs of output caps.
5. replaced C104 (10uf), C106 (470 uf) n C101 ( 47uf) with Nichicon gold series caps All these caps' value were accordingly to the original orange color caps which i removed from the amplifier's driver.

Actions of items 1 , 3 to 5 were done before the poping sound. So they were not the cause)

(So the poping sound occurred after i replaced the 2 pcs of power supply caps).

Is it advisable for me to replace The input capacitor ( tantalum type )- may i ask this cap is under which item on the schematic diagram ?

Once again appreciate for your time and sharing your knowledge with me. Meanwhile i would see what are the possible things i can do to this amp like -- measure the value of the resistors etc .

Best regards,


Re: Quad 303 power amplifier hissing n pop sound.

Dear Wong,

Increasing the value of the output capacitors will increase the chance of having an inrush pop.
The amount of power on the loudspeaker will normally not excessive.
If it is a single pop it is likely normal behavior.
A well know behavior of the QUAD 303 is that switch on the power shortly after switching off will produce a louder bang.
It better to wait 20 seconds or so.

C100 is the input capacitor and can be replaced by a 680nF MKT or any other good foil capacitor.
Old tantalum capacitors can fail or behave oddly.

If the resistor trimmers like RV100 and RV101 ect. are old replace them and adjust them as written in the service manual.

C201 on the regulator board is an important one because is prevents the regulator to oscillate.
This oscillation can produce artifacts on the output.


lwong2's picture

Re: Quad 303 power amplifier hissing n pop sound.

Dear Paul,

Once again thank you for the advice, Both the caps ( items C201 n C100) are from Mullard which i have trust in their quality. As you said, due to age , the value may drift.

RV100 and RV 101 , i have not tried adjusting any pots so far. No doubt i am having a multi tester and a capacitance testor. i think i need to try

I shall look into your suggestion.

Best regards,