Quad 303 blew up

Surdo

Surdo

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Hi there,

Returned home last night, powered up my two mono-blocked Quad 303 amps which were disconnected at the wall because of storms. Briefly after it powered up, the right-channel amp produced some bursts of static through the speaker. I could smell electrical smoke and immediately pulled the power cord out. Haven't checked the fuse - but I'll definitely take it to a technician and won't attempt to run it again. The right speaker is OK and wasn't damaged.

The amp is an old one with the serial number somewhere in the 37 thousands (i think that gives it a 1970s vintage). I don't know what's involved in the fault, but I'm wondering if now is the time to do a revision of the components, especially the capacitors. I love the sound of these amps as they are and don't wish to modify the amp in any way (more than it is already with the switch-able mono mod). Can someone suggest a kit of components that is most true to the original specifications?

I've seen the kits by Dada Electronics and by NetAudio. Can anyone discuss the relative merits of the two, especially in respect retaining the original sound? Are there other options available to me?

Thanks,

Iain
 
Thanks Tenson. I opened up the amp and it looks like it may have already been recapped. In the main chamber there are 4 X Epcos 4700uF caps. I think the Dada kit uses these although from what I read they only supply 3. I uploaded a snap, please see:

http://reverberant.com/tmp/303_guts.jpg

I can remove and photograph the cards underneath the unit if it helps.

Is there any way to date the caps from the codes? The code that's only half visible in the photo is: 40/085/56

I did ask the previous owner if the caps had been changed but he thought they were original - he may have forgotten though.

The power fuse is unbroken. Couldn't see any visibly burnt components. The transformer smells OK. The burnt smell is at the other end where the cables enter.
 
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That amp has definitely been serviced.
The capacitors on the left and right driver boards are differest and the four large caps are not original.

How have the amplifiers been made into mono blocks?
 
Thanks Rob. I'm not sure how they were mono blocked. I asked the original owner about that too and he thought that they were done in parallel. There's a switch that allows you to select stereo or mono. When in mono I use the left channel input and the left channel speaker output and if this means anything to you, when in stereo, it hums if only the left input is connected, both need to be attached. I do know that they were modified at a certain shop here in Brasilia, so I can ask for more info. The owner of the shop has a good reputation in the city.

I noticed too that the left and right cards are different and the central card in the image 303_4.jpg seems to have newer capacitors. I'll check the other amp tomorrow.

Cheers,
 
The switch on the back looks like it simply connects both inputs together, so I'd assume parallel too. Unless there is a bigger switch somewhere though, you'd need to use both output sets not just the left channel.
 
Thanks. How would I do that? Connect a jumper lead between the left and right speaker outputs when in mono? Is this a worthwhile "mono-blocking" approach or should I look into having the amps modified further?

Edit:

or two speaker cables joined at the speaker end? Could the fact I was using only one speaker output have blown the amp?
 
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I don't know for sure how they were designed to be mono-blocked, but if they are meant to parallel then both inputs are driven and both outputs are used in to one load. Usually there is a small resistance in line with each output to balance the current from each output.

However, I think first you need to find out what has gone wrong. Can't you find any burnt parts?
 
an update:

Took the amp to a technical shop this morning (a different one from the one that did the mod) and asked for a quote on the repair and a complete exchange of the caps. Thanks for mentioning the mono-block business. I spoke to the technician about this and he said that it may need a small "bridging" circuit in addition to the simple summing of the inputs that's presently done - the bridge as I understood it, will do a phase reversal on one of the speaker output channels.
 
What he was suggesting involved the following in mono mode:

For each loudspeaker, I would connect the speaker cable (+ and -) to the two positive (red) terminals of the speaker outputs on its respective amp.

So he was definitely thinking of bridging the channels rather than putting them in parallel. He's just having a look at this stage to see what's been done and to search for the fault.
 
From what I'm reading now, parallel mono-blocking is the preferred option for the Quads. Bridging is also known as a "serial mono-block", correct? Thanks for your time on this.

...I'm starting to think that I'm better off dividing these two "mono blocks" into two separate stereo power amps.....
 
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An extract from Joost Plugge's article "Quad Mono Blocks" (available on the http://www.dadaelectronics.eu download page - login needed):

The parallel monoblock is simple in principle. Connect the inputs together and the outputs, that's it! To prevent large output currents from one amplifier to another, two measures have to be taken. First the output voltage of the two channels has to be matched, this is done by placing a volume pot meter in the input circuit. Second, some output resistors per channel are used to form a so called current sharing circuit. Some drawbacks, by connecting the input channels in parallel, the input impedance is halved, not all pre amplifiers like that. Also there is no loudness gain, the output voltage of the amplifier stays the same. It is better suited to low impedances, distortion as a result of clipping will be reduced.

A 303 example

Due to lower than modern amplifiers, channel separation, and reduced power output in low ohmic loads a popular Quad to parallel monoblock is the 303. A standard 303 will deliver 28W in 4 Ohm loads, instead of the expected 90W. This is the result of the very strict current limiters, a 303 is short circuit proof. In monoblock mode the output will be around 55W in 4 Ohm. In a 303 a current sharing circuit is not necessary, a 303 has output capacitors, so no DC current can flow from one amplifier to the other. Only the channel matching must be done. If you use a single amplifier, by removing one channel from the amplifier, the output will still be 28W, the limitation is not the power supply, but the current limiter. Off course the current limiter can be adjusted. But the parallel monoblock will always outperform a single amplifier, given the same adjustments or modifications are applied.
 
You want it like this. The resistors are the bits that need adding to operate stably.

parallel.jpg


However lets find the fault first!
 
Thanks!

Yes the tech will identify the fault first. I couldn't see any burnt components and left it with him to find.

Have passed on this diagram and asked for a quote applying the parallel design.

Using this technique, how is the speaker cable attached?
 
Assuming those output resistors are in-line before each output socket, you would attach the positive terminals together and the negatives together.
 
The only advantage from parallel mono operation is increased current into 4 ohm loads.
I wouldn't recommend it since to work well it requires the ability to fine trim the input signal on both channels for perfect balance, so you need to start adding potentiometers,

If the 'speakers aren't difficult to drive there is a far easier way to make a 303 into a mono amp and improve it's performance - remove one of the driver boards.

The 303 uses a regulated power supply and this allows each channel of the 303 to supply around 3 amps maximum. By removing (or disconnecting the power lines) to one channel you allow the ramaining channel to use the entire capacity of the regulated supply.

Unless using 4 ohm loudspeakers the single channel 303 should play just as loud as the parallel version, will perform better and not require further modification.

The 303 is not suitable for conventional bridging unless you were using 16 ohm (or greater) loads.
 

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