I have purchased a Quad 306 power amp to go with my Quad 34 preamp last week. I hooked them up yesterday to discover the 306 has really low volume and distorts when driven hard. Changing the balance on my 34 revealed that the 306 only uses the left input to feed to both channels.
To make sure my 34 isn't faulty, I've hooked up the 306 directly to my Motu M4 interface with exactly the same results: dual mono, low volume and distortion when driven hard.
I'm pretty handy with a soldering iron and have a Hakko 407 desoldering station as well. If needed, I can fire up an old oscilloscope too. The schematic seems pretty straight forward, and there are more than enough service notes and help topics on the internet.
Shall I attempt to troubleshoot and repair the 306 myself, or return the amp and try to find a working one?
To make sure my 34 isn't faulty, I've hooked up the 306 directly to my Motu M4 interface with exactly the same results: dual mono, low volume and distortion when driven hard.
I'm pretty handy with a soldering iron and have a Hakko 407 desoldering station as well. If needed, I can fire up an old oscilloscope too. The schematic seems pretty straight forward, and there are more than enough service notes and help topics on the internet.
Shall I attempt to troubleshoot and repair the 306 myself, or return the amp and try to find a working one?
I'm all for giving this legendary piece of equipment a second life, but I also think I paid too much for a defective device.
Then again, the seller claims it was fine before shipping, so it might as well be something trivial or mechanical.
It would have been nice if the dual mono problem was mentioned somewhere on the internet.
Then again, the seller claims it was fine before shipping, so it might as well be something trivial or mechanical.
It would have been nice if the dual mono problem was mentioned somewhere on the internet.
Are you sure your 34 preamp is performing as it should..? Do you have another dual-rca output source with adjustable volume you can try with the 306? NB a passive pre , i.e. a pot in a box, works brilliantly with the 306.
IF answer to both Qs is yes - then your 306 has been heavily messed with, or is defective.
What you describe is not how the 306s are wired, or were ever offered to be wired.
Fortunately you can find all the service documentation you need online.
IF answer to both Qs is yes - then your 306 has been heavily messed with, or is defective.
What you describe is not how the 306s are wired, or were ever offered to be wired.
Fortunately you can find all the service documentation you need online.
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Your description does read like a fudged repair job for quick disposal. Having said that, I had my pristine 306 shipped from the UK some years ago but due to the method of mounting the power transformer on the PCB with only aged and powdered foam damping any movement in transit, some driver transistor legs had sheared off and the sound in one channel was akin to your description.
To understand the issue, the "driver" transistors T7,8 are mounted flat on the main heatsink brackets with their leads projecting out over, then bent down and through the flexing PCB. With enough vibration, they simply break off - something that could happen during a long road journey or a cargo hold in a Jumbo Jet, as it did for me. I didn't notice this fault until I pressed the PCB downward along the sides and voila! The culprit was then obvious so a couple of TIP42 were in order and that rotted foam was replaced with a substantial piece of solid, high temperature rubber.
Do check this possibility by careful inspection with strong lighting before diving in and wasting time and money on guesswork and random parts replacements.
To understand the issue, the "driver" transistors T7,8 are mounted flat on the main heatsink brackets with their leads projecting out over, then bent down and through the flexing PCB. With enough vibration, they simply break off - something that could happen during a long road journey or a cargo hold in a Jumbo Jet, as it did for me. I didn't notice this fault until I pressed the PCB downward along the sides and voila! The culprit was then obvious so a couple of TIP42 were in order and that rotted foam was replaced with a substantial piece of solid, high temperature rubber.
Do check this possibility by careful inspection with strong lighting before diving in and wasting time and money on guesswork and random parts replacements.
I might have found the culprit, or at least something pretty strange.
The 306 has two jacks added. I was told they were parallel with the banana plugs, so I used them instead of the banana plugs because I had a few Neutrik jacks laying around and I find them to be more reliable.
I'm looking at the jacks now, and it seems that they are both wired in stereo, with a 470R resistor in series with the signal. This explains my dual mono, but it also means I shorted the output of the right channel through 2x parallel 470R resistors to ground. 🙁
I do feel like a complete idiot hooking up speakers without investigating the internal connections of this mod. I'll toss the mod out while I have it open.
Might this also explain the low volume and distortion?
The 306 has two jacks added. I was told they were parallel with the banana plugs, so I used them instead of the banana plugs because I had a few Neutrik jacks laying around and I find them to be more reliable.
I'm looking at the jacks now, and it seems that they are both wired in stereo, with a 470R resistor in series with the signal. This explains my dual mono, but it also means I shorted the output of the right channel through 2x parallel 470R resistors to ground. 🙁
I do feel like a complete idiot hooking up speakers without investigating the internal connections of this mod. I'll toss the mod out while I have it open.
Might this also explain the low volume and distortion?
yes it would - you'd be driving the speaker via a 470ohm resistor, and becasue that would deliver pathetically-low sound volume - you've wound the volume input up to the point where the amp was clipping.
Don't worry - if that's the case you will not have damaged anything at all.
(I do wonder what that mod was for - picking off a signal from speaker outputs to drive a pair of active subs? must be something like that...)
Don't worry - if that's the case you will not have damaged anything at all.
(I do wonder what that mod was for - picking off a signal from speaker outputs to drive a pair of active subs? must be something like that...)
It's alive! 😀
Volume is back, no distortion and stereo! It's a tad brighter than my 405 and the lows aren't as clear as the 405, but this is a big relief!
While I have it open, I'll change R13 from 9R to 36R to get 1.25v input sensitivity. I'll mod my 34 like stated in the user manual as well for 1.6v output.
I will also try to source new filter caps and up the value as well. Maybe that will add some clear lows?
I'm using a set of B&W 220i speakers, btw.
Volume is back, no distortion and stereo! It's a tad brighter than my 405 and the lows aren't as clear as the 405, but this is a big relief!
While I have it open, I'll change R13 from 9R to 36R to get 1.25v input sensitivity. I'll mod my 34 like stated in the user manual as well for 1.6v output.
I will also try to source new filter caps and up the value as well. Maybe that will add some clear lows?
I'm using a set of B&W 220i speakers, btw.
