My 306 has a very soft “rattle” in the left channel, with the input disconnected.
You really need to put your ear close to the tweeter to hear it, however, the right channel is dead silent.
All power-transistors seem the originals, and the amp sounds wonderful.
Changing to another mains outlet did not work, reversing line and neutral had no effect either.
It’s not mains hum. I connected a scope on the speakers and saw little spikes of a few mV “traveling” on the output with 20-140 ms interval. These spikes themselves seem oscillations of approx 15 MHz. See screenshot (with NL comments, sorry).
A Quad-guru advised to change T7-T8, however, I do not understand that…
Any other suggestions? Thanks!
You really need to put your ear close to the tweeter to hear it, however, the right channel is dead silent.
All power-transistors seem the originals, and the amp sounds wonderful.
Changing to another mains outlet did not work, reversing line and neutral had no effect either.
It’s not mains hum. I connected a scope on the speakers and saw little spikes of a few mV “traveling” on the output with 20-140 ms interval. These spikes themselves seem oscillations of approx 15 MHz. See screenshot (with NL comments, sorry).
A Quad-guru advised to change T7-T8, however, I do not understand that…
Any other suggestions? Thanks!
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The advice to replace T7,8 may actually be appropriate in this case. I had an apparently similar 306 problem, when the driver transistor leads sheared off due to flexing of the PCB - not just with my amp but also with a repair for another owner too. The leads were still in contact and static DC measurements were fine but there was a strange "zing" in the sound at peak music levels.
The problem related to the transformer being mounted on the PCB. The foam pads which would have damped any movement in transit or general handling, had disintegrated over many years. With vibration in air transit or just moving the unit about, the driver leads flexed and fractured but remained in contact when testing - a frustrating fault to locate but a suitable replacement was cheap - TIP42, IIRC.
The problem related to the transformer being mounted on the PCB. The foam pads which would have damped any movement in transit or general handling, had disintegrated over many years. With vibration in air transit or just moving the unit about, the driver leads flexed and fractured but remained in contact when testing - a frustrating fault to locate but a suitable replacement was cheap - TIP42, IIRC.
Could also be an input transistor gone noisy, or an external RF signal being envelope-detected due to the physical configuration of that channel.
Our environment is bathed with RF signals, with most of them digitally modulated, sometimes in bursts, like PLC.
I have PLC in my house, and I find it everywhere, whatever my efforts to keep it away
Our environment is bathed with RF signals, with most of them digitally modulated, sometimes in bursts, like PLC.
I have PLC in my house, and I find it everywhere, whatever my efforts to keep it away
thanks for the replies!
@ejp: the sound is barely audible, a soundfile will be difficult I guess, but will give it a try.
@Ian: the "rattle" is independent from volume; it's not at all audible when music is playing. I measured the scope plot with the input disconnected.
@ejp: the sound is barely audible, a soundfile will be difficult I guess, but will give it a try.
@Ian: the "rattle" is independent from volume; it's not at all audible when music is playing. I measured the scope plot with the input disconnected.
I've experienced not unlike what you show on the scope shot and it was caused by mains related interference. One channel can be effected more because of less than perfect grounding within the amplifier.
Paying attention to safety issues... why not couple up a transformer secondary to the scope and see if the pulses are present. Just use a small transformer with a free floating secondary.
Paying attention to safety issues... why not couple up a transformer secondary to the scope and see if the pulses are present. Just use a small transformer with a free floating secondary.
@Mooly, I also thought about mains-induced stuff. I therefore used a different mains outlet (in the same room; probably the same Phase from my grid); and also reversed Line and Neutral. All this had no effect. I'm afraid my house and grid may be full of RF...
The test to check the mains with an extra transforming sounds interesting, however requires careful preparation and process. Not sure I'm up to that in this stage...
The test to check the mains with an extra transforming sounds interesting, however requires careful preparation and process. Not sure I'm up to that in this stage...
Using a secondary winding is safe but it must be 'free floating' and not like one in an amp where it is coupled up to the circuitry. Otherwise you create a short when the scope ground is connected.
Even that is quite safe from a shock hazard perspective but not good for the amp or the scope 😱
Remember the voltage from the transformer is higher than you might think (thinking of scope input settings) and even a 15 volt AC winding would actually be over 40 volts peak to peak. So just make a little resistive divider and put that across the secondary, say a 10k and 1k and clip the scope over the 1k.
Different sockets will not fix the issue if that is what is happening.
Even that is quite safe from a shock hazard perspective but not good for the amp or the scope 😱
Remember the voltage from the transformer is higher than you might think (thinking of scope input settings) and even a 15 volt AC winding would actually be over 40 volts peak to peak. So just make a little resistive divider and put that across the secondary, say a 10k and 1k and clip the scope over the 1k.
Different sockets will not fix the issue if that is what is happening.
I just thought: we also use ethernet-over-mains in our house. That does not help I'm afraid.
If the HF stuff is entering my 306 via the mains; would it be possible to filter it in the 306 power supply? E.g. placing some "HF" MKT/MKP caps in parallel with the large rail electrolytic caps?
Or use an external mains-filter (depends probably whether the problem is common- or differential mode)
If the HF stuff is entering my 306 via the mains; would it be possible to filter it in the 306 power supply? E.g. placing some "HF" MKT/MKP caps in parallel with the large rail electrolytic caps?
Or use an external mains-filter (depends probably whether the problem is common- or differential mode)
Firstly you need to confirm that mains related interference really is the cause.
If it is then in my experience I found no amount of filtering corrected the problem. This was actually on a build of Doug Self's original blameless Class B. When this problem occurred it could also be heard as a change in 'tone' of the toroidal transformer.
All you can do is take it in steps and try and first off prove where the issue is and then look at solutions. A first step would be trying the 306 with nothing connected but speakers and with shorting plugs fitted at the inputs.
The problem may be more related to the spikes coupling into the ground connection (is the Quad 306 mains grounded?) and it is extremely difficult to remove because of the energy and rise time of each pulse.
Take it in stages and prove the problem first.
If it is then in my experience I found no amount of filtering corrected the problem. This was actually on a build of Doug Self's original blameless Class B. When this problem occurred it could also be heard as a change in 'tone' of the toroidal transformer.
All you can do is take it in steps and try and first off prove where the issue is and then look at solutions. A first step would be trying the 306 with nothing connected but speakers and with shorting plugs fitted at the inputs.
The problem may be more related to the spikes coupling into the ground connection (is the Quad 306 mains grounded?) and it is extremely difficult to remove because of the energy and rise time of each pulse.
Take it in stages and prove the problem first.
OK, will take the advice to attack this issue "step by step"
BTW: the 306 is connected to PE (earth, as we call it here). I tested it on the bench with inputs disconnected; not shorted. With my 34+FM4 connected, the "rattle" remains unchanged.
Probably the first step is to retest in an area where mains is "cleaner" than in my mancave, and where perhaps there's less RF.
Will update this thread with any new findings, thanks a lot so far!
BTW: the 306 is connected to PE (earth, as we call it here). I tested it on the bench with inputs disconnected; not shorted. With my 34+FM4 connected, the "rattle" remains unchanged.
Probably the first step is to retest in an area where mains is "cleaner" than in my mancave, and where perhaps there's less RF.
Will update this thread with any new findings, thanks a lot so far!
OK, that sounds like a plan.
Other things you could look at for interest would be to short the scope probe and its ground lead (I know, sounds a bit weird) and then place the probe tip on the amplifiers ground. Any pulses visible?
You could also look at the DC rails in the amp (use AC coupling on the scope) and see if you can see the same pulses.
Other things you could look at for interest would be to short the scope probe and its ground lead (I know, sounds a bit weird) and then place the probe tip on the amplifiers ground. Any pulses visible?
You could also look at the DC rails in the amp (use AC coupling on the scope) and see if you can see the same pulses.
You mean PLC I imagine?
In this case, getting rid of it is quite difficult.
First thing to do is to disconnect completely every PLC device in your house (I mean disconnect physically from the mains outlet, not just disable it) and listen to your amp output.
If the rattle disappears, you nailed the culprit, but hardening an old amp against it might be more difficult than it looks