Hello there.
I stumbled upon this forum when looking for problems on my nad 314. I have successfully fixed the problems I've had with the balance pot, and the CD input switch, by cleaning contacts. But a more subtle problem remains.
There is a quiet white noise present on the left channel, when there is no signal input, which is unaffected by the volume level. However, when I remove the bridge connector connecting the pre-amp to the power amp, the noise increases and becomes really loud. This does not happen on the right channel, it remains silent, also with the bridge removed.
I am no electronics expert and have only a basic understanding of solid state amplifier theory, but it looks to me as the pre-amp is actually grounding some noise source in the power amplifier. Or that there might be a faulty component in the power amplifier causing some voltage float which is then drained back through the pre-amp.
I've tried tapping all components and looking for loose solder joints. No obviously damaged components, capacitors, that I can see. Does anyone have any pointers?
Cheers!
I suspect this sort of behaviour may ring some bells with experts.
I stumbled upon this forum when looking for problems on my nad 314. I have successfully fixed the problems I've had with the balance pot, and the CD input switch, by cleaning contacts. But a more subtle problem remains.
There is a quiet white noise present on the left channel, when there is no signal input, which is unaffected by the volume level. However, when I remove the bridge connector connecting the pre-amp to the power amp, the noise increases and becomes really loud. This does not happen on the right channel, it remains silent, also with the bridge removed.
I am no electronics expert and have only a basic understanding of solid state amplifier theory, but it looks to me as the pre-amp is actually grounding some noise source in the power amplifier. Or that there might be a faulty component in the power amplifier causing some voltage float which is then drained back through the pre-amp.
I've tried tapping all components and looking for loose solder joints. No obviously damaged components, capacitors, that I can see. Does anyone have any pointers?
Cheers!
I suspect this sort of behaviour may ring some bells with experts.
Disconnecting the power amp input is bad. You should check the power amp input separately for noise with shorted input connection. Open gain point can pick up all kind of random electrical noise. However, make sure the preamp is disconnected before you short the amp input, shorting the output transistor(s) of the preamp will burn it.
Noises and random volume changes are hard to debug. You were correct to go tapping, unsoldered joints can be a problem. Also pushing on components with a wooden stick to see if the sound coming out of the speaker increases or decreases. The component that changes the sound, something wrong there.
That doesn't work you need a meter or a scope. I use an analog voltmeter with a 2vac & 20 vac scale. You have to put a .047 uf cap in series with the ground probe to keep the AC volt scale from showing a reading on DC.
Then you look at similar points on the quiet channel and the noisy channel. You start at the beginning, the input jacks and the small black transistors, and work towards the big transistors on the heat sink. The stage before the noise begins to be readable is where the problem is. Could be a resistor, or a transistor or diode, more rarely a capacitor. Sometimes interboard connectors, especially the type with push down connections, are the problem in the 5-15 year timeframe.
Be aware you should only use one hand probing, as voltage over 24 across your heart can stop it. Connect the negative of the meter through the cap to the speaker return with an alligator clip lead, so you only need one hand. Wear no jewelry on hands, wrists, or neck, 1 v at high amperage through a ring can burn your flesh to charcoal. Wear safety glasses or reading glasses, parts sometimes explode.
Happy hunting.
Noises and random volume changes are hard to debug. You were correct to go tapping, unsoldered joints can be a problem. Also pushing on components with a wooden stick to see if the sound coming out of the speaker increases or decreases. The component that changes the sound, something wrong there.
That doesn't work you need a meter or a scope. I use an analog voltmeter with a 2vac & 20 vac scale. You have to put a .047 uf cap in series with the ground probe to keep the AC volt scale from showing a reading on DC.
Then you look at similar points on the quiet channel and the noisy channel. You start at the beginning, the input jacks and the small black transistors, and work towards the big transistors on the heat sink. The stage before the noise begins to be readable is where the problem is. Could be a resistor, or a transistor or diode, more rarely a capacitor. Sometimes interboard connectors, especially the type with push down connections, are the problem in the 5-15 year timeframe.
Be aware you should only use one hand probing, as voltage over 24 across your heart can stop it. Connect the negative of the meter through the cap to the speaker return with an alligator clip lead, so you only need one hand. Wear no jewelry on hands, wrists, or neck, 1 v at high amperage through a ring can burn your flesh to charcoal. Wear safety glasses or reading glasses, parts sometimes explode.
Happy hunting.
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Thanks for your reply.
This amplifier has a bridge on the back, connecting pre-and power amp. Presumably in order to insert stuff, e.g. equalizer, or to use a separate power amp. Like I said, the problem exists only on one channel. It is white noise (not hum), and when the channel is disconnected, the noise becomes very audible. I can test shorting it, yes, but in my experience, power amplifiers with a disconnected input signal normally don't behave like this. Certainly the right channel doesn't. Something must be leaking noise into the un-grounded signal. I also suspect that connecting up the pre-amp does drain the input sufficiently well to almost silence it (probably over the pre-amp's output transistors)
I've done all the tapping and twisting already and there are no mechanical reasons for this. The noise is pretty steady.
I seem to recall that the most common source of white noise in electronics are actually resistors, if they get amplified sufficiently.
Anyway, I'll arm myself with the schematic (NAD 314 SERVICE MANUAL Pdf Download | ManualsLib) and try do do some divide and conquer with a probe and an oscilloscope if I can borrow one. But I thought it worth a shot to ask here if there were any typically obvious source of noise if the input were floating.
Cheers!
This amplifier has a bridge on the back, connecting pre-and power amp. Presumably in order to insert stuff, e.g. equalizer, or to use a separate power amp. Like I said, the problem exists only on one channel. It is white noise (not hum), and when the channel is disconnected, the noise becomes very audible. I can test shorting it, yes, but in my experience, power amplifiers with a disconnected input signal normally don't behave like this. Certainly the right channel doesn't. Something must be leaking noise into the un-grounded signal. I also suspect that connecting up the pre-amp does drain the input sufficiently well to almost silence it (probably over the pre-amp's output transistors)
I've done all the tapping and twisting already and there are no mechanical reasons for this. The noise is pretty steady.
I seem to recall that the most common source of white noise in electronics are actually resistors, if they get amplified sufficiently.
Anyway, I'll arm myself with the schematic (NAD 314 SERVICE MANUAL Pdf Download | ManualsLib) and try do do some divide and conquer with a probe and an oscilloscope if I can borrow one. But I thought it worth a shot to ask here if there were any typically obvious source of noise if the input were floating.
Cheers!
Okay, armed with the schematic, I confirm that ther eis a problem.
I'm measuring -250mv around R315 on near the left channel input. There is an imbalance in the supply voltages to Q301 and Q303, im measuring 350mV ath the upstream end of R319 and -830mv at the downstream end of R317. On the right channel, I measure a balanced +-530mV at the ccorresponding place.
I suspect that Q303 may be leaking, pulling the base low, causing current to flow through R303. Indeed, there are 210mV over that resistor, which is 22kOhm, so we have some 10uA going across the base of one of those. This is confirmed with an ampmeter between the base and ground. Now, grounding the bases brings back balance to the voltages at the transistor collectors.
So, am I ok to assume that the negative transistor, Q301, is faulty and leaking current?
(Refer to power amp cirquit diagram in the service manual above)
I don't want to ground the tansistor base since if the transistors are leaking, their
I'm measuring -250mv around R315 on near the left channel input. There is an imbalance in the supply voltages to Q301 and Q303, im measuring 350mV ath the upstream end of R319 and -830mv at the downstream end of R317. On the right channel, I measure a balanced +-530mV at the ccorresponding place.
I suspect that Q303 may be leaking, pulling the base low, causing current to flow through R303. Indeed, there are 210mV over that resistor, which is 22kOhm, so we have some 10uA going across the base of one of those. This is confirmed with an ampmeter between the base and ground. Now, grounding the bases brings back balance to the voltages at the transistor collectors.
So, am I ok to assume that the negative transistor, Q301, is faulty and leaking current?
(Refer to power amp cirquit diagram in the service manual above)
I don't want to ground the tansistor base since if the transistors are leaking, their
250 mv across R315 is rather a lot of current. I'd change C301 & C303 before messing around with the Q's. Electrolytic capacitors are known time fuses. Subjecting Q301 & Q303 to +-56 volts does rather stress them however.
Success! I replaced Q303 with no effect, but replacing Q301 fixed the issue. Luckily I had bought both replacements in case of me having misdiagnosed the problem.
This is my first successful diagnosis and fix of a transistor-related problem, which makes me very happy. Symmetric feedback circuits are a bit hard to reason about.
Btw, this particular model actually doesn't have R301 and C301, rather the input connects directly to R305.
Cheers!
This is my first successful diagnosis and fix of a transistor-related problem, which makes me very happy. Symmetric feedback circuits are a bit hard to reason about.
Btw, this particular model actually doesn't have R301 and C301, rather the input connects directly to R305.
Cheers!