Hi - I'm new to this forum and I'm hoping for a little guidance. I recently picked up a NAD T761 surround receiver (which is notorious for a variety of issues - the boards look great but the construction and soldering between them looks relatively slapdash) and was able to get the analogue section working which was the reason for the low sale price.
However I'm now trying to address a noise issue on all the digital inputs. Whenever the receiver locks to a digital input (optical and coax) in stereo or surround modes there is a constant noise on all outputs, preamp and speaker which is not present with analogue sources. The main component of the noise is c 1.1-2 kHz with a bunch of dirty harmonics if my ears and meter can be trusted.
I have the service manual - all the digital inputs run to the ac3 card. There's one service bulletin for this card which covers replacement of a PSU capacitor which overheats. However this mod has been performed, by the factory if the quality of soldering is any guide. In any case, this covers noise/distortion in surround modes and specifically states that stereo modes are not affected.
Can anyone suggest where I might start investigating this problem?
However I'm now trying to address a noise issue on all the digital inputs. Whenever the receiver locks to a digital input (optical and coax) in stereo or surround modes there is a constant noise on all outputs, preamp and speaker which is not present with analogue sources. The main component of the noise is c 1.1-2 kHz with a bunch of dirty harmonics if my ears and meter can be trusted.
I have the service manual - all the digital inputs run to the ac3 card. There's one service bulletin for this card which covers replacement of a PSU capacitor which overheats. However this mod has been performed, by the factory if the quality of soldering is any guide. In any case, this covers noise/distortion in surround modes and specifically states that stereo modes are not affected.
Can anyone suggest where I might start investigating this problem?
Very odd problem. Almost sounds like digital crap (from SPDIF receiver?) is getting into the analog audio. Hmm. Grounding problem maybe? Do supplies in that area check out OK?
The 5VA and 5VD are from different sources. Have a look at the 100n decoupling capacitors and with your scope check the inputs to the d-a supplies.
You might want to check Onkyo AV receiver repair on YouTube. Back in 2000 I remember people telling me the NAD receivers were based on the Onkyo's. Also when a 85 degrees capacitor fails within 2 years a 105 degrees replacement might need replacement after 14 years again?
Thanks all - I appreciate the experienced perspective. Hoping to pick up a 230V scope soon (I've recently moved to Australia) and dig in a little deeper. I did swap out the cap as per the mod on general principles and I'm now checking the (many) ground connections.
Just an off the wall comment but have you tried more than one digital source to feed the receiver... even if the source you are using appears OK when used with other gear ?
No - good thought, I've tried both optical and co-ax sources from three different devices (Apple TV, Rotel DVD & a horrible multi-region Yess DVD player)
Now have some time to get back to this issue - I can be more specific about the problem, I think. The digital noise only appears when I lock to a digital input, from any of my available sources. It's relatively low-level, appears as a whine which is masked by music at average volumes and appears on all of the outputs. It does NOT vary in level with the volume control which I find curious and hopefully significant. Neither does it vary in tone as settings change.
I realise I'll need a scope to trace this but I'm in a new country and looking for work so funds and time are limited - also the ac3 board is buried next to the power transformer (odd decision) and I have no extender cables so it's an excercise in caution.
I have a T753 with the identical problem. I did some looking into it but without extenders it is a real pita.
I gave up and solved the problem by pulling most of the cards out of the receiver and turning the 6 channels of power amps into 4 channels with some bridging. So 2x90W and 2x~200W?, perfect for biamping.
The power section is decent enough; although the output section is fan cooled the power transformer does get a bit toasty.
I gave up and solved the problem by pulling most of the cards out of the receiver and turning the 6 channels of power amps into 4 channels with some bridging. So 2x90W and 2x~200W?, perfect for biamping.
The power section is decent enough; although the output section is fan cooled the power transformer does get a bit toasty.
Hope you still have the cards - I've been in discussion with a tech (well above my pay grade...) who thinks he has an angle on this. Basically, the digital section of this receiver is an Onkyo relative and their receivers had a habit of cooking the 100u surface mount capacitors on the digital board, causing steady digital noise. He's taking a look at it next week and if we find a solution (or any other information) then I'll document it here.
You're right - it's worth having for the power amps alone. I got mine for c $25 US because two of the amps had "stopped working" - turned out to be dirty speaker connections and a dry joint on the main speaker board. The tuner is more than decent, however, though it does have the worlds' worst remote control!
I think NAD's philosophy of high sound quality at low cost blew up with this receiver series - the assembly is pretty shoddy.
Hi Imartine3
Do you have any info/link's regarding bridging the amp's ?
Non of the analog and digital input's on my T761 are working but the 5.1 input.
Brgd
Kai
Rod Elliot provided the basic info here: Simplest Ever Bridging Adapter for Amplifiers.
Electrically it is trivial, a couple of resistors and some wire.
The pain is disconnecting cables and pulling the heat-sink/amplifier boards out and then finding good solder points on or near the appropriate components.
Schematics are available on the web, ask the google. Below is for NAD T537, your amp will be different.
For bridging the surround left and right, add 10k resistor from base of Q806 (right channel input LTP) to output node of left channel, as close to common node of dual resistor R843 as possible. Connect the input of the right surround amp with 100 ohm to audio ground and drive the left channel, then take speaker output across the hot outputs.
Similarly, to bridge the front left and right, 10k between Q514 base (right LTP) and R523 (left output). Right input 100 ohm to ground, drive left input, speaker across hot outputs.
The bridged outputs have twice the output voltage as the remaining unbridged channels so you might need some level matching elsewhere.
Electrically it is trivial, a couple of resistors and some wire.
The pain is disconnecting cables and pulling the heat-sink/amplifier boards out and then finding good solder points on or near the appropriate components.
Schematics are available on the web, ask the google. Below is for NAD T537, your amp will be different.
For bridging the surround left and right, add 10k resistor from base of Q806 (right channel input LTP) to output node of left channel, as close to common node of dual resistor R843 as possible. Connect the input of the right surround amp with 100 ohm to audio ground and drive the left channel, then take speaker output across the hot outputs.
Similarly, to bridge the front left and right, 10k between Q514 base (right LTP) and R523 (left output). Right input 100 ohm to ground, drive left input, speaker across hot outputs.
The bridged outputs have twice the output voltage as the remaining unbridged channels so you might need some level matching elsewhere.
Sorry to be rude - I quite missed this. I have no experience in bridging power amps but those NAD amps certainly shift a lot of current! 🙂 Good luck researching this. BTW, does your EQ work on the 5.1 input?
My efforts to track down the noise have been fruitless so far...
Thanks Imartine3.
Regarding the 5.1 inputon my Nad, yes my cd and turntable works fine there.
I can splitt signal wire and Bi-amp my speakers, but hoping to bridge for more output power.
Kai
Regarding the 5.1 inputon my Nad, yes my cd and turntable works fine there.
I can splitt signal wire and Bi-amp my speakers, but hoping to bridge for more output power.
Kai
OK, back to exploring my NAD 761 problem after a long hiatus...
New information - now that I have the speakers to experiment with, I've found that the "digital whine" only happens on the main L&R outputs (and preamp outputs) - the centre and surround outputs are clean. It occurs at the same level regardless of volume setting and is not affected by the tone controls. I have a sense that it decreases slightly in level after the amp warms up a little.
Otherwise the amp sounds fantastic on analog inputs, to the extent that I'm even considering buying a Rotel surround preamp to drive it.
Here's a link to the service manual
New information - now that I have the speakers to experiment with, I've found that the "digital whine" only happens on the main L&R outputs (and preamp outputs) - the centre and surround outputs are clean. It occurs at the same level regardless of volume setting and is not affected by the tone controls. I have a sense that it decreases slightly in level after the amp warms up a little.
Otherwise the amp sounds fantastic on analog inputs, to the extent that I'm even considering buying a Rotel surround preamp to drive it.
Here's a link to the service manual
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