# NAD T743 receiver sub channel clipping - need upgrade?



## Blueeyedfrog (Dec 15, 2007)

Hi everyone,

I am finding that my current NAD T743 receiver is clipping (with BFD LED) in the subwoofer channel and so I have to set it at around -8 to -10db compared with the rest of the speaker channels which are set around 0db. I'm happy with the volume output of the mains when I crank the receiver up to 0db but I have to keep the overall gain down around -8db or the subwoofer starts clipping.

So I need a new amp right? The receiver doesn't have enough oomph to deliver the sub channel, compared to the other channels? I was thinking the next size up NAD receiver?

Cheers,
Blue


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## Wayne A. Pflughaupt (Apr 13, 2006)

Okay, I’m confused. If it’s your sub that’s clipping, why would you need a new receiver? Sounds like a better sub is what you need. :scratch:

Regards,
Wayne


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## Mike P. (Apr 6, 2007)

Hi Sam, is this the same sub amp you were using with the Peerless subwoofer?


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## Blueeyedfrog (Dec 15, 2007)

It's the same sub amp that I am using with the Peerless - a 240 watt plate amp, but in a few months I'll be upgrading to something like an EP2500. But in preparation for my new sub set-up (and two Mach 5 Audio IXL-18s), I'm thinking that I'll need a beefier amp to feed the new sub amp?

I _think_ I need a new amp because my amp is clipping - ie. the signal is clipping before it reaches the sub amp... which I presume will limit the output of the sub? At present I am able to prevent the sub from clipping by reducing the receiver output - but wouldn't it be better to be able to output a louder signal from the receiver that doesn't clip (similar to the output to the main speakers)? And so that's what makes me think I need a new amp (more headroom?)... Yes/no?


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## Mike P. (Apr 6, 2007)

I'd say your Peerless sub/amp combination is simply maxed out. Since you will be getting a EP2500 soon, wait and see if that is the problem. No sense buying a new receiver if it's not the issue.


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## Wayne A. Pflughaupt (Apr 13, 2006)

> - ie. the signal is clipping before it reaches the sub amp...


How did you determine that?

Regards,
Wayne


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## Blueeyedfrog (Dec 15, 2007)

Well, I think my receiver is clipping because my BFD input LEDs are going into the red - when they are up there my sub starts to make poping noises (unless I turn my sub amp down) - so that means my receiver is clipping, right :dontknow:?

I see your point Mike re waiting to see how the new sub system performs...


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## Wayne A. Pflughaupt (Apr 13, 2006)

Blueeyedfrog said:


> Well, I think my receiver is clipping because my BFD input LEDs are going into the red - when they are up there my sub starts to make poping noises (unless I turn my sub amp down) - so that means my receiver is clipping, right :dontknow:?


Nope. It means your feeding too much signal from the receiver to the BFD. Reduce the sub output in the receiver (hopefully that’s an option) to where the LEDs only hit the yellow occasionally; that will give you some headroom. Naturally, this will mean your sub will have to be turned up (using its gain control).

Note that this isn’t probably going to fix the problem with your sub. You see, low frequency distortion has to get much higher before it’s audible than mid-to-upper frequency distortion. So even if the BFD is clipping, it probably wouldn’t be audible. Popping noises from the sub usually means the driver is bottoming out, or perhaps that the amp is clipping. Equalization places additional demands on the amp and driver, so going in you have enough reserve in headroom to handle it.

Regards,
Wayne


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## Blueeyedfrog (Dec 15, 2007)

So I don't need a new receiver then? Everything will change when I get the new EP2500 / Mach 5 18"s - so wait until I run the new sub setup then, as Mike suggests?

By the way, could that mean my mains are clipping if my sub channel is feeding too much to the sub?


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## Wayne A. Pflughaupt (Apr 13, 2006)

One has nothing to do with the other. 

Regards,
Wayne


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## Mike P. (Apr 6, 2007)

Your going from a 12 inch Peerless/250 watt plate amp to an EP2500 / twin Mach 5 18 sonotubes. Are you ever in for a nice surprise! :yes: :hsd: :yes: Make sure you keep us posted!


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## Blueeyedfrog (Dec 15, 2007)

OK, so I have my EP2500 and naturally, 2 x IXL-18 sonosubs tuned to 15hz. I have used my BFD to give me a reasonably gentle rising house curve to 15hz and I'm happy with the way things are looking in REW. I should note that I had to apply a whopping -12db filter of 14 bandwidth (?) at 42hz to achieve this. 

I have set my receiver so that when the BFD is in by-pass mode it (the BFD) briefly goes into the red clipping LEDs at the loudest part of the movie (here, WOTW) at my loudest comfortable listening levels. To achieve this I had to set my sub channel level to -11db.

I have checked the phasing of the subs by using a meter and pink noise with all speakers running, to get the loudest SPL.

Using a radioshack meter I have set all channels to read 75db (using what I presume is pink noise generated by my receiver) - in order to do that I have to run my receiver with the gain control set at -17db. In order to get this 75db for all channels there is a big discrepancy between my mains and sub channel levels in the receiver - my mains are around -1 to +1 db and my sub is at -11db.

My normal loudest comfortable listening level is around -7db on the receiver.

I have both EP2500 channel gain controls set to full (connected with a 4 ohm load @ 750 watts per channel). However, I don't feel I'm getting enough bass to match my mains. When I take the BFD out of by-pass mode (and so actively use the filters), and run the receiver at my loudest comfortable listening level of -7db, neither the BFD nor the EP2500 display any red clipping LEDs, and I don't feel that the bass matches my mains.

I know that I could easily turn up the gain in the receiver sub channel to give me more relative bass but then surely this will induce clipping and distortion? I presume the EP2500 and IXL-18s are not the limiting factor since they are not showing clipping on the BFD or EP2500. So I need a bigger receiver amp to feed the EP2500, right?

By the way, would it make any difference changing the input setting on the BFD from -10 to +4, to allow the feeding of a higher level signal from my receiver?

Also, I have my front 3 main speakers set to large in the receiver as recommended in the NAD T743 manual (I could also set the smaller rears to large too but don't) and bass enhancement on. Could setting all speakers to small give more power to the sub channel and so reduce clipping into the BFD?

Also, without the huge -12 filter in BFD I get some really exciting shaking going on - my girlfriend felt compelled to come down to the basement to gently inform me the whole house was shaking :gah: - excellent news!


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## Athomas (Jun 17, 2008)

Hi There,
From previous experience, i would most definately set all speakers to small flavour (crossed over at 80hz IIRC)and give the bass to the sub. I would hazard a bet that it is actuall more beneficial set-up like this! 

Regards
Andrew


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## Wayne A. Pflughaupt (Apr 13, 2006)

> I have both EP2500 channel gain controls set to full (connected with a 4 ohm load @ 750 watts per channel). However, I don't feel I'm getting enough bass to match my mains. When I take the BFD out of by-pass mode (and so actively use the filters), and run the receiver at my loudest comfortable listening level of -7db, neither the BFD nor the EP2500 display any red clipping LEDs, and I don't feel that the bass matches my mains.


Well, typically if you aren’t getting enough bass, you need to increase the sub level.



> I know that I could easily turn up the gain in the receiver sub channel to give me more relative bass but then surely this will induce clipping and distortion?


You won’t know unless you try.



> I presume the EP2500 and IXL-18s are not the limiting factor since they are not showing clipping on the BFD or EP2500. So I need a bigger receiver amp to feed the EP2500, right?


If your receiver’s sub level is set way down at –11, then you have a long to go before you have to worry about not sending the sub enough signal. Besides, we’re talking about a pre-amp signal. There’s no guarantee that a “bigger receiver” will deliver a stronger sub pre-amp signal. In fact, generally speaking, pre-out levels are pretty much standard.



> By the way, would it make any difference changing the input setting on the BFD from -10 to +4, to allow the feeding of a higher level signal from my receiver?


I never can remember which setting is which, but if the one you’re using is pegging the meters, and switching to the other setting reduces the meter level, then do that. It would allow you to push a higher sub signal level from your receiver. But if your BFD currently is not red-lighting, then you’re fine.



> Also, I have my front 3 main speakers set to large in the receiver as recommended in the NAD T743 manual (I could also set the smaller rears to large too but don't) and bass enhancement on. Could setting all speakers to small give more power to the sub channel and so reduce clipping into the BFD?


The main speakers should always be set to small, even if they’re flat to 0 Hz. Otherwise, the bass output from them is going to have more or less the same peaks and valleys as the sub does, only you can’t equalize them away.

Anyway – if turning up the sub level doesn’t do it for you, then perhaps you can post a graph for us to have a look at.

Regards,
Wayne


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## Ilkka (Jun 8, 2006)

I'm guessing the sub out on your NAD T743 might be broken. I own the same receiver and mine's broken too (I don't use it though). It puts out around 8V DC!  I have heard there are many units with same kind of problems. Too bad, otherwise it's a great receiver...


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## Blueeyedfrog (Dec 15, 2007)

Thanks Wayne, I'll try your suggestions. Just to clarify, are you saying that the sub output from the receiver is only from the preamp? If so, I didn't realise it didn't use any of the receiver's main power amp.

Ilkka, re the voltage output - can I measure it simply with an el cheapo volt meter an see what the voltage is at the peaks (in the movie)? I presume that would be DC?

I presume that if it is a voltage problem that is can be (easily/cheaply) fixed?

Thanks everyone.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Blueeyedfrog said:


> Just to clarify, are you saying that the sub output from the receiver is only from the preamp? If so, I didn't realise it didn't use any of the receiver's main power amp.


Friendly tip, connections that use RCA cables between components (or 1/4” or XLR) are pre-amp. Connections that use speaker cable are the “main power amp.” 



> Ilkka, re the voltage output - can I measure it simply with an el cheapo volt meter an see what the voltage is at the peaks (in the movie)? I presume that would be DC?


Yup – DC. It would be difficult to measure using program material. Use a low frequency steady-state sine wave tone, and by all means turn off the EP2500 while you’re doing it!

I think you’d already know if it wasn’t working, as Ilkka has experienced – you’d be getting nothing at all.



> I presume that if it is a voltage problem that is can be (easily/cheaply) fixed


The last time I had a local dealer do a repair on my Yamaha receiver, they had a $135 minimum charge. Sometimes the manufacturer will offer economic rates (it cost only $35 for AudioControl to fix my equalizer), but you have to add shipping charges, which will probably be at least $35 one way, unless you happen to be close to NAD.

Regards,
Wayne


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## Blueeyedfrog (Dec 15, 2007)

Thanks for the friendly tip Wayne.

My purpose for checking the voltage out of the receiver was not to see of it is working or not but to see if the voltage was suitable - ie. around 1.5-2.5 volts rather than 6-8 volts. 

Just to clarify, what exactly is happening when I see the BFD input LEDs going into clipping? I presume that the preamp signal from the receiver is so loud/big that the the wave peaks are being clipped and so turned into square shaped waves (which produces distortion)? Therefore my problem is that I have too big a signal (too much voltage)? If so, what is clipping the sound - the preamp or the BFD?

If that is not the case, then what is causing the preamp to clip? Not enough power (do preamps use amplified power?) from the preamp? Or is it the source producing distortion (in my case I'm using the digital output from my computer via coax to the receiver) - computer soundcard? DVD software player? 

I'm a little in the dark here... 

Cheers


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## tenzip (May 4, 2007)

Wayne A. Pflughaupt said:


> Yup – DC. It would be difficult to measure using program material. Use a low frequency steady-state sine wave tone, and by all means turn off the EP2500 while you’re doing it!


I'm pretty sure you want to use AC for measuring an audio signal. Or are my painkillers messing with my mind again? On a sine wave, the DC component should (theoretically) be zero.

Edit:
Also, some meters may not respond well to low (<50-60Hz) frequencies. An analog meter won't care, but a digital might, depending on design of the meter. I'm not sure of this, but you could try a 60Hz tone for testing, just for the **** of it.


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## Ilkka (Jun 8, 2006)

Blueeyedfrog said:


> Ilkka, re the voltage output - can I measure it simply with an el cheapo volt meter an see what the voltage is at the peaks (in the movie)? I presume that would be DC?


Normal signal (music/movies) should be AC only. There shouldn't be any DC. You can measure the amount of DC component with a normal volt meter (just choose DC range). You don't even have to have any signal playing. Mine outputs a constant ~8V DC all the time! 


> I presume that if it is a voltage problem that is can be (easily/cheaply) fixed?
> 
> Thanks everyone.


No necessarily...


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## Ilkka (Jun 8, 2006)

tenzip said:


> I'm pretty sure you want to use AC for measuring an audio signal. Or are my painkillers messing with my mind again? On a sine wave, the DC component should (theoretically) be zero.
> 
> Edit:
> Also, some meters may not respond well to low (<50-60Hz) frequencies. An analog meter won't care, but a digital might, depending on design of the meter. I'm not sure of this, but you could try a 60Hz tone for testing, just for the **** of it.


Yes, of course DC component should be zero or very close to it. But since the receiver is broken, it's not. And that's what is causing these problems.


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## tenzip (May 4, 2007)

Ah. I did not read carefully enough/understand that there was a DC offset issue. That, of course, would be bad.

I thought he was just trying to measure the audio signal


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