Question Two NAD c268 Amps in Bridge Mode

I have the opportunity to pick up, for a very reasonable price, a second NAD c268 Power Amp in addition to the one I am already running. I am looking to run them as two mono blocks in bridge mode and am a little unsure of connectivity. I think I have it, but wanted to run it by the community to check.

I will switch both Power amps into bridge mode and run the left output from my Pre Amp to the Left (Bridge Mono) input of Power amp for left channel (PA L) and then left speaker cables to Left + and Right - speaker terminals. Right output from Pre goes to Left (Bridge Mono) input of Power Amp for Right Channel (PA R) and speakers L+ & R-.

I've enclosed a screenshot of the back of the Power Amp to (hopefully) clarify what I am talking about.

Many thanks.
Screenshot 2021-02-07 at 14.33.53.png
 

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Ugg10

Distinguished Member
Sound right to me based on the pictures. Beware that most amps in bridge mode will only drive nominally 8 ohm speakers so keep an eye on them for a while to check for overheating if you speakers are a difficult load. The BX5 look ok but no mention of their minimum impedance on the literature. You can also have some fun vertical and horizontal bi amping if you want to see which of the three configurations you prefer, but don’t forget to remove the jumpers for bi amping, and also don’t expect night and day differences, it will be subtle if anything.
 
Thanks for the reply Ugg10. I did try Bi-Amping a good few years ago with 2 Cambridge Audio Power Amps and I honestly couldn't tell the difference. I'm going to take a look at the amp on Wed or Thurs so will report back the results.
 

Redwing68

Novice Member
I have just bought a second C268 and bridged it with the NAD 658 as the pre amp. Set up as
I have the opportunity to pick up, for a very reasonable price, a second NAD c268 Power Amp in addition to the one I am already running. I am looking to run them as two mono blocks in bridge mode and am a little unsure of connectivity. I think I have it, but wanted to run it by the community to check.

I will switch both Power amps into bridge mode and run the left output from my Pre Amp to the Left (Bridge Mono) input of Power amp for left channel (PA L) and then left speaker cables to Left + and Right - speaker terminals. Right output from Pre goes to Left (Bridge Mono) input of Power Amp for Right Channel (PA R) and speakers L+ & R-.

I've enclosed a screenshot of the back of the Power Amp to (hopefully) clarify what I am talking about.

Many thanks.View attachment 1456299
Hi Mike, I have just bought a second C268 (another bargain) and am running the two as mono blocks in bridge mode. I set them up exactly as you suggest above using balanced connections. I use a NAD C658 as pre amp and my speakers are B&W 603. Works perfectly giving the meatier, rounder sound that I was after in a somewhat cluttered room.
 
Cheers Redwing. I'm hopefully going to see this Amp for sale tomorrow.
 
Just to bring this thread to a close. I visited the seller on Wednesday evening to look at the amp. I didn't go ahead with the purchase as the unit had clearly been opened several times and was in surprisingly poor condition for such a new amp. I may go for a new one in the future.

thanks for the replies.
 

Redwing68

Novice Member
A shame - I got mine as clearance from one of the main high street outlets, ex display with obviously little use over the last year and full guarantees etc. Saved £150 and as new. There are others out there.
 
I took the plunge and ordered one of these Amps online, it will arrive tomorrow. So for my next question:

At the moment my system is located on a side wall and the speakers either side of the TV facing my listening position. I can either locate the Amps in the Rack with the rest of the gear or locate them side by side under the TV cabinet. In the rack means using my existing speaker leads, 5m and 2m but a short interconnect. Under the TV means a longer interconnect (already got a 3m one) but much shorter, and equal speaker cables (1.5m)

I rather think option 2 is the better but am open to suggestions.
 

noiseboy72

Distinguished Member
I took the plunge and ordered one of these Amps online, it will arrive tomorrow. So for my next question:

At the moment my system is located on a side wall and the speakers either side of the TV facing my listening position. I can either locate the Amps in the Rack with the rest of the gear or locate them side by side under the TV cabinet. In the rack means using my existing speaker leads, 5m and 2m but a short interconnect. Under the TV means a longer interconnect (already got a 3m one) but much shorter, and equal speaker cables (1.5m)

I rather think option 2 is the better but am open to suggestions.
Shorter interconnects are normally the better option, as you will get less signal degradation with longer lengths on the speaker cable side, but quite honestly if you are running balanced signals a few metres is going to make no difference.
 
Shorter interconnects are normally the better option, as you will get less signal degradation with longer lengths on the speaker cable side, but quite honestly if you are running balanced signals a few metres is going to make no difference.
Yeah, it's a good quality balanced cable I was intending to use. I suppose any signal loss will be balanced out by the shorter, and equal, speaker cable runs.
 
So the second Amp arrived a week ago and I connected up with no issues, everything worked fine. I gave several days to positioning the amps in the two positions (in the rack with long and uneven speaker wire, or side by side under the TV cabinet with shorter, even speaker run). TBH I could hear no difference, but there is more ventilation under the TV cabinet, so that's where they are staying.

Thanks for all the comments and thoughts. My music seems to have a lot more room to move around in. Yes it is quite subtle, no higher highs or lower lows, just a more relaxed presentation.
 

DaveSiberut

Standard Member
So the second Amp arrived a week ago and I connected up with no issues, everything worked fine. I gave several days to positioning the amps in the two positions (in the rack with long and uneven speaker wire, or side by side under the TV cabinet with shorter, even speaker run). TBH I could hear no difference, but there is more ventilation under the TV cabinet, so that's where they are staying.

Thanks for all the comments and thoughts. My music seems to have a lot more room to move around in. Yes it is quite subtle, no higher highs or lower lows, just a more relaxed presentation.
Hi, how would you rate the C268, I'm thinking of buying one and connecting it to my NAD T758 to power my fronts to improve music playback?
 

DaveSiberut

Standard Member
I have just bought a second C268 and bridged it with the NAD 658 as the pre amp. Set up as

Hi Mike, I have just bought a second C268 (another bargain) and am running the two as mono blocks in bridge mode. I set them up exactly as you suggest above using balanced connections. I use a NAD C658 as pre amp and my speakers are B&W 603. Works perfectly giving the meatier, rounder sound that I was after in a somewhat cluttered room.
Hi, how would you rate the C268, I'm thinking of buying one and connecting it to my NAD T758 to power my fronts to improve music playback?
 

Ugg10

Distinguished Member
Hi, how would you rate the C268, I'm thinking of buying one and connecting it to my NAD T758 to power my fronts to improve music playback?
Quality of music playback through the front L/R speakers is dominated by the preamp section of whichever amp is connected to them. The power amp will make a very small difference in the sound when playing at normal listening volumes as you will be working well within the power amp limits. So if you add a power amp on the L/R preouts on your avr it is still using the avr preamplifier circuits for your music playback including all of a DtoA and AtoD circuits and processing. Therefore the usual way of improving music playback is to attach a dedicated stereo integrated amp with ZhT bypass and connect your stereo sources to that (streamer, cd, tuner etc) with surround sources connected to the AVR (sat, cable, blue ray, dvd etc). This then benefits from the stereo preamp for music.
 

Hear Here

Active Member
Shorter interconnects are normally the better option, as you will get less signal degradation with longer lengths on the speaker cable side, but quite honestly if you are running balanced signals a few metres is going to make no difference.
Sorry - I disagree. Better to use short speaker cables and longer (preferably balanced) interconnects. That's how all pro studios do things. You can move low-level signals huge distances without loss - look at the mic cables used to record an orchestra for example, or the long interconnects from the control desk to active speakers at concerts.

If the OP's speakers have separate bass and top end connectors, he could use his 2 amps to bi-amp. I realise he's tried this in the past with little improvement, but if he's thinking of using some form of room correction (Dirac Live, RoomPerfect, etc) he is best to apply this only to the bass, as it will spoil the top end liveliness. He could do this with 2 power amps (unbridged) if he uses an electronic XO after his preamp and sends the bass only to the DSP device and on to the one amp, but the top end directly to the other amp – bypassing the DSP.
 

Ugg10

Distinguished Member
Sorry - I disagree. Better to use short speaker cables and longer (preferably balanced) interconnects. That's how all pro studios do things. You can move low-level signals huge distances without loss - look at the mic cables used to record an orchestra for example, or the long interconnects from the control desk to active speakers at concerts.

However, worth noting that many professional / Studio cables are balanced XLR which are designed to be less susceptible to electrical interference and also run at a minimum of 4v rather than the rca unbalanced which run at anywhere from 0,25 to 2V and have less effective shielding.
 

Hear Here

Active Member
Astin Trew AT1000 Plus Pre Ampit's a good quality balanced cable I was intending to use.However, worth noting that many professional / Studio cables are balanced XLR which are designed to be less susceptible to electrical interference and also run at a minimum of 4v rather than the rca unbalanced which run at anywhere from 0,25 to 2V and have less effective shielding.
Even long single ended interconnects are better than long speaker cables.

The OP said "it's a good quality balanced cable I was intending to use." although if he is still using the Astin Trew AT1000 Plus Pre Amp (per his profile) this seems to be single ended only! It would be good to know, as balanced is far better than single ended if the components at each end use genuinely balanced circuts. Many are single ended but offer XLR sockets for convenience only. If his preamp is single ended, he may as well accept that he's best using single ended (RCA) cables as balanced (XLR) cables with adaptors won't improve his signal.
 

noiseboy72

Distinguished Member
Just to be clear, my original comment is regarding using the sort of cable lengths found in a typical domestic system, not a studio or concert system.

I have significant experience in producing classical concerts using 100m analogue multicores and powered speakers and trust me, there's significant signal degradation, even with the best kit available. Most modern systems will run digital multis and stage boxes, so mic cables are move typically 10m long these days. Amp racks may be up to 25m away from the speakers and will use 2.5mm cabling to multi kilowatt speaker cabinets. Active systems are popular, but it's less about quality and more about flexibility to drive each cabinet independently in terms of level, delay and eq.

The difference between 1m and 3m of a good quality speaker cable is negligible. It will not be affected by external interference or suffer any form of measurable signal degradation.

High impedance line level interconnects will be affected by things like cable capacitance and in the case of unbalanced cables potentially extraneous noise as well.

I maintain that with lengths of less than 5m, a low impedance, high voltage speaker cable will affect the sound less than a high impedance line level interconnect, balanced or otherwise.

The test is simple. Can you more easily hear a difference in the sound by swapping interconnects or speaker cable? I would wager for most people it would be the interconnect.
 
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Hear Here

Active Member
I have significant experience in producing classical concerts using 100m analogue multicores and powered speakers and trust me, there's significant signal degradation, even with the best kit available.
So you know better than me that concert speakers are fed their signal at low level and never at speaker level. No concert location would have their amps close to the source (control desk) and connect these amps to speakers umpteen meters away!

I agree that losses in either method are small, but best practice, even in domestic situations (if one has the choice), is to use long interconnects and short speaker cables. But of course this is only practical with monoblock amps or twin amps if bi-amping. Most people don't follow this best practice as it means cluttering up the speaker location with amps, rather than keeping all electronics in a single rack.
 

noiseboy72

Distinguished Member
I think it best to agree to disagree. I know that it's almost impossible to measure any degradation in 3m of speaker cable at the impedances and voltage levels involved, but quite easy to do with high impedance signal cables.

In concert systems utilising passive speakers, the amp racks will usually be located either centrally so that they can be easily monitored and re-patched if a fault occurs, or adjacent to the stacks if space and manpower permits. This can still be some distance away from the speakers. As an example, at the Royal Albert Hall the speaker cable run is around 85m to the down stage arrays and anything up to 240m for some of the local fills in the boxes! The input into the system is fully digital, being mainly Dante to give enough channels from the processing rack.

When I used do outdoor gigs we used Meyer Sound self-powered speakers running from analogue processors. The signal cable length could be anything up to 200m and we needed to run heavy mains cabling, which could cause us significant problems in terms of noise and voltage drop. Our reason for using self powered kit was more about reducing the amount of weight of the amp racks, as there's very little difference in weight between a passive and self powered box. Flexibility was another big part of the design philosophy, not any requirement to reduce the length of the speaker cables.

I think it's clear neither example bears any resemblance to a home system and I would suggest your studio analogy is not entirely accurate either. Our production studios had a central rack of Quad monoblocks that could be patched as required. The rack was located away from both the speakers and fx rack, but that was more about electrical supply separation and reducing the potential for the speaker lines to interfere with any unbalanced signals flying around from guitars and some fx units. We had active Genelecs as well, so of course they were coupled with xlrs.

If you can evidence that line level cables are less likely to affect the sound of a system as opposed to similar length speaker cables, please feel free to link to the article. I'll be happy to provide technical specs for both speaker and signal cables showing the effect of capacitance and heat loss in the various types.

None of this is to say that you cannot run km long line level cables - and that would be the preference over very long speaker cables due to heat loss from the lower impedance, but at shorter lengths the argument really doesn't hold much water IMHO.
 

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