NAD T 955 - A good choice for SDAs?

jimbo1421

The Yamaha AVR in my office rig is going out and I don't want to fix it. Instead I want to pull the Parasound HCA-1000A from my HT+two channel system into the office and replace that with a 3 or 5 channel power amp. I found a refurbished NAD T 955 for $700 which is the very most I could pay for an amp. Refurbished with a warranty strikes me as a better deal than say, a 10-20 year old Adcom for maybe $400.

Is the NAD T 955 a good match for my SDAs or any other likely speaker I might take a fancy to, like LSis? ? Is it common ground? (I could check this in the morning.) My most important audio source is vinyl but since I don't have space for a separate 2 channel rig, it also needs to do a good job with 5.1 home theater. Eventually I will replace the Outlaw 1050 with something like a Harman Kardon AVR and replace the NAD 106 preamp with a tube preamp. I like a warm sound that doesn't aggravate my tinnitus.

Before I go for the NAD are there other 3 or 5 channel amps that I should consider? Whatever I get I would like to last a good long time.

Regards,

Jim

drumminman

Refurbished with a warranty strikes me as a better deal than say, a 10-20 year old Adcom for maybe $400.

Depends on what "refurbishing" means. What was replaced, what work was done? Or was it only checked out to see if it worked and met specs. Same with a 10-20 year old Adcom. It's tough to say one is a better deal than another without more specifics.

On paper it looks like like a nice unit. I'm guessing it's common ground, but it's easy enough to verify.

Another idea is to get a nice used two channel amp for your SDA's, and a three channel for your center and surrounds. The surrounds draw very little power, the center a little more. Going this direction you would be able to stay within your budget and give the SDA's more of what they need.

Denzel

I would concurr with drumminman, I use an Adcom GFA545 (100w x 2) for my SDA 2As and a GFA 2535 for my center and surrounds (Monitor 5jr+), got plenty of power all the way around. 60w x2 and 200w x1 for the 2535 (it is basically 2 x GFA535 assembled in the same chassis). I picked up reconditioned Adcoms for about $550 for the pair (545 & 2535). Make sure the Adcoms caps are checked out, after 10+ years they can leak, thats what I found with most of mine.

pitdogg2

Use your best judgement. this is a statement from more than one authorized NAD repair centers website.

"T-761,762; entire "T" series
Poor quality products beset with many, many problems. We do not accept these for repair, advise replacement."

I have had NAD products was very happy with older NAD products and would LOVE to get a few master series pieces
but with the above statement from those who repair I'd take them at their word on this....

jimbo1421

Thanks for your input drumminman & Denzel.

If I went with a 3 channel amp, would it be better to keep the front speakers - left, right and center - all on one amp and let the AVR handle the surrounds. I don't have room for two external amps plus an AVR. If it makes no difference, then I would keep the Parasound HCA-1000A in the HT+2 channel system and get a new stereo receiver for my office. Then, later on, replace the Outlaw 1050, which also has its problems.

I will check on the meaning of "refurbished". I have assumed that it meant factory inspected, and cleaned up and/or repaired, with the manufacturer's warranty.

Jim

pitdogg2

If I went with a 3 channel amp, would it be better to keep the front speakers - left, right and center - all on one amp and let the AVR handle the surrounds.


yes they get the most use and demand the most from your equipment

jimbo1421

pitdogg2 wrote: »

Use your best judgement. this is a statement from more than one authorized NAD repair centers website.

"T-761,762; entire "T" series
Poor quality products beset with many, many problems. We do not accept these for repair, advise replacement."

I have had NAD products was very happy with older NAD products and would LOVE to get a few master series pieces
but with the above statement from those who repair I'd take them at their word on this....

Do you have a link for this, pitdogg2?

Jim

pitdogg2

http://www.angelfire.com/art2/stereorepair117/NAD.htm


you can google others and call to ask their opinion's on the T955 but it is part of the T series a lot of the newer gear from NAD is having some problems. I've seen 2 of the bee series and some of the newer receiver's that had many many folks upset that they bought items that cost far more to repair than it is to buy new. Most get them repaired under warranty and sell before any more problems.

jimbo1421

pitdogg2 wrote: »

http://www.angelfire.com/art2/stereorepair117/NAD.htm


you can google others and call to ask their opinion's on the T955 but it is part of the T series a lot of the newer gear from NAD is having some problems. I've seen 2 of the bee series and some of the newer receiver's that had many many folks upset that they bought items that cost far more to repair than it is to buy new. Most get them repaired under warranty and sell before any more problems.

Thanks for the heads up and the Stereo Repair link.

Jim

jimbo1421

pitdogg2 wrote: »

Use your best judgement. this is a statement from more than one authorized NAD repair centers website.

"T-761,762; entire "T" series
Poor quality products beset with many, many problems. We do not accept these for repair, advise replacement."

I have had NAD products was very happy with older NAD products and would LOVE to get a few master series pieces
but with the above statement from those who repair I'd take them at their word on this....

I sent this e-mail to Stereo Repair:

Hello,

I am considering buying a refurbished NAD T955 but wonder about this quote from you NAD page:

T-761,762; entire "T" series
Poor quality products beset with many, many problems. We do not accept these for repair, advise replacement.

Does this refer to everything with the 'T' prefix or just the AV receivers. Just above this quote you say:

ALL RECEIVERS AND AMPLIFIERS:
NAD used some rather poor quality caps in most of the product line. These can be upgraded to higher quality caps for an additional 50.00 when repaired depending on model and will increase the sound quality quite a bit. Please contact us for specific details

So do you work on T955s? I have not found much in the way of condemnation of this particular model online. Can you point me to a more comprehensive discussion of repair issues? I currently use a NAD 106 preamp and NAD 412 tuner, bought used several years ago, no problems.

Would you wave me off the T955?

His encouraging reply:

The amps in this series are really pretty good. The problem in the rest of the line is NAD did not have the in-house technology or resourses to do surround sound so they outsourced to the "Shanghai noodle factory" with terrible results. the amps are really good, being old school and simple.

I am still considering my options. When I rebuild my SDA crossovers I will have the option of removing the polyswitch. Is the "Soft Clipping" feature a good substitute for tweeter protection. I wouldn't think I would need tweeter protection for music, but would I need some kind of protection from explosions on movie sound tracks? I will turn up the volume to hear the conspirators whispering together, but then they blow up the train and it sounds tweeter blowingly loud. Do those of you with vintage speakers in your HT keep the polyswitch?

Regards,
Jim

drumminman

The wisdom around here is that if you use good quality (read clean) amplification, and do not drive the amp to clipping, the polyswitch is unnecessary and degrades the sound somewhat.

I have a set of RTA 12C's (vintage 1984) with rebuilt Xovers that I use for the front L-R mains in my HT. For Tweeter protection they have the forerunner of the polyswitch, a 0.75 amp fuse. I have it very loud at times watching stuff with alot of explosions and have never blown a fuse. They're powered by a Carver TFM-42 usually (375 wpc), though presently I'm burning in a new Odyssey Dual Mono amp.

Next time I feel like soldering I plan to remove the fuses and replace them with a resistor.

pitdogg2

That is very good to know Jim. I have also looked at the T955 in more than one case and in each case they had been worked on so my bid was not near as high as it ended up going for. I in most cases will take a chance on most stuff except when it nears brand new price that is where I draw the line. It is encouraging news to say the least. Take the plunge and report back.

PSOVLSK

I've had the NAD T-955 for a few months now and I've been happy with it. I run a complete LSi setup, so I can't really comment on how it would sound with SDA's.

I think you are correct in your definition of refurbished, but it wouldn't hurt to confirm that with the seller.

jimbo1421

I ordered the T955 from Spearit this morning. They have more in stock for those interested. Thank you all for your help.

Jim

jimbo1421

This beastie arrived today, well packaged, double boxed, etc. So the first thing I checked was the resistance between the negative speaker terminals. I set my multimeter to ohms, not K-ohms or M-ohms. Between the negative terminals I measured 197 ohms. This isn't right, is it? Touching the leads together I measure 0.0 to 0.3 ohms. I also tried plugging the unit in, while leaving the switch off. Same result.

The sales guy at Spearit forwarded this e-mail from NAD:

Hi Jim,
The T955 is a common ground amplifier. See Bob Moran's reply below.

Hi Dick,

Thanks for the welcome back! I'm just back today and catching up on e-mails.

Yes the T955 is common ground. I reviewed the SM to confirm the Black Speaker Binding posts are all chassis ground.

He included the attached diagram. So what gives. Any advice will be appreciated.

Regards,

Jim

drumminman

That doesn't sound right. Should be 0 or close to it.

You sure your meter is working properly?

And you touched each lead to a different negative terminal, not the positive? IIRC when I checked for common ground on my 2 channel amp the reading between the negative terminals was 0.

If yes to above sorting this out is above my pay grade :cool: Hopefully others will chime in.

Schurkey

Interesting how they cannot spell "BINDING" properly.

First Guess: Sourced from China with all the expected issues therein.




My main concern other than COO is that NAD is claiming the same wattage per channel into 8 ohm as it is into 4 ohm loads.
http://nadelectronics.com/products/home-theatre-amplifiers/T-955-Five-Channel-Power-Amplifier

T 955 Specifications

* 5 x 100W Minimum Continuous Power (8 / 4 Ohms); all channels driven simultaneously

To me, this is a hallmark of a current-limited amplifier. It may be rated for 4-ohm loads (better than some mass-market junk that can't handle 4-ohm loads at all!) but it doesn't have the suds needed to double the current as demanded by half the impedance. It is therefore half as powerful into a 4-ohm load as into an 8-ohm load.

jimbo1421 wrote: »

I checked was the resistance between the negative speaker terminals. I set my multimeter to ohms, not K-ohms or M-ohms. Between the negative terminals I measured 197 ohms. This isn't right, is it? Touching the leads together I measure 0.0 to 0.3 ohms. I also tried plugging the unit in, while leaving the switch off. Same result.

While not shown on the schematic they sent you, some amplifiers use a relay to disconnect the speaker terminals when the amp is shut off. MAYBE your ohmmeter results will be different if the amp is powered-up. Be careful...

drumminman

One other thought - the amp should be off and everything disconnected - I think. Try this and see what reading you get.

jimbo1421

drumminman wrote: »

One other thought - the amp should be off and everything disconnected - I think. Try this and see what reading you get.

Thanks, I tried it this way as well as plugged in but power off. Same result.

Jim

jimbo1421

My main concern other than COO is that NAD is claiming the same wattage per channel into 8 ohm as it is into 4 ohm loads.
http://nadelectronics.com/products/h...ower-Amplifier
T 955 Specifications

* 5 x 100W Minimum Continuous Power (8 / 4 Ohms); all channels driven simultaneously
To me, this is a hallmark of a current-limited amplifier. It may be rated for 4-ohm loads (better than some mass-market junk that can't handle 4-ohm loads at all!) but it doesn't have the suds needed to double the current as demanded by half the impedance. It is therefore half as powerful into a 4-ohm load as into an 8-ohm load.

Hi Shurkey,

I wish I had quoted the specs fully at the outset of this thread.

T 955 Specifications
5 x 100W Minimum Continuous Power (8 / 4 Ohms); all channels driven simultaneously
150W, 200W and 260W IHF Power into 8, 4 and 2 ohms, respectively
Mono-block, Modular construction
Differential, Class A input circuit topology
High Current Holmgren™ Toroidal Power Transformer

Does the IHF power (200W) improve this story? I also saw that Mono-block spec, which is why I asked the dealer to confirm the common ground issue. The Spearit manager is checking again and will also find out if I can strap the grounds together.

I have also looked briefly at the AI-1 interface, but since it is a DIY project, if I mess it up I would void the warranty.

Jim

Schurkey

jimbo1421 wrote: »

Does the IHF power (200W) improve this story?

Doesn't hurt.

The way I "read" those specs is the amp has reasonable storage capacitance--it will throw power into a low-ohm load SHORT TERM by bleeding the storage capacitors dry. But the transformer is way undersized, and can't provide low-ohm/high-current power long-term.

Could also be a problem with inadequate number/quality of output transistors; or inadequate heat-sinking that prevents low-ohm/high-current power delivery.

Maybe I'm wrong...

jimbo1421

I am still confused by the common ground issue since there seem to be multiple definitions of the term. But the only definition that counts in my situation should be Polk's, right? As I have read here in these forums, there should be no resistance when measuring between negative speaker terminals. I get 197 ohms. Could NAD define common ground differently, but in a way that doesn't work for SDAs?

I found this from Darque Knight:

If you are going to use SDA's you have only four options with regard to amplification:

1. Use a common ground amp where the ground terminals are tied to the same ground potential.
2. Use a non-common ground amp or mono block amps that are capable of having the ground terminals tied together (strapped).
3. Use the AI-1 interface (if your particular SDA model can work with the AI-1).
4. Run the SDA's without the interconnect cable.

By now you must have come to the realization that "common ground" can mean different things to different manufacturers. That is why Polk was so explicit in explaining what they meant about common ground.

I have not found an explicit definition of "common ground".

Furthermore, the consequences of being wrong are:

Originally Posted by heiney9
I just really need to know the risk to the SDA's if run in a non-common ground config.

1. Loss of SDA effect.
2. Damaged drivers.
3. Damaged crossover components.

Originally Posted by heiney9
The amplifier in question is certainly robust enough to handle just about any type of load.

But for how long and at what output levels and signal frequencies? Furthermore, what is the point of having a nice set of SDA's and a nice amp and having to "walk on eggshells" when using them?

Is the common ground part of the signal path in the SDA-1B or CRS+ circuitry? If so, then putting 197 ohms of resistance into that path can't be good. Do I understand this correctly for a non techie?

Jim

drumminman

I can think of two options at this point: contact NAD directly and tell them your situation. I would email them a copy of the Xover design so they can understand the signal path with the SDA's. They should be able to advise you on the amp design, why you get that reading and how to proceed. Though I'm sure Spearit has good intentions NAD designed it - they should know.

The other option - IF - you're SDA's can be used with the AI-1 interface, is to build one.

It's no more complicated than hooking up speaker wires, but there is a little soldering involved. Darqueknight has a great DIY thread with pics.

Many of us did this with the 800VA transformer with outstanding results even with common ground amps. If it were me, and the speakers can be used with the AI-1 that's what I'd do.

Removes all guessing and worry.

jimbo1421

Schurkey wrote: »

Doesn't hurt.

The way I "read" those specs is the amp has reasonable storage capacitance--it will throw power into a low-ohm load SHORT TERM by bleeding the storage capacitors dry. But the transformer is way undersized, and can't provide low-ohm/high-current power long-term.

Could also be a problem with inadequate number/quality of output transistors; or inadequate heat-sinking that prevents low-ohm/high-current power delivery.

Maybe I'm wrong...

Thanks, Schurkey. Since PSOVLSK was satisfied with the T955 performance with his LSi setup, I assumed that the T955 handled 4 ohm impedance well.

Jim

jimbo1421

drumminman wrote: »

I can think of two options at this point: contact NAD directly and tell them your situation. I would email them a copy of the Xover design so they can understand the signal path with the SDA's. They should be able to advise you on the amp design, why you get that reading and how to proceed. Though I'm sure Spearit has good intentions NAD designed it - they should know.

The other option - IF - you're SDA's can be used with the AI-1 interface, is to build one.

It's no more complicated than hooking up speaker wires, but there is a little soldering involved. Darqueknight has a great DIY thread with pics.

Many of us did this with the 800VA transformer with outstanding results even with common ground amps. If it were me, and the speakers can be used with the AI-1 that's what I'd do.

Removes all guessing and worry.

Thanks, drumminman.

My understanding is that the AI-1 interface works with pin/blade interconnects (my CRS+) but not blade/blade (my SDA-1B). If this amp won't work with both then I will return it.

I think what I really need to know is Polk's definition of "common ground". Does it mean zero ohms of resistance between the negative speaker terminals or is 197 ohms OK? If zero ohms is the requirement, then I will return this amp.

Jim

Schurkey

jimbo1421 wrote: »

I am still confused by the common ground issue since there seem to be multiple definitions of the term. But the only definition that counts in my situation should be Polk's, right? As I have read here in these forums, there should be no resistance when measuring between negative speaker terminals. I get 197 ohms. Could NAD define common ground differently, but in a way that doesn't work for SDAs?...

...I have not found an explicit definition of "common ground"...

"Common ground" means there's continuity between the negative output terminals, as they're electrically "common"--tied together. Problem is, a "true" common-ground has essentially no resistance between the negative outputs...but in real life, there may be considerable resistance for any one of several reasons--poor connections, design choices, etc. Polk expects--and SDA speakers NEED--virtually zero resistance, the amp manufacturer may choose to provide 20+ ohms for whatever reason. As the resistance increases, I suppose a person could consider the amplifier "semi-common-ground; or partially-non-common ground.

This is COMPLETELY DIFFERENT than the way "bridged" amplifiers operate. By nature, a stereo set of bridged amplifiers cannot be common-ground since there is no "ground" for the signal.

jimbo1421 wrote: »

Is the common ground part of the signal path in the SDA-1B or CRS+ circuitry?

YES. It's how the opposite-channel SDA signal gets back to the correct amplifier channel. Any resistance and the cross-channel current has trouble getting "home". Has to follow bread crumbs, and the electronic birds have eaten the breadcrumb trail. Once the electrons get "lost", they hang around the pool hall and cause mischief.

jimbo1421 wrote: »

If so, then putting 197 ohms of resistance into that path can't be good.

IF (big IF) you REALLY have 197 ohms, you are in trouble. TWENTY ohms of resistance between the channels of my Aragon 8008 caused a terrible degradation of sound quality on my 1Bs. No damage whatsoever--but it sounded crappy. In my case, a home-made three-dollar jumper cable (tying the negative terminal of one channel to the negative terminal of the other channel, and therefore providing a near-zero-ohm connection), completely rectified the situation. This solution may--or may not--work for you. It WILL NOT WORK on a bridged amp.

I'm still wondering if NAD has a relay in the output circuit that provides a near-zero ohm "common ground" when the amplifier is turned ON. The schematic doesn't show a relay, but it also doesn't show any cause for 197 ohms, either.

Got a photo of how you have the test leads connected to register near-200 ohms between the negative speaker outputs? No possibility of a decimal error and you really have 0.197 ohms? Maybe one or both output terminals aren't CORRECTLY grounded?

jimbo1421

Schurkey wrote: »

"Common ground" means there's continuity between the negative output terminals, as they're electrically "common"--tied together. Problem is, a "true" common-ground has essentially no resistance between the negative outputs...but in real life, there may be considerable resistance for any one of several reasons--poor connections, design choices, etc. Polk expects--and SDA speakers NEED--virtually zero resistance, the amp manufacturer may choose to provide 20+ ohms for whatever reason. As the resistance increases, I suppose a person could consider the amplifier "semi-common-ground; or partially-non-common ground.

This is COMPLETELY DIFFERENT than the way "bridged" amplifiers operate. By nature, a stereo set of bridged amplifiers cannot be common-ground since there is no "ground" for the signal.


YES. It's how the opposite-channel SDA signal gets back to the correct amplifier channel. Any resistance and the cross-channel current has trouble getting "home". Has to follow bread crumbs, and the electronic birds have eaten the breadcrumb trail. Once the electrons get "lost", they hang around the pool hall and cause mischief.


IF (big IF) you REALLY have 197 ohms, you are in trouble. TWENTY ohms of resistance between the channels of my Aragon 8008 caused a terrible degradation of sound quality on my 1Bs. No damage whatsoever--but it sounded crappy. In my case, a home-made three-dollar jumper cable (tying the negative terminal of one channel to the negative terminal of the other channel, and therefore providing a near-zero-ohm connection), completely rectified the situation. This solution may--or may not--work for you. It WILL NOT WORK on a bridged amp.

I'm still wondering if NAD has a relay in the output circuit that provides a near-zero ohm "common ground" when the amplifier is turned ON. The schematic doesn't show a relay, but it also doesn't show any cause for 197 ohms, either.

Got a photo of how you have the test leads connected to register near-200 ohms between the negative speaker outputs? No possibility of a decimal error and you really have 0.197 ohms? Maybe one or both output terminals aren't CORRECTLY grounded?

Bread crumbs! I knew I was missing something. I understand now. :biggrin:

Really, thanks for all your help, Schurkey. The 197 ohm measurement was confirmed by NAD who also told me that I can strap the negative speaker terminals together, thus creating a real common ground. I have done this and installed the T955 in my system. After a break in period I will report back on how I like it. How long should the break in period be?

My initial impression is that the NAD has a softer presence than my old Parasound SCA-1000A, perhaps due to the age of the old amp. I estimate that the SDA sound stage is about the same, judging from my memory of Kraftwerk's Computer World.

I think I need to add a fan to my set up, one that could clip to the glass shelf above the amp. Any suggestions?

Jim

drumminman

Re: fan, I just bought this 120mm fan, ac powered with a speed control: http://www.acinfinity.com/speed-control-fans/120-mm-kits/ac-infinity-speed-control-fan-kit-120mm/

It moves a decent amount of air, and you can turn the speed down to the point where you can't hear it from across the room even without background sound (TV, music, etc.). It still moves enough air to keep equipment barely warm to touch.

Has dual ball bearings and a rated life of 67,000 hours!

jimbo1421

I just ordered it, drumminman. Thanks.

Jim