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Old 10th February 2009, 02:32 PM   #1
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Location: Singapore
Default NAD 2600 overload - how to upgrade

Hi,

I was listening to some favourite music just now, (NAD 2600 bridged mono, driving a 8 ohm 15 inch sub). got a bit carried away with the volume knob, and suddenly realised that there was no bass! Looked at the amp rack, the red 'protection' light on the NAD was 'ON'.

Quickly turned the volume down, and the light went off after a few seconds, and everything was fine. I think i just found the limit of the amp.

From what i understand about the NAD power envelope circuit, with dual voltage rails, the PSU sags when loaded. Does this mean 150W RMS continuous rating for the 2600 is from the lower voltage rails?

How does the protection circuit work? When the PSU voltage rails sags excessively, or due to output current limiting?

What would be the best way tomodify the 2600 to improve the transient performance? IE, what is the weakest link now? More capacitance in the power supplies? Larger transformer? Or it is limited by the output transistors?

I'm not looking to up the continuous power rating, more like the transient load performance, so i was thinking along PSU capacitors upgrade.

Currently, think there are 2x 10,000uF 120V and 2x 10,000uF 80 V capacitors.

Thank for the input and comments!!
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Old 10th February 2009, 06:27 PM   #2
sreten is offline sreten  United Kingdom
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http://user.faktiskt.se/phon/misc/2600.pdf

Hi, Personally I think your wasting your time, it does what it does, /sreten.
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Old 11th February 2009, 01:54 AM   #3
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Thanks for the link to the service manual.

Agreed that the performance is limited by the design / components useds etc, but going from a weakest link analysis, what would be the a worthwhile mod/upgrade?

A regulated power supply mod?

The amp was recently repaired, a whole new set of output transistors were installed. I'll check the part numbers used when i get back, maybe they were under-specced.
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Old 11th February 2009, 04:11 AM   #4
SQLGuy is offline SQLGuy  United States
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Based on your description, I'd guess you overheated it. If that's the case, since it's already a class-H'ish amp, the only thing I could think of that would get more out of it would be bigger heatsinks (or a fan).

Then the power supply would probably become the weakest link, and, if you upgraded that, you'd start exceeding current capability on the outputs, and then....

Like Sreten sort-of said, I don't think you're going to find any cheap and easy roads to significantly more power here. A regulated power supply might lower the noise floor, but it's not not going to increase power... in fact, it should cost you a few watts to do it.

The one possibility, depending on how the amp is situated in your "amp rack" would be to relocate it somewhere where it will get more and/or cooler airflow.
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Old 11th February 2009, 10:40 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

Its not an amplifier ideal for a subwoofer. Its design depends on the
statistics of music signals, the statistics of low bass only signals are
very different to full range music. Its not an amplifier I'd use bridged.

That is : it is best used as a stereo full range amplifier, it this mode
it should work best without overheating and give best bang for buck.

You want to redesign it ? Redesign the overload protection ?
Your severely chasing your tail IMO and will not get very far.

FWIW underspecced output devices usually = one dead amplifier.

/sreten.
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Old 11th February 2009, 02:16 PM   #6
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OK, just checked the output transistors : the guy put in 2SA1943 / 2SC5200 transistors. Apparently they are higher voltage and current specs than the amp design values.

It did not overheat, just a momentary transient that probably drew too much current. The amp was barely warm when i checked it after the brief protection kick-in.

Any benefits at all for transient current draw by increasing the power supply capacitance?

So, i guess this was not the best amp for the job. I'm currently using a pair of NAD 214s (80wpc) for passive biamp on the mains. Should i sell the 2600, or sell and 214s and use the 2600 for the mains? Any difference in power?

What would be a good recommendation for the new sub amp? I'm in Singapore, so one of those pro audio kilowatt amps are likely out of the question. I recently saw a B&K ST202 for sale, and occasionally some ADCOMs and PARASOUNDs come up for sale. I supposed the Adcom GFA 555 is highly recommended. Any other suggestions?

Thanks a lot!
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Old 11th February 2009, 02:29 PM   #7
SQLGuy is offline SQLGuy  United States
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The Adcom GFA-555ii is quite a lot of amp for the money. The one bit of preemptive maintenance you might want to do is to replace the 2SA1210's and 2SC2912's (on the input board), as they run quite hot and tend to die. If they die, you're running 80V DC out the back until you replace them.
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Old 18th February 2009, 02:12 PM   #8
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Doing a bit of thinking, am i barking up the wrong tree looking for a stereo power amp to drive a sub?
Should i be looking for a dedicated subwoofer amp instead?

It must one that can comfortable handle a 4 ohm load, and a parametric EQ would be nice. Minimum 500W into 4 ohms, 1000W would be better, need a 240V version.

For the GFA555, would cost me about USD350, but that is without the crossover, and PEQ etc.

So far i have only found 1 plate amp that is 1000W. Rest are usually 500/600W.

http://www.parts-express.com/pe/show...number=300-808
http://www.reckhorn.com/index.php?ln=en&prod=a400
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Old 18th February 2009, 02:49 PM   #9
jaycee is offline jaycee  United Kingdom
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Yes, you would be better with a dedicated plate amp.

500/600W is fine. 1000W is *impossible* and is marketing crap usually pushed by car amp manufacturers. 500W RMS into 4 ohms would provide enough SPL to reduce your house to rubble, if the neighbours dont call the cops first.
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Old 18th February 2009, 03:07 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

Model the subwoofer in WinISDPro to see how much juice it can take
within its excursion limits, and select amplifier power accordingly.

/sreten.
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