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 Post subject: NAD PE voltage question
PostPosted: Fri Oct 19, 2018 2:19 am 
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Hey all, I've been looking for a new power amp to drive a pair of KEF reference 103.2s. I'm currently using a NAD 214 @80 watts, but I'd like to try moving to something around 150 watts. I've been considering something like a NAD 2400 or 2600.

The thing is, the 103.2s have a protection circuit built in, and the manual warns against using amps that send more than 60v because higher voltages can damage the circuit. Reading up on the PE amps, it seems something like the 2400 or 2600 will very likely exceed 60v as volume increases. Is that right?

I get the feeling I should probably look for a different option, but I don't know enough to know whether I'm being appropriately or overly cautious. I'm grateful for any advice.


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PostPosted: Fri Oct 19, 2018 5:09 am 
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two2the8 wrote:
Hey all, I've been looking for a new power amp to drive a pair of KEF reference 103.2s. I'm currently using a NAD 214 @80 watts, but I'd like to try moving to something around 150 watts. I've been considering something like a NAD 2400 or 2600.

The thing is, the 103.2s have a protection circuit built in, and the manual warns against using amps that send more than 60v because higher voltages can damage the circuit. Reading up on the PE amps, it seems something like the 2400 or 2600 will very likely exceed 60v as volume increases. Is that right?

I get the feeling I should probably look for a different option, but I don't know enough to know whether I'm being appropriately or overly cautious. I'm grateful for any advice.

Use pretty much any 150 w/ch amp, don't drive it into clipping, and you will be ok. Including those NAD amps. I'll explain...

The installation manual for the 103.2 reads as follows:

"POWER AMPLIFIERS: The Model 103.2 will operate satisfactorily with power amplifiers capable of delivering 30-150 watts into 8ohms. Amplifiers larger than 150 watts may be used but on no account should their rating into 8ohms exceed 250 watts, as peak voltages greater than 60V may damage the protection circuit."

The voltage rises as you turn up the volume. There is a commensurate increase in current as power level rises. They are simply saying that at some point, with a very powerful amp, that the current will exceed the circuit's capability to deal with it. With a 60V peak into an 8ohm load the power is 450 watts. (Current is 7.5 amps.) By suggesting 30-150 watt amps (into 8 ohms) and not rated higher than 250 watts, KEF is making sure that you are staying away from that limit.

In the case of some NAD amps, they are capable of delivering instantaneous power levels (ie. high current) far in excess of their nominal output ratings. But until they do, they are going to behave like any other amp. The difference is that if you overdrive most other amps they will be clipping - which in itself is potentially damaging to any speaker - whereas the NAD will not be clipping but will simply deliver a clean signal that exceeds what the speaker can handle. (And in the case of the KEF, the protection circuits could be damaged - rather than the drivers in an unprotected speaker.) A bit of a damned if you do... situation.

Personally I'd prefer to have the peak power capability - to avoid clipping - and exercise caution and not crank it up to the point where I'd need to be worried.

Jeff

ps. I've used, and still use, a variety of NAD amps in my many systems, including the incredibly capable NAD 2200. I'd love to pair it with any of those old KEF Reference Series speakers.

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PostPosted: Fri Oct 19, 2018 8:54 am 
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Thanks Jeff, that's very helpful.

I was reading a bit about the 2600/7600 series over on AK (http://audiokarma.org/forums/index.php? ... hts.55417/), and I stumbled on this bit:

"In regards to the "Power Envelope" marketing term... I think the amplifier has three voltage rails, 55, 70, and 90 volts. If the impedance switch is set to 8-16 ohms, the rail used is 70 volts. If the switch is set to 4-8 ohms, the 55 volt rail is used. At some threshold which I don't know, when that rail voltage is approached, the amp switches over to the 90 volt rail, for up to 200 msec."

Here's my back-of-the-napkin calculation, for what it's worth.

If the bit I quoted above is right, 55V @ 7.5 Amps should give something approaching 412.5 watts. I'll just guess that the amp switches rails somewhere between 40-50V. At 7.5A that gives me 300-375w of room to play with before I get above 60V.

The KEFs sensitivity is 86db, and I sit 3-4m back, which means I'm getting something like 74db efficiency at my listening position.

If all of that is right(-ish), 300-375 watts gets me somewhere between 98-100db. If I listen around 86db, that gives me something like 12-14db of headroom for peaks before I'd have to worry about any switching that might occur.

Does any of that seem right? My grasp of these things is a bit shaky, but I'm learning (I think) :roll: :lol:


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PostPosted: Fri Oct 19, 2018 8:03 pm 
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Years ago I owned a pair of KEF 103.2 and a NAD (I think it was a 2200) amp. I had no problem with either.
If I got a little heavy handed with the volume control the KEF's protection circuit would kick in and the volume would be reduced.
The protection circuit in the KEF's was quite efficient and likely saved me a couple of times.
It can be beat though. Turn it up loud enough with the right (or wrong) music and you can "weld" the protection closed.
I never managed to do it but the guy I sold the KEF's managed to do it with a 25 watt receiver.
IIRC it wasn't too expensive to get them fixed.
I really liked the KEF's. I had them on a Target stand filled with steel shot. They sounded great with almost everything.

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PostPosted: Fri Oct 19, 2018 9:07 pm 
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Location: Brantford, ON, CA
I love my NAD 2200. I use to run it with Bose 901's, which many friends left thinking they just left a Concert and very rarely could run it so PEP would kick in and when it did it was like the old MAXEL ads. https://youtu.be/Zk71h2CQ_xM

I came across this one day. I thought some may enjoy.

https://youtu.be/CA582cvAfjA


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PostPosted: Fri Oct 19, 2018 9:27 pm 
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Location: Montréal, QC, CA
two2the8,

The NAD 2600 has 3 output voltages; 62, 72 and 95v on each rail.

62v for lower impedance 4-8Ω.
72v for normal operation.

The 95v is added when the current (and not the voltage) reaches a threshold.

Then, it depends on the load.
A speaker at 8Ω could have a lower dip impedance at a certain frequency than another 8Ω speaker.
Too much current passing the output emitter's resistor triggers the 95v system.
Too much current can happen at 25v, 49v whatever...

two2the8 wrote:
The thing is, the 103.2s have a protection circuit built in, and the manual warns against using amps that send more than 60v because higher voltages can damage the circuit.
Reading up on the PE amps, it seems something like the 2400 or 2600 will very likely exceed 60v as volume increases. Is that right?
Yes. But only if you crank it up very loud. You shouldn't need to do that, see why:

The KEF 103.2's are 100w speakers (150w program). But they don't go very deep, 60hz or so.
Then, you don't need all the muscles of the amp.

I'm pretty sure a NAD 2600 won't ever switch to the 95v mode with those KEF at normal loud volume.
With a more powerful amp than the speakers, you likely won't ever enter in the clipping zone.
Know how to play with the volume and enjoy the music.

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PostPosted: Sat Oct 20, 2018 12:48 am 
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Right on, that is really helpful Nakamichel. I had a feeling my math was a bit wanting ;)

Sounds like the consensus all around is that I'll be alright. That's good to hear.

Cool to see that video, TD 100 MKII. The 2200 looks to perform pretty well!

Mahatma1, IIRC your recommendation in another thread helped to swing me toward buying these speakers earlier in the year. Thanks for that, I dig em too :)


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