Using NAP 140 with passive preamp ?

Posted by: tomek on 27 February 2001

I would like to use Nap 140 with one passive preamp. When i am turning on my Nap 140 witout Nac 72, than there is a loud noise from the loudspeakers and Nap 140 is not switching in "normal" modus. What should i do if i want to use Nap 140 with passive preamp ? I have to connect some pins of the nap din output. But which one and how ? confused

Posted on: 27 February 2001 by Laxton

Not a good idea. Naim designed their preamps to prevent their poweramps from oscilating if i'm not wrong. Someone correct me on this.:)

Regards
Laxton

Posted on: 27 February 2001 by Andrew L. Weekes

In Naim's words (almost), the operating conditions of the power amp's are determined by the pre-amp.

This means Naim pre-amps are OK with other kit, but Naim power amps must use Naim pre's to maintain safety / stability margins.

Andy.

Andrew L. Weekes
alweekes@audiophile.com

Posted on: 27 February 2001 by David Dever

quote:
I would like to use Nap 140 with one passive preamp. What should i do if i want to use Nap 140 with passive preamp ? I have to connect some pins of the nap din output. But which one and how ?

I have yet to hear a passive attenuator that sounds any good. There are plenty of technical arguments for using an (active) preamplifier:

1. Gain (of course)
2. Input impedance that does not vary with volume setting (buffered inputs)
3. Buffered tape outputs (NAC 32/72 and above) provide consistent source impedance for tape decks, etc.
4. Output impedance that does not vary with volume setting

quote:
When i am turning on my Nap 140 witout Nac 72, than there is a loud noise from the loudspeakers and Nap 140 is not switching in "normal" modus.

This is normal operation when using a Naim preamplifier without a dedicated supply (I presume you've also got a NAC 72, hence the reference above). The relay activates once the preamp supply voltage has reached a stable 24 VDC level, thereby protecting the amplifier/speaker from any unpredictable oscillation or spurious noise caused by the preamplifier circuit, or, in your case, the passive attenuator loading, etc.

Hope this helps,
Dave Dever, NANA

[This message was edited by David Dever, NANA on WEDNESDAY 28 February 2001 at 03:32.]

Posted on: 27 February 2001 by bam

Hang on you guys. I don't get the bit about the pre-amp setting the operating conditions of the power amps. Where did this idea come from?

The Naim amps (except the 500?) use an op-amp type architecture with very high negative feedback and a controlled gain response that rolls off above the audible range. The amp inputs are capacitively coupled to ground with a series resistor to form a low-pass input filter. This circuit should be unconditionally stable wrt the input source impedance. If this is not so then please advise.

Passive pre-amps, which are essentially resistive attenuators, are a simple and very linear way of controlling volume. There is just one problem - the series resistance of the attenuator needs to be very small compared with the input impedance of the amp whilst also presenting a high resistance to the CD (or chosen source). This is a bit of a compromise. The input impedance of a 250 is <15k at 20kHz, so you would like to drive this with 1.5k or less. But this means presenting your CD with a 3k load (using a 3k pot). This may be too low for some players - you'll have to check this.

BAM