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NAD 208 THX resource

Discussion in 'd.i.y.' started by earthfield, Aug 2, 2019.

  1. earthfield

    earthfield earthfield

    Hi guys...

    Is there someone out there who specialises in repairing
    and upgrading these beauties back to sonic excellence?...
    within The British Isles?

    Please message me.

    It would probably be a good idea to add your definitive experiences with 208's here...
    Very little about these astonishing flagship amps is on the web.

    The Nad 208 THX was based on the Studer A68, a highly regarded monitor amp of its day.
    with 100 watts per channel into 8Ω (stereo), 350W into 8Ω (mono).
    Frequency response: 30Hz to 15kHz
    Total harmonic distortion: 0.1%
    Damping factor: 75
    Input sensitivity: 0.775V
    Signal to noise ratio: 100dB
    Speaker load impedance: 4Ω (minimum)
    Dimensions: 19 x 5.24 x 13.20 inches
    Weight: 20kg

    The NAD 208 THX aimed for something more robust, the result:
    Power output:
    Stereo Mode: 250W RMS per channel into 8Ω , IHF Dynamic Power 600W.
    Clipping Power 280W RMS.
    Mono Mode 500W RMS into 8Ω, IHF Dynamic power 1500W.
    Frequency response: 20Hz to 20kHz
    Total harmonic distortion: 0.03%
    Slew Rate >100V/usec
    Damping factor: >200
    Voltage Gain: 29 dB, Rated power 120dB.
    Input sensitivity: for 1 Watt 100mV, for rated power 1.6V
    Input Impedance: R =20KΩ, C = 700pF
    Signal to noise ratio: 120dB Rated Power, 96dB ref.1W
    Speaker load impedance: 2Ω (minimum)
    Dimensions: 435 x 175 x 370mm
    Weight: 17.3kg, Boxed 20Kg.


    The best resolution Service manual pdf can be downloaded from https://www.vintageshifi.com/repertoire-pdf/Nad.php

    jonas hi end audio modifications channel on you tube is definitely in the right direction...



    The LM4562 Op Amp upgrade sounds good, but what was done to that power supply to "clean it up"?

    Best from another forum:
    “The NE5532 is sufficient for systems with SNR in excess of 90-100 dB, so it's still 'good enough' for audio and the like. It is also very cheap and besides the NJR (2068, 4556) there are few parts in the same price range with overall similar performance.

    The LM833 is only slightly better in some specs than the NE5532 (notably it has a bit more phase margin at unity gain, the '5532 has only like 45° or so PM which leads to overshoot and poor stability with capacitive loading in follower applications). They are overall very similar chips and should, in most cases, be interchangeable without much difference. The 833 doesn't really perform better in terms of THD or noise, they are nearly indistinguishable.

    The newer LM4562/LME49720 series improves on many parameters at once cf. NE5532, e.g. noise, bandwidth, THD and output current. They have, however, higher current noise and cost ten times as much. They were designed for systems with 120+ dB SNR. Note the high bandwidth of these devices (which leads to high open loop gain at audio frequencies, which in turn increases feedback and thus reduces THD, which is how they achieve the extremely low THD of these devices - high feedback and a very linear op amp to start with), which means that you need excellent bypassing and careful layout ; these are "high speed devices" and somewhat prone to oscillation by insufficient bypassing or too high feedback network resistances (which create too much phase shift with parasitic capacitances).

    There are some other contenders, but most have, in usual applications, one or more (important) disadvantages over the 5532. So for the great majority of applications the 5532 is just fine, but as a bipolar op amp it's not well suited for high source impedances (JFET territory).

    All in all your circuit and it's layout and environment will matter much more than the specific choice of op amp, be it 5532 or 2068 or 4562. Mind you, be aware that you often can't just swap op amps, especially against faster ones, without adjusting the circuit.”

    A reply:
    I would throw the NJM4580 (JRC4580, RC4580) in as a contender, but only as a rough equivalent to NE5532 or LM833. It is about the same spec-wise and you will find it in many mid to high end Japanese Hi-Fi. I bought a bunch for ~$0.20 each (TI made RC4580) to replace some much noisier spec Mitsubishi M5218 op-amps in a AVR maybe 12 years ago. I was quite pleased with the result.





    I haven't typed it all into google translate, seems to be about the protection circuit path on a repair.












    NAD as they currently operate are void of real service in these isles.
    Stone wall corporate website, etc, etc.

    The other options appear to be in Germany but the logistics are not appealing for three amps! :

    "For a professional support in case of service the company Bergmann is at your disposal in Germany.
    There you will also get replacement parts for older NAD devices and also replacement manuals.

    NAD Central Service for Germany and authorised special workshop for brand repairs
    Bergmann Electronic Service GmbH, Dieter Bergmann
    Hanauer Highway 98 /Hanauer Landstraße 98
    63538 Großkrotzenburg
    Tel .: 0 61 86 - 80 66
    Fax: 0 61 86 - 82 61
    Mail: info(at)bergmann-electronic.de "
     
    Last edited: Aug 3, 2019

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