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RE: Tweeter difference problem with 3.3Rs and more

HI Satie,

Part of the tweeter mystery is solved!, - see above!

Yes it seems the 208 is the only serious amp NAD ever made (well the 218 is quite good too, - see below), - the 208 was made in England, and really over engineered, and I've heard many times that it should have been priced way more than they sold it for new. It was made between 1992 and 1996. So yes it is a sleeper that many people miss as its a NAD and its overlooked. I experienced it 11 years and sold that one, - used it in a home cinema setup, - it was seriously good, - very powerful but a good quality sound as well, - great compromise. I searched for another 3 years ago as I knew how good they are for the money. The good thing about it is its a very very powerful amp in a pretty light package (only 17.5kg)
I guess if you upgraded a lot of the parts in it for better quality ones and/or a few tweaks it would be a fantastic sounding amp. It does sound pertty good in stock form as well. It does have a fast treble like that review said. NAD also made the later Chinese made 218THX which uses bipolars instead of MOSFETS. It has a similar config of a reservoir of small caps, - that amp is heavier weight wise and it also does have a even more dynamic sound than the 208 and a very good bass response (see the Stereophile review of it) but from what I have read the treble is rougher sounding than the more refined MOSFET based 208. The 218 is the better choice probably for Home cinema (but the 208 is no slouch at all with Home cinema too) but I guess the 208 is the far better option for use with HI-Fi speakers like Maggies,

With the optional bridging option mode for the 208 you have an insanely powerful combo with two of these. And I would have though even bridged a pair of them would still be stable into 4 ohms and the class G switching mode STILL works in bridging mode too, - so huge transient reserve. OCCASIONALY you see them cheap, - I will look for another. I once saw one sell on Ebay for about £250!, - but that was pretty cheap, - I paid £450 used for mine from a Hifi shop. The price was £1000 new in 1992.

I conversed with a Dutch guy a couple of years ago about the 208 and he emailed me the service manual- its not on the web. It has all the schematics in it etc, - I'll dig it up and send it to you so you can have a browse, in case you might find it interesting.

I tried out the PLLXO (with headphones connected via resistors from the NAD speaker output), - sounds great!, - I compared the crossover point by listening and switing from the PLLXO to the Behringer going the HP and if we assume the Behringer is accurate in its setting I reckon the PLLXO cross over sounds to be about the same as 280Hz HP 2nd order Butterworth on the Behringer. So perhpas its a little low maybe?, - Should I just try to blend the bass with the PLLXO this way or might it be better to calculate sound values that would give me a slightly higher crossover of around 340Hz like you suggested for the 3.3R HP. Of course I;m not saying you calculated the values wrong!,.- just an observation in comparison to the Behringer. I can hear for sure this really works and all the nasty hiss I get with the Behringer is obviously eliminated. I used the NAD for testing but that has 20K input impedance like the Quad 606 does.


The Behringer really is very hissy when used in my system (with RCA to XLR connectors and HIfi, not pro level). I need to get it upgraded, - its pretty much unusable for the mid/treble section even with the lower gain NAD as you can still make out the hiss. I am going to carefully listen to it when using the NAD on the bass panel and the PLLXO on the mid/high (with the QUAD) to see if any hiss is still audible through the bass panel.
If I go for the mods I might go for the transformer based output stage from Audiosmile rather than the transistor based output stage, - the Transformer based one might be a good idea if I am minimum hiss.
I know part of the problem with the Behringer output stage is because its balanced and is higher output and this contributes to all this hiss. I am going to have to have a look myself on the DIY audio forum and see if its possible to reduce the output level myself. I made up a cable excluding that 1-3 XLR short and it DOES work, - the sound of the music is 6dB lower, but the hiss is EXACTLY the same level as it is with those pins shorted!!, - so its even worse S/N ratio, - so I will be using the shorted 6dB louder connection for now.
I am kind of tempted too to try your idea of using the PLLXO for the bass LP in 2nd order and 1st order on the high HP, - but obviously to do this I really need to experiment with the Behringer first using 2nd order LP and 1st order HP in order to get good sounding crossover points, then calculate I know the HP on the original MGIII is about 800Hz first order rather lower 350 ish Hz of the IIIa and 3.3r. The bass LP is still 3rd order on the original MGIII at 300Hz, 300Hz on the IIIa as well and 240 on the 3.3R. Bearing this is mind I understand the mid/tweeter internal crossover have different values on all these speakers as well.

I need to experiment to see if its viable to use these order of one lower for each branch with the 3.3R but if it is I might give the full PLLXO a build.

You are right about the polyester caps, - not great, - I will get the Cricklewood stuff soon, - I need to go there to get some more lead based solder anyway (no where else). I think the popypropylene caps I got for C1 from Maplin might be OK, they were pricey at about £2.50 each and are big!, - sort of rectangular shape. Probably not as good as the Ansar audio grade ones from Cricklewood though. At least I can see with this test how this really works, - I have some RCA splitter but they are cheap, - I need to get the Audioquest ones.

Anyway making progress on the tweeter issue!
Gonna test out the PLLXO at the weekend, - can’t wait!
Cheers,
Colin

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