DIY Transistor Amp Using NVA Boards

[pre65]

If you need any moral support, i may have one or two monitor speakers with Purifi drivers there - they will have a bit of Boofboof, so you won't feel out of place.

Well in that case…I’ll think about it

Please do Steve, I (for one) would be interested to hear your NVA clone.

[Cressy Snr]

Well three days into life with the Missions and the treble has come out beautifully. The speakers were a little rolled off last Thursday, but now they are nicely balanced, with no sibilance or harshness anywhere in the upper-mid/treble area.
I found this phenomenon interesting so removing the tweeter, so I could see the crossover, I found a simple first order job, with an inductor on the bass driver and a cap plus padding resistor on the tweeter. There’s also baffle step compensation in there.

The tweeter cap is a 4.7uF Tesla electrolytic, so that probably explains why the treble extension improved over time. The cap, not having been used in years will have taken time to re-form. There may be a bit more treble improvement to come, but they are sounding pretty damn’ good whatever.

There could be an argument for replacing the tweeter cap with a higher quality film/foil item, but why mess about? That way lies madness. Too many different makes of cap.

The NVA/Mission combination sounds excellent. The amp, now it is playing into more suitable speakers, is doing nothing wrong at all and seems to be providing fatigue free musical enjoyment, completely different to the Naim amp I had, twenty years ago.

As Phil has inferred, I might as well bring the Missions and the NVA clone to Owston in October. Obviously the system won’t cause any jaws to drop, but there’ll be no problem carrying the Missions to the car, which is a big bonus at my age.
Re the amp itself, I’ll use it with the cover off, so the panel can take a look and make suggestions as to how the twin power supplies might be improved on. If they don’t mind that is.

Haven’t stopped grinning for two days.

[Ray P]

Steve, I have a pair of Mission 773 floorstanders you're welcome to take off my hands, FOC, if you want to explore a little more? They're like these;



They're in good running order and cosmetically pretty reasonable with no dings to the cabinets. The leather effect covering on the baffle has shrunk a little on one, so shows a sliver of the baffle material, and there are a couple of small scuffs. The gril cloth has some small scuffs but would be easy to replace I think. I can take some pictures if of interest?

[Cressy Snr]

Steve, I have a pair of Mission 773 floorstanders you're welcome to take off my hands, FOC, if you want to explore a little more? They're like these;



They're in good running order and cosmetically pretty reasonable with no dings to the cabinets. The leather effect covering on the baffle has shrunk a little on one, so shows a sliver of the baffle material, and there are a couple of small scuffs. The gril cloth has some small scuffs but would be easy to replace I think. I can take some pictures if of interest?

Fantastic Ray!
I’ll have them. If you bring them to Owston, I’ll certainly take them off your hands.
Yep, take a few pics.

[Ray P]

Here you go Steve. They're n the spare room so a bit dusty but you'll get the general picture. I also have the stablising feet somewhere here when I find them.

Here's a couple of general pictures;





This shows where the leather stuff on the baffle has shrunk;



And these are close-ups of the little scuffs they've got - shouldn't be a problem to a man of your talents.

[Ray P]

Size wise, they're about 86cm tall, 17cm wide and 27cm deep.

[Ray P]

If anyone from up your way is passing through Somerset on the M5, on the way to/from Devon or Cornwall, I'm happy to meet them at Taunton Deane Services so you get them sooner.

[Cressy Snr]

Do you fill the bottom halves of the cabs with ballast, or am I thinking of a different model?

[Ray P]

There's no ballast in them Steve. I don't think they're intended to have it either as i can't see an obvious way of getting it in. TBH, I've not really done anything with them beyond checking them - I picked them up from a guy at work who was moving abroad and was just going to take them to the tip.

[Cressy Snr]

Must be thinking of a different speaker then.

As I said, I can definitively use them, no problem. I’m all for recycling.

[Ray P]

Steve, I'm listening to the Mission 773s with my SissySIT amp that I've just been working on.

Not ideal because using my mobile phoe as a source there's not enough gain so I've pressed my Noir HPA into service as a preamp, still not quite enough to drive them ard but certainly listenable volume.

It shows that they've not been used for years and it's a new amplifier too, sound is a little dull and the bass is a bit light but some hours and a bit more power into them should loosen things up a bit - already sounding better after just half an hour.

[Cressy Snr]

Steve, I'm listening to the Mission 773s with my SissySIT amp that I've just been working on…

…It shows that they've not been used for years and it's a new amplifier too, sound is a little dull and the bass is a bit light but some hours and a bit more power into them should loosen things up a bit - already sounding better after just half an hour.

The Mission 731s did the same, so there should be a bit more improvement to come.

[Cressy Snr]

Took a break from decorating and got the stands for the Missions done. I just have to decide on the finish for the bases.
Right now I have a yard to clear, a load of tools to put away and I’m knackered. But I’ve achieved something, so I’m happy.
Just need the poles to be black and white and I have a set of traffic lights.

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[Cressy Snr]

It didn’t take long for me to conclude that the tops of the stands look like crap.
The solution was a couple of large toroidal transformer washers:

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The stands now look a whole lot better:

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The construction is dead simple. The bases are a four layer glue-lam made of 12mm ply. The stands are held together by an M8 threaded tie rod passing up the pipe and it holds everything in compression. Years ago I had a pair of B&W stands under my KEF Coda IIIs that used the same principle.

As you can imagine, these much taller versions are far from rigid. Their main resonant frequency is around 3Hz - way below the audio band. However despite the lack of rigidity, the sound is superb. It was very good with the wooden top platforms, but with the washer platforms, the sound is outstanding for a pair of budget speakers.

Also because I designed the Fane full range speakers for my own room, they could just be thrown in and would sound good. With the Missions, I have had to re learn how to position speakers in a room. They ended up 10.25in out from the back wall, toed in a couple of degrees and spaced 66in apart.

I couldn’t be happier with the result.

[Cressy Snr]

Like all speakers, the Missions are front heavy. They are wider than they are front to back, so even more front heavy than a more modern speaker. Due to the stand’s top platform now being circular, there isn’t a lot of resistance to the tendency for the front to try to tilt downwards, ie only a single point at the underside of the front.
I had visions of the blu tack gradually giving way at the back and the speaker crashing forwards off the stand in the middle of the night and killing the cat.
Anyway, I’ve solved that with a couple of 3mm aluminium strips epoxied to the big washer: that should stop any toppling shenanigans.

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