h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Not sure I follow you, Pheroneous. You're still using the Tannoy 638s, right? And if I know Tannoy, they'll have two pairs of speaker terminals, one pair for bass, one for treble ('scuse me if I'm in sucking eggs territory). The fact that the tweeter is concentric with the bass cone doesn't prevent it from being a separate unit - that's why you pay lots of money for them! So if you want to use the Typhoons, connect (say) the A3 speaker outputs to the Tannoy's treble terminals, then take the A3's preamp output on phono cables to the Typhoon's inputs, and connect the Typhoon's speaker outputs to the Tannoy's bass terminals.

This is called bi-amping, and is considered the better way to do it than using one amp per side, because the amp handling the treble should have an easy ride with no bass to pump out and therefore give you smoother treble. It should work because you have amps from the same manufacturer which should match for level. Don't know if the Typhoons have a level control, actually, but if they do, you can then set the balance of bass and treble to your own taste.

Of course, you can swap it around and have the Typhoons drive the treble and the A3 the bass. That's what i meant earlier about seeing which sounded best. Or if you're enjoying what you've got just now, adopt the old maxim - if it ain't broke, don't fix it. If you're like me, though, you won't be able to resist trying it eventually!

it's a hi-fi a magnificent magnavox,date of manufacture 1948,wasn't that somewhere around the golden age of radio?

I take your point, Spiny, but I think I will be able to resist! (At least for now)

I am not sure how the connections would work, because the Typhoons used those big 'balanced' multipin input connectors from the Preamp, and also, although the A3 PreAmp can be bypassed, I dont think I can get it separately connected.

The Tannoys have three 8" speakers (hence 638). The first is a concentric, the second bass only and the third is what they like to call a passive bass. (i.e. it is unconnected, just works from the trapped sound waves, and seems to me a waste of space, a bit of marketing. It does very little) In use, the first is, to all intents and purpose, the only one doing any work, it must be responsible for 90-95% of the sound. I don't think I have access to the separate cones. There are two connections, sure, but as I remember, one is for the concentric, the other for the bass, which really doesn't work very hard.

The Magnavox sounds like fun james. I'm surprised though that anyone was making hi fi that big in the 40s. But that's America for you

Pheroneous, I have some thoughts on what you wrote, but I'll have to reply later when I get the time.

Right, found a quiet corner.

Pheroneous, the Tannoy dual concentric is different from other speakers in that it's two speakers in one. The tweeter is in the centre with its own voice coil, and that, via the crossover, is connected to one pair of binding posts on the back of the box to connect to your amp. The tweeter lives inside the mid/bass cone, and it is the cone itself which controls the dispersion of the tweeter - the cone's flared shape acts like like a horn for the tweeter. This mid/bass cone has its own voice coil as well, and it, plus your other bass driver will be fed from the bass output of the crossover. So if you only connect to the treble binding posts, you're only powering the tweeter. Try it - you'll get a sound like somebody else listening to a pair of headphones.

This means that your other set of binding posts are not just feeding the other bass driver, but the mid/bass part of the dual concentric as well. Your third passive driver works a bit like a port on a reflex design. It's designed to resonate at very low frequencies so that it enhances the response down there. Because this combination gives you lots of area to push the air about, it doesn't have to work as hard as smaller speakers do, so it might not seem like it's doing much, but if you put on a recording with lots of deep bass at a high volume and rest your fingers on the drivers, you'll soon realise how much is going on.

Now to the A3 - I thought that MF were keen on bi-amping, so they give you a pre-amp out, which shouldn't disconnect the power amp but should give you a variable level signal controlled by your volume control. All you would need to do would be connect this to the input of your Typhoons. But...you've got XLR inputs on them - that does make it tricky. You can get phono to XLR leads, but maybe not readily. I make mine up, but then I'm handy with a soldering iron. Oh well...if you come up with a solution, let me know. Hope this was useful.

I am not sure about the tweeter being separated by the crossover in that way, I hadn't seen it like that, nor do I remember it being 'tweeter only' with only one terminal connected, but I will try. (I remain envious of your D500's, did I say that before? A Tannoy engineer I know reckoned the D700's were the best speakers that they ever made, including the Prestige stuff. But he was an audio engineer. What do they know!)

If you are right, then its an interesting possibility!

You will, however, have to explain to me how I can get Mrs P to understand the need for two cables to be stretched across the Living room floor!

Well, that's how bi-wiring works. You split the crossover by lifting the link between the terminals (on mine it's two gold-plated rods joined at the top by a plastic bar saying Tannoy - probably the same on yours) so that the upper and lower terminals are no longer connected together. The two bits of the crossover are then separated so that one pair of cables from your amp drives the bass speaker through the bass bit of the crossover and the second pair of cables drives the treble speaker through the treble bit of the crossover. If you don't want to bi-wire, you put the link back down and use one pair of speaker cables. The split into bass and treble still happens, but now it's inside the speaker.

The advantage of bi-wiring is that you get improved power handling, and less interference in the treble circuit from heavy bass currents. The sound should be smoother, yet more dynamic. I converted an old pair of Audiomaster speakers I've had for years for bi-wiring, and the difference was amazing. We're not talking subtle!

Once you have bi-wireable speakers, you can then go further and bi-amp them, as we've discussed. You try to put your amps as close to the speakers as they'll go, then use the shortest possible speaker cables, because the shorter they are the better. The logical extension of this is to put the amps inside the speakers, then you have what's called active speakers, which are used a lot in recording studios. So you could try the A3 to left and right tweeter terminals (normally the top pair, and be sure to pull the link UP if you do this), and Typhoons to left and right bass terminals. That setup should really kick ass!

Yes, I'm proud of my D500s, but the 638 is virtually the same driver complement in a box that doesn't have the nice rosewood veneer. And hard though it may be to imagine, the tweeter in yours is the same as mine and it really is the little thing in the middle, and it really is separate. The dual concentric is not a cheap unit, but that's why it sounds so good and gives such pinpoint imaging. BTW, you may think the rosewood veneer means domestic acceptability, but I had a lot of work to do underneath the floor with cables to persuade Moira to have a rig of this size! Still, once they're in, you don't have to do it again. You could always get some 4-way cable intended for bi-wiring if you can't get under the floor - it can look quite neat.

Okay, I've gone on a bit, so I'll stop being evangelistic now. But I can't help myself, because I'm an audio engineer too....

Thanks Spiny. Most of that I know, it was only the statement that the tweeter is sepately wired to the surrounding mid-range/bass cone that worried me.

The speakers have always been bi-wired, in Typhoon days and now. I have QED 79 strand to the tweeter and some fairly hefty audioquest, I think, to the bass. I have a 'cellar' of sorts under the Living room, but its only about two feet high, and as I am about two feet wide, I don't go down there any more often than I absolutely need to!

In my case, the speakers are some 5 or 6 yards from the amp, so economics comes into it, Interconnect being very silly money, whereas speaker cable is only silly money. (Don't give me all that tosh about using mains cable, I really want to believe everything Mr Van den Hul tells me - have you ever come across his technical literature, its wonderful!)`

I cannot find any indication of a separate pre-amp availability, but I have yet to find the instruction book. If it is there I will definitely try it (Bi-amping, I mean), I would certainly like more bass presence at low volumes. Next time, its valves for me!

the magnavox may be from 1958 not 48,i've another one i got for parts (the correct turntable)with the same amp, looks like one 12ax7 and six 6v6's.if i can get both of these up and running behind the SAE preamp,it might be interesting.as to the 638,s if i were you my next upgrade would be to replace the passive woofers with like aggressive drivers.

Alright, Pheroneous, hope I've convinced you what a strange and wonderful beast the dual concentric is, and given you some pointers towards bi-amping. Let us know how you get on.

james, I don't know much about the 6v6 - what power output do you reckon 6 of them will give you?

i think this is a push-pull type configuration,so about 20 useable watts maybe.the 6v6gt is a fairly common tube in alot of guitar amps,two of them puts out about ten watts .i don't know enough about these things,and the only person i know who does insists on replaceing all the caps and resisters to bring them back to specs.that's all fine and good and useally produces the desired results,but doesn't tell me where the hummmmmmmmmmmmm,is comeing from

Hi, I hope you don't mind me butting in, but do you own an oscilloscope ? If so, check the HT in the power supply to see if there is significant ripple. If there is, that's the problem and you should look to reservoir capacitors and power supply chokes for replacement. If it doesn't have ripple there, you almost certainly have an earth problem. If you have the circuit diagram, try making additional earth connections from parts of the circuit eg the signal input earths - do this direct to the earth in the psu, not via the chassis. If there is a small amount of resistance between the signal earth and supply earth, hum is produced. ( even tenths of ohms is enough ). Hope this helps, or perhaps I'm doing some granny egg sucking teaching ?

a sillyscope smilely,,only test eqipment i've got at the moment is a signal generater.and i dont know how to use it.i,ve always wanted a sillyscope,maybe someone still has my grandfathers in their attic.

In which case, get hold of a high voltage capacitor, only 1-5 microfarads is needed. Connect it in series with a cheap speaker, and connect the other terminal of the cap to your ht supply ( connect the other speaker terminal to the psu earth ). The cap blocks all DC, and any AC is audible as a hum. If the level of hum is much lower than that through your main speakers, the p/supply is not the problem, so try the earthing tips. If the hum is similar in level, then it's your capacitors or choke.

Warlie, you mad dog! You know no fear!

I agree with what he's saying, james, but these voltages are gonna be big and will certainly knock you across the room or even worse if you're not careful. By all means try what Warlie's saying, (I would add that you should make sure to observe the correct polarity of your blocking capacitor, by the way), but don't (egg-sucking?) move the connectors around while the amp's on unless you're wearing rubber gloves. Another good tip is to keep one hand in your pocket so that any shock you do get can't go through your heart. Sounds like drastic precautions, maybe, but there's not enough of us in this club yet to risk losing a member!

Seriously though, your mate's not far off the beam. Capacitors dry out and therefore reduce in value, so that means their hum-reducing ability is impaired. And even if they don't dry out, sometimes the polarized ones (which is most of them in a valve amp) lose their polarity orientation if they've not been used for a while and cause all sorts of problems on switch on. So caps are always worth replacing.

Valves wear out as well, of course, and it's not uncommon to get a low resistance, or even a short, between heater and cathode, which can lead to bad cases of hum. One old trick used to be to put a "hum-bucking" variable resistor across the two sides of the heater supply with the slider connected to ground. You could adjust this for minimum hum, so it might be worth looking to see if there's one of those.

Lastly, if it's a really old unit, sometimes the speakers didn't have a permanent magnet round the voice coil, but used a second coil energised by the power supply. An elegant solution to hum used to be to feed a little bit of AC from the heater chain into this energised coil, but in antiphase. That way, you could tweak the amount fed in and cancel out the hum. So you may find a control for that as well. I always think of that as a technology that was abandoned too early before it had had a chance to benefit from modern research, a bit like the steam engine!

Just had a bit of fun tonight (house to myself) listening to Thomas Dolby's "Aliens Ate My Buick" on vinyl. If any of you haven't heard it, I commend it unto your turntables.

not to worry been pokeing around these things,for some time.admitedly without a clear understanding of what everything does.alot of problems can be obvious ,resisters that are still smokeing or half burnt up,a little less obvious bad solder connections .thanks for the info ,don't know what to start on first,oh should i get to the point of replaceing a power cap,i know these things can give a shock long after a unit has been unplugged,what is the proper way to degauss or dicharge the stored voltage?

I usually just short them out with a screwdriver, which probably isn't the best thing for them. You should really use a low value (470R would do it), but it's kinda difficult to arrange that. I've never damaged a cap by using a screwdriver, put it that way. If you do use a screwdriver, you might have to short out more than once, as the capictor's own inductance seems to store some energy after the first bang. Or maybe some leaks back into it from the others in the circuit. Good luck anyway, and glad to kmow you won't kill yourself!

A 100 watt domestic light bulb connected to 2 thickish insulated cables is a very effective way to discharge almost anything without drawing a massive current.

That's true.

It's amazing what they get used for - I've heard of them wired in series with tweeters for PA units. The idea is that if they draw too much current, they heat up, and as they heat up their resistance increases sharply, so the current gets limited automatically.

The Studer C37 tape recorder, still a much sought-after machine, had a few light bulbs inside it, which I think were used to achieve a certain time delay for the record cicuits so that they wouldn't produce clicks on the tape when you did a drop-in during record. Something like that anyway.

I've even heard of people hanging them from the ceiling and lighting their living rooms with them. What will they think of next?

i was suprized to find what looks like the bulb out of the domelight in my car being used to fuse the tweets on a old pair of bose 301s i have.now it makes a little more sense.all my speakers may be useing this setup soon.i wonder out of all the light bulbs i could use which one might sound best.