Tannoy cheviot -x over rebuilds

[Brian28]

Gents , im a Mechanical engineer , not an electrical .. so i need some advice ..

I want to rebuild some Cheviot crossovers , is it as easy as buying the same value components, but better quaility ? Or is there more to it ?

[walpurgis]

They may benefit, but what makes you think the crossovers need rebuilding?

In any case, I'd suggest speaking to Paul at RFC (trade section). He has rebuilt these and can qadvise. He will probably have any parts you may need.

[Brian28]

Hi , just the fact that they are getting old now , and with better new componants would hope an improvement in sound quaility..

[walpurgis]

As I said. Speak to Paul.

You will find him posting here today about Tannoy crossovers.

http://theartofsound.net/forum/showt...527#post813527

[Reffc]

Brian

they need carefully, completely dismantling. Don't lose the spacers for the inductors (air gap spacers) whatever you do when dismantling and be careful not to snag/snap the inductor and autoformer wiring which is quite delicate. The switches need taking apart and cleaning which is fiddly but worthwhile. The only components I'd recommend changing are the caps as the originals weren't always very tightly specified and whilst they do the job (in the case of the later brown plastic bodied polys), there are better available for not a lot of cost now. You will find it a squeeze fitting everything in if you deviate from the box capacitors.

Something inexpensive such as Solens or Mundorf Z-caps even would better the originals and for not a lot of money. The other worthwhile upgrade is to replace the lighting flex with some fisual biwire cable and a new 4 pin connector, plus replace the pretty awful spring binding posts for new gold plated binding posts with some speaker wire to the solder inputs (0.75mm cross section to fit into the hollow solder pillars). If you've never tackled one of these before, best to allow at least a full day to carefully refurbish them. Resistors are fine and do not really benefit from change. The PCB is really quite poor quality compared with what can be made today hence not worth investing on boutique components.

[Brian28]

Hi Paul , i was just reading your trade posts ..

Thanks for taking the time to read mine ..

I havent delved into the cabinets yet , probably take a look tomorrow ..

I have heard they are wired inside with almost bell wire .. i was going to keep as with all my interconnects ,and rewire the internals with .8mm solid core pure silver wire ..

You said the PCB's are quite poor quaility , so i was wondering if you would supply a kit of parts , new PCB's and components for me to build .. ?

[walpurgis]

Paul will no doubt answer, but I think silver wiring inside Tannoys is likely to be unwise. Silver is prone to highlighting higher frequencies and 0.8mm is really too thin for speakers in any case.

[Reffc]

Silver wiring is unwise for a reason most don't even consider. It has a nasty habit of corroding badly at junctions where exposed, due to hydrogen sulphide in the atmosphere in very small amounts (mainly) which blackens it eventually. I really don't understand why people still think that there's some mechanism for it benefiting SQ. It really offers no benefits whatsoever over a good OFC cable except for being a little more conductive and only then if it's a genuine pure silver wire. Contrary to popular belief, making less than 5% of the total speaker circuit in a silver wire (ie the bit connecting crossovers to drive units) cannot, in any way, affect the signal response in terms of modulation or filtering to change the sound somehow. It doesn't work like that. What silver can do, if in longer runs and sheathed in a ptfe is create conditions for tribolectric charge due to the valencies of silver and the ptfe dielectric. This theoretically can cause ringing in upper frequencies but get the dielectric right and it has no effect at all on frequency response. Most wire you buy is anything but pure as well. Most of it is a Nickle Silver (Sterling Silver). If you look at the losses per metre run in say 0.75mm OFC, it is so minuscule as to be barely measurable with the average multimeter. I would strongly advise sticking to a multistrand OFC of 0.75mm or 1mm. You don't need it any thicker for that short internal run.

Back to the silver. Just about all the wire you buy will be Sterling Silver, as 3 nines plus pure silver is usually very soft and malleable and can break very easily in small thin strands, hence it is often alloyed, sometimes with Zinc but more often with Copper, usually 92.5% Silver to 7.5% Copper to produce Sterling Silver. For higher voltage, higher current systems circuits such as loudspeaker circuits, it offers little to zero benefit over a pure OFC, but more drawbacks. For that reason, I would not recommend it but use it by all means if you have some and a good silver solder. It will make no difference to what you hear whatsoever given the equivalent geometry to a slightly thicker OFC. The voice coil in your Tannoys uses many metres of very fine copper and is part of the exact same circuit. I can't speak for amplifier design, but this is my advice as far as speaker circuits goes.

The original wiring was actually 3-strand lighting flex of about 20 or 21 AWG. It is worth replacing because by now, it may be corroded (it wasn't a pure copper like an OFC), so any quality 0.75mm OFC will do. A good solution is the Fusual bi-wire (4 - core) but you need to trim a little from the termination thickness (ie remove some of the strands) to common the earths at the PCB end of the standard crossovers, and to solder to the 4 pin plugs. Whilst I agree with Geoff that less than 0.8mm or so is generally too small for loudspeaker circuits, that is usually when applied to speaker cable runs of more than several metres with lowish impedance speakers. Your Cheviots have a minimum impedance of something like 7.2 Ohms and are easy to drive, so half a metre to 750mm within your cabinets will be absolutely fine done in 0.75mm. If you wish to use SE amplifiers, I would always recommend you minimise system losses where possible and stick with a minimum of 1mm within the speakers.

As far as a kit of parts is concerned, I have provided these to people previously, selling the parts at RRP cost, made little to nothing on the deal and had one of my designs stolen and used quite unscrupulously (commercially) after doing someone a good turn, so I hope you understand if I no longer offer to do this. What I can do is to provide the caps and resistors you need if you want to re-use your autoformer and bass inductors, and I can provide the PCBs for that, You would have to drill holes for fixing the autoformer and inductors. You then hard wire (do away with the switches) using the electrically flat setting which I can help you with. Consider it a project.

There is nothing wrong though with just refurbishing the originals. The real benefits in response lie with another route entirely hence it would be far simpler to just refurbish the originals. The benefits to response with the HPD 315 aren't fully realised anyway unless they are used in a much larger cabinet. You can re-tune and improve the Cheviots and actually, the best benefits come not from boutique parts, but from getting the cabinet tuning correct and improving cabinet stiffness, and reducing resonance.

Here are my tips:

Tear out the internals (all of them) from the cabs. Buy yourself some "Silent Coat". This is a superb vibration deadening material made from a butyl rubber visco elastic polymer. As it is not marketed with "hifi" in the tag, it is also very affordable! Line the rear and side plus top and base of your cabinets but NOT the front baffle. Next, introduce a ply brace cut so that it ties sides, back and front together. Place this just below the drive unit, no more than an inch from the frame, secured with glue to the cabinet (before you do the damping). It can have a large hollow in the middle so only needs to be an inch or so wide by 18mm thick. Failing that, use hardwood battens to effect the same thing.

Next, carefully remove the port. Tannoy tuned the cheviot far too low and it behaves more like a sealed box as a result, with none of the loading benefits at low frequencies. The cabinet is too small for the port length to be effective. It needs to be shortened to just 55mm from front face of baffle to back of the port. That is the correct port length for that enclosure and that driver. It will improve "slam" and bass energy appreciably.

Next, replace standard foam (mattress foam which was never a lot of cop) with the more efficient 50mm wedge tiles (coeff' > 0.75 @ 1KHz ) to the rear, sides, top and base.

Finally, remove and refurb your standard crossovers.

This will yield maximum benefit for minimum spend and targets key areas of acoustic response which replacing a copper wire for a silver one will have zero effect on, same goes for using boutique components. Once you have listened to the improvements, the next worthwhile improvements are gained by externalising your crossovers. If at that stage, you want to try a PCB solution, I'd be happy to assist you.

Hope this makes sense and is of help.

P.

[Arkless Electronics]

There is no point at all to silver wire (and obviously I'll never agree that it sounds bright! Old wives tale and technically impossible for a type of metal to boost treble) as it is only slightly better a conductor than copper and so one can just buy thicker copper wire and save a fortune.

[pgarrish]

I'm sure Paul is right about not using fancy resistors.... however it might be worth replacing them all anyway simply because they might not be the right ones! we found a few odd components on my crossovers when they were rebuilt including caps and resistors of the wrong values....