Dansette Major De Luxe volume problem - 2

[awc]

Thanks for the further replies. There are really two issues it seems to me when looking for a universal solution to our little problem, i.e. a solution for players with fully insulated mains transformers (as with this player the Dansette Major De Luxe) and everything else i.e. those with auto transformers or AC/DC supplies.

I agree the 100nf cap in the case of the Dansette Major and its ilk isn't required, but I was hoping to construct a universal preamp which could be used for all sorts of players without component changes.

One big problem is I think the appalling lack of safety by some manufacturers (including Dansette with their "Viva" and "Popular" models which have one side of the mains connected directly to the cartridge pins!) Even "posh" manufacturers like HMV weren't blameless, witness their Model 2009 where they went to great lengths to protect your fingers from a nasty shock with bits of plastic and rubber covering the "hot" parts! The deck is of course a Garrard RC121 MkII which has interchangable heads, so what happens when the head was changed for another with no such protection?

Is the consensus of opinon then, that the opamp or similar preamp way round the low output cartridge problem on single stage valve players is only safe when used on players with fully isolated mains transformers and that the autotransformer - AC/DC players cannot safely be so adapted?

[Edward Huggins]

I would agree in principle that the fully isolated mains chassis are the most appropriate to use the pre-amp with. But of course there were just so many made by Alba, Portadyne, Fidelity et al - that didn't and so a "one fits all" solution is not possible unless it was completely insulated and battery powered. Edward

[ms660]

So what's the problem with a preamp powered from it's own transformer supply and used in a live chassis record player?

Lawrence.

[Hartley118]

I agree that with an isolating transformer, it should be OK. Instinctively, though, I'd be inclined to go for the ECL86 solution for a live chassis job. More electrically robust and possibly cheaper.

Martin

[wd40addict]

One trouble with this is that since the Russians and Chinese don't appear to currently manufacture an ECL86 they're getting a bit pricey.

[Herald1360]

Originally Posted by awc

Is the consensus of opinon then, that the opamp or similar preamp way round the low output cartridge problem on single stage valve players is only safe when used on players with fully isolated mains transformers and that the autotransformer - AC/DC players cannot safely be so adapted?

Not really. Adding a preamp with its supply of a mA or so derived from the HT supply of a live chassis set would not affect its original safety or lack of it as long as the cartridge is connected to the preamp input the same way as to the original amp input.

Going to a UCL82 or similar valve solution would be exactly the same, safety wise.

[awc]

I have now completed the construction of a preamp based on the further revision as posted by Herald 1360 in post 71, using the voltage doubling power supply as shown on the attachment to that post.

The revisions to the original Maplin kit now mean the printed circuit board supplied with the kit is no longer of any real use and so I decided to construct the revised design on a bit of good old fashioned veroboard which means that that components for the power supply can also be put on the board leaving only 6 wires to be connected up when putting the preamp in a player.

I am attaching a photo of the completed preamp, powered from a 6.3v filament transformer, which I am pleased to say seems to work quite well although I don't have a player with a single EL84 on board to hand at the moment to check it out properly.

I am buying some more Veroboard which will have 6 x 25 strips which will give a little more room, because as can be seen, the prototype is a little crowded!

Will report back again when I have built another and tested it with a EL84 player amp.

[Hartley118]

Well done Alex. It looks very neat. I agree that by the time you've extensively cut & strapped a PCB, Veroboard is a neater solution. That original Maplin kit made a good starting point for your project though.

Martin

[awc]

I have made up a further prototype of the preamp (see attachment) on a 9x25 hole piece of stripboard and confirm that it is working just fine to increase the output from a medium output cartridge to a sufficient level to drive a single EL84 gram amp.

What I need to do now is find a safe way of using the preamp with a "live chassis" player using for instance a UL84/85 combo or any UCL type in a transformerless AC/DC circuit.

I have in mind inserting a .022uf Y2 rated cap between the "ground" connection on the preamp and the volume pot on the existing amp or where the output from the original high output cartridge would have originally been connected.

The preamp would be powered from a separate mains transformer, probably cannibalised from one of the power packs with a transformer in them used for old phones and similar gadgets, long since disposed of.

The preamp will accept input voltages from 3v to 32v DC, so any mains transformer giving a AC output from 3v to 11v will do.

Any thoughts?

Thanks

Alex

[Edward Huggins]

This looks really neat and unobtrusive. There is a ready demand for a small production batch of these. I can't understand why someone is not doing this already. A simple forecasting equation based on the number of extant single-stage players "out there" might mean it could be viable to make 50-100 of these a year. Edward

[julie_m]

You don't need a transformer to power the preamp, as it's going to be common with the live chassis anyway. A simple capacitor / Zener PSU, fed from the UY85 anode, will be fine.

[PJL]

Would it not be easier to replace the EL84 with an ECL86?

PS: I have one of these Dansette's and the cartridge is still working!

[awc]

Hi Edward. I agree that there probably is a demand for this type of easy to make and fit circuit. It isn't of course my baby so to speak, I have relied on others to recommend a suitable circuit, I have simply obtained the relevant bits and pieces and worked out their position on the stripboard. For various reasons I am not in a position to make these up for others to buy but I can give a list of components and source of same and where they are fitted on the stripboard so that others who want one, can construct it.

Hi Julie. Thanks for the suggestion regarding powering the preamp in an AC/DC amp. I am afraid you will have to help me out though, by providing a circuit with required component values, if I am to have a chance of putting the thing together.

Hi PJL. I agree it might be easier to substitute the EL84/UL84 for a suitable double valve in chassis constructed amps, but there is a real problem when dealing with amps constructed on a printed circuit board. Using this preamp does I appreciate introduce an element of solid state into a valve amp, but this is offset in my opinion by the fact that this preamp is easy to fit in a EL84 amp, requiring just the unsoldering of the original feed wire from the cartridge tagboard and the soldering in of 6 wires. This also means that the originality of the player can very easily be restored if a subsequent owner wishes to do so.

[awc]

Hi all

Attached please find the final circuit written up with a copy of the components required, in case anyone wants to get the bits and pieces in. I am working on the stripboard positions and cut-outs and will post these details as soon as I can.

Alex.

[julie_m]

Just another thought about adapting this for live-chassis players: There may be room to poke in an overwinding around the motor bobbin, and use this to supply power to your preamplifier module. It would need some experimentation to get the right number of turns (start with 10 turns, and aim for about 9 V off-load). Unfortunately, I've not got easy access to a suitable machine to try this myself right now.

[awc]

Hi Julie

Thanks for your further thoughts, but perhaps I ought to explain that the brief to myself when embarking on this project was to construct something that was simple to make with cheap, readily available components and which could be easily fitted to suitable players by persons, such as myself, of limited ability.

We seem, with the considerable help and advice from other members on this Forum, to have achieved this in respect of the EL84 preamp, so my only outstanding need, is to find out what modifications need to be made to enable the preamp to be safely fitted to auto-transformer and AC/DC amps using the same criteria.

Your previous idea with the capacitor/Zener PSU etc. seems to fit the bill, if you could show me the circuit changes needed to the existing amp.

Many thanks.

Alex

[awc]

Hi all

I have now completed the Mk6 version of the Ceramic Cartridge Preamp which just entailed moving a few components around to neaten things up a bit and minimise the need for wire links and track cuts on the copper side. I am attaching a photo of the completed board with component locations shown, together with a photo of the copper tracks and a component location list. If anyone spots any errors, please let me know.

I have to admit this exercise has made me fully appreciate the time and effort put in by those that do this sort of thing full time as even this simple circuit has taken up more of a chunk of my life than I ever imagined when I started and that's of course even with the invaluable help of others on this Forum.

I would be interested to hear in due course from anyone who has a go at making one of these, i.e.how they get on and their observations on its effectiveness as a solution to the problem it is designed to solve.

Most components were sourced from Bright Components on Ebay and RS, although they should be readily available elsewhere.

Thanks again to all who helped with the circuit design.

Alex

[Herald1360]

That looks like a very neat job now.

For use with live chassis type amps, a single rail version would be easiest. Probably powered from the HT via a simple resistor-zener shunt regulator. Use the same coupling/isolation arrangement from the cartridge as originally fed the valve amplifier input, so safety will be at least as good as originally.

Something like attached.

[awc]

Many thanks for your reply. The amp I have in mind to try out the live chassis adaptation using the revised power supply you suggest, scandalously has no protection for the poor user, as it allows a reverse connected mains lead to put 240v on one of the cartridge pins!

As you may recall, I was proposing to mitigate this by putting a .022uf Y2 cap in the ground lead between the output from the preamp and the input to the original amp. Would this do the job, do you think?

Thanks again for your invaluable help.

Alex

[wd40addict]

The diagram doesn't seem to currently show the strapping of the unused op-amp section which could lead to odd behaviour.

As the LM358 common mode input range includes Gnd maybe connect pin 6&7 together and connect pin 5 to Gnd?