Dansette Major De Luxe volume problem - 2

[Michael Maurice]

Is there any reason why this shouldn't work as a power supply run off the heater winding?

[julie_m]

That would certainly be what my first effort at a power supply would look like .....

My own Dansette Major De Luxe power transformer has just 3 leads on the secondary, so one side of the heater is definitely at chassis potential.

[awc]

Have made up a power supply as per Michael's design in post 41 and found it gives about 8v on load. The mains transformer in this player though stills gets very hot (too hot to touch) so may look to power it up separately.

My final assessment of the two variations of modifications to the original Maplin Kit favour the re-routing of the output pot rather than replacing R2 with a reduced value resistor.

Although this means a bit more work on the circuit board, I find that swopping R2 for R9 and having R2 as a pot and coupling the output direct to C2 gives the greatest flexibility.

Going the other route and leaving R9 as is and just changing the value of R2 seems very critical and may mean a different value of resistor is going to be needed to suit an individual cartridge/amp combination.

I am attaching a couple of photos showing both sides of the altered circuit board.

I have found that there is quite a critical point of adjustment needed to balance the point of max volume versus distortion. When sorted however, I found the sound quality (for a Dansette Major) to quite acceptable.

[Hartley118]

I think you're nearly there, though you're still being cruel to the mains transformer (and a rectifier diode).

Michael's power supply circuit will work well for you, but you'll see that it has 3 output connections: +V, 0V (GND), and -V, whereas your original circuit board has only 2 power supply connections: +V and GND.

So I suspect that at the moment you have -V and GND connected together which is short-circuiting the transformer secondary through a diode. No wonder you're hearing sizzling!

What I suggest you now do is go to your amplifier board again and make a couple more changes:

1. Disconnect pin4 of the op amp (the -V pin) from GND by cutting that bit of PCB track so that pin 4 is isolated.

2. Connect pin 4 of the op amp directly to -V on Michael's power supply. That will stop the short-circuit and the sizzling transformer.

3. Remove R3, which is biasing the op amp to work from a single rail supply. Michael's circuit gives you a split rail (+ and -) supply, so you now only need R4 to tie the op amp input to GND.

That should now work with a cool transformer and, with a +9V & -9V split rail supply, should be capable of a good 14 volts peak to peak AC output to drive the EL84, making its gain setting less critical regarding distortion.

If it gives trouble, check the health of the rectifier diodes - one may have passed away in the service of experimenting!

Martin

[awc]

Hi Martin

Oh dear, so I'm still in trouble!

Just to confirm, the connections to the heater taps on the mains transformer are to take the grounded tap to the junction of the + and - of the two capacitors only, with the floating tap connected to the two diodes which in turn go to the other two capacitor connections which then supply the respective + and - connections on the opamp. In my ignorance, I had thought that as the grounded tap only went to the capacitors and not a diode that the original problem would be sorted. What do I know!

Please let be know if you have comments on the above and I'll try out your suggested further circuit changes.

Thanks again

Alex

[Hartley118]

Yes that's right Alex, but don't forget that the junction of the two smoothing capacitors (marked 0V on Michael's power supply circuit) is to be connected to GND on the op amp board.

The top diode on the circuit produces the +V supply (probably about +8.5V relative to GND) and the lower diode the -V supply (probably about -8.5V relative to GND).

Best of luck,

Martin

[awc]

Hi Martin

Thanks for your further response.

Before I take the plunge and make the circuit changes you suggest, could you please confirm the following:

1. Given what you say, does the rectifier circuit as wired up presently, still suggest that there is an overload on the mains transformer?

2. If I do connect the 0v to ground, will I still need to make your suggested modifications?

Thanks

Alex

[Hartley118]

Hi Alex,

1. I'm not absolutely sure how you've got the rectifier wired at the moment, but there's a suspicion that you've got the -V supply shorted to ground. If you now wire the op amp circuit to the power supply as I've suggested, the extra load on the transformer should only be a few milliamps - nothing much compared with the valve heater current. If you see overheating, go back and check your wiring.

2. Yes, the 0V connection on Michael's power supply circuit should be connected to GND on the amplifier. You still need the previous modifications to the op amp circuit in order to make it work from the + and - 'split rail' supplies.

Martin

[Herald1360]

If you want a single sided positive supply use the circuit below- it'll give about 16V with 10mV ripple at 5mA load.

[Edward Huggins]

My God - this thread (now at Post #49) is driving me Potty. Why not go and buy a proper Cartridge and a nice bottle of Red Wine......??!! Simples. Edward

[Hartley118]

AFAIK, there's no 'proper' cartridge available today that will give the 5V rms or so necessary to fully drive an EL84 output stage. And, if there were, I guess record life would suffer severely because of its inevitably poor compliance and high tracking weight.

As in many such instances on this forum, it's leading Alex into an interesting journey of op amp circuit discovery which hopefully will turn out with a useful solution that amplifies the output of a readily and cheaply available ceramic stereo cartridge to drive a single stage amplifier.

Please don't knock it Edward. It's the way that we learn.

Martin

[Edward Huggins]

Yes I know, I know.....
As usual I veer towards the polemic to as to create some grit in the oyster within these threads.
But you can still import high-output cartridges - e.g Varco from the US or cloned TC8Hs from Australia....and these keep it very near to the original. You don't need 5V - no cartridge can deliver this. 1V was all that was ever needed by Dansette and all those others. And if you do still have a Dansette Major with its BSR UA8 or UA14 autochanger, record life will suffer anyway! Edward

[Hartley118]

I guess that would be a cartridge output of 1V rms per cm/ sec recorded velocity? If so, then a typical groove velocity of 5cm/sec would give 5V rms - enough to to give decent output from an EL84. And I suspect that your average pop 45 would exceed that!

Martin

[awc]

Hi Martin

The rectifier circuit is wired to the mains transformer in parallel with the existing wiring to the heater of the EL84, as per the top one of the attached diagrams with the grounded tap going to the junction of C1 and C2 and the floating tap to the junction of the diodes and the opamp then connected to the + and - on the output sides. This gives about 8v smoothed DC, C1 and C2 being each 1000uf.

This does work nicely when tweaked with minimal hum pickup, except that the mains transformer gets exceptionally hot. It obviously has already done so in the past, as wax has oozed out of the bottom. This is in my experience not an uncommon phenomenon, due in no small part to the silly location of the transformer, wedged as it is up against the side of the box and right underneath the motor board, with minimal ventilation.

The opamp is wired to the tagboard on the turntable and to the outside tags on the volume control, with the wiper going to the grid of the EL84 in the time honoured fashion.

Am I therefore overloading the heater winding with the rectifier wired up this way or is the mains transformer on the way out?

Thanks again

Alex

[Hartley118]

That sounds OK. You should be measuring approx 16V between the + and - terminals on that top diagram.

Are you getting that? If not, there's something wrong.

Martin

[Herald1360]

The top circuit should give 8-0-8 volts- fine for a dual rail opamp. Makes the input side easy- no need for capacitors. Probably best to capacitively couple the output though to the valve grid. I can't see how a few mA could have any noticeable effect on the transformer.

You will get trouble if the -ve side output is connected to chassis, however.

[awc]

Decided to go back to the beginning and take some voltage/current measurements. The first thing I measured was the ac voltage to the EL84 heater. 4.3 volts ac. Just in case, read it again with another meter. 4.25v ac or 4.8v with the valve removed.

Disconnected the new rectifier just in case it was pulling the voltage down. Read again. Still 4.3vac No wonder the dc on the other side of the rectifier was 8v instead of the expected 12vdc. Proves one thing. You can run an EL84 with only 4.3 volts on its heater. Time for a replacement mains transformer me thinks.

[Hartley118]

Does the transformer still overheat with the new rectifier removed?

Martin

[Edward Huggins]

In spite of my (provocative?) comment at Post#50, sooner or later some enterprising Member (maybe even a Newbie) will make a small batch of non-RIAA, single-chip, self powered, x4 amplification op-amps to resolve the growing problem of "no-more-high-output-cartridges". Given that importing a near equivalent type costs $45/50Euros - such a device could sell for £25/30 and future-proof many older portable players. Built cost, £10-12? Nice little earner! Edward

[awc]

I think we may be there.

I started by removing the grounded link to the opamp rectifier, but the voltage across the heater tags remained at 4.3v until I also removed the two diodes connected to the floating tag whereupon the voltage jumped up to the expected 6.3v. Replacing the EL84 only reduced the voltage a small amount.

I then disconnected the input to the grid of the EL84 from the opamp and tried it out. The voltage stayed the same at 6.3v and the amp seemed to operate normally.

With the op amp still disconnected, I reconnected the opamp rectifier, the heater voltage remained at 6.3v but the smoothed dc output rose to the expected 16vdc. So far so good. On the basis that the problem must be due to a short of some sort being caused by the ground return on the opamp, I inserted a .1uf capacitor on the opamp board between the ground output and the lead to the main amp pot. There is already a 1uf capacitor in the signal lead.

I checked the mains transformer after playing an LP and although it did get hot, I could keep my finger on it for a couple of seconds, previous it had been too hot to even touch.

I think I now have a solution to the medium output cartridge into a single valve amp problem. I need to try it out with a AC/DC player using a UL84 etc. but I am proposing the power the opamp in that situation from a scrap mains transformer from an old power supply of which there appear to be plenty knocking about.

Thanks to all who contributed to what at times became a somewhat lively debate!

Alex