Dansette Major De Luxe volume problem - 2

[awc]

I have been looking for an (easy) answer to the seemingly perennial problem of a suitable preamp to boost the output from the cheap and readily available generic "red" cartridges to a level sufficient to drive a EL84 and its ilk to an acceptably loud level.

There is a closed thread of the above name in which a preamp kit from Maplin using a LM358 op-amp was suggested. This appears attractive, not least because it is an easy to assemble kit costing around £6.50, but will apparently also run off of a wide range of voltages i.e. from 3v to 25v.

I duly made this up and tried it out, using a 9v PP3 as a power source. The result was predictably (because of the impedance mis-match) disappointing, as although it "worked" there was no apparent increase in volume.

In the closed thread, Herald 1360 suggests a redesign of the circuit to improve the impedance mismatch. I have built this on the original kit circuit board, it was in practice very easy, just requiring a change of 3 resistors and 1 capacitor and moving components around to suit the new design.

Although this does achieve the desired effect of increasing the output volume considerably, unfortunately this is at the expense of an unacceptable increase in distortion as well, not reduced by turning down the level of the on-board pot.

Can anyone suggest any further design changes that will keep the increase in gain whilst reducing the distortion level?

Thanks in anticipation.

Alex

[ms660]

FET input plus whatever.

Lawrence.

[awc]

Here is the circuit in question from Herald 1360.

[ms660]

I know zilch about OP amps so I wouldn't know what the input impedance is, someone else might clarify.

Basically a high impedance cartridge needs to see a high impedance load, the grid impedance of the EL84 won't be fussed about the source impedance, it just needs enough signal voltage to provide the expected output at minimum distortion.

Lawrence.

[julie_m]

The input impedance is dominated by R3 || R4 (any PSU should be effectively a short-circuit at AC), so about 1 MΩ.

[ms660]

Thanks for that, that's the input impedance sorted.

Just the drive to the EL84, any figure for the output of that preamp for typical cartridge input?

Lawrence.

[Hartley118]

I think that your op amp is overloading. Preset pot R9 can't correct it because it's located after the op amp.

I suggest that you use R9 as a variable feedback resistor to control the gain of the op amp in order to stop the overloading.
First remove R2
Cut the three PCB tracks that connect to pot R9, connecting the output connection feeding the main amplifier direct to C2 instead of via pot R9.
On the now isolated pot R9 connect a wire between the wiper S and track terminal A
Then wire pot R9 tags A and E to the two connections where R2 came from (between pins 1 & 2 on the LM358 op amp)

You should now be able to control the gain so that it no longer overloads.

Martin

You should now find that

[Edward Huggins]

Look why don't you import a BSR TC8H from the supplier in Australia, £55 or buy a rebuilt one here for £44 from "Adriantrax". It will come with a Mono stylus but you could fit a TC8S stereo stylus here, knowing that the vertical compliance will be poor. Best to use on older stereo LPs. Edward

[Michael Maurice]

Easier still buy a X5M it has a very high output.

[Edward Huggins]

Regret not good advice, the BSR X5M is only a Medium Output cartridge (c.250/300mV) and will just not be suitable. Edward

[Michael Maurice]

With all due respect Edward, I've been fitting these to Dansette's for some time, the resulting sound is always loud enough and I've had no complaints.

[awc]

I am proposing to have a go at the modifications suggested by Martin (Hartley118) but before I do, could I please just ask him to confirm that in his description of the work to be carried out, that his reference to "track terminal A" is the same connection as reference to "R9 tag A"

Many thanks

Alex

[GeoffK]

Be easier to replace R2 with a variable resistor and keep VR9. The AC gain is set by R2/R1.

[julie_m]

Actually, in non-inverting mode, the gain is set by (R1 + R2) / R1. If R2 is a dead short circuit, then the gain will be equal to 1.

The output of an ideal op-amp tries to assume whatever voltage it has to, in order to make the input voltages equal. By potential divider action, the inverting input will see the output voltage times R1 / (R1 + R2), and this must also be equal to the voltage at the non-inverting input. So the gain = (R1 + R2) / R1.

[GeoffK]

Thank you Julie, knew it was something like that, have to take a refresher course on Op Amps.

[Edward Huggins]

As you know the Dansette Major was designed to be used with high-output cartridges, c. 1 volt. I must agree with you in that I know you can use an X5M and it will give an adequate enough output on 78s and 45s, but on LPs (especially lower modulated stereo pressings) you need to advance the volume to full to get enough sound and this has a tendency to emphasise any hum present at such high volume settings. Edward

[Hartley118]

Originally Posted by awc

I am proposing to have a go at the modifications suggested by Martin (Hartley118) but before I do, could I please just ask him to confirm that in his description of the work to be carried out, that his reference to "track terminal A" is the same connection as reference to "R9 tag A"

Yes Alex, my reference to terminal A is the same as R9 tag A.

Best of luck,

Martin

[regentone001]

Originally Posted by Michael Maurice

Easier still buy a X5M it has a very high output.

I have to agree with this. I have an X5M in my Dansette Popular,I actually did a double check as I thought I had fitted an X5H, it is quite loud enough,too loud to turn the volume up full and I have never had to adjust the volume when changing from playing a 45 to an LP of vice verse.
Steve

[awc]

Right. I have carried out the modification suggested by Martin and have to report a substantial improvement. Not exactly Hi-Fi, but I reckon perfectly acceptable. The control over the gain is about right, a controllable balance between volume and reduced distortion. Final assessment of sound quality will have to wait till I have put the bits and pieces back in the box.

To reiterate, the cartridge is a generic "red" costing around a Fiver, with the Pre-amp kit from Maplin at £6.50 plus a couple of pence for two resistors and a cap. I am hoping to knock up a suitable power supply from the heater tap on the mains transformer to substitute for the PP3 currently in use.

Many thanks to everyone who put their thoughts forward and in particular to Herald1360 and Harley118 for their invaluable contributions to resolving the problem.

Alex

[Hartley118]

Glad the gain control mod worked.

One further suggestion: I think your 9V supply voltage for the op amp is on the low side, meaning that the op amp will overload before the EL84 can get to maximum output.

According to the LM358 data sheet, the output can only swing to within 1.6V of its supply rails. That means that with a 9V supply, the maximum output voltage swing will be (9 - (2x1.6)), or 5.8 volts peak-to-peak.

Now an EL84 in tip-top condition needs about 12 volts peak-to-peak input signal on its grid to give its maximum output, so we have a problem. The solution I suggest is to double up on your PP3 battery by connecting another one in series, giving your LM358 an 18 volt supply (it'll cope with up to 32V). With an 18V supply, the available output voltage swing if you wind up the gain will now be (18- (2x1.6)) or 14.8 volts: plenty to drive your EL84 .

Martin