Simple opamp or FET circuit to boost ceramic cartidge output

[pszolk5948]

Can anybody help, please?

I have a Dansette single valve (UL41) record player.
I was wondering if anybody has a schematic for either an op-amp, transistor or FET amplifier to boost the output of the cartridge. The player works but it's not very loud. The amplifier has been fully serviced so HT is correct, valve replaced etc.

I have a few cartirdges but not the high gain type.

Any help would be gratefully accepted.

Peter.

[paulsherwin]

There are lots of solutions depending on how high you want the quality to be. For a simple Dansette record player a very simple one transistor preamp should be adequate. Use a high gain low noise BC109 or equivalent, 2M2 resistor between base and collector, 4k7-10k in the collector, 0.1uF coupling caps in and out. You will need a resistor in the input to match the high impedence of the cartridge, something like 470k-2M2 - you can fine tune it to get the gain you need. You can probably find a DC supply from across the UL41 cathode bias resistor.

An FET would have higher input impedence but you don't really need that much gain. Similarly, an op-amp would probably be OTT for this application unless you have suitable parts to hand.

[Later] In case it isn't obvious, I've quickly drawn a diagram.

Paul

[Brian R Pateman]

Paul's single transistor solution is the way to go.

It will also have the advantage of running for years on a PP3. If you go for an Op. Amp. solution the power supply might be rather morer complicated as you may need split rails. The battey supplysolution is ideal as it introduces no noise. Any manis derived supply unless very well smoothed will introduce more hum and noise than enough.

[paulsherwin]

While there are advantages in running this from a battery, it should be easy enough to get the power from the UL41 cathode, maybe via a 10k resistor and a hefty smoothing cap (1000uF say). I wouldn't expect there to be much hum from this arrangement, and it will be much more convenient.

It would also be possible to derive power from the HT line via a suitable resistor given that the preamp will only be drawing about 1mA.

Paul

[Sideband]

That's almost identcal to an arrangement I used for years when I needed a similar pre-amp. With a 9V supply, the gain was far more than I needed and I dropped it down to about 3V. I reckon Paul's right about getting the supply from the cathode of the UL41. You'll have about 8 - 12 volts available so plenty to play with. 10K sounds like a good starting point for the feed resistor but you will need to decouple that as Paul suggested to keep the impedance of the supply as low as possible. Aim for about 3 -6V supply to the transistor and if you use something like a BC108, you'll have stacks of gain.

I would suggest that in the interests of keeping the input impedance correct for the cartridge, connect a 1Meg resistor in series with the input capacitor and connect a 100pF capacitor in parallel with the 1Meg. You can play around with the 100pF to give the amount of treble that you require.

Rich.

[evingar]

Originally Posted by Brian R Pateman

Paul's single transistor solution is the way to go.

It will also have the advantage of running for years on a PP3. If you go for an Op. Amp. solution the power supply might be rather morer complicated as you may need split rails. The battey supplysolution is ideal as it introduces no noise. Any manis derived supply unless very well smoothed will introduce more hum and noise than enough.

I have to confess to personally not liking batteries all over the place. Paul's solution is reasonable - Just to be difficult, I personally like OP-Amps as well. They are a good, easy way of getting a low distortion amplifier with very stable gain. There are techniques that can be used with standard op-amps to operate them from single rails. The following document from Texas Instruments gives good coverage of the technique :-

http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf

Additionally, there are Op-Amps specifically designed to work from single rails, such as the http://www.national.com/pf/LM/LM324.html

[kalee20]

Originally Posted by Richard_Newman

With a 9V supply, the gain was far more than I needed and I dropped it down to about 3V... Aim for about 3 -6V supply to the transistor and if you use something like a BC108, you'll have stacks of gain.

Definitely agree with getting the supply from the UL41 cathode. A battery is so inelegant.

However I don't like the idea of controlling the gain by reducing the supply to 3V or so. The UL41 will need a couple of volts of drive, so you need at least 6V of supply to your transistor amplifier if clipping and horrible distortion are to be avoided. Far better to reduce the gain by increasing the feedback around the transistor (add a series 0.1uF and 1M in parallel with Paul's 2.2M), or increase the 470k input resistor.

[paulsherwin]

There are lots of op-amps that could be used for this application and performance would be better than the single transistor circuit in several respects, but I would argue that it isn't worth buying in components to do this when most people will have a suitable silicon transistor to hand. The improved sound quality is unlikely to be audible in a Dansette.

I mentioned a BC109 transistor and this is ideal, but any general purpose small signal silicon transistor could be used. Just swap the power rails over to use a PNP type. The 2M2 bias resistor should be reduced to 470k-1M if the transistor has significantly lower gain as the bias requirement will be different.

Paul

[Paul Stenning]

The general rule with a circuit like that is that the voltage on the transistor collector should be about half the supply voltage, so adjust the value of the 2M2 bias resistor to achieve that.

[Jonster]

How about keeping it thermionic, and changing the UL41 for a UCL82, rewiring of course and using the triode section as a pre-amp?

[kalee20]

If you use a PNP, as per Paul Sherwin's post, swapping the supply rails is a bit harder when using the UL41's cathode voltage as a power source - but it can still be done.

I'd thoroughly recommend a BC109 NPN as he said earlier.

Incidentally, is the cartridge itself OK? It would generally be a high-output type, capable of driving the UL41 directly. So if volume is lacking, there could be a cartridge problem which may get worse. In which case, you may need to change it in the future, and you might as well use a lower-output, better quality one (which will definitely need the amplifier we're discussing).

[Dr-Watts]

No disrespect, but why not fit a 'propper' cartridge ?
When I do a restoration I use these guy's...
www.musonic.co.uk/cartridges

Hope this is useful

Dr. Dave

[pszolk5948]

Thanks to everybody that replied to my query. I am very grateful and will use the information provided. I will try the BC109 circuit first and see how that goes.
(I would have replied sooner but a fault with my router held things up.)

Once again, many thanks. Great site.

Peter.