Pre-amplifer for magnetic phono pick-up.

[Geordie McBoyne]

I've installed a Garrard SP25 Mark 2 record deck in the RGD 1046 radiogram. The pick-up is a Shure M75 which suggests a low output, let's say 20mV.
So there is a requirement for a pre-amplifier, I've got a few circuits to hand and it seems the valve of choice is an ECC83 arranged as a two stage amplifier.
RIAA equalising included in the circuit for magnetic pick-up. But what about using a pentode valve instead? We know the noise generated by such a valves will be higher than that of triodes, that's the way it is with multi electrode valves. However, more gain can be realised by the pentode and as the valve is driven from a fairly low source impedance, this fact alone might make the pentode valve a viable choice. Your views on this subject?

Geordie McBoyne.

[Terry]

I recall once building a 2-valve pre-amp to a Mullard design - it's probably in the book with all the other audio amp designs.

All I remember is it had 2 x EF86 and switched equalisation as required but the noise level was so high that I abandoned it!

I think I built a solid state design from my Wireless World diary instead!

[ppppenguin]

Sounds like a job for a couple of transistors or an opamp. Power either from the heater supply or even dropped from the HT with a zener to stabilise it. Apart from lower noise and hum, another advantage of solid state is less loading on the mains transformer.

If you must use a valve then look at the Quad 22, or Leak/Radford/Rogers designs of the 1960s.

[Mike Watterson]

Valve preamp for magnetic cartridge is seriously challenging, without an expensive pair of transformers. A cheap modern single rail low noise op-amp off 5V regulator (rectified 6.3V heater is about 8V) will now outperform most 1970s discrete transistor designs. About £5 for ALL the parts ...

[ppppenguin]

For a moving coil cartridge it's almost impossible without a transformer. Quad and others got good results with moving magnet cartridges such as the Shure M75 series mentioend above. Having said that, a Shure M75 of any kind isn't the ideal thing to stick in a SP25 whose pickup arm isn't good enough for the cartridge. Depends a bit on the stylus. A 75ED would be hopeless, a 75EJ might be OK. For a magnetic cartridge in a SP25 think about a Shure M44 or a Goldring G800.

Just go solid state, you know it makes sense.

[Cardigan]

*duplicate post deleted*

[Cardigan]

For a moving coil cartridge it's almost impossible without a transformer. Quad and others got good results with moving magnet cartridges such as the Shure M75 series mentioend above. Having said that, a Shure M75 of any kind isn't the ideal thing to stick in a SP25 whose pickup arm isn't good enough for the cartridge. Depends a bit on the stylus. A 75ED would be hopeless, a 75EJ might be OK. For a magnetic cartridge in a SP25 think about a Shure M44 or a Goldring G800.

Just go solid state, you know it makes sense.

The standard fitment in later Garrard decks was the basic Shure M75-6S. An M75EJ is a no-no but an M44 should be okay. Similarly, a Goldring G800 would be a little close to the edge, the G850 was more suited to the SP25.

I'd also add that the arm on a Mk. 2 SP25 is little better than most of the arms they were fitting to their contemporary autochangers at the time; it's the same as the arm on an AT6. A better solution would be a Sonotone 9TA/HC ceramic cartridge worked in 'velocity' mode with a suitable equaliser. The 9TA/HC is one of the best ceramic cartridges ever made.

If you wish to use a pentode, I'd recommend - assuming a 6.3V supply is available in a 'gram using 4V valves - a Brimar 6BS7. Designed for microphone amplifiers and cheap. http://www.r-type.org/exhib/aab0164.htm

[Valvebloke]

Valve preamp for magnetic cartridge is seriously challenging ...

This, I'm afraid.

It can be done though. The quietest circuits I've ever come across used triodes. If you have a stock of them then you may gain a lot by working your way through them until you find a quiet one (or two, if you want stereo and don't want to split a double triode between the channels). Back in the day the criterion only had to be that the noise from the pre-amp was rather less than the surface noise on the record. In the absence of a record the pre-amp can sound quite noisy. But once the stylus touches down the record noise dominates.

It goes without saying of course that circuit design, wiring layout and construction need all to be of the highest quality. The bass boost that the RIAA correction applies will otherwise generate an enormous amount of hum.

VB

[Mike Watterson]

Mind you, my 1935 HMV radiogram is a moving iron magnetic cartridge!
http://www.radiomuseum.org/r/hismasters_370.html
The moving iron Pickup coil  uses unbelievably fine wire, with about 42500 Ohms (42K5!) on about a 15mm bobbin about 5mm tall winding. It was open circuit, but it was where the internal bobbin wire connected to lead out wires had corroded, so I was able to replace them!
   
The "moving iron" isn't the steel needle, but a bit of metal attached to the needle holder. I found a bamboo "needle" worked very well.
It's a very heavy assembly.

If I was connecting an SP25 deck to a valve amp, I'd be tempted to use a ceramic cartridge?

Certainly it's nearly 40 years since I assembled DIY decks & cartridges, so don't listen to me. I have a Dual 55, and I presume it has the correct cartridge as it came from a deceased BBC Recording Engineer that became a Lawyer (before he died).

[Eddystonepete]

Hi Geordie, I,ve got a wee pre amp you can have, for free which would match the M75 into a flat response input of about 200 mv. Its called a Btech BT26 , google it to see if it would suit. It needs 12v DC preferably isolated to avoid hum loops. Yours for postage.

Peter