Pre-amp Circuits for turntable MM or MC reproduction
Apologies for the lack of updates over the last few weeks, our scribe has been off ill for a few weeks. Here’s to a new session of articles and hopefully a little bit of invigoration to our framework.
The true value of Vinyl
It’s amazing how times change – I was recently given a treasure chest of CDs, DVDs and records (vinyl) from my brother whom has left our shores for the UK and guess which media carries the most value?
In line with this and the comparative interest from local folk to listen to vinyl (read: huge interest) we thought it relevant to add some words on the subject.
Firstly, years back double album sets cost about R12.00 (in the mid-70s). Nowadays one is looking at about R600.00. A school leaver would be earning about R150.00 – R200.00 in the mid-70s, nowadays R6000.00. So no big deal there but why spend 10% of your salary on a new album? Because as a teen that’s what we did, there was nothing else to spend your money on except wheels of course but you were possibly too young for a licence. (a car cost about R4000.00 – R5000.00 in the mid 70s). It’s all relative of course except nowadays by searching through the web you can download and pirate most tracks of your favourate artist all for free. Do they sell stolen cars on the web? Most probably.
To play your vinyl you will need a good turntable, in South Africa this will vary in price and if you are lucky you should be able to pick up something in the vicinity of about R2000.00, moving up the ladder for not something possibly better but something which can be easier to repair or upgrade through kits, like the Technics decks.
Boost that signal without added complications of that dreaded RIAA
Pre-amplification comes in a few flavours, mostly chip based and with RIAA signal correction applied. Enthusiasts roll the chips to get the best listening experience. Some build discrete transistorised amplifiers believeing them to sound better. I won’t go into debate here except to say that a lot of this is user perception and of course, your own hearing. I hate to be told that “such and such sounds better” because that’s the biggest yarn in the book – if the specs tested through good quality equipment is tremendous then as a rule it should be, but watch the room acoustics.
For me tube preamplifiers and power mosfets do the trick.
In our teen years in the 70s I thought a friend’s sound system kicked butt. It was a 25W per channel semiconductor system. No names mentioned in case you owned one. As a family we had only an Akai M8 and a mono 15W 2xEL84 amplifier which I used for teen parties.
Vinyl, tubes and warmth
In reality it was the 15W tube amp which kicked butt, Mona Bone Jakon sounded much better on the tube amp as opposed to the transistorised amplifier. Even today I find Cat Stevens music comes alive through tube amplification – this may very well be a perception, much of what I heard through transistorised equipment in the early years sounded dull and lifeless. It’s definitely not the same today of course and to be strictly honest, budget audio in the early years didn’t necessarily give transistors a great name yet there were audio circuits which still to day are revered. John Linsley Hood is a typical example of such genius in his designs.
And of course, loudspeakers share a great deal of the blame as well.
OK, so where does this leave us?
Building a tube preamplifier is not going to break the bank. A big problem in most cases is that your amplifier nowadays does not have a turntable preamplifier – the output is a measly 5mV or so so you will need to either buy one or make one. If you are making one don’t be deterred by the “experts”. There are many chip preamplifier circuits which are supposedly budget but have really great quality – again my preference leans towards tube preamplification, possibly the ECC83 as a first timer’s choice.
Noise – or rather F%@C*(! why?
There’s a couple of things one will learn about building sensitive audio equipment which can really rile one as opposed to designing and building high powered amplifiers. The most obvious is going to be noise, hum, hissing, you name it.
Most tube preamps use DC to feed the heater filaments. I have had preamp equipment which used AC and as a factory built item one can only hazard a guess how they get it right but when one builds their own, it’s often a freaking nightmare. Understanding certain principles at the very beginning is only half the battle, putting it into practice is another. Using DC is often the solution before you pick up problems.
Having a separate power supply in tube equipment or batteries for chip preamps also cuts out hum unless you know how to reduce hum by rotating the transformer or screening. The worst for me is building something which works just fine … for an hour and then slowly the hum becomes worse and worse until it’s totally unacceptable. Here again, removing the probably cause is where to start – look at toroidal solutions for transformers.
Microphonics can be a bastard of a problem if you don’t have the necessary mechanical damper. Always go for the best quality. Of course this applies to tubes as well – don’t believe because the tube is hot off the press that it is a good working tube, especially nowadays – tube rolling wasn’t an art in the 60s, enthusiasts knew which tubes to buy and for what, certain tubes from the USA were in high demand, likewise UK, Dutch, Italian and Hungarian equivalents etc.
Because you are building a tube preamplifier remember the golden rule – the simpler the better. If you are starting of as a complete Rookie please learn the fundamentals first – impedance matching. Rudimentary rationale = output impedance feeds higher value input impedance.
Next: Balancing the head signal