Hacking your HT or audio Pre-amp

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Rebuilds and Tune-ups for Pre-amplifiers

There seems to be a stigma about buying lesser known brands. The fact that someone designed a device at home doesn’t mean it’s inferior. I had a Rotel amplifier which vaguely looked like a WooFlungKak and inside it actually looked the same. In fact it was the same except for the power supply where many short cuts had been taken. Taking out the bits, strengthening the supply and re-case the whole lot and you have a good home brewed amplifier. But I am looking at pre-amplifiers here, a commodity which isn’t that common on the shelves in thrift stores. After a few nasty surprises through Gumtree I decided on Crusaders and Converters.

Get an amplifier for about R500.00. Whip out the power output devices, slip in a NE5532 or better and you have a great sounding pre-amplifier. Audiophiles may cringe at this but look at it this way. Most older amplifiers have turntable inputs. Nowadays they are not that common. Buying a pre-amp kit costs about R200.00 and with PSU one would be looking at over R300.00. And it looks like shiite. Taking an integrated amplifier and doing a few hacks here and there to get just pre-out may not necessarily be cheaper but it’s going to look a whole lot better.

Remember the NAD 3030 and 1020 – have a quick squizz here. I have a Pioneer A-5 which I use to drive a Behringer amplifier.

The above mod is nothing new neither unique, it’s just a standard divider chain feeding an XLR socket. At half volume with no input there is barely an audible hiss. Although this amplifier is old, antiquated by today’s standards it took no recapping, the switches are in excellent condition but what is more important – very good quality reproduction. Typical of Pioneer and Kenwood of that era. By modern standards these amplifiers lack punch but who cares.

Need a DAC for your new pre-amp? The one above goes for about R300.00-R400.00 at BidorBuy and TakeaLot. One can take the board out of the box and mount it inside it’s new home but this may not be advisable, the box acts as a screen.

Tube preamplifiers

One can buy a good quality preamplifier from Mars Amps (keep it local) or look around for a faulty tube amplifier. Most of the older stuff had a turntable preamplifier built in, the power tubes may be knackered or the output transformer has shorted HV turns. (the old play ’em without speaker syndrome). The Akai M8 is a good starting point if you only want the pre-amps. Although the M8 uses two 6W mono block amplifiers one doesn’t need the EL84 output stage (don’t test without load).

Ebay always has a solution but stick to the tried and trusted – this means 100% positive feedback.

Marantz 7 schematic — Marantz 7 basic schematic

Ampslab image of original 7 console amplifier.

Hybrid tube and semiconductor

There is a lot of genuine interest in the tube versus semiconductor, tube front-end, semiconductor power and vice versa trains of thought and audio practices. Having personally experimented with the different configurations my pick would be tube phono preamplification followed by MOSFET output stage. There cannot be any factual observations here, mood, ambience, lighting and your hearing ability all play a role in what sounds best.

In conclusion: A good quality power amplifier is often easier to manufacture in many cases than a device used to boost that of a phono signal. Low noise and accuracy is best from a dedicated pre-amplifier. With modern components and spec sheets available some of the best is home brewed, whether FET, BJT or tube. The best bang for buck and eye candy is often right under our noses. And mostly, they just work.