Tandem Photography

Turntable Balanced Upgrade (Part 2)

So the turntable itself is back in one piece and working well. Its easy to fall into the trap of thinking, hmm, I’ve spent time and money on this now so it must sound better. I’m sure there’s little if any difference so far, though it may be running a bit smoother, the cables don’t look like crap and the power button doesn’t stick any more.

On to the sound…

The pre-amp kit has arrived from the UK, but before I build that we need a power supply for it. Rather than get an off-the-shelf switched-mode supply I opted to build a linear supply. I needed +/- 15V for the pre-amp. No current draw to speak of, but it does need to be fairly well regulated.

The design is a 240-30VAC torroid with a bridge recitifier, a regulator for the positive and negative rail and a few smoothing capacitors. This gives a +/- 15V output. Fairly simple…

This all goes in a beautiful grey plastic box. Look at the awesome dremel work  yet again. And it even has a red neon lamp on the front that comes on! It does work though, with both rails running at pretty much bang on 15. The pre-amp design actually calls for, ideally, 17V to get the most headroom. 15 should be enough… though if not its going to mean swapping out the torroid and/or the regulators. I’m never, ever, going to attempt to take this thing on an aeroplane.

I'm sure the TSA would love seeing this in my hand baggage

The choice of the 4-pin XLR is rather strange, but what the PCB for the pre-amp uses so I stuck with it for the source end. Also the heat-sinks are currently missing from the regulators until I can pick up some thermal paste.

Now onto build the pre-amp itself…

Balanced Turntable Pre-amp Upgrade (Part 1)

Disclaimer: Not in any way related to photography, but bleh…

A few weeks ago I ‘acquired’ some old-fashioned, black vinyl (actually one of them is red) records.  One of the main aims of this was to digitise some of this content (for some rare-ish recordings I can’t find in any digital format), so the aim here was to modify the turntable to give a reasonably good reference environment, with a balanced high-level output stage I could connect to an external firewire/USB audio interface on the PC. I didn’t own a turntable so tried to acquire one. I ended up picking up a mid-80′s vintage Technics SL-QX200 off of eBay for next to nothing. Apologies to any audiophiles reading this; I’ve got no interest in audiophile snake-oil ‘tweaks’; this project is entirely about getting the most bang for the buck!

Here’s one looking in better shape than mine:

So much Chrome...

I’d read that the early-mid 80′s Technics were well manufactured bits of kit. The SL-1200, while much used by DJs was orginally conceived as a reference turntable, and this build quality is retained in a lot of their other higher end bits of kit from this era.

The one I picked up, while good, had a few problems.

1. 30 years of dust and dirt
   
   Easily fixable with a screwdriver, some isopropanol, a cloth and some cotton buds. Having had the thing in bits for a week or so now though, I’m not 100% sure if I can remember where all the screws go. Fortunately, I managed to acquire the service manual. 

   The power switch was full of gunk, and was stuck. Not any more! Someone has also removed the original feet from the chassis at some point and replaced them with some nasty rubber coasters. These might need swapping out to improve the damping.



Not sure I can remember which screws go where...

1. Pre-amp/RIAA stage The turntable will be running an MC (Moving Coil) cartridge. Due to the lower weight, these are known for having a superior frequency response/dynamic range than a typical MM (Moving Magnet) cartridge. The downside is the very low (100-200uV) so we need a pre-amp stage before we can think about doing an A/D conversion.The audio interface being used is a MOTU UltraLite mk3.  This has 2 low-level, balanced  instrument (or phantom powered mic) inputs which we can use to test, but even then the gain stage on these isn’t enough.
   

   MOTU UltraLite mk3

   The second problem is the lack of an inverse RIAA filter stage. This reverses the pre-emphasis applied during the original mastering of the record, boosting the bottom end and cutting the top end.

   We can implement this filter on the MOTU, kind of, as it has a parametric and shelving EQ feature we can apply on the input, but I found it difficult to match the RIAA curve with any precision.

   So, the solution is an external MC/MM pre-amp with RIAA equalisation, preferably with a balanced output stage to cut down on noise as much as possible. The MOTU has 8 line level, balanced inputs, so we can certainly make proper use of it.

   Rather than designing a pre-amp myself I’ve cheated and gotten hold of a balanced pre-amp in kit form. You can see the specs on the manufacturer’s site here at Signal Transfer.

   

   RIAA balanced pre-amp

   This isn’t the one I’ve built as I’m still waiting for the bits to come in from the UK. Time to get back up to speed with the soldering skills….

1. PSU/Cabling Upgrade
   I’m not a big fan of over-priced 110% OFC free, directional, skin-effect resistant, needs 6-months to break in audio cable, but the bits of string attached at the factory, given 30 years of use were looking pretty nasty. Same for the power cord. I decided some better screen cable for the low-level outputs to the pre-amp and some EMI filtering on the mains input, with an IEC connector replacing the permanently attached cord would help improve things a bit. 

   This also allows to use the 3rd pin of the IEC connector for the ground connection. I haven’t tested this yet, but I’m hoping that everything in the system is using the same ground, there won’t be any loops set up. For now, the chassis and tone arm have been connected to the mains ground. We’ll see how well this works or if we need to break this connection and use an external ground.

   The rear of the chassis where the mains cable entered was dremel’d, the new connector (with an EMI filter in series) bolted in, and soldered to the torroid. There might be some scope in adding some more filtering to the PSU, but I really need to take some before/after ripple measurements on the DC side.

   Lastly, the existing RCA phono cable was removed and a better one connected. I did toy with the idea of installing a set of RCA connectors on the chassis (the original was directly soldered to the wires coming from the tone-arm), but as we’re dealing with low-level signals I though best to try to keep the number of physical junctions down, removed the RCA connectors from one end of the cable and soldered this to the same point.

   This is an awful iPhone photo this morning, but this is how it looks so far…

   

   Check out the awful, awful metal work

   Next, to re-assemble it, build the pre-amp and see how it sounds…