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Pro audio equipement such as mixers and sound interfaces work with signal levels referenced up to +4 dBu whereas consumer audio equipement (such as hi-fi amplifiers or cd players) works with signals refereced up to -10 dBu.

The following diagram represent a very easy to build transformer that will let you connect the output of a ‘pro’ mix-table to an ‘consumer’ amplifier for instance. Please note that I deliberately left the output to be balanced. You need to make sure the the input of the ‘consumer’ equipment you are connecting the output of this circuit to has a balanced input. If it hasn’t simply connect pins 3 and 1 of the output together to make the line out unbalanced.

Notes: 1 dBu is the voltage level which delivers 1 mW of power in a 600 ohm resistor. dBu and dBv mean exactly the same; dBv came up first but it was confusing people with dBV so dBu is now in use.

I used the following schematics from Jensen Transformers to make the diagram above:

• Jensen pro to consumer interfaces
• Jensen Line input transformer 4:1 stepdown for balanced bridging inputs

I found it very difficult to buy Jensen transformers from UK but luckily I found an alternative with OEP audio transformers (I used an OEP A262A2E wired in 4:1).

Intrinsically, a transformer will not introduce ground loops and other undesirables to your sound path! And it won’t make your sound sound weak like resistor attenuators tend to do.

It would also be very easy to rewire the transformer into a 1:1 configuration if all you need is to de-couple two systems together without introducing ground loops (like in a sound stage to front PA scenario for instance)…

End result with the circuit above (x 2) housed in an aluminium box

Following my previous Powermac-mini post, I would like to should you how I wired and coupled the mini to the powermac.

The challenge here was to have the ATX power-supply put inside the powermac to start at the same time the mini is started (and to stop when the mini stops) and also to get the front LED and buttons of the powermac to work with the mini. Grincheux’s dedicated blog powermacmini.wordpress.com gives all the info you need to do all that so I won’t go too much into details about it.

Bellow are pictures showing how I modified the original powermac quicksilver’s front buttons board in order to be able to wire it to the mini. The idea is to wire the power-button in parallel with the reset switch so that it’s easy to wire the reset wires to the on/off momentary connector on the mini’s main-board.
Then on the other side I wired the two unused wires on the 10 pin IDC connector straight to the LED.

Conclusion

I am thrilled with this mod. The powermac quicksilver case looks stunning under my desk, I’ve got plenty of room for hard drives, the mini runs cooler and I can’t hear it any more (if you’ve got a mini sitting on your desk you know what I’m talking about)! Oh and I’ve got a PCI-express 1X expansion slot!

After having swapped my mac mini’s internal SATA hard-drive for an external 3.5″ one I got really pleased with the performance boost but also progressively more and more annoyed with the noise produced by the bigger hard-drive, sitting right next to me on the desk.

A few month later I happen to browse Grincheux’s powermacmini blog and got instantly seduced by the concept: Replacing the guts of Powermac G4 with a mac mini! Why not any other PC case for this mod? Well, it’s mostly a question of aesthetics: I really like look of powermac computers.

I chose to experiment on a Powermac G4 quicksilver and it didn’t took me long to get one from eBay. You will find bellow a few pictures of the body work and installation of the mac mini inside the case. For more detailed information please jump to powermacmini.wordpress.com.

Hole cutting

The first thing that had to be done was digging out a hole at the back of the G4’s case in order to be able to connect things to the mini when it is positioned inside.

Internal support

Then, I started converting an old aluminium plaque into a support that would maintain the mini in place inside the G4 case. The left picture bellow shows one of my first satisfying attempts. The one the right shows the final version of the plate (with the paint totally removed so that the plate can act as a passive heatsink for the mini by dissipating the heat that builds up on the mini’s rubber pad). I also cut a section out the aluminium plate to make room for a PCI express 1x female socket…

Powermac optical drive issue

I didn’t foresee something quite frustrating: with the mini mounted like illustrated on the pictures above, it’s impossible to close back the Powermac’s case since the end of the optical drive conflicts with the mini. In the end I was forced to cut a small portion of the mini out. This is the only irreversible modification I performed on the mini, but although irreversible, it doesn’t interfere with the mini’s operation at all, and in fact you can even put the mini’s case back on it and you can’t see a thing (unless you look at the mini upside down).

The VTB1 preamp from Studio projects is a great piece of kit. I use it to record guitar and vocal tracks. It has a handy tube blend control that lets you how much of your sound you want to be going through the transistor section and how much you want to go through the valve.

As you can see form the pictures of its electronic board, the VTB1 is built to last. And very well designed.

Tube swap

Out of the box the VTB1 comes with a Chinese 12AX7 tube that tends saturate a bit to rapidly for my liking. It wasn’t long before I replaced it with a ECC803 S from JJ Electronics. The 803 S valve has a much smoother transition into clipping and slightly compresses your sound before distorting.

IC swap

I am happy with the way the stock transistor section of the VTB1 sound but there are a few discussions on the web about swapping some the ICs on the VTB1 for better components. I haven’t tried any of these yet and I’d love to hear from you if have tried this.

Blue glow

Second mod I applied to the VTB1 was to unsolder one of the ends of resistor R30 in order to permanently turn off the two blue LEDs glowing behind the tube. The reason I did this is the LEDs are powered with AC current and are very close the tube. Surely it isn’t a good thing to have an AC circuit radiating right next to the tube! Second reason to turn the LEDs off is I didn’t like the way they flicker at 50/60 Hz (being AC powered). Turning the LEDs off also reduces stress applied to the power supply.

On/off switch

There is in my opinion one design flaw with the VTB1: it doesn’t have an on/off switch! Well, not anymore. There is just enough room in between the 12VAC and the line out XLR sockets at the back of the preamp to add a toggle switch!

As you can see on the picture bellow, adding an on/off switch is a bit of a dirty job since you’ll have to cut two tracks coming from power socket, then solder two pieces of wire form these tracks to the on/off switch.

Bellow is a picture of the end result.

I dicovered the ART DI/O last year as I was looking for a external DAC/ADC to extend the number of I/O on my external soundcard using the two SPDIF input/output connectors.

What attracted me in the DI/O is it was a very good quality and cheap DAC and tube driven ADC, with plenty of room for mods/improvements. The DI/O uses 7V (+4dBu) analogue input & output levels which is fine in my case since I connect it to a Mackie mixtable… It wasn’t long before I got one from ebay…

The unit rapidly became a new reference for me. This DAC outputs tight bass, well defined mediums and realistic without being overwhelming highs. Just the sound I’ve been looking for!
A few weeks later I didn’t enjoy listening to the analogue output of my powerbook anymore and realised the Airport Express’ analogue output - although very descent - produced tiring muddy bass. In comparison to the Art DI/O, the analogue out of my Tascam soundcard at the time sounded very good, but lacked a bit of presence in the mediums.

The next logical step was to route every digital audio stream available through the ART DI/O’s DAC. On one hand, I could have spent loads on a descent coaxial S/PDIF switches (the expensive ones re-sync the streams and have transformers on all I/O to avoid ground loops), or go optical and use an inexpensive 3 way optical switch to switch from my CD Player, to the Airport Express, to another optical source!
The problem with the optical solution is that the ART DI/O doesn’t have any optical input, hence you have to use a converter to transform the optical signal into a ‘coaxial’ one…

I found an litle box at Maplin that just do what I wanted. It’s a TOS Link to coax digital converter (the picture of adaptor should appear on the right of this paragraph). The little box worked a charm but wasn’t very convenient since it requires a power adaptor (and my mains extensions are overcrowded).

I managed to resist the temptation to open the SPDIF converter for a week. I wasn’t surprised to find almost nothing inside: just a few ceramic capacitors, one electrolytic capacitor, a diode, a few carbon resistors and a small IC.
I sometime wonder if people don’t design stuff for other stuff unintentionally: the optical convertor board was just the perfect dimensions to fit vertically inside on the sides of the ART DI/O.

Components that can be removed form the TOS Link convertor board:

• The red LED (D3) and it’s resistor
• The diode (D1)
• The DC in socket
• The cinch socket (coaxial output)
• The 96 Ohms output resistor (R3) it’s important that you remove this one otherwise this resistor will be connected in parralel with the digital input terminator resistor of the ART DI/O and you will had a perturbed digital signal!

Bellow are a few pictures of the end result:

I’ve summarised the mods in a PDFs: ART DI/O optical TOSLINK input mod diagram.

Other mods

As far as I read across web, most people use the DI/O as a DAC, in beetween their CD player and their HiFi amplifier. In that case you are only using half the features of the little black box and you can very easily improve its performance by removing the 12AX7 tube (tube generates a lot of heat => hotter enclosure => hotter DACs => more noise generated)!

The first mod I made on the unit was to add an On/Off mini-switch next to the AC in socket.

I also upgraded the power transformer with on of bigger amperage (I honestly couldn’t hear any noticeable improvement).

On the digital dauther board of the ART DI/O, I replaced the C9 capactior (4700uF 50V) with one of 10000uF in order to improve the locking capacity of the DAC when used in ‘Ext sync’ mode. This is really a must try of you use the dac in that mode! Before I changed it, turning off a light in my flat would cut the sound comming out form the DAC. With this doubled capacity the DI/O almost never looses doesn’t lose sync in ext-mode!

Last but not least, stock ART DI/O units have a 100 Ohms digital input resistor (R10) terminators, which should be 75 Omhs according to the SPDIF specifications (and if you use a 75 Ohms coaxial cable). I replaced this resistor with a metal film 0.6W 75 Ohm resistor (it is located on the I/O board close to the digital input sockets).

Evaluation

I’m really pleased with the results. It’s impossible to use both the optical and coaxial inputs at the same time but it doesn’t bother me too much. I can’t hear the difference between the SPDIF and Optical inputs. And I freed one mains socket under my desk since I no longer need to power the SPDIF to optical converter!

It’s really surprising to rediscover songs… just use a different (better) DAC an you’ll hear subtleties you never heard before!

Hope you enjoyed this post.

Benjamin.

Links:

• Summary of ART DI/O Mods on Audio Asylum.
• Digital audio converters at Maplin’s