Analog Output Options, Capacitor or Transformer? That is the question...

Introduction

From all the equipment I built so far, there has always been one major factor in sound quality and that is the use of couple capacitors. Many say (rightly) the best capacitor is no capacitor, and where you can actually do this, you should. Even in the case of this DAC, if the next stage you use actually has input capacitors, you would not need them at the DAC side of things. Normally this is not the case however, so you need some way, to produce an output without any DC left over. I have been trying with DC servos in the past, but they make things unnecessary complex and on top you have instead of a capacitor a power supply in the signal path, so no 100% solution, one way or another.

The capacitor

OK, let's accept for the moment, you need a capacitor. Than there are only two questions to answer, how many uF and what type of capacitor....? Well, the last part is VERY tricky. Going through web sites and audio forums, it seems every one has his own personal choice. Like myself ;-) So what comes next is my personal view on things.... I used many many couple C's and from high level, it is pretty safe to say, that Electrolytic capacitors in general should be avoided, the are cheap and sound cheap. Grainy and loose a lot of detail. At least you should use a film cap. Now many many types are available and I would say, use your budget as guidance. But also the following point of view; I know people spend far over 100 Euro for Interlinks, which might be a good investment beyond doubt, but THAN, I would also invest that sum of money for highest possible grade output capacitors. There is no hard rule here what is the best, some time a complete chain benefits a smooth old oil capacitor when the amplifiers and speakers are bit to pointy or the other way around. Long story short, I prefer the Mundorf silver gold oil ones. They are just so neutral, so 100% detail rich, so good :-) They have their price, but totally worth it!

The standard Mundorf capacitors I deliver with the Kit are very reasonable compromises. They sound pretty good to start with and will give you an immediate good feel of the sound quality of the DAC and the clear differences between CD quality and HD sound tracks. From there it is truly my advice to start experimenting and see what improvement you can achieve in your setup by using some high(er) end capacitors.

What Value you need? Now for the value... Specially with the high end stuff, you do not want to make the value unnecessary large, as it will unnecessarily drain your wallet! The value is only to be determined by the classic RC formula, telling you where you will have your low frequency cut-off. Cut-Off meaning 3-dB level with 45 degrees phase shift. Actually you want this to be 5 to 10 times below the actual lowest frequency your system can reproduce. The formula is very simple: Fc = 1 / (6,28 x R x C) Where C is the value of your Capacitor in Farad and R is the input impedance of your next stage in Ohm To make things simple, at the left I show a small tool to find the desired value... Highlighted is a practical example where 10kOhm and 7 Hertz will require a couple Capacitor of 2,2 uF. This also tells you, that the same capacitor with a 100kOhm input will deliver a cut-off frequency clearly below 1Hz. I think it explains itself. Click for larger Table Of course this table can be used for every other RC cut-off calculation you might need between amplifier stages or what ever....

A helpful table: Just look up the input impedance of the next stage, pick your -3dB point for low frequencies and voila, see the value you need.

The Transformer Output Actually the Sowter Transformer was meant mainly to help on a 4x times higher output. I was prepared to make a compromise on the sound. After the listening tests, conclusion is, that it is almost the other way round. So the transformer is the one to stay and is a great option. An output transformer, where there is some residual DC left over, even if it is only few mV, is very hard to design. I talked with few great names in the audio world and to be honest, only Brian Sowter was ready to try this. Thanks to Brian, as the result is outstanding! At the right you see the original test plot of my prototypes. The achieved bandwidth is breathtaking. Nevertheless the high frequency roll-off is enough to smooth out the signal. Actually this leads to a blacker background and very liquid sound. Important notice, to achieve the needed low output impedance, you need minimum 4 DAC decks for decent low frequency response.

Original Plot from Sowter: This plot shows the great frequency response of the transformer. This was measured with 10k Load and 40 Ohm input impedance

Transformers are a complex technology and I will not go in deep detail. nevertheless I wanted to show the basic specs (there is more of course;-) for the technical interested readers. They will see, that this is far from being a standard transformer. This one has been pushed to the limit and worth every penny (literally, as Sowter is in the UK ;-)

Basics of the Sowter transformer

• Ratio: 2.000
• Turns: Prim 240 / Sec 480
• Core: Gapped Mumetal, Package: Mumetal Can, Termination: Color coded leads
• GENLOAD @20 Hz Sec. SC: 10 ohms Max
• GENLOAD @20 Hz Sec. OC: 429 ohms Min
• LEVEL for THD=0.5% @ 20 Hz: 10.2 dBu Min
• Primary inductance (H): 3.4 H Min
• Primary resistance: 1.6 ohms Max
• Secondary resistance: 35 ohms Max

Transformer dimensions

• The can is 72mm X 63mm x 45mm
• The brass grommet in the center of the 63x 45 side and protrudes approx 15mm from 72mm dimension
• The diameter of the brass grommet is 9.5mm.

DDDAC Testreport with AP

• High Bandwidth from below 20Hz till over 200kHz with almost no phase shift in the audio band
• At full scale output with the DDDAC1794 (1,2V rms) THD below -100dBr (equals 0,001% THD) for 1kHz
• At full scale output with the DDDAC1794 (1,2V rms) THD below -70dBr (equals 0,03% THD)for 50Hz

Keep reading ...... the story continuous with other opinions ( testimonies )