Concerning DC-offset From Vintage Multibit DAC

This post is about sharing my experience for the present of DC offset at the output of a DAC, especially when we build it by our own, we want to make sure it is safe to the equipment after it. Since the DAC is considerably the top-chain in the audio system, so it is important to look this issue in the first place.

DC offset is a nightmare, especially when the equipment after the DAC such as preamp or the power amplifier is not using any capacitor or transformers in the signal path to blocking the DC present at the input. Just a small amount of DC, let's say 100 mV at the input of an amplifier will multiple by the gain from the amplifier, plus its current, will harm to the speaker's driver. In most cases, it will burn the driver's coil.

DC is inaudible, but it moves the driver cone. It creating the heat to the woofer driver coils and burn it. It is safe for the tweeter since the capacitor included in the cross-over network inside the speakers will protect it, act as DC blocking to the driver.

This post will explore the vintage multibit DAC that naturally have DC offset at the analog output and how to solve these problems.

Types of vintage multibit DAC

The type of vintage multibit DAC can be divided by how the PCM data is converting to analog. This method will be easy to understand what DAC type that naturally produce DC offset at the output.

The R2R or resistor-to-resistor ladder DAC is the most common and popular name for vintage multibit DAC type. This data conversion technique is simple to implementing, but very difficult to make it correct because the resistors tolerance is very critical to the data conversion. This tolerance will impact to the linearity and sound quality.

I will list down DAC manufacturer that implement this R2R technique, which are limited for serial data input only, such as:

1. Burr-brown (now under Texas Instrument acquisition) starting from PCM56, PCM58, PCM61, PCM63, PCM69, PCM1702, PCM1704.

2. Analog Device such as: AD1856, AD1860, AD1862, AD1865.

Surprisingly only those two manufactures produce the vintage multibit DAC in R2R basis.

R2R DAC type is free from DC offset.

So, you can build DAC using those IC without worry about DC offset present at the analog output. The output DAC offset will always stay at zero. Just use a straight wire to connect from the DAC IV stages to the RCA output, we are ready to listen the music.

Although we still can find almost all CD player from big manufacture which using those R2R vintage multibit DAC IC's still using capacitors at the output for DC blocking.

What is the reason for that?

These capacitors not only to protect the equipment after the CD player's DAC to avoid any claims from their customers because of any components fails that creating DC-offset reason, but the main reason because those CD players always including the muting circuit at the output to avoid any pops and noise sound from the CD tracks changes. The present of those capacitors is to protect the op-amps IV stages from short-cut to ground as the muting circuit does when active.

The second type of vintage multibit DAC is Dynamic-element Matching (DEM) which only came from Philips DAC, such as: TDA1540, TDA1541, and TDA1543.

DEM is not R2R DAC! Please don't mixed up.

DEM is bit divider, not using resistors (unlike the R2R DAC) but instead using transistor as an active divider, except in TDA1543 which only using the passive divider. DEM is more advanced because it has better linearity, avoid the issue on miss-matched resistors in the R2R DAC above.

This DEM DAC always have DC offset at the output.

The DC offset can be various from 200 mV up to 3 volts which need careful attention to null those DC.

How to eliminate the DC-offset?

As I know, there are 3 ways to eliminate this DC offset and the simplest way is using capacitor. Capacitor is passive components that using insulator or dielectric between the two conductive materials. The nature behaviors of capacitors are stores energy, that is why after the capacitor is always suggested to use resistor to ground as shunt-voltages.

The main problem using capacitor is because it colored the sound. So mixing and matching capacitor very important to make sure less coloration to the sound quality.

We want to use film capacitor instead of electrolytic capacitor because it is better and more linear, but life is not perfect. Film capacitor is very expensive and bulkier than the electrolytic capacitor.

So, few electrolytic capacitors for audio that I can recommend to use are:

1. Elna Cerafine (Red-gold mark)

2. Audio Note bipolar (gold-black mark)

3. Audio Note standard (black-gold mark)

4. Rubycon Black Gate STD, F, NX (very expensive)

If capacitor at the signal path is not your cup of tea, maybe use transformer can be the answer.

Using the transformer to separate between primer and secondary will null the DC offset. Transformer can be used for increase the gain, but I prefer use it as buffer, without any gain to avoid any additional noise.

Again, using high-quality transformer like Lundhal or Sowter transformer is not cheap, mixing and matching also require to make sure less coloration to the sound quality.

The other method to eliminate DC offset is put DC servo circuit in the signal path by using transistor and trim-pot. This active circuit takes the opposite voltage polarity to adjust the DC in the signal path to become null. This method also has some drawbacks.

First drawback: using DC servo is not 100% can eliminate the DC offset.

In my case when trim the DC servo in my TDA1541 DAC, one channel can set to null but the other channel keep stay around 160 mV. I also found that both channel DC servo trim is interrelated, which when one channel adjusted will impact to the other channel. The balance for both channel at 33 mV which still have DC offset.

Second drawback: we don't want any components in this DC servo fail. Let's say if the transistor or the trim-pot fails, then huge amount of DC will pass through to the signal line to the output of the DAC.

In summary, we are not living in the perfect world.

The DEM type DAC is an advanced type of DAC that eliminate the non-linearity problem in the R2R type DAC but the DC-offset caused from this DEM need to null by using non-linearity components such as capacitor or transformer.

While the R2R DAC is more popular but the matching resistors inside this DAC become the problem to manufacture and the result is inequality sound performance from one to another.

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.