Kierkegaard
Electronics

During the work for Sokrates I have become a real fan of differential circuits because of their striking advantages namely

- no hum, buzz, click, pop and less masking of the signal because of greatly decreased ground noise
- less cross talk

In total differential designs result in a much cleaner signal.

Ground noise, ground loops and grounding techniques are always blamed when it comes to noisy signals. However, ground noise and loops are virtually unavoidable in HiFi installations and so the noise will always contaminate the signal of single-ended circuits although that might not be obvious right away.
So instead of blaming the inevitable, what about using the proper technology to render the issues harmless ?
Differential circuits and signal transmission simply leave no opportunity whatsoever (if done properly) for ground noise to combine with the signal because ground is not used as a signal return path like in single ended designs. Instead there is a third and dedicated wire not carrying all the dirt that is to be shunted to ground.
Anyway, this passion has now been re-ignited by an article in
Linear Audio Vol 5 by Bruno Putzeys about analogue differential signal processing.

During the recent years I have missed all this notion about chipamps although they really come with a set of nice features such as overload protection, thump-free start up/shut down and thermal sensing and tracking. This is all good instant stuff, just add water.
Although there are already about one zillion designs out there, some of them are simply overdone and others are missing basic but essential stuff like proper supply bypassing, let alone the PCB layout. Also, what I have not come across so far (without looking too hard especially in the deep dark bowel of the usual suspect forums), are differential designs (and I hope there are no good reasons for that).

As a result of all this I have designed three different configurations to cover the typical use cases. The buffers are always LM4562.

There is a single configuration for small woofers, midrange drivers and tweeters:

single

A bridged amp for 8 Ohm woofers and subwoofers:

bridged

And a parallel design for 4 Ohm woofers and subwoofers:

parallel

The sound is really stunning and compared to single-ended amps the fine details of music, microphone position during the recording and embedded reverberation tails come through much cleaner and more defined.

These chipamps have another huge advantage: You have much better control about the gain structure of your system starting out from the pre-amplifier through a cross-over into the power amps.
The gain of the LM3886 can be as low as 10 times. This in combination with a low impedance design greatly reduces hiss, which needs to be observed in balanced topologies. It also constitutes a lot of good feedback with all the known advantages such as low distortion, lower output impedance etc., etc..

The only drawback compared to class-D is the requirement for larger heat sinks. However, with a gain of only 10 times and a supply volatge that just fits the needs of typical speakers, the LM3886s run surprisingly cool (luke warm).

In total, these amps are an extraordinary bang for the buck. They are the reason why I sold all my single-ended SymAsyms !

 

Measurements (parallel configuration as an example)

Parallel_9W_4Ohm
Parallel_9W_IMD

Last updated 27-Mar-2017

THD+N, 9W into 4 Ohms

The peaks at 100Hz and its multiples is ripple from the unregulated power supply.

CCIF/ITU-R 19KHz+20KHz IMD