AFW1R2 / Active 3-Way Crossover

this project / page is under construction

 

Inhaltsverzeichnis

Introduction

2018/08/01

As often happens in life, the test setup AFW1 has become a permanent solution and this crossover has been working very well for me for many years. It has also been successfully rebuilt several times. A development with which I am quite satisfied.

In the meantime I have been working on the topic active crossover from time to time. Some of these ideas I also published here and took them out again after a while – they have never been used. Some designs only became CAD data, others I built up and was not really satisfied with the result.

For the RQM-system I necessarily need an active 3-way crossover and so I took up the topic again in mid 2018. The result I named AFW1R2 because there are a lot of similarities with the AFW1 – the R2 stands for revision 2.

Of course the audiophile demand has grown in relation to the AFW1 and so it has become a 2 cabinet solution with a very complex unregulated power supply in one cabinet and a second one with the actual crossover, built with discrete operational amplifiers as I also use them in the DPV1.

Unlike the AFW1 this crossover is not flexible designed, it was developed for my application and has no external controls. All necessary adjustments must be made with the lid lifted – a result of the experience of recent years. I learned how difficult it is to adjust a crossover correctly and how fast you can then change something unintentionally with external controls. The crossover is levelled by me – thanks to my friend Heiner and his Acourate – exclusively with the help of acoustic measurements.

Description of the Electronics

2018/08/02

As with the AFW1, the filters of this crossover have a Salen-Key topology with 24dB Linkwitz-Riley filter characteristics. As active elements I use discrete operational amplifiers – similar to those in the DPV1. So that there is enough space for such an amount of electronics, the amplifiers are mounted on plug-in cards.

At this point I ignored one of my principles and built the new crossover – at least internally – asymmetrically. The input circuit converts the incoming balanced signal to the internal unbalanced signal and vice versa at the outputs of the crossover. Towards the outside this crossover has to be operated symmetrically. Another common feature with AFW1.

The two input amplifiers are designed as instrumentation amplifiers. They not only convert the balanced input signals into unbalanced output signals, but this stage also amplifies the signal and limits it at the maximum frequency. All 3 operational amplifiers of this stage are discrete. The output signal is then forwarded to the 3 filter groups – Highpass, Bandpass and Lowpass.

As already mentioned above, all filter stages are constructed in a Salen-Key topology. Two 2nd order filters are connected in series to obtain a 24dB Linkwitz-Riley filter characteristic. The 1% resistors in the filters are designed as low resistance as possible – low noise behaviour. I use only 1% 10nF and 100nF MKP 1837 types from Vishay as capacitors. I accept “crooked” crossover frequencies to be able to avoid the parallel connection of resistors. It really doesn’t matter if the crossover frequency of the subwoofer is exactly at 125Hz or “only” at 127Hz, as long as the crossover frequencies in the corresponding channels are identical.

The subwoofer channel is designed in mono. The input signals of the left and right channels are summed and then fed to the low-pass filter. In this channel I use integrated operational amplifiers. I do not believe that this has an effect on the sound at the low frequency <150Hz.

The bandpass channel, which processes the signal for the stacked quads, has no level setting and thus defines the basic level of the crossover. The other two channels – high and low pass – must be adjusted to this level.

The balanced output stages are formed by 2 discrete operational amplifiers. They are assisted by integrated OP’s which, as servo-controllers, provide output signals without offset voltage. Therefore I can omit pure coupling capacitors in the complete crossover.

The discrete operational amplifiers run with a voltage of ±25V. These are supplied by discrete voltage regulators per channel, as I use them in my preamplifier. The sub-channel has an elaborate power supply with integrated regulators, they supply the usual voltage of ±15V for OP’s. This power supply also powers the OP’s of the servo-controllers. The grounds of all three controllers are identical and are formed by a large continuous ground plane.

All three power supplies have their own but identical unregulated power supply with capacity multipliers as front end. These are supplied with their own secondary voltages from a toroidal transformer. There is the usual remote control for my designs to switch on and off and a circuit to determine the correct phase position. Of course, a 230V/AC DC filter is also integrated in front of the primary winding of the transformer.