MT Audio Design PXO-2001

Active Crossover with Line Level Pre-amplifier

This page basically contains the same information as has been published before and will be updated when the XO-2001 (which is a part of this construction) is finished and out on the market.

I have decided to design an active filter unit for use with subwoofer(s). I will make this design universal so that it can be used with almost any bass unit even if the main purpose is to improve the sound with my Quad ESL-63/Gradient SW-63 system in which the supplied filter unit is not entirely to my liking. You can read more about my present solutions on the 'Quad Tweaks' page and these solutions are very good indeed, but I think the result can be improved even further with active filtering of high quality.

Just when I was going to start the PCB CAD work I got an interesting proposal for the design from Werner Ogiers, he asked me why I don't include a line level pre-amplifier in the design. I gave the idea a thought and indeed this is interesting, nothing much to add really and one cable less. With two extra tubes I get a very good line level stage, this must be better than to have an extra pre-amplifier.

Preliminary PXO-63 front panel design

The most important part of the active filter design is to make the high-pass filter good enough to disappear in true 'high end' systems, and also to make the high-pass filter steeper in order to improve the sound from the main speakers. The Quad ESL-63 sound improves quite a lot with a first order filter at 115Hz and I can only imagine how they will sound with my proposed fourth order filter at 100Hz. It is my experience that ALL speakers and especially full range speakers and two way systems improve their midrange and treble sound quality very much when the low frequencies are no longer present. This is the problem that three way loudspeaker systems try to address, but with passive filtering this is not an easy task because of the impedance variations at low frequencies and the large values on coils and capacitors that must be used. With an active unit the filter response will follow the calculated response almost exactly.

I have followed the discussions on the news groups and it seems that most subwoofers are used only at low frequencies with the main speakers running as full-range and this is not the best approach for audio nirvana. The reason for running them like this is of course the low quality filters supplied with the subwoofers and also the fact that the filters are usually located in the subwoofers and require long cables. With active filtering some important factors are introduced for improved sound quality:

The main speakers are relieved from low frequency signals and the sound will be a lot cleaner and the box coloration (with ordinary speakers) will be reduced.

The main amplifier is relieved from low frequency signals and the sound will be cleaner.

The maximum SPL is almost 6dB higher with the same main amplifier if the subwoofer is up to it.

Integration between subwoofer and main speakers is simplified.

With the steep filters used in this unit the subwoofer will not disturb the midrange.

Features in the MT Audio Design Pre-amplifier/Active Filter

Flow chart description of PXO-63

MT Audio Design Pre-amplifier/Active Filter KIT

My aim is to make this pre-amplifier/filter unit available as a kit with assembled and tested PCBs and a special designed box. I will try to make it as cheap as possible but I must make a small profit, otherwise I will not be able to finance the project. It is a hobby project and my aim is not to earn a lot of money but I am risking a major part of my savings and I will not (can not) do this one more time. The pricing depends mainly on the PCBs, transformer and box cost (NO labour cost for the construction!). I intend to make it good looking since I think that this pre-amplifier/filter will mainly interest those who are not into DIY and want a functional unit that can be put among other 'high-end' equipment without looking cheap. I will investigate the pricing on screen printed metal fronts and customized metal work. The assembly must be easy enough for anyone capable of holding a screwdriver and reading a manual. I will not compromise with parts quality, polypropylene capacitors and GOOD quality OPAMPS will be used and the power supply will be made as good as possible. Since you can not listen to the pre-amplifier/filter in a shop I plan to make it possible to return the unit if you are not satisfied, with no other cost than the freight (two-way), and in order to make such an offer I can not afford to make it less than perfect.

It will be possible to order the unit with other crossover frequencies for use with for example Martin Logan speakers that need a higher crossover point.

Those who know the sound from Quad ESL-63 and tube amplifiers know my preferences in sound and I intend to make this filter in the same quality league, time will tell if I succeed. No success, no sale.

I am interested in feedback on this design and also want to know if there is an interest for an active filter of this kind. Please e-mail me with suggestions and if you are interested in buying a filter.

When the design is ready I will make a cost evaluation. It will take some time since all the work has to be done on my spare time.

Aluminium profile used for enclosure

I have decided on the enclosure design which will be an oval shaped aluminum box with a screen printed brushed aluminum front panel and a black rear panel. The width is 200mm and the height is 110mm, the depth is 300mm. I am trying to find a larger knob with good finish for the SW level adjustment. Aluminum can be anodised in several ways and I have not decided on which kind I will use, another option is to use powder paint on the box. The knobs are also aluminum and can for instance be gold anodised if it is not too expensive.

MT Audio Design Pre-amplifier/Active Filter Construction Details

The picture below is my pre-processing idea of the PCB mounting in the box.

Preliminary mounting drawing

Tube pre-amplifier section with an ECC82 cathode follower followed by a totem pole ECC82 stage. The gain for the total unit (with filter) is around 20dB. The main volume control is placed between the two pre-amplifier stages to make it possible to get a 500kOhm input impedance and also to reduce the swing in the totem pole stage. It would have been possible to place the volume control after the totem pole stage but than the required swing is 12 times higher and I think it is worth the extra cost to reduce it. The pre-amplifier output impedance is below 2kOhm.

Fourth order Linkwitz/Riley crossover at 100Hz, which means that the crossover is phase correct and at -6dB.

Switch selectable (on/off) fourth order low cut Linkwitz/Riley filter, -6dB at 16Hz (internal PCB switch).

Switch selectable mono for use with single subwoofer or warped records.

Switch selectable 6dB/octave slope for use with dipole subwoofers (internal PCB switch).

Delay filter meant to be used with subwoofer(s) placed at different distance (almost always the case) than the main speakers or with subwoofer(s) that are not phase correct at 100Hz.
The phase delay is adjusted in the range 10-175° with a front panel motor driven potentiometer. the procedure for use is the same as for the SW and Main level controls described further down.

Switch selectable 100Hz oscillator with adjustable level (on rear panel) for easy adjustment of subwoofer level and delay.

High-pass filter with tubes. Two ECC82 cathode follower stages are used. The output impedance of the filter is below 1kOhm.

The DIY OPAMP version will have high-pass filter with high quality OPAMPS on the same PCB as the low-pass filter. This filter unit will not be expensive.

The tube filaments will be regulated 12.6V and the high voltage supply will use a CLC configuration with a 5H choke, previously I planned to use active regulation but my experience from the Audio Note DAC with choke regulation has made me change my mind. I have chosen the ECC82 in the cathode follower stages because it is dominated by the 2nd harmonic (more so than most tubes), there are other tubes with less 2nd harmonic (and THD) but since the circuit uses local feedback, the 2nd harmonic will be low enough and higher order distortion components should be very low. I have used JJ ECC82 in my Jadis DA-30 for some time now and the sound is superb. Data is available at the JJ Electronic web site and used correctly this is a very linear tube. I have spent much time simulating the circuits, trying to find the optimum operation point with reasonable load voltage and the result looks promising. I use a simple cathode follower circuit with only two resistors and no bypass capacitor and I think this is the best way of doing it. With more components the feedback function affects the loadline and this may result in more high order distortion. The totem pole ECC82 in the pre-amplifier stage is an appreciated circuit with low distortion and also a low output impedance to make it possible to drive almost any power amplifier.

Switch selectable first order low cut at 10Hz, 25Hz, 40Hz. This is meant to be used for correcting recordings with exaggerated low frequencies (quite common) or to increase maximum SPL. It will cause a slight phase shift but I don't think it is necessary to compensate for this, the 10Hz setting is the one to use for serious listening.

Motor driven potentiometers for use with remote control. For cost reasons these will be wire controlled with switches and wires connected to the filter box.
I have ordered a reasonable priced motor driven potentiometer and intend to use this instead of the previously specified ALPS potentiometer in the subwoofer circuitry and the lower quality shall not be of any concern here, using ALPS potentiometer is a bit of overkill.
The pre-amplifier level control will use a motor driven ALPS potentiometer, the control is by wires as the other potentiometers. maybe I will make a separate IR remote control unit for use with the PXO-63 later.

The switches on the PCB for filter alterations and output choice will be gold plated jumpers, I have searched for reasonably priced switches of good quality and these does not exist. To keep price down and quality up, I have chosen to use jumpers instead. These are cheap and probably affects the sound less than a good quality switch, and the setting is a one time procedure or at least a rare occurrence. With a good manual this is as easy as switch selection. The front mounted selectors for mono and extra low-cut filter will still use good quality switches.

I will only make a single end version since I do not believe that balanced action is any good with filter units. The reason for this is that in balanced operation you add a negative and positive part in anti-phase and if the two parts are not EXACTLY the same you will add the difference to the output. This is likely to happen in a filter design where the filter characteristics of the two parts will vary slightly due to component tolerances (not critical in single end) and this will add strange sounds that are in no way harmonic. For use with balanced equipment I recommend Lundahls balanced to single end connectors ore other good balanced to single end converters.


Some Simulations on the Design

The simulations that does not involve the tube amplifier section are published on the 'XO-2001' page.

Fourth Order Tube High-pass Filter

Tube amplifier high-pass

Above is the 4th order tube high-pass filter. This is also a Linkwitz/Riley crossover with zero-phase at crossover.

Pre-amplifier and Fourth Order Tube High-pass Filter

Total tube amplifier section with high-pass filter
Above is the tube pre-amplifier with tube high-pass filter, the amplification is 8 according to the SPICE simulation (18dB gain). TubeCad calculations gave a gain of 20dB, the real amplification factor will be established later.

High Voltage Supply

I have used the PSU Designer II from Duncan Amps for the power supply simulations, a very nice freeware program available at Duncan's Amp Pages .

Plain capacitor filter 

The picture above shows a plain capacitor filter (1000uF) and what happens when the current demand changes from 40mA to 35mA. The ripple is quite high and also contains high frequency components.

Double choke filter schematics

Double pi filter 

The picture above shows my double pi-filter and what happens when the current demand changes from 40mA to 35mA. The ripple is minimal (~1nV) and contains no high frequency components which can be seen in the close-up below. 

Double pi filter ripple 

Ripple after the first choke 

The result with only one choke is pretty good too but the extra cost for one extra LC stage is only $25 and I think it is worth it to get the stunning performance, nV ripple level and pure sinusoidal output is hard to get with other construction methods. I considered using a regulator followed by an LC stage but I would have needed large heat sinks to make it possible to use with the different voltages in different areas of the world. The double choke solution is simple and with no active components in the PSU no switching components can pass the filter and ruin the performance.


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