May 25, 2017

Celestion Ditton 66 mod

Celestion Ditton 66 is a speaker that many enthusiasts have refurbished. 

A client came to us with a modified version. All the original drivers had been replaced with new drivers that its owner hoped would work well.  

As you can see, the front baffle has been modified with chamfered edges, which is a good idea, especially with a 3/4" tweeter which is highly sensitive to baffle edge diffraction. A Hiquphone 3/4" dome replaces the original along with a Morel midrange dome. Bass is handled by an Acoustic Elegance TD12X and is augmented by a Scan Speak passive radiator. 

Testing the original crossover with new drivers

Our first step was to test the existing crossover with the new baffle and drivers. As expected, we found a significant mismatch. 

Above: Gated farfield response of the new drivers with the original crossover. Green: Mid dome Blue: Tweeter Black: Tweeter and mid network.

Here we can see a number of issues, all of them quite common when changes are made without a crossover redesign.

1. The dome has a broad hump in the low end of its passband, which effectively shelves down the treble at 1.4k by 3 dB.
2. The top end of the dome has a hump centred around 5k - this is critical because the intended crossover is here. 
3. The treble response falls with rising frequency, resulting in a loss of perceived top end detail.
4. Mismatch between tweeter and mid levels.
5. The high and lowpass slopes overlap so that when combined, there is a broad hump from 4 - 14k in the treble response.

In short, we have a mess! 

Moving on to the woofer, we can see that the existing crossover is asking a great deal of the dome mid. The red plot shows the farfield response with the nearfield shown above. 

Choosing a crossover strategy

There are a number of ways we can move forward with a project like this.

1. Fully passive - we could have simply redesigned the crossover. Many people like this option because it allows flexibility to use any amplifier. The downside here is that this requires many large and expensive components. Low grade caps and cored inductors are common to keep the cost down. However, with very high end Acoustic Elegance woofers, we would consider it a bad idea to bottleneck them in this way. 

2. Fully active - this is a very popular option amongst DIYers. For this speaker it would have meant 6 amp channels (3 stereo amps). This option is most attractive where it allows drivers to work together which might have a difficult relationship in a passive design. 

With this speaker, we have drivers that can easily work together in a passive design. However, we felt the best option overall was a hybrid approach.

3. Hybrid crossover - passive tweeter/mid crossover actively bi-amped - here the client was able to use the two amps he already had. We designed a new passive which is simpler than the original. It contains a high pass for the tweeter and a lowpass for the mid. This new and much simpler network made it feasible to use higher end compnents whilst sticking to a modest budget. 

The new passive network

This is the new tweeter/mid passive network:

We were able to make it simpler by omitting the midrange high pass network - this would be handled by the MiniDSP active crossover. A further advantage we like about this approach is that we can shape the response further without adding more passive components. 

The response shown here includes DSP filters which provide further response shaping to the mids in particular. The response is +/- 1.5 dB.

Augmenting the bass response

An active implementation for the bass network has advantages that are well known. We avoid throwing money at big expensive passive parts and we also avoid wasting a great deal of amp power. Instead, we can use some of the headroom gained to augment the bottom end response. 

The final response is shown here:

Above 200 Hz the response is +/- 1.5 dB


In some speakers we find that a better subjective balance is achieved with a response that varies from flat on axis. This proved not to be the case here. This speaker sounds best with a flat treble response. However, the advantage of DSP implementations is that this can be easily changed. It's a simple matter of creating a different profile in which the bass and treble level can be modified.

Enclosure fill and the crossover

Did you know that the enclosure fill and the crossover have a relationship with each other? This proved to be true with this speaker. We found the amount of fill in the original to be inadequate. We wanted to raise the crossover point as we knew it would benefit the dome and the woofer would allow it. The problem was that enclosure resonances were interfering with the top end response of the woofer. If it were a sealed enclosure, we could simply fill the whole box. However, to preserve the operation of the passive radiator, the amount of lining is limited. The very large passive bass network also limited the amount of lining that could be used. Removing it meant we could use more lining, resulting in a smoother top end response.

How does it sound?

When it all came together, we noticed a well balanced sound. Smooth yet detailed top end with a nice integration throughout the entire range. The bass is clean, controlled and full bodied. Soon after our client had spent some time with them, we received a call. Usually this means there is a problem to solve - happy customers tend to enjoy listening to the music rather than call back to tell us about it. In this case the news was good. A happy customer describing the sound in very similar terms to those I've used. Even his wife commented on the improvement, noting that a particular track now sounds better than ever.

March 7, 2017

Melbourne room acoustic workshop

Would you like to know how your room performs acoustically? Would you like to know how to measure your room and more importantly, how to understand what the results mean?

We're offering the chance to learn about room acoustics in the room that matters most - yours. We're offering this with a twist. In a typical room analysis session, we work one on one. To keep it affordable, we limit the time and cut to the chase, moving to the interpretation and what to do about it. You don't get the chance to learn how to take the measurements and how to interpret them.

Hosts wanted

The best place to start is as a host. As a host you will learn about your room and come away with clear direction specific to your room, system and aspirations. Since all participants pay to enter, you pay less and you're getting more time. It also promises to be an enjoyable social event in the tradition of the Aussie audio GTG (get together) BBQ.

You need:
  • an audio system that you enjoy with either music, movies or both
  • enough space for 5 participants to attend
  • availability from 10 am - 5 pm


Participants wanted

As a participant you get to learn about room acoustics in a system and room that is different to your own. If you've already used our room analysis service, then you may like to learn more about how it's done, as this involves content we don't normally cover. The best way to learn and retain what you've learnt is with a hands on approach. Every system and room is different.

If you would like to attend, either as a host or participant, register your expression of interest via our contact form.

February 24, 2017

PSE ultra passive crossover

Introducing the Ultra crossover:


One of our PSE horn customers requested an ultra high end version of the passive crossover. These parts are in cost-no-object territory and they are among the best available. Where possible we used Mundorf Supreme Silver Gold and Oil caps. All cable runs are beneath the board, which serves not only to avoid visual clutter but also to assist with holding the caps in place. All caps are secured with adhesive rather than cable ties. Inductors are Mundorf foil types.

January 16, 2017

When your IR repeater is slow and unresponsive

So you've decided to finally get your act together and make your AV system spouse friendly. That might mean hiding all your audio equipment inside an entertainment unit without having to open the doors for your remote to work. Perhaps you've even installed it all in a dedicated cabinet completely hidden from view. You need an IR repeater.

Above: a wireless IR repeater.

The concept is simple. A receiver senses the signal from your remotes and then relays them to your equipment via a number of emitters attached to each device. Some relay the signal wirelessly, others via included cable or though existing HDMI or ethernet network cable. Integra AVRs also allow you to direct wire the signal.

Sounds so simple, what could go wrong?! If you use Foxtel, you should be aware that not all IR repeaters are compatible, so check before you buy. Another potential problem is interference from devices near the IR receiver. When this occurs, you'll notice that the response is very slow. You may have to press a button multiple times and it appears to work very slowly, very hit and miss.

To test if this is your problem try this:

1. Move the IR sensor away from all other devices

2. Aim the remote at the sensor at close range (say 100mm away) - aim carefully

3. Does it work now? If so, gradually move the unit closer to its intended position. You may find that the problem comes back as it gets close to other devices.

4. Is there an alternative position that avoids the problem?

5. The sensor may require shielding. A quick experiment is to line it with aluminium foil as shown below.

We know, it's not pretty! We suggest this not as a long term solution but as part of your troubleshooting process. If this solves your problem, then you know you have an interference issue. If you have DIY inclinations then you might consider a longer term mod, like installing the IR receiver in a DIY metal box or you might come up with some simple DIY shielding. Many at this point would simply take the device back.