Even local prices are not that great:
https://www.wagneronline.com.au/cro...ts/audio-speakers-pa/ic1810-83866/1000382/pd/
and that is for 10mH, the max available at WES
overseas buy might be worth considering
I’ve always wondered if you use subs with these OB designs, especially Bitches Brew. I think you may be right about wasted energy, especially after the 12db/oct loss and increased excursion. But for film or dance music, it can be quite noticeable, having deep subsonic tones.
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Another interesting option:
if you buy 4 of these from DigiKey (prices in $AU), including free delivery for orders over $60
https://www.digikey.com.au/en/produ...AeUwBZQtnwBXAE6EQAX1IBaBKhAZIOAiXJUQMGinaqrQA
you'll have your inductors for the OB plus 2 for a pair of Zenductor amps, see Pass forum.
Of course the Q of the filter will change and will need to be investigated, but worth considering.
@D1sco the Bitches Brews are designed to go down to 20-25Hz and they don't have an infrasonic filter. If I were playing them super loud in a large room I would roll them off below 30Hz.
But if a system clearly cannot produce competent bass below 40Hz, and if it doesn't have the inherent protections of a sealed box, you can waste tons of excursion and produce almost zero sound. This occurs all the time with bass reflex systems, which can play 5-10dB louder if you filter out signals below the tuning frequency. The same applies to dipoles. Dipoles play much cleaner if you remove signals below their useful range.
But if a system clearly cannot produce competent bass below 40Hz, and if it doesn't have the inherent protections of a sealed box, you can waste tons of excursion and produce almost zero sound. This occurs all the time with bass reflex systems, which can play 5-10dB louder if you filter out signals below the tuning frequency. The same applies to dipoles. Dipoles play much cleaner if you remove signals below their useful range.
Shipping to Isle of Australia often costs more than the product!
Is that what people call Australia tax? 😂
I agree there’s no point trying to play those low frequencies- just ends up sounding like flapping cones in the wind.
IMHO this is why people with dipoles end up going (back to) sealed box bass.
At least below <50Hz it is a sound decision
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@tktran303 correct. That's why in the Walnut Dipole design I picked my battles carefully. Fully committed to strong 40-50Hz and sharp filter below.
This has been called a Passive Assist for ages, but mainly utilized by a series cap alone and a Qtc of just above 1.0, with the cap value selecting the rolloff range.
Criteria usually require a Qms between 7 and 10, and long Xmax. It works with the self resonance of the driver and aids by placing a peak in the lower range, much like your proposed usage. The main difference is that the coil is usually not used and therefore does not cause the impedance dip issue.
The main problem is that using the large capacitors comes with a pretty big insertion loss when using NPE or back to back polarized caps. From experience, this is pretty audible, and it affects dynamics pretty readily. Using a film cap can fix this issue, but film caps this size get even more expensive and massive in size. Paralleling a film cap with NPE caps to reduce the loss by lowering ESR is not effective enough below the Fc of the film cap used, so the film caps have to be large to be effective too. Electronic Concepts UL30 series are as they call them Unlytic, and are in the 3" diameter standard computer style size and have low ESR. They are able to stretch the film to a thinner insulator so that they can make them in this size format. I have some to test in use, but have not as yet. Ebay seems to have them at good prices.
https://www.ecicaps.com/film-capacitors/ul30-series/
Criteria usually require a Qms between 7 and 10, and long Xmax. It works with the self resonance of the driver and aids by placing a peak in the lower range, much like your proposed usage. The main difference is that the coil is usually not used and therefore does not cause the impedance dip issue.
The main problem is that using the large capacitors comes with a pretty big insertion loss when using NPE or back to back polarized caps. From experience, this is pretty audible, and it affects dynamics pretty readily. Using a film cap can fix this issue, but film caps this size get even more expensive and massive in size. Paralleling a film cap with NPE caps to reduce the loss by lowering ESR is not effective enough below the Fc of the film cap used, so the film caps have to be large to be effective too. Electronic Concepts UL30 series are as they call them Unlytic, and are in the 3" diameter standard computer style size and have low ESR. They are able to stretch the film to a thinner insulator so that they can make them in this size format. I have some to test in use, but have not as yet. Ebay seems to have them at good prices.
https://www.ecicaps.com/film-capacitors/ul30-series/
Interesting indeed - and thanks to @perrymarshall for such a detailed and thorough description. It's sort-of reminiscent of KEF's (and others') occasional use of a series capacitor to achieve a somewhat milder effect.
A current-drive amp would be one possibility. The other obvious alternative is the series inductor - which offers good control in the bass and the benefits of current-drive at higher frequencies (if covered by the same driver, for example like the Manzanita), but needs an expensive high quality, high-inductance coil. What are the requirements for the quality of the (shunt) inductor used here, in comparison to that?
Thanks for the write up! For years I have been using a big inductor in series with the OB woofer to correct for the OB bass losses. You can see the idea put to use in this thread:
https://www.diyaudio.com/community/threads/fast-fun-inexpensive-ob-project.110583/latest
There is also a technique I tried years ago call T-Bass (IIRC). This was written up on another forum. It uses a transformer and a cap to boost the bass. As with all bass boost, there is no free lunch.
Your peaking filter and the T-Bass trade higher voltage for a lower impedance. The series inductor trades away sensitivity. But even active and DSP come at the cost of voltage. You need an amp able to supply the higher voltages required by the DSP.
https://www.diyaudio.com/community/threads/fast-fun-inexpensive-ob-project.110583/latest
There is also a technique I tried years ago call T-Bass (IIRC). This was written up on another forum. It uses a transformer and a cap to boost the bass. As with all bass boost, there is no free lunch.
Your peaking filter and the T-Bass trade higher voltage for a lower impedance. The series inductor trades away sensitivity. But even active and DSP come at the cost of voltage. You need an amp able to supply the higher voltages required by the DSP.
Here is the T-Bass circuit plus associated discussion regarding the -questionable- claims of the circuit: https://www.diyaudio.com/community/threads/t-bass-drive-for-ob-lf-drivers.130679/
I've played with this quite a bit. Generally you should aim for less than 1 ohm DCR. The higher the DCR, the lower the Q of the filter and resistance makes it harder to get a bass boost at all. That said, some resistance is acceptable and does serve to keep the impedance from going down to zero at the resonant frequency.
VituixCad models DCR very well.
Also, @wolf_teeth - thanks for bringing up capacitor ESR. We should factor that in as well. For what it's worth, the 500uF capacitors I used in the Walnut Dipoles have an ESR of about 0.4 ohm, which reduces output by 0.5-1dB.
I need to be clear that the Marshall Bass Boost LC circuit is VERY different from
1) a series inductor, which attempts to cancel out the 6dB slope of the OB cancellation in combination with a very high efficiency driver, or
2) a series capacitor, which KEF and others have done. This can be optimized in combination with T/S parameters to protect an acoustic suspension system with specified 3rd order slope.
Neither of these can boost the output, can only cut.
I can't think of many situations where a series capacitor for an OB Low Frequency section would be helpful.
Current drive is a very good approach to OB and will have somewhat comparable effects.
"Neither of these can boost the output, can only cut."
This is incorrect. Using a series capacitor will boost the bottom end by making it 3rd order in combination with the Q and rolloff compared to that of a rolloff without the cap in place. The F3 is lower than without, so therefore has more output than without.
I understand what you are saying in terms of impedance dip and resulting boost, but the truth is the lower F3 is there. The cap cuts the lower bass below that of the cap Fc, so i get where you are coming from there too.
This is incorrect. Using a series capacitor will boost the bottom end by making it 3rd order in combination with the Q and rolloff compared to that of a rolloff without the cap in place. The F3 is lower than without, so therefore has more output than without.
I understand what you are saying in terms of impedance dip and resulting boost, but the truth is the lower F3 is there. The cap cuts the lower bass below that of the cap Fc, so i get where you are coming from there too.