Passive LF boost circuit.

Vix · « **Reply #160 on:** 7 Sep 2008, 07:21 pm »

Hi Graham,

Today I have tried the new values for the T-bass circuit, as you had suggested. Beta sounds much better, and I could keep it that way, if there wasn't another problem popping up: Sony amp. At higher volumes, its protection got triggered and it stopped working, and started again at lower volume. So, I'm afraid that I'll have to abandon the the T-bass circuit, at least for some time.

Another problem is the Visaton B200 itself. It seems that I began to hate this driver. Its response is getting on my nerves. I have tried various inductors/resistors,Zobels, but just to some degree of success. At present it has no dust cap, and has some mock-up phase plugs, that can easily be removed (but then it sounds even worse). Sometimes it seems to me that I'll have to abandon this driver combination and move on to something else...

Thank you for the support,

Vix

Graham Maynard · « **Reply #161 on:** 7 Sep 2008, 07:50 pm »

Hi Vix,

The main problem with the B200 is its beaming/ energy pattern, due to size and cone shape.
This was my reason for using the foam pieces, to break up symmetrical surface wave patterns and absorb/diffract some central output.
http://server6.theimagehosting.com/image.php?img=Fingers.a9d.jpg
click on image for full size.
Also my reason for running D'Apollito, which also modifies mid frequency surface wave patterns.

Even if foam is used, there can still be an 'edge' to this drivers response which is hard to tolerate.
Now I don't know if you might have a scrap transistor AM radio you can salvage the ferrite antenna out of, but if you can wind say 50 turns of wire along a ferrite rod using ordinary connecting wire of 1 to 3 Amp rating to make a low value choke and insert this in series with the driver, this 'edge' can be filtered out.
The beauty of this arrangement is that you can add or remove turns whilst you are listening, and thereby acheive best sound.
This might be worth a try before you give up on the B200.

Cheers ....... Graham.

Vix · « **Reply #162 on:** 7 Sep 2008, 08:42 pm »

Hi,

Exactly this "edgy" sound is what I find hard to tolerate. I will try the mod with the ferrite AM radio antenna, and the with the foam pieces. As I could see, you also put a foam inside a pole piece and also glued a thin foam instead of the dust cap? Sorry, but I can't see clearly on the pictures how did you put/fix the foam inside a pole piece?

Thanks a lot,

Vix

Graham Maynard · « **Reply #163 on:** 8 Sep 2008, 08:24 am »

Hi Vix,

We all know that driver size leads to beaming, but we are not obliged to put up with that beam or listen off-axis ! This being where foam can help.

I found the foam centre to be better to listen to than a plain dustcap. Though when using a foam finger close to and right in front of the cone, a foam centre need not be fitted.

In my photo the radiation from the lower half of the dustcap was dispersed/diffracted. Thus the axial HF beam focusing reduced.

Separately I personally feel that there is a tuned series high frequency resonance between electomechanical and underhung voice-coil/magnetic circuit impedances, and that inserting a low value series choke of 35 to 100uH (depending on individual requirements) is enough to reduce the Q of this resonance without affecting the main AF, such that overall reproduction is then improved.

Overhung drivers already have impedance which is not enclosed by the magnetic field, and so already have sufficient series filtering.
Maybe impedance at the field gap promotes high frequency ringing with underhung construction and non-conducting ferrite magnets ?

Good luck with the tweaking.

Cheers ......... Graham.

painkiller · « **Reply #164 on:** 9 Sep 2008, 06:58 am »

That foam finger trick is really clever. It actually works very well, and it doesn't have any apparent downsides other than poor WAF. A simple remedy for a tiresome upper midrange.

Graham Maynard · « **Reply #165 on:** 9 Sep 2008, 08:47 am »

Hi painkiller,

Glad you find the foam works. There is still much to do with single drivers in order to make them satisfy better than mid/tweeter combos.

I am trying to bring many aspects together in my present constructions, but 'yes' those fingers do look odd, and they are the first thing to catch the eye from a distance.

If they could be much longer and running the entire front of a baffle as a decorative statement (except where they tune airside wave activity) there might be much better WAF.

Cheers ........ Graham.

mightym · « **Reply #166 on:** 23 Sep 2008, 11:35 pm »

Graham,

I find this thread fascinating. I've been planning a low cost OB to " get my feet wet " so to speak.

At this moment I'm looking at the Eminence Alpha 15a, and the Goldwood GW 1858 in a fairly shallow U frame for the sub, crossed between 100 and 200 Hz, and an 8" wideband to 5K or so, with a tweeter above.

I am still trying to learn crossover theory, and don't really totally understand the principles involved in your T-Bass passive contouring circuit, but it looks like a winner WRT what I want to produce.

I plan to acquire some sort of measurement system, and start with the values you previously recommended for the Eminence Alpha 15A ( I'm leaning toward this, as It is on sale, and could be used in other alignment if I'm not happy with the OB).

I'm sure that when I get started, I'll be pestering you with questions Re: the circuit, and tuning.

Thanks for sharing it with the world.

John

painkiller · « **Reply #167 on:** 24 Sep 2008, 05:38 am »

That Goldwood 18" driver sure looks like a good candidate for OB bass combined with the T-bass. Especially with the measurements made by MJK on his website. Looks like the Alpha 15A efficiency is a bit optimistic in the datasheet.

That would certainly explain my impression of lack of bass in my current setup.

My own system has been put on hold for a while. I hope to resume experimenting in a month or so. Ribbon tweeters and extra bass drivers. aa

Graham Maynard · « **Reply #168 on:** 24 Sep 2008, 09:22 am »

Hi John,

Not been doing much audio here recently either.

The most important thing for OB bass is displacement, and as Painkiller says that Goldwood 18"er wins out.
With the lower Fs it will drive deeper before roll-off too.

Of course the heart of your OB must be the widerange driver, though it is matching the two driver levels with a smooth crossover to suit your own requirements which will take time to get right.

The T-bass gives boost at/below Fs where higher driver impedance normally causes the OB roll-off which folks say you need a sub to overcome.
I don't need a sub - but then I am using 15" + 18" per 18" wide baffle (fits just fine with careful cutting).
Having done this I would not go for 2x 15" per baffle (does not go as deep) or 2x 18" (lacks upper bass coherence).

Time to jump in John - not just get your feet wet !

Cheers ......... Graham.

JohnCZ · « **Reply #169 on:** 5 Feb 2009, 10:52 pm »

I am about to assemble the components for the T-bass circuit but I want to be sure that I get it right. My OB project consists of the B200 and an Alpha 15. The post I have found to be a correct match to my OB is project that Graham posted is this:

http://server6.theimagehosting.com/image.php?img=T-bass.f83.png

I have found an Avel 250v 25v/25v transformer at Parts Express. The question I have is regarding the 1500 uf cap. On a couple of other posts it was suggested that a cap value of 470 to 660 could be used. Could I use (Parts Express) 100V Non-Polarized Crossover Caps? They have values that go up to 500uf and are very reasonable cost wise. I would normally use Dayton or Solen caps, but they would be quite a bit more.

I also assume that a conventional 18 gauge air core inductor (6.4mh) would be ok.

John

Kludden · « **Reply #170 on:** 12 Feb 2009, 10:14 am »

Hi Graham!
i have a OB with alpha 15, built after Scorpions suggestion.
Now to my first question on this forum. I have done a T-bass and it works

.
Should I connect my passive crossover behind the T-bass?
With only the T-bass i have to much high frequense.
Regards
Kludden

panomaniac · « **Reply #171 on:** 12 Feb 2009, 07:07 pm »

Graham usually says that the T-bass should go closest to the amplifier. So you put the crossover after the T-Bass.

Is that right Graham?

jeffac · « **Reply #172 on:** 13 Feb 2009, 06:22 am »

Good to see another one in use

Not Graham but that's not quite it, others can chip in though. Choke for bass low pass is placed after the T-bass circuit. For mid/high freq drivers, use separate wires from before the T-bass, at the amp probably preferable. Somewhere at the beginning of this thread, Graham has a link to the circuit, as well as part of the crossover used in his Spirit speaker and posted in that thread.

cheers.. jeffac

Kludden · « **Reply #173 on:** 13 Feb 2009, 09:41 am »

Thanks!
I will try that.
Regards
Kludden

Graham Maynard · « **Reply #174 on:** 15 Feb 2009, 08:51 am »

Hi Kludden,

The posts from Panomaniac and Jeffac are correct.

The T-bass must have low impedance drive, which means thick leads and a high damping factor SS amplifier.

If you wish to cut mid frequencies then any low pass must be on the LS side of the T-bass. However if you use a series L and a C across the capacitor you will introduce the same phase change as will a conventional crossover, and this can lead to a lack of low-mid blending.

If the LF driver has too much mid output then do what you can to deal with this.
In place of the C in an LC crossover use a Zobel = R+C in series.
You could also and/or try a capacitor in parallel with the L to notch out a mid frequency peak - 2.2uF to 22uF depending on L and LF driver.

( Sorry for delay. Wife has been fretting for days trying to pick a new carpet for the lounge, and of course skirting has to be resealed/painted. 2yr TV broke; colour gone - not degaussing. 6yr washing just packed up - motor not running. Never blooming ends ! )

markC · « **Reply #175 on:** 15 Feb 2009, 02:47 pm »

If using a tweet with the B200, why not cross over @ around 2k and be done with the beaming issues?
I have a bsc circuit and a zobel on the B200. No issues with it to my ears. You just loose efficiency. If you can afford the loss of efficiency, this is the way to go IMO.

JohnCZ · « **Reply #176 on:** 26 Feb 2009, 11:47 pm »

Graham,
Finally collected all of the components necessary for my T-Bass circuit (for my Alpha 15s). It is simple enough based on your illustrations of the circuit. However, the problem I have is with the transformer.

http://www.parts-express.com/pe/pshowdetl.cfm?&DID=7&Partnumber=122-630&ctab=1#Tabs

The diagram on the transformer indicates 2x115v on one side and the other side - black & gray at 0v and red & yellow at 30v.

Your diagram indicates 0v, 12v and 24v lines (used in the circuit). How do I determine which leads to use for the circuit on my transformer?

John

Graham Maynard · « **Reply #177 on:** 27 Feb 2009, 02:40 pm »

Hi John,

Obviously the mains primary windings are left disconnected and insulated.

The transformer secondary windings need to be wired in series with say the grey and yellow connected as the T-bass transformer centre tap (12V on the circuit drawing), the amplifier to black (0V), and the loubspeaker circuit to red (24V).

Look forwards to reading your comments.

baMarek · « **Reply #178 on:** 27 Feb 2009, 05:04 pm »

Hi!

It's my first post so Hi to everyone.

Graham, I found your thread by accident but your design looks like something that would make me happy bunny. Big thanks for sharing that and such a great support you provide.

Im currently running OB with single Alphas 15A and vintage coaxial black cone Sabas. All is driven by a massive (almost 100pounds) chinese tube amp and diy DAC. Although black Sabas seem to be superior to any other Sabas they are all fantastic speakers and I simply do not want to look for any other contemporary drivers that could (only potentially) compete. Comparing to SAbas, Alphas are sloooowwww, muddy and no fun at all, just like Vix wrote earlier. Basically I found that I cannot run Sabas and Alphas in the same range (even with 2nd lowpass xo) because they are too slow. I tried different amps with the same results.

Other issue is an obvious lower than specified SPL of Alphas. Even without roll off they would never play as loud as Sabas, hence I use resistor to attenuate my German beauties (I cant believe I said that about something that's German

).

I recently ordered some transformer and want to give your circuit a go. If it works I will probably order 4 other woofers and thinking of mentioned Madisound EMI1550. Managed to convince my lady that making the speakers bigger is a good idea so the only thing that can stop me is the freight price to the UK. We'll see. What LC values of your t-bass do you think are appropriate for parallel EMI? David used some huge choke and I hope it is not necessary.

Marek

JohnCZ · « **Reply #179 on:** 27 Feb 2009, 10:56 pm »

Graham,
I assembled my T-bass circuit per your diagram.

http://server6.theimagehosting.com/image.php?img=T-bass.PNG

I cannot get a signal to the bass. I've tried a couple of other wiring combinations and still no signal.
Here is the schematic printed on the side of the transformer:

blue----------- I -----------Black 0v
gray----------- I -----------Red 30v 4.17a
2 x 115v
violet-----------I-----------orange 0v
brown----------I-----------yellow 30v 4.17a

Is there something that I am missing?
John