Passive LF boost circuit.

jeffac · « **Reply #180 on:** 1 Mar 2009, 03:09 am »

Hello John,

I'll give this a go seeing Graham will be pushing out zzzzeds at the minute. It can be confusing first time around

and it will help me too if Graham can step in with either a 'that's it' or 'you're a drongo'.

Forget about the transformer primary wires and just insulate each so they can't make contact with anything inadvertently

With the transformer type I'm using, secondary wiring colours and winding orientations are

1. Amplifier +ve signal (red) --> transformer white/black linked together
2. Transformer red --> bass driver +ve (red)
3. Transformer orange --> T-bass choke --> resistor --> linked into amplifier -ve signal (black) --> low-pass choke --> bass driver -ve (black)
4. Capacitor between and amplifier +ve signal (red) (mine connected where this is linked to the transformer black/white) and transformer orange (connected before the T-bass choke)

Hope that's correct,

if not.

Graham will be better able to advise on choke and capacitor values than me.

cheers.. jeffac

JohnCZ · « **Reply #181 on:** 1 Mar 2009, 05:09 am »

Jeffac,
CLS responded to my problem the other day and gave me the right information. I did have the wires correct but I may have not had them assigned to the right values based on the schematic. The schematic I am using is this one:

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

If we use your diagram, I have the leads grouped correctly. My Red is marked 0v -- My Orange is marked 30v -- and tied together my Black is 30v and White is 0v.

Looking at the schematic above the T-bass diagram list connections to the transformer at three places indicated as 0v, 12v and 24v.

Should I assume that the 0v connections is - Red
and the 12v connection is Black & White
and the 24v connection is Orange

Obviously my transformer is a little different and is rated at 30v/30v. Does connecting black and white together produce 15v?

jeffac · « **Reply #182 on:** 1 Mar 2009, 06:08 am »

Hi JohnCZ,

Used as a power transformer, you could use each secondary separately. Using the colour scheme in the pic, and if your transformer is 30-0-30, red-black used to give 30V 4A, white-orange used to give 30V 4A. Parallel red-white and black-orange to give 30V, double the current capacity 8A. Series connect black-white to get 60V across red-orange OR +30V/-30V if black-white is used as a centre tap ground reference point OR black-white (30V) and orange (60V) if red is used a ground reference point. I think this could be reversed, red (60V), if orange is used as a ground reference point. You'd need a 15-0-15 transformer to get 15V but from what you've posted, yours looks to be 30-0-30 so 30V is the minimum you can get.

I think this is right, and I hope it helps, but please someone jump in as I'm way too new to this game to be really offering such advice.

cheers.. jeffac

JohnCZ · « **Reply #183 on:** 2 Mar 2009, 10:17 pm »

Graham, CLS, Jeffac,
Thanks again for your help - I solved the problem - a cold solder connection.

One other question - What values in the circuit controls the cutoff for the bass. The bass seems to have a little more midrange in it than I want. Just need to know which direction (up or down) the values of the caps or inductors should go.

John

Graham Maynard · « **Reply #184 on:** 3 Mar 2009, 07:32 am »

Hi John,

Midrange output is related to driver characteristics because the T-bass is for OB bass correction.

You can try a series inductor between the T-bass and driver (not L+C because this introduces deleterious phase change); plus a Zobel across the driver voice coil terminals.
Say a 4mH series inductor with about 4.7uF connected across the inductor terminals; plus say series connected 4.7 ohms and 22uF for the driver Zobel.

All values approximate, and as usual should be trimmed to suit an individual application.

Also a LF driver can be reversed so that its magnet is outwards, with the cone pointing into a soft cushion insert about one foot behind the baffle. This gives extra roll-off without electrically distorting the applied waveform.

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

baMarek · « **Reply #185 on:** 3 Mar 2009, 12:00 pm »

Assembled the circuit last night and I must admit it works really well. My Alpha 15A played much louder from 35Hz up but it started to roll off surprisingly low, at about 85Hz. Values I started with: choke 9.4mH, 1000uF cap, 1Ohm resistor in series with the cap, no resistor in series with the choke.

I think it's time to think about some decent woofers.

Graham, what values whould you start with for 2 parallel Madisound 1550?

Marek

Graham Maynard · « **Reply #186 on:** 3 Mar 2009, 04:15 pm »

Hi Marek,

Strange as it may seem, the T-bass L and C are not directly scaled with driver impedance as is the case with an L-C crossover network.

The T-bass L and C work together to create the dip for the open baffle response, and are not matched to driver impedance alone. So use what you already have and trim values once you are up and running.

Those 1550s must be one of the nicest sounding for OB with transformer bass !

Looking forwards to hearing about your findings.

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

baMarek · « **Reply #187 on:** 3 Mar 2009, 04:46 pm »

Hi Graham,

I was actually thinking of a choke value as this affects the budget the most. I understand the bigger L the lower the boost starts (and roll off towards mid). Using 20mH+ choke with Alphas 15A probably does not make sense as they start at around 35Hz so pretty high.

Those Madisound are tempting but just like you i feel how low the pound fell and it is a pity you can only buy them in the US.

Something interesting, I tried your circuit with my tube amp and the other SS one. Normally the latter has much more power in the lowest section but strangely they played at a similar level and the tube amp sounded more controlled. Could have this been caused by the fact I used 4Ohm output from the tube amp and 8Ohm from the SS one?

Graham Maynard · « **Reply #188 on:** 8 Mar 2009, 07:49 pm »

Hi Marek,

I cannot remotely advise on choke value, between 5 and 10 mH is a fair aiming point.

Whether a particular amplifier sounds best is going to be greatly influenced by the nature of any internal NFB loop and the transformer quality. With tubes the output can already be phase shifted and less damped at 30Hz, hence LF output can sometimes 'sound' better without actually measuring so.

Cheers ...... Graham.

baMarek · « **Reply #189 on:** 8 Mar 2009, 11:42 pm »

Hi Graham,

What is great about your design is that it actually improves the response of bass drivers a lot. I do not hear the awful carton-like sound above 100Hz anymore, at least it is not as disturbing as it used to be. The quality of LF was so much better that I ordered 4 speakers from Madisound hoping it would finally end my bass searching journey. Once they arrive in 2-3 days I'll be able to tell you more about their performance in comparison to the standard Alphas.

The circuit will not work with every amp but strangely my tube amp was perfectly fine with driving the load. Though I guess the real test will be with 2 woofers on each side.

Thank you again Graham, you've made some people much happier.

Marek

baMarek · « **Reply #190 on:** 1 Apr 2009, 11:28 pm »

Hi,

I finally assembled my Madisound woofers and I am somewhat confused. While the T-bass was helping a lot with the single Alphas it doesnt do much with the 2 parallel. I started with C=6.4, R=1R in series with L=500-1500uF. There is 2-3dB gain around 45Hz but also a huge peak around 180Hz. The peak is not caused by the T-bass but it's lauder with it.
Any thoughts?

Cheers,
Marek

painkiller · « **Reply #191 on:** 8 Apr 2009, 08:21 am »

About the Alpha 15A's. I've been living with these for some time now. They are good entry level drivers and they are dirt cheap. They can deliver some deep bass, but when trying to push them towards 30Hz they reach Xmax (and beyond) pretty quick. It sounds really bad at realistic levels. The only reason I have kept them is because I couldn't afford an upgrade.

I'm looking at the Beyma 18LX60 which seems like an amazing driver. http://www.mamut.net/gitarnett/subdet243.htm What are your thoughts on these drivers Graham? Do you think they can do well with the T-bass?

Graham Maynard · « **Reply #192 on:** 8 Apr 2009, 09:31 am »

Hi painkiller,

Yes those Beymas do look good.
It comes down to what you can afford/obtain.

Hi Marek,

180Hz is in the realm of a driver/baffle/corner reinforcement frequency.
What is your amplifier ?
The T-bass works best with <100 milliohm source impedance.
A lower R would likely be necessary in series with for capacitor when driving parallel loudspeakers.

Cheers ........ Graham.

sba · « **Reply #193 on:** 12 Apr 2009, 05:00 am »

Hello Graham,

I picked up two vintage 15" drivers for my first OB (3-way) project, and I'm wondering if they will work well with your T bass circuit.

f's 40
qts 0.25
qms 4.0
qes 0.27
xmax 0.76
spl appox 101 db

The mids will be Eminence Beta 8s and the tweeters will be vintage jbl paper cone LE26s in waveguides.

Thanks

Graham Maynard · « **Reply #194 on:** 12 Apr 2009, 09:41 am »

Hi sba,

Yes, and Good Luck.

I'm trying to get my websites up and running again after they became totally lost, but boy am I struggling with inefficient software !

My 'T'-bass circuit description page is here, but presently I simply cannot get the circuit diagram to upload. (Maybe I should try it as a hyperlink.)

http://www.gmweb2.net/new_page_1.htm

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

sannax · « **Reply #195 on:** 20 May 2009, 10:20 am »

First of all...Thanks Graham for shearing this with us

I have read the thread and I HAVE to try this, but have some question regarding my own setup. I am sorry if this is mentioned before and appoligies for my newbiee questions (there will be more of them).

1. I have a Keiga 5230 subamp that runs 2 Augies in paralell (the amp then sees 4 ohm). This is a mono bass setup. (I know I should think of driving it in stereo

) Do I need 2 power transformers of 500Va 2x40V or only one because of monodrive?
2. If I choose "to go stereo", do I then need 2 power transformers of each 500VA or is it enough with 300VA?

Edit: I know have my hands on 2 transformers with 800VA 2x50V and 2 transformers with 300VA 2x50V...are any of them useful?

Graham Maynard · « **Reply #196 on:** 20 May 2009, 04:23 pm »

Hi sannax - 800VA and 2x50Vac - you are so very lucky !

The 300VA will likely have too much series resistance for parallel loading.

One 800VA trafo will be fine for two 8 ohm drivers in parallel, but is your amplifier capable of driving into a 2 ohm load ?

Two drivers in parallel will be 4 ohm nominal, but with the transformer circuit this will be further reduced and if the amplifier is not heavy duty it might struggle above its half power rating.

If you are going to drive at less than party levels your amplifier might be okay.
I will check if you send me a link to its specification.

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

painkiller · « **Reply #197 on:** 28 May 2009, 06:58 pm »

Long time...

I'm now in the middle of a huge project. We're talking physically huge. 32" x 27" constant directivity horns from 500 Hz to 20 kHz at over 111dB sensitivity. After compensation filtering I guess I'll end up at around 100dB. The big question is how to solve out the bass, when going for OB.

I have some options. One option is a 100dB efficient 18" JBL pro driver with Fs=20Hz, pretty high Xmax and low Q. With the T-bass I guess I would achieve pretty good bass output at the cost of heavy amplifier loading. The dispersion angle would be a perfect match at 500 Hz, but what about the distortion? Is such a big driver really useful at 500Hz?

Another option would be using the Lambda Acoustics Dipole 15. These look like great drivers! Huge Xmax, Fs=20Hz, Q=1. They are also 16ohm for paralleling several drivers to achieve higher output, since the efficiency is only 90dB. When combining 2-3-4 of these drivers per channel together with the T-bass I believe one would definitely get the needed bass output. Perhaps this is an easier amplifier load, due to the high Q. But then again I would have to do something to fill the hole between 150 and 500Hz, which would be easily done with an efficient 15" driver.

Any advice on which road I should choose? Or any other smart solutions available?

sannax · « **Reply #198 on:** 28 May 2009, 11:15 pm »

Quote from: Graham Maynard on 20 May 2009, 04:23 pm

Hi sannax - 800VA and 2x50Vac - you are so very lucky !

One 800VA trafo will be fine for two 8 ohm drivers in parallel, but is your amplifier capable of driving into a 2 ohm load ?

Two drivers in parallel will be 4 ohm nominal, but with the transformer circuit this will be further reduced and if the amplifier is not heavy duty it might struggle above its half power rating.

If you are going to drive at less than party levels your amplifier might be okay.
I will check if you send me a link to its specification.

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

Thanks Graham! Here are the specs of my plate-amp.
http://www.madisound.com/catalog/product_info.php?products_id=371

Tomorrow i am picking up two 800VA's...looking forward to that

Graham Maynard · « **Reply #199 on:** 4 Jun 2009, 09:01 am »

Sorry for delay sannax.

That amp should provide enough output to X.max drivers on an OB prior to it suffering from impedance dip clipping.

Enjoy.

Hi painkiller.

I can't see an 18" or even 15" meeting your 500Hz upper driver. Especially as a driver with Fs 20Hz tends to be low and slow circa 500Hz.
If you have the 18s and constant directivity drivers already then can I suggest you optimise these together, then fill the 'gap' with a 10" mounted above with simple filtering and level adjustment. This would be similar to my best so far arrangement, though for LF I used 18" + 15".

Cheers ........ Graham.