'T'-bass drive for OB LF drivers. - diyAudio
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Old 1st October 2008, 12:04 PM   #1
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Default 'T'-bass drive for OB LF drivers.

The 'T' bass circuit is attached in open baffle form.

A first construction using the values shown will give an idea of its capabilities.

The 2x 470uF (low ESR electrolytic) capacitor tunes the choke to the baffle/rear reflection peak which can become obtrusive around 100-120Hz. This can allow an OB to be moved further back into a corner. Try different capacitor values to adjust the 'cut' frequency.

The cut runs from around/above driver Fs and becomes maximum at the baffle/room corner SPL peak frequency. The resistor in series with the capacitor controls the degree of cut at this frequency.

The choke controls the voltage step-up which arises below/around driver FS where the driver impedance becomes high, and often fails to return to nominal at low audio frequencies. The choke value can be increased to reduce the boost frequency, but this depends on baffle (U/H frame etc.) size. It is the slope with respect to roll-off which is most important, not just the boost frequency itself, for this circuit is acting in series with the driver(s), and it is the driver(s) which limit the 'boost' capabilities the step-up ratio can provide. Choosing 'too low' a choke value will produce a kinked boost resultant which no longer optimally matches the driver/baffle roll-off. The resistance of this choke should also be low to allow LF 'voicing' via resistor adjustment. If the choke does not have a low resistance, then the resistor in series with it may be omitted.

The resistor in series with the choke plus the resistor in series with the capacitor together control the degree of boost/cut arising to balance the first half cycle transduction losses against driver resonance derived SPL increase. There is an initial phase coherent series choke induced boost to counter driver subtraction of first half cycle energy, which becomes stored within the suspended cone and contributes towards resonance, followed by damping due to the series tuned C+L input circuit which limits continuing energy input into the loudspeaker system at its resonant frequency. Resistor values which are too low will provide a response with too much first half cycle emphasis - hence values should be individually selected to suit the driver cone mass, baffle form, Qes etc., also to suit personal preferences, because a slightly over emphasised boost might actually be preferred to compensate for some unavoidable loss at the very lowest of reproducable frequencies.

The line transformer may be between 250 and 500VA, between 2x 18V and 2x 40V depending on driver choice, and with its mains voltage primary left OC. The lower ratings would suit one or two Aplha-15As in parallel, the higher ratings larger Pro drivers. Of most importance is obtaining a transformer with low secondary winding resistance, which generally means toroidal types.

The series output choke should be chosen to suit the selected LF driver; 2mH being shown here as a mid value starting point. Its value may be lower with parallel drivers, higher for those having good MF sensitivity, or may even be omitted with drivers already having significant voice-coil inductance.

The 10uF in parallel with the output choke makes it act like a parallel tuned circuit to introduce roll-off in the typical 1-2kHz breakup region. The final series connected R+C components form a Zobel to counter driver impedance rise with frequency.

The driver is your choice. I have had feedback of the circuit working well with several different types.

The amplifier MUST be a SS NFB type with good damping figure. It should be 4 ohm continuous rated for use when this circuit drives an 8 ohm driver on an OB, and 2 ohm continuous rated when two 8 ohm drivers are driven in parallel. If of sufficient quality this same amplifier may also feed the wideband driver via a series capacitor, say 47 to 220uF, though with a damping resistor connected in parallel with the wideband driver voice coil, say 8.2 ohms accross an 8 ohm nominal driver. Once the wideband driver has this damping resistor connected it is easy to reduce its sensitivity by inserting a resistor in series with the capacitor, say 2.2, 4.7 ohms etc., maybe with a 470nF to 1uF in parallel with the series resistor to maintain overall balance if a supertweter is not being used.

The T-bass circuit may also be used for IB too - between amplifier and crossover though without the series output choke. The kind of energy balancing AF response this 'T'-bass circuit produces *cannot be matched by EQ*, because EQ does not act with the loudspeaker during waveform time.

Cheers ........ Graham.
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Old 3rd October 2008, 11:43 AM   #2
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Only those who actually construct and listen to this circuit will appreciate the advantage it provides.

It is possible to increase OB output at LF by using a high 'Q' LF driver, but the amplitude gain arises due to driver resonance and passive crossover interaction, typically in the Fs to 2x Fs frequency range, but below Fs the relative output then falls away much more sharply as a result.

The 'T'-bass circuit provides a similar lift, but the drive is more phase coherent with input, and with a decent transformer the boost below Fs is maintained down through 20Hz.

I here show force related driver current via a representative woofer equivalent circuit, because driver reactance renders a voltage waveform viewed in isolation less directly relevent.

Cheers ........ Graham.
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Old 3rd October 2008, 12:00 PM   #3
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And some current traces via a representative woofer equivalent circuit.

Voltage responses are not shown here because it is the current which energises the voice coil, and current becomes phase shifted by driver reactivity. As illustrated here by the direct connection to a voltage output amplifier.

If too much energy transduced via an initial current peak becomes stored a LF driver, then the reproduction becomes muddied. On the other hand current drive does not provide enough 'first cycle' drive to accurately amplitude drive the cone mass.

Adjusting component values adjusts the level of first cycle boost, slope, peak and dip frequencies etc.

Cheers ...... Graham.

PS. If your browser won't resolve this - try fullscreen (the F11 key ) or right click and save the picture to desktop then (double)left click to open with the normal Windows viewer.
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Old 3rd October 2008, 12:09 PM   #4
Zen Mod is offline Zen Mod  Serbia
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I have 2 autoformers (220:110V , 1,2KVA) ;

can I use them for that ?
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Old 3rd October 2008, 12:36 PM   #5
SimontY is offline SimontY  United Kingdom
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Thanks for the explanation to T-bass Graham.

I love your circuit, it's working well on my test baffles (4 x 7" woofers, series-parallel).

Question: what happens if the transformer is too small?

Mine are 100VA and I can't detect any obvious problems.

Simon

ps - drums, in particular, sound fantastic with this circuit on an open baffle speaker
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Old 3rd October 2008, 02:07 PM   #6
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Any details or pics of your OB setup Simon? Hope to see it at the 08 fest
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Old 3rd October 2008, 02:10 PM   #7
SimontY is offline SimontY  United Kingdom
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Quote:
Originally posted by richie00boy
Any details or pics of your OB setup Simon? Hope to see it at the 08 fest
Hi Rich,

Any more subwoofers on the go?

Here's my OB story so far:
My dipole experiment

I'm afraid most of it is me thinking out loud and learning! But there are a few pics. More to add actually as I've just been tinkering.

T-bass is what it's all about though, as you'll see if you read it through.

Simon
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Old 3rd October 2008, 02:28 PM   #8
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Hi Zen Mod,

Give them a try - they sure won't be saturating !

A single winding resistance of circa 0.1 ohm is desirable, at 0.2 ohm some LF loss of attack becomes just detectable, and even up to 0.5 ohm will work well - though maybe with different resistor/capacitor/choke values.

Hi Sy,

Glar to hear you are enjoying the improved OB bass.

Your 4 driver series/parallel will also be improving efficiency, means you get decent output without too low a load on the amplifier.

When the transformer is too small it just does not work as well as a larger one will, and if of too low a voltage it might saturate and cause an amplifier to clip.

If yours is working fine, just make sure you go as big as you can if you upgrade.

Cheers .......... Graham.
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Old 3rd October 2008, 02:31 PM   #9
SimontY is offline SimontY  United Kingdom
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Thanks Graham,

Can you explain the capacitor in parallel with the series (low-pass) inductor? You mentioned a tuned-circuit, but this doesn't help me

Simon
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Old 3rd October 2008, 03:03 PM   #10
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Hi Simon,

Many LF drivers beam or resonate or suffer cone breakup modes before they roll off. This can cause a peak which interferes with mid driver output. T
The capacitor forms a parallel tuned circuit with the series inductor. This can cut output by 30dB at the peak frequency and or assist LF driver roll-off.

Very easy to try. To broaden the range of frequencies being cut, but also make the degree of cut less shallow you could try a resistor in series with the capacitor, say 1 to 4.7 ohms.

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