[Graham Maynard]

Hi Boudy,

Yes I am refering to the electrical Qes factor alone, for it is this characteristic (in series with driving impedance) which controls the driver's voice coil, the cone, its transient response and its mechanical damping capabilities.

The B200 either needs its own full size baffle or a series connected baffle step compensation circuit to reproduce correctly, and even then it has limited LF displacement capabilities.  If you don't mind listening at even lower levels then the B200 could be driven by this circuit, and the component values shown would be a fair starting point.  However, making its cone move more at LF will further limit its linear output capabilities at other frequencies.

The B200 is specified as +/-3.5mm and it is capable of this at LF.  However, its genuinely linear X.max is only +/-2mm, and this means that mids and highs will be distorted by any bass excursions exceeding +/-2mm, which the voice coil drives right through unless the listening level is limited or the LF is filtered away;  this being exactly the opposite of what the T-bass circuit has been designed for.

I note you have a B200 desktop U-baffle, so if you use this transformer boosting circuit you must either accept a reduced listening level, or use it to feed a separate LF augmenting driver directly beneath it on the same baffle;  maybe driving a BG20 with its own separate choke and resistor/capacitor crossover circuit, which leaves the B200 separately driven and unaffected by the raised bass output.

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

[boudy]

Thanks again Graham.

Here's the context, sorry to pull this discussion in a somewhat different direction.

I've been using 2 way prototype OBs for the desktop that have 8" Goldwood 8028 woofers (driven by a Sampson amp) and the Fostex F120A full range (driven by the Trends amp). The source is a Squeezebox 3, which feeds the digital input of a DCX2496 digital crossover. While this system sounds pretty good, it does not have the clarity and presence of my main system (206ESR BIB, huge!). I listen to a wide spectrum of music and like a solid bottom end.

I recently setup the B200's (with BSC and Zobel) to see if they could replace the 2 way speakers and allow me to simplify and upgrade the system. I suspect that the 2496 is the weak link in this system, however, I was hoping to be able to eliminate it by going to a single driver rather than improving it (analog output mods, digital input mod, PSU, etc).

So it looks like I should stick with the existing 2 way OB and experiment with the T-Bass circuit on the woofer. And invest in the improvements to the Xover.

Then I can experiment with the B200 in the HT system, where an SVS subwoofer handles the LF and where the baffles can be larger.

[Graham Maynard]

Hi Boudy,

If you have enough components and a transformer you can press into service, it might be worth trying the circuit out on your BIB !

Might give the reproduction even more 'presence' (will definitely take it lower) as well as give you a chance to hear what the circuit/driver can/can't do.

Cheers ........ Graham.

[Graham Maynard]

Hi Rudolf,

Thanks for your e-mail; will digest/reply shortly; different computer for e-mails.

I've thought that a horizontal ripole could be used as a LF extending sub beneath a plain dipole woofer whilst at the same time being the physical base for the main baffle.

I've tried the U-frame (with damping) and many other arrangements with a dipole, but the LF reproduction cannot be balanced in the same way as when using the transformer circuit on a plain dipole woofer, so I guess my present path is already plotted.

Another way of looking at it is that the choke and the capacitor in the T-bass circuit can be used to adjust the LF response 'timing' in much the same way as can the phase control within the equaliser part of a plate amplifier;  where the eventual reproduction becomes optimised as opposed to any theoretical requirments being technically refined.

I do like the concept of cardioid LF in relation to dipole operation, especially if this might mitigate a first corner/wall rear reflection induced peak circa 100Hz back into a room, and especially if it were made tunable for individual LS sitings within different rooms.  However this would be more complex again and require monopolar output behind the baffle to be subtracted from the dipole output at the required frequency (constant energy), or possibly a variable TL arrangement, and I will not be burdening 'she who must be obeyed' with this kind of investigation, so this is not on my proposed path either.

I've still got to finish discussing the capacitor/resistor value determination. 

Cheers ........ Graham.

[Rudolf]

... I've tried the U-frame (with damping) and many other arrangements with a dipole, but the LF reproduction cannot be balanced in the same way as when using the transformer circuit on a plain dipole woofer, so I guess my present path is already plotted.

I have rescued a rusty-but-trusty 50 VA transformer with double 10/12/15 V taps from my basement and will eventually try it with my H-frames. But this will take some time to happen.

... I do like the concept of cardioid LF in relation to dipole operation, especially if this might mitigate a first corner/wall rear reflection induced peak circa 100Hz back into a room ...

I always believed that the first rear/side wall reflections of a dipole should be used to add to the bass response. Of course this asks for some freedom in the placement of the dipole to get the right shape and amount of that reflections.

Rudolf

[Graham Maynard]

Hi Rudolph.

It would be great if you could try the circuit.
I have already ordered a pair of low Qes Beyma SM-115N to use on one baffle with the T-bass.

Over the last week or so I have been on other forums discussing opinions relating to Qes etc.

Some writers have the opinion that higher Qts (controlled by Qes) drivers are okay for OB;  however this depends upon whether one or more LF drivers are to be used and whether SPL or SQ is the priority.

Some writers have the opinion that EQ can correct driver response on a open baffle, I think not *unless* the driver (or a LS system) has low Qts.

I also note that Peter Comeau, writing in the July-08 Hi-Fi World magazine is covering dipole bass generation.
He has tried both the Alpha-15A (Qes= 1.53) and the Beta-15A (Qes= 0.63) on OB, and says when the Alpha is in a room of moderate dimensions it maintins LF output, but that its articulation is blurred.  He says the Beta brings much of the clarity back, but with a less subjective extension.  To be continued next month.

It is my intention to use low Qes drivers to maintain the articulation, but two of them to gain the LF extension.

I don't have much time at present, but have tried some more electrical simulation of the T-bass circuit.  I noted some very interesting results which match what has been heard - and a quite different dynamic (first cycle) response, which standard EQ simply cannot achieve !

I regard these simulated findings to be valid because the original circuit derivation and the listening came before the simulation;  the point being that we need to be extremely wary of simulated responses when the simulation comes first, for these does not always tie in with the reproduction qualities eventually heard.

This can happen due to dynamic (first cycle) responses not being observed via a steady sine energised type of readout;  those first cycle responses are completely missed by the time any 'r.m.s. sine' or 'dB-SPL' level settles enough to provide a measurable reading at any frequency.

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

[Graham Maynard]

At last I have managed to sort out some of what is happening with this circuit.

Whilst easily measurable voltage and current amplitudes are important when driving a LS, so too are source impedance and current phase/group delay with respect to an original voltage waveform.  I am not however in a position to relate driver current/voltage waveforms to the transduced sound waveform reaching our ears at a listening position.

This T-bass circuit does not just increase the LF amplitude, it improves the reproduction quality as well.  Why ?

I have tested this circuit with mid-bass drivers having a Qts circa 0.4.
David above noted improvement via his drivers having a Qts of 0.23.

Reproduction quality improvement has not yet been checked/reported with a higher Qts LF driver (>0.7), though there is no reason why improvement should not be possible due to to the way in which waveform amplitude is increased below Fs, whilst output can be reduced by the circuit to counter an increase arising around Fs due to driver Qts.  Hence reproduction quality with a B200 + Alpha-15A baffle combination is still likely to be improved, though would eat into a single Alpha's X.max limited capabilities.

A B200 + Alpha-15A can be matched using MJK's latest software, which I believe relies upon SS amplification.  The LF response of this simple combination might be further extended by using a tube or current source amplier, but then the LF definition could suffer as a result of the Alpha's natural Q becoming less well controlled.

Here is a simulation circuit for looking back into the 'T'-bass component assembly from the viewpoint of a 6 ohm loudspeaker in order to examine the reactive nature of the effective source impedance being generated.  From the outset this reactive nature was intentional in order to counter LF driver responses due to their own unavoidable natural reactivities.
(Other simulations will follow using an approximate equivalent LS circuit in order to more realistically examine electric current flow generated by the 'T'-bass when driven by a SS amplifier.  It is not likely to work similarly if driven by a tube or SS current source amplifier, unless that tube amplifier is a push-pull triode or ultralinear tetrode type with global NFB.)

Transformer winding loss resistance does not change the shape of the the plots shown here because the impedance is a function of the C, L and R values chosen;  however transformer resistance does reduce amplitude plots as the effective series resistance increases proportionately.
Transformer primary resistance can be countered by changing the resistor values in series with the C and L components, where all the values need to be adjusted to match individual driver+baffle+room installations anyway. 
Transformer secondary resistance is likely to be small compared to LS driver resistance, with their series connected values added together anyway.

The reverse energised examination circuit is shown here;-
http://server6.theimagehosting.com/image.php?img=TB.imp.test.gif

The impedance and phase plots for the component values here;-
http://server6.theimagehosting.com/image.php?img=TB.impedance.gif

The three grey lines represent; top - 0.6 ohm;  centre - 1.3 ohm;  lower - 3.3 ohm.
_______________________________________ ____________________________________

Below circuit resonance the 'T'-bass output impedance is due to the choke impedance, its resistance, plus series resistor and transformer winding resistance. 
Here the summation tends towards 1.3 ohm by 10Hz.
The 'T'-bass gain when loaded with and matched to a LF driver system is approx 5dB, but the source impedance generated by this circuit is increased too. 
Hence the LF SPL below driver resonance is increased more than might be expected due to circuit voltage gain plus an effective increase in the driver's Qes due to an increase in the driver's source impedance.

Another aspect relates to sub-resonant LF driver impedance.  This rarely falls fully back to the nominal value below resonance - an aspect which further contributes to SPL losses during LF transduction efficiency.  Thus the increased voltage drive provided by the 'T'-bass, even with its increased output impedance, provides a better amplifier/loudspeaker impedance match below an OB mounted driver's Fs. 
Also, the load presented to an amplifier by the 'T'-bass circuit when driving an OB LF driver does not fall as low as might be expected due to any simplistic calculation of a 1:2 ratio transformer driving a nominal value resistor load.
_______________________________________ ____________________________________

Close to driver resonance the 'T'-bass circuit output impedance increases - here to 3.3 ohm - a value which can be set by adjusting the C, L and all resistance values. 
Even when the equalising impedance at the primary is adjusted to transform with unity output gain, the reproduced SPL can still be increased due to the raised 'T'-bass output impedance effectively increasing driver Qes.  Yet current provided to the driver via the C+L+R tuning may be adjusted to reduce output at Fs, and thus counter the resonant energy storage within the driver which gives rise to increased SPL but degrades the drivers dynamic response.

'T'-bass throughput to low Qts drivers may thus be adjusted to match the driver and increase its LF output, yet it can also help to counter a high Qts driver resonance induced 'tonality' due to the driver's naturally resonant characteristics which arise in 'driver controlled time' and not 'music waveform controlled time'.

S we must not forget that where a driver has a higher Qes, or develops a higher Qes due to drive circuitry (type of amplifier, crossover, 'T'-bass etc.), then the driver's Qms can have an increased impact upon not only the SPL, but also the reproduction quality due to the effective increase in driver Qts.  Whilst the 'T'-bass circuit is capable of increasing LF output, it is equally capable of allowing too high a proportion of LF 'gain' arising due to the increase of effective driver Qes/Qts at Fs, and the driver can be rendered incapable of transducing/reproducing cleanly - actually more like a driver in an undamped vented enclosure !
_______________________________________ ____________________________________

Above the driver resonance, drive impedance tends towards 0.6 ohm.;  this being the total resistance in series with the capacitor = R plus transformer winding resistances.

Here the circuit gain is unity (no step-up via the transformer) and the source impedance is close to normal, thus the SPL is not increased. 
However a capacitor induced increase in dynamic first cycle current flow can counter first cycle loss due to a driver's natural impedance/mass induced dynamic loss. 
Ordinary EQ cannot prevent this entirely natural loss of first cycle dynamic attack within a driver, nor indeed does this circuit, it does however compensate in a manner where the initial capacitor induced boost can be adjusted to match the loss during that first cycle.
See the current flow simulations in the post below.
_______________________________________ ___________________________________

So the 'T'-bass circuit can increase LF output in two ways.
1)  Below resonance it steps up the driving voltage.
2)  Around resonance it allows the driver Qts to become increased. 
It can also improve reproduction by linearising current flows which would otherwise remain reactively modified by driver characteristics if driven by either predominantly voltage or predominantly current source amplifiers, and it can compensate for dynamic loss within a driver.
_______________________________________ ___________________________________

A low moving mass plus low Qts driver driver can reproduce more accurately because it subtracts, stores and releases less of the transduced waveform energy within its own electro-dynamically energised time frames, but this makes for a less SPL 'efficient' transducer on OB.

The 'T'-bass circuit allows low Qts drivers to run more like higher Qts drivers at lower frequencies only, and may be tuned for 'best' overall dynamic/SPL characteristics in a way which could not otherwise be achieved without much more powerful amplification, complex NFB circuitry and/or additional series/parallel driver loading components. 


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

[Graham Maynard]

More simulations follow here.  Click on the images to open with maximum resolution.

http://server6.theimagehosting.com/image.php?img=TB.V.I..png

This shows the simulated 'T'-bass circuit feeding an equivalent driver via a series choke (or the driver could be one having a higher voice coil impedance).  In order to show the changes in both voltage drive and current flow with frequency due to the 'T'-bass circuit, a second virtual driver is directly connected to the amplifier as a 'normally connected' reference.
(Waveforms cannot be simplistically subtracted in order to show any difference due to their phase responses being reactively modified.)

The Mauve trace shows the 'T'-bass output *voltage* characteristic.
The Red trace is the *current* characteristic for the driver+choke when connected directly to a SS amplifier.
The Green trace is the *current* characteristic for the driver+choke when driven by the 'T'-bass circuit.
The Grey lines indicate current flow via 6 ohm and 90 ohm resistors.

There is an increase in output around and below driver Fs, and a slight reduction above which can become integrated with a crossover response.
_______________________________________ ______________________________________

In the following simulations;-
The Blue trace shows the amplifier output *voltage* waveform;
The Red trace shows the amplifier driven LS driver *current*;
The Mauve trace shows the 'T'-bass output *voltage* waveform;
The Green trace shows the 'T'-bass driven LS driver *current*.

http://server6.theimagehosting.com/image.php?img=TB.20Hz.30Hz..png

http://server6.theimagehosting.com/image.php?img=TB.40Hz.50Hz..png

http://server6.theimagehosting.com/image.php?img=TB.70Hz.100Hz..png

Descriptions to be added, especially with regard to the first cycle responses illustrated.


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

[Graham Maynard]

I have added simulations to the last post, and further clarified some wording within the last three posts above.
There is still more to add to the last post relating to the simulated driver current waveforms, so do please check again after a day or two.

This writing has become far more involved and longer than I could ever have expected.
I suppose I am trying to justify/show how/why the 'T'-bass circuit actually improves low frequency reproduction, yet the simplist way for anyone to understand is by simply setting one up and enjoying the results.

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

[Graham Maynard]

Not able to do audio at the moment.

This is interesting -
http://www.nousaine.com/transfer.htm

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

[Graham Maynard]

Nobody else tried this circuit yet ?

If we look at Linkwitz's 'Seas W22EX001' table about 3/4 way down this page;-
http://www.linkwitzlab.com/frontiers_2.htm

it becomes clear just how much extra displacement and kinetic energy becomes necessary with an OB driver at LF.

Due to driver cone mass and suspension characteristics there is a need for increased first cycle drive around resonance due to some of the energy available to the driver being stored, this energy being subtracted from that available via the voice coil for the initial first cycle amplitude displacement. 
That stored driver energy subsequently increases the steady sine/SPL output we normally see measured and published and muddies reproduction.
The higher the Qes of the driver - the greater the loss of initial first cycle displacement - also the greater the smearing (loss of clarity) of the reproduced dynamic response as the driver relies upon its resonance to increase its low frequency amplitude output.

In a post above I showed simulations;-
http://server6.theimagehosting.com/image.php?img=TB.20Hz.30Hz..png
http://server6.theimagehosting.com/image.php?img=TB.40Hz.50Hz..png

Note the increased 'T'-bass current drive (green trace) with respect to the same equivalent LS (Fs~33Hz) and its direct current drive (red trace), also the normalisation of the increased first cycle drive which arises due to initial cone mass/suspension driver storage.
The electrical phase shift and subsequently reduced current draw about a driver's resonance is an entirely natural phenomenon when a LS is driven by a SS NFB amplifier, and with a high Qes driver it leads to SPL being reduced when compared to output when driven by say a non-feedback tube or current source amplifier;  yet neither amplifier type alone can be best for LF drive, and this is where the 'T'-bass comes in.

Adjusting the capacitor and inductor values tunes the frequency of the LF boost, whilst the resistors control amplitude.  The tuned 'T'-bass drive can counter a driver's natural transduction inadequacies, yet this can also be over-done to the point where first cycles sound overly punchy - maybe good for parties and impressionable youngsters, though not for genuinely clean reproduction.

Another way of implementing the transformer boost - which appears more intuitive - would be like this. 



However the transformer would need to be of really excellent quality/size to take full SS output for OB drive with LF electronic 'music' waveforms.  My original circuit deliberately had the LF choke in series with the transformer primary for this very reason;  saturating transformer cores and SS NFB are not a sensible combination.
This second circuit has not been tested.  From an amplifier/drive point of view it is more efficient, but it is bound to present a different tonality because the L and C are parallel instead of series tuned, and thus I cannot say whether this will sound better or not.  Maybe a permanent resistor (of 47 ohms?) directly connected between the amplifier and the LS would also help.

Further simulations suggest that the 'T'-bass circuit could be beneficial if used with higher Qes drivers too !  It can reduce output at frequencies where there is resonance whilst increasing it below, and all this without needing to buy another amplifier, which if directly connected cannot sound as good anyway !!!


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

[panomaniac]

Hey Graham.

The sims you did look just like mine, but in reverse.  That's because I did voltage, you did impedance.  So the same stuff, really.

The curve can be changed a LOT by modifying the LCR values.  Just sim it and see.

I'll try to circuit again as soon as I get a chance.  It's certainly intriguing.

[Graham Maynard]

Just showing XLBaffle simulations for four 15" drivers on an upright baffle;-



and the individual driver responses;-


 
Click on image download for full resolution.

The Alpha-15A can 'measure' well on the baffle, but its more 'level' output is gained due to cone resonance !
Hence my choice to go for 2x Beyma in parallel on a single upright baffle for +6dB output, and 90dB(relative) at 20Hz with crossover and 'T'-bass as necessary to match a B200 running widerange with LF cut.
The Beymas will give more bass, with minimal resonance, and will have greater output and displacement per Watt than the more expensive cone mass tuned single car driver shown here. 
The MTX car driver is the most suitable I have found for OB use.
(The Beymas were ordered 3 weeks ago, but still waiting.)

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

[Graham Maynard]

Which transformers to use?

Answer - typical SS amplifier PSU toroids >250VA rating.

I have tried different types including 'old fashioned' clamped E-I core heavyweights. 

The older types have higher winding resistances for the same power rating and tend to lose out just a little on waveform amplitude and quality at lower drive frequencies (<50Hz).

However all do work.  The older higher resistance/ lower inductance types must be allowing driver Qes to increase slightly more, such that the driver makes up for the electrical inefficiency at the lower extremes, but the same cone displacement is not there on close miked kick drum.

Choosing a higher voltage component provides a higher winding impedance, also lower frequency capabilities without risk for saturation, and with something like a 500VA component the winding resistance will remain low even up around 2x 35 or 40Vac.

So my recommendation for the transformer is toroid, between 300 and 500VA, and between 2x 25 to 2x 40V.

Why so large ?
Because they must transform without saturating at frequencies way below the 50/60Hz they were designed to run at.

A 300VA 2x 35V I presently have here has 0.2 ohm resistance and >60mH per winding;
a 500VA 2x 40V 0.1 ohm and >50mH per winding.

Both work well.  The 300VA being fine for single 8 ohm driver working; the latter better for use with two 8 ohm drivers connected in parallel.

Also 500VA toroids have a low winding resistance loss to cost ratio; so winding my own on old clamp cores will simply not make any sense.

Changing the transformer most definitely changes the component values which become necessary to optimise LF boost and output, thus with a good transformer the capacitor value might be better reduced in value as with David's findings above.  Say starting at around 1,000uF.

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

PS.  Anyone who has not yet tried this circuit for OB LF driving really cannot begin to imagine what they are missing out on. 

The parts should cost less than an additional driver, yet reproduction with a single driver can be improved beyond that with two LF drivers due to the way in which it better matches the driver's response to baffle/ room level changes.  This circuit counters the phase/ tonality mismatch between a widerange and LF helper as is so often observed and reported when a conventional crossover is fitted to the LF driver to counter its raised upper bass output.

[Graham Maynard]

Still no one else tried this circuit yet ?

Yes it costs to construct, but there are significant differences related to reproduction quality.
_______________________________________ ___________

I could NEVER get a larger driver to sound 'right' when used as an 'augmenter' on the same OB as a fullrange driver when the drive to the larger unit was low pass filtered for its frequency response to match and blend in with the response of the main driver;  this whether the main driver was used as fullrange or with highpass as widerange. 
Also it did not matter whether the LP filtering to the larger driver was active or passive, or whether the LF driver had its own amplifier.
_______________________________________ ____________

Recently I have been trying to improve upon the T-bass using a simulator to shortcut on real life testing (noise) time. 
I now have something to try, and will report the results if successsful.

When doing this I decided to look at the phase responses for LF driver augmentation arrangements, as would be typical with say an Alpha-15A and a full/widerange on the same baffle.
_______________________________________ _____________

If the frequency response of the voltage to the augmenting driver is shaped using L+C low pass circuitry to cut voltage drive above say 50 to 100Hz, then the voltage input to that LF driver is often in lagging quadrature circa 250Hz and completely out of phase by 800Hz.

This is why 'simple' augmentation does not sound right.  The bass sounds 'slow' !

Yes you can get the phase of some low bass to match the phase of the mid/high-mid ranges on the main driver, but that crucial mid/high bass and low mid region reproduction loses coherence due to the relative output phases of the two drivers shifting with frequency and the output of the larger driver interfering. 
Thus the characteristic 'body' sounds of voices and instruments take on a characteristic which is due to the nature of the crossover circuit which is filtering the LF drive, and the use of better drivers cannot overcome this.
Additionally, a plain crossover 'C' directly connected across any driver voice coil has a separate tuning effect upon the driver which is independent of signal voltage drive where that drive has series impedance (as with a choke) !

In the T-bass circuit I have the C and L balancing against driver reactivity so that the LF voltage step up due to the transformer below 50 to 100Hz smoothly becomes a reactive potential divider network above.  This returns drive voltage phase back to zero circa 800Hz, with the drive lagging only about 20 degrees at 250Hz - where the 'conventional' crossover is often in full quadrature and making the bass sound 'slow'.

I also tried simulating lots of different large driver parameters with the T-bass circuit, and it became clear that it is the driver itself which determines just how low and efficient the boost will be - this because the T-bass circuit is effectively in series with the driver itself.

This is the reason for David above gaining a more useful advantage with his low Fs Madisound EM1550 15"ers than most of us could.  I was worried about toroidal transformers being bought before I had been able to test what would be best - but he must mave already heard the better sound already, and he did indeed choose correct components for his drivers.

So, whilst the T-bass will help for OB augmentation with all drivers, it will provide better improvement with low Fs, low Q drivers; of course the larger the linear air displacement the better too.
(Just a reminder here that NFB controlled SS amplifier drive is essential - not tube/valve.)

Cheers ....... Graham.

PS These phase changes are not separately identifiable within the electrical impedance chatacteristics for the composite LS system, nor via a microphone measured (or simulated) SPL response.

[panomaniac]

Hey Graham,

Don't forget that the T-bass circuit adds some series R, so Qes is going up.  That's got to change the response of the driver.
What's the series R of the transformer windings you use?  (mine are packed up, so can't measure now)

[Graham Maynard]

Hi Michael,

Yes indeed, the wound components do add series R, and this can allow a low Qes driver to behave more like a higher Qes driver, thus producing greater output from Qms alone, as well as from the increased LF voltage drive.  I did mention something like this in the above.  The LF SPL is increased more than might be imagined.

Everything on the primary side of the transformer is going to have its series impedance quadrupled with respect to the driver, which is why most tube amplifiers will not be as good.

My 500VA 40V primaries are about 0.1 ohm each, but the driver will 'see' approx 0.5 ohm for the transformer between itself and the amplifier.  Also the resistance of my 0.5 ohm choke winding will look more like 2 ohms as well, and this being series additive to its inductance at any frequency.  In some simulations, the source as 'seen' by the driver was close to 3 ohms, and this increased driver output at the natural resonance frequency.

The choke end of the transformer is connected to a series resonance tuned node, which is capable of lower impedance via the capacitor at or above Fs.  Thus the output at and above driver Fs can actually be reduced, so that the increase in source impedance allowing the driver to self resonate can be controlled by adjusting component values.
_______________________________________ _______________

I tried 2 circuits which the simulator showed will increase LF SPL - result - they don't *sound* right !

Yes the amplitude at say 30Hz can be further increased, but drum thwacks sound more muffled and the trade off for increased organ music level is simply not worth it.

I had simulated leading edge start-up with 30Hz sines and noted that the response for the proposed circuits had a typical resonantly induced increase throughout the first cycle.  I also tried to convince myself that the ear would tolerate a 2dB difference in amplitude between first and second half cycle amplitudes because all harmonics would be shared via another driver; but NO.

It is still the leading edge volume displacement of a LF waveform (first 90 degrees) which reproduces the character of drums or plucked double bass etc. and this has absolutely *nothing* to do with steady sine measured or simulated amplitude/frequency responses.  This also being why a resonant LF enclosure cannot sound as clean as an aperiodic for the same low note instruments, and why crossovers for sub/main drive can ruin the leading edge of a LF waveform. 

I have shown to myself that SPL charts and steady sine measured amplitude responses are almost irrelevent when it comes to revealing cleanly reproducing a first LF cycle as well as for mids and highs, which is why bigger really is better because phase shifted reinforcement methods take time to build up and always fail to displace enough air in the listening environment until their delayed augmentation becomes simultaneously acting in time;  this being my reason for choosing the low Qes drivers in the first place.

Next step - to find a low resistance choke which will not cost a small fortune.  This to further degrease the source impedance 'seen' by the driver and thus increase the LF leading edge displacement, rather than have driver 'Q' doing its own thing at Fs. 

The driving waveform needs to be in control, not the driver itself.

Unfortunately for OB this cannot be achieved by normal active or crossover networks without the level changes also introducing phase change !
About 10 years ago I ran a LF equaliser which I called 'e-bass'.  It equalised similarly to this circuit without introducing phase change to steady sines, but it did require a separate LF driving power amplifier.  Presently I am trying to avoid implementing any additional amplification. 

Everything is a compromise bound together within an amplitude+ frequency+time continuum;  unless viewed in total isolation you can't change one characteristic without affecting another within the dimension of the third. 

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

[panomaniac]

That''s low DCR - lower than I would have thought.
You're using the primaries?  I thought you were using the secondaries.  Or did I miss soemthing?

Some day in the not to distant future, I'd like to measure what is going on with the T-Bass circuit.  Both electrical and acoustical measurements.  Maybe use an audio sample as the waveform, like a kick+snare, plucked bass, etc.  Would be fun to measure what's going on.

[Graham Maynard]

Hi Michael,

I have just realised where a misunderstanding can arise via my circuit description.
The original transformer primary (circa 2 to 7 ohms) remains unused = open circuit

The primary winding presently used with my T-bass application is the 0.1 ohm winding connected between the amplifier and choke;  this *originally* being a transformer secondary.

The secondary winding for the T-bass output is the other 0.1 ohm winding, which also was a secondary.

The LS 'sees' 0.1 ohm + 0.1 x (2^2) ohm = 0.5 ohm for the transformer source alone.
The LS would 'see' 1 ohm from a transformer with dual 0.2 ohm transformer windings.

Both of the source resistances represent potential divider loss in series with the LS; hence I am using the 500VA component because I have two 8 ohm drivers wired in parallel, whereas a 300VA transformer would be quite adequate for a single 8 ohm driver.

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

[panomaniac]

Gottcha, thanks!