Tweaking Sextets.

Peter,

I asked ages ago if you wouldn't mind answering questions about some of your old 'speaker work. As requested, I've put it in this forum.

I have 'Mk1' Heybrook Sextets that were later upgraded to 'Mk4' spec. with a new bass driver and crossover and, apparently, extra stuffing in the bass enclosure. The crossover diagram is as attached (values kindly confirmed by Guy Sergeant).

Years ago I externalised the crossover, and upgraded the components. The capacitors are now all Solens and the Inductors are 'Falcon' wound, and have slightly less DCR than the originals. This was a good improvement, but I want more!
Namely, more bass depth and(!) punch, and hopefully a little more midband clarity.

Each bass enclosure is well stuffed with three 10" x 8" x 2" slabs of foam and two big teased out handfuls of fibre. Each midrange enclosure is slightly less well stuffed, having one foam slab and one handful of fibre.
The 'speakers are attached to the 8 Ohm (which I believe may be closer to 6 Ohm) taps of my amplifier.

I am replacing the skinny 'Mk1' hookup wire.
I intend to experiment with the damping and try the 4 Ohm taps.
I have also thought about tweaking the tweeter's resistor up slightly to drop HF energy.
(And I had a mad five minutes wondering about cutting out the back of the midrange enclosure!)

Have you any tips or suggestions?
I am loathe to build another brand new crossover, but would welcome any suggestions for tweaking the existing values.

[petercom]

I don't recognise that xover - it looks as though it was done well after I left, but then memory plays tricks sometimes and it was a long time ago.

The original Sextet used a bass driver made specially for Heybrook in Wales and had twin port tuning with a foam lining in the ports to control turbulence and add a modicum of resistance. When the driver manufacturer went over to car audio only we reverted to a SEAS bass unit (mk2). This didn't suit the tuning quite as nicely as I remember.

The Mk 3 was tuned extensively to overcome these problems, particularly in the crossover department.

You can, by all means, increase the resistance to the Tonegen treble unit. The first Mk1s had a switch which did this - either 1.5 or 1.8 Ohms I think. The 3.9uF to the unit is highly critical of component choice. I suggest the Soniqs SAX 3.3uF plus 0.68uF (the combo is still within 5%).

How is the 125uF to the mid provided? That's a lot of Solens if it is those! I would suggest 100uF Alcap plus Soniqs 15uF and 10uF PXX if not.

Cutting a hole for the midrange chamber is inadvisable - you will change the crossover for the mid to bass too much. You could make it aperiodic, though, as I did in the Mission e82 and Pilastro by adding a short port and filling it with a spiral of our aperiodic foam. The port can be relatively small in diameter.

Once you have done that you will need to adjust the value of that 125uF by bringing it down by 30% or so. As you keep reducing it the midband clarity should increase due to the reduction in LF power getting to it. When it gets too small, however, you will start to loose the cohesion between bass and mid, noticeable on spoken male voice. Radio 4 is a good test, I find.

Experiment with the amount of foam in the bass enclosure. Reducing it might bring in a little more bass thump. Replacing it with heavy duty carpet felt could work better. It's a suck it and see approach, I'm afraid.

[NealG]

Peter, like Matt I have a pair of Sextets (MKIV's I think) they gave way to Spendor S6e's. I keep eying the Sextets wondering if the midrange could be improved, it's too veiled for me. I haven't sold them as I still like the Toingen tweeter and bass. In fact I find the Bass a little too heavy in my room, I may well try lowering the 125uF cap. How could the bass be tamed as well?

Many thanks!

Thanks Peter,
The damping is definitely suck it and see, taking a quarter of it out has given a bit more 'boomp-h', but I know I'll have to keep experimenting. And yes, the midrange of my XXO is rather large (80, 20, 20, 5) - as per Neal I may take out the 5uF and see.

I was half joking about the openback, but that's an interesting idea about the port, I'll sleep on that. For the tweeter a better capacitor is definitely in order, and tweaking the resistor up by a couple of points or so might just swing the balance.


Neal,
Can you remember/re-measure your tweeter resistor's value?

[NealG]

1R I think Matt.

[Brian]

What are the xover points for this speaker?

Brian,
I believe that Mid to Tweeter is between 4.5kHz and 5kHz. Bass to Mid is, very approximately, somewhat under 300Hz (according to a test tone)

Peter,
Thanks for your input earlier, and for the relevant 'damping' article in the magazine. For your interest, the circuit I attached above was originally based on my measurements, some values were then corrected (10% or so) when I received a scan of the 'Sextet 4' crossover. This was supplied by Guy Sergeant and also has the following words on it:
LINING CHANGE: BASS DOUBLE WOOL IN BASS ENCLOSURE
BASS DRIVER: FIT CAP AND COAT WITH PVA

So, I've finished playing inside the cabinet:
I finally replaced the cheap Mk1 internal wiring mine still had going to the mid and treble with different stuff.
I rearranged the midrange cabinet damping. The 'foam pad' is now wedged tight against the top and back walls, the fibre (hippy wool, I now believe) has been teased out and fills the rest of the enclosure with a nice depth of it behind the cone.
I did similar in the bass cabinet, by wedging two of the pads tight against the top/back/side walls, the other at the bottom and the masses of wool teased out into the centre of the cabinet and behind the cone.

The sound differences were pleasantly surprising.
The midrange has cleared up with the new wire and better damping, it has lost its slight clattery and blurry nature and stereo depth/width has improved.
I wasn't expecting as much from the bass but now, with a properly damped enclosure, I have bass rumbling across the floor without the box honk I'd introduced earlier.

They are still not perfect, but they are a lot better.

So back to the crossover.
Is it purely first order with an impedance equalisation circuit across the woofer?
Tweaking the tweeter resistor up will almost certainly happen, my initial measurements had this at 1.1 - 1.2 Ohms which is slightly higher than specification, and Peter's Mk1s had a higher value still.
Removing midrange capacitance is still an interesting idea.
It has also been suggested, that altering the value of the resistor across the woofer could alter the bass character, any thoughts?

[petercom]

You can adjust the resistor in the circuit across the bass unit up to 15 Ohms and this will alter the bass character and increase the midrange output of the bass unit. Try 10 and 12 Ohms and hear what happens.

The crossover is first order electrical using the natural roll off of the units to provide second order acoustic slopes with the exception of the bass crossover which is a highly damped second order electrical which also does a dual duty in helping to equalise the impedance rise through the midrange.

Peter,
A belated thank you for your input.
I, as yet, haven't felt it necessary to tweak the crossover values as the changes I made to the distribution of the internal damping addressed most of my issues. (Tweaking my power amplifier sorted the rest!)

I've attached Dominic Baker's test results of the Sextet MkII just for interest. Apologies for the just readable quality!

Neal, I remember you had produced your own graph for the impedance curve of your Sextets, which I've lost, did it differ greatly from tghe HFW one?

[planet10]

I'm not at all familiar with Heybrooks... they never made it to this part of the colonies... how is the midrange enclosure constructed?

As to opening up the enclosure, it is not crazy at all -- this is something i've suggested before for the IMFs, carefully stuffing the long tube until it became aperiodic.

dave