Doped drivers, why?

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Dr Bunsen Honeydew
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Re: Doped drivers, why?

Unread post by Dr Bunsen Honeydew » Fri Apr 14, 2017 11:24 am

Just gather what you want how you want on a word processor and copy it across when Terry does his magic.

Or do a new post with all the alterations and I will bin the other one.
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Ithilstone
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Re: Doped drivers, why?

Unread post by Ithilstone » Fri Apr 14, 2017 12:15 pm

that was the idea for time being -
hope Terry will come up with something as it will be great for digging deeper as well as cooperation to keep it in one place
and when we get to the end you can lock the thread and remove all posts below of main one
I already run into some dead ends and will have to ask some additional question to fill the gaps
but that will have to wait till I have core ready
Nothing to see here...

Simon Hickie
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Re: Doped drivers, why?

Unread post by Simon Hickie » Fri Apr 14, 2017 4:51 pm

Meanwhile, at the risk of coming over a bit 'Rumsfeld', it seems to me that for a Doc Mod project either using a used speaker or a new pair the issues fall into two broad categories: Knowns and Known Unknowns. My analysis is as follows.

KNOWNS.
1. Most bass, bass-mid and mid drivers have cone break-up issues above a certain point.
2. Conventional systems address this by as a minimum rolling off the top end with a coil.
3. Adding in a coil introduces phase changes and an additional component into the signal path.
4. More components offer the opportunity for sound degradation.
5. Acoustic wadding, stuffing etc. in a speaker cabinet modifies the performance of the speaker, acting as a filter.
6. Doping a driver Doc style addresses those cone break-up issues thus negating the need for a coil allowing the driver to run fully open. The principle was/is employed successfully by Dynaco from the late '60s and by Robin Marshall (Monitor Audio, Epos).
7. Doping the driver makes it thicker thus reducing the risks of rear backwave breakthrough.
8. Doping reduces driver efficiency a bit.
9. Adding steel to a cabinet interior adds mass and also enables energy to be absorbed from the driver backwave. Various Royd models, the Townshend Glastonbury (aluminium rather than steel) and the Leema Xen are examples where steel is used.
10. Tweeters are likely to need padding to match the bass-mid sensitivity
11. Using a single resistor for padding is regarded by the Doc as sounding superior to a conventional two resistor Lpad.
12. The tweeter needs protection from bass frequencies. A single cap producing a normal 6db/octave first order roll-off is the way to achieve this
13. A 3.3uF cap with a 12 ohm resistor in series will begin the tweeter roll-off at about 2800hz assuming 5 ohm impedance at the roll-off point. This will give about 10db tweeter attenuation.
14. A lower value resistor will reduce the tweeter attenuation and also raise the roll-off point with the same value cap.
15. A higher value resistor will increase the attenuation and lower the roll-off point with the same value cap.
16. Tuning needs to be done by ear with music.


KNOWN UNKNOWNS

1. The bass-mid roll off point will depend on (a) the driver and (b) the doping.
2. The loss of efficiency from the doping process cannot readily be calculated or measured. Evaluating it has to be by guesstimation and ear.
3. The efficiency of the tweeter will not be known unless it is a recognised model with relevant T&S data.
4. The impedance of the tweeter at the effective roll-off point may not be known. This will impact on the roll-off point for the selected cap and resistor.
5. The exact final value of capacitor and resistor can only be finally determined by ear due to all of the above. The recommended 3.3uF cap and 12 ohm resistor are as good a starting point as any.

I'll give some thought to a 'tuning guide' to try to put into words the effects of various results including (a) tweeter padding too much, (b) tweeter padding too little (c) tweeter roll-off too early (d) tweeter roll-off too late (e) combinations of all of these. I'll need to experiment with my selection box of capacitors and resistors, but have no time over the next week and a bit. Suffice it to say (again) that the 3.3uF cap and 12 ohm resistor pretty much hit the spot with my drivers.

The bottom line is that this is all very much as the Doc advises. Beyond the remove stuffing, steel line the cabinet, replace the crossover with cap and resistor and dope the drivers steps, the rest is down to the individual modder based on the unit they are choosing to modify and their ears.

Musical accompaniment to these musings: Gryphon - Midnight Mushrumps and Red Queen to Gryphon Three. Brian Gulland's bassoon sounds delightfully woody.
“The reasonable man adapts himself to the world: the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.” George Bernard Shaw.

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Re: Doped drivers, why?

Unread post by karatestu » Fri Apr 14, 2017 5:59 pm

You need to add in some details about doc modding 3 way speakers. Doc advises wiring the bass & mid in series. The other option is in parallel with a cap (33uf ?) on the mid i think. Also if wired in series the mid does not need a back box. Don't know if it needs one when wired in parallel.

Stu
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Dr Bunsen Honeydew
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Re: Doped drivers, why?

Unread post by Dr Bunsen Honeydew » Fri Apr 14, 2017 6:04 pm

You shouldn't need a back box, because of the driver doping.
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Jonty2547
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Re: Doped drivers, why?

Unread post by Jonty2547 » Fri Apr 14, 2017 6:44 pm

karatestu wrote:You need to add in some details about doc modding 3 way speakers. Doc advises wiring the bass & mid in series. The other option is in parallel with a cap (33uf ?) on the mid i think. Also if wired in series the mid does not need a back box. Don't know if it needs one when wired in parallel.

Stu
Did you finish your project Stu? I'd be interested to hear your findings on either parallel or in series and which you preferred.
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DAC-Bushmaster MKII. CDP- Sony X555ES. Speakers-B&W 705 with LS1.

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karatestu
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Re: Doped drivers, why?

Unread post by karatestu » Sat Apr 15, 2017 9:25 am

Jonty2547 wrote:
karatestu wrote:You need to add in some details about doc modding 3 way speakers. Doc advises wiring the bass & mid in series. The other option is in parallel with a cap (33uf ?) on the mid i think. Also if wired in series the mid does not need a back box. Don't know if it needs one when wired in parallel.

Stu
Did you finish your project Stu? I'd be interested to hear your findings on either parallel or in series and which you preferred.
No not yet. I have hit a stumbling block which is work - enquiries through the roof, not enough hours in the day. Being self employed I can't pass the buck. I will get it finished eventually just don't know exactly when. Stay tuned. Sadly with my project I have to do some major work with the baffle as I can't use the layout of it as is. Oh and the repair job I had to do when parcel farce dropped them from a great height. Wankers.

Stu
Naim, Linn & Harbeth fanboy

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