Speaker Motors

[Cressy Snr]

Been reading an interesting bit of my guitar amp handbook (understanding tube amplifiers and getting great sounds)
A chapter on getting a cranked tone in the studio without getting yourself deafened in the process, talks about output attenuators that can be used at the end of the chain. These allow the guitarist to replicate the same tone and distortion they'd get flat out on stage, but at levels that could be used safely in a recording studio, either with amp combo or direct injection.

Most of these insert some sort of resistive load in the speaker channel which is no good for audio use. However the Weber "VST MASS" output attenuator in particular piqued my interest. This device, rather than inserting a high power resistor network in the speaker channel, uses a speaker motor as the series element, which offers a proper reactive load that mirrors the way a speaker interacts with the amp output stage in a way that a purely resistive load does not.

This could be an interesting way to make my 8 Ohm Fanes into a 16 Ohm load for the OTL once it is finished.
I have a couple of big magnet Faital 6PR### drivers, one of whose cones I put through with a screwdriver, lying doing nothing and no use for anything but the tip.

One inside each cabinet fastened to the holey brace and wired in phase, in series with the main driver and with no direct outlet to the outer air.

Might work, might not, but no harm in trying. If it works, I won't have to build another set of cabs, if it sounds like a rubbery pile of blancmange, then I've lost nothing, as the change is completely reversible with no damage to the existing cabs.

[Mike H]

I agree should really be a reactive load that has electrical behaviour that mimics a real loudspeaker, and not just a resistor.

[Cressy Snr]

OK, so today, I've been mainly modding my Fanes, just to see if this particular guitar amp bodge could be applied in a hi-fi audio situation. Of course one could create the same thing by using a series wired pair of drivers; one behind the other, in an isobaric config, but that'd mean buying another pair of Fane full rangers and also having to build another set of cabs; neither of which I'm in a position to start forking out for.

What I've ended up with is very interesting. DCR of each speaker system is 13.5 Ohms, so a nominal impedance of 15 Ohms is not beyond the bounds of reason. What we have with this, because the internal 6" driver is attached to a horizontal holey brace is effectively, a small up-and-down-firing open baffle speaker, operating at both 90 degrees to, and inside the cabinet of a large sealed box speaker.

Connected to my NVA A20/P20 combination, the volume is of course lower for the same setting, but I'm pleased with the sound. Interestingly, the on-axis, tipped up response of the main Fane full range driver is no longer evident, which initially seemed curious, but it's explainable; at least I think I can explain it.

There is a bit more ease and liquidity to midrange instrumentation and electric guitars in particular sound spot-on (no surprise there then) loads of harmonics revealed within the distortion envelopes. Vocals come over with natural tones and free of residual shoutiness. Bass instruments can be followed more easily with acoustic bass twang and buzz more obvious; demonstrated to good effect by Ed thigpen on Oscar Peterson's "We Get Requests"

The configuration shows great potential and I've one or two ideas on how I can refine the concept.

[Cressy Snr]

I agree should really be a reactive load that has electrical behaviour that mimics a real loudspeaker, and not just a resistor.

Seems to work well too. See above
I've found the need to pull the cabs a couple of inches further out from the rear wall. The backs of the cabs are a whole four inches away from the wall now.

[Cressy Snr]

......demonstrated to good effect by Ed Thigpen.....

Not Ed Thigpen, he's the drummer! I mean Ray Brown.

[Cressy Snr]

So because these 6" drivers were effectively scrap, due to torn/pierced cones, I decided to go the whole hog and use them as just the speaker motors, as used in the Weber MASS guitar amp output attenuator.


Here, they're mounted inside the cabinet, screwed to the holey brace.


The speaker is still an excellent sounding system; no longer a speaker within a speaker, merely a way to create a 16 Ohm amplifier load and at 16 Ohms is now of course, the forthcoming 13E1 OTL will have an easier time driving this system.
I'm looking forward to hearing the speakers on the end of that particular amp.

[Nick]

But how is that better than just using a 8R resistor?

[ed]

I'm curious as well....

I know nothing about the impedence concept but I would have thought that the new motor will be flapping about without a care in the world with repsect to the fane because you've destroyed half the mechanical load which governs the movement with respect to the signal...i.e the mechanical Q...

so at best I would guess that it would introduce a bit of chaos with respect to the acceleration and braking of the fane...

but as I said, I know nothing of such things.....just sayin

[Cressy Snr]

But how is that better than just using a 8R resistor?

A resistor will get hot. It also will mess up your amplifier damping factor and its AC impedance will remain constant with frequency unlike the speaker motor which behaves like an inductor, and that could curtail your top end, so neither method is ideal. But with the tipped up response of the 12-250-TC that may be a good thing. What you really want of course is a 16 Ohm driver. Unfortunately I can't really see Fane making a 16 Ohm version of the full ranger, just because I happen to want one.

I've added power resistors to speakers during the late 70s, and without exception the bass went to pot and there was a smell of burning. They're fine for padding down a tweeter for example, but are a dead loss for bass units. As my drivers are full range units the resistor option is not tenable.

There's always the chance, in fact there's every chance that another flailing, reactive element in series with your main driver could result in a mess and TBH it's a bodge to get a 16 Ohm speaker, but it works in this case as the spider on the deconed unit is very, very stiff. Sometimes you have to improvise and use what you've got available.

[IslandPink]

Umm... but you've only got 1W coming in from the OTL - or is this for other amps ? So there's not going to be any problem with heat.
The 6" driver without the cone will have a higher Fs than it is was as standard, so the impedance peak will be in the upper bass or lower midrange. It's like a variable resistor with frequency. It'll modify the frequency response you get from the Fane and that might work out OK , so that'll be a suck-it-and-see approach, which is worthwhile.

[Cressy Snr]

Hi Mark.
ATM I'm running a 25WPC NVA A20 into the Fanes. The FR seems fine, no peaky bass that I can hear, and of course in this situation it's suck it and see. All bets are off with this thing. I like going out on a limb sometimes. That's when interesting things happen.

[pre65]

But how is that better than just using a 8R resistor?

A resistor will get hot. It also will mess up your amplifier damping factor and its AC impedance will remain constant with frequency unlike the speaker motor which behaves like an inductor, and that could curtail your top end, so neither method is ideal.

So, what if you had an inductor (choke ?) with a DCR of 8 ohm ?

Something like a Hammond 159ZA

https://www.mouser.co.uk/ProductDetail/ ... r14EwXDA==

I'll get my coat.

[Nick]

So, what if you had an inductor (choke ?) with a DCR of 8 ohm ?

Something like a Hammond 159ZA

https://www.mouser.co.uk/ProductDetail/ ... r14EwXDA==

I'll get my coat.

The DCR has nothing to do with it. A 300mH inductor like that will have a impedance of 188 ohm at 100h and 1k8 at 1kHz etc.

I would expect the resistor and the speaker coil will generate identical amounts of heat.

But if it works it works.

[chris661]

But how is that better than just using a 8R resistor?

If you used two identical speakers in series, you've doubled the impedance seen by the amplifier at all frequencies. Perfect potential divider.

If you put an 8ohm resistor in series with a speaker, you've added 8ohm where sometimes the speaker's impedance will be 6ohm, sometimes 40ohm. That's not a proportional increase any more, so you'll get an uneven voltage drive to the speaker.
As Steve noted - the bass ended up rubbish. My bet would be that the resistor was attenuating quite a bit, except where the speaker's impedance peak is (somewhere in the 70-80Hz range at a guess), where attenuation would be minimal. Result = one-note bass, like using a too-small sealed box with a high Q-factor.


Now, using non-identical speakers would give interesting results. My inclination would be to play with adding mass to the internal driver to make sure its resonant frequency matches the Fane's. That way, you'll have a shot at keeping the potential divider working well throughout the bass. That'll need either impedance sweeps, or a bit of trial and error.

Chris

[IslandPink]

Yes, that clarifies a few things. If you run both drivers in series with cones and output, they could be different and with different resonant frequency and work OK, because any drop in output from one driver where the second driver has its impedance peak, is covered by the second driver having more output. I think Thorsten Loesch had an OB project a few years ago with two different bass drivers in series.