Building an attenuator(Weber style with speaker motor)

[rockgardenlove]

So, I'm interested in making an attenuator as people are always telling me to turn down the volume.  I'm most interested in teh Weber style attenuators that use a speaker motor to provide more realistic attenuation.  How do I go about building one?  Do you need a special value speaker motor?  I have a spare 11 ohm(wierd...) speaker that I can cannibalize...
So yeah...what do I do?

[Bernardduur]

I was also thinking about making a attenuator (as I am also always too loud).

I've read the archives and have seen the Mass schematic; problem is that this schematic is for a 25W amp; my amp is 50W so I need a 50W speaker; my question:

- My other speaker is 4 ohm; does this Mass speaker also needs to be 4 ohm?
- What are the values of the other items. I know they are 100 ohm, but I guess they need to be of an higher wattage. Double the wattage?

Oh, sorry to hijack the thread btw

[rockgardenlove]

No problem, quite helpful in fact, I need to know the same stuff.  I have a 30 watt amp.

So yeah, anybody know?  And can you post the schem please?

[Bernardduur]

Check this thread. Love the pic

It is the voice coil, spider, small magnet,  and voice coil wires. It is exactly like a speaker, unmodified; so I guess it is just a speaker with the conus and dust cover removed not touching the surface of the box by the mounting part.

Shame they don't sell the motors anymore

[rockgardenlove]

I emailed Weber, he said he may start selling them again in a month or so...cool!

If you wanted to use a normal speaker though, what kind of specs would you need?  What influence does the speaker motor ohmage have?  What about the wattage?

Thanks

[aron]

It is a speaker. Then the equivalent mass of the speaker cone is on top of the speaker so the "mass" moves like a speaker so there is buzz. You need to isolate it in a little box.

It's good as far as attenuators go.

[rockgardenlove]

Hmm yeah, what ohmage should the speaker/resistors be?  Is there some formula to calculate it?  I know how to do it with fixed resistors, just not with a speaker.

Thanks!

[Bernardduur]

I guess the speaker should be of the same ohmage as the other ones; I guess the ohmage of the resistors is like it is pictured in the schematic, only the wattage should be changed I guess.

[d95err]

Check this thread. Love the pic

Is the schematic posted in that thread really correct? With the volume knob at max, the 100R resistor and the pot forms an L-Pad with an impedance of around 160 ohm.  This should result in very little power going to the real speaker. Does the Mass attenuate the signal even when the volume is at max?

I'm trying to figure out how a Mass is constructed (and I can't find any schematics on the Weber site), but that schematic doesn't make sense to me. Could anyone explain?

[Doug_H]

That schematic is "correct" in the sense that it is the example schem that Ted offered for people buying Mass motors that wanted to build their own attenuators. The series 100 ohm resistor helps isolate the loading effects of the pot/spkr from the Mass load on the amp, which helps maintain a constant load on the amp independent of the pot setting. The disadvantage is that, as drawn, it will  knock off at least roughly 20db (for an 8 ohm spkr) even when the volume pot is maxed. (For an 8 ohm spkr: 8 ohm || 100 ohm pot = ~8ohm which with the 100 ohm series resistor gives you a 100/8 divider.)

For the 10-13W amps I build this was way too much attenuation. So I experimented and ended up subbing the 100ohm resistor with a 27 ohm power resistor I had lying around and that worked much better for me. This increased the "volume at max loss" from ~-20db to ~-10db and was a much more useable volume for my purposes. It also decreased the "loading isolation" but I felt the effect was negligible. I keep thinking there must be a better way of "isolating" the volume pot "tap" from the Mass load, maybe with some active electronics or something. Then again, it's a very simple circuit that works well and maybe it's just a matter of tweaking it for your individual needs.

Otherwise, for a low wattage amp a variable L-pad may make more sense. Grab a handful of power resistors and a rotary switch and come up with 2 or 3 volume levels that you find useful. For more "accurate reactive loading" you could use some inductors too, then again it may not be that significant depending on what you are trying to do.

Doug

[Ed G.]

I bought the MiniMASS and although it sounded good, I found it knocked off too much treble, so I changed the 2.2uf bypass cap with a 10uf cap, and it retained highs throughout the entire range, it's just enough to keep it consistent.
However, and this may just be the nature of the attenuation beast, but at moderate attenuation levels, my amp just sounds too thin. It lacks that low-end sonic 'punch' and I'm afraid that there's nothing that can be done about it, or is there?

[MartyMart]

I use a "Palmer Sponge" in the studio (PDI-06) which works quite well at reducing
output whilst you "crank" the amp for overdrive tones, it does change the sound a little,
not brightness, but there's some "punch" and what I'd call "speaker spank" that gets
reduced too much.
Perhaps it's just that the amount of speaker "movement" is reduced and there's less
"air" being thrown at you ... ??
Nice little unit tough, on the whole

MM.

[d95err]

However, and this may just be the nature of the attenuation beast, but at moderate attenuation levels, my amp just sounds too thin. It lacks that low-end sonic 'punch' and I'm afraid that there's nothing that can be done about it, or is there?

The main reason it sounds thin is because of the way your ear works. You hear less bass at lower volume levels. If you record the sound you'll find that there is much less difference. The microphone doesn't have the same limitations your ear has.

Perhaps someone could come up with a clever way to add some bass boost at lower volume levels. Probably difficult to do in a passive circuit though.

[d95err]

That schematic is "correct" in the sense that it is the example schem that Ted offered for people buying Mass motors that wanted to build their own attenuators. The series 100 ohm resistor helps isolate the loading effects of the pot/spkr from the Mass load on the amp, which helps maintain a constant load on the amp independent of the pot setting. The disadvantage is that, as drawn, it will  knock off at least roughly 20db (for an 8 ohm spkr) even when the volume pot is maxed. (For an 8 ohm spkr: 8 ohm || 100 ohm pot = ~8ohm which with the 100 ohm series resistor gives you a 100/8 divider.)

Exellent description (as usual). Thank Doug! The I suppose the Hi/Low range switch on the Mass probably selects between different series resistors. The Weber website does not mention that the Mass attenuates the signal even att full volume setting, so I thought it did. It makes more sense now.

I suppose you could use a rotary multi-pole swith to add series resistance to the Mass motor while reducing the series resistance to the real speaker when going from low to high volume.

[Doug_H]

The Weber website does not mention that the Mass attenuates the signal even att full volume setting, so I thought it did. It makes more sense now.

Yeah, I don't know if you can really draw any conclusions from that schematic about what he finally marketed as a finished product. He offered that up as a simple example for hobbyists that wanted to experiment with mass motors, back while he was still developing the product. I noticed Ed's comment about the 2.2uf cap and am curious how that was used. There are different attenuator topologies, L-pads, bridged-T and etc. I'm going to do some research and maybe find a different way of implementing the mass. I'm working on a 13w wreck proto right now that is just too loud but my current mass setup knocks the volume down too far. I'm going to see if I can "improve" the attenuator circuit in way that will work better for my purposes.

Doug