Light bulb attenuator/compressor

[PRR]

> when i was playing with stuff a long time ago, i had a fluke i used to read the resistance of the celestion in my princeton... and it was definitely hitting 0 ohms at times while i was playing...

You had the ohm-meter across the speaker *and* OT, *WHILE* you were playing??

> silly hippy is dangerous to be around i think

Snort some orange-juice and put your brain in gear.

The ideal OT has zero resistance (for perfect efficiency). In a real world it has a part-ohm resistance, say 0.6 ohms. 0.6 in parallel with 8 ohms is 0.56 ohms. Many ohm-meters won't read right below 1 ohm; some Flukes do but good sub-Ohm readings need special technique. So it _should_ read way-low.

Now think about how the Ohm-Meter works. It has a power source, puts a *known* voltage/current out the probes, and reads the current/voltage developed in the circuit being tested.

If there is ANY other power source involved, the meter does not know that and gives bad readings.

My Fluke will not read ground-resistance around my house. Always shows 2.8 Megs from power-ground to dirt-conductor. After much head-scratch, I put it in Volts. Most of the paths I tested had copper at one end and iron at the other. This plus damp dirt makes a 0.6V battery! Already there is a power-source the meter doesn't know about. But on AC Volts I get 2.5V to 3V AC. This is voltage-drop in my very-long line from the street. While the ohm-meter "should ignore AC", it is trying to measure <0.28V of DC, and apparently 10X that much of AC over-whelms it (which is reasonable, since I should not be reading resistance in the presence of stray voltages). It turns out that 2.8Megs is just the max reading for this model of Fluke (yet not high enough for it to read OPEN).

You've got several volts of AC (audio) across the speaker. One time it is high and the meter shows a high reading. Another time it is low or negative and the meter may try to give a negative reading. Something in there knows that negative resistance is probably a screw-up and rounds-up to zero. But it is reading a meaningless combination of meter self-assumptions and unexpected audio voltages.

> seen dc on an output transdormer

If it is "DC", then it will be there ALL the time. Or at least more than an instant.

If you mix AC signals with a DC-reading meter, sure it will cross zero sometime.

If you want to know speaker impedance, set up a signal generator to a power amp and then put 8r resistor in series with the speaker. Put 1V AC across the combination. Measure voltage across speaker. If it is 0.5V, the speaker must be 8 ohms. If it is 0.9V the speaker is 72 ohms. If zero, the speaker is zero ohms. Try the entire audio range. Try small and large drive voltages.

For extra tediousness, use o'scope so you can watch the current/voltage over the whole wave.

If you drive the speaker to "slap" with special drive waveforms you *can* get a reading lower than the DC resistance. This is stored energy in the cone mass/stiffness "generating" counter-voltage. I don't think this has been confirmed in real-life audio signals.

> high resistance is much of a problem for an amp's output section

Generally only in tube/valve amplifiers. And mostly when playing HARD. Transistor amps mostly do not have output transformers. The few that do work at such low voltages that undamped transients are not likely to exceed the ~~500V insulation. But a 400V tube, played HARD, with no load, can kick 4,000V spikes across the insulation. This happens same-way that a car ignition system kicks-up a 12V supply to 400V spikes at the primary (then transforms them to 40,000V spikes). If there is current in the coil, and you break the circuit instantly, the voltage rises to infinity. With practical switches and with lossy iron-core coils, the kick-up may only be 10X. (>20X in an ignition coil because they _want_ large kick.)

> rate of resistance change is like for the electricity to heat conversion in the lamp?

Like what Jonathan says, couple tenths of a second.

But also symmetrical. Cools down about as fast as it heats up. Most LDRs have considerably more decay than rise, which is fairly useful in audio compression. In fact the "un-natural" timing of the lamp may have been useful in the tweeter-protector application: the "wrong" sound led the user to suspect hot tweeter instead of other problems in the signal path.

[chptunes]

A long time ago.. Paul R forgot more than I'll ever know about electrical matters.

.. ...

I may tinker with the light bulb scheme that Paul posted.. if I do, I let you guys know what I think of it...

[pinkjimiphoton]

^^^

indeed.

paul... thanks for all the explanation. i am actually comprehending much of it, and it explains my previous delusion...perfectly!!

thanks bro!!

[gritz]

A long time ago.. Paul R forgot more than I'll ever know about electrical matters.

.. ...

I may tinker with the light bulb scheme that Paul posted.. if I do, I let you guys know what I think of it...

If you do get around to it then I'll be interested to hear how it goes. Paul's observation that the "cooling" time of the filament lamp is similar to it's "lighting up" time means that in conjunction with it's sloooow response it can act as an RMS (i.e. energy or perceived volume) detector (I'm simplifying things because I believe that ballpark comes first and details can often wait...). While it doesn't offer a "sustain 'til Christmas" compressor, a few dB of reduction on sustained notes after letting attacks through at full bore might e.g. lead to a musical "sag" which may bring a bit more life to those teeny power amp IC's that we often press into service as late night practice amps. In any case it's certainly worth messing with, because it might be one of those subtle things (albeit not so subtle as $1000 speaker cables / capacitors!)

*Stream of conciousness alert*

It's just dawned on me that a lot of folks out there might have a box full of old EL34 / other power valve pulls. The hot resistance of the filament is *about* 4 Ohms and the cold resistance rather less (to my eternal shame I don't have a tube power amp right now to nick a valve out of for measurement purposes and the hot:cold ratio may be a bit less than for lamps 'cus perhaps heater filaments aren't run quite so white hot...). The filament dissipates about 10W on full chat and is generally very robust. It may be a candidate for a power soak limiter, possibly? Perhaps the filaments might be a bit massive and slow, but if not then couple of EL34s that light up in time with your palm mutes would look great...  

[maxbauer]

hy, the wien bridge speaker compressor/expander looks very interesting, i do build amps and guitars,
but here i am left abit confused, how does this work the way the speaker is connected between the outputs of the the transformer,
any help very much appreciated,

I'm specifically interested in building a lightbulb compressor, not any other type of attenuator.

Something like a Wien bridge (with other resistor values):



I wonder how much compression you can get out of it.

Edit: Actually is more like an expander than a compressor, much like you describe, but it has the advantage that if any of the lamps fail, nothing "bad" will happen. To get a compressor you should put the lamp in series with the speaker.

[PRR]

Welcome.

> how does this work the way the speaker is connected between the outputs of the the transformer

Make all four resistors/lamps the same value, there is no voltage across the speaker, no output.

Make them different, there is output.

Specifically: make the lamps zero, you have full voltage to the speaker. Very-low, you have most of your speaker voltage.

When the lamps heat-up, their resistance rises from very low to around (if properly selected) 10 Ohms. So heat shifts the speaker voltage from nearly-all to nearly-none.

[pinkjimiphoton]

thanks for the endless info and clarifications, paul.
dude. i have learned so much from your posts its not funny,
and i'm forever in your debt.

that kinda sums up what i found years ago... 60 watt incandescent bulb is around 8 ohms when hot, so i had sent kevin from london power an email asking about replacing speakers with lightbulbs as a reactive load. he said it should be fine, tho i never did end up trying it... i had envisioned using 4 in a series parallel arrangement to replace an 8 ohm load to use as a "dummy load" but never ended up getting around to it.
sorry for the ot... back to the original topic. peace!

[PRR]

> 60 watt incandescent bulb is around 8 ohms when hot, ... replacing speakers with lightbulbs as a reactive load.

120 Volt 60W lamp when lit is obviously 0.5 Amps at 120V or 240 Ohms. (240V 60W lamp is 960r.)

Incandescent lamps have negligible reactance at audio frequency.

I had a 120V 60W which measured 15 Ohms *cold*.

I put it on the 16 Ohm tap of a 15 Watt tube amp. Above a dozen Volts out, I got impossible results. At 15V I could see the filament dim-red. Obviously not "cold". I later verified it was going over 50 Ohms above 12V.

The time-constant around dim-red is several seconds. In others tests I have dumped 120V through a lamp into a dirt-rod (don't try this at home! Get the other guy to try it.) It would take a few seconds for the lamp to reach part-glow and the current stabilize. I'm not sure what the musical use is, of an effect that don't happen until a bar or two *after* a loud peak.

[pinkjimiphoton]

>

In others tests I have dumped 120V through a lamp into a dirt-rod (don't try this at home! Get the other guy to try it.) It would take a few seconds for the lamp to reach part-glow and the current stabilize. I'm not sure what the musical use is, of an effect that don't happen until a bar or two *after* a loud peak.

LMAO

[pinkjimiphoton]

reminds me of the time i mis-wired a sewing machine "speed" pedal trying to control the leslie out of an old organ.
i didn't quite understand at first why the walls were vibrating and the lights were going dim as i worked the pedal.

ooopsies.

[highwater]

reminds me of the time i mis-wired a sewing machine "speed" pedal trying to control the leslie out of an old organ.
i didn't quite understand at first why the walls were vibrating and the lights were going dim as i worked the pedal.

ooopsies.

That reminds me of a story I read about a guy developing industrial motor controls:

These aren't lab stories, but they're funny all the same. I used to work at a drive system engineering firm, where they designed and manufactured the parts for the industrial lines that do things like make giant rolls of steel or toilet paper. There was a guy there who had worked out on the shop floor who told me a few tales of disaster that he'd seen in his time there.
1. In the days when digital motor controls were new, there were still a few bugs to work out. One day, one of the engineers was testing his work by powering up a drive system with a motor attached (the motor was mounted to a metal skid not too different from a shipping pallet, and these aren't little dinky motors – these could be anywhere from a few horsepower to a few hundred). Anyway, the current limiting portion of the control software hadn't been set up properly, so when he set it to spin the motor up to normal speed, the drive attempted to reach that speed *now*. A large BOOM rebounded off the walls of the shop: the immense torque rotated the entire motor, causing it to tilt sideways on one edge of the skid, and then when the circuit breakers heated up enough to trip a second or so later and shut off the motor, the whole thing came slamming back down.
2. There was another incident involving a runaway motor in which the drive system spun up the motor, but didn't level off. A few seconds later, a chunk of the rotor finally broke off, punched through the motor housing, and lodged in the ceiling 30′ up.
3. Apparently, there was yet another incident in which someone powered up a motor, but it spun up too quickly or oversped and came loose from its mounting to the floor and began rolling across the shop floor. As the story goes, my friend looked up to see a guy running in panic across the shop, the runaway motor tumbling after him.

Found in the comments at this post about ruined lab equipment. Yeah, I bookmarked it.

[pinkjimiphoton]

a party ain't a party til the poltergeists trip the breakers. lol...

please remember i am a ten toed freek of a monkey with semi-opposable thumbs and a breadboard, so DON'T TRY THIS AT HOME.....

for real.
thanks for the link, i find stuff like this amusing. one time in a machine shop i used to work at, a bud was working on some kind of ridiculously expensive part for the government on a vertical turret lather <bullard>... kinda like a super king kong sized record player/cutter machine.
he was behind schedule and cutting hard n fast. but forgot one of the mounts on it or something... next thing we know, there's this like, 200k silver ufo flying across the shop, to crash into a nearby wall.
i got to clean up the mess. whatever it was, it was harder than the concrete walls were

good times.
anyways sorry for Yet Another Pointless Hijack