Attenuator question

[phatt]

Surely they sell Lpads in Tandy,, it will make life so much easier as you don't need to mess with any other components.

 5 watts can be done with a simple speaker pot or L pad.
These are *NOT* like a normal pot they are a specialized unit designed specifically for the purpose of attenuation.
Two *Seperate* rehostats with one common wiper.
This might help you to see what goes on inside.
Cheers phil.

[phatt]

I had another pic but keep getting internal server error
I'll try again later.
Phil.

[will316]

I believe that's what I've got. It's all mounted on a board, has a faceplate and knob so you can mount it in the wall. You've got hot and ground in and hot and ground out to your speaker. I hooked several different 8 ohm speakers to it and put my meter on the other side to get an idea of what my transformer would see. The readings were way too low. I disconnected the speaker and ohm'd it at the terminals and got a normal reading. Is this a case of me taking my measurements incorrectly? I don't understand  how an 8 ohm speaker would ohm out correctly but ohm out incorrectly with the attenuator hooked up. Is this a case of the trans will see the correct load with everything in place? I mean how could this control cause the ohm reading to change? I assumed that the speaker's impedence would satisfy the transformer and having the attenuator in line between the two would only raise the ohm reading. I'm kinda stuck here. If someone could briefly explain I'm sure I'd understand. As far as going through all the trouble, it serves to help me grasp a better understanding. My girl thinks I'm crazy, but I would love to be a Jim Marshall, Hartley Peavey, Leo Fender, etc. If anything, I'd be happy as a clam just owning a small shop and keeping garage bands going. I could get so much satisfaction just throwing together little 5-10 watt transistor units that kept music alive for poor kids. You never know, one man's hobby/livelihood could potentially help to inspire the next EVH, Randy Rhoads, Dimebag, etc. Music is most sacred to me but as with all my hobbies(firearms, cars, etc.) I also like to know how it works, build one myself, diafgnose, and repair or modify as needed. I'm sure everyone here's got that in them too.

[phatt]

they look like this;
http://www.amptone.com/wallmountspeakerattenuator.htm

They should state that they are for an 8 Ohm system and wired correctly yes everything will be fine.

If you wire them the wrong way then yes when you turn down to zero the amp gets a dead short,,, not that a dead short matters to a Valve Amp but it sounds crap.

Note the two rehostats are different values.
this maintains a relitive Z load to Amp.
Remember that DC Ohms is not the same as Z Ohms
*Z is AC resistance* which is freq dependant and DC Ohms is exactly that. Ohms
Phil.

[spud]

Will,

I used that to build an attenuator, it's a Wall Volume Control for speakers.  I got the 25w mono model from my local RS.  I was advised over at ppwatt forum to use a  4.7uF non-polarized cap across the in and out terminals to preserve highs at lower lower volumes.  It seems to work ok except when you turn it way low it tends to lose highs so I might need to use a diff cap.  Here's some pics of what I did with it.  The schematics are for a different type of attenuator that I never built not for this super simple one. 

http://s252.photobucket.com/albums/hh7/spudjds/Public/Builds/MISC%20Projects%20and%20Ideas/Attenuator/?action=view&current=100_0575_1.jpg

It's in a Radio Shack project box and I put and in 1/4" jacks for in and out and a chicken head knob - I also marked the front with a white magic marker with min/max and a tic for each click of the volume control - mine is stepped and not a continuous volume control/pot - at min it's off (no sound) and max is 0 resistance so wide open.  Hope this helps.

Jim

[will316]

Thanks, guys. Jim, that's the same unit I've got. Looks like I've got some reading ahead of me to completely grasp all this. My job as a tech has taught me alot about electronics and such, but obviouslly I have alot to learn! I assumed impedence was strictly pull the leads, ohm it out, etc. I wasn't aware of differences between ac and dc ohms. Thanks for the info, Phil. Appears I've learned something today, now my head hurts! lol I've gotta lookback through the archives and note reference guides, books, etc. Now my interest is piqued to the max.

[joecool85]

Thanks, guys. Jim, that's the same unit I've got. Looks like I've got some reading ahead of me to completely grasp all this. My job as a tech has taught me alot about electronics and such, but obviouslly I have alot to learn! I assumed impedence was strictly pull the leads, ohm it out, etc. I wasn't aware of differences between ac and dc ohms. Thanks for the info, Phil. Appears I've learned something today, now my head hurts! lol I've gotta lookback through the archives and note reference guides, books, etc. Now my interest is piqued to the max.

AC and DC ohms are the same.  Ohms are ohms.  It is the impedance that is different.  Resistors are measured in ohms and it won't matter if it is AC or DC, it'll be the same.  Speakers are measured in impedance and it will be different ohms because speakers not only use AC power, but also the resistance changes depending on the speaker's position in the voice coil.

[will316]

Now we're getting somewhere. I can grasp that. I wonder how many folks don't understand what a motor is. Many people probably just think of the motor that runs their washing machine, condensor fan motor, etc. In my work I've been put to test trying to explain the fact that a refrigeration compressor is a motor. The folks that don't understand that would probably keel over if I also explained that a speaker is a motor. Seriously, while I do have a basic grasp of circuits, electronics, etc. I don't claim to be an expert. My schooling covered the bare bones basics of electrical theory and focused mainly on diagnosis and repair of refrigeration. I've met other tecnicians that have absolutely no grasp of electronic theory who were spectacular techs. I've also met guys who probably held degrees in electronic theory yet were piss poor techs(they tended to over think stuff). My philosophy kinda falls somewhere in the middle. While I don't doubt electronic theory I also keep in mind that it's just that-THEORY. What I mean is- according to theory, some things I've seen should not work, yet it runs like a champ. I'm sure some of you guys have built or seen projects that on paper, defy all logic and shouldn't work yet you plug it in and it makes beautiful music. I'm probably losing some of you but the ones that understand are probably smiling and nodding their heads.

[rowdy_riemer]

Well, keep in mind that resistance is a type of impedance.

[phatt]

Sorry I was not very clear;
*Impedance is the Resistance of a component at a given frequency*
Measure the DC Resistance of a 600 Z Ohm microphone,,, the reading will be quite different.

Try Reading this one,,
http://www.teamrocs.com/technical/pages/resistance_impedance.htm

Or just google it ,, "ac resistance vs dc resistance"

Phil.

[rowdy_riemer]

I think the term impedance includes resistance and reactance. AC resistance is reactance. You've got inductive reactance and capacitive reactance. All are measured in ohms. Both types of reactance and resistance must be considered to figure total impedance.

[phatt]

You will find heaps of related info on the Jensen pages,
http://www.jensen-transformers.com/

Heaps of stuff under *Aplications > White papers and Schematics.*

Also *Rane* used to have a mass of technical terms all laid out in alphabetical order but I can't seem to find it now.

But look here ;
http://www.rane.com/

Under *Support* > Library,
Heaps of stuff to help broarden your knowledge base.
Phil.

[will316]

Now I've built the unit. I used 1 8ohm 20 watt resistor going from the hot leg coming from the amp going to one side of the rheostat. Wiper leg from rheostat to hot leg out. Ground coming in to opposite terminal on rheostat to ground going out. Tested it witha little Gorilla practice amp. No sound and amp appears to be going out on thermal overload. It dies completely-no led, nothing. After a few minutes, the amp will run fine. Is this a case of bad wiring, way too much resistance in my unit for the Gorilla (10 watter 4 oh I believe), or a case of solid state amps not being usable with attenuators? I just used the Gorilla because I didn't wanna blow up the tube amp. I'm also wondering if I could use 1 10 watt, 10 ohm resistor instead? Also wondering if this scheme would work: 1 10 watt 10 ohm resistor soldered across the outer legs of a 25 ohm 3 watt rheostat with hot wire going in, to outer leg of r'sta. Wiper leg to hot out. Ground coming in to opposite outer leg to ground going out. Also wondering if the r'stat body needs to be grounded like guitar pots are. Any help is appreciated. I also built one with the Radio shack l pad and it works just fine. The only reason I wanna build from scratch is education and possibly financial if I can sell them.

[phatt]

Hi, Sounds like a dead short if Amp is shutting down.
Check your wiring.
BTW, please draw a simple diagram rather that trying to explain it all.

A 50 Watt Lpad as used for HiFi crossovers is all that is needed for Amps up to about 10 Watts.

Adding more is needless complexity and more chance of mistakes.
Phil.

[will316]

Gotcha. I'm on a phone but with symbols and such I should be able to make crude diagrams. Didn't think to ask-should my input and output jacks be insulated?