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Attenuator Schematics?


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11/11/2000 5:59 PM
aron
Attenuator Schematics?
I posted this on my forum, but I was wondering if anyone had any schematics for an attenuator. I just want something to knock down the level a little bit with my Bassman.  
 
 
 
 
 
Thanks,  
 
 
 
Aron
 
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11/14/2000 3:51 AM
Steve A.

Aron:  
 
 
 
    Check this one out:  
 
 
 
http://www.blueguitar.org/power_sk.jpg  
 
 
 
    I have one half-built in a metal Radio Shack box but it isn't a very high priority for me since I have a Marshall Power Brake... But I better finish it up before I misplace all of the parts! BTW the new L-Pad's from RS use a rotary switch rather than a pot...  
 
 
 
--Enjoy!  
 
 
 
Steve Ahola
 
11/14/2000 5:35 PM
aron

Do you think this will work with my Bassman since it's 50? watts?  
 
 
 
Thanks,  
 
 
 
Aron
 
11/15/2000 10:02 AM
Steve A.

Aron:  
 
 
 
    Here's the one that Bruce Collins drew up (no pesky Radio Shack l-pad to burn up):  
 
 
 
http://www.blueguitar.org/volreduc.gif  
 
 
 
    So this is for your brownface bassman with a 4 ohm OT tap? You could make one up using six 8 ohm/20 watt non-inductive resistors available from Radio Shack for 1.49 each: just wire up 3 parallel pairs of the 8 ohm resistors together, and then hook them up like the speakers in a 4x12 cab (with the fourth "resistor" being your speaker cab).  
 
 
 
    That is less confusing than it sounds! It is easier to picture if you are using an 8 ohm load. Wire up the three 8 ohm resistors AND the 8 ohm speaker as the four speakers in a cab. Only connect the leads that would go to the 8 ohm speaker to a jack... (For 4 ohms, just double up the 8 ohm resistors in parallel.)  
 
 
 
    In any case I believe that this arrangement would divide the power output going to your cab by 4. So a 50W bassman would be sending only 12.5W to the speaker... still loud enough to get you evicted if you crank it up late at night!   :(    
 
 
 
Steve Ahola
 
11/15/2000 5:49 PM
Frank Clarke

I built the Bruce Collins volume reducer without inductors or switch, you can place any number of them in series.  
 
Instead of (16ohm) in parallel with the (8ohm resistor and speaker in series) I now have (16ohm and 32ohm in parallel) in parallel with (24ohm and 8 ohm speaker in series). This is supposed to be (11ohm) and (24+8ohm) in parallel, which is about 8ohm. I think that is about 1/15 volume which is handy with a 15-watt amp.  
 
You could scale up the resistor wattage to suit the amp.  
 
Something more complicated would be a purchased item, but I'm open to suggestions.
 
11/15/2000 9:16 PM
aron

>I think that is about 1/15 volume which is handy with a 15-watt amp.  
 
 
 
How did you figure out how much wattage the final output will be?
 
11/16/2000 7:01 AM
Frank Clarke

(11ohm) and (24+8ohm in series) in parallel,  
 
 
 
Aaargh...  
 
 
 
24+8=32ohms, so the speaker gets 8/32 or 1/4 of the current.  
 
 
 
1/r = 1/11 + 1/32  
 
1/r = 4/33 (nearly), so the 1/32 gets a quarter of the current, a quarter of which goes to the speaker, so 1/4 * 1/4 = 1/16. So 1/16 wattage through the speaker.  
 
 
 
I think 1/2 vol is -3db, so 1/16 is -12db.  
 
 
 
I think the series inductors reduce treble, but it doesn't sound wildly overbright with a 12" speaker.  
 
 
 
You would want to measure that the amp side actually is 8ohms, and that the resistors aren't going beyond pleasantly warm when in use. I used a plastic box to reduce shorting concerns.  
 
 
 
It is a little scary plugging the thing in the first time though :).
 

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