Attenuator Help

[Matt420740]

Hey guys.  Been looking at some schematics for attenuators. 

The different methods I've been playing with -

1.  Resistive load using 2 or more resistors

2.  Resistive load using L-pad

3.  Resistive load using resistors AND L-pad.

I understand the first 2 methods pretty well, and have come across several calculators to help determine the resistor values needed.  The 3rd method however, has me a little unsure, and I'd like for some of you more knowledgeable guys to enlighten me. 

First here is a link to Bane's site with a calculator for the resistor values if just using resistors alone -  http://diy-fever.com/misc/l-pad/

Here is a link to the schematic of the resistors + L-pad - http://blueguitar.org/new/schem/misc_mod/power_sk.jpg

As I said my questions are about the 3rd method. 

1st question - Why is there resistors AND an L-pad being used here, instead of just resistors OR an l-pad?   

2nd - Why is it okay to swap in a 25ohm wire wound pot as shown?  I though the purpose of the L-pad was to let the amp see a constant 8 ohm load.  Or do the resistors do that job here, like in the first method?

3rd -  How would I go about figuring values if I wanted to build a 16ohm version?  Do I just use the calculator for the 1st method, or does the added L-pad/pot change things?


Thanks guys, sorry for my noobish questions.

[bancika]

L-Pad is not just a pot, it's got two conductive elements to make the load constant. It's like on my fixed version, but one element varies R1 and the other one varies R2 in a way that makes the load close to constant.


On that schematic you already have two resistors that form close to 8ohm load (5 in series and 4.7 in parallel with the speaker). What that pot does is to blend the speaker with that 4.7 resistor. On max you'll have 4.7 in parallel with 25 ohm paralleled to the speaker which gives around 4 ohms. Parallel that with an 8 ohm speaker and you get around 2.7 ohms. Add 5 ohms you already have and you get 7.7 ohms load and about 9db attenuation.

For 16ohm version scale everything with 2.

I don't know what would be the difference between pot and l-pad there. People like the way airbrake works, maybe you could look into that
http://ampgarage.com/forum/download.php?id=8823

Cheers

[PRR]

> resistors + L-pad - http://blueguitar.org/new/schem/misc_mod/power_sk.jpg  - Why is it okay to swap in a 25ohm wire wound pot as shown?  I though the purpose of the L-pad was to let the amp see a constant 8 ohm load.

Work it out.

Since there's more than 2 parts, work out small sections, then put them together.

25 ohm pot with 8 ohm speaker. Full-down, just 25 ohms. Full-up, 25||8= 6 ohms.
(Quite a swing.)

That's in parallel with 4.7 ohms.

25||4.7= 4 ohms
6||4.7= 2.6 ohms
(Less swing.)

That's in series with 10||10 or 5 ohms.

5+4= 9 ohms
5+2.6= 7.6 ohms.

NO amplifier cares the difference 7.6 or 9 ohms.


Note the fallacy on all these "constant resistance" pads. A real speaker has VARYING impedance. (If it were constant, we'd probably just say "resistance".) Maybe 50 ohms at 90Hz, 7 ohms at 400Hz, 20 ohms at 5KHz. (You can find curves.) Tube amps, some of them, work off that variation and the amp+speaker together make a "sound". Also the speaker looks-back into the amplifier, which may be a high or low inpedance, but hardly-ever the nominal load impedance. When you use any sort of pad, you lose that synergy and direct connection.

[ayayay!]

Note the fallacy on all these "constant resistance" pads. A real speaker has VARYING impedance. (If it were constant, we'd probably just say "resistance".) Maybe 50 ohms at 90Hz, 7 ohms at 400Hz, 20 ohms at 5KHz. (You can find curves.) Tube amps, some of them, work off that variation and the amp+speaker together make a "sound". Also the speaker looks-back into the amplifier, which may be a high or low inpedance, but hardly-ever the nominal load impedance. When you use any sort of pad, you lose that synergy and direct connection.

Best advice I've read in a long time Paul.  Props. 

[petemoore]

  +1...close enough for me.
    Nothin' beats the security of hardwired output/speaker.
     Some transformers are expensive too:
   Jack/plug/cable/plug/jack/loadbox/jack/plug/cable/plug/jack...
    I exposed some of my transfer coils to that kind of dangling participle-age.
     Excellent warranty just doesn't have the same show 'n tell factor that the actual working amplifier does.
   But the flattenned dynamics [compressor recommended]...if you're really into that particular sound, and can't do without it, I found it sounded...increasingly flattened if anything...but I didn't try 'em all and opted for amps 1, 2, then the bigger #3 amp etc....would rather just have the nice-right amp[s than play around with dangle-wired OT's.