Help with newbie's circuit: attenuator + line out, treble boost

[cherny]

Hi all,

First time poster here, and absolute beginner at electronics. I'm planning to build an attenuator with bypass, treble boost, and line out, fixed at 8ohm. This device goes between the amp output and the speaker input. I modified the Weber MiniMass circuit (http://freestompboxes.org/members/soulsonic/schematic/WeberMiniMASS.gif, thread at http://www.freestompboxes.org/viewtopic.php?f=2&t=2287), and would like to check if my modifications (including component values) are correct, before I fry my tube amp   

Modifications I made to the MiniMass:
1. Removed the voice coil motor assembly
2. Replaced the Volume rheostat Rh1 with an 8ohm 100w L-Pad
3. Removed the impedance switch circuit (the SW2 block). Since the L-Pad is 8ohm, this should give me a fixed 8ohm impedance in the entire circuit, right?
4. Changed the line out resistor values from R1=10k R2=4k7, to R1= 4k7 R2=470. I actually don't understand why the original MiniMass circuit doesn't have the line out resistor values at a 10:1 ratio as recommended elsewhere, can anyone explain?
5. Moved the line out circuit to after the bypass switch

Seems pretty straightforward, what I'm most unsure about is the resistor values in the line out block. If anyone can confirm that my circuit is correct, I'd be most thankful...

[MrStab]

i don't have much to contribute beyond the observation that you might wanna consider the mechanical action of the switch if bypassing speaker loads. personally, i definitely wouldn't want a break-before-make kinda switch, even for a millisecond, but i don't know if a make-before-break system would be safer. those terms are often applied to rotary switches and the likes, too - footswitches seem to be even less varied in their function.

just one paranoid amp owner to another!

[PRR]

> the L-Pad is 8ohm

Be sure it IS an L-pad, gets hot. Has vents.

There are also transformer-based speaker attenuators. These reduce the load on the amplifier when less power is wanted. Transistor amps do not mind, but hard-driven tube amps DO. They may be rated "FOR 8 Ohm 100 Watt", but are not a 8 Ohm 100W resistor.

Clip-lead a speaker to the pad and measure the amp side for Ohms. A transformer-based design may measure under 2 Ohms (for DC, higher for audio). A true L-pad will wander from 6 to 10 Ohms as you turn the knob. ("8 Ohm" speaker is probably 6 Ohm DC, and the L-pad value is liable to run a bit high for lower settings.)

The bypass switch is fine IF you only switch when "silent". The danger comes when the amp is working HARD and the load is disconnected.

[cherny]

Thanks for your input MrStab and PRR, I didn't consider the split-second of no speaker load during bypass switching.. I'll be sure to only bypass when the amp is silent! Also wasn't aware of transformer-based attenuators.. I'll check it out.

I've another beginner question regarding this circuit: since the signal branches off into the line-out circuit, and through the two resistors (R1 and R2) to ground (assuming the lineout plug isn't connected), does this degrade the original signal going into the speaker in any way? Is part of the signal lost, or is the signal unaffacted since it is blocked by R1 and R2? How about when the line-out plug is connected, is the signal going into the speaker affected?

Also, are the the voltage-divider resistor values chosen for the line-out conversion (R1 and R2) affected by the rest of the attenuator circuit? I know they tend to be in the 10:1 neighbourhood, so does the rest of the circuit affect what resistor values to use?

[PRR]

> signal branches off into the line-out .... degrade the original signal going into the speaker  .... part of the signal lost ... when the line-out plug is connected

You forgot to ask: what if the sound-guy puts a dead-short on the Line Out?

Go ahead. Short R2. Now R1 is in parallel with 8 Ohms (speaker or pad).

8 Ohms in parallel with 4,700 Ohms is 7.986 Ohms.

Can your amplifier tell the difference?

Can your best ohm-meter tell the difference??

It is like 0.1%.

If you have a 3,000 pound car, do you worry that a 4-pack of beer will change the handling?

If you get a $100 gig, but pay a dime to park, does that hurt much?

Un-short Line Out and the difference is insignificantly less. Like 9 cents to park.