DIY attenuator build

[xFallen]

I am not sure which forum this belongs so if the mod(s) need to move it please do so and let me know where it lands.

I found this schematic somewhere on the internet a while ago and thought it looked interesting. I validated it to the extent of my admittedly limited abilities and decided one day I would build it. If memory serves, it was billed as a KF inspired design and may well be some version of an earlier Airbrake-type circuit.



One thing that I was not sure of was the difference between position 6 and position 5. Position 6 is clearly a bypass, but it seemed to me position 5 would be also. EDIT: Nevermind. Figured out position 5.

Other than drilling a hole in the wrong spot (oops, but recovered from it) and having to bend the resistor mounts in order to fit them into this case, the build was uneventful.

After measuring everything to ensure it was what I expected (albeit maybe not correct!), I tested this first on my recently built 5F2a. In practice, there is no audible attenuation in switch position 5. What is the epxected behavior?

Everything else works just as expected. With the switch in the variable position, I can dial in the fully cranked 5F2a down to where the sound from the pick hitting the strings is just as aduible as the speaker in the amp.

This is the front and the back just after holes were drilled and components test fitted.





Here's the innards. The fixed value resistor is up top, the tapped resistor at the bottom, the rheostat to the left and and the switch on the right. Input is on the right, which is left when the box is right side up.



My tap values are 6.1, 11.0, 16.2 and 24.8 Ohms and the fixed resistor is 24.3 Ohms.

Here it is just after being wired up but before the wires are fully routed and tied in place. I used 16AWG stranded wire.




I worked on this finish for hours as you can clearly see.



One thing that is

Barry

[Jag]

Hey!!!

Is that an INSULATED crescent wrench in the first picture?

Nice work, post how it stands up over the long haul!

[xFallen]

Why, yes, yes it is. I actually did use it to hold the KEP nuts while I got them started as I could not get a ratchet or nut driver in there.

As to the circuit, so far so good. I measured the resistance and it is pretty close to 8 Ohms up to the 4th position where it hits 12. On the variable setting it gets somewhat out of range for an 8 Ohm load at the extreme end so I doubt I will use it much but I think you could use it on a 16 Ohm output very easily.

It sounds fine with the expected 'flatness' I have come to expect from any attenuation device I have heard so far. It seems to sound better than the built-in attenuator in my Budda SD30 combo.


Barry

[bob p]

there's another thread that discusses the generic L-type attenuator that was made famous as the Airbrake. you can find it here:

http://music-electronics-forum.com/s...ght=attenuator

IMO this circuit works okay for 8 and 16R loads, though I wouldn't use it on 2 or 4R loads.

[xFallen]

Great info, thanks. I did actually find that during research. Was it you that built the version from the PDF that had the fixed resistor off the switch to the input as opposed to between ground and one pole of the switch? They should be electrically equivalent based on what I could see. Are there any pros and cons to one way vs the other?

I was tempted to try one of Ted's MASS units but thought I would go this way first.

I don't suppose you have a Spice model for this circuit lying about...


Barry

[bob p]

sorry, no spice model.

yes, i built mine according to the original PDF schematic.

what size hammond box did you end up using?

[xFallen]

I used one recommended on a site somewhere. Sorry, my mind is not recalling where. It is the Hammond 1444-14 enclosure. I looked up the case and it was 9"x5"x2" which after some quick estimates seemed plausible.

As it turns out, using the Ohmite resistor mounts does not leave enough room with the way I laid out the components. You'd probably want maybe 6-7" of width if you place the swicth and rheostat on the centerline of the minor dimension. You could mount them on the base or top and have enough room, but I really wanted to drill the holes in the side of the case as I felt that was the superior location for the screw heads. Rather than drill holes in the top or bottom, or use a different enclosure, I bent the resistor mounts so the resistors fit without interfering with the rheostat.

Were I to build another, I think I would choose a different enclosure, or maybe spend more time and come up with my own layout rather than attempting to mimick another one. Whether it turned out any differnt or not, who knows, maybe what I have is about what it needs to be.


Barry

[Jag]

I've got a MASS and I like it better than the Airbrake. It's kind of a toss up for me between the THD unit and the MASS. I went with the MASS because it can match different impedances as opposed to the HotPlate which seems to require a different unit for different impenaces.

I don't really use mine a whole lot.

[drewl]

Nice work, I've built a couple 4 ohmers for my Fenders, and use my 16ohm Hotplate alot with my marshalls and AC30.

What's the current rating on that switch?
It looks like a cheap plastic type I bought to build a clipping device for a guy's Hammond.

[xFallen]

Thanks.

The switch is by Lorlin and seems decent quality, I think. Model number is CK1455, silver plated contacts and the like.

Copied form the spec sheet:

• Working voltage: 250VAC or 250VDC max.
• Dielectric strength: 1,000VAC @ 25°C
• Current capacity: 5A continuous
• Contact rating: 150mA/250VAC/DC DC




Barry

[bob p]

one thing that i noticed about this type of attenuator is that it seems to sound best when you're using the variable taps on the 25R power resistor, compared to when you're feeding the signal through the rheostat. this could be for impedance matching reasons, for F-M reasons, etc. i think that most people are in agreement that this design tends to sound best at minimal levels of attenuation, and tends to lose some of the magic as the attenuation level increases.

i tend to use mine more for quieting the amp down a little, like when i need "downsize" a bigger amp for a smaller room, or when i need to protect my 2x12 cab with low power celection blues from a bigger amp. i find it less satisfying for extreme forms of attenuation, like quieting a big amp down to bedroom levels.

in that vein, i think that some people may want to consider building the attenuator wtihout the rheostat. the rheostat contstiutes a large portion of the expense in that circuit, and IMO its only needed for those ultra low volumes where the attenuator does't sound that good. i've thought about building a couple more as "light" models without the rheostat. ymmv.

[bob p]

Were I to build another, I think I would choose a different enclosure, or maybe spend more time and come up with my own layout rather than attempting to mimick another one. Whether it turned out any differnt or not, who knows, maybe what I have is about what it needs to be.

Barry, when I was building mine, I looked around for quite a few different boxes. What reall amazed me was how economical it was to use the admittedly cheap looking Hammond boxes, and how cost prohibitive some of the other better looking (ventilated) boxes turned out to be. If you have leads on other suitable boxes, please let me know. My attenuator continues to reside in an old recycled project chassis that looks like swiss cheese. Well, at least its ventilated.

[xFallen]

one thing that i noticed about this type of attenuator is that it seems to sound best when you're using the variable taps on the 25R power resistor, compared to when you're feeding the signal through the rheostat. this could be for impedance matching reasons, for F-M reasons, etc. i think that most people are in agreement that this design tends to sound best at minimal levels of attenuation, and tends to lose some of the magic as the attenuation level increases.

i tend to use mine more for quieting the amp down a little, like when i need "downsize" a bigger amp for a smaller room, or when i need to protect my 2x12 cab with low power celection blues from a bigger amp. i find it less satisfying for extreme forms of attenuation, like quieting a big amp down to bedroom levels.

in that vein, i think that some people may want to consider building the attenuator wtihout the rheostat. the rheostat contstiutes a large portion of the expense in that circuit, and IMO its only needed for those ultra low volumes where the attenuator does't sound that good. i've thought about building a couple more as "light" models without the rheostat. ymmv.

I agree with everything above, or at least, what I read and recall at this particular moment ;-)

Quick calcs indicate that by the time the variable attenuator is kicked into play the mismatch is great (EDIT: as in a large non-desirable gap).

Were I to do this again I think I would use two tapped resistors and choose attenuation and match points with some attempt at making the load reactive, perhaps with a MASS-type device in series or parallel with one of the loads. I agree the rheostat is likely a marginally useful feature of the circuit.

I have a Budda Dual Stage 30, the power stage switched amp with the with built-in attenuator. Thew attenuation in it only sounds good to my ears at -3dB and maybe -6 dB. Levels above (below?) that compress the tone too much and make it sound somewhat fizzy. The amp without the attenuation is among the best I have ever heard, bar none, except maybe the Twinmaster I owned before it. This attenuator I just built sounds better than the Budda attenuator to my ears even at extremes of attenuation. I cannot explain why since I believe theyare similar circuits, except that maybe the Ohmite resistors may offer some dynamic feature to the load with their built in inductance whereas the resistors in the Budda may not. Purely speculation on my part and may well be incorrect.


Barry

[xFallen]

Barry, when I was building mine, I looked around for quite a few different boxes. What reall amazed me was how economical it was to use the admittedly cheap looking Hammond boxes, and how cost prohibitive some of the other better looking (ventilated) boxes turned out to be. If you have leads on other suitable boxes, please let me know. My attenuator continues to reside in an old recycled project chassis that looks like swiss cheese. Well, at least its ventilated.

At least you thought about ventilation. I didn't think about it until I had the box ordered and it had shipped. Maybe I could send the box to Fergie's for some louvers.


Barry