Attenuator 8ohm 16ohm

[FunkyGibbon]

Hi,

I built an Airbrake attenuator for my 8ohm, 45W amp, and was very happy with the results. I used those little gold-coloured, wirewound, aluminium-bound resistors, instead of the adjustable ones in the originals. They're available from RS-Online. They screw to the chassis, and I put some heat-transfer paste in between the resistors and the chassis to aid heat dissipation. I also placed a heat sink on the back of the chassis. This may have been over-kill as, even with my amp at full-tilt, the whole thing only gets a little warm, not hot. I didn't include the 'bedroom' mode as I don't live in a flat/unit/apartment.

You will see from the schematic at ampgarage that for position 1, the load that the amp sees drops to 6ohms. This might be fine, but it will probably put the output transformer under unnecessary strain. In any case, I was not satisfied with this. I gave it some thought, and modified the schematic, as shown below. These are not ideal values, but the nearest values available in the above-mentioned wirewound resistors.



It still drops to 7ohms at position 3, so probably could use some further improvement.

In addition, as I've been thinking about building an AC30, it would be good to have a 16ohm version as well. It was a simple matter of doubling all the values, then finding the nearest standard value:



Christopher

[sanction13]

This looks great for a friend of mine.

I had a look at ampgarage for the original schematic but couldn't find it. Could you please point me in the right direction?

Thanks

[iccaros]

I could not find the resistors at radio shack, (RS) my guess

but I found these at mouser
http://www.mouser.com/ProductDetail/Arcol/HS25-16R-1/?qs=sGAEpiMZZMtlubZbdhIBIEZhzOrJrOmDZt5OCwpDlYA%3d

I need one of these for playing in Church.. We do some heavy stuff sometimes, but to get my amp that dirty requires a lot of output..

thanks for sharing..

[FunkyGibbon]

Airbrake files here:

http://ampgarage.com/forum/viewtopic.php?t=8733

RS Online is RS Online:

http://australia.rs-online.com

Those ones at Mouser look pretty similar.

Christopher

[egasimus]

^^ From what I hear, Radio Shack has next to nothing in the way of DIY. I'm thankful there's not a single Radio Shack in Bulgaria
Although I hear they're trying to get back into the DIY market - yesterday I saw this, which, as far as I understood, was by RShack. Didn't really watch it though, the picture looked crappy enough.

And it's kinda... odd that, in the US, you have churches where you play in bands, and the church usually has some gear on its own, too. We have this instead. Maybe this is where our church's 'gear money' went...

And, eventually, on-topic: recommend a 100-120W attenuator?

[FunkyGibbon]

RS Online is not related to Radio Shack. I believe "RS" refers to Radiospares, as they started as an Australian Radiospares distributor.

[egasimus]

Yeah, I grew up with a 1000-page RS Components catalog as my picture book 

[Pigyboy]

^^ From what I hear, Radio Shack has next to nothing in the way of DIY. I'm thankful there's not a single Radio Shack in Bulgaria
Although I hear they're trying to get back into the DIY market - yesterday I saw this, which, as far as I understood, was by RShack. Didn't really watch it though, the picture looked crappy enough.

And it's kinda... odd that, in the US, you have churches where you play in bands, and the church usually has some gear on its own, too. We have this instead. Maybe this is where our church's 'gear money' went...

And, eventually, on-topic: recommend a 100-120W attenuator?

I think if you double the wattage rating of all the resistors you will get 100w attenuation with a fairly big safety margin.

[FunkyGibbon]

Re. adequate power ratings for the resistors, see the following diagrams:





I hope my calculations are correct. To make sense of it, look at the columns "P1", "P16", and "P2". This refers to the power in the first pair of resistors, the 16/8.2ohm resistor, or the second series of resistors respectively. They show that with 100W in, the first 12ohm/ 27ohm resistor needs to be at least 72W, the 8.2ohm resistor in the 8ohm version needs to be at least 30W, the 16ohm resistor in the 16ohm version needs to be at least 13.6W, while the maximum power in the second series is about 23.6W.

So, yes, if you double all my power ratings for a 100W version, the resulting values will be more than adequate.

Christopher

[Pigyboy]

Hi Christopher,
Thanks for all the info.
Do you have pictures of your build?

Cheers,
Chris

[sanction13]

Thanks for the link.

RS Australia is affiliated with Allied in the states: www.alliedelec.com
Search the manufacturer part number: HS25 12R J and you'll find them. Part of their extended range.

[FunkyGibbon]

I was reluctant to post pictures, as I haven't finished the chassis properly, and my wiring's not great, but here goes:









Note that this is not my modified version, but simply the Airbrake with fixed instead of adjustable resistors.

Christopher

[iccaros]

Thanks for the link.

RS Australia is affiliated with Allied in the states: www.alliedelec.com
Search the manufacturer part number: HS25 12R J and you'll find them. Part of their extended range.

\

Thanks

[petemoore]

  Hard to tell from the pictures but it looks like a reflow with a bigger/hotter iron might make better looking solderjoints.
   Not recommended...Don't let on that I told you: Sometimes supplimental heat can make the almost hot enough iron a few scoches hotter...it works but is 'dirty method'. Tip of the flame is where the carbon is lowest and the heat is most concentrated/highest, just back from the tip a ways is a good place to add flame-heat. Otherwise electric stove can get the iron well above it's normal peak temperature.

[FunkyGibbon]

Hi Peter,

I think two factors explain the soldering: non-lead solder sometimes does flow very well; because I've been using pre-tinned wire from Small Bear for so many years, when it came to using this thicker wire as a one-off for this project, I completely forget to tin it!! It didn't even occur to me until I was about 90% done (and puzzling about why my soldering had taken a sudden turn for the worse, not that I'm normally particularly good)! Gives me another reason to build versions 2 and 3.

Now, was your tip to heat up an electric soldering a little further using the hot plate of an electric stove? For how long would it retain that extra heat? Do you mean a portable stove that you can put next to your project? Since I'm using non-lead solder, I might go with your first suggestion: bigger/ hotter soldering iron.

Thanks,

Christopher

[FunkyGibbon]

Errr... I meant to say that lead-free solder sometimes does NOT flow very well.

I've added another resistor, and updated the values to give a total load that the amplifier sees (RT) that is much more consistently close to the value of the speaker, and ALSO gives a more linear attentuation as you turn the knob (dB). 8 and 16 ohm versions:





I hope it's of some use,

Christopher

[Coda]

I'm just about to order the parts to build this. Surprised at the lack of activity on this thread though. Circuit looks kosher but wondering if anyone else has made one?
Gonna pair it with my home-built 18W Marshall clone (even with a VVR it's too loud for a flat lol).

[tonight, we ride]

Tons of people at the ampgarage forum have built them. Nice folks and very helpful if you have questions.