Simple Attenuators - Design And Testing

Gene Ballzz

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Hey @JohnH ,
That looks sweet! Just lookin' & thinkin', would it not be possible to change the switching around a bit, with a three position switch to allow for -3.5db, -7db and -10.5db?
Just Curious?
Gene
 

JohnH

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Hey @JohnH ,
That looks sweet! Just lookin' & thinkin', would it not be possible to change the switching around a bit, with a three position switch to allow for -3.5db, -7db and -10.5db?
Just Curious?
Gene

I think so, but I think it either needs a 3 pole rotary, or , it could be done with a 4pdt on-on-on mini-toggle.

Either way, I think it would need the extra two resistors to add the usual -3.5db stage after stage 1, to combine with -7db to make -10.5db in the third switch position
 

Gene Ballzz

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@JohnH ,
Yeah, I see, and by the time we go there, might as well build a full M2 unit! I must add though, that my method of running parallel for just a -3.5db, reactive cut works pretty well and is fairly/semi fool proof. Having it all on switches is better for the masses, using the 86 principle! ;)
Keep On Keepin' On,
Gene
 

Joe Patti

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I created an account here just so I could thank everyone who helped develop these designs.

I decided to shoot for absolute over-build and doubled the wattage of everything so I don't have to worry about heat, and so far it's handling my SLO 100 with ease. The fan and Heatsink were added cause I had them on hand and I figured the cooler the better.
 

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JohnH

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Hi @Joe Patti, that's an awesome unit! Do you find the fan makes a big difference? also, how hard do you like to drive that amp? Cooling on these for the 100W amps is an ongoing study.
 

Joe Patti

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Hey @JohnH

I started by running this unit with just the fan as I was waiting for the thermal epoxy to arrive for mounting the heatsinks. The fan by itself moves enough air through the enclosure to cool it down pretty quickly after a 30 minute jam session, but the box still heated up to what I would call an uncomfortable temperature with the SLO masters on about 7.

After adding the heatsinks, I plugged the unit into my KT66 powered JTM45 which, even with the amp dimed didn't make the load box enclosure warm. I'll have another opportunity to try it with the SLO tonight in my practice space and will report back how things go, but so far the heatsinks have done the most to keep the heat off the resistors.
 
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Barnsley Boy

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Evening chaps,

Finally finished and tested my 8 ohm M2. Wow! that's all I'm going to say! @JohnH thank-you so much for the design and @Gene Ballzz many thanks for rescuing me from making a dire attenuator mistake, and diverting me to this thread!

Holes yet to be drilled in the lid, but I'm very happy about how it has turned out.

IMG_5747-1.jpg

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Joe Patti

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I swear, I have never heard this amp until I got to turn it up past halfway today...

I built this Hiwatt DR103 clone last year during peak introvert season and it turned out great, but I have never had the opportunity to really hear it opened up cause it is just so much loud. Today during a break I plugged it into my sealed 2x10 loaded with Greenbacks and it was quite a sound to behold.

The box was still getting pretty hot as I was hitting chords and pushing the amp into a fair bit of both pre and power amp saturation. Settings in the picture are what I was running.

I would feel comfortable running this for a while under observation just to ensure it doesn't go too nuclear, but the heat seems fine. I can still touch the top and sides of the box without pulling away, but I'll get a temperature reading on it at some point just to see where it's at under a load like this.
 

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Barnsley Boy

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@Joe Patti I'm lovin the build. I'm now toying with the idea of a tastefully designed heatsink bolted to my lid!


@JohnH - I'm gonna sound like a right lightweight here, but the JCM900 is still too loud for me to fully crank, so I could do with adding another switched stage. Is this feasible? and would it just be a case of replicating and adding onto the end of the signal chain?

So adding another -14db cut would just add another R5 and R6? Physics lessons were a long time back - I'm sure it's a simple ohms law calculation, but my head's still spinning with the opportunities this is opening up!
 

Joe Patti

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Joe Patti

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That is a lovely build - was it a kit? I'd love to tackle something like that - maybe a lower power combo version.

Yes, it was a Mojotone kit. They actually added a 50W version a la DR504 shortly after I did this one but their wait times for unassembled kits is up to 8 weeks last I looked at 'em, so be mindful of that if you look into it.

I generally prefer to source my own parts instead of kits, but the mojo kit was the easiest way to source a chassis and faceplates that wasn't from the UK. Shipping on stuff was killer so I put the project off until I found the Mojo kit.
 

Barnsley Boy

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Great projects everyone!

@Barnsley Boy , yes you can add another stage. You can use another of any of the resistor stages, and you can insert it anywhere after Stage 1.

I'm gonna squeeze another one into the case. Good job I left a space for an additional switch! If I were to go the whole hog, what values would a -28db stage be? That would give me a good range of options and more importantly save the wear and tear on my eardrums (not from the amp..... from the wife!!!)
 

JohnH

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I think with another-28 db, you need 3 resistors. I can work that out (what's ohms are you using?) but really that's very very low with all the other stages. But I suppose it's the next logical step, and it'd give you all the next steps like -35, -38.5 etc
 

Barnsley Boy

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With all 4 stages engaged, I have 7+3.5+7+14= 31.5db cut. If there was a -28db cut, yes it's a massive drop (-59.5db total), but it's worth it to open up the other switching options to get intermediate values. I'm anticipating that I'll find a suitable sweet spot and maybe reconfigure the unit to suit.

I can work that out (what's ohms are you using?) but really that's very very low with all the other stages.

Not sure what you mean by "what's ohms are you using?"?
 

Marcomel79

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Hey @JohnH ,
I think I'm gonna start a new thread, simply for completed builds. I'll ask for only those who have finished a build, with pics to post, and direct those with questions to this thread. It might be kinda fun to actually know how many completed builds there are out there?
Watcha Think?
Gene
Very good idea Gene! I see from tracking that my coil has arrived in Oslo today, so im guessing ill be adding it before the weekend....
 


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