I have to brag for a sec

[Byrdman]

Here is the full M4 design posted with John's latest component values. Most of the component differences are due to availability, not impact on results...

Simple Attenuators - Design And Testing

Hi pietro, I reckon the case should not be grounded, and with the plastic Cliff jacks that we recommend, then the case is not in circuit at all. @JohnH , I have a "proposed" guess as to why we've encountered issues when the jacks get "grounded/connected together" through the metal enclosure...

marshallforum.com

Edit to add, including the bass resonance piece also gets pricer and will take up a lot of space as the impedance goes down. Those are pretty big film caps and inductors for a 2 ohms based build but much more practical at 16 or 8.

No worries, all I'm looking for is 8 Ohm in and 8 & 16 Ohm out

 

[Byrdman]

Here is what I built. Basically I blended the M2 and M4 designs to get a simple attenuator that also had the -3.5 and -7 db options for the first stage. I did not include the ability to use cabs with different impedance, or the extra first stage inductor because I don't think it is necessary with the way the plugged in cab will add the low resonance peak and the way I and many of us roll off the lowest bass anyway.

After I did this, John updated numbers for 2-16 ohms based on component values he found more available at the time. I had a spreadsheet with what I ordered from where, but I can't find it. Not sure how useful it would be as it is now a couple years out of date.


View attachment 48360


View attachment 48361

If possible, can you take a pic of the guts on yours, I would appreciate it. No hurry, I wont be able to start on this project for a few weeks anyway. Doesnt have to be up close, just a general layout pic is all I need.

TIA

 

[Chocol8]

Not possible really. I screwed most of the resistors to the lid (floor because I built it upside down) and then covered them with a big heat sink and then crammed it into the case body with a couple small blower fans to suck air across the heat sink. I used a decent quality (cpu) heat sink grease between the resistor bottoms and the case and the resistor tops and the heat sink then bolted it all together. Taking the things apart would be a sticky mess! Possibly worse than degooping a Dumble or Klon!

There might be some pictures of my layout in the Marshall forum thread from around July 2023, but I am pretty sure I changed things up a couple times when I went to build. I would not do the layout the same way again FWIW, and would choose a larger case and only put the heat sink on the first stage resistors. With 7 db in the first stage, those resistors are absorbing about 80% of the output power and the remaining 20% is spread over the other resistors and the speakers so they don't have the same heat concerns.

Also, obviously most people are building without heat sinks, but they are also building much more ventilated designs. I didn't want any holes on top to keep spilled liquids out and went with a small amount of powered airflow over the heat sink to keep things cool. I would still do that for a 100 watt build, but I think a bigger passively cooled case is just fine for a 50 watt build. Also, my fans are on a separate 9v circuit, but the thread now has some designs to power a fan from the extra power coming from the amp. I don't know if the voltages are high enough to drive the fans sufficiently with a 2 ohm build, but for an 8 or 16 (100 watt) I would definitely add the fan circuit and not have to rely on external 9v power.

 

[Byrdman]

Not possible really. I screwed most of the resistors to the lid (floor because I built it upside down) and then covered them with a big heat sink and then crammed it into the case body with a couple small blower fans to suck air across the heat sink. I used a decent quality (cpu) heat sink grease between the resistor bottoms and the case and the resistor tops and the heat sink then bolted it all together. Taking the things apart would be a sticky mess! Possibly worse than degooping a Dumble or Klon!

There might be some pictures of my layout in the Marshall forum thread from around July 2023, but I am pretty sure I changed things up a couple times when I went to build. I would not do the layout the same way again FWIW, and would choose a larger case and only put the heat sink on the first stage resistors. With 7 db in the first stage, those resistors are absorbing about 80% of the output power and the remaining 20% is spread over the other resistors and the speakers so they don't have the same heat concerns.

Also, obviously most people are building without heat sinks, but they are also building much more ventilated designs. I didn't want any holes on top to keep spilled liquids out and went with a small amount of powered airflow over the heat sink to keep things cool. I would still do that for a 100 watt build, but I think a bigger passively cooled case is just fine for a 50 watt build. Also, my fans are on a separate 9v circuit, but the thread now has some designs to power a fan from the extra power coming from the amp. I don't know if the voltages are high enough to drive the fans sufficiently with a 2 ohm build, but for an 8 or 16 (100 watt) I would definitely add the fan circuit and not have to rely on external 9v power.

I appreciate it! I'll check out that thread on Marshall Forum. That thread sucks a little IMO because it could be better organized. I'm not going through 200+ pages...

I asked it over there, but I dont need a line out at this time so I'm going to bypass the pot, resister, and 1 output jack. I have one on my HB PA-100 and I have never used it. Did you install it on yours? Do you really use it much if you did?

 

[Chocol8]

No line out. The circuit I built is the first picture I posted on the last page. Basically the minimal M2 with no extra outputs, no bypass, no line out. The only extra I included was the first stage switch so I can do only a 3.5 db cut instead of the minimum 7 db cut of the original M2.

I would not use the line out as I have other devices that can do the job better.