Simple Attenuators - Design And Testing

[jfromel]

I just ran my first battery of reamped attenuation recordings with the Lotus through my AC30 after several weeks of usage at rehearsals with the Princeton Reverb, Pro Reverb, Deluxe Reverb, and the AC30 in various spaces. The JohnH/Lotus is killer! Really a marvelous little box that’s quickly become a daily tool in my guitar rig.

 

[MasterEvan07]

Thats really encouraging feedback on the Fromel Lotus M2, coming from someone such as yourself who knows what works and with your range of amps. And as I noted before, I reckon the only way to get an AC30 to sound right through an attenuator is to get the attenuator right!

The proof is in the pudding, as the saying goes. From a lot of NFB in the Fenders to the unrestrained rawness of an AC30, it handles and reproduces them pretty spot-on accurately. Room perception is one thing, and it’s a big component of “feel,” but actual tone being reproduced, whether through a PA or into a DAW or whatever, sounds just like what you’re putting into it. I’m pleased to say the recordings demonstrate that quite accurately.

I’ll be sending John the not-normalized files to help illustrate just how significant the volume changes are. The white noise kinda skews perception a bit without contrasting context but it’s just that the actual recording is so quiet, while still perfectly reproduced, that the noise floor comes to the fore.

 

[JohnH]

hi @MasterEvan07
Thanks thats awesome and what you described above is exactly what it's designed to do!

 

[jfromel]

The proof is in the pudding, as the saying goes. From a lot of NFB in the Fenders to the unrestrained rawness of an AC30, it handles and reproduces them pretty spot-on accurately. Room perception is one thing, and it’s a big component of “feel,” but actual tone being reproduced, whether through a PA or into a DAW or whatever, sounds just like what you’re putting into it. I’m pleased to say the recordings demonstrate that quite accurately.

I’ll be sending John the not-normalized files to help illustrate just how significant the volume changes are. The white noise kinda skews perception a bit without contrasting context but it’s just that the actual recording is so quiet, while still perfectly reproduced, that the noise floor comes to the fore.

 

[JohnH]

Yesterday was Fathers Day here in Australia. So my daughter, who works as a concept artist , gave me a card with a drawing of her playing through an M2 attenuator, the new Fromel Lotus version!

 

[jfromel]

Yesterday was Fathers Day here in Australia. So my daughter, who works as a concept artist , gave me a card with a drawing of her playing through an M2 attenuator, the new Fromel Lotus version!

View attachment 156740

She plays a Firebird V!, obviously you are a great dad.

 

[Sgoryan]

Excited to start my build very soon. I'm planning on building the basic version for 4 ohm to work with my Fender Blues Deville. I'm mounting it on a large heat sink, and might add a fan if its still getting too hot.

I got all the parts together but have one piece that's out of spec. For the inductor I was only able to find a a .47mH in 18AWG (instead of the .45), I know I can unwind this, but don't have the measurement tools to make sure its correct at the moment. Will .47 work or should I do something else?

 

[jfromel]

Excited to start my build very soon. I'm planning on building the basic version for 4 ohm to work with my Fender Blues Deville. I'm mounting it on a large heat sink, and might add a fan if its still getting too hot.

I got all the parts together but have one piece that's out of spec. For the inductor I was only able to find a a .47mH in 18AWG (instead of the .45), I know I can unwind this, but don't have the measurement tools to make sure its correct at the moment. Will .47 work or should I do something else?

0.47 on the coil is fine!

I am guessing it's a 2x12 DeVille? I would recommend a fan and beefing up the reactive stage to 100w spec. @JohnH and I have been discussing and revising the fan circuit from half wave to full wave rectifier with a huge reservoir on the filter cap. The schematic below uses 3 1000µf caps (3000µf) and two fans in parallel but if you have room in the box, a single 3300 or 4700 (min 50VDC rating) would be preferred and will work fine with only 1 fan. For a 4Ω build use a 5VDC fan and a 7805 regulator.

Advantages this fan circuit has over the previous version.
Smoother start and stop of the fan and a regulator to make sure the fan doesn't get too much juice. The bigger the filter cap the longer the fan will spin when it's not getting power. The fan circuit is also completely isolated from the signal path.

 

[JohnH]

hi @Sgoryan , thanks for posting. I agree that 0.47mH is fine for your coil. Good luck for the build!

 

[stickyfinger]

0.47 on the coil is fine!

I am guessing it's a 2x12 DeVille? I would recommend a fan and beefing up the reactive stage to 100w spec. @JohnH and I have been discussing and revising the fan circuit from half wave to full wave rectifier with a huge reservoir on the filter cap. The schematic below uses 3 1000µf caps (3000µf) and two fans in parallel but if you have room in the box, a single 3300 or 4700 (min 50VDC rating) would be preferred and will work fine with only 1 fan. For a 4Ω build use a 5VDC fan and a 7805 regulator.

Advantages this fan circuit has over the previous version.
Smoother start and stop of the fan and a regulator to make sure the fan doesn't get too much juice. The bigger the filter cap the longer the fan will spin when it's not getting power. The fan circuit is also completely isolated from the signal path.

View attachment 156778

Interesting! I just finished up your previous fan circuit about a hour after you posted this. It worked fine and could get a 50watt DSLto push the fans at 6v. I didn't like how the fans would come on and off if there were short pauses in playing. I was most worried about short life of the fans coming on and off but I have no idea if that could cause that?? My first thought was add more capacitance in the filter to 470uf.

I was previously running my two fans off a 9v battery (actually reads 8v) and getting about 7v wired in the fan circuit. This cooled my unit almost completely. Using your old fan circuit at 6v and the fan coming on and off my unit was significantly warmer that with using the battery. I also experimented using a 5v wall supply and that kept my unit cooler than the fan powered ciruit.

I am using a FR207 diode as that's all I had and this is the fans I'm using. https://www.amazon.com/dp/B00N1Y4U8U?ref=ppx_yo2ov_dt_b_fed_asin_title.

Ill probably try this new circuit at some point. I appreciate you sharing!

 

[stickyfinger]

0.47 on the coil is fine!

I am guessing it's a 2x12 DeVille? I would recommend a fan and beefing up the reactive stage to 100w spec. @JohnH and I have been discussing and revising the fan circuit from half wave to full wave rectifier with a huge reservoir on the filter cap. The schematic below uses 3 1000µf caps (3000µf) and two fans in parallel but if you have room in the box, a single 3300 or 4700 (min 50VDC rating) would be preferred and will work fine with only 1 fan. For a 4Ω build use a 5VDC fan and a 7805 regulator.

Advantages this fan circuit has over the previous version.
Smoother start and stop of the fan and a regulator to make sure the fan doesn't get too much juice. The bigger the filter cap the longer the fan will spin when it's not getting power. The fan circuit is also completely isolated from the signal path.

View attachment 156778

With the regulator will you be able to choose a resistor value to get the fans spinning at a good speed say 9v with a 15watt amp and keep that 9v with a 100w amp? That would be ideal in my build.

 

[jfromel]

With the regulator will you be able to choose a resistor value to get the fans spinning at a good speed say 9v with a 15watt amp and keep that 9v with a 100w amp? That would be ideal in my build.

That's exactly the idea.Still working on final design but the idea is to get the plan plenty of smooth power without overpowering it.

 

[adrian078]

I was thinking of getting an enclosure like this for my 100W M2 build. I intend to mount a 60mm PC fan on the back and drill holes in the bottom. Any thoughts on this case, in particular the cooling, will it be enough? Or should I go with one of the hammond boxes and drill loads of holes + a fan?

aliexpress.com/item/1005002584253785.html

 

[JohnH]

hi @adrian078 Its a nice looking box, and I think I saw it posted before, which means maybe somewhere on this thread, or the 'completed' one. Hard to find it though!

If its an M2, without extras, it looks big enough but itll be a squeeze for a 100W. The base looks to be thinner than the top, but youll have to mount everything in the base so that the top can slide over it all from front to back, hence spread of heat over the thinnish base is a question. Thats unlike a Hammond where you can drop the lid on from above (and make the lid the base). Fan cooling definitely, can help work out how to power it from the amp, or run a 9V supply to it.

 

[jfromel]

I was thinking of getting an enclosure like this for my 100W M2 build. I intend to mount a 60mm PC fan on the back and drill holes in the bottom. Any thoughts on this case, in particular the cooling, will it be enough? Or should I go with one of the hammond boxes and drill loads of holes + a fan?

aliexpress.com/item/1005002584253785.html


View attachment 156999

I love that box. I was thinking of putting the Lotus in that but the added machine work was a lot.

 

[adrian078]

hi @adrian078 Its a nice looking box, and I think I saw it posted before, which means maybe somewhere on this thread, or the 'completed' one. Hard to find it though!

If its an M2, without extras, it looks big enough but itll be a squeeze for a 100W. The base looks to be thinner than the top, but youll have to mount everything in the base so that the top can slide over it all from front to back, hence spread of heat over the thinnish base is a question. Thats unlike a Hammond where you can drop the lid on from above (and make the lid the base). Fan cooling definitely, can help work out how to power it from the amp, or run a 9V supply to it.

Thanks! I've laid it out. It is a bit tight but doable I think. Saying that, if anyone sees issues with this size box, I'll change it to the hammond 1550N

Which layout, if any, do you think will work best? I will need to tweak the order of resistors a little to have the shortest wiring runs.


Layout A: I could spread out the resistors and mount the inductor on the lid (inside). Maybe have a plug to disconnect it.




Layout B: Squash the resistors together more and mount the inductor on the bottom of the chassis with the resistors.

 

[JohnH]

Id think it might have to be layout B. If you pre-mount the coil up to the top, it looks like it might get wiped out when you slide the case together in step 2 below:

 

[adrian078]

k it might have to be layout B. If you pre-mount the coil up to the top, it looks like it might get wiped out when you slide the case together in step 2 below

Excellent observation and thanks for the save! I completely missed that. Would have been pissed if I'd put everything in and went to close it up...haha

 

[stickyfinger]

Excellent observation and thanks for the save! I completely missed that. Would have been pissed if I'd put everything in and went to close it up...haha

You should be able to mount the coil on a nylon standoff of some sort from the bottom over the resistors.

All so I do not like those 100w resistors. They are not comparable to size of the name brand at 100w and the bolt holes should be opposite like the 50w so when you bolt them down the thermal paste gets spread out evenly. I haven't tried them so these are just my observations.

Ideally and why a Hammond box is better is you mount the resistors to the top and the top acts as a source for heat management. On the bottom you lose all that and are now relying on the fan.

Its a good looking box and I considered it too but decided on the Hammond.

 

[JohnH]

I've used the Chinese 100W resistors. I think they are ok. In an M2 design R1 gets about half the amp power, and I allow x3 that in the spec. But rounding up, it ends up as x4. But an alternative is to use twice as many 50W, which may be neater overall and can be spread out wider.