Simple Attenuators - Design And Testing

[musicheals]

@JohnH
Is there a way, to use your M2 also as a dummy for recording?
If yes, where is the best place, to put in a line out?

Thanks a lot
musicheals

 

[JohnH]

@JohnH
Is there a way, to use your M2 also as a dummy for recording?
If yes, where is the best place, to put in a line out?

Thanks a lot
musicheals

Sure, if you want to use it with no speaker, just set if to max attenuation and connect it to the amp. The amp is loaded fine, since at that setting it sees hardly any of the speaker anyway. You can put your ljne-out circuit across the input.

If you want to keep a speaker running though to monitor while you record, you can take a line out from the speaker end, maybe using the second speaker output. You may find that the signal level there is already at a good level to go to a mixer. An advantage of that way is you pick up some more of the real speaker character.

Both ways have been tested. The best values for a pot with series resistor are around 5 to 10k each, depending on what you are using.

Best to experiment to see what works best for your set-up.

 

[musicheals]

That's great! I thought I have to build me reactive load.
Today I will order all parts. When I finished your attenuator, I will review and send some photos...

Thanks a lot John
musicheals

 

[dbishopbliss]

I finished assembling my M2 yesterday. I used the same enclosure as Gene Ballzz found back on page 75. One big difference, I installed the jacks on one end and the switches on the other end. I did NOT drill any additional holes for ventilation. I wanted to see how hot it would get.

My amp is a 1962 Sivertone 1482 with circuit tweaks I made. It is similar to a Tweed Deluxe in terms of tube complement and power. Generally, I set my amp at about 2 and my bandmates complain it is too loud. It is louder than the drummer and louder than the other guitar player with an AC30 (he has a master volume). I usually have to turn it down so low that it seems to lose all character and sounds really thin... I think everyone knows that spot on their amps controls.

The attenuator worked great. I had the default -7dB engaged plus the -3.5dB. So 10.5dB was perfect. I did not notice any loss in response or frequencies, just a slightly lower volume, BUT it did not have that thin sound that it would get if I simply turned it dow. After 3 hours of band practice today I can report the enclosure was cold. I kept checking it every couple of minutes, just in case. Seriously, not warm in the slightest. Again, this enclosure has no holes for ventilation. So, if you have a 20-watt amp that you don't run dimed you might be able to able to save yourself some complexity.

 

[JohnH]

That's great to hear, and congrats for getting it together.

You observations on heating make sense.

On the temperatures, I think the relationship with power input is different to that for perceived volume vs power, and that's consistent with the physics. When I'm feeding in about 30W I can make it warm, with continuous thrashing, but that's not my playing style! But I think a cranked 50W needs all the passive cooling it can get, and at 100W, id reckon some active cooling or much bigger build may be needed.

 

[dbishopbliss]

For those that like photos (like me), here are the glamor shots.

The 100W 15R resistor (R1) was much larger than I expected. I tried to mount it as centrally as I could on the top of the enclosure so it would be able to dissipate heat efficiently. I have to put slightly off center so I could mount the L1 inductor next to it. The resistor is mounted with 4-40 bolt and locking nuts using thermal paste (the white goop). The inductor is mounted with hot glue.

The rest of the resistors were mounted to the bottom. Again, I located the 50W resistors centrally so they would have more surface area to dissipate heat. I spread out the remaining resistors as much as I could. I only used one screw per resistor. I figured they will not have any stress on them. The locking nuts held them securely and should not loosen over time.

I used DPDT switches for no particular reason other than someone said they used the same ones for an attenuator on Parts Express. On one hand they are pretty large and probably overkill. On the other hand, bridging across both polls made it easy to solder the connections from the resistors.

I cut and soldered the wires before putting on the end-plates with the switches and jacks. I probably could have mounted the plates on first and then cut the wires to exact size. Initially, I was thinking I could leave them long in case I needed to take the plates off but in retrospect I don't think I will ever do that. Doesn't impact performance in any way so I'm not going to worry about it now.

Another thing I think I would do is change the order of the switches so that the values were sequential: -14dB -7dB -3.5dB. Just makes things easier to remember.

I wish I could find an enclosure like this one that was slightly larger. There is not enough room to build an M3 with the additional inductor and capacitor. I have a hammond enclosure that is 5x7x3 that should work fine but it doesn't look as nice. I have some heat sinks laying around that are pretty big. I might mount them to the sides of the enclosure so the attenuator can handle a larger amp.

 

[JohnH]

hi @dbishopbliss , thanks for posting the build pics, it looks very effective!

Those Arcol 100W resistors look like they use a fair amount of real estate, with the flanges on each side. The eBay ones just have one flange, more compact but less heat transfer.

One thing I can mention: in the circuit, the stages go -7db reactive, -7, -14, -3.5. There are minor electrical reasons for that. but physically, if you wish, you can mount the switches in ascending order of value, if that appeals.

I'm glad it's working well and letting you crank up to the sweet spot!

 

[Mark Soltow]

A big thank you for posting up the M2 circuit.

Big day today as I finished construction and tried it out. Not to mention my first post on the Marshall Forum. (Edit--sorry about double posting the pictures)

This took a little longer than I thought, due to the amateur metal fabrication. I wanted something a little heftier than the project boxes most people use. Wiring went pretty slow too, 18g stranded was a pain with the little holes in the Arco resistors. Not having any amps over 15 W, I used lower rated resistors.

Anyway, tried it out with a project amp (6V6 push pull) and it sounds....attenuated. Never used an attenuator of any kind before so didn't really know what to expect.

Playing at 60 dB SPL is a different sensory experience than playing at 85-90 dB SPL. Once you get past that and focus on what you're playing, it's actually pretty cool.

Well worth it in my opinion.
,

 

[JohnH]

hi @Mark Soltow , thanks for posting. It looks to be an awesome build, nice to have plenty of space to work in!

That system for mounting the coil looks ideal for the bobbin-less type, being zip-tied to a perf-board, which is then bolted to the case with a stand-off gap. That avoids inductance and loss issues with both the bolts and the case.

 

[dbishopbliss]

This took a little longer than I thought, due to the amateur metal fabrication. I wanted something a little heftier than the project boxes most people use.

That certainly looks hefty! I thought about doing something similar. I bet the aluminum cost as much as all the other parts.

 

[JohnH]

Double Trouble! Attenuator M2x2

Here's a new variant based on M2. It's aimed at getting a more versatile option for a unit to work across different amp and cab impedances.

Previously:

Designs M and M2 were based on a fixed amp impedance tap, and then work with various cab ohms.
M2V added an extra stage, to convert to x2 or x1/2 impedance at the front, at the expense of soaking up an extra 3db of power.

Attenuator M2x2

I wanted to see if a box can be made that works natively at 8 or 16 ohms, with no added losses, as well as convert to different cab ohms.

It's based on two 8 Ohm M2 circuits:



They are arranged symmetrically to make a mirror image at top and bottom. With both engaged, the two 8 Ohm circuits make a 16 Ohm one, with no quirks.

The stage switches S1, S2 and S3 are now DPDT (each with A and B poles), to operate on both halves at once. But mostly, this type of switch is what we use anyway since they are common units.

S5A and S5B is a DPDT that shunts the lower circuit, giving effectively an 8 Ohm M2, which is how the diagram is shown above.

S4A and S4B is another DPDT to reduce damping and adjust impedance, to lift and correct the tone when using a 16 cab with and 8 Ohm tap.

But whether or not S4 is engaged, it's safe to put any 4, 8 or 16 cab into the output, using either 8 or 16 input.

All the values of each half are as M2, with the lower circuit references R101 instead of R1 etc. All the 100 series resistors can be half the power rating of the upper ones, since they always share the work. L101 should be the same as L1 ie 0.9mH 18 gage air cored.

So although there's more wiring, its not hard to use and the extra parts are actually only a fairly small part of the total cost.

There's also an even more versatile one possible, with the same two circuits, that adds a native 4 Ohm front end by putting the two circuits in parallel. But this needs a 4-pole switch for S5, with high current, and I figure that most amps have at least an 8 or a 16 tap.

I worked this out mainly just as a puzzle to see where it might lead. Maybe it will be useful for some, though for most, a basic M2 will do the business.

If anyone is tempted to try it, the way it works is fully proven based on previous designs, so no issues there. In this case it becomes essential (instead of just advisable) to use insulated jacks eg (Cliff type), and it'll definitely need a bigger case.

 

[dbishopbliss]

But whether or not S5 is engaged, it's safe to put any 4, 8 or 16 cab into the output, using either 8 or 16 input.

Please comment on what happens if you use a 4 or 16 ohm cab with an 8 ohm M2. Is it safe? I'm guessing it is sort of similar to mismatching a speaker cab so probably safe unless your amp is pushing extremes. What impact would you expect it to have on the response and/or sound?

 

[JohnH]

It's definitely safe for the amp. Given the first reactive stage is fixed, the nominal ohms seen by the amp stays close enough to 8. The greatest variation is if all other stages are off. Then the ohms seen by the amp are about 7.3, 8.3 and 9.7 for a 4, 8 and 16 Ohm cab.

The tone is closest to normal with an 8 ohm cab. With a 4 ohm cab, it's a tiny tad brighter, easily compensated in the amp, with treble or presence, if you want to. A 16 ohm cab is a bit more muted in the high treble, and its a more noticeable change. That's why the 8 Ohm M2 circuit has a special 16 ohm out, that adjusts for this.

 

[iefes]

Here's a new variant based on M2. It's aimed at getting a more versatile option for a unit to work across different amp and cab impedances.

Hi John,

great you're never getting tired of working out further evolutionary stages of the attenuator. Even though for me that's not the most elegant approach as it involves many parts, it's a really nice idea and it's good to know it would work like this. It's great your publishing all those ideas, a lot of stuff for us to learn and think through and eventually all these ideas can be used, combined etc. to work out suitable designs for different needs!

So thanks for your ongoing research on this!

 

[musicheals]

That's great! I thought I have to build me reactive load.
Today I will order all parts. When I finished your attenuator, I will review and send some photos...

Thanks a lot John
musicheals

I got all the parts now. I didn't order in China. I bought at my long time supplier Reichelt in Germany. It was no big difference in price. Only the big guy was around 7€. I attach a picture...

Thanks
musicheals

 

[JohnH]

I got all the parts now. I didn't order in China. I bought at my long time supplier Reichelt in Germany. It was no big difference in price. Only the big guy was around 7€. I attach a picture...

Thanks
musicheals

Thanks, and that does look like a great place to buy from in Europe. eg Heres a page of their metal case resistors:

Wirewound resistors in metal housing at reichelt elektronik

I saw suitable coils there too. Good luck with your build.

 

[musicheals]

Thanks a lot John! The coil I bought there also. It was a 1mH and I unwinded it until it was exactly 0.9mH

 

[hoost]

Hi John,

Thanks for all of your hard work and contributions on this and the various other projects you've come up with for us guitarists (I've built and installed your buffer in one of my guitars).

I built what I think is the M1 version of this attenuator a couple of years ago and it works well.

I've since acquired a Tone King Sky King which has two built in attenuators and I really like having them on board and I like the functionality of the rotary switch.

I'm working on building a single channel deluxe reverb in a princeton reverb chassis, and I would like to include this attenuator on a rotary switch as a sort of master volume.

I've reviewed the new M2 design as well as your previous design which incorporates a rotary switch, and come up with the following:

My goal is to have a six position rotary switch with the following db reductions: 0, 7, 14, 21, 28, 35

Am I on the right track here? Do the values look right?

Thanks again for all of your help and all of your contributions.

Sincerely,

hoost

 

[musicheals]

@JohnH
I finished the M2 today. Tomorrow I will test.
I attach some photos...

 

[musicheals]

Here the inside