Simple Attenuators - Design And Testing

[mike_lawyer]

I have all of the components set in my box, about to begin the wiring. Maybe a silly question, but the air core inductor has a lead that is on the outside of the ring of wire, and a lead that goes to the middle of the ring of wire. Does it make any difference which lead is wired to the input jack and which one goes to the resistors?

 

[JohnH]

I have all of the components set in my box, about to begin the wiring. Maybe a silly question, but the air core inductor has a lead that is on the outside of the ring of wire, and a lead that goes to the middle of the ring of wire. Does it make any difference which lead is wired to the input jack and which one goes to the resistors?

Hi Mike, its a good question but no it doesn't matter. The coil is not directional.

 

[Mcentee2]

I did some work on this, to find out what would be a good EQ curve to apply to the direct out from an M or M2, and turn it into a reasonable cab-sim signal.

Here's where I got to:

View attachment 102603

What I did was to set up a broad strummed signal with lots of frequencies in a loop. I recorded the direct out across the attenuated speaker and also recorded a mic'ed sound, all into Audacity. Then I exported the two normalized frequency plots into Excel, and subtracted them. That's the orange line and in principle, that would be the EQ curve to convert from direct to close mic'ed. Then I was playing around with some circuit designs to try to capture that in a physical piece of kit and ended up with the red curve. I then applied that curve back to the direct line out recording in Audacity. It sounded pretty reasonable, obviously not quite the same as the mic'ed signal.

I reckon that red curve is the sort of EQ that might work for what you want, though I've not built the actual physical circuit that I was designing.

Likely a better solution would be a simple IR box like a Mooer Radar

Many thanks! I thought there was a chance you had looked at this. I dug out my old captures, but none if these were from a line out.

Looks straightforward enough to feed a reverb

 

[Lancer X]

And so it begins…



Accidentally bought one resistor with big doofy leads on it…

 

[mike_lawyer]

Ok, I have run into some problems. I finished all the soldering, ran through the schematic and it looks like I had everything wired correctly. The problem is with the first 7 db switch, it doesn't do anything when I flip the switch. The 14 db and 3.5 db switches seem to work fine.

When wiring, is everyone running two wires to the center lug of the DPDT switches, or just one? I am running a wire to the lug of the first resistor, then a wire to the center of the DPDT switches. I wouldn't think it would make a difference, but maybe I wired it up wrong.

I have tried to upload photos of my build, but it says the uploaded photos are too large.

 

[JohnH]

hi Mike - Have you done resistance tests before testing with the amp?

 

[Gene Ballzz]

Ok, I have run into some problems. I finished all the soldering, ran through the schematic and it looks like I had everything wired correctly. The problem is with the first 7 db switch, it doesn't do anything when I flip the switch. The 14 db and 3.5 db switches seem to work fine.

When wiring, is everyone running two wires to the center lug of the DPDT switches, or just one? I am running a wire to the lug of the first resistor, then a wire to the center of the DPDT switches. I wouldn't think it would make a difference, but maybe I wired it up wrong.

I have tried to upload photos of my build, but it says the uploaded photos are too large.

Most comfusers provide a function/option to resize your photos. I usually choose to make a copy or duplicate photo and then resize that, to retain the original. You will normally find the option by right clicking the pic, for a contextual/drop down menu.
We'll Get You Sorted,
Gene

 

[mike_lawyer]

The wiring diagram on page 123 with the 28 db reduction, is that the way everything should be wired on the M2? I just saw that wiring diagram, it is slightly different than I have it wired up. Might be what is causing the problem.

 

[Lancer X]

Pro tip for aspiring builders: apply your thermal compound before fastening down all your resistors, kids!

 

[mike_lawyer]

[URL="https://www.marshallforum.com/media/albums/attenuator-photos.1443/"][/URL]

 

[mike_lawyer]

 

[JohnH]

When wiring up, Genes diagram on p111 is very good. But, its important to trace it through relative to the schematic for yourself and dont try with the amp until the checks and resistance tests are done. For this, plug in speaker, plug in cord to input without amp and measure across the jack that would go to the amp. It should be 7 to10 ohms in every setting, for an 8 Ohm M2 , x2 for a 16 Ohm build

 

[mike_lawyer]

Thanks, I am going to try and wire it up like Gene outlined it. Any tips for making bus wires across the DPTP switch pads?

 

[Gene Ballzz]

Thanks, I am going to try and wire it up like Gene outlined it. Any tips for making bus wires across the DPTP switch pads?

I use #18 gauge solid buss wire or simply strip the wire long that I'm using to the resistors, tin it snd put it across the two lugs. The solid buss wire works best for me! That way I can simply solder my wires to that jumper instead of trying to get tinned, stranded wire through those tiny lugs!

HTH,
Gene

 

[IvanMan]

Hello. I've build in the past a complete resistive attenuator (with my own calculations) but with a no-consistent frequency response.
Now I've found your project and I'm very interested to applicate it, but (if possible) re-using the high power resistor I bought.
But they are difference by my available resistor and the suggested value in your M2 project, for example R1 --> 33 Ohm vs. 30 Ohm, R2B --> 33 or 47 Ohm vs 39 Ohm, , R3 --> 33 Ohm vs 30 Ohm, R4= 22 Ohm vs. 20 Ohm, etc.
The questions are:
1) Is there a tolerance in the suggested value or
2) Are there formulas or and Excel spreadsheet to calculate different couple of value for series / parallel and match my available resistor
3) May be the use of my resistor is limited to pure resistor part, and for the reactive part I try to combine my resistor to obtain the suggested values?

Another question is about the inductor: is it possible to diy. In the past I realized (with success) a couple of output transformers. Someone can help me in finding instructions to build the inductor(s)

Many thanks and compliments for the project. Very, very interesting

PS1 I attach a couple of picture of my previous realization. Please note the aluminium part I used to dissipate - it is very efficient also without drilling holes
PS2 Only for information the resistor I've at home are 50W, with the following value / q.ty:

0,01	1​
0,22	1​
0,47	1​
0,82	1​
1,20	2​
1,50	5​
2,20	6​
12,00	5​
18,00	1​
22,00	2​
24,00	1​
33,00	4​
36,00	1​
47,00	2​
68,00	3​
100,00	2​
150,00	1​
470,00	1​
750,00	2​

 

[Barnsley Boy]

The wiring diagram on page 123 with the 28 db reduction, is that the way everything should be wired on the M2? I just saw that wiring diagram, it is slightly different than I have it wired up. Might be what is causing the problem.

The correct one is on page 126 - I know that one definitely works!

 

[Barnsley Boy]

With all the different cab options that the 8Ohm M2 can do, just note that it has to always run from an 8Ohm tap on the amp. If that leaves out some amp/cab combinations that you need, a front end can be added to convert.

Hi @JohnH,

Rookie question, but I'll ask it anyway, below are pictures of the back of my VTX:







Would I be able to use my 8 ohm M2 for this amp? Or will I need to build a 4 ohm version? I was a bit confused about what you meant when you said about running from an 8 ohm tap. My JCM900 has an 8/16 ohm switch on the back which I would presume selects which tap to use. The VTX only has the speaker jacks as indicated.

Also, could I get away with a 50 watt M2? or would I need a 100 watt version to accommodate the 65 watt power output? The amp is incredibly loud, and as I've mentioned previously, can only be used at home with a volume control in the FX loop. It would be good to do away with this and warm up the valves to really get it going using the attenuator!

 

[velvet_man]

This thread has so much info in it, it's hard to know which is the most current. Can someone point me to the most recent/best schematic for maximum attenuation into a 16 ohm cab?

EDIT:

I should note I only use this amp for band rehearsals and gigs, so it's never used at home (i.e. I don't need "bedroom" volume, just not destroying my eardrums volume).

 

[JohnH]

Hi @JohnH,

Rookie question, but I'll ask it anyway, below are pictures of the back of my VTX:

View attachment 102731


View attachment 102732


Would I be able to use my 8 ohm M2 for this amp? Or will I need to build a 4 ohm version? I was a bit confused about what you meant when you said about running from an 8 ohm tap. My JCM900 has an 8/16 ohm switch on the back which I would presume selects which tap to use. The VTX only has the speaker jacks as indicated.

Also, could I get away with a 50 watt M2? or would I need a 100 watt version to accommodate the 65 watt power output? The amp is incredibly loud, and as I've mentioned previously, can only be used at home with a volume control in the FX loop. It would be good to do away with this and warm up the valves to really get it going using the attenuator!

It looks like it needs a 4 Ohm load (and reading the manual, does it also have a 2 Ohm option?). So an 8 Ohm M2 is not right. But if you add three parts switched in at the front end (a coil and two resistors) you can convert the input from 8 to 4. These parts will absorb 1/2 the power, and they could be a separate box. That will take the heat off the main unit.

https://www.marshallforum.com/media/attenuator-m2v-200420.11998/full?d=1600635241

The values are on the top line of the table. This one does 16-8-4, but you could simplify just to have 8 and 4 and a switch to connect the new parts in parallel with the input.

 

[JohnH]

Hi @IvanMan , welcome to our thread.

The values are very carefully optimised to maintain tonal consistency at all settings and provide equal steps. I suggest 5% tolerance is OK. Maybe you could get some higher value and lower powered resistors to put in parallel with what you have to bring down the values?

Designing this needs close control of both input and output impedance at each setting, taking into account the amp and the speaker as a reactive load. Its too complex for a formula, I do it using a huge spreadsheet that plots every setting, and also SPICE models.

I guess you can wind the coil, if you can keep it tight and have a way to measure it afterwards. Personally I wouldn't trust formulas for inductance except as an approximation.

Good luck!