Simple Attenuators - Design And Testing

[kenrinc]

Thanks John,

Resistance measurements are dead on. 8ohm. It seems that R2A and R2b take the brunt of the power as they get really hot and R1 just gets warm. My chassis is not the greatest though. New box coming soon.

K-

 

[JohnH]

There could still be something to check out. With a nominal 50W coming in, R1 gets about 28W and R2A and R2B get 8W to 10W each. Those are max values, not all at the same setting though. Overall resistance measurements will vary a little at each setting.

 

[matttornado]

Hello. I'm sorry if this been covered before but are ways I can tame the highs by adding a capacitor(s) somewhere? If yes, where would it go and what values / voltage ratings would be a good start?

 

[JohnH]

hi @matttornado

Can you remind me again which version and Ohms you use?

For M2, the balance of tone is set by R2A and R2B. The base design targets a neutral tonal change, with the same balance of treble as the full volume sound. If you want less high treble, you could bypass R2B with a cap and resistor in series. I probably wouldn't recomend just a cap since it can throw off impedances too much. Switching it across R2A instead of R2B will lift treble.

Could have presence lift, presence neutral and presence cut using a cap, resistor and an on-off-on toggle.

What sort of change do you seek?

The other place is at the output, across the speaker, again a cap and resistor in series, all in parallel with the output.

A similar effect is if you plug a 16 Ohm cab into the 8 Ohm output of an 8 Ohm M2, where the treble gets softened. That's why there is a compensated 16 Ohm output.

I can work out values but would need to run numbers once you can describe a bit more about what you are after.

 

[matttornado]

Thanks for the reply! It looks like I have the M4 with two inductors. I built the 16ohm 100 watt version.

I guess what I really would like is something similar to dialing back a tone control on a guitar, to take some of the shrill away at higher volumes.
A toggle switch likes you suggested would be cool as I don't really need it at low bedroom levels.

 

[JohnH]

hi @matttornado , thsts design M, the first prototype, which I still use myself. I'll figure out an option or two for it.

 

[matttornado]

hi @matttornado , thsts design M, the first prototype, which I still use myself. I'll figure out an option or two for it.

Thanks!

Here's the one I built.

 

[Gene Ballzz]

@matttornado ,
The nomenclature on that drawing can indeed be confusing! While John has actually designed a much more elaborate "M-4" unit, with multiple features, that drawing is of the initial "M" design, followed by a "dash" and then the phrase "4 stage Reactive Attenuator" and not actually referring to the "M-4" per se. FYI & FWIW, the "M" is actually just an enhanced version of the "M-lite" that includes full bypass switching and a circuit that tonally compensates for using a 16 ohm speaker on an 8 ohm attenuator.
Simply Clarifying,
Gene

 

[JohnH]

Hi @matttornado

With that M attenuator that you posted above, 16 Ohm version, to add a tone option to reduce treble, Id suggest a 2.7uF cap in series with about 30 Ohms, and switch that to connect it from 'ground' to the right side of R2. It will trim down the treble rise. If its too much, try a larger resistor.

For a 50W amp Id suggest 25W for the resistor and the cap needs to be non-polarised and preferable a film or poly cap.

 

[matttornado]

Hi @matttornado

With that M attenuator that you posted above, 16 Ohm version, to add a tone option to reduce treble, Id suggest a 2.7uF cap in series with about 30 Ohms, and switch that to connect it from 'ground' to the right side of R2. It will trim down the treble rise. If its too much, try a larger resistor.

For a 50W amp Id suggest 25W for the resistor and the cap needs to be non-polarised and preferable a film or poly cap.

Thanks for taking the time look into that and replying. So with my 100 watt amp I should use 50 watt / 30 ohm resistor, right?

 

[JohnH]

yes that sounds ok. If you have any spares around, you could lash something together to test it with lower power, to see if the tone change is what you want

 

[stickyfinger]

I've read the entire thread and I don't recall what the M stands for in the name?

 

[JohnH]

I've read the entire thread and I don't recall what the M stands for in the name?

A good question! I was just following where the design and testing led, starting with the letter A, not knowing where it might lead. Each variation got the next letter. At M, it seemed like it had arrived somewhere, and I liked the way 'M' sounds. It's M for Marshall, or MI5, or James Bond's boss. So I stopped with the alphabet there, and since then its been fine tuning and added features based on the same principles.

 

[stickyfinger]

Any reason not to parallel R1 with two 60ohm 50watts instead of going series for 16ohm?

 

[JohnH]

Any reason not to parallel R1 with two 60ohm 50watts instead of going series for 16ohm?

If its a 16 Ohm M2, then two 60's In parallel is fine to make 30 Ohm for R1. 60 isn't usually a common value, so it might be easier to get 56.

 

[fitz]

Late to the party, but I've decided to join the club.
Got the John H 16 ohm M2 Ballzz kit & a 17x8x3 Hammond amp chassis w/ lid to put it in.
(Thanks again for your guidance @Gene Ballzz !)
Going to mount everything to a piece of 1/4" 6061 that I'll cut to fit in the chassis on some stand offs.

Still need to get some mounting hardware and misc. stuff like a fan & such.
Fairly good chance I'll encase the whole thing in a low-profile Marshall-ish head shell style box.

 

[chocol8]

Finally got my missing parts yesterday, so I should be able to build one or both this weekend. Looking forward to posting in the completed thread!

 

[fitz]

Got the mounting plate cut and laid out - plenty of room...

Quick question on the wiring of Out 4:
Does R9 go tip to ring on the speaker jack for Out 4?

 

[JohnH]

hi @fitz

Yes that's right. So until anything is plugged into Out 4, R9 has no effect and when a standard mono jack is inserted, the ring terminal gets taken to ground by the barrel of the jack. It just fixes a slight extra treble and bass when using a lower ohms speaker.

The layout looks nice and spacious and that AL slab should spread the heat very well.

For the coil, particularly since its on relatively thick Aluminium, best to raise it up on some non-metallic spacer (wood is fine). Usually about 5mm is enough but with your base I'd suggest more if possible, maybe 10mm. Otherwise the coil creates a small induction cooker in the aluminium! This doesn't really make much heat but it throws the coil value off and adds unwanted damping. Also, avoid ferrous bolts through the coil.

Good luck, I reckon it should wire together in a nice open and relaxingly clear build!

 

[fitz]

hi @fitz

Yes that's right. So until anything is plugged into Out 4, R9 has no effect and when a standard mono jack is inserted, the ring terminal gets taken to ground by the barrel of the jack. It just fixes a slight extra treble and bass when using a lower ohms speaker.

The layout looks nice and spacious and that AL slab should spread the heat very well.

For the coil, particularly since its on relatively thick Aluminium, best to raise it up on some non-metallic spacer (wood is fine). Usually about 5mm is enough but with your base I'd suggest more if possible, maybe 10mm. Otherwise the coil creates a small induction cooker in the aluminium! This doesn't really make much heat but it throws the coil value off and adds unwanted damping. Also, avoid ferrous bolts through the coil.

Good luck, I reckon it should wire together in a nice open and relaxingly clear build!

Thanks @JohnH ,
The Ballzz kit came with just about everything I'll need, including a non-ferrous nut & bolt for the coil.
I put the coil on a hard plastic spacer that's 3/8" thick, but I could easily add another - the bolt is plenty long enough.
There are also some smaller diameter, but same thickness, spacers between the corners of the plate and chassis box.
Extra coil spacer shown beside the ready to wire enclosure.