Simple Attenuators - Design And Testing

[Barnsley Boy]

Nice one @JohnH .

Under criteria 1: 4 ohm speaker load with the added front end, at max attenuation 18 ohm, with -7db attenuation 12 ohm

Under criterial 2: I've got 17.7 ohms at max attenuation, and 15.5 ohms at -7db

Not sure why the first set of readings is so high, but the 2nd set seem to be close enough. I'm gonna connect it all up and give it a gentle test run. Just need to make sure I flick the the right switches (best case scenario I get some beautiful power tube based overdrive, worst case it's good bye Putin!)

 

[JohnH]

@Barnsley Boy , hang on! if its supposed to be going into a 4 ohm amp and its reading that high st thd front then its not right! Seems maybe the new circuit is in series I stead of parallel.

 

[Barnsley Boy]

@Barnsley Boy , hang on! if its supposed to be going into a 4 ohm amp and its reading that high st thd front then its not right! Seems maybe the new circuit is in series I stead of parallel.

Looks like it could be a schoolboy error on my part John, I'll open her up and do some further investigations! Many thanks!

 

[Barnsley Boy]

Problem traced to user error! I'd plugged the speaker into the input socket instead of the output socket. I'm now getting 3.4 ohms, so all good to go.

DOH!!!!!

 

[andreio]

@JohnH thank you so much for posting the schematics.
I only have rudimentary knowledge of physics so I would like to ask you if this attenuator can also work as a dummy load when set to maximum attenuation levels.
Btw, here's my attempt. Unfortunate placement of the 50w resistors as they get a lot hotter than the big one and the top plate is thiner:

It sounds great !!!

 

[JohnH]

Hi @andreio , thanks for posting and welcome to our thread. I glad your build is working. Yes you can use it as a dummy load, all switches to max attenuation. At that level, the speaker is taking only about 1/1400 of your amp power, so it its not there at all the amp doesn't feel any difference and so you can unplug the speaker.

If parts are getting hot, a couple of suggestions:
Are you using thermal paste under the resistors? I don't see any scraps of it oozing out. If not, it will help a lot to suck the heat out of the resistors into the case. A very thin layer is all that's needed. At this stage you could just do this to Stage 1

Some vent holes where possible in the lid, and also in the base. I see some small holes, but if you can safely drill them a lot bigger - mine are 8 or 10 mm.

 

[JohnH]

Cab is 8ohms. 2 x 16 ohms speakers wired together.
Thanks so far. Don’t go to any trouble over this.

Problem traced to user error! I'd plugged the speaker into the input socket instead of the output socket. I'm now getting 3.4 ohms, so all good to go.

DOH!!!!!

Is it going now? I think you are first, leading the way with this front-end gadget! You may need to turn up volume on the switches a couple of steps to get your usual volume since the added box for 4 ohms is taking some power.

 

[Barnsley Boy]

Hi @JohnH I've tried it with both of my Peavey's and everything works fine. I need to spend a bit more time (preferrably when there's no one in the house) with the amp up at higher volumes, but I can report back that it sounds just as good as the "un-front-ended" version. The VT sounds great, the VTX has got a bit of what I can only describe as fizzle or fly trapped in a net curtain sound as the notes decay. I'm not sure if this would warrant a change of power tubes. I got the amp off ebay for the ridiculous sum of £77 (or 44 of your Australian Dollar units!). As I don't know much about the history of the amp apart from some tell-tale signs of extreme tube heat on the adjacent panel, I'd hazzard a guess that it has been used at some pretty high volumes. It may be prudent for me to replace the 6L6's whil I can still get hold of some.

Could this extension box become a "thing" I wonder? Modular attenuation stages - add another stage or 2 if you change to a more powerful amp!! I really liked the idea of the switchable 3.5db boost extension box on a previous post.

 

[Barnsley Boy]

Scorching photo:

 

[JohnH]

Hi @Barnsley Boy , thanks its great its working!

There's now quite a range of add-ons to a basic M2 design, which can be added, or not, to suit needs. I think simple is best, and to add just the gizmo's that are necessary. Here's what I recall from the thread so far, starting with a basic M2 as shown in Post 1.

1. 3rd output to use 16ohm speakers in an 8 ohm M2, so it then works with 4, 8 or 16 ohm speakers.

2. Line-out for direct recording or PA (needs a cab-sim or IR box)

3. Resonant front end (design M3), for possibly more accurate response when used as a load. Consensus so far is that this is not needed (makes no audible difference) when just running to a cab.

4. Footswitch stage, a switchable extra -3.5db stage seems preferred.

5. Switch Stage 1 between -7 and -3.5db, for loud but just a small reduction

6. Full bypass, particularly for fixed or built-in versions

7. Switchable or plug-in-able extra front end to convert 16ohm input to 8, or 8 to 4., or 8 to 16 etc

8. Extra attenuation stages to get really low volume

9. Fewer attenuation stages for simplicity on small amps, or for gigging when very low volume is not needed.

10. Dedicated single-stage fixed version.

11. Extra power rating and fan for running big amps

12. Cocktail umbrellas for efficient cooling and to celebrate the true legacy of Rock.

Any other tweaks that have been tried, or wished for?

cheers
John

 

[Barnsley Boy]

No. 12 - my personal favourite

 

[john_skaar]

hi @john_skaar , welcome to our thread.

Either of those coils looks to be fine, the ohms are as expected and the 0.04 Ohms difference makes no difference at all. (but in general, less ohms is better for the coil, but those two are so similar that there's no point in using Ohms as a decision factor between them). BTW, the local supplier here in Sydney would sell one at 0.8Ohm, also fine.

Thanks for the info John!

I've ordered the 0.70 ohm version, hopefully it should be with me sooner than 3 weeks!

Keep up this gold mine of information!

 

[Barnsley Boy]

For anyone in the UK, a good source of air core inductors can be found here:

Audio Inductors - Air Core

Audio inductors, air core. We supply an extensive range of audiophile quality air core loudspeaker crossover inductors made by Intertechnik / Audyn and Monacor. These high end components are manufactured in Germany to tight tolerances of 1%, 2% or 3% using high purity OFC wire.

willys-hifi.com

and here:

Loudspeaker Crossover Inductors and Coil Bobbins

Loudspeaker Crossover Inductors and Bobbins. Ferrite and Air Cored Inductors, Choice of Bobbins sizes, Wire Sizes and Custom Inductances

www.qtasystems.co.uk

as @Gene Ballzz would say ......... "just inducting"!

 

[vintageman]

Hi John, I want to buy components for the M2 attenuator from tme.eu. Unfortunately they do not have an 18R (R2B) resistor available. Can I use 15R and 2R7 resistors in series ?

 

[JohnH]

hi @vintageman , yes that's ok. There are also ways to put two in parallel as an alternative. eg 22 and 100, or 20 and 180 to get 18

 

[lordjester]

Just finished a small version of the M2, 3,5/7dB switch plus 3,5dB and 7dB stages. It is small, light - just perfect for gigs - I probably don`t need more options. And it works perfectly! Thanks again John!

 

[vintageman]

Just finished a small version of the M2, 3,5/7dB switch plus 3,5dB and 7dB stages. It is small, light - just perfect for gigs - I probably don`t need more options. And it works perfectly! Thanks again John!

Hi lordjester, can you please provide a schematic for M2 3.5/7 with additional stage 3.5dB and 7dB? Scheme M2 with 3.5/7 switch what was presented here had the following stage 14dB.

 

[vintageman]

Hi John, what SW are you using to design the layout of the Hammond box?

 

[lordjester]

S

Hi lordjester, can you please provide a schematic for M2 3.5/7 with additional stage 3.5dB and 7dB? Scheme M2 with 3.5/7 switch what was presented here had the following stage 14dB.

Sorry, I don‘t have one. I just used the schematic of the M2 with 3,5/7 switch, but leaving out the 14 db stage and all other options like 16 Ohm Output and line out.

 

[JohnH]

S

Sorry, I don‘t have one. I just used the schematic of the M2 with 3,5/7 switch, but leaving out the 14 db stage and all other options like 16 Ohm Output and line out.

Yes all good. Just to confirm, you can add and delete stages no problem to suit what you need.