Simple Attenuators - Design And Testing

[Barnsley Boy]

Pretty close! The full logarithmic formula goes:

power ratio = P1/P2 = 10^(decibels/10)

or, decibels = 10.log(P1/P2)

In this case
P1/P2 = 10 ^(-59.5/10) = 0.00000112

power is therefore 50W x 0.00000112 W = 56 microW

I expect you're finding a setting to use quite a few dB's louder, but if you engage this full attenuation, can you still hear it?

@JohnH ...... a slight tweak to the calculation

10 ^(-59.5/10) = 891250.9381.
Assuming P1 is the unattenuated power (in my case 50w), then the calculation should read:

P2 = 50/891250.9381 = 56 microwatts.

Just out of interest, I populated the table below showing the 17 increments of reduction (assuming the first one is with no attenuator connected) and the associated power reduction.



It's fascinating firstly how quickly the power drops, and secondly how bl**dy loud even 0.4 of a watt is!

With all stages engaged, what I would class as DEFCON 5, I would say that it is at a level where you could almost play the amp with everything cranked and it wouldn't wake a sleeping baby. Things get a lot hairier very quickly when the switches get flipped. It would be great to play at a volume more suited to the amp, and push the speakers to get that proper Marshall goodness, but without a soundproof enclosure and a decent pair of ear defenders, I'm going to have to wait!

My ears have never been the same since a Godfathers St. Valentines Day Masacre gig 5 years back. Tinnitus is a terrible thing, the ringing sound never stops!

 

[JohnH]

Hi @Barnsley Boy , thanks for the table! Isn't human hearing amazing that it's possible to hear that power level?

 

[Lancer X]

@Gene Ballzz we'll call this pretty much finished. (Switch 2 changes cathode bypass and cap options for V1A, and Switch 3 adds negative feedback and elimination of the bypass cap.)

Just need to relabel the controls: Input, Bright/Normal, Volume, Tone, Master, Voicing (Lead/Clean/Tweed), and Gain (Lo/Mid/Hi). Bought white-on-black tape for Susan's label machine, but we moved recently and it's still hiding at the moment...



Sorry to hijack - back to attenuatin'...

 

[Gene Ballzz]

@Gene Ballzz we'll call this pretty much finished. May tweak the B+2 voltage again later, but will try it a while like this for now. (Switch 2 changes cathode bypass and cap options for V1A, and Switch 3 adds negative feedback and elimination of the bypass cap.)

Just need to relabel the controls: Input, Bright/Normal, Volume, Tone, Master, Voicing (Lead/Clean/Tweed), and Gain (Lo/Mid/Hi). Bought white-on-black tape for Susan's label machine, but we moved recently and it's still hiding at the moment...

View attachment 102365

Sorry to hijack - back to attenuatin'...

So COOL!

Who made the "Lancer" logo plate? I'd love to have one, different logo, of course!

Thanx 4 Sharon, She's A GOOD Girl, In The Worst Ways!
Gene

 

[Lancer X]

Who made the "Lancer" logo plate? I'd love to have one, different logo, of course!

This Etsy shop. They use a "Fender" font, and these are cut from stainless. It looks really nice:

Custom Guitar Amp Badge, Speaker Cabinet Badge, Custom Metal Name Badge, Personalized Name Badge, Guitar Amp Emblem - Etsy

This Hooks & Fixtures item by DroplightIndustries has 355 favorites from Etsy shoppers. Ships from Chalfont, PA. Listed on Aug 18, 2024

www.etsy.com

I have a cool idea for my final "company" logo, but that reveal has to wait for the Bassman Micro build...

 

[Gene Ballzz]

This Etsy shop. They use a "Fender" font, and these are cut from stainless. It looks really nice:

Custom Guitar Amp Badge, Speaker Cabinet Badge, Custom Metal Name Badge, Personalized Name Badge, Guitar Amp Emblem - Etsy

This Hooks & Fixtures item by DroplightIndustries has 355 favorites from Etsy shoppers. Ships from Chalfont, PA. Listed on Aug 18, 2024

www.etsy.com

I have a cool idea for my final "company" logo, but that reveal has to wait for the Bassman Micro build...

Thanks for that,
Gene

 

[XTRXTR]

Here's a maths puzzle, for anyone to try: If BB is running his 50W amp, at the max attenuation of -31.5db, plus the added -28db stage, how many Watts are reaching the speaker?

I did this without using any log calculation.

I could do the log but I thought this was a good enough approximation and it follows the idea that for every -3db you get half of the power. 31.5db/3 is 10 with a remaining 1.5 or its half way to 11 times or 10.5.
50x.5^10.5 = 34.5 milli-watts
and now we do a similar thing with the additional attenuation of -28db 28/3=9.33
the final equation then
50x.5^(10.5+9.33) = .000053523-->54 micro-watts

 

[XTRXTR]

I think my calculation is slightly off because -3db isn't exactly or precisely half but is close at 0.501.
50*.501^(10.5+9.33) = 0.000055687-->56 micro-watts

 

[JohnH]

I think my calculation is slightly off because -3db isn't exactly or precisely half but is close at 0.501.
50*.501^(10.5+9.33) = 0.000055687-->56 micro-watts

All good! yes -10 db is exactly 1/10th power, per the log definition. The other simple values that I listed are very close but not absolutely exact.

 

[ibmorjamn]

@JohnH ...... a slight tweak to the calculation

10 ^(-59.5/10) = 891250.9381.
Assuming P1 is the unattenuated power (in my case 50w), then the calculation should read:

P2 = 50/891250.9381 = 56 microwatts.

Just out of interest, I populated the table below showing the 17 increments of reduction (assuming the first one is with no attenuator connected) and the associated power reduction.

View attachment 102341

It's fascinating firstly how quickly the power drops, and secondly how bl**dy loud even 0.4 of a watt is!

With all stages engaged, what I would class as DEFCON 5, I would say that it is at a level where you could almost play the amp with everything cranked and it wouldn't wake a sleeping baby. Things get a lot hairier very quickly when the switches get flipped. It would be great to play at a volume more suited to the amp, and push the speakers to get that proper Marshall goodness, but without a soundproof enclosure and a decent pair of ear defenders, I'm going to have to wait!

My ears have never been the same since a Godfathers St. Valentines Day Masacre gig 5 years back. Tinnitus is a terrible thing, the ringing sound never stops!

Yes, this
“

Tinnitus is a terrible thing, the ringing sound never stops!

 

[luxspring]

Hello.
New member who joined after reading about the JohnH attenuator project while researching on Google.
I have read a fair amount on the pages but not had the time to manage all 125 !!

The background to getting involved is that I have a friend who asked me if I would be able to build him an attenuator and the JohnH sounds amazing.
I have limited electronic skills but I have built a couple of amps from kits so I am keen to have a go.

The M2 seems to be the one most suited to his needs so that is the one I will try to build.
One thing I would like to know before I proceed is whether the M2 in the standard config will allow him to play his Vox AC30TB at bedroom levels.

Many thanks in advance for any help and advice, I'm really looking forward to the challenge.
Cheers

 

[JohnH]

hi @luxspring and welcome to our thread. Yes an M2 should be ideal for an AC30. It's rated for 50W amps. It should be able to set any level down to about a low to moderate TV volume with max amp power.

A key thing to check first is what ohms value to build to match the amp and speakers. I think AC30's usually have two 8 ohm speakerinvited in series so 16 ohms, but this needs to be checked.

On page 111, @Gene Ballzz posted a helpful wiring diagram. If you follow it, be sure to trace through it in relation to the schematic on post 1. It will help you check the build and adapt it to the case that you have.

 

[Splawn]

My low watt M-Lite for my 2.5 watt LTP P-P EF80 Plexi/2204. I divided all the wattages by 10. Cost was about $25.

 

[Guitar-Rocker]

Question. On a M-Lite, can the always "on" -dB cut of -7dB be reduced to a -3.5dB always on cut?

 

[luxspring]

hi @luxspring and welcome to our thread. Yes an M2 should be ideal for an AC30. It's rated for 50W amps. It should be able to set any level down to about a low to moderate TV volume with max amp power.

A key thing to check first is what ohms value to build to match the amp and speakers. I think AC30's usually have two 8 ohm speakerinvited in series so 16 ohms, but this needs to be checked.

On page 111, @Gene Ballzz posted a helpful wiring diagram. If you follow it, be sure to trace through it in relation to the schematic on post 1. It will help you check the build and adapt it to the case that you have.

Hi John, and many thanks for answering personally. It’s greatly appreciated.
Great tip re the layout by Gene

You are correct re the Vox, it is 16 ohm, however I will build the 8ohm version with the extra Jack for a 16ohm cab as I know he has other amps he may use it on which are 8 ohm.

A question re the switches. I am in Europe with 230v mains. What amp rating do the switches need to be ?

Many thanks

 

[JohnH]

Hi John, and many thanks for answering personally. It’s greatly appreciated.
Great tip re the layout by Gene

You are correct re the Vox, it is 16 ohm, however I will build the 8ohm version with the extra Jack for a 16ohm cab as I know he has other amps he may use it on which are 8 ohm.

A question re the switches. I am in Europe with 230v mains. What amp rating do the switches need to be ?

Many thanks

That's OK but need to check that the AC30 has an 8 ohm output option that is reasonably accessible - I'm not sure. The tap schematics show that it probably has an 8 ohm tap, but it may be configured to only work with another cab as an extension to the internals as well rather than as a standalone . Another small thing to note, when you use the 16 ohm out on an 8 ohm M2, there's about another 2db overall reduction. If the AC30 is the main use, I'd suggest to build it at 16 Ohms. In any case, need to match the attenuator to the amp output ohms.

On switches, only relatively low voltages reach the switches , like 20-30V max. Switches are generally rated at mains voltages 120 or 240V. So you can look at the specs for 120V for switches since we are not actually switching mains, for which we want 5 Amps ac current. Most toggles will do this.

 

[JohnH]

Question. On a M-Lite, can the always "on" -dB cut of -7dB be reduced to a -3.5dB always on cut?

Not directly. But there are a few ways to get a 3db or 3.5db setting. What do you intend this for? Do you have an M-lite? if not I suggest to build an M2, its simpler.

With any of them, you can get a -3db setting by using it in parallel with the speakers, setting it to max attenuation, and setting the amp to half the ohms. It works if you have the right output taps and there's nothing to change on the attenuator

It is possible to switch the M2 front end down to -3.5db, then if you want -7 or more, switch it back to standard setting. Its more complex with the older M and M-lite designs since they have two coils. You can also have a dedicated -3.5 box.

 

[Barnsley Boy]

Hi @luxspring, welcome to this great community!

Just to put my 50p in the slot based on my experience building an 8 ohm M2:
1 - Buy the biggest enclosure that you can, a bit more air circulation isn't a bad thing, and some decent working space to see what goes where
2 - I had great success pop riveting the resistors to the case
3 - Good tools to have - step drill bit and an automatic centre punch.
4 - If your friend really wants to play his 50 watt Vox at bedroom levels, add the -28db stage to the shopping list
(it opens up a few more volume control options)
5 - Planning is everything, leave a bit of room in case you want to add something else (I had to jiggle things about a bit to add the -28db stage.
6 - @Gene Ballzz 's layout on page 111 is a fantastic starting point, don't be afraid to draw it out yourself so that you can get to grips with which wire goes where
7 - I found the mini toggles a bit too fiddly to work with, so opted for some big ones and spade connectors

Finally, post pics of the completed build on (you guessed it) the completed build page!

N-Joy!

 

[Guitar-Rocker]

I built the M-Lite. When trying it out yesterday -14dB cut was the most I would ever want out of the 14watt amp that I built the Lite for. I was wondering if I could revise it to always a -3.5dB when not in bypass, then have the three switchable stage options as 3.5 each also?

 

[luxspring]

That's OK but need to check that the AC30 has an 8 ohm output option that is reasonably accessible - I'm not sure. The tap schematics show that it probably has an 8 ohm tap, but it may be configured to only work with another cab as an extension to the internals as well rather than as a standalone . Another small thing to note, when you use the 16 ohm out on an 8 ohm M2, there's about another 2db overall reduction. If the AC30 is the main use, I'd suggest to build it at 16 Ohms. In any case, need to match the attenuator to the amp output ohms.

On switches, only relatively low voltages reach the switches , like 20-30V max. Switches are generally rated at mains voltages 120 or 240V. So you can look at the specs for 120V for switches since we are not actually switching mains, for which we want 5 Amps ac current. Most toggles will do this.

Good points John.
I will do a bit more digging to get the exact spec of the AC30.
And as I haven't completed the parts order, I will hang back in case I need to build the 16Ohm version

If I do build the 16Ohm version, can I connect an 8ohm amp as well ?