Simple Attenuators - Design And Testing

[JohnH]

hi @tfletchii , They can be 50W because when in -7.5db mode, what was a single 100w resistor for R1in the base M2 is now shared by two 50W in parallel, and in -3.5db mode, the first stage takes less power. All of that is for a 50w amp. Does that cover yours?

 

[vintageman]

Hi John, I found this forum and I read almost the whole thread. Thank you very much to you and others involved for this great project.

I have been longing for a good attenuator and I would like to build an M2 version. I would like an attenuator for the combo, which is a copy of the Princeton Reverb amp with a power of 15W (2x 6V6 tube). The speaker is a Weber California 12 (80w power for more headroom sound from amp). I would like an attenuator for playing in the bedroom or recording at night. I would like to achieve attenuation eg from 15W to 7W, 3W, 1W, 0.5W.

I have a few questions. When I look at the drawing "J Hewitt 31/12/2021", is it possible to cut the resistors by half for my amp 15W? So it would be R1A and R1B at 25W or use one R1 15R 50W. R2A and R2B 25W, R3-R11 then 15W? R13 must be 1W or half (0.5W)? I'm from Europe, so I'll probably order from tme.eu, where some resistors have only a tolerance of 5%. Will it have a major impact on the circuit ?

 

[JohnH]

hi @vintageman , welcome to our thread. Yes if you are just using a 15W amp, you can halve all the power rating specs on the resistors, but still use case-mounted ones screwed to the case and with thermal grease.

Also, keep the coil as wound with 18gage wire.

5% spec is fine for the resistors.

Good luck!

 

[vintageman]

hi @vintageman , welcome to our thread. Yes if you are just using a 15W amp, you can halve all the power rating specs on the resistors, but still use case-mounted ones screwed to the case and with thermal grease.

Also, keep the coil as wound with 18gage wire.

5% spec is fine for the resistors.

Good luck!

Hi John, thanks for your prompt reply. For my low power amp it will probably be good to use stage1 -3.5dB / -7dB. Is it possible to make an attenuator with three stages? I don't think -14dB is needed for my 15W amp. Maybe to completely mute the speaker and use a line out ?

Stage1 -3.5dB / -7dB
Stage2 -7dB
Stage3 -3.5dB

I apologize for my questions, I want to find out more information before I start building.

 

[BlueX]

For my low power amp it will probably be good to use stage1 -3.5dB / -7dB. Is it possible to make an attenuator with three stages? I don't think -14dB is needed for my 15W amp. Maybe to completely mute the speaker and use a line out ?

I use my M2 attenuator, built with all three steps (-3,5, -7, and -14 dB), together with my 20W SV20H. When all three steps are "on" I would still disturb a person trying to sleep in the next room. Possible to speak normally where I'm playing though. Maybe that can give some hint on "effectiveness".

 

[vintageman]

I use my M2 attenuator, built with all three steps (-3,5, -7, and -14 dB), together with my 20W SV20H. When all three steps are "on" I would still disturb a person trying to sleep in the next room. Possible to speak normally where I'm playing though. Maybe that can give some hint on "effectiveness".

Thanks for the feedback. Then the M2 attenuator with all three steps will be suitable for me as well.

 

[JohnH]

Yes I use mine often at -31.5 db, with my VM. It's nice and clear in the room, and the volume is not even maxed at the amp. On paper, my VM is about 38W, which is just 4db louder than an equivalent 15W amp.

So if you're playing at home, I'd suggest that -14 db stage will still be useful sometimes, even if you switch one or two of the others off.

 

[donwagar]

For my SV20 I left the -14 stage out, but I built it for gigs and practices. (And I have the additional -3.5 in a footswitch). For home use I'd keep the -14 stage.

 

[russbryant]

Hi John,

I posted a short while ago and mentioned that I would be building an M2 after I get a tube amp. Well I got it on Sunday. A second hand Marshall DSL40CR. I just about have the workshop whipped into shape after moving from Arizona to Virginia so I'll be ordering parts soon.

Thanks,
Russ in SW Va

 

[JohnH]

hi @russbryant , I reckon that's a great amp. Good luck with that and the M2 project!

 

[Lancer X]

Curious if any of you have considered a modification such that the attenuation levels could be hot-switched without a resulting "pop" or other effect detrimental to the speaker health? Maybe something like a high pass voltage divider on the output, or the like? I'm an amp electronics beginner, so please forgive my sophomoric stab at a solution.

Commercial attenuators with dial controls seem to be okay to hot-adjust. (Or am I mistaken about that? I certainly used my Minimass that way.) Perhaps the problem is only due to the discreet jumps in switched attenuation levels?

 

[lordjester]

I don`t have any switching noises with mine, no pops, nothing, not even with the bypass switch....

 

[donwagar]

I don`t have any switching noises with mine, no pops, nothing, not even with the bypass switch....

Neither do I, and I use the -3.5dB footswitch too.

 

[BlueX]

attenuation levels could be hot-switched without a resulting "pop"

I don`t have any switching noises with mine, no pops, nothing, not even with the bypass switch....

Neither do I, and I use the -3.5dB footswitch too.

Same answer here, no switching noise. Could the problem be with your switches?

 

[JohnH]

hi @Lancer X , it's ok to operate the main stage switches -14, -7.5 and -3.5db 'on the fly'. There is enough permanent continuity through the circuit to control this when any one switch is operated in a particular moment of time. The foot switch option is the same and it's also true of the -7/-3 5 switch on stage 1 shown recently.

Thd only switch I think should not be operated unless the amp is off or on bypass is the 'full bypass' switch, if you have one. It's a good reason not to have one IMO, so I don't draw it on the base diagrams. I don't think the bypass switches on commercial units work any differently really. The issue is fly-back voltages back into the amp. I know enough to be aware of a possible risk but not enough to say it isn't one!

 

[tfletchii]

hi @tfletchii , They can be 50W because when in -7.5db mode, what was a single 100w resistor for R1in the base M2 is now shared by two 50W in parallel, and in -3.5db mode, the first stage takes less power. All of that is for a 50w amp. Does that cover yours?

Hey John,
I wired everything up and it is working as planned. For playing at home this switch is unnecessary but I think it will be a great addition for those of us using your attenuator out on gigs. I'm gonna play a few dates with it and report back. If I actually find it as useful as I think I will probably just build one of the single stage attenuators on page 118 , with a bypass switch because it would be so compact.

 

[john_skaar]

Hi @JohnH ,

Great information here! Quite a few pages to trawl though in hope of finding an answer so thought I'd just ask straight up:

I'm about to order an air core from a supplier in the UK. I've narrowed it down to 2 options and want to check if the resistance in the specification will make a difference. There's a 21 day lead time (built to order) so I don't want to order the wrong one!

The two options are:
1) Jantzen Air Core Coil 1.8mH AWG18 0.74 Ohm (part no. 000-1131)
2) Jantzen Air Core Coil 1.8mH AWG18 0.70 Ohm (part no. 000-1942)

The only other differences between the two are the dimensions (one is taller/thinner and the other is shorter/wider) and a resulting 20g difference in weight, but this would only add a practical factor in deciding what enclosure I need.

The question is, would a small difference of 0.04 Ohms matter, or should the resistance be something completely different which makes neither of these suitable?

Thanks in advance!
John S

 

[JohnH]

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.

 

[Barnsley Boy]

Maybe you could try the simple 3-part addition for the 8 ohm M2 to work at 4ohm. It's a lot less to build than another full one. Then, you could decide if you'd like a separate 4ohm unit?

Hi @JohnH
Eventually got round to building my extension box to convert the 8ohm M2 into a 4ohm for my Peavey. I've plugged it all in, and just wanted to check what resistance I should be measuring at the output jack. I don't want to blow anything up .... them valves are gonna be like gold dust! I have the speakers attached, and I'm measuring 17.8ohms with full attenuation engaged, and 12ohms with all the switches disengaged. I'll post some pics of the "son of" on the completed attenuator thread. I managed to find a nice little inexpensive enclosure on Amazon that has a similar look to the main unit.

Keep up the good work!

 

[JohnH]

Hi @Barnsley Boy ,

The most important check is the ohms that the amp sees. So you have an 8Ohm M2, with an added front end circuit that adds another 8ohm reactivevload in parallel. Is that right?

1. 8 Ohm speaker plugged in, no amp connected, extra front end engaged, you should get around 3.5 to 5 Ohms measured across the jack that will go to the amp. With a 4 ohm speaker, not much different but slightly less.

2. Measured at the attenuator output, no speaker and no amp, I'd expect around 18 Ohms at max attenuation and about 15 at -7db, with the extra front end on.

Do those check out?