My DIY attenuator

Nicholas-L

So one day I decided it would be an awesome thing to try an build my own attenuator for my amp ?....(cough I mean beast ???)

Getting the parts...

Starting assembly

Time to test it

At -12db I started hearing some tone loss and strangely I quite liked it ? , but it's still not enough to tame this 100W amp ?

So I guess I need to do some more things to it.

Manfred-Klose

Those resistors are gonna get hot.

you need to add a heatskrink system and some form of ventilation.

but looking good so far, its a very different design from the tube tamer.

Nicholas-L

Luckily the box is one massive heat sink and I drilled a few holes for vents but I want to add a fan or two in the mix..

Nicholas-L

The design is a 4 stage L-Pad design. Each stage cuts -3db. Maybe some of the more knowledgeable folk here might tell me if it's a good idea to add a Volume control in the mix and perhaps what size and power rating?

I made this to run on 8 ohm loads. It has a 2.2 ohm (series) and a 20 ohm (parallel) in the mix to get the -3dB for each stage. Each stage is suppose to have a 100W rating.

So any ideas on an added volume control would be much appreciated ?

Will most likely be adding caps in the mix to get some added treble response.

makepeace

In my opinion, based on the amount I've reading done about the subject, you're never going to get completely satisfying results with any purely resistive attenuator with that monster, and most likely with any amp. Unfortunately, to my knowledge and experience, this is where things start becoming a problem for the DIYer, and where lots of research and work has been put into it by the bigger companies that don't make their schematics available, it makes it inviable for the DIYer to put enough time, energy and money into to make it worth not just buying one from Weber or the like or even from Graeme on the forum. For example you have manufacturers putting speaker motors sans cones into their attenuators to dissipate power with out making a noise! And then you're not even touching on what happens to the dynamics of your driver when its moving a fraction of the amount of air it was designed to.

Anyway, on to your ideas.

What confuses me slightly.. is the power ratings of each of your pads. Why 100W each?

You're attenuating 100W by a factor of 12dB to call it 6W. So you're dissipating approx 94W by means of 4 stages, thus the power rating for each stage should be 94/4=23.5W - call it 30W to err on the side of caution. But 100W per stage?!

Problems relating to why you're off ball with the output power could relate to the tolerances of your resistors and the unknowns surrounding your output stage sans attenuator. I'm aware that a fair amount of accuracy is required in terms of getting close to the calculated values to get your calculated attenuation and even resistance changes caused by your coefficients of resistance temperature and voltage is likely to cause an imbalance, ie you need 2.34 ohms and 19.39 ohms for -3dB, but you have 2.2 ohms and 20 ohms and the difference is multiplied by 4 stages. You're either changing your attenuation or your output impedance, both of which will affect the final power transfer in a way that's not easily quantifiable as you don't know the exact output impedance of your output stage, or the input impedance of your speaker. Anyway, 6W is still pretty loud, and you need to remember that generally, the more signal you attenuate, the greater the impact is on the sound.

As far as a "volume" control goes, I wouldn't suggest it. Whats wrong with just switching the number of attenuation stages you're using? With a variable attenuator, you're always going to have the chief problem of the power rating of your variable resistor. Big wirewound pots are big. And expensive.

Anyway, if you were starting from scratch, my advice would have been to go bridged T rather, and build a single stage, with the attenuating resistors R3 and R4 switchable via a rotary switch. Possibly half power, 5W output and 1W output. With the bridged T, you're able to get closer to your out and in impedances and you are also dividing your power across more resistors which is likely to sweeten expense.

OR you could give in to a gas strike and build a 5 watter ?

edit woops: I realize now why you need 100W handling per stage, but thats kind of a silly design come to think of it.. You only need 100W handling when you're using 1 stage, which in your case would be fairly infrequently as you'd still have way to much power with -3dB.

Nicholas-L

Thanks Makepeace.

I am actually busy building a 5 Watter ? , but I guess I'll have to go and do some more research into this... or try to build a VVR into my amp.

ezietsman

Nicholas L wrote: Thanks Makepeace.

I am actually busy building a 5 Watter ? , but I guess I'll have to go and do some more research into this... or try to build a VVR into my amp.

Can your SLO clone not run with only half of its power valves, hence 50 watt output? If so you can likely use it with heavy handed attenuation. (After the 5 watt, try an 18 ? )

Nicholas-L

I still want to add a triode/pentode switch for 100W/50W or I could probably just pull 2 of them... But I'm planning on doing an 18W with a VVR ? next. Arjun has told me a bit about yours and from the youtube vid I think that would be where it's at for me ?. Ewald I would love to check your amp out someday if I may?

ezietsman

Nicholas L wrote: I still want to add a triode/pentode switch for 100W/50W or I could probably just pull 2 of them... But I'm planning on doing an 18W with a VVR ? next. Arjun has told me a bit about yours and from the youtube vid I think that would be where it's at for me ?. Ewald I would love to check your amp out someday if I may?

Jolly Roger, Plumstead Main road, Saturday 28 Jan, 9pm. The monkey is doing a demo ?

Otherwise we can just organise some jamming or something.

Nicholas-L

ooooh sounds good. I'm going to drag Arjun with.... just for incase I get lost ??? lolol :roflmao:

ezietsman

Nicholas L wrote: ooooh sounds good. I'm going to drag Arjun with.... just for incase I get lost ??? lolol :roflmao:

Arrrrrrr! Bewar' de dragons!