Simple Attenuators - Design And Testing

[BlueX]

Where do you buy parts from? I'm in Europe so it's a bit difficult to find parts for a reasonable price. So far I have been looking at Mouser, they at least have everything available.

Digi-Key and Mouser are both good, also if you live in Europe. They both offer free shipping to consumers for orders above about EUR 40. I've got some shipments from Digi-Key within 48 hours from placing the order, from the US to northern Europe. Prices might be lower on far-east websites, but all products I've got from Digi-Key and Mouser are good.

Distrelec is a Europe-based supplier, but only offers free shipping to businesses. I also find their assortment somewhat limited.

 

[Gingerale08]

Digi-Key and Mouser are both good, also if you live in Europe. They both offer free shipping to consumers for orders above about EUR 40. I've got some shipments from Digi-Key within 48 hours from placing the order, from the US to northern Europe. Prices might be lower on far-east websites, but all products I've got from Digi-Key and Mouser are good.

Distrelec is a Europe-based supplier, but only offers free shipping to businesses. I also find their assortment somewhat limited.

Thanks, those are the ones I have found as well. I think I'll order from Mouser once I get my parts list sorted out since they seem to have pretty reasonable prices.

The inductor seem to be a bit of a special item though, I might order one from eBay.

 

[BlueX]

The inductor seem to be a bit of a special item though, I might order one from eBay.

Yes, these can sometimes be hard to find. You could search for Jantzen air core coils. They come in many different values.

Air Core Wire Coil - Jantzen-audio.com

High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.

www.jantzen-audio.com

My last purchase of coil was from https://www.audiohobby.eu in Estonia. Low price for the Jantzen coil, but check shipping cost.

 

[Gingerale08]

Yes, these can sometimes be hard to find. You could search for Jantzen air core coils. They come in many different values.

Air Core Wire Coil - Jantzen-audio.com

High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.

www.jantzen-audio.com

My last purchase of coil was from https://www.audiohobby.eu in Estonia. Low price for the Jantzen coil, but check shipping cost.

Thanks, I searched around on their European dealers and that Estonian shop seemes like a good option. I think I'll rather buy a well-known product than something from eBay that is only slightly cheaper (if you consider VAT, customs and so on.)

Found this in Poland as well: https://blackdotaudio.eu/szczegolyProduktu.php?kat=90&item=6323&lan=4&cur=2 Seems like 0.9 mH and AWG18 is quite common but when you need AWG16 or bigger it gets a bit harder to find, but not impossible.

I checked out your 100W build in the other thread with completed builds, AWG17 worked fine for you?

 

[BlueX]

AWG17 worked fine for you?

Regarding sound, no problem. I haven't pushed that attenuator with a 100W amp yet, only lab tests with AC through a variac (posts #3731, 3733, and 3739 in this thread). No problems.

 

[Gingerale08]

Regarding sound, no problem. I haven't pushed that attenuator with a 100W amp yet, only lab tests with AC through a variac (posts #3731, 3733, and 3739 in this thread). No problems.

Cool, thanks for the info. I'll probably not "max out" the attenuator either but it's good to know that it can handle it.

 

[Gene Ballzz]

Where do you buy parts from? I'm in Europe so it's a bit difficult to find parts for a reasonable price. So far I have been looking at Mouser, they at least have everything available.

I get the resistors from chinesiawanoreanam, off of fleabay. The search can be tedious to find which vendor is currently shipping them at friendly prices and actually has all the needed values, but I feel it is worth the effort. On the other hand, the suggestion from @JohnH to go with all 50 watt resistors (doubling and/or quadrupling to get the needed values) is a really good idea, for managing the size/footprint, if sourcing from Mouser or DigiKey.

Below are the links to the switches I'm currently using. While the vendor and their shipping may not be suitable for you, at least you can see the brand, part numbers and specs. The screw terminals allow removal of the screws to provide nice big holes to use as junction points for two or more fairly good size wires. Through building multiple units, I've found that wire junctions are best done with most of them at the switch lugs. I personally, like the DPDT as we increase the current capacity a bit and add redundancy by jumpering the two poles/sides of the switch together.
Genuine CARLING brand with highest current rating:

Toggle Switch - Carling, DPDT, 2 Position, On-On, Terminals | Antique Electronic Supply

Carling, 2 position DPDT toggle switch with screw terminals. On-On. Note: On-On switches may sometimes be referred to as On-None-On.

www.tubesandmore.com

Non-Carling:

https://www.tubesandmore.com/products/switch-toggle-dpdt-screw-terminals

Or from DigiKey, In-Stock:

https://www.digikey.com/en/products/detail/carling-technologies/2GL54-73/483654

Hope This Helps,
Gene

 

[JohnH]

Digikey in Australia wants $32 for one of those! (ouch)

 

[Gingerale08]

I get the resistors from chinesiawanoreanam, off of fleabay. The search can be tedious to find which vendor is currently shipping them at friendly prices and actually has all the needed values, but I feel it is worth the effort. On the other hand, the suggestion from @JohnH to go with all 50 watt resistors (doubling and/or quadrupling to get the needed values) is a really good idea, for managing the size/footprint, if sourcing from Mouser or DigiKey.

Below are the links to the switches I'm currently using. While the vendor and their shipping may not be suitable for you, at least you can see the brand, part numbers and specs. The screw terminals allow removal of the screws to provide nice big holes to use as junction points for two or more fairly good size wires. Through building multiple units, I've found that wire junctions are best done with most of them at the switch lugs. I personally, like the DPDT as we increase the current capacity a bit and add redundancy by jumpering the two poles/sides of the switch together.
Genuine CARLING brand with highest current rating:

Toggle Switch - Carling, DPDT, 2 Position, On-On, Terminals | Antique Electronic Supply

Carling, 2 position DPDT toggle switch with screw terminals. On-On. Note: On-On switches may sometimes be referred to as On-None-On.

www.tubesandmore.com

Non-Carling:

https://www.tubesandmore.com/products/switch-toggle-dpdt-screw-terminals

Or from DigiKey, In-Stock:

https://www.digikey.com/en/products/detail/carling-technologies/2GL54-73/483654

Hope This Helps,
Gene

Great, I actually looked at something similar at Mouser.

I'm putting together an Excel sheet with everything for my parts order...

 

[Gene Ballzz]

Great, I actually looked at something similar at Mouser.

I'm putting together an Excel sheet with everything for my parts order...

Very important to use CLIFF UK style jacks that completey isolate all electrical connections from the chassis/enclosure. We prefer using stereo/TRS type, even though the ring only gets used in a couple instances. That extra spring loaded tab helps grip the plug more securely, even though not electrically connected. Also important that if choosin a different switch, it have good current rating in addition to being only two position ON-ON and NOT three position ON-OFF-ON. Some vendors call them two position ON-NONE-ON.
Just Remindin'
Gene

 

[Gingerale08]

Very important to use CLIFF UK style jacks that completey isolate all electrical connections from the chassis/enclosure. We prefer using stereo/TRS type, even though the ring only gets used in a couple instances. That extra spring loaded tab helps grip the plug more securely, even though not electrically connected. Also important that if choosin a different switch, it have good current rating in addition to being only two position ON-ON and NOT three position ON-OFF-ON. Some vendors call them two position ON-NONE-ON.
Just Remindin'
Gene

Yes, I checked out completed builds in the other thread and read in this thread about those jacks so I have added them to my list.

Good catch about the switches being on-on, thanks! Seems like the 2GL54-73 is called on-none-on on Mouser for example. The one I planned to use from the start was only rated for 5A so I'll change to the Carling instead.

I have built mostly pedals before and there you seldom have to bother about anything over 18V, this is another ballgame

 

[JohnH]

Hi @Gingerale08 and @Gene Ballzz

So I'm puzzling out a layout for a full fan-cooled 100W M4, from which it will be easy to step back to a simpler version. The M4 design at 50W has a mix of 50W and 25W resistors, so the 25's will go up to 50 and the 50's become a pair, series or parallel (TBC) to get the value. If its in a Hammond case, it may need one of the largest, which is 10" x 10" x 4" . It's work in progress....

Ginger, for yours, I understand itll be an 8 Ohm build, so it needs to run out of an 8 Ohm amp tap. Would you be adding the extra outputs to adjust for 4 or 16 ohm speaker sets?

 

[Gingerale08]

Hi @Gingerale08 and @Gene Ballzz

So I'm puzzling out a layout for a full fan-cooled 100W M4, from which it will be easy to step back to a simpler version. The M4 design at 50W has a mix of 50W and 25W resistors, so the 25's will go up to 50 and the 50's become a pair, series or parallel (TBC) to get the value. If its in a Hammond case, it may need one of the largest, which is 10" x 10" x 4" . It's work in progress....

Ginger, for yours, I understand itll be an 8 Ohm build, so it needs to run out of an 8 Ohm amp tap. Would you be adding the extra outputs to adjust for 4 or 16 ohm speaker sets?

Thanks again. I really appreciate that you take the time to do that! I'm really looking forward to see what the end result will look like.

For my build, I think I will add a 4 Ohm out, that would be useful every now and then. I don't think I need a 16 Ohm though.

 

[Mattman8001]

Hey guys,

My apologies if this isn't an appropriate topic for this thread.

I'm planning on adding part of this circuit into an amp I'm building for myself. I've got enough space for my circuit in the head. Any design considerations that need to be thought of?

The amp head is a cathode biased PP with 2 x 6V6s and is more or less a Benson Monarch clone with a different tone stack and a choke in the power supply. 15 watts. 8 ohm output only. I've got the amp up and running and I'm thinking it's probably time that I build the attenuator part.

- I was planning on just using Stage 1 and Stage 2 resistance values and reducing the resistor wattage ratings in proportion for a 15 watt amplifier. I've already made a two stage attenuator for 15 watt amps and have been happy with the volume levels.
- I was planning on using air cooled resistors and I'll use parallel resistances when sufficient wattage isn't available for a given size.
- I was thinking of wiring a 270 ohm 5 watt resistor from the OT secondary tap to ground for speaker protection and hopefully to allow for powered switching of the attenuation on/off and for going from -7dB to -21dB.
- Unless I'm thinking of this wrong, I'm just going to use a normal style jack and forgo the Cliffs.

Anything wrong with my thought process? Any pitfalls you guys can think of?

 

[JohnH]

Hi @Mattman8001 , the thinking seems generally good.

To make the attenuator work, it needs Stage 1 engaged, which is -7dB. So the best extra stage to add should be stage 3, being another -7dB, otherwise there's a big -14dB gap. But then I ld suggest having three stages to give you a better range if you want it Or just build them all!

On insulated Cliff-type jacks, we recomended them for the seperate builds, to work with any amp. But if your amp circuit is such that the output jacks are intended to be grounded to the chassis, then I can't see a problem continuing that through the attenuator using a metal jack.

 

[Mattman8001]

Hi John and thanks for the reply! Really appreciate what you’ve done here.

Would it be possible to just tweak the first reactive stage’s resistance values for -14 db without needing to mess with the inductor’s value or would adjusting r1 and r2a/b’s resistance for double the db loss require a tweak of the inductor’s value as well?

Maybe I should look at the math at how it all works! Is there anywhere you recommend reading to better understand the math and how this circuit works?

 

[JohnH]

hi @Mattman8001

The first stage is dedicated to -7dB and that's where all the planets align and the sun shines out! Not sure why you'd want to make it -14dB because then you'd have a very big step from not attenuated to -14dB which is power x 1/25. with no steps between. But If you did, the easiest way is to hardwire the two resistors from stage 3 after it, adding -7dB reactive to -7dB to get -14dB.

The circuit is based on classical electrical theory from the 19th century, and in that respect it predates the amplifiers that we use it with by about a century! I learnt what I know about it at Uni about 40 years ago but I dont know what is the best reference to read, except all the theory is online. Analysis is easiest done using a Spice simulation model, eg LTSpice where you build the circuit on the screen, and it runs the complex-number maths to solve it. Such a model starts with an output impedance from the amp, then has the attenuator itself, then a circuit model that represents the speaker and its variable impedance.

I see that Toneking build their Ironman attenuator into their amps, and it basically the same as the full stand-alone version. That's the approach id take in your case. there's not much difference in work just to have all the stages and then you have a versatile system.

 

[Pippylong]

Hi John

I'm currently ordering parts to build the attenuator. Big thanks to everyone involved so far.

I'm about to order the inductor from audiohobby.eu. They sell 3 versions of the 18awg 0.9mH.

.5ohm, .46ohm, and .53ohm.

Is there any advantage to one over the others?

Many thanks P

 

[JohnH]

hi @Pippylong
Thanks for your interest. I had a look at that store and it looks like a useful source of parts. Actually many stores don't even have 0.9mH and they have 3. All very similar, they look to vary with width and height. All are fine and only vary by +/- 0.04 Ohm. There is no significance in that small difference for us. If the ohms difference was greater, I'd suggest the one with the lowest ohms.

 

[Pippylong]

Thanks for the advice.

For anyone in the UK highficollective has the inductors as well.