Simple Attenuators - Design And Testing

[matttornado]

@matttornado ,
FYI & FWIW, before life with The @JohnH attenuators, I used a Weber MiniMass to do a two amp, wet/dry rig. It had a post attenuator line out that I used to drive the input of an effects unit (Lexicon MX300) and out of the Lexicon into the effects return of an MG100HDFX amp This bypassed the preamp of the MG amp. It was by far the best and most natural sounding way to do delay, etc. An added benefit was that even though the preamp of the "wet" MG was bypassed, it still allowed me access to the onboard effects of reverb, chorus, flanger and delay. It was fantastic in many/most ways, EXCEPT setup, tear down and wiring on a daily basis was a royal P.I.T.A. Having the line out after the attenuator circuit allowed me to controlled a balanced volume for both amps, all from the attenuator.
Just Sharin'
Gene

Thanks Gene! I actually run a line out into the FX Return on My DSL 40CR and it sounds pretty good! The only thing is that some effects don'tsound too good or work too well in that scenario. TC Electronic effects work great though.

 

[parsnip]

That's what I'll do! Thanks.

 

[Dretot]

Hello John and forum, I got a new fan installed and working. I'm all wired up, checked, double, triple checked. Everything looks good. How do I test it before connecting to my amp?

 

[JohnH]

Hi @Dretot, thsts great news.

Best to do some resistance tests:
Yours is a 16 ohm build?
So plug in a 16ohm cab into the output and a 16ohm speaker cable into the input. Measure resistance across tip and barrel at the plug that will go into the amp (but no amp connected yet). It should read between 14 and 20 ohms in all 8 settings of the three switches.

Then try with the amp, starting with amp at lowish volume.

 

[Dretot]

Thanks John,
My build is 16 ohm with 1 switch, -3.5dB and -7dB. In the up position I'm getting 20.3 ohm and down position 18.8

 

[JohnH]

ok yes I forgot! Those seem good, so ok to give it a try when you're ready!

 

[Dretot]

Success! My JohnH works perfectly! I gave it a thorough testing yesterday and it sounds fantastic. There is no degradation of tone that I can perceive. I played it mostly at -7dB so I could crank up my amp (100w Marshall w/2 power tubes pulled + variac) and it never got hot, just warm. Also, I learned a lot about my amp by finally hearing it cranked up. I preferred it at 3/4 volume with gain around 2 o'clock. Higher vol and gain settings got a little mushy.

Thank you JohnH for the design, layout and help along the way. I could not have built this without it. If I ever win the lottery I am going to send you a BIG check! Also, thanks to the members here for posting details on their builds and lists of parts, etc. (teleman), so much good and helpful info in these threads.

I'll post a couple pics in the "Completed JohnH Attenuator" thread.

dre

 

[JohnH]

hi @Dretot , I m really happy that it's working well. Probably the first built that way, and there havnt been many with the -3.5dB stage. I think it's a useful addition to fill the space between -7dB and full volume. Thanks for trusting the design and posting a nice build!

 

[uncle sixer]

Sorry if this is a dumb question, but can someone give a mouser part number for decent toggle switches and jacks? There are so many to choose from it is making me a little dizzy.

Wire gauge for connections inside the box... 16? 18?

And enclosure size... sounds like the Hammond 1590D is pretty ideal for a basic build. That is the one that is 7.38 x 4.7 x 2.05 inches?

Thanks for any advice, building a few for my friends and myself soon I hope (one 4ohm, two 8 ohm, one 16 ohm... 50 watts)!

 

[Gene Ballzz]

Sorry if this is a dumb question, but can someone give a mouser part number for decent toggle switches and jacks? There are so many to choose from it is making me a little dizzy.

Better pricing, etc, from AES (or if you have an account, thier CE Distribution arm). I've been using the switches in the first link, but am planning to change to the higher current ones in the second link. Once you take out the screws, the holes are larger, for using as multiple wire junctions. Placement gets a little tricky, for clearance.The best jacks are in the third link.

Switch - Carling, Toggle, DPDT, On-On, Short Bat | Antique Electronic Supply

Carling, DPDT 2 position toggle with solder lugs (6A, 125V), .50" toggle length (shorter bat). On-On. Note: On-On switches may sometimes be referred to as On-None-On.

www.tubesandmore.com

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

Get these in stereo (the extra terminals grip the plug more securely, even though you don't use/wire them) and they're available in black & white

https://www.tubesandmore.com/products/14-jack-cliff-solder-lugs-switched

Wire gauge for connections inside the box... 16? 18?

#18 gauge for a 50 watt build.

And enclosure size... sounds like the Hammond 1590D is pretty ideal for a basic build. That is the one that is 7.38 x 4.7 x 2.05 inches?

I prefer the slightly extra space in the Hammond 1590DE enclosure.

Diecast Aluminum Enclosures (1590 Series)

Hammond's 1590 series of die-cast aluminum enclosures have a longstanding reputation for quality and ease of use. Casted from high quality alloys, these enclosures are tough enough to withstand every day abuse, but easy to machine for connectors, switches, and other components. The 1590 series...

www.hammfg.com

Thanks for any advice, building a few for my friends and myself soon I hope (one 4ohm, two 8 ohm, one 16 ohm... 50 watts)!

I've built more than a dozen of these for myself and friends and will likely continue to do so. In the process, I've developed drilling templates for both Mouser/DigiKey and Chinese resistors, as well as procedural conventions. I'm also available by phone/email to share tips to make it easier. PM me/start a conversation, if you like, to share contact info. My boxes end up looking similar (with some minor updates) to the pics below:

Let Me Know?
Gene

 

[uncle sixer]

Gene, that thing is a damn specimen... I might be in touch!

 

[Gene Ballzz]

Gene, that thing is a damn specimen... I might be in touch!

Understand a couple quick notes:

A> The DC power is not required to operate the attenuator, it is there only to power the cooling fan!
B> Though the pics show a BYPASS switch, I do not recommend one, for multiple reasons. Most of these reasons surround possible "operator error" that could potentially cause damage to an amp!
C> Cooling is critical, as this design dissipates wattage as heat! While a 50 watt unit will not likely even need the fan usage with amps below 30 watts, as we approach 50 watts, they can get pretty darned hot!

Let Me Know?
Gene

 

[BillJ]

Hi @JohnH, is there somewhere I can buy the air core inductor from in Australia? I haven't been able to find anywhere. And the 2 links you provided on p174 don't actually match the 1.8mH requirement. One of those places does have 1.8mH available, but not in 16 awg, they have them in 15 or 18 awg, and being in the US, the cost to get one here is astronomical, it ends up being over $200.

 

[JohnH]

Hi @BillJ
I've been going to wagneronline.com.au, based in Sydney.

I could suggest something. But first, what Ohms are you planning go run the amp at (16?), also what amp power do you want to build for?

 

[BillJ]

Hi @BillJ
I've been going to wagneronline.com.au, based in Sydney.

I could suggest something. But first, what Ohms are you planning go run the amp at (16?), also what amp power do you want to build for?

16 Ohms, and the amp is 30W, but better to go for 50W to be on the safe side.

 

[JohnH]

Great, 18 gage will be fine. There's a few options but I'd suggest this:

https://www.wagneronline.com.au/inductors-1mm-18awg-air-core-200wrms/speaker-components/audio-speakers-pa/3014/fl/

You might find that Wagner would sell a few things thst you can't get from a local Jaycar. Jaycar is good for Al boxes, bolts, feet and I don't mind their plastic jacks (get TRS ones, they grip better), thermsl paste, solder, wire. I dont much like their larger toggle switches though.

 

[SmutjeHannes]

Hey guys, I found this forum and post when searching for an attenuator for my Fender 59 Bassman Combo, which expects a 2 Ohm speaker.
So far very detailed and interesting info for me as sound engineer with limited knowledge in electronics and a lot still to process.
I found out that someone already built a 2 Ohm version of the attenuator but one thing I couldn't find out:

Can I built the 4 ohm M4 variant and then be able to use the 2 Ohm out for my Fender?

Or is the 4 ohm attenuator only for 4 ohm amps and the 2 Ohm out for 2 Ohm speakers I want to connect to that 4 ohm amp?

Many thanks in advance
Hannes

 

[Gene Ballzz]

@JohnH can either confirm or deny, but I believe it to be a walk in the park to simply include a 2 ohm output, in addition to the 4 ohm output. This is especially the case with a "native" 4 ohm amp output build. IIRC, all that is needed is a resistor and an extra jack! A handy feature of the way John laid/wired this out, is that you can either use a single 2 ohm load out of the 2 ohm jack, OR a 4 ohm load out of each of both of the jacks, as John's arrangement parallels the two jacks, once a cable is plugged out of the 2 ohm jack.

And FWIW, you could use any impedance out of the output jacks (no bypass switch allowed here) as the first "always on" reactive stage, effectively isolates the amp's output from the speaker load. The only downside of other impedance loads is a probable, small tonal compromise.

Happy Building!
Gene

 

[JohnH]

Hi @SmutjeHannes

Welcome to our thread and thankyou for your interest in our attenuator designs.

What Gene says is all good, but there are a few things to decide when working out a build that will suit you best.

As Im reading your post, your amp has a 2 Ohm output and you want to show it a 2 Ohm load so it matches? From the designs shown on Post #1 and those linked to it, You'd need a 2 Ohm attenuator build. Out of that, you could run a 2 Ohm or a 4 Ohm speaker. The basic designs assume amp tap = attenuator build Ohms, and then you can use different cab Ohms X2 or x1/2 after that.

There are designs to add a variable front end to convert from 4 to 2 at the input, which uses another 3dB of power. They aren't in post 1 though but I can find them.

In principle, the whole design can be scaled from 16, 8, 4 and 2 ohm variants, and by adding extra jacks and a few more parts, you can change the speaker cab and use x2 or x1/2 the speaker ohms. So an 8 Ohm build can be used (with an 8 Ohm amp), with 16, 8 or 4 Ohm cabs and a 4 Ohm build can be used with 8, 4 or 2 Ohms etc. When you use the x2 or x1/2 Ohms, you lose another 2 or 3 db of power from each setting. This isn't an issue unless you are running at the least attenuated setting (eg -7dB for an M2), because you can turn it up on the switches. But some users are needing just a very small amount of reduction.

For the M2 design (loudest attenuated setting -7dB), all the values for 16, 8, 4 and 2 Ohm builds are in Post #1, in a table halfway down the page.

You mentioned the M4 design also. M4 is really a collection of added features than can be selected or not. I've tabulated it for 16 and 8 Ohm builds. In principle, it can also be scaled for 4 and 2 Ohm builds, including the output Ohm Options. I haven't posted the values for 2 and 4 however., although I have worked them all out.

Things to consider with M4 and low Ohms:

M4 includes an extra coil and capacitor to model the bass resonance of the cab. These are fairly bulky and expensive parts and if the purpose is just to attenuate a guitar speaker, the consensus from builds so far is that they aren't really needed. The speaker itself creates the right resonance and this is demonstrated by several tests on the M2 design. It may be of use if the unit is used as a load box or as a direct line out, but even then, a bit of low EQ downstream can compensate. The bass resonance only affects the fundamental tone of the very lowest guitar notes, down the low end of the fretboard. Also, Ive aimed it at closed back cabs (resonance about 110 hZ). For an open-backed cab (eg a Fender combo), it would really be even lower at about 80 hz (low E on a guitar). The point Im getting to is that as you go down to 2 and 4 Ohms, coil and resistor values reduce, but the bass cap gets bigger, and maybe, you dont need it anyway!

In M4, there is a switch to allow a -3.5dB setting as the lowest attenuated instead of -7dB. Mostly, if you want to attenuate, -7dB is OK, -3.5dB is a very small step. But some need this. It affects some power ratings and components downstream though.

My guess, for the Bassman, would be to build a simple and minimal M2 , for 2 Ohms, rather than an M4 with good switches and jacks to deal with the high current that runs at these low ohms. Consider whether you need a -3.5dB setting, in which case I can help with values.

 

[TXOldRedRocker]

@JohnH and @Gene Ballzz , I just want to chime in a say thanks on behalf of all of us. You guys sure do put in the time to help people like the new member above. You’ve helped me for sure!