Simple Attenuators - Design And Testing

Gene Ballzz

Well-Known Member
Joined
Feb 19, 2013
Messages
3,816
Reaction score
4,606
Location
Las Vegas, NV
Hi Gene, I looked at that before, to try to work out a set of values to buy from typical Chinese sellers - see the first diagram. I went for 5 Ohm at R8 and the diagram shows R7 staying at 33 Ohm. Theres no tonal consequence (less than 0.1db at any frequency) but it shifts the attenuation down by about 0.15db. When you step through the range, then the series of increments available at each step, intended each to be -3.5db, then vary from about -3.3db to -3.7db. With the specced values the range is about -3.5 to -3.6 db, so a tad more consistent.

If you change R7 from 33 to 30, with R8 at 5, then its back to virtually the same as specced.

Thanks for that info. R7=30Ω & R8=5Ω. Makes for "Mo Betta" ordering, availability, etc!
Thanks Again,
Gene
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
Following on from above, here's a diagram for those who love diagrams:

M2 8 ohm response.gif


This the calculated response of a basic 8 Ohm M2, driven from an 8 ohm amp tap (based on my VM), stepping through all the attenuation steps from full volume down to nominally -31 db. 'db inc' is the difference from the previous step, to show consistency, 'R440' is the impedance at 440 hz, and 'bass rise' and '5khz rise' show the output peaks at low and high frequencies, relative to mid frequencies.

The thick red lines are the response of a full-volume G12M closed back cab, modelled to match measured impedances (data from Mike Lind - TGP)

The central plots show frequency response of the signals reaching the cab, on the right side is what the amp sees. In this model, there's no bass resonance circuit, so you can see that diminishing from the point of view of the amp, but its there in the speaker output because its generated by the speaker itself.

The 16 Ohm data at the top (blue background) adopt Output 3, which is tonally compensated for 16 Ohms in an 8 Ohm M2
The central band in green is 8 ohm for speaker, amp and attenuator
The lower band in yellow is for a 4 Ohm cab into an 8 Ohm M2. This has no specific extra compensation and you can see a db or so of extra high treble and a couple of db of extra bass peak (its possible to add a couple of parts to compensate if wanted)

The whole thing is from a giant excel sheet that runs a macro to step through all the variations and paste them onto this chart. It takes a minute or so to run a new set.

I use this system to check out variations in the design and to keep close control of all parameters, to ensure that at every setting, tone is maintained accurately. By being very picky with optimizing each of the numbers, I can control small changes in the design to a level which cant be heard, so that when all are combined, it all sounds as consistent and as seamless as possible.
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
@JohnH
What is row #1, that is greyed out? Is that parallel function?
Just Askin'?
Gene

That's the -3.5db resistive stage working on its own, without Stage 1, as it did in Design M shown on post 1. It works pretty well, but I stopped promoting it since it creates wiring fiddle and a few quirks when using a 16 ohm cab with 8 ohm attenuator (in that case, the amp has to be set at 16 too) It's still in my excel file though.
 

Gene Ballzz

Well-Known Member
Joined
Feb 19, 2013
Messages
3,816
Reaction score
4,606
Location
Las Vegas, NV
@JohnH
Please confirm or deny an assumption I have, concerning the 16Ω, "Out 3" an 8Ω, M2 attenuator. My "assumption" is that the switching function of that jack need not be used to introduce and remove R10 from the circuit?, Instead that switching function is accomplished by the sleeve of a Tip/Sleeve plug connecting the Ring to the Sleeve of the Tip/Ring/Sleeve jack and vice/versa when removing said plug?

This brings to mind a somewhat related question. Is there a similarly simple solution for correcting the small tonal anomalies of using an 8Ω speaker with a 16Ω, M2? Or does that get more complicated?

Thanks John, As Always,
Gene
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
@JohnH
Please confirm or deny an assumption I have, concerning the 16Ω, "Out 3" an 8Ω, M2 attenuator. My "assumption" is that the switching function of that jack need not be used to introduce and remove R10 from the circuit?, Instead that switching function is accomplished by the sleeve of a Tip/Sleeve plug connecting the Ring to the Sleeve of the Tip/Ring/Sleeve jack and vice/versa when removing said plug?

This brings to mind a somewhat related question. Is there a similarly simple solution for correcting the small tonal anomalies of using an 8Ω speaker with a 16Ω, M2? Or does that get more complicated?

Thanks John, As Always,
Gene


All true, the only extra thing used is the ring connection, which gets grounded by the sleeve of the incoming plug. No switched contacts are involved and need not be provided when buying the jacks.

Yes there is an even simpler adjustment if you want to further tweak the 16 Ohm build for 8 Ohm. The difference in tone is due to the 8 ohm speaker seeing a higher output impedance from the 16 ohm M2 than it would get from an 8 Ohm M2. The fix would be to add a 39 Ohm resistor in parallel across the speaker. This should trim back the treble and bass resonance in proportion to the mids. It could be added as a switch, or using a second jack, wire tip and sleeve direct to the other jack and put the 39 Ohm one end to tip and other end to the new ring lug so it goes to ground when the plug is inserted.

Youd have two jacks, one dedicated to 16 Ohm (or a brighter output for 8 ohm), and the other for corrected 8 Ohm. Or put a 16 cab into each and it will correct the pair for the 8 Ohm total.

But Id really like it if you could test this with a lash-up of some kind first, just temporarily wire the resistor across the 8 cab and see if it does as predicted?

The ultimate version of all this would be the 8 Ohm M2, with just two parallel jacks, and a switch to adjust tone for 16/8/4 cabs.
 

Gene Ballzz

Well-Known Member
Joined
Feb 19, 2013
Messages
3,816
Reaction score
4,606
Location
Las Vegas, NV
Ha Ha John! This debate between us over 8Ω vs 16Ω will likely continue forever! :duel: My stance is that in my universe, 16Ω is the more prevalent cabinet and/or speaker. Also, the 16Ω unit allows for more flexibility (especially with the 8Ω tweak you just mentioned) for use in parallel, as many amps do not have a 4Ω tap. Can I also assume that the 16Ω tweak for an 8Ω M2, as well as the above mentioned 8Ω tweak for a 16Ω M2 do not affect the impedance seen by the amp, in any appreciable way?

I must say though, that I just recently revisited your design incorporating both an 8Ω and 16Ω M2 combined into one unit! I'll need to pour over it a bit more, but at face value, it is quite intriguing! Might be the way to move forward, should this ever become any sort of commercial enterprize! The added components are not very expensive and it would not require a very much larger enclosure!

On the other hand, it sure is nice having a friend to sensibly , politely and logically debate such picayune details with! :cheers: Maybe someday we'll actually at least get to speak with each other on the phone, although I think I'd truly enjoy, even more, having a cold one with you in some seedy gin mill with a cool band playing! :dude:

Again, Thank You!
Gene
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
Hi Gene, yes all the ohms as seen by the amp stay within reasonable range with tbe various tweaks. For 8Ohm versions, I try to keep what the amp sees between 7 and 10 Ohms at any setting. You can see that in tbe charts I posted yesterday, on the 'R440' column.

I tend to focus on the 8Ohm versions since that's what I need to cover both my rigs, which are both based on 2x12. But it looks like most tube Marshall combos come with a 16 ohm speaker but do Fenders usually come with 8?

That double-barrelled 8/16 version is appealing, since the main switches and jacks can be the same as a basic version. It can be made either with two 8ohm circuits or 2x16, and it'd work 'natively' at either 8 or 16 Ohms in either case.

cheers
John
 

Gene Ballzz

Well-Known Member
Joined
Feb 19, 2013
Messages
3,816
Reaction score
4,606
Location
Las Vegas, NV
@JohnH
I get your whole "8Ω thang" but was simply bustin'yer cojones. And yeah, Fenders are a different story, although my 5E3 clone has a selectable 4/8/16Ω output transformer! Some Bassman and Super Revereb amps were actually designed for optimal use at 2Ω. In case you hadn't noticed, the one nice thing about the updated software at theis forum, is that you can now use the Ω omega symbol more than two or three times without getting an error message! One big downside is that the "outline" format has gone away! It's funny though, as many other websites that use otherwise identical software still have the outline available! :scratch: My guess is when it comes to renewal/upgrade time, there is a selectable grocery style list to choose from. At least they did not remove some of our coolest emojis! Although this one :slap: and this one:nutkick: will likely be the first to go, given the nature of the "PC" vortex that our world is irretrievably spiraling into!

Outline Wishin'
Gene
 

Gene Ballzz

Well-Known Member
Joined
Feb 19, 2013
Messages
3,816
Reaction score
4,606
Location
Las Vegas, NV
All true, the only extra thing used is the ring connection, which gets grounded by the sleeve of the incoming plug. No switched contacts are involved and need not be provided when buying the jacks.

Yes there is an even simpler adjustment if you want to further tweak the 16 Ohm build for 8 Ohm. The difference in tone is due to the 8 ohm speaker seeing a higher output impedance from the 16 ohm M2 than it would get from an 8 Ohm M2. The fix would be to add a 39 Ohm resistor in parallel across the speaker. This should trim back the treble and bass resonance in proportion to the mids. It could be added as a switch, or using a second jack, wire tip and sleeve direct to the other jack and put the 39 Ohm one end to tip and other end to the new ring lug so it goes to ground when the plug is inserted.

Youd have two jacks, one dedicated to 16 Ohm (or a brighter output for 8 ohm), and the other for corrected 8 Ohm. Or put a 16 cab into each and it will correct the pair for the 8 Ohm total.

But Id really like it if you could test this with a lash-up of some kind first, just temporarily wire the resistor across the 8 cab and see if it does as predicted?

The ultimate version of all this would be the 8 Ohm M2, with just two parallel jacks, and a switch to adjust tone for 16/8/4 cabs.

@JohnH
While at face value, I really like the idea of putting the output speaker impedance/tone compensation on a switch, some concern comes to mind. Let's say for an example that an 8Ω M2 is connected to a 50 watt amp with a 16Ω speaker, in compensated mode. What would be the ramifications if a bypass switch was included? If bypass was activated, might not the 25 watt resistors R10 & R11 go up in smoke? Same question for a 16Ω M2 with a bypass and 8Ω compensation? I'm simply trying to allow for unintended and/or unexpected circumstances. Remembering of course, that guitarists are only a step or so away from being drummers! ;)
Thanks,
Gene
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
hi @Gene Ballzz , I think the resistors and the compensating components like R10 and R11 could be made to be OK. They are not taking a lot of power, really just tweaking things. The worst outcome could be with a bypass switch, if a guitarist lets a drummer set up their amp. If say an 8 ohm amp and 8 ohm M2 is wired to a 16 cab, but the bypass is engaged, then you have a 16 cab straight into a 8 ohm amp. But It'd bypass everything so no resistors would be involved. And, I doubt if any other attenuators can do much about that either.

Another issue with a 4/8/16 switch on the output is, I think its tricky to work out the switching with a simple 2-pole switch. I might have another look at that though.
 

XTRXTR

Well-Known Member
Joined
Feb 14, 2021
Messages
966
Reaction score
1,105
Location
Some City, USA
A DPDT switch can be wired for a triple select but you must find one that can handle the current which is likely large.
1656383507335.png
 

ebswift

New Member
Joined
Jun 10, 2022
Messages
1
Reaction score
1
All my parts came from Aliexpress fairly quickly, I got the inductor from Soundlabs Group and the switches from Jaycar. The case was some old specialised computer unit which is fairly large for the job and should be able to handle the heat very well. End support panels were 3D printed with heat thread inserts, and 1mm aluminium folded over the top and screwed on. All the holes for the resistors were tapped (also used thermal paste), and the inductor has a 3D printed 10mm base screwed on with nylon screws. This is a basic 16 Ohm M2, I haven't tested the line out yet. I haven't done much testing, but when I feel the case it remained cold to the touch.

I made this for my Vadis 600R, a 60W Aussie amp made in about 1965 with a 4x12 cab stuffed with Rola 12PEG's that I bought as 'electronics' non-working really cheap, but I put the effort in and got her singing again. Interestingly, cranked through the M2 this amp doesn't really seem to break up, it seems to just run clean. Anyway, thanks John for the design effort and all the followup info you have provided through the forum.

The vinyl didn't like me turning screws onto it as you can see in the closeups, but it looks pretty good from a few feet back. If the OCD catches up with me I might re-print them some day.

AM-JKLU3udOA8cFaTESwwp0ZS2xn_qUAjC4qByHToLa61btqtS6YpVa2chMtfFFy-nVBRAIavovPwz8Li4ooek5xsn_5Yk7tU0XqnFuN23CLVmv3c74hdeQdK9HEf6-JH3NJ0WKw5YO1Y8ZDsOuRv2-KKsud5A=w703-h938-no


AM-JKLWRFp0UUey9j1ZG3jOG8zAD4K-5ABs1D3mehqgqHVU2TXZLFe7X4SnfVBFczIDMkBd4ZBTJfDtfL0Dwl3gkU4AVN4LL0z-pailNWoHo2hXOU2WUcZnKD3KYbp22YiTch58ms9kWPLdN-GSYIrX05Y9ucA=w1250-h938-no


AM-JKLV_4NxNkzgl56UUmRKcWyqjYipjNOuveqNUJkNbN2jPcDEvsP6xtFO8zhC-teakVDvPfwmdqu2IIyj9_TVMbuYk2bOZ_PsPtB6jqZzC44OpdaSNEgWocgDAjV3Gu2Nrbo4hZdf3hGoLtttXfEaeI6ppeg=w1250-h938-no


One thing of interest is that my amp had some low level hissing, and a hum that came along with the reverb dial, but the hiss is 99% gone (and the hum 100% gone) with the attenuator hooked up, even though it used to be there with the volume turned right down low.
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
thanks @ebswift , it looks great. Nice to see another build down here in Oz and its useful to note the coil supplier here too. I was buying from Queensland Speaker Repair but they stopped selling the coils. The other one I've noted is Wagner online in Sydney. I like the finned case.

Suppressing the output hiss and hum on a powerful amp is a nice extra benefit of attenuation.
 
Joined
Jan 9, 2021
Messages
16
Reaction score
7
The M2 design should have an attenuation of a little over 30dB. So the power gain should be roughly 10^(-30/10) = 1m. Resulting in an attenuated output of 50 mW. Okay, with a lot of distortion maybe 100mW.

Mine doesn't sound like that, it is still pretty loud when I crank the volume of the amp up. (Marshall JCM800, 2205, built in '86).

Is this suppose to be like that? I checked with the schematics multiple times. Measured to check what I expect with the switches on and off.
 

JohnH

Well-Known Member
Joined
May 31, 2008
Messages
5,113
Reaction score
3,334
Location
Wilton NSW
Hi @Gert-Jan van der Heiden

It's surprising how loud a very small amount of power can sound through a guitar cab. I quite often play my amp set at about 6 or 7 , attenuated to -31.5db. I reckon my amp might be putting out about 20-30W, and the power reduction at -31.5 db is about 1/1400, so I'm getting maybe 14 - 20 mW!. It sounds about TV volume, and mixes well with unamplified vocal or acoustic guitar.

There's a few more checks you can do:

1. Set up some kind of consistent signal into the amp (could be a riff in a looper, or a steady tone). Then mic the amp and record into a computer. Set the levels so the recording is not distorting at full power, then keep everything set and step down through the attenuation. If you want to start this test at full volume, you'll need to switch to standby to engage the attenuator.

I did this test in post 1, and you can hear and see the results. The dB's were correlating well with expectations.

2. You could run a sound level meter app on a phone and note the changes in sound dB's between settings. I haven't tried this, but note that a phone is unlikely to cope with a full volume amp nearby. But you could try it with amp volume set low, just to see what the attenuator does.

3. You could use a meter to read a.c. Volts across your speaker. The voltage ratio is the square root of the power ratio. Lets say you tried this test between the -7db and -31.5db attenuator settings. That's a power ratio of 1/280 so the voltage ratio at the speaker is about 1/17
 
Last edited:

Gene Ballzz

Well-Known Member
Joined
Feb 19, 2013
Messages
3,816
Reaction score
4,606
Location
Las Vegas, NV
@Gert-Jan van der Heiden

I've looked long and hard at your build pics and don't see anything glaringly obvious. Nice work, BTW. I've got a dumb question, as 'm not familiar with those particular switches, did you confirm, with a meter, which lugs did what "before" installing them? I have encountered some SPDT switches that the center lug is NOT actually the common lug. If operation/lug positions were different than expected/assumed, you'd be chasing your troubleshooting tail forever!
Just Thinkin'
Gene
 
Joined
Jan 9, 2021
Messages
16
Reaction score
7
@Gert-Jan van der Heiden

I've looked long and hard at your build pics and don't see anything glaringly obvious. Nice work, BTW. I've got a dumb question, as 'm not familiar with those particular switches, did you confirm, with a meter, which lugs did what "before" installing them? I have encountered some SPDT switches that the center lug is NOT actually the common lug. If operation/lug positions were different than expected/assumed, you'd be chasing your troubleshooting tail forever!
Just Thinkin'
Gene
Thank you Gene for taking your time to check this. Actually, I did make a mistake, I swapped the values of resistor R4 and R6. I have a 16 ohm version, so in my case it was 30 and 20 ohm. Not that drastic change.

I did check the switches to confirm the common connection, as the data sheets where not that clear. I had to do some assumptions on the part numbers. No nice part number built up documentation.

I also checked by simple hooking up a multimeter to check the resistance. Al nicely around 16 ohms, not matter what switches where on or off. Also tried 2 multimeters and they both couldn't cope the 16 ohm input of the cabinet. 8 went fine. Not sure what that was about. I also didn't dive into the industry standard of measuring impedance of a speaker, I can imagine this is measured at some AC frequency, not DC.

Also checked the air coil, it is 0.7 ohms as it suppose to be, so now shorts in the coil other than the long wire in a loop.

I haven't checked the amp itself. No idea if it's biased right or that the plate voltages are correct. The inside and outside look really original. Even the big caps, and they measure fine. ESR value is very good. Don't know what it is, but I like it when you see these old stickers with names on them. Or the original stamps, you really can feel someone worked on this back in the days. Same thing when working on my car from '69. Yes, and I'm sorry they're old haha, if "only" from '86.
 
Joined
Jan 9, 2021
Messages
16
Reaction score
7
Hi @Gert-Jan van der Heiden

It's surprising how loud a very small amount of power can sound through a guitar cab. I quite often play my amp set at about 6 or 7 , attenuated to -31.5db. I reckon my amp might be putting out about 20-30W, and the power reduction at -31.5 db is about 1/1400, so I'm getting maybe 14 - 20 mW!. It sounds about TV volume, and mixes well with unamplified vocal or acoustic guitar.

There's a few more checks you can do:

1. Set up some kind of consistent signal into the amp (could be a riff in a looper, or a steady tone). Then mic the amp and record into a computer. Set the levels so the recording is not distorting at full power, then keep everything set and step down through the attenuation. If you want to start this test at full volume, you'll need to switch to standby to engage the attenuator.

I did this test in post 1, and you can hear and see the results. The dB's were correlating well with expectations.

2. You could run a sound level meter app on a phone and note the changes in sound dB's between settings. I haven't tried this, but note that a phone is unlikely to cope with a full volume amp nearby. But you could try it with amp volume set low, just to see what the attenuator does.

3. You could use a meter to read a.c. Volts across your speaker. The voltage ratio is the square root of the power ratio. Lets say you tried this test between the -7db and -31.5db attenuator settings. That's a power ratio of 1/280 so the voltage ratio at the speaker is about 1/17
ok, that is an extensive reply! Thank you very much. I don't own a mic believe it or not, apart from the one built-in my macbook. I just recently hooked up my Marshall and guitar again, after many years of dust collection. I have invested in a wah pedal, looper, audio interface, midi keyboard, bunch of cables, tools for setting up my old guitar. So a bit strange, such a setup and no mic.
I have this amp for a long time. In the first years I used it at my parents house at rural environment. So, hearing this amp again at some decent volume (on the knobs) brings back nice memories of that sound. Also, I totally get what people say that you can feel the air moving.

I have my amp in the living room, and all walls are made of thick brickwork with a layer of plaster over them. So, the sound isn't absorbed anywhere. Basically, my room is a big cabinet too. I didn't knew that a cabinet could get so loud with so little power. So again I learned something new here. Now I should learn how to play a guitar... This attenuator surely helps a lot with that.

I think I should try #3 to check. I do have an oscilloscope but no function gen. Maybe I can generate a sine wave gen with an STM32. I better start looking for my ear protection, because a sine wave that loud isn't gonna be a pleasant journey haha.

I am aware of ground loop hazard with the scope. Your compatriot Dave from the EEVBlog taught me that. As the chassis of the amp is connected to mains earth and ground. So, I should use no other voltage ref than ground and I should be good.

But I really should start with the amp. See if it is biased ok and the voltages are correct. That's good anyway, as I'm cranking it up now. I did check the electrolytic caps, and replaced most of them.

I do think the attenuator is working just fine. No earth leakages or shorts, all resistors measure just fine, resistance over the resistors are correct, switches short out the correct places, and it is actually attenuating a lot.

I have trouble searching through this thread only, I use google for that (using site:....). But could I add another stage? I haven't figured out how you managed to maintain the impedance of 16 ohms the same. Very clever circuit sir.
 


Top