Stereo 8Ohm L-pad into mono 16Ohm L-pad

[jamesm1000]

Can someone advise me how to wire this 8Ohm stereo L-pad to run as a 16Ohm mono one, i cant figure out how to wire the two central contacts, i'm struggling to understand wether they should be parallel or series.

To answer questions about WHY!! Im attenuating a tiny terror into a 16Ohm orange 2x12, and im in the UK where 16Ohm L-pads seem hard to come by.

Thanks in advance and sorry my first post is a question, I have looked for an answer but as most people are in the US and easily capable of getting a cheap 16Ohm L-pad its not really a common question.

This is the L-pad.
http://cpc.farnell.com/jsp/displayProduct.jsp?sku=LS00548&CMP=e-2072-00001000

cheers

[Gurner]

Since you've no responses...I'll start.

The product you linked to had no datasheet, so I've not a clue what it does, but just going back to basics, if your product presents an 8 Ohm load to a stereo circuit, then you'd need to connect these two 8 ohm loads in series if you want a 16 ohm load presented to a mono circuit.

[PRR]

[Mike Burgundy]

This is what the innards of a variable L-pad look like:


What it does is present the amplifier or passive crossover (between terminals 3 and 1) with the same (-ish) impedance while attenuating the speaker.
Left most variabl;e resistor is 8Ohms, rightmost one is rather high (1k5? didn't do the math ;P ).
At it's max setting, the 8Ohm is out of circuit, and the speaker is parallelled with the high resistor. This yields something close enough to 8Ohms, and (almost) maximum power to the speaker.
At it's min setting, the speaker terminals are both shorted and the 8 Ohm resistor is in series, still presenting * Ohms to the amplifier.
That's one mono one.

If I'm not mistaking PRR's solution does yield 16Ohms but doesn't do variable attenuation - at both extremes the speaker is parallelled with the high resistor, and this combo is min series with the 8 Ohm resistor. Attenuation, just not variable.
I don't really see a way to do this with a stereo L-pad, though I might totally miss something.
That said - valve amps are quite sturdy as far as impedance mismatches are concerned - I wouldn't worry about a 1:2 mismatch myself. Just don't leave the speaker terminals *open*.
Using an 8 Ohm pad will result in presenting the amp with a just-under-16Ohms load at full tilt, and an * Ohm load at full-off. Don't see any reason why that wouldn't work, there's no crossover filters that could be thrown by this.

[PRR]

> PRR's solution does yield 16Ohms but doesn't do variable attenuation.....

Does this re-drawing help?



BTW: I strongly suspect jamesm1000's "L-pad" is really a large stereo potentiometer. This can be checked by putting 8 or 10 ohms at the "speaker" lugs of one section, and looking into the "amplifier" lugs while turning the shaft. A perfect L-pad would read 8 ohms from one end to the other (for convenient 10-ohm load, tapering to 10 ohms at full-up). I suspect this is more like a 25 or 50 ohm pot: will read a little low full-up and a lot high at full-down.

[Mike Burgundy]

'ere ya go, that was what I was missing. Reverse the two in series. Thanks!
AT60ST is a Monacor 2x100W L-pad by the way, should be ok.

*edit* - I am very interested in the outcome of any measurements though. I know Monacor to produce very decent stuff, and I'd be surprised to see them market a low value pot as an Lpad.

[PRR]

> what I was missing. Reverse the two in series

Yup, I tried to show that in schematic with the opposite arrows (arrow points to "up", even when pot is drawn upsidedown), and it could have been inferred from the image and smeary red lines,  but I shudda called it out.

[Mike Burgundy]

Hey, that's redundant encoding right there - I only noticed after your second post

[Butcher]

Hey Guys, I realize this thread is dead and buried but I am hoping someone could please give me some info. I could start a new thread but since most of the info is already here I thought I would start here first. I started off wanting to do the same thing as the OP but then thought I could take it one step further. I would like to add a switch to be able to change from 200W 16 ohm mono to 200W 8 ohm mono. I am an electronics idiot but I like to do things myself and learn along the way so please try to keep it simple. Having looked at it I think the neg wire would pass through both No. 1 lugs and through to the speaker and the Pos. would come into both No. 3 lugs and exit both No. 2 lugs. Could someone please tell me if this is correct. Thanks in advance!!

[PRR]

> from 200W 16 ohm mono to 200W 8 ohm mono

I don't understand.

200 Watts is a BIG tube(?) amp.

16/8 Ohm mis-match is not a big deal.

What are you really trying to do?

[Butcher]

Thanks for the reply,
From what I have read oversizing the wattage doesn't matter, it just handles the heat better. I have also read that you should double the amp wattage with the L-pad, 100W head= 200W L-Pad. If I am using a stereo L-pad why not match the ohms as best as I can was my thought process. As I said i am a noob at electronics and any or all of this could be wrong it is just what I have been able to source from the web so far. Any input is appreciated.

[Butcher]

Would my wiring work?

[Butcher]

I thought this was a pretty simple question. 
In my mind if I have a switch to change the wiring from the schematic that PRR laid out for the OP and switch it to the stereo l-pad in parrallel I should be able to switch from 8 to 16 ohms resistance at 200W.
I have seen attenuators with switching from 4 to 8 to 16 ohm. I thought I would at least have 8 & 16 since I was buying a stereo L-pad
Anyways.
Is there a fundamental problem my old brain is missing?
Is there a problem running a 200W L-Pad with a 50 or 100W head?
Any help appreciated.
Thanks,

[Butcher]

Well I guess either I stumped the band or messed up on some forum etiquette. In any event this will be my last post.
Sorry to have cluttered up your forum. 

[bluebunny]

Would be a shame to see you disappear so quickly.  I don't think you stumped the band nor messed up some etiquette, you just didn't give time for the right person to come around with the answer.  I'm sure Paul will be back soon, and he'd love to see an answer to the question he posed you.  For my part, your question about overrated pad on smaller amp, then I'd say yes, go ahead.  And Paul already said 16 ohm vs. 8 ohm is no big deal.

Do stick around.  This is a forum where people stop by when they can (and there's a wealth of knowledge to be had here).  Just remember it's not a call centre! 

[Butcher]

Well it's true I'm not known for my patience. You never know what you are gonna get when you join a new forum, so far wasn't getting a warm and fuzzy feeling and always sucks to be the new kid at school. 
I don't think my question is that much different than the OP and the answer he got was not don't bother matching the impedance so I figure that coupled with everything else I have read including the amp instructions to match the cab to the head I thought was a good idea to do so. As I said I am new to this stuff, I am more mechanical than electrical but I like building my own things and learning along the way.
Thanks for your reply!! 

[bluebunny]

Stick around.  Warm'n'fuzzy will be along shortly...   

[Butcher]

I can hardly wait. lol

[PRR]

> In any event this will be my last post.

Sorry that we had another long snow-storm here.

You can't always get 25-hour service. I'm waiting 70++ hours and counting to get through to my Honda dealer.

The 16 from two-8 problem had a non-obvious but simple solution.

Making an L-pad switchable is NOT trivial.

Perhaps the best approach is two separate L-pads, one for 8 one for 16.

I am sorry that you are offended. That was not my intention.

[Butcher]

NP I heal quickly. Lol
I hope you didn't get too much snow.
We just had our first day into the 50's or 12C.
I can't wait till all the snow is gone!!!
If it is easier to build two I will build one of each, shouldn't take too long doing them at the same time.
Just received my first DIY TS808 clone in the mail today... Looking forward to the finished product.
Thanks for the info and reply.