L-pad for a 100w amp

[Rodav]

Hi there, I want to build an attenuator for my 100w amp (16 or 8 ohms switchable) and was going to follow a schematic I found that uses an 8 ohm 100w L-pad. However it's become quickly apparent from what I've read that it needs to be bigger or it won't handle a 100w amp. How can I get around this ? Could I put a resistor in front of the L-pad ? I found the Fostex R82B 200w 8 ohm L-pad. It's made for a tweeter, is that suitable for my purpose? Are all L-pads simply variable resistors or are there differences depending on the purpose?
I only have basic electronics knowledge, hence the number of questions.

[The Dude]

Here is another thread on the subject. Hopefully it helps.

https://music-electronics-forum.com/...-l-pad-wattage

In regards to the Fostex R82B, literature says "This power rating is only for tweeters. Using at lower frequencies it would be 100 watts.", and that is not peak power, so I wouldn't try to use it alone. Also, it looks to be a dual L-pad, so divide by 2 and now you're down to 50W per pad.

https://www.madisoundspeakerstore.co...2b-200w-l-pad/

If we're talking guitar amp, I recommend building or just buying a power soak unit made for your purpose, which will require more than a simple L-pad. The Weber Mass Attenuator is a good unit.

Also, you say it's a 100W amp, but we don't know which amp. Is it tube or solid state? Is the power rating peak or RMS. Is it a guitar amp, bass amp, home stereo amp, an amp for aquarium speakers, etc.? If you can provide more information, we can provide better answers.

[Chuck H]

Along the same line as what The Dude mentioned... 100W is a 'nebulous' inference. The devil is in the details. Some 100W Marshalls running at full tilt truly need an attenuator capable of 200W if both the amp and the attenuator are to survive. That said, I run my 20W amp (actually about 15W clean for proper rating but peaks well past that when clipping hard) into a reactive load made to handle 50W and it just gets warm with hours of play time. WRT attenuators it's imperative to over rate. The Dudes suggestion of the Weber Mass is a good one. The standard "L-pad" options made for home audio don't often sound very good used as attenuators for guitar amps anyway. Sometimes you just need the right tool for the job. Cheeping out and plugging a 100W tube amp into a home audio 'L-pad" made for 100W is going to be a compromise in tone (at least in my experience) and possibly dangerous to the amp. Use the right tool for the job to avoid safety, tonal and damage risks.

JM2C

[Rodav]

Here is another thread on the subject. Hopefully it helps.

https://music-electronics-forum.com/...-l-pad-wattage

In regards to the Fostex R82B, literature says "This power rating is only for tweeters. Using at lower frequencies it would be 100 watts.", and that is not peak power, so I wouldn't try to use it alone. Also, it looks to be a dual L-pad, so divide by 2 and now you're down to 50W per pad.

https://www.madisoundspeakerstore.co...2b-200w-l-pad/

If we're talking guitar amp, I recommend building or just buying a power soak unit made for your purpose, which will require more than a simple L-pad. The Weber Mass Attenuator is a good unit.

Also, you say it's a 100W amp, but we don't know which amp. Is it tube or solid state? Is the power rating peak or RMS. Is it a guitar amp, bass amp, home stereo amp, an amp for aquarium speakers, etc.? If you can provide more information, we can provide better answers.

The Dude, thanks for your reply. The Weber mass is very rare here in Australia, there are none on Amazon.au or Ebay.au and they would be very expensive if they were available (the exchange rate almost doubles the price of US made items). Bugera makes one for sale here at $200 but being a full-time non-famous musician that is still a big chunk of my income. I might go that route nonetheless if I can't build one for less than $100. Isn't that what most commercially sold attenuators are - just switchable resistors or a variable resistor (L-pad) ?
It's a Marshall 100w (rms) JCM900 4100 model - it has a tube preamp and tube power amp. Some of the recording studios that I've been in had an isolation booth for the amplifier. I was able to fully crank the amp and it sounded great. I want to run it through an attenuator in live performances.

[The Dude]

Please update the location in your profile so we know where you are. Here is one of many DIY attenuator articles found on the web. A little searching will probably get you more.

https://guitar.com/guides/diy-worksh...wn-attenuator/

Chuck H has also kindly posted a schematic in the thread I linked in post #2 and there's lots of information there. I haven't run numbers, but my guess is that you probably can't build one for the cost of that Bugera unit you mention.

Attenuators are BASICALLY voltage dividers as you say, but good ones will have frequency compensation, etc.

Just a thought: I might start by modding the amp with a half power switch and associated circuit saving some attenuator cost.

[Chuck H]

= I might start by modding the amp with a half power switch and associated circuit saving some attenuator cost.

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^

I do make my own attenuators and my own sounds great to me. YMMV. The resistive attenuator posted is good for a little trim but sounds less dynamic at high attenuation levels. Replacing the purely resistive elements with a reactive load in the same circuit is just the bees knees IMO. It's all over the forum with searching but I'll dig up some old stuff for the reactive load version if Rodav want's to make one.

[Rodav]

Please update the location in your profile so we know where you are. Here is one of many DIY attenuator articles found on the web. A little searching will probably get you more.

https://guitar.com/guides/diy-worksh...wn-attenuator/

Chuck H has also kindly posted a schematic in the thread I linked in post #2 and there's lots of information there. I haven't run numbers, but my guess is that you probably can't build one for the cost of that Bugera unit you mention.

Attenuators are BASICALLY voltage dividers as you say, but good ones will have frequency compensation, etc.

Just a thought: I might start by modding the amp with a half power switch and associated circuit saving some attenuator cost.

Thanks, that's the schematic I was talking in my original post. I've already done the searching and am trying to find out how what I could use that will handle a 100w rms amp as a 100w L-pad is not big enough. A half power switch is a good idea though, I'll look into that. I've already worked out the cost of an elevenator with a 100w L-pad and it's on budget.

[Rodav]

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^

I do make my own attenuators and my own sounds great to me. YMMV. The resistive attenuator posted is good for a little trim but sounds less dynamic at high attenuation levels. Replacing the purely resistive elements with a reactive load in the same circuit is just the bees knees IMO. It's all over the forum with searching but I'll dig up some old stuff for the reactive load version if Rodav want's to make one.

Hi Chuck, send them my way if you've got them. Although cost saving is a factor, I also enjoy building little projects like this.

[Mick Bailey]

Could I put a resistor in front of the L-pad ?

You always have to bear in mind the output impedance of the amp. It is possible to reduce the power before the L-pad in order to relieve some of the load it takes, though because the L-pad can't be altered the choice of resistors is limited. For an 8 ohm L-pad it's possible to use a 4/8 ohm divider in front of it. There are a number of downsides, though - a) the maximum power output to the speaker is significantly reduced when the L-pad is on maximum, and b) with a 'straight' L-pad turned right up it's fairly transparent, but adding a divider in front means the amp always 'sees' a mainly resistive load and this has a bearing on the sound.

[dai h.]

some Weber attenuator schematics in this thread:

https://www.freestompboxes.org/viewtopic.php?t=2287

related thread:

https://www.freestompboxes.org/viewtopic.php?p=112791

since the amp is a 100W, how about pulling two tubes (not a huge difference in volume but less wasted power), maybe reduce the AC going to the amp slightly? (R.G. Keen's Vintage Voltage (something like that) comes to mind as a possible way to do that less expensively (compared to a Variac)). Also (this is mentioned in the above thread), but (my non-expert understanding) when using fixed resistors only better results seem to come with using a higher than rated impedance (the amp output is less damped--couple of examples--the Ho (Ultimate) Attenuator IIRC used a fixed 30 ohm load (for 16 ohm tap), EdVH apparently used 16 ohms fixed R on 8 ohm tap for 1984(album) (at least according to David Friedman)). (Possible downside? higher voltage due to higher load on the primary side of the output transformer) Maybe don't use the L-pad if you just need a set volume reduction (L-pad looks expensive). (Budget I guess is a concern so) using cheap bipolar aluminum electrolytic caps (for compensation to brighten sound) would seem cheaper than inductors.