Building a load box

SunDevil

I'm looking to build a resistive loadbox than I can then feed the lineout from into a power amp

Does this look like the correct interpretation of the description I've borrowed from our friends at TGP for such an animal with a lineout (with thanks to Holger from Komet)

Also, are the power ratings on the lineout 'appropriate' - really don't know if this is overkill or too low (for a 50W amp)?

http://i1381.photobucket.com/albums/ah232/SunDevilarker/Load box 3_zpsj7rnyocc.jpg

Apologies for the crappy diagram!

SunDevil

Just giving this a nudge to see if I can get some confirmation before I hit the RS site

Bidley

@ICBM or @ecc83 should be able to help

ecc83

Well, 225W IS a bit OTT but if you have it handy? Then, maybe you are thinking of a bigger amp in the future? Lot of questions tho..

50W amp but what impedance are you going to set it to for a 12 Ohm load? ICBM has said loads/soaks sound best as a rule if they are a bit higher than the set Z but 12R seems too high for 8R to me and way too low for 16R?

Now, I am a bit OCD where loads are concerned. By definition the amp is likely to be belting its beans off and the very LAST thing you want is a load burn out and go OC! So, for a 50watter I would go for 2x22R in parallel = 11 Ohms assuming an 8R tap. Each R being at least 50W rated and go for the Alyclad jobs and bolt them to a heat sink, they are nigh on indestructible.

The 82k + 5k will give you about 25dB of attenuation and thus around 1V rms at 50W in (for a 12R load). That might not be enough drive for some applications if you wanted to keep the amp power down. There is no "emulation" so that line out is going to sound fizzy as eff. A basic filter of say 100nF across the pot track would help.

Would add 10quid or so to the project but I would seriously consider a 1:1 "600 Ohm" isolating transformer* on the output, hum loops are the bane of these systems. OEP make some decent traffs at fair prices, RS do them but I bet CPC/Farnell would be a cheaper bet for all the components.

Would really need some re-jigging of the attenuator circuit. My (400W capacity) 200W load box used a 100V line speaker transformer.

Over to IC!

Dave.

ICBM

12R is probably about right for an 8-ohm load really. The impedance of a real 8-ohm speaker at high frequencies rises a lot higher than that, and it's probably not a bad average. 10R would also be fine though, and is easier to get if you don't have that one already.

225W is overkill for a 50W amp but there's really no such thing as overkill for this application if you can afford the resistor and a big enough enclosure for it. Bear in mind that the power rating also depends on an adequate heat sink - the free air rating will be no more than half that, possibly less… and a fully-cranked 50W amp can put out up to 100W, so although it *is* more than you need, in the worst case it's not that much so.

The other two values seem about right, but as Dave said I would consider an isolating transformer to prevent a ground loop with the power amp.

Dodge

This sounds like the perfect opportunity for a TFB Community build project.  As some point, I intend building either Randall Aiken's  reactive load box, or perhaps the load part of the Palmer PDI03 (inductor and resistors in series) with a line out for my AC30 (scaled for 16 ohms of course).

Anyone else up for it?  My Palmer PGA04 sounded so much better as a load box than a simple resistive load.

SunDevil

Thanks for the replies gents

I get the idea of the isolating transformer, but not sure on spec - would something like this be correct: 

http://uk.rs-online.com/web/p/audio-transformers/2106481/

Assume this sits ahead of the 82k resistor?

The Aiken reactive load looks really interesting - it's essentially the Power Break but with up-scaled components and a slight tweak to the resonant peak to match the cab. There's a 115 post thread on this, but in essence, it comes out at the below spec

http://i1381.photobucket.com/albums/ah232/SunDevilarker/Reactive load_zpspxmkec8u.jpg


Dissecting the thread as best as an un-trained idiot can, this appears to be the key to making this work effectively is air core inductors (to avoid saturation) and to ensure the big capacitor has a ripple current rating sufficiently high not to have it's performance impacted at audio frequencies.

It looks above my pay grade, but I'm sorely tempted..

vale

     

vale

Dodge said:

This sounds like the perfect opportunity for a TFB Community build project.  As some point, I intend building either Randall Aiken's  reactive load box, or perhaps the load part of the Palmer PDI03 (inductor and resistors in series) with a line out for my AC30 (scaled for 16 ohms of course).

Anyone else up for it?  My Palmer PGA04 sounded so much better as a load box than a simple resistive load.

i'm 30w class A as you dodge, so have same maths to do on load values.
got insomnia & searched around the aitken on the webz..
found this indepth 12p thread at tgp. mucho spice simulations & photos, not that i understand any of it yet. but looks like something worth digging into.
https://www.thegearpage.net/board/index.php?threads/aikens-reactive-dummy-load.1072793/

please could a kindly boffin advise what spec from my amp will determine the values of components i need? i have a basic multimeter so could take a reading at the speaker if required?

ICBM

Simple really... the resistance needs to be somewhere between the amp's output impedance and double that - but preferably less than one and a half times - and the power rating needs to be at least double the amp's rated power, up to about four times if you're not confident the casing is going to provide an adequate heat sink.

Within that range it really doesn't matter, impedance matching is not all that critical - especially not for a purely resistive load, which are the least stressful for the amp - and power rating is just a safety factor, which can easily be made 'more than enough' if you're not too constrained by build cost.

vale

thank you kindly ICBM. i was looking at this thread for basic resistive load http://www.tdpri.com/threads/load-box-for-silent-recording.666353/
& i spose i couldn't believe my luck that something i want to build looks so simple. but will feel reassured in trying it now.

Almach

Two cooker rings = 5Kw

ICBM

Excellent .

I've made a couple of high-power loads from an old kettle and a fan heater in the past, for when I was working on very-high-power amps like a Hiwatt 400… ~450W clean and over 750W full overdriven! Usefully, a 2-3KW 240V heating element is in the 20-30 ohm range (actually a little less when cold) which is ideal for a 16-ohm load.

SunDevil

Dusting off the loadbox design from last week.. does the below reflect what @ecc83  and @ICBM are talking about re adding a bypass cap on the pot and 600 ohm isolating transformer ahead of the output jack?

http://i1381.photobucket.com/albums/ah232/SunDevilarker/2015Jr/reactive/Load box with trans 02_zps2ath69j6.jpg


Could I also just sanity check (as you've no doubt gathered, I haven't a Scooby what I'm doing here in reality, but I'm out of work with time on my hands to experiment)

Does this transformer look correct?
http://uk.rs-online.com/web/p/audio-transformers/1237236/

I'm assuming that both sides of the transformer need to be grounded (hence the 4 lugs) even though I've not put this in the above diagram?

Does the transformer 'care' re polarity when connecting it up?

Cheers

Baz

ICBM

That's basically correct. The transformer polarity doesn't matter, but the output side must *not* be grounded - that's why you need the transformer, to prevent a ground loop. The two ends of the coil go to pins 2 and 3 if it's an XLR output (or tip and ring if it's a 1/4"), or you can connect to a mono 1/4" provided it's a type which is isolated from the casing - either one with a plastic frame or with an insulating washer set.

SunDevil

Makes 100% sense - thank you!

I'm going to knock one of these up over the next week, so I'll come back on this thread with the results

Dodge

I reckon you could add the inductor part of the Palmer load boxes too - my old one sounded SO much better as a load box than a purely resistive one.  It's just an inductor in series with the resistor, I haven't got the specs to hand but I'll dig out my notes when I get home.

ecc83

Yes, with a +25dBu capability into 600 Ohms it is actually something of an overkill.

Yes, winding to ring and tip of jack but I would also connect a capacitor of around 10nF (not at all critical) from the OP jack's sleeve to the casing. This keeps a cable shield bonded to earth at RF should you use a TRS balanced lead.

Another gilding of the lily could be a phase flip switch (yup! KNOW it should be "polarity")

Dave.