Impedance question

[Mango Moon]

Wow. There's a stupidly expensive solution to a problem that shouldn't even exist.

I can certainly speak to it being "expensive" (which it was!), but I can't really speak to the assertion that its stupid or unnecessary. It provided me with a sorely needed variable Line Out that enabled me to employ any given amp head as a central (dry) tone generator in a wet/dry/wet rig where the dry signal is sent via Line Out to rack-level FX processors (wet signal), and then to two outer speaker cabs via a separate solid-state amp. Its also allowed me tremendous freedom in optimizing tone by pushing the tubes in higher-powered tube amps (e.g., 30-50w) to their 'sweet-spot' at livable (near bedroom) volumes. Those two aspects alone made it worth the investment for me in spades!

[Tom Phillips]

The Aracom PRX-150 sure seems to be an interesting unit. I had not previously heard of it. The posted videos indicate impressive performance. It would be nice to hear it live vice the camcorder sound track.
Thanks for posting the info.
Tom

[Mango Moon]

Yes, the PRX-150 is a fantastic unit, and it has performed unbelievably for me. A search of its history and performance on-line will show that a vast majority of people have found the device to perform fantastically including some highly technical and critical evaluations of its sonic characteristics. The debate will always rage, but I know this much, I'll never part with mine.

Anyway, thanks again to everyone for their many valuable contributions here.

[J M Fahey]

To be able to really adapt a 2/4/8/16 ohms amplifier to a 2/4/8/16 ohms cabinet the Aracom should have inside a transformer with all those taps (plus ground) and I doubt it does.
Not even Aracomm says so or even hints at that.

Their only claim is that the attenuated signal can be so manipulated, which is a very different thing.
*That* is easy, because you simply switch resistors.
Cheap and light.

But nowhere they say that you set to "Bypass/Non attenuation" and it still matches impedance .
Which they could, *if* they had the aforementioned big high quality transformer.

For example; suppose you have an amplifier with a fixed 4 ohm impedance and you want the attenuated power to drive a 16 ohm speaker cabinet. With other attenuators this is typically not an option, however the PRX150-Pro is specifically designed for this purpose. You simply set the attenuator's input impedance to 4 ohm and separately set the output impedance to 16 ohm.

That's easy.

If you want to "adapt" your 4 ohms speaker to a 16 ohms output just add 12 ohms in series; if you want to do the opposite you connect a 5 or 6 ohm resistor in parallel and so on for any other combination.

But I very much doubt the Aracom can transfer the full output power from an amp to a mismatched cabinet.

While the simple homemade box suggested above can, at least with 2 8 ohms cabinets to a single 16 ohms output.

EDIT: I simply don't understand that an expensive custom builder, who makes his amps by hand one by one, who has a waiting list, can't offer it with a 4 ohms option.
I mean, I understand you can't get a custom Peavey Classic 30 or Fender HRD, because they are made by the thousands, but .... on a custom handmade one ....

[bob p]

JMF, there are all sorts of things that you could find inside of a big expensive box like that. The device could be done the "right" way or it could be done the "wrong" way. Because the marketing description is purposefully vague I have doubts as well. We could sit here all night and speculate about what's inside, or we could just ask for a definitive answer about what's inside and put an end to the mystery.

Mango Moon, how about a picture of the guts? If we knew what was actually inside then we could give you an accurate answer about it's ability to perform proper vs. improper Z matching.

On a related note, I saw the "patent applied for" claim at the vendor's web site, so I looked for patent information online. Didn't find anything that would go along with the vendor's PAF claim.

[g1]

To be able to really adapt a 2/4/8/16 ohms amplifier to a 2/4/8/16 ohms cabinet the Aracom should have inside a transformer with all those taps (plus ground) and I doubt it does.

This could be the case, however, this is a big unit weighing 17lbs, selling for around $700. A weber z-matcher sells for around $100 for the 100W version. Maybe not the kind of quality transformer you speak of, but surely there could be the equivalent of 2 of those Weber boxes in this unit, no?

[bob p]

Has anybody considered that we might be being manipulated as pawns in a marketing game? I need to see some gut shots or I'm out.

[Mango Moon]

Yup, yup, it never fails (i.e., that train's never late!). I call "troll(s)".

Look, with all due respect to Mr. Fahey, he seems to be an argumentative sort (his earlier comments were relatively condescending and abrasive), but he's welcome to investigate whatever he likes. I (however), am not interested. I happen to know Jeff (the designer of the PRX-150) at Aracom, and as I've already said (very clearly), I've used the device to 100w on several high-powered amps with variable impedance for well over two years now!?@#$%&*? Um, am I not speaking the King's English here? What more would the esteemed Mr. Fahey like to know? You guys can talk all of your "theoreticals" till the cow's come home - what matters is this . . . it works, and it works damned well. If your puzzled about why, call Jeff. If you want proprietary schematics and internal photos, call Jeff.

As far as 'why' respected engineer Alan Phillips at Carol Ann Amplifiers doesn't offer his popular OD2 head or any of his other fine offerings at 4 ohm, well, I don't know! Call him too - that'd be my suggestion. Maybe you can educate him from your roost there atop Buenos Aires.

Bottom line, I'm sick to death of being railroaded by arrogant wannabes who haven't taken the time to research jack-squat before they start dismantling a perfectly reputable device. And just because none of you had ever heard of it before, doesn't mean I'm some sort of clandestine salesman with an ulterior motive. How warped is that? The fact is, I have no relation to Jeff or anyone at Aracom (haven't spoken to him in two years or more), no financial interest in anything Aracom, and more importantly, Aracom sells plenty of PRX's to those in the know without purchases from this lofty fraternity. My suggestion? Do what I do, go get your learn-on.

[Mango Moon]

But nowhere they say that you set to "Bypass/Non attenuation" and it still matches impedance.

If you're able to speak to Jeff at Aracom and obtain the Owner's Manual for the device, you can take a moment to read pages 6 and 7 ("Rear Panel Controls"), and you'll learn that the variable impedance dial is functional only when the attenuator is active (not bypass) and set to position F (Variable/Load). Use of the word "bypass" was inaccurate and a poor choice on my part, and I did my best to edit that at the time of writing. Keep in mind that I'm not the Ph.D. engineer that you are. Next?

[Steve Conner]

What is the matter with you guys?! For the record, I've used the series cable trick and it worked fine for me. Speakers will sound slightly different in series than they do in parallel, but then again, adding a 4 ohm tap to an OT will make it sound different too.

The Marshall Power Brake had a transformer inside, so I could believe that the Aracom box would have one too, especially if it weighs 17lbs.

[J M Fahey]

If you're able to speak to Jeff at Aracom and obtain the Owner's Manual for the device, you can take a moment to read pages 6 and 7 ("Rear Panel Controls"), and you'll learn that the variable impedance dial is functional only when the attenuator is active (not bypass) and set to position F (Variable/Load). Use of the word "bypass" was inaccurate and a poor choice on my part, and I did my best to edit that at the time of writing. Keep in mind that I'm not the Ph.D. engineer that you are. Next?

Dear Mango Moon, please don't understand me wrong, I'm here also to learn and definitely for fun.

But when I read a technically incorrect , or at least technically garbled/misleading statement, I ask for clarification.

1) One point to be considered: *without* reading the (unavailable on line) User Manual, I deduced that the "impedance matching" only worked on attenuated signals; you have just confirmed my belief. Thanks!!

2) don't excuse for using the word "bypass", it's printed in big bold letters on the Aracom panel picture you kindly supplied in post #25.

3) analysis will stay "theoretical" , of course, until offered some hard facts, that's for sure.

4) FWIW the Attenuator might have 1 or 2 transformers inside, it certainly weighs enough (17 lbs) , but this was not even *mentioned* until post #36 !!!
Talk about flying blind !!!!!!!

Please don't be pissed off at the kind advice you are being given here.

And we are all trying to help you with the Technical problem you posted at the very beginning:

but I don't really NEED an attenuator in this application, so if possible, I'd like NOT to use it strictly for its impedance regulation function.

Understand we feel dismayed when your final decision is to use it, after all, and in an "attenuating" setting .

But ok, it's your setup, nothing binds you to anything that's suggested here.

Peace

[bob p]

Bottom line, I'm sick to death of being railroaded by arrogant wannabes who haven't taken the time to research jack-squat before they start dismantling a perfectly reputable device.

The conventional wisdom on posting etiquette (here at least) is that if you're asking questions about a device or referring to it, then you have the responsibility to take a few basic steps to make life easy for the people here who you are asking to spend their time trying to help you:

The first step is to name the device that you're referring to, so that nobody has to make guesses in answering your questions. Mango Moon started off this thread without disclosing anything about the amplifier or the attenuator. It was left as a mystery, and people had to squeeze her to get that information. That hampered everyone's ability to help her.

The second step is to not refer people to external web sites, so that they have to go on a fishing expedition to find documentation for the product that you have questions about. When asking a question, you need to provide links to online schematics and/or owners manuals when they are available, to minimize the effort that people have to take to help you. That didn't happen here either. In spite of that simple courtesy not being extended, a number of people here went the extra mile in trying to help. They spent their time going on a snipe hunt to find this information. In the spirit of no good deed going unpunished, when they came to some logical conclusions, they took a fair amount of flack for their efforts at being helpful and providing honest, unbiased answers. That should never happen.

The third step that people should follow is to remain polite when they don't like the free advice that they've been given. It's important to bear in mind that the free expert advice that you can get here doesn't cost anything more than the price of being civil in accepting the answers. When anyone ventures out onto the internet they need to remain aware of the fact that they're might receive answers that they don't like. That goes with the territory.

I have to admit that I see a bothersome trend evolving here. I've seen an unacceptable amount of name-calling going on in the Theory & Design forum in the past few days. End-users come here for help. Sometimes they don't like the answers that people provide for them, and they respond by thrashing the people who are trying to help them. That's just bad manners. It needs to stop.



Looking at the way that this thread evolved, I have to admit that I feel like I was being played for a fool, so I decided not to stay on the ride. We were asked some technical questions, while key pieces of important data that were required to formulate a proper answer were intentionally left out. Vague references were made to a magical black box to pique our interest. When the consensus of expert opinions all reached the same conclusion (that a series adapter was better), then Voila! -- the thread magically morphed into an advertisement for an $850 attenuator.

I intentionally asked for gut shots because I thought the request had diagnostic value. An end user wouldn't really care one way or the other about photos, while a posing marketeer would react in violent opposition. JMF made some objective assessments of what might be inside of a black box, and he got attacked for doing so. That kind of response does not strike me as characteristic of an end user. YMMV.

Aracom sells plenty of PRX's to those in the know without purchases from this lofty fraternity.

Fraternity?!? Where's my beer?!? Anybody up for carrier landings???

[Steve A.]

Regarding that comment in the diagram that says not to mix an 8R and a 4R cab in series -- what's the problem with that?

With this thread turning into a MEGATHREAD perhaps your question has already been addressed but the "problem" would be that one of the cabs would produce a lot more sound than the other.

Steve

[bob p]

Thanks for mentioning that, Steve. I guess the answer to that "problem" is to make sure that your cables are long enough to put the loud cab next to the drummer.

[Steve A.]

The Marshall Power Brake had a transformer inside, so I could believe that the Aracom box would have one too, especially if it weighs 17lbs.

I have an 8 ohm Marshall Power Brake- they also make a 16 ohm version and they are not interchangeable. (I don't know what problems you would run into with an impedance mismatch.)

* Using a junction box has a downside -- it places you at risk that a speaker cabinet (or cabinets) could become unplugged if/when somebody trips over them.

How would that not happen with a speaker cord plugged into the back of an amp head? The idea of using a locking connector for the speaker has one big drawback- if you trip over the cord the amp will probably come crashing down to the floor! The issue of a speaker getting unplugged from a tube amp was discussed in great detail here (or at AMPAGE- I forget which.) I think that solution was to mount something like a 100R resistor on the speaker jack so that there would be some load if the speaker were disconnected. The math was done and the proper size of resistor was posted based on the output of the amp.

Yikes! I see that you mentioned the resistor for protection at the end of your post, Bob. My bad!

Steve Ahola

P.S. While MM has used the PRX-150 extensively with no problems that doesn't necessarily tell us how well the device would work with extreme settings, like connecting a 16R speaker load to a 2R output jack, or vice versa. It would be nice to be able to look at the patent application to see what sort of principles are being used. (Speaking of which, there could be a delay between the submission of a patent application and the publication of that application in the patent database. Or is there a process in which they must first give a preliminary approval to an application?)