Replacing Ultimate Attenuator noisy inductor

[fretwizard]

Hi

I've found this very interesting thread on an Australian Forum.
http://www.guitargear.net.au/discussion ... 91.45.html" onclick="window.open(this.href);return false;
I've subscribed to the forum to ask some questions but the admin didn't enable my account yet . A month has passed.

So I'll ask here.
They guy basically cloned the UA and made a way better build.
He's also wound his own inductor. It's a 2.1 mH aircore coil and it shouldn't buzz like the UA usually does.

It seemed very simple to me so I looked for a 300w 2.2mH aircore inductor and I've found one used for hi-fi crossovers. Made in UK.

I wired it in and I tried with a 30w amp. As soon as I flicked the 'plexi switch' the amp blew the HT fuse , as if there was no load. I checked my connections but honestly if you looked inside you would know ho easy it is. It's only 3 wires. And even if you swap the inductor terminals it doesn't matter.

I've put a new fuse in. Same result, the amp sounds very fizzy as if there was no load and then the fuse blows again.
I've wired the old inductor back in and everything works again?

Do you have any idea? The UA inductor is a ferrite core inductor. the wire is thin, maybe gauge 20? or 22?
I tried to pot it with some nail polish but the buzz reduction was minimal.

The other inductor is 4 times bigger and the wire gauge is much larger too... does this matter to the load? It's still wired in series with the resistors anyway...

[Tone seaker]

Maybe contact HO and get a new inductor. They have a life time warranty on them. I have never had a problem with buzzing on my UA

[vanhalen5150]

i tried to get on that forum as well, for that same thread. Nothing yet.

Did they ever post a schematic? There are things in that attenuator that make no usable sense to me. It looks like a transistor based reamp. Thats why you need the inductor. At the same time, a lot of us slave with no inductor and everything works fine.

I don't think potting the inductor would do much, but if you are going to, you need to heat up some wax and do the whole thing.

The enclosre he is using comes in a kit from china. Really well built for projects such as this. Its actually the same one Im using to build one now.

High end speaker emulators like the Palmer use a 250mH inductor, but I dont know the wattage.

[fretwizard]

Maybe contact HO and get a new inductor. They have a life time warranty on them. I have never had a problem with buzzing on my UA

Do you think I can ask Ho directly instead of Mark?.
When I first received this thing the voltage selector switch was already broken. It's a cheap chinese switch that AFAIK you can only buy in big batches... Mark told me to hardwire the voltage for europe myself

I just wanted to fix those 2 things and sell it.

[fretwizard]

I don't think potting the inductor would do much, but if you are going to, you need to heat up some wax and do the whole thing.

Yeah I figured that was this only way. The nail polish on the first 2 layers of winding did make a bit of difference though...
The guy didn't post any schems yet, and the thread is fresh, like last month... I wondered when somebody would start cloning these. Interesting the part about the gooped circuit that is completely redundant...

I just don't have the tech background to understand why just the difference in wire would make such an impact on impedance. I also measures the resistance of the 2 windings and it's quite similar although the specs say otherwise. But even if tone is 1.5 Ohms vs 0.6Ohms... would that really make the impedance too high or too low?

This is the inductor I bought, it's massive compared to the stock one, but small enought to sit in the chassis, next to the tranny.
It's actually a different brand and a bit cheaper than the one below, but it's identical...
http://www.ebay.co.uk/itm/OFC-Hi-Fi-cro ... 1c19705a81" onclick="window.open(this.href);return false;

[rgorke]

This one seems even less expensive.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... ink:top:en" onclick="window.open(this.href);return false;

or maybe this one:

http://www.ebay.com/itm/OFC-HiFi-Crosso ... 35bc3ad2f7" onclick="window.open(this.href);return false;

So, can we figure out this build?

30 ohm load with one of these inductors in series, then through a set of push-pull transistors.

The clip he posted was pretty awesome sounding.

[evh0u812]

Hey, the UA clone was my little project. Ive been hessitant to post schemaics as im building a variant myself with quite a few tweaks. The bi polar transistors are set up like a cathode follower with zero gain. what you put in is what you get out, so while it technically is a re amp, it wont give you a boost in gain as if you were running into a regular SS power amp.

Regarding the inductor you cant pot it in wax as the stupid amount of heat the undersized load and especially the dropping resistor on the board for the fan will just melt it and make a mess everywhere. best bet is to either unwind the original and re wind it making sure to laquer or superglue the coil as you go. or wind your own air core inductor using the same tecnique. air core will be bigger but less prone to saturation than an iron core, hense quieter.

[evh0u812]

As for the vallidity of the inductor in the circuit, I never used one when re amping my plexi either......BUT the high end suffers due to the purely resistive load the amp is seeing, you just never notice it because you EQ the amp to suit. With the UA you can bypass the attenuated signal and this immediately shows the difference in the high end response of the circuit without the inductor in play ( this is what the plexi switch does). Sure you can EQ the amp to suit and it sounds fine, but switching back and forth requires the EQ to be adjusted every time, which makes the UA seem anything but transparent. I found 1.8 - 2mH to be about the sweet spot for inductance in this circuit. Im also toying with the idea of using a smaller value inductor and a capacitive network to make up the high end..... this is still just a theory ATM.

This is MarkII with switchabe impedance and full DC over current protection. 500 watts of properly heatsinked load rather than two 50watt resistors strapped to an off cut of thin aluminium. No Fan, doesnt need one, barely gets luke warm with a cranked 50 watt plexi after an hour of playing.

[vanhalen5150]

It's funny your using the exact enclosure I am. Did you get that off eBay?

So is the purpose of the big toroidal tranny just for stages? I know the UA and Alex use one but is that merely as a multiple tap purpose?
I owned a UA for a total of 24 hours and returned it. It wasn't even in the proximity of the likes of the others that are on the market.
The simple design I've been toying with works with 2 dials and a single impedence of 16ohms. The simplicity of the design seems to work well with a 50 watter do far. I haven't tried a 100w yet, but its loaded for 400watts. Heat sinking things that are round needs some thinking though.

From the heat produced I have to wonder how long the coils in speakers would last at full volume. They certainly were never designed to.

[vanhalen5150]

What gauge wire is in those inductors? How do they just not melt? Perhaps I'll incorporate one. What if its in parallel? Why does it have to be in series?

[vanhalen5150]

Maybe contact HO and get a new inductor. They have a life time warranty on them. I have never had a problem with buzzing on my UA

Do you think I can ask Ho directly instead of Mark?.
When I first received this thing the voltage selector switch was already broken. It's a cheap chinese switch that AFAIK you can only buy in big batches... Mark told me to hardwire the voltage for europe myself

I just wanted to fix those 2 things and sell it.

Contact Ho directly.

[fretwizard]

Hey, the UA clone was my little project. Ive been hessitant to post schemaics as im building a variant myself with quite a few tweaks.

Dude. Great job.
So you finally got the toroidal transformer fitting in the enclosure?

I got the 2011 UA version and it's build a bit bigger and better than the one you dissected. Although as soon as I pulled a wire a cold solder joint broke.
The other problem i had was that the PCB is mounted on the transistor heat sink. Whoever mounted it didn't cut the transistors pins short enough and pushed them to fit the board on the single screw hole on the heat sink. Very soon after I received the thing I could hear something bouncing in the enclosure. The screw came off and I realized the transistor pins were broken because of the PCB bouncing. I could shorten the pins on one transistor and resolder it, but the other on broke too close to the unit on one pin, so I had to use a piece of pin and a good amount of solder to keep it together.

It's been holding for a year now, playing a 30watter into it. The only thing that made it got it hot was the 100w plexi I just built.

It looks like the resistors are connected to the input jack sleeve on one end (or to ground? don't remember...).
The other end goes to both the inductor and the plexi switch and then back together.
And then from there to the <bedroomswitch-volumePot> circuit and the PCB. So it looks like when the plexi switch is OFF the inductor is bypassed, because the shortest path to ground is through the resistors-switch segment? correct?

Anyway I can't figure out why it would stop working just with a higher rated inductor of roughly the same value...

Also do you think you could tell what the transistor are?
I remember you said the values on the originals have been sanded off.
I just need to replace the one I had to hack up...

[evh0u812]

[quote="vanhalen5150"]So is the purpose of the big toroidal tranny just for stages? I know the UA and Alex use one but is that merely as a multiple tap purpose?quote]

In the case of my UA clone, the torroidal supplies 30v rails to power the bi polar transistors, unlike the the Alex which uses the torroidal as a simple inductor, all be it with some neat little variations.

[vanhalen5150]

So is the purpose of the big toroidal tranny just for stages? I know the UA and Alex use one but is that merely as a multiple tap purpose?quote]

In the case of my UA clone, the torroidal supplies 30v rails to power the bi polar transistors, unlike the the Alex which uses the torroidal as a simple inductor, all be it with some neat little variations.

Ok, I see. With no fan I was thinking you no longer needed a power supply.

[evh0u812]

The inductor is wired directly in series with the 100w load resistors and you are correct about the plexi switch simply shorting it out.
Replacing the inductor shouldnt be the cause of your problem if the new inductor is fine. Wire it in and put a multimeter onto the end of a speaker lead plugged into the UA's amp socket. It should read approximately 30-32 ohms. Im guessing you either didnt clean the enamel off the ends of the wire of the inductor properly before soldering, or its internally open circuit.

If you dont get a reading, flick the plexi switch to bypass the inductor and see if you get the 30 ohm reading then.