Ho Electronics Ultimate Attenuator Schematic

[bmsiddall]

Greetings from Australia.

Bought the UA second hand a couple of years ago but didn't get around to using it.  Now my needs have changed I thought i might as well flip it to someone who could use it.  After hooking it up found it works in bypass but is completely silent when switched on (fan is working so 240V is available).  It seems to be a dummy load fed into some solid state reamping so i didn't really want to poke around unless i had the schematic (googled unsuccessfully).  Can anyone help?

Ho Electronics domain is for sale so i'm guessing the original builder has retired/moved on.

Cheers,
Brett

[Rob Strand]

You might be better off tracing it yourself.
The circuit won't be that complicated.

I don't know how it works.   It's could be using power MOSFETs a variable resistors, in which case the big transformer is largely a ballast.   

Another theory is it wires the power MOSFETs as a buffer in which case the transformer is supplying power to the speaker.   I don't see any large filter caps so that put a spanner in the works for the buffer theory.   Something along the idea of this, where the speaker is at the load L1.  Except with U1 through U6 replaced with some other circuit. (Not a great example, the output stages of some power amps might be better.)

https://www.radiolocman.com/shem/schematics.html?di=150753


Maybe like this,


From what I can see there's many variations of the circuit/builds. 

On the amp side you might find fixed value power resistors as a dummy load.

As far as the bug goes.   Probably easiest to follow the signal from the input through to the output as best as you can.

You might find some internal fuses.  They could be blown.   They could be blown *because* the MOSFETs are blown.

[bmsiddall]

Thx Rob, appreciate your input (NPI  ).

[Eb7+9]

I got given one of these about ten years ago and at my last visit at his shop I asked him about it ...

turns out to be a power load, attenuator (pot) and 20w ss amp ... inspired by EVH's original rig

YuKong Ho closed his shop a number of years ago now

[Rob Strand]

I got given one of these about ten years ago and at my last visit at his shop I asked him about it ...

turns out to be a power load, attenuator (pot) and 20w ss amp ... inspired by EVH's original rig

Interesting.  I noticed different models seem to use different circuits.   One, perhaps the top model, did looked like it had a solid state power amp.  The smaller models didn't look like it, that's why I was thinking a buffer. 

[teemuk]

I once saw a schematic for one of these and it was basically just a 20-ohm resistive dummy load followed by a volume control pot and simple push-pull complementary emitter follower stage (you know, the one you'll find as the final stage of 99.9% of transistorized power amps). No voltage gain stages as it already had the high input signal amplitude.

Not recommended to build. Aside from the fact that the whole unit was constructed amateurishly, it's crude diode biasing was godawful for any reliable thermal tracking, there was no short circuit protection for the "power buffer" so one can destroy the unit totally in an eyeblink, the output stage was seriously hindered in regards to power transistor base drive current (it did not even employ darlingtons), the 20-ohm loading is incompatible with many tube amps and a purely resistive dummy load is just.... sigh.

These are the types of "designs" to forget.

[Rob Strand]

Here's one of the higher power models,
https://www.mylespaul.com/threads/ultimate-attenuator-makeover.107912/

On this one you can see a ferrite bobbin inductor which might be part of the dummy load.

Large transformer.

But I can't see a rectifier or filters caps.   The heatsink extends under the board
so no room for any decent sized caps.

Transistors on a heatsink.   Could be BJTs and not MOSFETs.
If you look close I think you can see two 1N914's resting on the output devices.
So likely to be BJTs.

PCB pretty sparse which goes with the buffer theory.
However,  another one I saw had two output devices on one PCB and two on another.
One of those boards looked like the buffer board and the other looked like it could be a full amp.

Never that same circuit twice! 

Smaller version which looks like a buffer and filter caps visible.
https://el34world.com/Forum/index.php?PHPSESSID=45lf5a3kuu34j2fpj2sagvvtf7&action=dlattach;topic=17793.0;attach=46805;image

[teemuk]

If you look close I think you can see two 1N914's resting on the output devices.
So likely to be BJTs.

Yes, two diodes between bases of the output transistors to establish class-AB bias; one diode voltage drop per each base-emitter diode voltage drop. The most rudimentary setup. Diodes are put close to BJTs to roughly track the temperature.

This is how things were built in times when SS gained notoriety for bad tone and unreliability: The thermal tracking in thus manner is poor (there were better methods devised ages ago) and it basically needs a "cold bias" to operate reliably. Which then leads to crossover distortion buzz at low volume levels. And need I remind, no short circuit protection whatsoever and no protection for the speaker happens the transistor short and "latch" to supply rail.

You can really sketch the schematic in about 20 minutes.