And huge films caps on the bass boost, but no resistors. What values are you using at CBB and R3/R4? I'd be interested to see the graphs.

pre mosfet resistors are all vishay dale 1/2 watt. when i order most of my parts for stuff i generally get the heavy duty parts that fit the pin spacing. Retrospectively, there is no great need with this amp design. Interestingly its the first PCB i have encountered where the body thickness of the resistors caused a spacing problem rather than just the leg spacing. ( resistors at different mount heights to overcome)

The big resistors are also 3 watt ones.
Big Film Caps are Vishay .22 microfarad ( stock i guess )
default values for R3/R4. No resistors as it goes throu pot. (variable bass boost) although even with this, there is not much bass "boost"

does this sound right ? -- it has about the right amount of bass overall for my tastes at about mid way on the pot. but there is no huge difference if i have the pot on min all the way to max.

There is nothing really "different" about my one - some of the resistors are .1% but i doubt that would make much difference to anything.

cheers

Jon

But already the next step gives me trouble.
However I turn the BIASOPG, nothing happens - DC voltage stays at 2,3mV.

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[Quoting Sathimas]

However I turn the BIASOPG, nothing happens - DC voltage stays at 2,3mV.

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If this still doesn't help, then check your Q3 and Q4 JFETs and BIASOP trimpot. Make sure they are installed in the proper orientation and reflow any suspicious solder joints.

My soldering iron is currently heating up to reflow the solder joints.

How would I check the Jfets in case I'll have to?
Maybe you can give me some link to a website or something.
___________________________

Just resoldered all joints in the area - with no effect

[Edited by Sathimas on 04-07-2008 at 12:13 PM.]

Then, try reflowing the joints of Q2, R6, R7 and the BIAS trimpot. Power up again to see if this fixes the problem.

If you still have the problem, then check the 2N5486 JFETs. Unfortunately, you must remove them from the board to do this. Power down and discharge all caps before removing any part. Measure the JFETs' Idss to see that they're within the datasheet limits. The way to measure Idss is found on the β22 website, in the "Device matching" section.

[Edited by amb on 04-08-2008 at 04:42 AM.]

I razored this particular trimpot open and you could actually see that as i had adjusted it past its click position the lead screw would not pick up the threaded part of the wiper.
amb's advice is correct - its also a relativley simple amp to debug.

tis all good fun... wait till you get the thing working, (I finished the circuit board last week and still having loads of fun with it)
my next job is finding a good case to put her in

good luck !!

I'll put the project to rest for a while then - University is starting again right now
and I don't have enough time for that now. I only started the check because I hoped that all would work

Thank you for your help nonetheless.

This is my first build so apologies for the newb question. Most LEDS from my local supplier seem to be rated at about 4V drop / 32mA.

Using the advice given, I get an RLED resistor value of 625 ohms and a power dissipation of 1.6W. This seems so far from the examples given I wanted a sanity check. I do have a 2W resistor that would do the job however.

Thanks!

[Edited by mnemonix on 04-12-2008 at 08:35 AM.]

You would probably find that unacceptably bright. You'll probably find that a 32mA LED will provide more than enough brightness at 10mA (2.2k, 1/4W resistor), and probably just fine at 5mA (4.7K resistor). If you have some resistors or a pot lying around, try playing around and seeing what value gives you the brightness you like (you don't need to worry about the wattage for short usage)

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[Quoting amb]

I recommend trying the default RLED value of 10K ohms first. Most of the time it will be bright enough. Only if it's too dim then you experiment with lower resistor values.

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That's the advice you gave me on the first page of this thread..... it was 100% spot on.

while i wait for my opamps to arrive (Farnell sent me an empty foil bag )i've done the unpowered tests for shorts AMB covers on his site. I'm reading resistance for the tests from the opamp pin sockets, is this correct ? they're clearly not shorts but the instructions don't tell me what to expect.

off topic... what changes are necessary to carry out projects here requiring transformers in the uk given our 230V mains ?

[Edited by mnemonix on 04-14-2008 at 08:20 AM.]

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[Quoting mnemonix]

what changes are necessary to carry out projects here requiring transformers in the uk given our 230V mains ?

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If you're building a power supply, just get a power transformer that has a 230VAC/50hz primary, and the same secondary as the one specified. If you use amb's power supplies, this is specified in the instructions. If you use Tangent's STEPS supply, it's designed with a 2X115V primary transformer and you can adapt it for either voltage using wire jumpers.

You also want to make sure that any passive parts (notably capacitors) that are located BEFORE the transformer's primary are rated for at least 230VAC. You'll find that most are.

And if there's an AC line fuse, it should be half the value that you'd use on 115.

Other than that, everything remains the same.

One dirty secret is that you can power the M³ pretty well using a laptop computer power supply... better to find one that's a bit more than the IBM standard 18V... I had a "universal" laptop supply from Maplin that was switchable up to 22V, and I powered my M³ for a couple months with it while I was building a new power supply.

It worked very well. I think I could discern the difference once I'd built my expensive high-tech discrete regulated supply and started using it. But maybe that was just the difference between 22V and 26V. Or maybe I just WANTED to hear a difference, because a £100 deluxe regulator just has to sound better than a £30 laptop supply, doesn't it?

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[Quoting Dougigs]

It worked very well. I think I could discern the difference once I'd built my expensive high-tech discrete regulated supply and started using it. But maybe that was just the difference between 22V and 26V. Or maybe I just WANTED to hear a difference, because a £100 deluxe regulator just has to sound better than a £30 laptop supply, doesn't it?

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As it happens a friend who designs and builds custom tube stuff for musicians & studios jumped in and made me a power supply when I showed him the M3, which he was very complimentary of besides describing it as insanely over-engineered (i think that was actually kind of a compliment too). I tried to interest him in the idea of designing a tube headphone amp, but he's somewhat wary of audiophiles and their "special" needs - it's paradoxical that those who create and capture the music are often far less precious about the process than those who then simply listen to it!

Thanks again...

Cased up and finished, as promised!