ETI 5000 MOSFET Power amp

[ralphs99]

The PCB looks very good Suzy!

I only have a few small comments that you may like to consider.

The copper pours around the mounting holes and some of the tracks may a bit too close to the screw heads unless you are planning on using insulating washers under them.

You may wish to change the 10R resistor for a lower value or make it a higher power type. I have seen these self destruct due to ground currents.

It is a good idea to have a marking on your overlay that shows electrolytic polarity outside the component so that you can inspect the board after assembly.

It can also be a good idea to have a link to the power stage power rails that you can isolate them for fault finding.

A test point where you can ground a CRO probe is wise investment.

The holes for what I'm guessing are spade connectors for the power entrymay be a bit small.

The PCB makers prefer a board outline on Mech 1, either a very
small trace of 1mil or the width of the router bit.

All really minor stuff.
I look forward to seeing the stuffed PCB's.

Cheers,
Ralph.

[suzyj]

Hiya Ralph,

Nice to see someone with an interest

Good idea regarding isolating the power rails - I was vaguely thinking it would be nice to have a couple of links so that everything but the output transformers could be run off a separate (lower power) supply. That way, one could drive the gates of the power FETs beyond their drains. Is that ever done? It works nicely in simulation, and would allow the use of a lower voltage power transformer, for the same sort of distortion/power. Though one imagines that the onset of clipping would be rather harder.

In any case, a couple of links on the supply rails are a good thought.

As for sizing of terminal holes etc, I simply went by the standard hole sizes that we use at work - out 1/4" terminals have a pair of 1.2mm tabs, that fit nicely in a 0.05" hole.

I've also bunged a couple more designs up, if you're interested.

A preamp board:

PDF file
GIF file
SCH file
PCB file

and a power supply board:

PDF file
GIF file
SCH file
PCB file

There's aso a Q&D autocad drawing of the whole box:

http://www.atnf.csiro.au/people/sjackson/Amplifier.dwg

I'll bung the control board and the video switch board up tomorrow (assuming I remember to put them on my USB key).

Everything's pretty rough at the moment. I generally use keep-outs to define the edge of boards while I'm drawing them, then draw in the mech1 stuff just before I send out the panel. I've still got some work to do cleaning up silkscreens etc as well.

Cheers,

Suzy

[ralphs99]

Hi Suzy,

I know there are some pro power amps that use multiple rails, so I'm guessing they are probably doing just what you suggest. It's certainly a good idea where you need the efficiency.

I've had a look at your power supply. I like it a lot! The soft-start circuit is a nice touch. Very clever the way you have used the split rails on the '358 to power the relays.
Circuits that rely on short term overload capability always worry me however. Maybe a PTC would be a better choice instead of the 20W resistor?

I'd like to see a larger spacing between the mains terminals and the mounting screws. Maybe you're planning on using plastic hardware, but I'm sure a lot of people will want to build your design, and you can't always assume they're gonna build it the same way.

Lastly, don't forget to terminate the unused inputs on the '00.

Cheers,
Ralph.

[suzyj]

Here's a turn-up for the books.

I was just chatting with my supervisor (who recently generously gave me a gorgeous pair of Infinity RS-5b speakers), and it turns out the first amplifier that drove my Infinity speakers, back in the mid-80's, was... wait for it... an ETI5000!

My supervisor built a pair when he was at Sydney Uni. Apparently in order to get really good distortion levels took quite a bit of trial and error, mainly in optimising the power wiring (and grounding). He spent quite a bit of time with an old HP audio spec-an, fiddling with power rail connections and such-forth.

So now I know I'm on the right track

Cheers,

Suzy

[suzyj]

Hi guys,

I've (finally) got my boards back, and am part way through populating them. While I wait for my capacitors to wing their way from Digikey, I thought I'd post a couple of pics:




Cheers,

Suzy

[clem_o]

Looks great!!

[suzyj]

Hi guys,

Here's a couple of photos of a populated monoblock, mounted to its heatsink. They have a nice weight to them. I'm now building the rest of the amplifier, complete with a pair of thumping great toroidal transformers.





It's taken a while, as I don't have terribly much spare time to devote to the project these days, but it's plenty rewarding.

More details of the design are on a webpage I put together, at http://www.littlefishbicycles.com/poweramp/index.html

Cheers,

Suzy

[clem_o]

Wish we had parts of that quality here!

So how does the final product sound??? I'm sure you've run it with your preamp, even if not at great power levels...


Cheers

[suzyj]

I must confess that I haven't yet powered it up. I figured I'd do all the power supply wiring properly first.

I'm actually a little leery of just applying power to it - I imagine there could be terrible consequences if something isn't right. What's the usual procedure for smoke testing large amps? I imagine a few ohms in series with the transformer would probably prevent most fires...

Cheers,

Suzy

[clem_o]

Hi Suzy,

My usual suggestion is to put series incandescent lamps on both rails. If anything 'funny' happens, the lamps light up, and you know that you've saved some $$$... lamps tend to have a longer life than series resistors when used this way.... :-)

Start with zero-biased outputs, do a quick 'scope test to verify its actually working, then bias it. The rest of the testing, including resistive and resistive-capacitive loads can be used to verify that your amp isn't going to misbehave and kill your loudspeaker's HF units...

Cheers!