In article , Peter Chant
wrote:
Jim Lesurf wrote:


Any reason you did not build an emitter follower instead as a
buffer? Or is that just a bit too complex for your aims above?

Chose this approach my aim was to make it as 'simple as possible' for
people who had not previously built anything. So a 'minimal' design.
Must confess that having thought of using transformers I became
curious to see how well the cheap 'CPC' ones might work. :-)

I see your point there. I made a simple head phone amp years back.
Gain via common emitter stage forllowed by an emitter follower for
current gain. I did not get around to the Mk2 as I found myself using
headphones less. It had two significant problems:

* Appaling power on / off thump - basically 4.5V step into headphones.
* A lot of crosstalk, presumably via the bias cct on the voltage gain
stage.

Yes, that was one of the things I had in mind. The modern 'solution' tends
to be dual rails ... thus making the setup more complex.

The latter was inelegently partly solved by a large electolytic across
the power rails.

That can reduce the thump (and reduce crosstalk if the psu is poor) but
then means a series resistance in the rail as well - unless you decide just
to poke the psu hard at every turn-on. 8-]

An alternative is ye olde 'volume pot with the on-switch at the 'off' end
of the travel'. But does anyone still make these with decent specs?

Lession - a usable circuit would be more complex to solve the above
issues.

Using the transformers also meant I could avoid needing a bit of veroboard
or some other board. In turn meaning no need for attachments for the
internals. With the tiny transformers I just let them rattle about in the
box. If that is a bother, I'd have just padded out with some foam. As the
French (nearly) say, "Not magnificent, just the railway station" :-)


Of course, there is no reason I or someone else can't now suggest
'improved' versions - like better transformers or using buffers.
Similarly, adding crosstalk bleed networks which some may prefer for
headphone listening. :-)

But surely transformers are expensive and have hysterisis losses?

In the case of the prototype I made I'm not sure all told if the
transformers are much more expensive than a nice amp and psu that had
minimal distortion, no thump, etc. And the output seems OK to me. But I'd
agree better transformers might be desirable, and more costly. One of the
things that did surprise me about the transformers I used was that the
winding resistances were higher than I'd expected. Also I suppose that does
help protect the DAC from an unintentionally shorted output. 8-]

[snip]


FWIW later tests and all listening have been with 4 x NiMH AA
(1700mAh) cells as the power supply. These have so far lasted a couple
of hours but I've no idea how much longer they will manage before
needing a recharge.

A PP9 seems to run mine for a reasonable time, even though the output
was class A - but I suspect that I could buy a PSU for cheaper than the
battery if I looked carefully.

Yes. I chose batteries because I wanted it to be portable and for use with
my laptop. Only after I made it, I decided it was decent enough to use with
my AV/audio items as well! FWIW The NiMH cells I'm using are still running
on the first charge so I don't know how long they will last. I'm also
hoping to find some 'better' transformers sometime I can try out. Now
curious to see if something along those lines is available and worthwhile.

And the AU-D3 seems to be designed for 5V rail. So a PP9 would be risky due
to the higher voltage. FWIW I've asked the makers for more info on the
AU-D3 so may know more later. If so, I'll report anything useful.

Slainte,

Jim

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