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From: Eeyore on 16 Dec 2008 06:59


Bob Eld wrote:

> I went to one show where they were using wireless mics and all of a sudden
> somebody was ordering a hamburger and a coke. A bit of fm interference from
> the joint across the street. They switched out to wired mics for the next
> set.

There was a specified band for wireless mics once, but the FCC has allowed it to
be used for other stuff under commercial pressure form the likes of the IT guys.

So forget wireless.

http://fohonline.com/index.php?option=com_content&task=view&id=1885&Itemid=1

Graham


From: Eeyore on 16 Dec 2008 07:00


BobW wrote:

> The wireless stuff would have to be a fairly sophisticated technology like
> spread spectrum to avoid that type of interference.

No, there was a dedicated band that the FCC fucked.

Graham

From: Michael A. Terrell on 16 Dec 2008 12:18

John Larkin wrote:
>
> Not to change the subject (Heaven forbid!) but The Brat and I were
> sitting in the Bar of America in Truckee on Saturday night, me
> chomping down a $12 burger and a Stella draft and watching vehicles
> slide around in the snow.
>
> I noticed a guy fooling with a lot of sound gear nearby. He had a
> guitar in a case, a set of drums, and maybe 6 or 8 electronics boxes,
> including some sort of Peavey equalizer/amp, an Echoplexer (?), a Bose
> box, and three other things I didn't recognize. The floor was covered
> with power strips and cables and more little boxes with switches and
> pedals and stuff. I started counting knobs and quit after 75 or so.


Echoplex. They were a tape loop echo effects machine. Most of the
ones I saw used tubes. Basically, it uses two heads to record & play
back the input. The playback head is on a tack, to adjust the delay. The
bias to the erase head was adjustable, to set how many passes it too top
erase a signal from the tape loop. They were a pain to service, because
the cheap assed musicians would use them till the heads were worn out,
with bad tape loops that hissed like crazy. Then they wanted everything
replaced for $5, including the labor. :( They were all world class
cheapskates. That was why I got out of the music business, long ago.


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The crazy, and the insane.
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From: morris.slutsky on 16 Dec 2008 14:52

>    And that's another thing, the current gains of these Darlington
> pairs has to be carefully matched to be sure the positive and negative
> half-cycles are the same at the output. On the other hand, such an
> imbalance may simulate the sound of a single-ended tube output. :)
>
>    You can make this into a an equivalent but more
> conventional-looking schematic by moving the ground to the 'usual'
> place, swapping each Darlington pair of transistors to the other rail,
> and driving the bases (which will be near their respective rail
> voltages) from the input through capacitor copupling. I'm pretty sure
> this would work the same way (except it won't have response to DC,
> which you don't really care about, though this one doesn't anyway
> because of that 'fake center-tapped' power supply), and the schematic
> would 'look right' to everyone who has studied conventional audio
> power amplifiers.
>
>
>
> >Graham

Yeah, I could move things to their 'usual' places and it would look
more normal. But then I'd need to use more components. Two more
capacitors? That'd be . . . more parts! This is a minimum components
idea. I'm willing to hand-select driver transistors, although yeah, I
don't mind a little bit of 2nd harmonic imbalance at all.

Voltage gain is about 4. That doesn't sound like much. But that'll
take a line level signal up to 9V RMS really easy, which is enough to
clip this thing out. Boosting the gain by changing the resistor ratio
would also push the bias way, way down in a bad way. But here's the
cool part - if I use a higher-voltage supply, then I can change the
resistor ratio for higher gain and keep the existing Class AB bias.
So it's sort of . . . invariant? Like, there will be enough gain from
line level, even if I use a bigger supply.

Right now I'm thinking that this could work nicely with a JFET preamp,
using J201's maybe. Yeah, the preamp will need it's own power
supply. But not a very powerful one. Heck, whatever wall wart I can
find in the sub-$5 range could donate it's organs to the cause.

Someone mentioned capacitor coupling - I like capacitor coupling for
guitar amps, I really do. Just like the tube amps with transformer
coupling - you can't put DC through it. But do you want to? Fact is,
a lot of musical waveforms (heck, even the human voice!) are
asymmetrical and can easily generate DC upon clipping. Speakers don't
like that DC. Sometimes it's nice to have an amp that won't put the
DC out. And if the same cap can be power supply filter and output
coupling, that's just cool!
From: Phil Allison on 17 Dec 2008 19:06

<morris.slutsky(a)gmail.com>
>
>
> Schematic:
> http://img213.imageshack.us/img213/2480/10wpowerstageym6.png
>
> Resistors R1-R4 work very hard here. They set gain, linearity, input
> impedance, output impedance, and bias. This thing idles along at
> about 150 mA through Q3 and Q4, according to SPICE. A reasonably warm
> class AB.
>
> SPICE thinks that this is actually fairly linear.


** Shame how SPICE lets you operate devices well beyond their ratings and
capabilities.

The maximum collector voltage is being exceeded for the BOTH driver
transistors as neither can tolerate 70 volts as your schem requires.

Their puny current and power dissipation ratings will be exceeded in real
operation too, since loudspeakers present quite a reactive load at some low
audio frequencies.

I strongly suggest you use the TIP31C /TIP32C pair in place.

These are rated at 100 volts, 3 amps and 40 watts max & Hfe is also high at
over 100 and fairly linear in the range up to 500mA.




...... Phil



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