Dodgy App Note Cct.

[Phil Allison]

Winfield Hill is Riding a Donkey wrote:

---------------------------------------------

Phil Allison wrote...

** Win brushes aside another of his idiot mistakes.

Everyone is supposed to know a "Mackie 16 ch desk"
doesn't have any low-level mic inputs?

** Wins asks, having snipped his reading error out of sight.

Back in my
broadcast days, all mixers had dynamic mic inputs.

** So does the Mackie - 16 of 'em.

https://www.long-mcquade.com/files/6678/lg_1604VLZ3-3Qtr.jpg

Good low noise pres too and 48V phantom.

Guitar amps use 1/4 inch jacks, same as for LINE inputs on desks.

At age 17, I spent a summer designing and building
a mixer for our local FM station, which they used
for five years until they could purchase a new one,
having blown their budget on a new GATES transmitter
and antenna. My mixer had many dynamic mic inputs.

** Using tubes and input matching transformers ?

Or some of 'dem new fangled Silicon BJTs ?


..... Phil

 

[Winfield Hill]

jlarkin@highlandsniptechnology.com wrote...

On 9 Nov 2019 06:49:59 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

Phil Allison wrote...

A large enough signal at the input would still destroy
the preamp and anything else on the same DC rails.
I have seen this happen, a near new Mackie 16 ch desk
written off after a guitarist plugged his 100W Marshall
into a line input.

The input had 1A diodes direct to each rail an so pumped
it up to a lethal level. Beyond economic repair. One
2.2kohm in series with the input would have saved it
easily.

The preamp sports a 1nV input noise level. A 2.2k
resistor has about 6nV of Johnson noise. A 47-ohm
value would have about 0.9nV, but fail to provide
protection you're seeking (THAT suggests 10 ohms in
their 1510 datasheet). It's often suggested when a
protection diode is added, connected to a supply
rail, that a big zener to ground be placed on the
supply, rated a few volts higher. Or a simple BJT
network could shunt the higher current to ground.

A elegant way to limit input current, is to use two
depletion-mode MOSFETs, back-to-back. See AoE III,
Figure 5.80, page 361. There we show two LND150,
and an R4 = 1k resistor to set current-limit to 1mA.

Hopefully this scheme can be used without adding too
much noise, but 1k isn't much better than 2.2k. With
no resistor, the maximum current is equal to Idss, or
3mA for an LND150. But LND150 has Rds(on) = 650 ohms,
and 2x = 1.3k, far too high. Another favorite of ours
is the BSS126, but it has Rds(on) = 320 ohms. Awwkk,
we want both low Idss and low Rds(on).

The BSP135 has Rds(on) = 30 ohms, excellent, but its
Idss = 20mA value is a minimum, no maximum is given.
However, it has a very low Vgs, so a small R4 = 27
ohms could limit the maximum current to 25 to 50mA,
and total resistance would be 87 ohms, adding 1.2 nV.
Or use 56 ohms. The withstand voltage is +/-600V,
so one could safely hook it up to the AC line.

Low Rds-on, low Idss is all we want. Sounds like some
sort of Phemt. But phemts aren't very appealing as
input protection devices!

DN2530 is a nice little fet, 12 ohms at Vgs=0, mostly
off at -2 or so.

Yes, 12 ohms is nice. But it'd take Vgs about -2.25V
to limit the current to 25mA, so R4 = 91 ohms. That's
115 ohms total, 1.4nV, not too much worse than 87 ohms.
It's a 300V part, DN2530N3 in convenient TO-92 package,
and the DN2530N8 is a 1.6-watt SOT-89 SMT package.


--
Thanks,
- Win

 

On 9 Nov 2019 19:43:02 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

jlarkin@highlandsniptechnology.com wrote...

On 9 Nov 2019 06:49:59 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

Phil Allison wrote...

A large enough signal at the input would still destroy
the preamp and anything else on the same DC rails.
I have seen this happen, a near new Mackie 16 ch desk
written off after a guitarist plugged his 100W Marshall
into a line input.

The input had 1A diodes direct to each rail an so pumped
it up to a lethal level. Beyond economic repair. One
2.2kohm in series with the input would have saved it
easily.

The preamp sports a 1nV input noise level. A 2.2k
resistor has about 6nV of Johnson noise. A 47-ohm
value would have about 0.9nV, but fail to provide
protection you're seeking (THAT suggests 10 ohms in
their 1510 datasheet). It's often suggested when a
protection diode is added, connected to a supply
rail, that a big zener to ground be placed on the
supply, rated a few volts higher. Or a simple BJT
network could shunt the higher current to ground.

A elegant way to limit input current, is to use two
depletion-mode MOSFETs, back-to-back. See AoE III,
Figure 5.80, page 361. There we show two LND150,
and an R4 = 1k resistor to set current-limit to 1mA.

Hopefully this scheme can be used without adding too
much noise, but 1k isn't much better than 2.2k. With
no resistor, the maximum current is equal to Idss, or
3mA for an LND150. But LND150 has Rds(on) = 650 ohms,
and 2x = 1.3k, far too high. Another favorite of ours
is the BSS126, but it has Rds(on) = 320 ohms. Awwkk,
we want both low Idss and low Rds(on).

The BSP135 has Rds(on) = 30 ohms, excellent, but its
Idss = 20mA value is a minimum, no maximum is given.
However, it has a very low Vgs, so a small R4 = 27
ohms could limit the maximum current to 25 to 50mA,
and total resistance would be 87 ohms, adding 1.2 nV.
Or use 56 ohms. The withstand voltage is +/-600V,
so one could safely hook it up to the AC line.

Low Rds-on, low Idss is all we want. Sounds like some
sort of Phemt. But phemts aren't very appealing as
input protection devices!

DN2530 is a nice little fet, 12 ohms at Vgs=0, mostly
off at -2 or so.

Yes, 12 ohms is nice. But it'd take Vgs about -2.25V
to limit the current to 25mA, so R4 = 91 ohms. That's
115 ohms total, 1.4nV, not too much worse than 87 ohms.
It's a 300V part, DN2530N3 in convenient TO-92 package,
and the DN2530N8 is a 1.6-watt SOT-89 SMT package.

We use the SOT-89.

Looks good for 3 watts, properly copper-poured.

https://www.dropbox.com/sh/lfqcg32y0obx4qk/AACqwtqAb6fP7z67b9hXKWE1a?dl=0

It makes a nice linear cap bleeder. Saves time.





--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Winfield Hill]

jlarkin@highlandsniptechnology.com wrote...

DN2530 is a nice little fet, 12 ohms at Vgs=0, mostly
off at -2 or so.

Yes, 12 ohms is nice. But it'd take Vgs about -2.25V
to limit the current to 25mA, so R4 = 91 ohms. That's
115 ohms total, 1.4nV, not too much worse than 87 ohms.
It's a 300V part, DN2530N3 in convenient TO-92 package,
and the DN2530N8 is a 1.6-watt SOT-89 SMT package.

We use the SOT-89.
Looks good for 3 watts, properly copper-poured.

https://www.dropbox.com/sh/lfqcg32y0obx4qk/AACqwtqAb6fP7z67b9hXKWE1a?dl=0

Hey, John, 3W very nice, yes, but 15 square inches!?!
How about only 1 square inch, which is still a lot to
give up. Is that the 1.6W spec?


--
Thanks,
- Win

 

On 10 Nov 2019 09:15:56 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

jlarkin@highlandsniptechnology.com wrote...

DN2530 is a nice little fet, 12 ohms at Vgs=0, mostly
off at -2 or so.

Yes, 12 ohms is nice. But it'd take Vgs about -2.25V
to limit the current to 25mA, so R4 = 91 ohms. That's
115 ohms total, 1.4nV, not too much worse than 87 ohms.
It's a 300V part, DN2530N3 in convenient TO-92 package,
and the DN2530N8 is a 1.6-watt SOT-89 SMT package.

We use the SOT-89.
Looks good for 3 watts, properly copper-poured.

https://www.dropbox.com/sh/lfqcg32y0obx4qk/AACqwtqAb6fP7z67b9hXKWE1a?dl=0

Hey, John, 3W very nice, yes, but 15 square inches!?!
How about only 1 square inch, which is still a lot to
give up. Is that the 1.6W spec?

One could via down to an internal plane and/or a copper patch on the
bottom of the board.

I wouldn't actually use it at 3 watts, but the specified 1.6 looks
conservative.

More copper area has diminishing returns, due to thermal spreading
resistance. An infinite sheet doesn't have zero theta. The patch area
of the device matters a lot. Vias to other spreading layers helps. 2
oz copper would cut theta a lot.







--

John Larkin Highland Technology, Inc

lunatic fringe electronics