Use bleeder resistors in audio opamp design?

[snip]

That would be about the same time I found the problem... I was making

300 Baud "muff" modems for GenRad's portable test equipment ;-) [snip]

Been there, done that... actually both descriptions ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

[about forcing op amps into class A operation with output current bias]

If you need a lot of sinks, it's easy to generate another, circa -24V, supply, and strap base of NPN to (-15V), then an emitter resistor to the (-24V) programs the sink current of the collector.

Since these currents are regulated, there oughtn't be any ripple due to load at the (-24V) supply. Also, the sinks are better compliance at the (-) rail than most op amps; your source-versus-sink decision might be guided by the improved behavior-near-rail that this offers.

For the cautious, a base resistor can be added to eliminate all-silicon sneak paths.

And kill the likely RF oscillations.

John

s.

ts

age

and

t

So, what's your opinion of using electros back to back to form non- polarized caps?

I hear back to back electros work well at reverse low voltages of 1.5 or so, and act as diodes at higher voltages. I've tested some small values with reverse voltage and they didn't leak much.

-Bill

Phil is himself, and doesn't stand nonsense, a bit overboard in his responses, but I worked on building sites as a teenager, the language doesn't fuss me. Some people have short fuses (fuzes?), I don't have to buy into that.

In this particular case I dunno if he was impatient for my reply to his query (where to buy 1% 47n, which I answered) or what?

I don't see what else he's fussed about unless it's because I don't see the point in replying to posts I agree with, nor do I see a point responding to Rich G. when he was talking about a different point to what I'm interested in. I find myself quite busy lately (surprisingly for someone left the workforce in '93), so I don't spend a lot of time here.

It's nice to pop in the odd questions and see responses, but I'm not here to play flame games. Unless I am ;)

Grant.

If you are making active filters, your approach is simply wrong. There are filter topologies that use more op amps but allow total flexibility on the value of the caps. In the dark ages of modem design, there were discrete active filters built out of nothing by

0.1uf caps. It was far cheaper to get 1% caps of one value and then build around that.

In this example, it takes 10 op amps for 5 poles. But op amp can be cheaper precision caps.

There are other topologies besides leap frog that have the same flexibility in cap value, but leap frog is always the lowest sensitivity solution.

Interesting thought. Worth a look at.

Except the next available higher voltage is from power amplifiers, very ripply, so there's some interesting stuff for me to think about in there.

That too.

Grant.

What's wrong with using bipolar caps, the leakage? I've never liked back to back as they sort of expect diode action to keep the electros nice, so what does that non-linearity do to a signal? Or the cap itself after a while.

Then there's all those circuits out there with little, none or slight reverse bias on tantalums in audio signal paths, how do they sound after a while?

Grant.

"Grant"

** Thanks - they were formerly very hard to get items in Australia.

Farnell had some ( expensive) 1% polystyrenes up to 10nF and that was it.

.... Phil

"Grant"

** My oversight, now corrected, in not replying. ** You have a reading comprehension problem.

The concept of "context' seems beyond you.

..... Phil

"Bill Bowden"

So, what's your opinion of using electros back to back to form non- polarized caps?

** Ask the NG, not me - s*****ad.

I've seen a trick used, which hooks the junction of the two back-to-back bipolar caps through a high-value resistor, to a voltage source which biases both caps in their preferred direction (e.g. might be the negative supply rail in an audio amplifier). This prevents the dielectrics from ever being reverse-polarized and keeps them healthy and happy over time.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

Yup! ALL my filter designs at GenRad used 0.01uF/1%/COG :-)

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

--
http://www.youtube.com/watch?v=gc_1LRh9ObU&feature=related

That's a huge number of parts, most of which affect gain and offset. I like S-K filters because they can have a DC gain of exactly 1, independent of any resistor values.

John

Geezer rock on scratchy vinyl. Probably doesn't hit anywhere close to

20 KHz.

John

Too right :)

There's more practical factors in my mind, like building a well known topology that will work first time.

Grant.

S-K filters are hyper sensitive to component variations. I never use that configuration. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

tors.

in

n a s

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

SK filters are for people that don't know how to design filters.