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RE: volume pot

206.209.220.20

In Reply to: RE: volume pot posted by Tre' on August 08, 2012 at 20:03:50

Many, here's three:
1) Current limitng. Say the end user puts a RCA Y cable on the preamp outputs to get a poor mans mono combine. You really don't want the two OPAMPS tied directly together.
2) Cable impedance. Not really an issue for audio but in video and other HF applications you want to match the cable impedance so the buildout resistor plus the circuit impedance should equal the cable impedance. Most good video line driver circuits have sub ohm impedance. Look at any commercial video amplifier. There is always a 75ohm resistor between the cable and the output stage. There was/is one baseband audio application however for build out resistors. If you are building a transfomerless 600ohm cable driver, you typically put a 300ohm resistor in each leg of the balanced line. This approach is obsolete however as most professional line level audio systems since the early 1980s are voltage matched and no longer power matched.
3) Rise time control. I worked with some 100mhz ECL switchers designed to drive cables. Good design practice includes a 22-33ohm resistor in each signal pair. This reduces the rise time just enough to prevent excessive ringing on shorter cables.

Edits: 08/09/12

Follow Ups:

RE: volume pot - Jim McShane 12:12:08 08/09/12 (3)
"1) Current limitng. Say the end user puts a RCA Y cable on the preamp outputs to get a poor mans mono combine. You really don't want the two OPAMPS tied directly together."

Why not? What is the reason for that statement?
- RE: volume pot - gusser 12:32:37 08/09/12 (2)
  It's not good practice to short two low impedance outputs together [see edit] especially when the output impedance is much lower than you need. If one side is sourcing current and the other side is sinking you want something to buffer that current. Now granted the resistances should be matched within I would say 5% for general audio use.
  EDIT: I should say two low impedance outputs that do not have the exact same signal present.
  
  Edits: 08/09/12
  - RE: volume pot - Jim McShane 13:23:15 08/09/12 (1)
    A simple isolation resistor on the op amp outputs will do well to prevent problems in most cases if any indeed arise.
    
    Or (and especially if you force the op amp into class A) an output buffer is pretty much a requirement, especially if you expect to drive a low impedance or highly capacitive load.
    - Same thing? - gusser 13:40:42 08/09/12 (0)
      I thought that's what I was referring too. I call it a "build out" resistor. You call an "isolation" resistor. - Same thing.
RE: volume pot - Tre' 12:10:08 08/09/12 (0)
Thanks.

#1 was what I was going to guess.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

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