Resistor % for High Quality inline Pad

[Mulmany]

Does anyone know whether a 1k Ohm 5% resistor will adversely affect the balanced input circuit?

So if you have a +/- 50 ohm margin on two resistors on pin 3 and pin 2 of an inline pad, will it created any phase/amplitude issues at the balanced line input circuit?

Basically how precise of a resistor is needed for a HIGH QUALITY pad?

[paully]

You should get the answer on this page Goldpoint Level Controls . You need close tolerance for noise cancellation.

Best, Paul

[Mulmany]

thats close to what I need. So it would be a 1k Ohm @ 0.1%, 25ppm.

[David Kulka]

Chances are, the gear on either end of the pads will probably have balance specs way worse than .1%. The resistors will not be the weak link in the chain, most designers would be comfortable with 1% tolerance.

[Francis Vaughan]

Originally Posted by David Kulka

Chances are, the gear on either end of the pads will probably have balance specs way worse than .1%. The resistors will not be the weak link in the chain, most designers would be comfortable with 1% tolerance.

In some metrics yes, but in others no. Transformer based systems will present a naturally balanced setup that will be clearly degraded by 1% tolerance in a pad.

Actively driven output stages typically have close to zero output impedance - and so the absolute variation of impedance they have will be vanishing, so again, the tolerance of the pad resistors will make a difference. Where you may see some variation may be in some pre-amps - where the input impedance may be defined by only 1% tolerance components. That preamp will already be compromised due to this - with a clearly worse common mode rejection ratio than might be otherwise possible, and so a pad with loose tolerances will make less difference. But a really good pre will have good CMRR, and a close tolerance pad still important.

[Jim Williams]

5% resistors will degrade CMRR to 40db. 1% resistors will give -60 db CMRR. go to matched 1% to .1% and you get -80 db CMRR. Some mic pre's have CMRR better than -90 db 20~20k. Any pad less than that will degrade CMRR (common mode rejection ratio or noise immunity).

Jim Williams
Audio Upgrades

[Rick Sutton]

I've always wondered about this......
Is it acceptable to use say 5% (or even 1%) resistors and match them up in pairs by measuring their resistance with a Fluke digtal VOM? Are there other issues beyond the original resistance?
Thanks, Rick

[Ethan Winer]

Originally Posted by Rick Sutton

Is it acceptable to use say 5% (or even 1%) resistors and match them up in pairs by measuring their resistance with a Fluke digtal VOM? Are there other issues beyond the original resistance?

Yes, another issue is how the resistance might change when you heat it up while soldering. Resistors are cheap! Your time and expected results are not. If it were me I'd just buy 1% metal film resistors and be done with it.

--Ethan

[paully]

Originally Posted by Ethan Winer

If it were me I'd just buy 1% metal film resistors and be done with it. --

>>>>>>>>>>>>>>
. Not to mention the inevitable dirty/corroded contacts on connectors and the rotary switch contacts. Keep the connecting wires to and from the attenuator as short as possible, use quality wire, and you should be fine with 1%.

Best, Paul

[Matt Syson]

Hi
Ideally the resistors should be matched as well as possible but by the time you have your 'wonder resistors' assembled the chances are that other factors will degrade the possible CMMR response. 1 percent types are probably good enough for most tasks. If you are using a attenuator in the first place then you are already accepting a compromise. Use the attenuator at the receiving end of the cable and it will reduce the effect of any imbalance partly because the interferance will be in the 'high level' portion of the circuit. Relatively few input stages manage superb CMMR over a 20-20K bandwidth although arguably it is necessary FAR beyond this as it is your mobile phone, TV and radio signals and a whole host of 'junk' that you are trying to repel, very few input stages have immunity to this noise relying on CMRR alone.
Go for 1 percent and stop worrying, or get your multimeter out and check the match (rather than absolute value) AFTER soldering the thing together.
Matt S

[Jim Williams]

Originally Posted by Rick Sutton

I've always wondered about this......
Is it acceptable to use say 5% (or even 1%) resistors and match them up in pairs by measuring their resistance with a Fluke digtal VOM? Are there other issues beyond the original resistance?
Thanks, Rick

Yes, temp co's. That is values that drift with temperature. Most MF's are 50 ppm, some 25 ppm. Vishay bulk foils are under 1 ppm. When measuring resistors to match, be careful not to handle them first as the body heat will cause the measurement to drift. A 4 digit meter will allow you to match within 1 ohm.

Jim Williams
Audio Upgrades

[tINY]

Remember that 1% resistors have a 1% tolerance over temp, voltage, current, and lifetime.

In samll-signal applications like this, you can expect better matching than 1% if pairs are pulled from the same lot and mounted in the same place.

I'd still use 1% or 0.5% metal film types, though....



-tINY

[Mulmany]

Thanks for the replies.

I ended up getting some mil spec 1.05k ohm .1%, 50ppm from SMC. Mouser is out untill August and I can't wait that long.

If anyone cares to know? The Red One camera cannot accept +4dbu line input and so must be padded down with home made inline pads or the relatively expensive RED cables with the pads built in.