Effect loop pedal impedance

[Buffalo Tom]

Hello. I need an effect loop pedal for inserting various guitar effects and control both send and return levels. It should be optimized for both guitar and keyboard/synth. So is the impedance switch a good idea? Or should I just keep it 1M for keyboards too. Is there any disadvantages with the high Z? I want the pedal to have as low noise as possible. And whats a suitable return impendance? Low or High...

Thanks for your time

[PRR]

Keep it 1Meg for anything. (That's actually a broadcast line proposed spec but rarely followed.)

The 1Meg may raise hiss when left OPEN but why would you do that?? When plugged to keyboard it "sees" the 470r-2k or so of the electronics, in parallel with the 1Meg, for a satisfactorily low impedance.

5532 is a great lo-hiss device for <6k sources, but isn't TL07 a hair better for >50k guitar? (Either ought to be fine for synths/epiano/organ)

If you DID need two input loadings, it would be sweeter to leave the 1Meg on permanently and use a SPST to tack on 47k (or 49.318k) as needed. So the input is never left unchained.

[Buffalo Tom]

Keep it 1Meg for anything. (That's actually a broadcast line proposed spec but rarely followed.)

The 1Meg may raise hiss when left OPEN but why would you do that?? When plugged to keyboard it "sees" the 470r-2k or so of the electronics, in parallel with the 1Meg, for a satisfactorily low impedance.

5532 is a great lo-hiss device for <6k sources, but isn't TL07 a hair better for >50k guitar? (Either ought to be fine for synths/epiano/organ)

If you DID need two input loadings, it would be sweeter to leave the 1Meg on permanently and use a SPST to tack on 47k (or 49.318k) as needed. So the input is never left unchained.

Thanks Paul. I will remove the switch and go with 1M. And I figure thats a good value for the return impedance resistor (R9) too.

[antonis]

Just for 1M impedance, wisely proposed by Paul..

NE5532 doesn't like bias resistors of values greater than 100k or so..
Of course, for a +/- 15V buffer it shouldn't be an issue - unless AUDIO input signal amplitude is marginally close to Vee minus Vbias offset..

If case of the above probability, split R2 into 2 X 47k and bootstrap their junction via a cap from opamp out/inverting input..

[Buffalo Tom]

Just for 1M impedance, wisely proposed by Paul..

NE5532 doesn't like bias resistors of values greater than 100k or so..
Of course, for a +/- 15V buffer it shouldn't be an issue - unless AUDIO input signal amplitude is marginally close to Vee minus Vbias offset..

If case of the above probability, split R2 into 2 X 47k and bootstrap their junction via a cap from opamp out/inverting input..

The NE5532 with +/-15V and 1M bias resistor was something I borrowed from this schematic
http://guitar-fx-layouts.42897.x6.nabble.com/file/n32135/Suhr_Minimix_II.jpg

[antonis]

It's up to you...
(but also borrow C5 decoupling cap - it's there NOT only for R10/C5 LPF)

P.S.
What I said above concerns the probability of 800mV (800nA max X 1M) misbias might be an issue..

[PRR]

> probability of 800mV (800nA max X 1M) misbias

800mV is not too probable; if +/-15V supply as noted even 800mV is no real problem; 800mV won't even blow-up his electrolytic caps (and he could check the DC and turn-around if that feels better).