Input Buffer Circuit advice

Theurge

Theurge

Member
Hi All,

I have a Rocktron Rack Interface whose buffered guitar input I stopped using as the input impedance is really low (quoted in the tech specs as 470k) which was making a noticeable difference, especially to my Strat. For various boring reasons I want to start using this again, but don't want the extra space/ power supply taken up with an additional buffer. So I want to raise the impedance by altering the circuit.

I have the schematic, and in the section dealing with the input buffer, I think I've found the resistor that determines the input impedance: resistor R149 on the diagram (attached).

Bear in mind, I have no real understanding of electronics, and have only done some rudimentary research on op amp buffers.

Without resorting to math (eek!) can someone check out the schematic and tell me if:

a) that definitely is the resistor determining the input impedance
b) can I just swap this for a 1 meg resistor?
c) will I need to change anything else (to preserve the amount of gain produced etc)?

Thanks, and keep it simple for stupid!

Si
 

Attachments

  • Input Buffer.pdf
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That input opamp NE5532 is of a bipolar type with a comparatively low input impedance, around 300k afaik so that one resistor doesn't make a huge difference. The NE5532 is a good old workhorse low-noise opamp but today there's quite a number of modern low-noise high input impedance types that one could choose from.
 
ChristThePhone said:
That input opamp NE5532 is of a bipolar type with a comparatively low input impedance, around 300k afaik so that one resistor doesn't make a huge difference. The NE5532 is a good old workhorse low-noise opamp but today there's quite a number of modern low-noise high input impedance types that one could choose from.
Click to expand...

So, your saying replace the opamp rather than changing the resistor? Could you explain please, as simple as possible, why changing that resistor wouldn't make a huge difference? As far as I understand, that's what the pickup signal would 'see' before the opamp, so changing it to a 1meg resistor would create a more suitable (for my needs) impedance regardless of the input impedance of the opamp that follows.

Also, every opamp buffer circuit I've seen uses 1meg resistors (or higher) in that spot to set the impedance. And that current resistor value (470k) matches the tech specs quoted in manual exactly. As I said, I lack much electronics knowledge, but it does seem that there's therefore a 1 to 1 correlation between the two.

Hope what I'm saying makes sense

Thanks
 
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The pickup sees the opamps input impedance in parallel to the 470k resistor. The two resistances in parallel effectively mean a input resistance of about 180k. You could raise the input impedance to about 300k by leaving away that resistor entirely but not above. If you want a 1M input resistance there's no way around changing the opamp type to JFET which comes with an input impedance of a couple of M so the additional input resistor would become the defining factor.
 
Cheers. I see what you mean now about it being in parallel.

One thing that’s been puzzling me about the unit is why they’d want to design the circuit with such a low impedance?
 
That opamp was the standard back in the days and there was no real alternative with sufficiently low noise floor at a competitive price. The TL072 was cheap but too noisy. Today I would rather choose e.g. the OPA1642.
 
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ChristThePhone said:
That opamp was the standard back in the days and there was no real alternative with sufficiently low noise floor at a competitive price. The TL072 was cheap but too noisy. Today I would rather choose e.g. the OPA1642.
Click to expand...

Thanks for all your help. Two last questions, if you don’t mind: any other recommendations and (at the risk of sounding stupid) will they physically fit on the board as a straight swap?

Thanks loads.

Si.
 
Uhm, no. Today's modern opamps are not produced in DIP through-hole packages anymore but SMD only. There are adapter PCBs available to make an SMD chip fit into a DIP socket. Requires some soldering, though.

Edit: if you can get an LT1113 or an OPA2228 then it would be a plug-in replacement
 
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Just wanted to follow this up to say thanks for the advice. I tried both op-amps (installed a dip socket for quick swaps) and swapped the parallel resistor to 1 meg. Both worked like a charm, but with subtle differences. Settled on the LT1113.

As a total noob to electronics, this, and the reading I've been doing on it lately, has got me really curious, and may be the start of a whole new realm of geekdom. My wife will be pleased!

Thanks again.

Si
 
Glad it works for you. Keep posting your discoveries. I know there are a few DIY geeks around here that would be interested as well.
 
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