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Old 2nd March 2006, 02:14 PM   #1
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Default QUESTION ABOUT Rin IN ¨EL CHEAPO¨ PHONO PREAMP

Hi all.
I´m tweaking my Analog Addicts phono pre -a simple and superb little marvel-; I`m using it only as MM preamp, so the MC was deleted.
I would like to know what the function of the 681k input resistor is; that resistor is paralleled with the input cap and with another input resistor of 51k1.
I don´t know exactly if the removal of that part -the 681k resistor- (apparently redundant in my MM setup, because of the value of the other resistor, but I`m not an expert in this issue) would be a wise move.

Thank you very much in advance for your comments.
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Old 2nd March 2006, 04:08 PM   #2
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Old 2nd March 2006, 04:39 PM   #3
Raka is offline Raka  Europe
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A hint: Try to calculate the value of a paralleled 681K and the 51K.
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Old 2nd March 2006, 04:40 PM   #4
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You can go ahead and remove the 680k. And replace the 51k with 47k.
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Old 2nd March 2006, 05:18 PM   #5
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Default Rin...

Thanks, Raka and Analog. I had notice previously for the calculations of the paralleled resistors that the Rin was more or less 47k, so I supposed that eliminating 681k would be possible.
But then I begin to think that maybe the 681k resistor was more related to the RIAA eq than to the input... I´m not very skilled in circuit design, you know, but I was noticing something strange, or maybe superflous.

Now I know that it can be ommited, previously replacing the other resistor with 47k.

Thank you very much for your input.
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Old 2nd March 2006, 09:24 PM   #6
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Default Re: QUESTION ABOUT Rin IN ¨EL CHEAPO¨ PHONO PREAMP

Quote:
Originally posted by federico moreno
Hi all.
I´m tweaking my Analog Addicts phono pre -a simple and superb little marvel-;
I`m using it only as MM preamp, so the MC was deleted.
I would like to know what the function of the 680k input resistor is; that resistor is paralleled with the input cap and with another input resistor of 51k1.
If you have say 30-50 cm from RCA input jack to op-amp
and have MM resistor 51k1 and those components at the input jack,
and an internal piece of connection wire from this jack to circuit,
then is good to have 680k resistor as CLOSE as possible to op-amp INPUT PIN


If you have all components soldered close to op-amp,
than you do better replace 680k and 51k1
with a single 'normal 47k', which is put near to op-amp input pin.

In this case I would prefer to solder sheilded wire coming from MM phono, directly to the circuit.
And not at all use RCA jack.
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Old 3rd March 2006, 02:26 PM   #7
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Thank you very much, Lineup.

In the actual setup I have tried to minimize the signal paths to the best of my knowledge, but your input is a good tip just in case that a subpar layout was made.
Can I perform the same trick -that is, put an additional high value input resistor, close to the opa Vin, matched to obtain the recommended Rin, to better the performance of a VSPS that I made a year ago? That preamp has long traces between RCA`s and opamp, and is prone to hum an some HF hiss.
Another question -pardon for my curiosity, but I`m trying to understand better all those issues-: What the function of the 681k resistor is?
Is a way to avoid parasitics, or to better match the impedances of the components because of the low level signals involved?
I ask this because if it`s function is optimize the performance of long signal paths, is a really good tip to know, just in case...
In some line level preamp, I have seen, if I remember well, 47k resistors to gnd at the rca inputs, then some low uF dc filtering, followed by the pot, and then, at the opa input, some 47k again...
The designer of that pre was giving a special recommendation to those 47k resistors at the RCA plugs, saying that their inclusion vastly improved the performance of the circuit.
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Old 3rd March 2006, 04:13 PM   #8
lineup is offline lineup  Sweden
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Quote:
Originally posted by federico moreno
Thank you very much, Lineup.

In the actual setup I have tried to minimize the signal paths to the best of my knowledge, but your input is a good tip just in case that a subpar layout was made.
Can I perform the same trick -that is, put an additional high value input resistor, close to the opa Vin, matched to obtain the recommended Rin, to better the performance of a VSPS that I made a year ago? That preamp has long traces between RCA`s and opamp, and is prone to hum an some HF hiss.
Another question -pardon for my curiosity, but I`m trying to understand better all those issues-: What the function of the 681k resistor is?
Is a way to avoid parasitics, or to better match the impedances of the components because of the low level signals involved?
I ask this because if it`s function is optimize the performance of long signal paths, is a really good tip to know, just in case...
In some line level preamp, I have seen, if I remember well, 47k resistors to gnd at the rca inputs, then some low uF dc filtering, followed by the pot, and then, at the opa input, some 47k again...
The designer of that pre was giving a special recommendation to those 47k resistors at the RCA plugs, saying that their inclusion vastly improved the performance of the circuit.

Thorsten Loesch made this circuit and built it.
I can see two possible reasons for he uses 2 resistors instead of one.

As you see, he might have used a switch, MM or MC pickup.
Now when using a switch, you MUST HAVE resistors on both sides.
In the moment of switching, otherwise it will be a short period, where input of OP-Amp/transistor has no bias.
This could make some nasty things, loud bumps or even smoke!

So when having switch, use 1 resistor on each side of switch.
Or only one resistor close to OP-amp input.

But with other end of internal cable/wire unconnected to a source
all that lenght of wire will potentially act as an antenna.


Other reason for using 2 resistors, to form one impedance is termination of cables.
You know that 50 Ohm video cables are terminated in both ends.


And it has to do with capacitances and also that a cable unterminated at one end can act like an antenna.
Picking up hum and noise.


OP-amps like to have everything close to their pins.
Input Bias resistors and power supply pins bypass caps.
Especially JFET input Op-amps, like OPA2134 ... TL071 .. OPA627.

Actually I think it is same as with MOSFET transistors:
You put a 'Gate stopper resistor', and attach it with as short as possible lead as close as possible to the Gate-pin.
Then if needed you put a wire to PCB in other end of resistor.
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Old 3rd March 2006, 05:30 PM   #9
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Quote:
As you see, he might have used a switch, MM or MC pickup.
Aha! I understand... in that way the opa input never gets unbiased, not even when the switch is halfway and being operated...
And when you´re in any of the two selection positions of the switch, the opa is ¨seeing¨ some 47k of Rin because the two resistors are always in parallel.
Thanks for the tips about resistor location in switching schemes; sometimes one tend to overlook some relevant issues, till the smoking PCB sends a smoke signal telling ¨you dumb!¨

Besides that, yes, I have noticed that OPAS are very fussy regarding distances to asociated components: they like everything near -with the notable exception of ac transformers and wires-; opa637 is a notable example, making noises, getting horny and oscillating -including in relatively high gains- if not properly decoupled and caressed.
But if one take some caution, it sings like a bird!
I still remember my woofers slowly going up and down everytime the pot was moved in a previous line amp high gain design; that problem dissapeared when I decoupled them properly...

Very nice tips again, people. Thank you very much.
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