Guitar opamp Pre-amp input resistors - diyAudio
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Old 29th August 2013, 11:13 PM   #1
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Default Guitar opamp Pre-amp input resistors

Hello, noob here.

I am using 1/2 TL072 at a gain of 8 as a pre-preamp feeding a high gain tube pre, for the purpose of reducing noise floor hiss.

In an effort to reduce it further, it has been found the input resistors are contributing a good portion of noise, regardless of whether anything is plugged in or not.

Experimenting, reducing the parallel value of the 68k and 100k resistors, to ~10k, the hiss is greatly reduced.

What I don't understand, and could not find out by research, was how the parallel pair and the coupling cap suppress RFI, or if reduction would cause any performance issues. I wish to reduce the values to 6.8k and 10k, but don't know if the cap value should be adjusted as well.

P27B Input Section .JPG

I know there are quiter opamps, but I wish to order lower value, low noise resistors at the same time as the opamp. I am tempted to go with foil$ but my budget won't allow. Paralleling is another option which averages the thermal noise; thus suppressing the total when they add.

Also, would there be any advantage to using the other half as a buffer?

Any comments welcome. Thanks.
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Old 29th August 2013, 11:44 PM   #2
Zero D is offline Zero D  United Kingdom
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Quote:
In an effort to reduce it further, it has been found the input resistors are contributing a good portion of noise, regardless of whether anything is plugged in or not.
Hi,

The 68K & 100K should't be contributing any noise on their OWN, if nothing is plugged in ! It's possible some RFI or other interference is being picked and is the cause ?

Quote:
how the parallel pair and the coupling cap suppress RFI,
They don't.

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or if reduction would cause any performance issues.
It shouldn't, but the gain will be less.
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I wish to reduce the values to 6.8k and 10k, but don't know if the cap value should be adjusted as well.
I would to 470nf.

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I know there are quiter opamps,
Use a low noise FET input OpAmp because in high impedance circuits they actually produce LESS noise

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1% metal film resistors will be just fine
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Paralleling is another option which averages the thermal noise; thus suppressing the total when they add.
It's the resistance that REALLY matters. Higher = more noise

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Also, would there be any advantage to using the other half as a buffer?
Where ?
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Old 30th August 2013, 12:59 AM   #3
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Thank you for the tips!

Quote:
Originally Posted by Zero D View Post
The 68K & 100K should't be contributing any noise on their OWN, if nothing is plugged in ! It's possible some RFI or other interference is being picked and is the cause ?
This shoud be checked. The problem seems to be the 100k only. The 68k is very close to its value and shorting that one does nothing; just touching the 100k definitely picks up some RFI and a little 60 cycle. I will switch the 100k with a low noise and see what happens. If nothing, then some shielding. The aluminum box is open during these experiments.

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Originally Posted by Zero D View Post
It [effects of reducing input resistor value]shouldn't, but the gain will be less.
Sounds like the pickups would be loaded down. Didn't consider that. Thanks.

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Originally Posted by Zero D View Post
Use a low noise FET input OpAmp because in high impedance circuits they actually produce LESS noise
Ok, got two different ones in the chamber, but where is the "high" impedence that you refer?

Quote:
Originally Posted by Zero D View Post
It's the resistance that REALLY matters. Higher = more noise
I meant, in lieu of using a higher wattage rating, use multiple resisitors in parallel--whose total resistance is equal to the single component--as an alternative.

Quote:
Originally Posted by Zero D View Post
Where ?[other half as buffer]
A unity gain at the output of the first half, since there is some loading by the feedback circuit.
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Old 30th August 2013, 01:23 AM   #4
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Just a couple of things to consider. Changing the value of the two "input" resistors will have minimal effect due to the 1 meg resistor being in series.
And the corner frequencies of the input and feedback circuits are kinda bass-ackward. Input Fc is about 3Hz, and feedback Fc is about 33Hz. In that configuration the opamp could be fed signals that the feedback network will never see, in other words, increased noise and distortion. Put a 33uF cap in place of the 1uF in the feedback path to fix it, the input Fc should be at least two or three times the feedback Fc for proper operation.

Mike
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Old 30th August 2013, 01:45 AM   #5
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Quote:
Originally Posted by Michael Bean View Post
Just a couple of things to consider. Changing the value of the two "input" resistors will have minimal effect due to the 1 meg resistor being in series.
The 1 Meg is in parallel with the input transducer. The one feeding the non-inverting input is 1k.

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Originally Posted by Michael Bean View Post
And the corner frequencies of the input and feedback circuits are kinda bass-ackward. Input Fc is about 3Hz, and feedback Fc is about 33Hz. In that configuration the opamp could be fed signals that the feedback network will never see, in other words, increased noise and distortion. Put a 33uF cap in place of the 1uF in the feedback path to fix it, the input Fc should be at least two or three times the feedback Fc for proper operation.
Feedback path filtering is not intuitive for me yet. However, I will make a note of this, since high gain designs can suffer from seemingly small details. It also makes perfect sense.

Thanks Mike!
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Old 30th August 2013, 02:38 AM   #6
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Quote:
The 1 Meg is in parallel with the input transducer. The one feeding the non-inverting input is 1k.
No, the 1 Meg is in series with the input transducer. The inputs on that opamp are something like twelve gigaOhms, so the 1k resistor one the non-inverting input is, for all practicle purposes irrelevent in this regard, there's no current through that 1k resistor.

Mike
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Old 30th August 2013, 02:55 AM   #7
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Quote:
Originally Posted by Michael Bean View Post
No, the 1 Meg is in series with the input transducer. The inputs on that opamp are something like twelve gigaOhms, so the 1k resistor one the non-inverting input is, for all practicle purposes irrelevent in this regard, there's no current through that 1k resistor.

Mike
Yes, it occurred to me afterward that's what you meant. Even that is voodoo for me at this time.

What I did is take on a rather complicated project for first time, without studying enough of the fundamentals.

At any rate, I tried closing the cover and got some howling. Snipped a few unnecessary leads on the "ghetto wiring", placed a flat copper shield over the opamp board, and there is no howling with the cover almost closed. The tube sockets are on the cover adjacent to the tube-preamp board.

The socket of V1, which is now the second stage, should also be shielded, along with its peripheral components on the tube-preamp board. It was found when the amp was all-tube, V2 was rather sensitive and benefited from shielding. I used an aluminum stomp-box housing upside down without the cover, and it worked good; mostly for the 60 cycle hum. But if it picks up that, it must also be prone to RFI.

Also, the feedback resistor commonly referred to as R1 (the smaller value) is currently a carbon slide pot. I think it should be gotten rid of. Touching the metal housing makes noise, and picks up some 60 cycle hum from the open toroid that has yet to acquire its metal can.

Some wiring should also be shielded, and the perf board also has unecessary lengths of lead ends in hopes to save the components for reuse after they are pulled.

The preamp is very sensitive. It should be completely encapsulated with mumetal or "mag shield" and copper which meets with my "overkill" tradition. 3/32" aluminum has its limitations.

Thank you for the clarification on the resistor relationship.
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Old 30th August 2013, 03:11 AM   #8
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It's all good, we're all still learning, we all start at the bottom and work our way up, and nobody ever reaches the top.
One thing that might help is to make sure that your circuit paths don't create any large loop areas, for example, the leads on a 9 volt battery should be twisted together to keep stray inductance low and noise pick-up antenna effect minimal. This principle should be applied wherever possible within the whole circuit to keep noise and distortion low.

Mike
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Old 30th August 2013, 03:26 AM   #9
Zero D is offline Zero D  United Kingdom
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Quote:
just touching the 100k definitely picks up some RFI and a little 60 cycle.
It will when not connected. It's the 1Meg resistance that causes the issues you hear. The input impedance has to be 1Meg though to load the guitar coil correctly. The GC resistance should be in parallel with the 1Meg, as far as i'm aware, so the actual input resistance as seen my the OpAmp is around 10 times less. Well it would be without those extra resistors you put in series.

You don't need the 1k resistor.

But do NOT get hung up on resistor noise, in fact you can forget it, & about using resistors in parallel. Also 1/2 - 3/4 Wattage resistors should be fine.

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Ok, got two different ones in the chamber, but where is the "high" impedence that you refer?
I overlooked the fact that you're already using an FET OpAmp = TL072. If it wasn't i was talking about changing it for one.

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A unity gain at the output of the first half, since there is some loading by the feedback circuit.
You could do, but i don't think it will make any difference.

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Touching the metal housing makes noise, and picks up some 60 cycle hum from the open toroid that has yet to acquire its metal can.
It will, so don't. You need to screen the whole circuit in a metal box & ground it via the signal ground. Also when the pot thread is screwed onto it the problem will disappear.
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Old 30th August 2013, 04:01 AM   #10
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Originally Posted by Michael Bean View Post
It's all good, we're all still learning, we all start at the bottom and work our way up, and nobody ever reaches the top.
One thing that might help is to make sure that your circuit paths don't create any large loop areas, for example, the leads on a 9 volt battery should be twisted together to keep stray inductance low and noise pick-up antenna effect minimal. This principle should be applied wherever possible within the whole circuit to keep noise and distortion low.
Yes, but I should have acquired some books on guitar preamps, and read a few more articles and forum posts. If I had, I would have not used the bootleg SLO-100 preamp board, since it's much easier to modify a turret board (Aiken's suggestion).

This amp is sensitive to loops and lead placement. I really should start setting the final harness and see what happens.

Everything is twisted or shielded. I would use twisted/shielded pair, but there is only one place that it can be used, and it's so far down the chain, it makes no difference. The second conductor could be grounded to improve the shielding. I heard of the technique used when the shield is ground and connected at both ends.

If it looks like I will stick with the opamp-preamp arrangement, then I will order a real board made for this kind of thing. ESP has one and is reasonably priced. It has sockets for two opamps, but I can use the second one for an FX loop recovery stage.

The carbon slider was replaced with a metal film but to no affect. Anyway, the board and most of the peripheral opamp components are antennas, for sure.

There are now "low noise" capacitors by Wima. What distributor has them, I don't know? I would use them around the opamp just for GP.

Thanks much for your suggestions and encouragement.
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