JLM DiNGO Questions

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Gold
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JLM DiNGO Questions

Post by Gold »

I have a couple of questions about some of the finer points of construction this. I am using these as Input Amplifiers and Output Amplifiers in my console. I have a 5k bridged T rotary switch for gain.
Image

My questions have to do with R1 and R2 and R21 and R22.

I think R1 and R2 are there for a DC bias path for the Op Amps. I've seen similar circuits without these resistors. Question one is: are they necessary?
Question two is If they are necessary how do the affect the input impedance? Are they seen in parallel with R3 and R4 since R3 and R4 are looking at virtual earth ?

Now the output. I think R21 and R22 are there to provide a drain path for C12 and C13. I have basically the same questions as the input. If the output is always connected and never left hanging are R21 and R22 necessary. If they are necessary how do they affect the output impedance?

BTW I am using 0.1% resistors in positions R3-R6. I am also using 0.1% resistors for R19 and R20. I like the "dumb" output stage. Everything in the system is balanced.
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JR.
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Re: JLM DiNGO Questions

Post by JR. »

Gold wrote: Tue Dec 12, 2017 3:41 pm I have a couple of questions about some of the finer points of construction this. I am using these as Input Amplifiers and Output Amplifiers in my console. I have a 5k bridged T rotary switch for gain.
Image

My questions have to do with R1 and R2 and R21 and R22.

I think R1 and R2 are there for a DC bias path for the Op Amps.
That DC path is blocked by C1 and C2 so not for benefit of the op amps. Perhaps to discharge output caps from previous gear.
I've seen similar circuits without these resistors. Question one is: are they necessary?
perhaps to prevent clicks. or not.
Question two is If they are necessary how do the affect the input impedance? Are they seen in parallel with R3 and R4 since R3 and R4 are looking at virtual earth ?
In parallel but input Z of of diff amps depends on which input or how you look at it (normal, CM, etc).
Now the output. I think R21 and R22 are there to provide a drain path for C12 and C13. I have basically the same questions as the input. If the output is always connected and never left hanging are R21 and R22 necessary. If they are necessary how do they affect the output impedance?
yes... if no transformer.

If following gear is constantly connected they are not doing anything useful.
BTW I am using 0.1% resistors in positions R3-R6. I am also using 0.1% resistors for R19 and R20. I like the "dumb" output stage. Everything in the system is balanced.
Precision Rs (and Cs) will help CMRR on input differential... output build out Rs not as critical to use precision.

JR
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Gold
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Re: JLM DiNGO Questions

Post by Gold »

Thanks John. I guess I'll forgo R1 and R2. I'm using 10k resistors for R3-R6. I want a 10k input impedance (regular) so putting R1 and R2 in parallel will lower the input impedance which I don't want to do.

I'll also leave out R21 and R22. The monitor path of the console is passive. I figure since there are decent length cable runs and a trip through a monitor attenuator on the way to the cable run to the speaker amps the accuracy of the source impedance will help.
Gold
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Re: JLM DiNGO Questions

Post by Gold »

I have a question about DC offset at the output. For testing I'm using 5532's in both positions. R19 and R20 are both 47.5R. C12 and C13 are both 470 uF. When I measure DC offset on the 5532 at Output pins 1 and 7 I get 0.001VDC or there about on both legs. The meter is floating and meter common is on audio common. I get the same readings on each side of R19 and R20. If I remove C12 and C13 and jumper the pads I get the same thing at the output. As would be expected. Every time I install C12 and C13 the measured DC offset at the output increases. Usually by an order of magnitude like 0.01VDC.

Why? I didn't expect any DC to get through the cap let alone have the value increase.
Gold
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Re: JLM DiNGO Questions

Post by Gold »

A follow up question. Do op amp DC offset characteristics change with environmental conditions or over time? If I measure a value low enough to eliminate the output capacitors should I expect that value to hold?
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JR.
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Re: JLM DiNGO Questions

Post by JR. »

Gold wrote: Fri Dec 29, 2017 10:16 pm A follow up question. Do op amp DC offset characteristics change with environmental conditions or over time? If I measure a value low enough to eliminate the output capacitors should I expect that value to hold?
Generally yes....

JR
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Gold
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Re: JLM DiNGO Questions

Post by Gold »

I phoned a friend about the question about measuring more DC offset after the caps. I thought it might be a measurement error but my friend said it was probably a residual charge across the caps. I should install R21 and R22 to bleed the charge. Now I know.
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Re: JLM DiNGO Questions

Post by Gold »

After installing R21 and R22 the DC offset measurements are what I expected. I then switched op amps from 5532's to LME49720. I'm getting no more than 1mV DC offset on the eight IC's I've tested. I've been told that less than 2mV offset generally isn't a problem. Do you think it's safe to eliminate the output caps?
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JR.
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Re: JLM DiNGO Questions

Post by JR. »

I can't say that I have been following close attention to changes made and unmade, then made again.

To answer the specific question <2mV of DC is generally not a problem.

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Gold
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Re: JLM DiNGO Questions

Post by Gold »

Thanks John. That answers the question.

A bit OT but is there a standard graph that shows how source impedance affects noise performance?

I have a 5K source impedance before the BJT input LME 29720. I want to see if a JFET input opamp would be quieter. What do I look for?
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