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Feedback resistor size - opamps
Feedback resistor size - opamps
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Old 26th November 2013, 01:36 PM   #11
DUG is offline DUG  Canada
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Is the op-amp able to drive 156R.

Not all can do this.

Re: post # 4 from dirkwright

Find out what the op-amp you are using can drive and go from there.

What op-amp were you considering using?

What is your load?
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
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Old 26th November 2013, 02:35 PM   #12
xslavic is offline xslavic  Moldova
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i intend to use only opa1612 because i like how it sound,has also very good specs
-i always set the gain opamp -1st, as multiple feedback LPF (with 56 and 100) i WOULD get excellent q factor and very low noise.
Also intend using Opa1612 for buffer ,inverting unity gain voltage follower.
The problem is that datasheets don't specify always minimum feedback res like for LT1028.
My output voltage should be 6V RMS direct connection to amplifier that are found as kit.
input can be a smartphone or My audiophile DAP the DX50 ,intend to use same layout for DAC I/V
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Old 26th November 2013, 02:45 PM   #13
Mooly is offline Mooly  United Kingdom
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Feedback resistor size - opamps
The data sheets will specify a maximum output current and that value can be used to derive the minimum feedback network resistance/impedance the device can drive. For minimum distortion you probably need to increase those calculated values by at least 100%. And don't forget the load of whatever the preamp is connected too.

For a unity gain inverting buffer an acceptable "Rin" partly decides your choice because that resistor value determines the input impedance.
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Old 27th November 2013, 01:40 AM   #14
DUG is offline DUG  Canada
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As Mooly suggested, the data sheet shows about 40mA max current...maybe 45mA if you can keep it cool.

6Vrms is 8.484Vp

8.484Vp / 0.04A = 212.1 ohms.

This number would be what I would use for the absolute lowest value of resistance for the load. (feedback resistors and connecting cable capacitance and amplifier input impedance)


There is little data on power dissipation or junction to ambient (or board) thermal impedance.

You might heat up the amp when operating at this level with this load.

If you still want to try it, please keep a finger on the amp.

If it is too hot to touch, there is a problem. (IMHO)

You could reduce the supply to about 12V to reduce heating if it becomes a problem.

Good luck.

Doug We are all learning...we can all help
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Old 27th November 2013, 01:59 AM   #15
sreten is offline sreten  United Kingdom
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Its utterly inane to to use a feedback loop that is harder to drive
than the intended load here, here minimum 3 times the load.

A high impedance feedback loop will add noise, so for a
headphone amplifier 5 to 10 times load is very reasonable,
though you can go higher and most do, without any real issues.

Power amplifiers get along fine with a ~ 30K feedback loop,
with an ~ 1K input resistor defining noise, so shedloads
less lop loading, many headphone amps the same.

rgds, sreten.

Last edited by sreten; 27th November 2013 at 02:26 AM.
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Old 27th November 2013, 09:38 AM   #16
AndrewT is offline AndrewT  Scotland
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40mA is a typical short circuit output.
When you look at output voltage vs load resistance you will find that gross distortion sets in soon after going beyond half the short circuit current.
i.e. the usable maximum output current is probably around 20mA.

A 600ohm resistive output load in parallel with a 2k feedback resistor results in ~460r
6Vac pushes ~18mA through that combined load.
You then need to add on an allowance for capacitance, both parasitic and deliberate, that takes more current from the output stage.
regards Andrew T.
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Old 27th November 2013, 10:44 AM   #17
DF96 is offline DF96  England
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There is more to electronic design than taking your favourite opamp and placing it in a seriously suboptimal circuit. A poor opamp in a good circuit is likely to outperform a wonderful opamp in a bad circuit. Circuit theory comes first. Get that right, then start to worry about minor issues like opamp fashion.
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Old 1st January 2014, 07:16 AM   #18
gman76 is offline gman76  United States
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Lets get specific. How much gain in the opamp circuit? What is the load? If this is a headphone amp, it is likely the gain can be set to ~5. So, I wouldnt worry much about noise gain. If your headphones are >35 ohms, you can probably use an LT1497 which current limits at 125mA or so. I'm building a simple headphone amp based on the 1497. Powering it from a micro USB, takes +5 in and create a -5 rail. No coupling caps required is the big benefit here, caps take up space and provide a significant source of potential distortion (depends on dielectric type, voltage rating, etc). In my case, I hv audio technica M35s which are 65 ohms and almost purely resistive (i measured). I needed something small and light to provide a gain of 5 (or so) from my iphone while I sit at a computer. Sometimes I need to really crank the tunes. I'll get back with results after the build.

Last edited by gman76; 1st January 2014 at 07:20 AM.
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Old 19th January 2014, 02:43 AM   #19
gman76 is offline gman76  United States
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Update: Here's the board I built (1.8"x2.8") that is USB powered that derives the +5V to generate a split supply, +/-5V. I have audio technica M35 headphones which are 65 ohms. I wanted to have a gain of 5 to boost the level from my iphone, and both intended to be powered from a computer's USB ports. Sometimes I like to crank it at work. The max output is ~2.5V rms which is close to the max allowed for the headphones, 100mW.

I tested the amp and looked at the supply rails while pushed to the max. The inverting switcher does have occasional problems under severe rock passages with heavy bass. More decoupling is needed. Also plan to use a mini USB connector instead of micro B. Also looking into seeing if the 3483 can be operated in continuous mode (runs burst mode now). While music was running, I put a scope on the output. When I saw the drops outs of the -5V rail, I was somewhat surprised how little effect it had on the sound. The rail was dropping hundreds of mV. The op-amp's PSRR at work. But as you can see from the scope shot, the peaks were hitting about 3V which was very loud. Let me add, this little amp sounds very clean.
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