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Searching for high audio quality low noise low power opamp for DI box upgrade
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Mark Hennessy:
As I was gently trying to steer you towards, and as Alex said outright, the problem is those 100k resistors.

You say you're not a pro, so you hopefully won't mind if I suggest you read up about thermal noise: https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise

Basically, the higher the value of resistance, the more Johnson (thermal) noise it makes. There are online calculators that save you having to do the maths. For example: http://www.sengpielaudio.com/calculator-noise.htm

A 100k resistor at room temperature makes about 5.7uV of noise - that's about -103dBu. Assumed bandwidth of 20kHz.

By contrast, a 10k resistor makes 10dB less.

Now, as Alex pointed out, swapping op-amps might only make a small difference - he suggested 2dB, which if realised in practice is hardly noticeable in reality. Whereas changing all those 100k resistors for 10k resistors will make a lot more difference. Resistors are cheaper than op-amps, which I hope is good news!

That was the point I was trying to make earlier - there are many DIs using the same TL06x op-amps that don't have noise problems, therefore the problem with the DI20 is not the op-amp. I hoped that logic was simple to follow...

So what would I change?

Those 100k resistors need to go. I'd make them 10k or less. In fact, I'd make the first one (R13B) 10k, and R13D 1k. I'd do the same for R13A and C. But of course, R13 is a resistor pack, so that might not be physically easy...

Also, R19 and R21 also need to be reduced from 100k to 10k. If you were feeling especially diligent, you might also increase C6 and C9 by a factor of 10.

Having done that, the input of IC1A is now a bit exposed, so I'd add protection diodes from the junction of R13B and R13D to ground and the supply rail.

With this done, the DI will present an input impedance to unbalanced sources of 21k when the pad switch is set to 0dB, but it will fall to ~10k when the pad is engaged (if you want to use the pad, you'll obviously have to reduce R9 and R7 by a factor of 10, naturally). But if you're feeding this from an FX or preamp pedal, then Zin of the DI is not an issue.

I'd also increase R2A and R2B to 1k or more, as the TL064 struggles to differentially drive a 1k2 mic pre-amp. A lot of DI boxes make this mistake, resulting in more distortion than is necessary.

Doing this will reduce the noise considerably more than any op-amp swap could ever do. The noise contribution from the 100k resistors is swamping the op-amp noise - even a relatively unsophisticated op-amp like the TL064.

Frustrated with the "schoolboy errors" in many commercial DIs, I make my own. It's really not difficult. I get the impression that in big companies, the DI design jobs get given to the new engineers while the experienced guys work on the high-value gear. That's why small companies like Orchid, who I mentioned earlier, make excellent DIs, as like me, they've actually thought it through...

Does that help?
David Hess:

--- Quote from: 3dBi on September 03, 2019, 09:03:52 pm ---If there is any other component change ideas for better, do tell me.
--- End quote ---

It is not a matter of changing some values.  Within the design parameters they set, the values are correct for good performance.

Instead a different circuit configuration would be required.
Mark Hennessy:
If you wanted to maintain a high input impedance - for use with an instrument pickup directly - then you could do it quite easily. You'd lose the ability to feed a balanced signal in, and would have to short the earth lift switch. The the pad switch would no longer function. If those constraints are OK, here goes:

Replace R19 with a short. Remove R13 A to D completely, and install a 10k resistor from the +ve end of C16 and pin 3 of IC1A. Increase R21 to 1M.

This changes IC1A from a differential amplifier into a simple follower. Input impedance is now 500k (R21 in parallel with R3D). Simple.

I'd still increase R2 A-D to 1k or even 2k2. This reduces loading distortion. I'd also still consider protection diodes at the input to the IC1A. C16 can be reduced to 10n or thereabouts.

These changes would give the lowest noise possible from the circuit as it stands. But you could lower it a bit more by talking IC1C out of circuit - to do this, connect point X5 to the output of IC1A - not sure how practical that would be as it's been a while since I was last inside one, but if it's possible, then give it a go. In fact, you could do this if following the instructions in post #10 instead.

Most DIs use a dual op-amp, the first being a unity gain follower, the second being a unity gain inverter. - the "hot" coming from the first op-amp and the "cold" coming from the second. I have no idea why Behringer used 3 op-amps in their design - it just adds noise for no obvious benefit.

For an example of a simple dual-op-amp design, have a look here: https://sound-au.com/project35.htm

Note that Rod doesn't aim for a high input impedance - not idea why, but that's easy to fix. Also, R8 and R9 are a bit on the low side for a TL072 working into a 1k2 differential impedance. But apart from that, you can see that a DI is a very simple thing to make - I've literally no idea what the DI20 designer had in mind. If they were set on a differential input, they could have used a pair of op-amps to buffer the incoming hot and cold, then made a diff-amp using sensibly low-value resistors (10k or less), then used the 4th section to generate the inverted cold output signal. There's no need to use an op-amp to make a virtual earth (IC1D) and they could have implemented the battery LED flash circuit differently.

The nice thing about this being a dual DI is that you can leave one channel original while making changes, so comparing the before to after is dead simple - no need to rely on memory.

But if the above suggestions sound like hard work, then a different DI is the easiest fix. Sadly, "op-amp rolling" is often a waste of time, as the problems with are usually elsewhere - and that's definitely the case here.
001:
Honey
this semy-balanced DI is piece of shit
try transformer-based unit
Bassman59:

--- Quote from: 3dBi on September 02, 2019, 08:41:52 am ---Hello, I have Behringer DI20 direct box and I wanted to make it a bit less noisy

--- End quote ---

Hi. Professional live-sound mixer-person here (who designs electronics as the day job).

You should use the Behringer DI20 for its intended purpose, which is to adjust the angle of stage monitors. Lift the end of the wedge, put the DI box underneath and then let the wedge rest on the DI.

Now go out and buy a Countryman or a Radial or a BSS DI box.
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