Searching for high audio quality low noise low power opamp for DI box upgrade - Page 1
Forum section: electronics / projects

author: 3dBi
author_rank: Contributor
timestamp: September 02, 2019, 08:41:52 am
content: Hello, I have Behringer DI20 direct box and I wanted to make it a bit less noisy, so decided to swap out crappy TL064 opamps. I had in hand OPA4134, so i soldered them in. All is well until you try to power the box with 48V phantom power, I think two OPA4134's draws too much current (Iq = 4mA per amp) and that is why circuit fails. Battery power works good as it should. I've attached a schematic, so you can check, if my logic is sound.So, could anybody point me in direction to high quality low power opamp? I found OPA1692, but quad version OPA1694 is still in development.Closest low current audio opamps I could found is OPA1664 with Iq = 1.5mA per amp, but it's nowhere close 200µA per amp on original design.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 02, 2019, 08:56:38 am
content: OPA4196 or OPA4191CheersAlex

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: September 02, 2019, 10:29:15 am
content: That particular DI box is notoriously noisy.Many other DI boxes use the TL06x op-amp, yet they don't have a noise problem. I've built many myself. The TL06x is perfectly adequate for this role. The TL07x is better if you can afford the higher current consumption (which you usually can if you use a dual rather than a quad).Rather than focusing on the op-amp, look to the (rather dumb) design of the DI20 to see why an op-amp swap is likely to be disappointing

author: 3dBi
author_rank: Contributor
timestamp: September 02, 2019, 10:37:43 am
content: I must have got a good DI20, it worked for me very acceptable with original TL064 too, noise I was talking about is opamp hiss, all these opamps (TL06x, TL07x, TL08x) have it so better opamp should get better results. In my main usecase I can loose phantom supply feature too, as it is powered by power supply trough battery connection all the time and reside on my bass guitars pedalboard. The reason I'm asking for this, is for any other usecase I might have one day, where phantom powering could be useful.Using dual opamp is out of question, because of existing board layuot. It has two quad opamps in SOIC-16 package.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: September 02, 2019, 11:01:40 am
content: Yes, I know it uses quad op-amps. I mentioned duals because many DIs use a dual opamp. Some even use a single (usually in conjuction with an output transformer).From memory, the DI20 is at least 10dB noisier than other DIs using the TL06x op-amp (I have the actual figure written down elsewhere - I have researched DI boxes extensively recently).So how do other DI boxes manage to be so much quieter, despite using the same op-amp?The DI20 is acceptably quiet if you're lucky with the signal level and don't need to put too much gain after it. Running from batteries limits the headroom, which might require the attenuator to be switched in, which in turn needs the desk gain to be increased, and up comes the noise. Really, it's a very poor design, and it's quite hard to improve it you need to maintain a high-ish input impedance. But if you're feeding this from a pedal, then hi-Z in isn't an issue, so that at least gives you some scope to reduce the noise...I don't know if they're available where you are, but this is a much better DI for not much more:http://orchid-electronics.co.uk/classic_DI.htm- it's only single, but based on what you've said about your application, that might be all you need. There's an in-depth review of it on SOS here:https://www.soundonsound.com/reviews/orchid-electronics-di-boxes- despite the excellent test (and subjective) results, it uses a TL072 - nothing fancy needed in a DI box that's properly designed.I'm just hoping to save you a lot of messing around and disappointment

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 02, 2019, 02:27:56 pm
content: Quote from: 3dBi on September 02, 2019, 10:37:43 amUsing a better opamp (i.e OPA4196 or OPA4191 I've suggested earlier) could improve the SNR by about 2dB, as the main noise sources are those 100K resistors around the input opamps, however the sound quality may improve as well, as the TL064 is not a very nice opamp sound-wise.CheersAlex

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: September 02, 2019, 04:04:49 pm
content: Quote from: Alex Nikitin on September 02, 2019, 02:27:56 pmIn a different context (e.g., output stage of a CD player), then maybe. But in this context? For starters, what will we likely find in those other things screwed to his pedal board?Granted, the TL06x - like the 07x and 08x - is horrible when driven into clipping because of input stage polarity swapping. That's why I'd always avoid running a DI on 9V battery power - some instrument pickups output pretty high levels. Probably less of an issue here if the OP is using pre-amps and FX pedals that are also probably running on 9V...But they also struggle to drive the 1k2 input impedance of a mic input stage - something else that the DI20 designer seemed to be unaware of.While I haven't tested them myself, the datasheet indicates that the OPAx196 and OPAx191 also struggle to drive low impedance loads. Of course, the load seen by each output op-amp section is 475R (100R plus 600R in parallel with the 1k phantom power pick-off resistors), which is pretty tough for even an NE5532. But easy to fix if you're aware of it.I stopped using DI20s years ago - they're just awful. Some other Behringer DIs are pretty reasonable by comparison - I'm not anti-Behringer as such. If I had a DI20 and wanted to make it half-way reasonable, then I'd swap a bunch of passive components but leave the op-amps in place as you're really into the realm of diminishing returns by that stage. The result would be something with a much lower input impedance whenever the pad is engaged, but given that the pad should only be needed with active sources (not pickups), that's a reasonable compromise.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2019, 03:05:17 am
content: So you want a lower noise replacement for the TL064. The design is not optimized for low noise but the TL064 sets an easy bar for improvement.Going by TI's selector, that leaves the OPA4196 or maybe OPA4171 if a little more current is acceptable. Analog Devices does not really have any suitable parts.

author: 3dBi
author_rank: Contributor
timestamp: September 03, 2019, 09:03:52 pm
content: If there is any other component change ideas for better, do tell me. I'm not a pro in electronics, to calculate impedances and upgrade designs myself, so I'm very open to suggestions. I will definately try to swap opamps again, if I can get them. If not, 4134 will stay and phantom will be disabled.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: September 03, 2019, 11:07:35 pm
content: 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_noiseBasically, 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.htmA 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?

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2019, 03:12:18 am
content: Quote from: 3dBi on September 03, 2019, 09:03:52 pmIt 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.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: September 04, 2019, 03:33:30 pm
content: 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.htmNote 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.

author: 001
author_rank: Super Contributor
timestamp: September 04, 2019, 04:15:08 pm
content: Honeythis semy-balanced DI is piece of shittry transformer-based unit

author: Bassman59
author_rank: Super Contributor
timestamp: September 05, 2019, 04:18:28 pm
content: Quote from: 3dBi on September 02, 2019, 08:41:52 amHi. 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.

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 05, 2019, 05:26:11 pm
content: Quote from: Bassman59 on September 05, 2019, 04:18:28 pm