Adding DRV134 balanced circuit to the preamp.

[nguitar12]

Hey guys so recently I have completed my build on the ina217 preamp.
I am now considering adding a unbalance to balance circuit using a DRV134 chip.



However I am not sure if there are anything good on doing this. From what I understand a fully balanced preamp. The hot and cold signal should be amplify by different opamp. Obviously INA217 have converted the mic signal into a unbalanced one. It is too stupid to convert it back into balanced signal using DRV134? Will it consider to be a 'fake balanced" signal?

If not. Will the quality of the sound increase if balanced circuit is added?

[tubegeek]

Using separate op-amps for hot and cold is done in the typical 3 op-amp "instrumentation amplifier" design, is that what you are referring to?



There's nothing "fake" about converting unbalanced to balanced using DRV134, that's exactly what it's designed for. Some designs keep the signal balanced all the way through the preamp but this is unusual nowadays, and not really necessary. Notice that the classic instrumentation amplifier produces an unbalanced output.

I saw an article in Linear Audio, Volume 5. "The G Word, or How to Get Your Audio off the Ground" by Brunp Putzeys. This article describes some board layout techniques for treating unbalanced signals in a fashion similar to balanced signals, you might want to seek that article out or some of their other articles about mic preamp design.

Will the quality of the sound increase if balanced circuit is added?

The answer depends on the rest of the system.

If the output of your preamp is doing something that balancing will help with - for example, driving a long cable, then balanced output will improve the noise behavior of the system so long as the input it is driving is also balanced.

If not then you won't really get a benefit. So if the next item in the signal chain is an unbalanced input a short distance away, it will not usually make a difference or in fact cost a small noise penalty compared to unbalanced out.

This is why mastering studios are wired unbalanced->unbalanced these days: no need for balanced, and slightly lower noise.

[nguitar12]

Thanks after reading your reply I decided to add the DRV134 into my circuit. I will keep the unbalance out from the INA217 in case of need.

I am using this preamp on a recorder with balanced XLR input. However the length will not be too long probably 2-3m in total. So will a unbalanced signal keep the signal as original as possible  since it do not need to pass through an extra circuit?

On more question. I have some problem on the cap selection on DRV13. Can I use a polarized cap in replaced of the non-polarized one? i don't seem to be able to find a non-polarized cap have such high value. What is the different on using the non-polarized one?And if a polarized cap can be use. What is voltage rating of them?

[nguitar12]

[ reposted please delete ]

[tubegeek]

The data sheet for the DRV134 would be worth looking at, I think.

The way nonpolar electrolytics are constructed is with two caps in series, facing towards the opposite polarities. To make a 10uF non polar electrolytic you can take two 22uF standard electrolytics and connect them with either both + leads connected together or both - leads. That'll get you an 11uF NP electro.

A likely source for a prepackaged 10uF NP would probably be a parts supplier for speaker building like Parts Express, non polar electros are often used in cheap & cheerful crossovers.

[nguitar12]

The data sheet for the DRV134 would be worth looking at, I think.

The way nonpolar electrolytics are constructed is with two caps in series, facing towards the opposite polarities. To make a 10uF non polar electrolytic you can take two 22uF standard electrolytics and connect them with either both + leads connected together or both - leads. That'll get you an 11uF NP electro.

A likely source for a prepackaged 10uF NP would probably be a parts supplier for speaker building like Parts Express, non polar electros are often used in cheap & cheerful crossovers.

Thanks for your tips on making nonpolar cap. So in this case will a standard polar cap works on DRV134? If so what is the voltage rating?

[tubegeek]

Thanks for your tips on making nonpolar cap. So in this case will a standard polar cap works on DRV134? If so what is the voltage rating?

The data sheet says: "For applications with large dc cable offset errors, a 10-µF electrolytic nonpolarized blocking capacitor at each
sense pin is recommended as shown in Figure 29." That doesn't seem to be a very common problem, in other words, that cap may not be needed at all.

[nguitar12]

Thanks for your tips on making nonpolar cap. So in this case will a standard polar cap works on DRV134? If so what is the voltage rating?

The data sheet says: "For applications with large dc cable offset errors, a 10-µF electrolytic nonpolarized blocking capacitor at each
sense pin is recommended as shown in Figure 29." That doesn't seem to be a very common problem, in other words, that cap may not be needed at all.

Thanks for you kind help. I decided to connect two 22uf 16v electrolytic cap in series as suggested.
However I am not quite sure whether a 16v rating is enough as I am keeping the 48v phantom power.
I am assuming that 48v is not passing through the DRV134 right?

[tubegeek]

The caps in the input circuit adjacent to the phantom power keep the phantom power out of the preamp and in the mic where it belongs. The caps we've been discussing seem to be to protect the DRV134 from DC in whatever it is connected to.

[nguitar12]

The caps in the input circuit adjacent to the phantom power keep the phantom power out of the preamp and in the mic where it belongs. The caps we've been discussing seem to be to protect the DRV134 from DC in whatever it is connected to.

Thanks for your detail explanation. I will use a 16v rating cap in this situation.

[PRR]

> a fully balanced preamp. The hot and cold signal should be amplify by different opamp.

I think you are over-simplifying.

Even the few "totally balanced" twin-tube and transformer preamps cross-connect the two sides.

> converted the mic signal into a unbalanced one. It is too stupid to convert it back into balanced signal

Not "stupid". "ALL" audio electronics (few exceptions) convert to unbalanced for all internal operations, going balanced only at the input and output jacks.

Balanced is really for rejecting external garbage. Microphone wire signal is very weak, thus is likely to pick up garbage (power hum/buzz, clicks from motor switching, high-level signals such as speaker lines). In small studios in electrically-quiet rooms, even mike signals "can" be run unbalanced. I used to do it in auditoriums and churches over long distances.

However "balanced" does solve some problems with garbage getting into signals, also with interconnecting un-balanced with balanced connections, and low-cost "balanced" solutions (like DRV134) have made this common in modern equipment. So you "want" a balanced output. However as TG says, it is wise to keep the un-balanced signal available for connections which do not "need" balanced signals.

The DRV is certainly not "fake". It is the modern standard for "balanced" signals, and equivalent to (often same-as) what all modern electronics use to give "balanced" outputs.

I am an old guy. In my opinion, the Gold Standard for truly-Balanced operation is a large expensive Transformer. They tolerate many odd connections, they block huge ground differences (like different buildings on different power lines), they absorb radio interference. They also limit bass and treble extension and add bass distortion. Very-good audio transformers are super-expensive, also may be large and heavy.

[nguitar12]

> a fully balanced preamp. The hot and cold signal should be amplify by different opamp.

I think you are over-simplifying.

Even the few "totally balanced" twin-tube and transformer preamps cross-connect the two sides.

> converted the mic signal into a unbalanced one. It is too stupid to convert it back into balanced signal

Not "stupid". "ALL" audio electronics (few exceptions) convert to unbalanced for all internal operations, going balanced only at the input and output jacks.

Balanced is really for rejecting external garbage. Microphone wire signal is very weak, thus is likely to pick up garbage (power hum/buzz, clicks from motor switching, high-level signals such as speaker lines). In small studios in electrically-quiet rooms, even mike signals "can" be run unbalanced. I used to do it in auditoriums and churches over long distances.

However "balanced" does solve some problems with garbage getting into signals, also with interconnecting un-balanced with balanced connections, and low-cost "balanced" solutions (like DRV134) have made this common in modern equipment. So you "want" a balanced output. However as TG says, it is wise to keep the un-balanced signal available for connections which do not "need" balanced signals.

The DRV is certainly not "fake". It is the modern standard for "balanced" signals, and equivalent to (often same-as) what all modern electronics use to give "balanced" outputs.

I am an old guy. In my opinion, the Gold Standard for truly-Balanced operation is a large expensive Transformer. They tolerate many odd connections, they block huge ground differences (like different buildings on different power lines), they absorb radio interference. They also limit bass and treble extension and add bass distortion. Very-good audio transformers are super-expensive, also may be large and heavy.

Thanks for your detail explanation. I successful combined the DRV134 circuit into my preamp. I don't see any improvement on the sound quality except the output (db) is higher. Maybe the difficult is more significant when the cable is longer. I am still keeping the unbalanced output.

Is there any bad when both balanced and unbalanced output is in use at the same time? Will both the output become weaker as they are sharing the same signal source?

[PRR]

The INA127 can probably drive a 2K load (read the specs).

The DRV input is 10K impedance (read the specs).

There will be hardly-any bad effect until load on INA comes below 2K. Allowing 10K for DRV, you can drive another 2.5K of load. Modern unbalanced inputs are 10K-22K. You could hang four such loads on the INA output without strain. Small studios rarely need one source to four places. A mike preamp should typically go right to the "tape deck" (or digital thingie), plus maybe a monitor load.