DI Circuit w/ Active Volume

[Bo Deadly]

Consider the following circuit (click to enlarge):

Normally I would just copy an existing design but I want to make a DI for guitar that has certain features (like being able to attenuate so that it can drive a Fuzz and not just get massive blocking distortion).

The immediate concern I have is the noise gain of the active volume control. It's range is +-14dB but the noise gain is +16db and -1.5dB. So in the attenuated position, the noise gain would be 13dB more than a simple non-active control + buffer. Right?

So how is this sort of thing done normally? Is there a particular circuit that I should look at?

Criticism about other parts of the circuit are welcome. The switch with caps and 1M pot on the input are just supposed to setup the input impedance environment. This will ultimately connect to a relay switching network to other boards which could be anything. So the optional servo or coupling cap ensures zero offset.

 

[PRR]

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[Bo Deadly]

<snip schematic>

What is the gain adjustment range of this circuit? Does it attenuate and amplify equally or what?

 

[abbey road d enfer]

The immediate concern I have is the noise gain of the active volume control. It's range is +-14dB but the noise gain is +16db and -1.5dB.

+16, yes, -1.5, no. Min NG is +1.5dB (NG cannot be negative).

So in the attenuated position, the noise gain would be 13dB more than a simple non-active control + buffer. Right?

No. If you want to have a control range that goes up to +14dB, you need to have gain after the passive VC (potentiometer); the gain circuit there would have a noise gain of 16dB. The resulting noise would always be there, even with the pot turned down.

So how is this sort of thing done normally? Is there a particular circuit that I should look at?

So many ways to skin a cat...

The switch with caps and 1M pot on the input are just supposed to setup the input impedance environment.

I hope the 1k and 22u after the pot don't stay there...

 

[abbey road d enfer]

What is the gain adjustment range of this circuit? Does it attenuate and amplify equally or what?

Gain varies between total attenuation (minus infinity) and ca. 38dB. Beware that the output impedance varies significantly vs. gain (from 0 to about 2k). The use of a 4558 doesn't make it particularly low-noise, which is not an issue with electric guitar.

 

[JohnRoberts]

Consider the following circuit (click to enlarge):

Normally I would just copy an existing design but I want to make a DI for guitar that has certain features (like being able to attenuate so that it can drive a Fuzz and not just get massive blocking distortion).

The immediate concern I have is the noise gain of the active volume control. It's range is +-14dB but the noise gain is +16db and -1.5dB. So in the attenuated position, the noise gain would be 13dB more than a simple non-active control + buffer. Right?

So how is this sort of thing done normally? Is there a particular circuit that I should look at?

Criticism about other parts of the circuit are welcome. The switch with caps and 1M pot on the input are just supposed to setup the input impedance environment. This will ultimately connect to a relay switching network to other boards which could be anything. So the optional servo or coupling cap ensures zero offset.

Modern op amps are pretty quiet (I am not familiar with the ones you are using and too lazy to look them up) and how much gain do you anticipate using?

In my last active DI (last century) I used TL07x bi-fet op amps, they were around 3uV input noise and reasonable. I suspect now we can buy quieter bi-fets.

I wouldn't waste too many brain cells looking for the lowest possible noise gain topology, perhaps focus more on managing input ground (signal 0V) to output signal 0V differences (real world noise).

A balanced output seems useful since you will be feeding balanced inputs almost exclusively.

JR

 

[Bo Deadly]

Modern op amps are pretty quiet (I am not familiar with the ones you are using and too lazy to look them up) and how much gain do you anticipate using?

In my last active DI (last century) I used TL07x bi-fet op amps, they were around 3uV input noise and reasonable. I suspect now we can buy quieter bi-fets.

I wouldn't waste too many brain cells looking for the lowest possible noise gain topology, perhaps focus more on managing input ground (signal 0V) to output signal 0V differences (real world noise).

You mean ground loops? This would be for guitar. Seeing as how a guitar or even guitar pedal running on battery is floating, It's not obvious to me that I can doing anything other than use a simple single ended input like I have and make sure everything is shielded well. The jack will not be connected at chassis because it's not balanced so I don't want to connect ground internally at two places. Shielded cables will be used to get from the jack to the DI board (and possible for the inverted input of the second op amp).

Or perhaps there some sort of clever noise cancelling technique like maybe putting a choke in there somewhere? I can see a pretty serious peak at 180 Hz on the analyzer. And it doesn't take much gain to actually hear it. Of course it's not hard to get hum from a guitar. I suppose I am just killing brain cells but I have always enjoyed such activities.

A balanced output seems useful since you will be feeding balanced inputs almost exclusively.

The other boards it will connect to (through said relay switching) will be nearby in the same enclosure so I can use "star" or "hierarchical" ground to minimize ground potential differences.

What about the Baxandall active volume control?:

This is also in Small Signal Audio Design Fig. 9.21. It requires an extra op amp but I suppose I could just use the output cap and drop the servo. Simulation suggests the servo just adds noise anyway (unless the servo cap is huge).

 

[PRR]

> What is the gain adjustment range of this circuit?

I had assumed that if you are "designing" a circuit, you could figure simple resistor ratios.

(And on the quickest inspection, you should see the attenuation goes to nominal Infinity-- it should be obvious where to put a stopper resistor.)

No, the exact parts there do not match your specifications, but the topology may be appropriate. Season to taste.

 

[JohnRoberts]

You mean ground loops?

no.. I would have said that if I meant it.

Some battery eliminators used with pedals inject noise into SE grounds that need to be managed (follow the path the current will).

This would be for guitar. Seeing as how a guitar or even guitar pedal running on battery is floating, It's not obvious to me that I can doing anything other than use a simple single ended input like I have and make sure everything is shielded well.

I said to balance the output. DI stands for "direct injection" into a console input, typically mic input, so classically DI was a transformer with voltage drop in turns ratio to better match mic input voltage and impedance.

I designed and active DI (last century) because it was cheaper than sourcing a "good" high quality transformer, and higher impedance to not load down a lead guitar pickup as much as transformer DI would. They aren't bad if enough ratio used in the transformer. The nominal 2k mic input impedance is raised by the turns ratio squared. 

If you are instead making a guitar preamp that is a slightly different animal with typically a hotter output voltage.

or just disregard.  ;D

JR 

The jack will not be connected at chassis because it's not balanced so I don't want to connect ground internally at two places. Shielded cables will be used to get from the jack to the DI board (and possible for the inverted input of the second op amp).

Or perhaps there some sort of clever noise cancelling technique like maybe putting a choke in there somewhere? I can see a pretty serious peak at 180 Hz on the analyzer. And it doesn't take much gain to actually hear it. Of course it's not hard to get hum from a guitar. I suppose I am just killing brain cells but I have always enjoyed such activities.
The other boards it will connect to (through said relay switching) will be nearby in the same enclosure so I can use "star" or "hierarchical" ground to minimize ground potential differences.

What about the Baxandall active volume control?:

This is also in Small Signal Audio Design Fig. 9.21. It requires an extra op amp but I suppose I could just use the output cap and drop the servo. Simulation suggests the servo just adds noise anyway (unless the servo cap is huge).

 

[abbey road d enfer]

What about the Baxandall active volume control?:

The first circuit you posted is the Baxandall volume control; the latter is a variation advocated by Doug Self. IMO its added complexity is not reflected in the performance.

 

[Andy Peters]

What about the Baxandall active volume control?:

The load presented to the guitar pickups is a 50k pot, which sets the circuit input impedance. That's way too low for a passive pickup.

 

[JohnRoberts]

The load presented to the guitar pickups is a 50k pot, which sets the circuit input impedance. That's way too low for a passive pickup.

It's actually 50k in parallel with 100k at full volume so more like 33k Yes that is low for a lead guitar pickup.

I used 100k in a parametric EQ front end that got used as a bass preamp by some artists (bass pickups are less fussy).  My Active DI used something like a 3M input termination.

A passive transformer DI will probably be in the 100k ballpark give or take depending on transformer turns ratio.

JR

 

[abbey road d enfer]

more like 33k Yes that is low for a lead guitar pickup.

Anyway a DI'ed lead guitar sounds like crap 

 

[JohnRoberts]

Anyway a DI'ed lead guitar sounds like crap 

;D ;D ;D ;D    Minor detail.... 

My old active DI had a thru (actually buffered) output so you could feed the amp and the console separately.

A mic on the amp grabbed the wet sound to mix with the dry signal (or not). Dry lead guitar is more than a little thin sounding by itself.

There are also amp modeling processors to feed the dry signal to.

but this is getting off topic.

JR

 

[My3gger]

It's actually 50k in parallel with 100k at full volume so more like 33k Yes that is low for a lead guitar pickup.

I used 100k in a parametric EQ front end that got used as a bass preamp by some artists (bass pickups are less fussy).  My Active DI used something like a 3M input termination.

A passive transformer DI will probably be in the 100k ballpark give or take depending on transformer turns ratio.

JR

Isn't even 200-300k ohms pretty low for general DI inputs? We've done a few tests regarding this, 1Meg seems to be quite a bit better. I never really understood why industry pushes passives, instead of active transformer balanced similar to Bo Hansen's DI.
Is it because of cost and passive not needing power?

 

[JohnRoberts]

Isn't even 200-300k ohms pretty low for general DI inputs? We've done a few tests regarding this, 1Meg seems to be quite a bit better. I never really understood why industry pushes passives, instead of active transformer balanced similar to Bo Hansen's DI.
Is it because of cost and passive not needing power?

This is pretty old product category (transformer DI). Active DI only became practical with modern solid state. Actually two product categories (or more) now,  including guitar preamps.

Passive guitar EQ generally load the pickups in the high several hundred K range and are not flat (they're EQ after all).

Transformer DI have practical impedance limits as very high turns ratios to make impedance high involve other moving parts. 

I used something like 3M for my active box because it was easy.. Some tweaky DI makers use crazy high Z but in my judgement it is not worth the trouble.

As Abbey suggested dry lead guitar by itself is not very usable.

JR

 

[abbey road d enfer]

Isn't even 200-300k ohms pretty low for general DI inputs? We've done a few tests regarding this, 1Meg seems to be quite a bit better. I never really understood why industry pushes passives, instead of active transformer balanced similar to Bo Hansen's DI.
Is it because of cost and passive not needing power?

Passive DI have a big advantage with galvanic isolation. Typically an active DI needs galvanic continuity if you want it phantom-powered, unless you use a DC/DC converter for isolation, as done on the BSS AR133. It is a rather costly option. Galvanic isolation is supposed to prevent risks of electrocution and hum loops. It is quite debatable. There are many cases where one can get electrocuted even with a galvanically-isolated DI, and lifting ground is not a guarantee of noiseless operation.
Some think there is some kind of magic in iron, but I just don't.