O2 amp - A question about INPUT gain / headroom.

[Cableaddict]

I need a bunch of decent, reasonably-priced headphone amps for my recording studio, & also for my band’s live cue system. The O2 seems like a strong contender. I currently have a few of the Massdrop “standard” version.

I have a question about the gain structure of the O2 amp design, and most importantly about the INPUT gain / headroom:.

As we all know, there are 2 GAIN OPTIONS:
Standard: 2.5x Low, 6.5x High
Medium: 1.0x Low, 3.3x High - suggested for use with strictly IEM’s.

Since I have to drive both 600 ohm studio cans, AND sub-32 ohm IEMS, I figured I needed the standard model, and I would simply use an L pad in front of my IEM’s. (With the added advantage of raising their effecting impedance.)

However, I’m having some trouble getting the right INPUT volume.
How hot can the input be before distortion occurs?

I’ve read that the “medium” (why don’t they just call it “low” ?) is recommended if you’re feeding the amp from a “high” voltage source, but does that mean the “medium” has less gain on its INPUT, or are all of the gain control is on the output of the circuit?

I feed my studio headphone amps with a +4 nominal, balanced signal. And I DO have some issues with the input folding. Should I be considering adding an input pad? - And is that in fact what one would find in the “medium” circuit?

Either way, can I easily change the circuit, (a few resistors, I assume?) to make it a “medium” low gain model?

- Thx.

 

[Bamboszek]

In case of O2 maximum input level at power supply is Vin = Gain / 7. At battery it is Vin = Gain / 4.5
From my experience x1 and x3 gain is perfectly fine with most of headphones and home sources. Higher gain levels such as x6.5 are only useful for driving demanding headphones from weak sources like some portable players.
Using lowest possible gain means less noise, slightly less distortion but most importantly best channel matching on potentiometers is archived at higher levels (like from 12 o'clock and onwards).
Anyway, gain setting can be changed easily with replacement of two resistors.

 

[JohnYang1997]

Just use as low gain as you can get away with. It will have less distortion.

 

[Cableaddict]

In case of O2 maximum input level at power supply is Vin = Gain / 7. At battery it is Vin = Gain / 4.5

Thanks, Bamboszek.

- but I don't understand that. What is that in volts?
(A +4 signal is somewhere around 1.5v, with (I think) max peaks at around 1.7v, though I probably never get there.)

----------

...
Anyway, gain setting can be changed easily with replacement of two resistors.

OK, but HOW? I know my way around a soldering iron, but I can't look at a schematic (which I don't even have) and just figure this out.
Help a brother out !


And again, is this changing the input gain (thus input headroom) or just the output gain?

 

[Timbo2]

My understanding is that on a 2V source you should be fine on AC power with gain of 3.5x or less.

http://nwavguy.blogspot.com/2011/08/o2-details.html

The Math – For those who want the exact numbers, or want to change the default gain settings, on AC power the Maximum Gain = 7 / Vin(max) and on (low) batteries it’s 4.5 / Vin(max). So if you have a 2 volt CD player, the max gain is 7/2 = 3.5X on AC power. If you have a 0.5 volt LOD portable it’s 4.5/.5 = 9X running on the batteries.

More detail here:
http://nwavguy.blogspot.com/2011/08/o2-details.html#gainstageoverload

 

[Bamboszek]

Yes, it is in volts. +4 dBu is about 1.23 Vrms. So maximum gain to not overload input stage is 7/1.23 = ~5.69. Today most equipment like DACs or CD players uses 2 Vrms standard so you will be safe with x3.5 gain.
Is is explained with more details at authors blog -
http://nwavguy.blogspot.com/2011/08/o2-details.html#maximuminput
http://nwavguy.blogspot.com/2011/08/o2-details.html#gainsettings
To change gain you need to replace R17 & R21 or R19 & R23. Gain = 1 + 1500/R where R is value of R17 & R21 or R19 & R23.
You can find already calculated values for desired gain value at schematics.

 

[Cableaddict]

Fantastic. Thanks, guys!

And just to clarify, (Not that it matters much, now) this gain change does NOT affect the input gain / headroom, correct?

 

[Bamboszek]

O2 is two stage design. Input stage provides gain and second stage is just 2x NJM4556 configured as voltage follower (unity gain) to provide current drive for headphones. In context of consumer gear we rarely speak about headroom. Especially when it is mostly digital today. It is more like input sensitivity of amplifier. Maximum input level means point where amplifier reaches maximum power and starts to clip signal.
Your O2 should be configured as such that 0 dBFS signal from your interface won't exceed O2's maximum input level at given gain.
BTW I don't think that O2 is best amplifier for studio. Not possible to rack mount it, only 3.5mm which are less reliable and require adapters for most studio headphones, only one stereo channel, bulky wall-wart style power supply while sound quality is not that critical for musicians monitoring.

 

[Cableaddict]

Thanks, Bamboszek. Exactly what I still needed to understand.

 

[Cableaddict]

BTW I don't think that O2 is best amplifier for studio. Not possible to rack mount it, only 3.5mm which are less reliable and require adapters for most studio headphones, only one stereo channel, bulky wall-wart style power supply while sound quality is not that critical for musicians monitoring.

Nah, it's perfect, except for the drummer & bass player, who could benefit from a little more bass solidity. (I just started using THX amps for them.) FWIW:

"Not possible to rack mount it" - No need. They get mounted on mic stands.

"Only 3.5mm" - Well, first, the Massdrop was available with a 1/4" jack. Second, it's very easy to retrofit a larger jack on the other units.

"Only one stereo channel." - That's all I need. For a "more me" system, you'd want to use a small submixer, anyway. The THX, by comparison, is rather bulky. I did, in fact, rack mount those, using home-made aluminum rack ears.

"Bulky wall-wart style power supply" - That's actually preferable, as it can stay on the floor, along with my custom XLR pass-through box. That makes the unit itself a bit lighter, & so easier to deal with on a stand-mounted short boom.

"Sound quality is not that critical for musicians monitoring." - Say WHAT now?

 

[Bamboszek]

So it looks like needs can be very different with personal preferences. I've build a few Objective 2 for me or my friends but never considered them a studio amplifier. Mostly worked with Behringer HA8000 headphone amplifier placed in control room routed to studio thru wall. 8 channels, really low price and handy mono mix button. Giving musicians access to volume control can be both good and bad thing. One can control volume to his/her needs but for example by changing mix volume on vocalist's headphones you can manipulate his singing level and in result timbre.

"Sound quality is not that critical for musicians monitoring." - Say WHAT now?

That's not secret that most of musicians are deaf

 

[Cableaddict]

Well sure, different studios do cue systems is sometimes radically different ways.
Your basic premise was not incorrect, it justy doesn't apply to how I do it.

And I personally do NOT do a "more me" system, for exactly the reason you state. I'm rather impressed that you know about that!

 

[Tup3x]

I have 1x & 3,3x gain version. I don't see any point in going with higher gain versions since I already have to turn the volume down considerably to have semi decent volume range at 1x gain. I use around 10-11 o'clock while AE-5 line out is set top 32%. (When using HD 6XX.)

 

[trl]

- but I don't understand that. What is that in volts?

Basically, O2 has the potentiometer placed "in the middle", between the input stage (voltage gain NJM2068 opamp) and the output stage (dual NJM4556 opams), so feeding the input stage with more than 1V RMS on a 6.5X of gain it will saturate the input stage that will start to distort a lot when passing 7V RMS on its outputs. (1V x 6.5 = 6.5V RMS)

Low-gain of the O2 was not designed for IEMs, but for regular desk DAC units (2-2.2V RMS output voltage), while the high-gain was designed for portable devices (0.7-1V RMS output voltage).