Headphone stage & preamp output

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carlmart
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Headphone stage & preamp output

Post by carlmart » Fri May 30, 2014 12:14 pm

The last two things I have to decide upon on this portable design are what will I use for the headphone amp stage and what for the main preamp stage.

Let's see the preamp main output, which is the most critical, considering that is what will be fed to recorders of any kind.

Nowadays it's easy to pick a balanced chip, like THAT's 1646 or the DRV134 or the SSM2142. AFAIK they are all compatible, and I wonder if someone ever compared them all and published the results.

The question is, for this design, I will probably be feeding mostly unbalanced inputs, so I wonder if I shouldn't go simpler. On my previous design I had used an OP275 feeding both outputs, and what I was wire the output as fake balanced, so I could connect balanced and unbalanced units.

For the headphone preamp I wonder what chip to use, but I think that any modern chip, like an OPA2604 or similar could drive any pro headphone, like a Sennheiser HD280 or Sony MDR7506 to excellent monitor levels, with no additional boost transistors at the output. Or does anyone think otherwise?

The condition for both chips are that I can use V/2 at the non-inverting input and that current is not that high when not feeding the headphones.

carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Fri May 30, 2014 5:35 pm

This what I call a "standard output module", quite common on many mixers I have seen.

The only difference is the 3-pos switch I'm using to choose for the output, working as mono/stereo/reference tone.

Corrections have not yet been made for using a single supply, and the chips have not been chosen yet.

As you also see I'm providing just two unbalanced outputs, with no provision for balanced options. It's quite likely the protoypes will be assembled like that, even if it's quite likely the pcbs will be prepared to accept fake-balanced or full-balanced options.

One question to ask is where the Vu-meter should be connected to. Best place should be at the output, I think, providing a dual color led over each pot connected to the output of the balanced mic input. Something like the bi-color led clipper down below. Values should be adapted to my supply and to sensitivity, of course, probably using a trimpot instead of R25.
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Standard output module.jpg
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ricardo
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Re: Headphone stage & preamp output

Post by ricardo » Fri May 30, 2014 5:47 pm

carlmart wrote:Nowadays it's easy to pick a balanced chip, like THAT's 1646 or the DRV134 or the SSM2142. AFAIK they are all compatible, and I wonder if someone ever compared them all and published the results.
Use the THAT chips. The others may be 'compatible' but are NOT equal .. especially if you are driving unbalanced stuff. See Waynes measurements in the Balanced Outputs thread. The others are really yucky into unbalanced loads.
On my previous design I had used an OP275 feeding both outputs, and what I was wire the output as fake balanced, so I could connect balanced and unbalanced units.
Some of the common ways of doing this are REALLY yucky into unbalanced inputs. (Should really start a separate thread on this)
For the headphone preamp I wonder what chip to use, but I think that any modern chip, like an OPA2604 or similar could drive any pro headphone, like a Sennheiser HD280 or Sony MDR7506 to excellent monitor levels, with no additional boost transistors at the output. Or does anyone think otherwise?

The condition for both chips are that I can use V/2 at the non-inverting input and that current is not that high when not feeding the headphones.
My choice is NJM4556.
http://nwavguy.blogspot.com.au/2011/08/ ... ments.html
http://nwavguy.blogspot.com.au/2011/07/ ... ocess.html

You probably want to use it (or maybe 2 off) for your V/2 too ... less the headphone current move your V/2 around too much.

Note the caveats about implementation being crucial.
Last edited by ricardo on Sun Jun 01, 2014 5:30 pm, edited 1 time in total.

carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Fri May 30, 2014 5:56 pm

One thing I can't find very much literature about is how to properly implement V/2.

Should you use one V/2 buffered chip for every connection or one for them all?
You probably want to use it (or maybe 2 off) for your V/2 too less the headphone current move your V/2 around too much.

Note the caveats about implementation being crucial.
Can you elaborate on this a bit more?

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JR.
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Re: Headphone stage & preamp output

Post by JR. » Sat May 31, 2014 8:50 am

There is no single way to implement v/2, it depends on the specifics of the circuitry. Sometimes a V/2 can be as simple as a resistor divider, for a higher current application and active buffer.

Keep in mind the v/2 is taking the place of a ground so needs to look and act like ground to the current flowing into and out of it.


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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Sat May 31, 2014 9:58 am

The only place I have seen this V/2 rule being applied is in Walt Jung's book, and he advises to assign values for the divider, which in parallel should be equal to the high resistor on the IC gain chain. So I wonder if I shouldn't add a divider to every stage following that rule.

What I think I will do is also use an inverting stage for the headphone amp, so I can add a separate pot for it. The arrangement shown on my design was for the +/-9v supply I had used.

In fact, to keep the same phase and not invert it, I think I will invert the input pins at the balanced first stage. That would be better than adding another stage to invert phase before the output, for filtering perhaps.

Even if THAT's 1646 was strongly recommended for the output, they are quite pricey and cables should be quite short on this preamp, less than a meter.

Ricardo's suggestion to go for the NJM4556 for all those chips is very very useful. The comparison papers were very enlightening. Thanks!

In any way those should be decisions to be tested during prototype's field applications.

carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Sun Jun 01, 2014 8:31 am

One decision I will have to take, besides that mentioned above, about using or not full balanced output chips, is how to control the level.

As it is now I'm using a three-position switch to select three gains, the lowest being line level, and then a pot just before the last chip to control output level.

One option would be to put a pot across Rg and use that to control output level. Many people use a multiposition rotary switch with resistor steps to do that, instead of a regular pot.

That is not solution I like too much, because of cost and size. But I'm aware that pots have a tendency, with time, to get noisy. The series large capacitor you need to put in series with the pot might alleviate that, but I'm not sure it cures it.

Of course this is something I have to raise with the THAT people, but I would very much appreciate your opinions over this matter. Summing it up, for and con:

1) Using a switch for Rg and a pot to set preamp output levels

2) Using a pot and series capacitor for Rg to set preamp output levels

3) Using say a 24-pos switch for Rg to set preamp output levels

Are there any other options that I don't know? Simple ones, not using digital stages to control levels.

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mediatechnology
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Re: Headphone stage & preamp output

Post by mediatechnology » Mon Jun 02, 2014 3:00 pm

Ricardo's suggestion to go for the NJM4556 for all those chips is very very useful.
+1 on that suggestion.

The NJM2114 (a "turbo" 5532) is also a good choice due to its 60 mA output current but they can be a little hard to find.
http://www.cn.njr.com/PDF/NJM2114_E.pdf
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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Sun Jun 29, 2014 6:53 am

It's a pity the Motorola MC33078 was not tested on that 2011 comparison.

I would like to know how it does compare with the NJM4556. The MC is the one THAT seems to use on their application projects when not using a THAT part.

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mediatechnology
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Re: Headphone stage & preamp output

Post by mediatechnology » Sun Jun 29, 2014 7:10 am

I use the MC33078 fairly often and it's a good part.
TI also makes it.
Never tried it in a headphone amp - the output current is limited with the minimum positive short-circuit current (Isc) of 15 mA.
The 33078 would require external buffer transistors.
It also has a 10V minimum supply.
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