Headphone stage & preamp output

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Wed Jul 16, 2014 10:16 am

I'm assuming, hopefully right, that I can set my gain, using a pot as I had done on my other designs, the ones with a boost output.

The idea of taking Rf to the junction of the 10 ohm resistors looks like a clever one. Neither was I planning on using the Zobel.

My original plan for the resistor was 49R9 instead of 10, but that will be one thing to try on the prototypes, and see which handles more caskets better.

Wouldn't the push-pull boost transistor be serving as buffers too?

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JR.
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Re: Headphone stage & preamp output

Post by JR. » Wed Jul 16, 2014 12:02 pm

carlmart wrote:I'm assuming, hopefully right, that I can set my gain, using a pot as I had done on my other designs, the ones with a boost output.

The idea of taking Rf to the junction of the 10 ohm resistors looks like a clever one. Neither was I planning on using the Zobel.
not really clever just circuit design 101
My original plan for the resistor was 49R9 instead of 10, but that will be one thing to try on the prototypes, and see which handles more caskets better.
Making these resistors larger will cause more voltage drop so provide less output voltage before clipping, all else equal. They need to be large enough, to develop only small current from expected DC voltage error between two opamps.
Wouldn't the push-pull boost transistor be serving as buffers too?
I do not understand this question...

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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Wed Jul 16, 2014 1:10 pm

In any case I still can't imagine if or how this parallel circuit will be affected if I connect Vref to the joint point of IC1 output and non-inverting pin in IC2.

I'm thinking it's not possible at all.

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JR.
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Re: Headphone stage & preamp output

Post by JR. » Wed Jul 16, 2014 1:44 pm

carlmart wrote:In any case I still can't imagine if or how this parallel circuit will be affected if I connect Vref to the joint point of IC1 output and non-inverting pin in IC2.
I still do not follow. There is no Vref in that schematic.

If you are laking about connecting a V/2 to + input of IC1 instead of ground that should work.

You do not do anything different with IC2. That is simple unity gain follower.
I'm thinking it's not possible at all.
??

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mediatechnology
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Re: Headphone stage & preamp output

Post by mediatechnology » Thu Jul 17, 2014 8:37 am

mediatechnology wrote:

mediatechnology wrote:One thing this series has going for it that the 5532/34 and LM4562/LME49710/LME49720 don't is that the MC33078 does NOT have differential input clamp diodes.

:o Evil! Truly EVIL !!?!

Not sure I'm following you ricardo.

Insclutable Oliental attempt decadent Western joke :ugeek:

OK, I LIKE OPAs with input clamping diodes. I tend to put them on TL07x cos they don't have them .. especially if they can be overloaded.
Image
NJM4556 Simplified Diagram

NJM do show differential input diodes in the 2114 which is a turbo 5532.
If they are consistent from one datasheet to the next, the 4556, shown above, does not have them.
If I were being paid to do this I would measure the 4556.
But I doubt it has them.

My original point is that it's very difficult to do some types of analog processing with parts that have them e.g. peak detectors etc.

There is true evil in the world right now; the absence of differential clamp diodes in an op amp is not one of them.
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Re: Headphone stage & preamp output

Post by mediatechnology » Thu Jul 17, 2014 8:45 am

Now, to address carlos' question...
(Sorry about calling you Carl.)

The last circuit you show, as JR has already pointed out, can have the non-inverting input of the top stage, IC1, raised to 1/2 Vcc. (Shown as ground in the drawing.)
The second stage, IC2, will follow; the circulating current in the ballast resistors from output-to-output will be small since the input offset of IC2, 6 mV max in an NJM4556, determines the current.

But, how do you intend to couple the combined outputs, which have a 1/2 Vcc offset, into the load?
With the output raised to 1/2 Vcc you will need either a coupling cap at the output or you will need to raise the sleeve (ground) connection of the 3.5mm headphone connector to 1/2 Vcc.

With regard to op amps:
I agree that the MC33178 is not the best choice; it's benefit, low power, doesn't provide advantages to the audio path.
I used them in the "One Knob Squeezer" Guitar compressor reference design for THAT and they worked quite well given that the design was used to showcase the low-power aspects of the THAT4316.
The whole gizmo came in at 5 mA.

In terms of noise, due to input current noise of the MC33178, the TL072 turned out to be far quieter in the input stage.
In a control path the MC33178 are fine if you need low power.
The MC33178 is very popular with Chinese wireless mic manufacturers.
I realize that doesn't make it a good choice, but the volumes on that part are very high.
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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Thu Jul 17, 2014 9:42 am

Thanks for your suggestions.

Of course I will be using coupling caps at the output. It's the price you pay for using 1/2 Vcc ref. But you even see coupling caps in regular +/- supplies, so I don't think there's much of a loss there.

And I will be using 4556 all over, not 33178 anymore. It has been proven that it may sound better.

But I will be doing, and my engineer provide should provide that, is try different piggy-back options for that amp, starting with a single 4556, then two paralleled ones and then the boosted version.

We will be trying two preamp versions: one regulated to 12v, and the other unregulated, just LC filtered at the battery output, which should provide at least 14.8v. THAT had already asked me on our first contacts why was thinking of regulating, as I could feed it directly from the battery, duly bypassed, of course.

Two other areas we should try different options are the limiter, one using the THAT limiter and the other the NJM2762, and the VU meter, one using an LM3915 and the other a microprocessor (my partner knows how to deal with them) with much more leds.

It's prototype time now, and we are heading for it.

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mediatechnology
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Re: Headphone stage & preamp output

Post by mediatechnology » Thu Jul 17, 2014 10:23 am

But I will be doing, and my engineer provide should provide that, is try different piggy-back options for that amp, starting with a single 4556, then two paralleled ones and then the boosted version.
The NJM4556 at high output currents (25 mA) has an output Vce saturation of about +/-3.5V (typ).
With a 12V supply that leaves about 5V p-p available or 1.77 V rms.
Neglecting losses in the ballast/build-out resistor(s) you should be able to get approximately 50 mW from a single 4556.
2.5V peak into 63 Ohms is about 40 mA.

Under worst-case conditions however (25 mA, Vce sat +/-5V) about 8 mW is available.

I suspect that you may wind up paralleling them but in either case encourage you to build it.
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carlmart
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Re: Headphone stage & preamp output

Post by carlmart » Thu Jul 17, 2014 12:02 pm

mediatechnology wrote:I suspect that you may wind up paralleling them but in either case encourage you to build it.
And I will...

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