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CG
PostPosted: Wed Jul 04, 2012 10:10 pm 
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Charlie - do you think that is too low?
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maxpou
PostPosted: Fri Jul 06, 2012 2:57 am 
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Hi,
thank you for reply Guys, after several test like increase or decrease the current in all transistor, increase or decrease the collector voltage in Q2, tried many RIV or CIV value and replace all transistors for BC550C and BC560C, I will test tomorrow a simple BJT differencial with 50 ohms input RIV and I will be back with my results. Maxpou
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maxpou
PostPosted: Fri Jul 06, 2012 3:06 am 
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zinsula wrote:
OK i bothered to check PCM1798:
And as Thorsten said, way too much output swing. With 2.4k I/V Resistor, you swing 9.6Vpp. You don't need so much, do you?


Hi Zinsula,
thank you for reply too, I want the output voltage of a standard CD player 2Vrms. RIV 2.7K I have 5.6Vpp with my scope. I will drive my preamp input. Maxpou
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maxpou
PostPosted: Fri Jul 06, 2012 3:08 am 
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Jocko Homo wrote:
Perfect example of why I don't like to post schematics. This idea was hatched in the days when DACs put out +/- 1 mA. And now someone wants to pump a boatload more current into one.

Oy vey..................


Hi Jocko,
What's the problem?
Maxpou
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maxpou
PostPosted: Fri Jul 06, 2012 3:13 am 
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CG wrote:
Some probably worthless comments...

Power supply:

Usually it's not the best idea to put a smaller value cap like a .01 in parallel with a large electrolytic. There will be parallel resonances where the impedance can get amazingly high. This is especially true with a proto board where the leads can be long. Don't believe me? Simulate it with all the parasitucs taken into account or actually measure the impedance or find the thread hereabouts where we posted some results.

You need to either add damping in the form of what some call snubbers or change the topology some. Here, the easiest thing to do would be to install a few Ohms between C11 and C17 and between C12 and C18.

Why do you put a 100 uF cap across your Zener diodes? The diodes are pretty low impedance and caps placed there won't do much. I suspect you'd do better by splitting R3 and R4 each into two series resistors of about 500 Ohms each, with the 100 uF caps to ground from the junction points of the 500 Ohm resistors.

You also might find you get lower noise by using LED strings instead of Zeners, too.

Finally, the 100 uF caps from the bases of Q1 and Q2 to ground may be causing instability or marginal stability in those stages. A couple Ohms between the rest of the circuitry on the left and the bases may improve things. Especially with a film cap in parallel with an electrolytic at the emitter of the output...


Hi CG,
thank you for you advise, I will resolve my IV stage and I will work on my PS.

Maxpou
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maxpou
PostPosted: Fri Jul 06, 2012 3:21 am 
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CG wrote:
What quiescent current is your output JFET pair running at?


HI,
Charles was not far. The IDSS of my jfet is 7.3mA and my quiescent current is 4.15mA.

Maxpou
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maxpou
PostPosted: Fri Jul 06, 2012 3:51 am 
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Hi Charles,
I sent you a private mail because it's offtopic here. I hope have news. Maxpou
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Jocko Homo
PostPosted: Fri Jul 06, 2012 6:40 pm 
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maxpou wrote:
Jocko Homo wrote:
Perfect example of why I don't like to post schematics. This idea was hatched in the days when DACs put out +/- 1 mA. And now someone wants to pump a boatload more current into one.

Oy vey..................


Hi Jocko,
What's the problem?
Maxpou


You don't understand it. You can't just "copy" it, and assume it will work.

The whole point about the simple I/V converter was to get you guys to sit down and think about how I-V conversion takes place.

The DAC wants to see a low Z. If it didn't, all you would need to do is put a 1K resistor on it, and voila! (Some folks will make it 10R, and amplify it. Works, but not my idea of how to do it.)

No, you need to move the current, to another node, and then stick the load resistor there. Moving current from one node to another. That is what you are trying to do.

The "simple" I/V was an exercise in how to do it.

I don't think you get a passing grade. Sorry.

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maxpou
PostPosted: Fri Jul 06, 2012 8:33 pm 
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Hi guys,
I tried with another circuit like this and I have the same result of THD. But If I tried my HP334 on other CD player I have a good result, consequently my THD analyser is OK. I work on that tonight and I will back. Maxpou


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Jocko Homo
PostPosted: Fri Jul 06, 2012 8:58 pm 
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Maybe you should get an oscilloscope, slowly increase the current, and watch what happens.

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maxpou
PostPosted: Sat Jul 07, 2012 1:09 am 
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Jocko Homo wrote:
Maybe you should get an oscilloscope, slowly increase the current, and watch what happens.


Hi Jocko,
do you talk of my last circuit? I have an oscilloscope here. Maxpou
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Jocko Homo
PostPosted: Sat Jul 07, 2012 1:23 am 
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YES!!!!!!!!!!!!!

Any of them, actually. I assume you understand compression and clipping. Watch, and you will see.

Then, you need to understand why those take place.

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maxpou
PostPosted: Sun Jul 08, 2012 8:02 pm 
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Hi,
I have tried to increase the current in my last circuit and the THD drop only of 0.45% to 0.35% at 1 Khz and 1.25% to 1.1% at 10 Khz. Maxpou
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zinsula
PostPosted: Sun Jul 08, 2012 8:27 pm 
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maxpou wrote:
[...]increase the current in my last circuit[...]

Your last circuit? The one with direct I/V conversion through 50 Ohm resistor?
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Jocko Homo
PostPosted: Sun Jul 08, 2012 9:38 pm 
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OK..............I need to clarify my answer.

Slowly increase the INPUT current, into your circuit.

"But how do I do that?"

Easy. (Really easy, if you have a test CD, with lots of different levels. But, let's assume you don't.)

Get a signal source. Make sure it has a variable level. Put a 1k resistor, from it, to the input of your circuit. You now have the opposite: a V/I converter. Should be easy to figure out.................1 V into 1k.............hey, we have 1 ma!

(Get the idea?)

Start at a low level, say 0.1 mA. Watch the output. Now, slowly raise it. If this doesn't give you insight, then my usual answer applies:

Get a new hobby.

Obviously, if your signal source is a fixed level, you ought to be able to figure out if you make the 1k into 10k..............

If not, see above.

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