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C5, C6, C7 and C8 should be 4700uF.

That is interesting. I can't readily imagine how your emitter follower regulator managed to oscillate. Perhaps, if the collector A.C. impedance were quite high. The collector should have an good A.C. ground in an emitter follower.

I recall Charles Hansen telling about oscillating emitter followers, not sure it was here or elswhere.

He also mentioned a book from Dennis Feucht, which helped him to solve the issue. Charles amps are open loop and triple BJT in the output stage, as far as I know.

Look here for some information from Dennis Feucht:
http://www.analogzone.com/col_1017.pdf
http://www.analogzone.com/col_1121.pdf
http://www.analogzone.com/col_0116.pdf

this:

You can also use LM431 instead of zener diode.

Hi Guys,
thank you for reply

Zinsula, I check my regulator and I don't have any oscillation, but for prevent this I can put a resistor in the base of Q3 and Q4. You mention Charles Hansen, I built this circuit after reading the comments of Charles post here http://www.diyaudio.com/forums/solid-st ... post379305 and my circuit is post 15.

Elso, I can replace my C5,C6,C7and C8 by nichicon serie FW 4700uF and why replace diode zener by LM431 less noisy?

Thank you! Maxpou

LM431 Datasheet states: Due to the sharp turn-on characteristics this device is an excellent replacement for many zener diode applications.
I think noise is lower too.

As long as Nichicon FW is stinking normal electrolytic it's OK.

Hi Guys,
I'm back after few months. I have work on my Jocko circuit IV stage with a PCM1798. Yesterday I analysed the THD with my HP334 and I was disappointed because with 1Khz I have 0.3% and With 10Khz I have 1.2%THD. I tested before my jfet buffer stage and I don't have the difference I tested too the Calvin suggestion without the jfet CCS but a BJT CCS, not improvement. I do all my test on a breadboard. I was hopind better. Somebody can help me. Thank you! Maxpou

p.s.: the CIV is 1200pF polystyrene for the moment.

What is the current swing your PCM does deliver?

Hi,



PCM 1798 IIRC had 7.8mA current swing (+/-3.9mA), that is a wee bit close for comfort IMHO for the current in the transistor, in fact, you may even find the circuit current clips with the PCM1798.

I would probably like to see at least ten times the expected signal current swing in the transistor of an open loop stage, to avoid linearity problems and issues with tolerances or thermal drift pushing the circuit out of linear operation.

Also, I may be wrong, but given the current, +/-3.9mA, the I/V resistor you show would try to generate +/- 9.38V peak signal, clearly something that will drive the circuit into clipping.

On the other hand, the circuit you show would do splendidly with a typical multibit DAC, with a PCM1704 and a 6mA lower and 9mA upper CCS it will give 2V RMS out spot on. In that case the full scale THD predicted by Tina-Ti is 0.005% with H2 dominant and nice rolloff towards higher harmonics, the only fly in this particular ointment being that harmonic series continues to quite high orders, though with a PCM1704 this will be well down in the DAC's noisefloor...

So it looks to me like you took a circuit designed for PCM1704 and did not re-dimension the circuit for the PCM1798.

As the datasheet is somewhat ambiguous I would suggest you need to start testing and measuring the PCM1798 output (maybe with passive I/V or with the normal datasheet Op-Amp circuitry until you understand peak signal current output and DC Offset current. Then you need to make sure to adjust the whole circuit (currents, voltages, I/V resistor values) are adapted to the requirements of the new chip. Or just get some much better sounding PCM1704...

Ciao T

_________________
Theory is when you know everything but nothing works.
Practice is when everything works but you don't know why.
Guruhood is when you know everything and everything works accordingly.

OK i bothered to check PCM1798:
Output current Full scale (0 dB) 4 mA p-p
Center current At BPZ –3.5 mA

If you have really 6.47mA through Q2, you end up with 1mA left at max negative current swing (6.47-3.5-2=0.97mA).
No good.

Be generous with current. Lots of current.
You can lower the rails if you have dissipation problems.

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?

Your high-pass pole is at ~280Hz?

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..................

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Yours,
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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...

I/V:

Most of the points have been covered by smarter people than I. I'd just point out that the original transistors might not be great at the current levels you need to run at with your DAC. I dunno, but you should check.

What quiescent current is your output JFET pair running at?

It will depend on the particular set of transistors, but the BL(ue) grade 2SK170's with 27 ohm source resistors will usually run at 5 to 7 mA.