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-   -   Preamplifier Dispre 2 - JFET (http://www.diyaudio.com/forums/analog-line-level/167033-preamplifier-dispre-2-jfet.html)

PMA 17th May 2010 06:34 PM

Preamplifier Dispre 2 - JFET
 
1 Attachment(s)
After listening tests of my pre standard II JFET preamplifer, and upon requests of the builders, I have decided to design a new version of the Dipre 2 preamplifier, Dispre 2 - JFET.

The main difference is that Dispre 2 – JFET has Toshiba 2SK170/2SJ74 devices in the input stage, not the bipolar transistors that were used in the Dispre 2 V4. This modification resulted in a design of a new PCB board, as it was impossible to simply replace BJTs by JFETs, circuit modification of the input stage had to be made. At the same time, I added two resistors to enable setting of the output stage idle current. These resistors allow, in case of high technology spread of output devices, to tune output stage idle current.

Dispre 2 – JFET can be also used as a headphone amplifier, without any components changes needed.

Dispre 2 – JFET has complementary differential circuit topology with self biased JFET input stage (this topology was introduced by John Curl). This maximizes linearity of the input stage and distortion suppression does not rely only on an overall negative feedback. To remove output DC voltage component, DC servo is used.

Output stage idle current is set at 40mA, and it works in a class A up to 8V into 100 ohm load. For standard link load 600 ohm and higher, the output stage works in a class A for the whole range of output voltage up to clipping.

Dispre2 - JFET has wide bandwidth and high slew rate again, similarly as Dispre 2 V4.

Regarding PCBs, please visit a thread here:

http://www.diyaudio.com/forums/vendo...available.html

or check my webpage: Dispre 2 -

bogdan_borko 17th May 2010 07:17 PM

Pavel, when designing a preamp what you consider the worst case input dc-voltage (offset) that servo must remove?
Cheers, Borko.:)

AndrewT 17th May 2010 07:33 PM

I would aim for 0.5% to 1% of half the rail to rail voltage supply.
+-15V PSU and servo correction for <=+-150mV.
If it exceeds the servo correction range then a DC detect and mute should trigger on the output.

PMA 17th May 2010 07:39 PM

1 Attachment(s)
Hi Bogdan, this is a good question. I will try to review older results and return with them later. According to my previous experience, it was more an issue with bipolar than with JFET input stage.

Now I am attaching a schematic with part numbers, to simplify possible discussion.

PMA 17th May 2010 07:41 PM

Quote:

Originally Posted by AndrewT (Post 2190192)
I would aim for 0.5% to 1% of half the rail to rail voltage supply.
+-15V PSU and servo correction for <=+-150mV.
If it exceeds the servo correction range then a DC detect and mute should trigger on the output.

Andrew, do you speak about input or output DC. In case it was output DC, I would suggest wider correction range.

Regards,

AndrewT 17th May 2010 07:47 PM

I am thinking input range error correction. With a 4times gain stage that would give an output correction range of +-300mV to +-600mV.

But I do want to hear your opinion/design parameters.

The 1% would allow a 100k from the DC servo feeding 1k0 in the feedback loop.

PMA 17th May 2010 07:53 PM

Quote:

Originally Posted by AndrewT (Post 2190212)
I am thinking input range error correction. With a 4times gain stage that would give an output correction range of +-300mV to +-600mV.

But I do want to hear your opinion/design parameters.

Thank you for the clarification. For the former version, Dispre 2 V4 BJT, I was correcting up to 600mV of the input DC offset voltage. For this JFET version I expect to correct less DC voltage.

Regards,

dodo 17th May 2010 08:51 PM

I have question ?
Is there any correlation between high input capacitance of the circuit and little bit boring (laid back) sound reproduction ?
Q. came to my mind after listning expirience with Borbely simple pre
( folded cascode , jfet input compl diff+ jfet VAS + jfet output buffer A-clas 10mA )

PMA 17th May 2010 09:00 PM

It is difficult to answer, but I do not think so. In case you drive such preamp from something with output impedance below 1kohm, I would not blame it on input capacitance.

Different topologies and parts do sound different, that's the reality.

dodo 17th May 2010 09:31 PM

in other words ask for additional cascode in the jfet input compl diff circuit ( as an option), will not meet our positive feedback ;)


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