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Old 14th July 2007, 02:18 PM   #131
AndrewT is offline AndrewT  Scotland
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Hi,
is the 10k (across BE of BD140) shown in the schematic correct?
it will pass only 60uA to the tl431. but minimum tl431 current is specified as 1mA and the only other current comes from the base of the bd140.
I'll try reducing 10k to 1k to see if there is any effect.
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Old 15th July 2007, 10:08 AM   #132
AndrewT is offline AndrewT  Scotland
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Well, I tried reducing 10k to 1k, letting 0.6mA into lt431, that hardly affected the result.
I added a 10nF cap across the pot.
This has helped a lot.
ccs=31mA and output=10mA (8.2V into 820r), the input can swing from 11.1V to 31.5V with no problem.
Still not quite right at higher output currents, but better than it was.
Will try swapping the PMA 1uF cap from BE of output transistor to across the pot.
I'll be back.
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Old 15th July 2007, 07:46 PM   #133
AndrewT is offline AndrewT  Scotland
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Hi,
success.
But I had to ignore two components in PMA's schematic.
The 10k shown feeding the tl431 and across the BE of the shunt BD140 had to be reduced below 600r, I used 300r resulting in 2mA through the 431.
I moved the 1uF cap from output to shunt transistor base to connect output to TL431 reference.

The circuit then pulls 50mA from the lab supply and feeds 10mA to 40mA into the output and shunts the remainder.

I tried various input voltages from 11V (8V out + 3) to 31.6V and over two output currents 10mA and 20mA.
Tried 15mA & 30mA outputs from 15V input to 31.6V.
Finally 20mA & 40mA from 20V to 31.6V

I even tried 24.5V output with 28Vinput to 63V input, lm317 max differential=38.5V.

All these tests ran with correct shunt regulation.

Next stage is to look at scope of the output for noise and oscillation. I'll keep my fingers crossed and even fit some sinks to the too hot to touch transistor/reg.
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Old 16th July 2007, 09:59 PM   #134
Gordy is offline Gordy  United Kingdom
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Andrew, there may not be many replies here, but I am sure that others are watching with interest too.
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Old 7th May 2008, 03:02 PM   #136
PMA is offline PMA  Europe
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Hi all,

there is a new version coming soon ....

New DISPRE preamp, successor to previous popular version
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Old 7th May 2008, 03:38 PM   #137
Bonsai is offline Bonsai  Taiwan
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PMA, the bias resistors are quite low here at 10k. how does this circuit handle being fed from a 'normal' value volume control pot of c. 50k? Do you recommend using a buffer like Grey's 4 JFET balanced version, or maybe something simpler like a straight JFET follower with current source load (a la Borberly for example).

Interesting design by the way - simple with great measured results - how does it compare to other stuff you have tried? I'd be interested to hear this compared to, say. a fully symmetrical JFET based circuit.
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Old 7th May 2008, 03:51 PM   #138
PMA is offline PMA  Europe
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Bonsai san,

thank you very much for your interest in this circuit.



It is still only simulated, but I will surely build a prototype in a couple of weeks/months. I intend to use blue Alps 2 x 10k, BUT this design will work with 2 x 50k without parameters degradation. BTW, in simulation it looks very, very well, even better, than a very well known preamp. -- Edit, I speak about new version, just missed the thread, sorry.

The old one was open, dynamic, with rugged output stage. I want to keep speed and dynamic properties, output stage, but I want to get very low distortion as well in the new design. The original design cannot handle 50k pot.

Oyasumi nasai (it must be late in Tokyo),

Pavel
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Old 9th May 2008, 04:15 AM   #139
Bonsai is offline Bonsai  Taiwan
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Pavel,

arigato Gozaimas!

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