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Old 10th April 2010, 02:05 PM   #201
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by vzs View Post
For 2V RMS out and ~0.08% THD I need +36V for the positive and -1V to -1.2V from the negative source (with Rs 75 and Rd 1k8).
this does not seem right.
I would expect -Vsupply to be less than +Vsupply but not that far apart. Maybe half voltage for maximum output voltage swing before serious 2nd harmonic adds in.
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Old 10th April 2010, 03:13 PM   #202
juma is offline juma  Germany
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Originally Posted by vzs View Post
... I want to use 2SK170V ...
What's the Idss of the k170v JFETs that you have?
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Old 10th April 2010, 04:51 PM   #203
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The simulated Idss measures LSK170C is 16mA (Vds 10V).
With sources at +36V and -1V and Rs 80 I biased to run at 13.5mA and with Rd 1.7k I got 2V rms out from 140mV rms in with 0.1% THD.

Should I use a lower Idss type?
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Old 10th April 2010, 08:59 PM   #204
juma is offline juma  Germany
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Quote:
Originally Posted by vzs View Post
The simulated Idss measures LSK170C is 16mA ....
Simulated Idss can be anything
If you decide to buy k170v JFETS, choose those with 13-15ma Idss and put them into circuit instead of BF862 without changing anything and you'll be fine (they are that similar).

Last edited by juma; 10th April 2010 at 09:01 PM.
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Old 11th April 2010, 01:32 PM   #205
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Quote:
Originally Posted by juma View Post
Simulated Idss can be anything
If you decide to buy k170v JFETS, choose those with 13-15ma Idss and put them into circuit instead of BF862 without changing anything and you'll be fine (they are that similar).
You are probably right that spice models are far from reality, I just measured the LSK170C model with G and S grounded and Vds 10V showed 15.8mA while LSK170B 8.4mA. At least this was OK

I'm a beginner with spice and while probably not perfect, these models I hope at least to give a hint about how will they behave in real.

Its good to know that are that similar, however, I have doubts that without changing anything I get the 14x time amplification I need. I plan to try with both and decide after which stays anyway

If I have to lower the voltage I could do it with series diodes or leds, so the PSU question is resolved

I'm thinking how could I modify it to be DC coupled...
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Old 11th April 2010, 08:50 PM   #206
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Originally Posted by vzs View Post
... I have doubts that without changing anything I get the 14x time amplification I need....
This preamp was designed for 2x gain (6dB). 14x is possible but only with massive modifications.
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Old 12th April 2010, 06:49 AM   #207
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Originally Posted by juma View Post
This preamp was designed for 2x gain (6dB). 14x is possible but only with massive modifications.
I really don't want to deviate from this thread which was started for this 2x gain preamp, but, could you help me to modify it to have 14x gain driven by 140 mV rms input? (I could start another thread or in private...)

I thought that the modification I wrote in #203 are enough to get 14x with certainly higher, 0.1%, THD. I did some calculations on paper and the values I wrote there seemed to fit for a 14-15mA 2SK170V.

Thanks anyway.
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Old 12th April 2010, 10:41 AM   #208
juma is offline juma  Germany
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What's the input signal level ?
What amp are you going to drive with this preamp (F4 comes to mind) ?

Last edited by juma; 12th April 2010 at 10:47 AM.
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Old 12th April 2010, 11:21 AM   #209
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The input signal level is 140mV rms (current output of AD1865N, +-1mA through a 200R).
As volume control after the preamp I use an R2R volume control, which has a variable input impedance of 25K - 90K and an output impedance ~10K (I will add a B1 buffer after the volume control).

I want to drive a Quad405 (ala Keith Snook DCD-Mod3) which has an input impedance of 22k and needs 1V rms for full output.
I wanted the preamp to deliver 2V rms out to be able to test it with lower sensitivity amps as well, however, if massive modifications are needed 1V rms out is enough.
As the Quads I'm using are fairly neutral I'm looking to add some warmth to them but I don't want to use tubes, so I want to try a common source JFet preamp, that's why a 0.06 - 0.1% THD at full output is OK for me.
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Old 12th April 2010, 01:06 PM   #210
juma is offline juma  Germany
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For that application I would choose another topology. Actually, what you need is a good I/V stage. You'll find plenty of them in digital line level/source sections.
Not that I'm telling you what to do, just trying to save you time

But if it had to be "this way or no way" I would make gain stage like this (attached sch.)
For negative PS (1.2V) I'd use rechargable battery cell and R3 should be 1W. Idss of Q1 is 15mA or so.
The buffer stage can be left unchanged.
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