RC Volume control for a lateral MOS AMP.. - diyAudio
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Old 28th March 2007, 09:21 AM   #1
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Default RC Volume control for a lateral MOS AMP..

Hi all,

I'm just finished the Nico RAS's lateral MOSFET amp (with 2SK1058+2SJ162);
Click the image to open in full size.

It works fine, there is no hearable noise or distortion and the sound is warm as needed. I will make a class A preamp for it soon.

Anyway, at the moment I need a remote volume control and input selector option for it. Today I saw this
http://eshop.diyclub.biz/product_inf...roducts_id=195

Its a very cheap one. It has an 100K ALPS motorised pot, but as I remember only a motorised pot costs at least 30Ä! But its 32$ all inclusive! Its some weird.

Has anyone used this kit before? There is no detailed datum on web site and I want to be sure thats a complete kit or not?

Thanks a lot for replies in advance..

Ozgur
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Old 28th March 2007, 12:58 PM   #2
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If you are just driving one set of lateral MOSFETs, R19 and R25 are unnecessary.
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Old 28th March 2007, 01:03 PM   #3
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You must be right. Because this system is actually designed for two or three pairs, thats my modification for one pair and I am not a professor on solid state.

Thanx..
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Old 28th March 2007, 01:09 PM   #4
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But I should ask another question to you.
When inputs are open and my ears were very close (in fact attached) to the tweeter I can hear a weak hiss tone, but when I move to 5 cm then there is nothing to hear. I can hear this "hiss" on my LM4780 GC also..
Is this an aceeptable noise?
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Old 28th March 2007, 01:18 PM   #5
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Sounds OK to me -- some folks actually prefer a little noise mixed in with their audio -- that's why carbon resistors are a hot-button item. (I don't know why, but I think that the HF noise helps people actualize the recorded music experience.)
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Old 28th March 2007, 01:19 PM   #6
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Ok then... Thanks again.
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Old 28th March 2007, 01:47 PM   #7
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Hi,
I find the DC operating currents in the front end a bit strange.
R9 has about 41V across it and carries 41/47=.8mA
Half of this current passes through each of R12 & R13 developing 1.7V across them.
The voltage across R23 is about 1.05V and this results in a current of about 0.47mA shared between the two halves of the second stage.
If the transistors have a gain of about 100 then their base currents will be about 0.0023mA<<<.4mA, so let's ignore it for the time being.

The current in the VAS can vary from 0.23mA up or down by about +-0.23, giving a range of zero to 0.47mA. Theoretically, if the signal is asymetrical then the maximum current in the positive direction can exceed 0.47mA.

This current range becomes the limit for driving the output FETs.
A high frequency signal has to charge/discharge the FET capacitance and I think this is too low.

The first stage current is probably acceptable and your lowish noise confirms it is operating within an acceptable window.
The second stage is starving the output FETs and I think there is much to be gained by increasing the second stage current substantially.

There is a saving that is preventing the second stage from current limiting.
The 82pF Miller compensation caps are slaughtering the slew rate of the voltage amplifier, preventing the amp from reproducing high voltage, high frequency signals.

Finally,
there is no protection to prevent voltage spikes from damaging the FET gates due to excessive gate source voltage.
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Old 28th March 2007, 01:51 PM   #8
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Should I add zener diodes to the gates?
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Old 28th March 2007, 02:10 PM   #9
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But there are already protection diodes on gates.... Both SK and SJs...
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Old 29th March 2007, 07:45 AM   #10
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Nobody will answer my real question?
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