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Old 10th January 2011, 05:46 PM   #11
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Originally Posted by mjf View Post
there are 8 pages - and more -

FET Preamplifier, Part 1

greets
This is an excellent source of info, very well written; thanks again!
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Old 10th January 2011, 10:35 PM   #12
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I've been playing with this circuit all evening and so far so Good I think; but I'm having some trouble with the biasing.
Click the image to open in full size.

I've been readin the JT Hawes design notes and following his process.

R1 = 4.4M (I don't have any 3M resistors; I dont think it makes any odds)
R3 = 680R (Reduce value of R3 for the higher current requirements of the 2SK170)
R4 = 220K
C1 = 4.7uF
R2 = I started with a value of 820R which attenuated the signal slightly and actually sounded quite good; which I can't understand because VD's Q-Point was only a few milivolts below the V+ And I would have thought it would clip.
I've been trying to tweek R2 to bias the amplifier so VD is about halfway between VS and V+; but I find I have to drop the value of R2 signifficantly to get a decent bias voltage; as low as 10R. But this pushes the gain right up; Ideally I'd like to be as close to unity as possible.
It does sound good though; just too much gain!

With R2 = 10R I get:
V+ = 8.3V (I need a new battery)
VD = 4.3V
VS = 50mv


Any thoughts on where I've gone wrong so far?
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Old 11th January 2011, 12:01 AM   #13
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This little circuit is maby better for your plans.

Hostarea git-preamp.jpg – Kostenloses Bilder Hosting

You don't have any biasing-problems and you can set the gain when you relpace R5 with a 100kohm potentiometer.
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Old 11th January 2011, 12:33 AM   #14
bigbeck is offline bigbeck  United States
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Originally Posted by JammyBStard View Post
I think a guitar has an output of up-to about 2v, so for a clean signal you wouldn't really want a gain of more than 3 on a nine volt battery.
I have both a Strat and Telecaster. Playing hard they only put out 200mv. Normal playing is about 100mv. I also have an Epiphone SG with Humbuckers and that puts out about the same. I think the 2 volt figure is for active pickups.
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Old 11th January 2011, 01:14 AM   #15
tinitus is offline tinitus  Europe
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definately passive vs active

so the question is, does your head amp have passive or active input, or both
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Old 11th January 2011, 07:53 AM   #16
Arnulf is offline Arnulf  Europe
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I have both a Strat and Telecaster. Playing hard they only put out 200mv. Normal playing is about 100mv. I also have an Epiphone SG with Humbuckers and that puts out about the same. I think the 2 volt figure is for active pickups.
Peak to peak or RMS (AC) ? RMS reading is useless because it doesn't tell you how much headroom you need, it's peak to peak reading that you're after and based on what I've seen recently when tinkering with a scope there's no realiable way to calculate PP reading from RMS as pickup output signal is not a clean sinewave.
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Old 14th January 2011, 10:00 AM   #17
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I've beeen looking around; But I can't find a definitive answer on max peak to peak voltage o/p of a guitar pickup; google turns up answers from 100mlV to 4v.
Has anyone tested on a scope before?

Quote:
Originally Posted by tinitus View Post
definately passive vs active

so the question is, does your head amp have passive or active input, or both
My head amp? My Amp is a tweaked amp-maker SE-5a

But the main reason i want a pre-amp is to sit infront of my effect pedals. which is also why I want low gain so i dont overdrive the first pedal.

I've been reading several docs on JFETS and common source-amps. But I could do with a decent text on designing a common source-amp and calculating the values.

I was trying to follow these examples FET Amp Designing but they are not very clear in places.

Does anyone have a link for a good design process?
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Old 14th January 2011, 01:11 PM   #18
Arnulf is offline Arnulf  Europe
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Originally Posted by JammyBStard View Post
I've beeen looking around; But I can't find a definitive answer on max peak to peak voltage o/p of a guitar pickup; google turns up answers from 100mlV to 4v.
Has anyone tested on a scope before?
50 mV PP with lightest touch of the strings using nylon plectrum, in excess of 2V PP when going as hard as I could. Nor all pickups are alike though. I'd say Google gave you a decent answer.
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Old 15th January 2011, 11:00 AM   #19
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Nice one Arnulf

Please correct me if you think I'm wrong, I'm trying to learn; but after much messing and reading I'm begining to think the 2sk170 is unsuitible for a low voltage amp in Common Drain Config.
It seems because of it's high transconductance (22mS), that everytime I get IdQ and VdQ biased correctly I've pushed the gain through the roof and it clips badly.

I tried the circuit in Common Source instead with a potential divider at the gate setting the Q point.
This seems to work well as a buffer with a Gain of just over 1 (I think) I need to go over the circuit again with more attention to detail; but it seems OK;

Is there any dissadvantage of using CS configuration insted of CD?
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Old 3rd February 2011, 09:45 PM   #20
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Originally Posted by JammyBStard View Post
Nice one Arnulf
Is there any disadvantage of using CS configuration insted of CD?
One potential disadvantage of common-source circuits is that they offer no voltage gain. In fact, CS circuits are slightly lossy. Usually the gain is almost 1, something like 0.95 to 0.99. In your case, the gain of 1 might turn out to be an advantage.

Another potential CS disadvantage is that the large source resistor produces a large negative bias. Your pickup must overcome this bias.

CS circuits offer two advantages: Isolation, due to high input impedance, and reasonably broad bandwidth. The isolation feature makes CS circuits great for buffering.

I examined the 2SK170 specs and you're right: This device has a rather high transconductance. (That is, at 10 volts and 1 kHz, which are the test parameters for that transconductance.)

Of course you can still vary the voltage gain. For example, with equal source and drain resistors, the gain drops to 1, just as with a CS. Plus, you now have a phase splitter. One advantage of the phase splitter over the CS is a lower negative bias. Plus, you can now choose either output phase. Or use both phases, but know that they cancel.

Your minimum Idss is some 2.6 milliamps (again at 10 volts). The Idss determines the recommended minimum current for stable operation. For example, let's assume that 2.6 milliamps will work as our average current with a 9-volt supply. (Probably a good assumption.) To make a phase splitter, use 820-ohm resistors for both the source and drain resistors.

I got 820 ohms by dividing and rounding to a standard resistor value...

(4.5 volts / 0.026) = 1,731
(1,731 / 2) = 866
866 is close to the standard value 820. We remain within the min / max
Idss specs.

If you want a little gain, divide the 1,731 unequally. With three-fourths as the drain resistor and one-fourth as source resistor, you achieve a gain of not quite three. Note that the FET's internal resistance and external loads reduce this gain.

When you're all through choosing resistors, you must still tweak the circuit. On the other hand, the phase splitter should work right away. My calculation method isn't perfect, but it's probably the easiest way.
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