FET based Acoustic preamp help - diyAudio
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Old 5th January 2009, 05:48 PM   #1
MondyT is offline MondyT  United Kingdom
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Default FET based Acoustic preamp help

Hi All

I am new to this forum but have been browsing for a while and see that there are some very experienced and knowledgeable girls and boys kicking around, I am very humbled!!

I am very interested in this AMZ Mini Booster, FET based guitar preamp/booster circuit…

AMZ Mini Booster

But I wondered if you clever guys and girls could help me. Am I correct in assuming that the input impedance of this circuit is 1.0MOhm as set by R1? But what is the output impedance, I am thinking this is 100KOhms as set by the pot R6? Is this so and if so, what would be the impedance without the pot?

I am basically trying to find a simple FET based buffer/preamp to go between my acoustic guitar Piezo pickup and the line input of a mixer (not a guitar amp that this circuit is designed to drive). I tried the simple common source circuit here…

Common Source FET buffer

Great sound, but not enough gain to drive the mixer’s line input. So I discovered the AMZ Mini Booster circuit above. This looks like the gain will not be a problem, but the output impedance could be too high and it may colour the sound a little too much (I am willing to try it however). My basic requirement is an input of 1.0Meg and output to match a 20K input impedance line level mixer input. The maximum output voltage of the piezo pickup is around 1.0V rms under hard playing conditions. I love the simple sound you get with FETs and so would really like a one or two FET circuit for the job

Any help or advice you can give on the circuit(s) would be much appreciated

Cheers
Ray
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Old 6th January 2009, 03:12 AM   #2
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You might check out a thread for FET-based electric guitar preamps. It's a fairly long thread, but it has lots of good meat in it. You can find it at:

New JFET guitar preamp project

Different individual FETs can have very different characteristics. These will require different values in the circuit. The thread has information about how to measure the characteristics of your FETs and adjust the values of the rest of the circuit.

Piezo pickups normally have a fairly high-level output, and shouldn't need much boost if you have the impedence high enough. They require a very high impedence input. The Rane AP-13's piezo input had an impedence of 5 Megohm. You may find better results with the piezo by increasing the input impedence.

-Vance
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Old 6th January 2009, 03:28 AM   #3
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Re:'"output to match a 20K input impedance '' - you need an output impedance in the order of 10x the impedance you're driving, so the mini booster should be OK...
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 6th January 2009, 06:31 AM   #4
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It's 40deg here and my brain's fried...of course I got that backwards...
the mini booster looks to be low z without the pot, so you could try it like that or with a lower value if you want to control level at that point
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 6th January 2009, 07:04 AM   #5
MondyT is offline MondyT  United Kingdom
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Hi Pete

Thanks for your reply from Down Under, i'ts appreciated!

I always thought that you needed an output to be 5 to 10x lower impedance than the load you are driving and that a high impedance output into a lower impedance load was not good. My knowledge is not too good however! Can you confirm as if I am wrong, this is good news!

Incidentally is the output of the mini booster 100K as I expected and could you tell me what the impedance would be without the pot?

Cheers
Ray
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Old 6th January 2009, 09:42 PM   #6
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Quote:
Originally posted by MondyT
Hi Pete

Thanks for your reply from Down Under, i'ts appreciated!

I always thought that you needed an output to be 5 to 10x lower impedance than the load you are driving and that a high impedance output into a lower impedance load was not good. My knowledge is not too good however! Can you confirm as if I am wrong, this is good news!



No, you are right - he was wrong - although I suspect it was more a typing error than anything else.

Quote:

Incidentally is the output of the mini booster 100K as I expected and could you tell me what the impedance would be without the pot?
I wouldn't have a clue how to calculate it, the constant current load messes it up rather a lot - but with a 100K pot on the output you really need a guitar type input to feed it to, not a line input.

If I wanted to know, I'd remove the output pot, and measure the output impedance.

If you're wanting to drive a line input, I would suggest adding an emitter or source follower on the output.
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Old 7th January 2009, 12:32 AM   #7
MondyT is offline MondyT  United Kingdom
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Pete, don’t worry my friend, I got it the wrong way round myself when writing my reply. I had to read it about 4 times until I was sure I had all “inputs” and “outputs” in the right order. And it is about -4deg.C here in the UK so I have no excuse!!

Vance, thanks for your reply and the link, this was the link were I originally found reference to the Mini-Booster circuit that I thought my be close to what I need. For some unknown reason I failed to notice there was more than just one page of threads and thought it finished on page one! A good read with most of it going over my head, but when I get time to study this link I hope to gain a little more insight in to FETs. Nigel has kindly pointed out what I feared would be the case for driving a 20Kohm line input, but I am not sure where to go from the first FET stage and I have no idea how to measure the output impedance of the Mini-Booster without the 100K pot!

Now JP Hudlsey’s schematic at the end of post #109 of this thread here…

JP Hudlsey’s schematic

seems to be very close to what I am looking for but I do not think the EQ section will be of any benefit to an acoustic guitar and so I would need to remove this. Could anyone tell me if I can use this circuit but just connect Q1 Drain Direct to the coupling cap C5, will this remove the EQ section? Also JP states the output impedance of the circuit Emitter-Follower is around 200R, so would it be possible to replace the 250K guitar pot on the output with say, a 5K pot to provide a better match to my 20KOhm input? Finally will this circuit then provide enough gain (with the emitter-follower just acting as a buffer) and is there a FET version of an emitter follower for the output as I would really like to keep the circuit all FET based if at all possible?

Once again, thanks all for your replies it is really appreciated here

Cheers
Ray
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Old 7th January 2009, 06:52 PM   #8
MondyT is offline MondyT  United Kingdom
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Join Date: Dec 2008
Hi All

Still searching!!

I found a two stage FET preamp idea that looks extremely close to what I need here....

Two Stage FET Preamp

Now this looks close but as always I wonder if the gurus here could answer a couple of questions before I commence obtaining some bits to try the circuit out (Credit crunch impoverished right now so need to get it mostly right before spending!)

1. Would I have to change any component values, resistors, FETs etc, for a 9v battery supply?

2. I am not sure, but it looks like this circuit may be a little too power hungry for 9v PP3 battery operation. Can anyone calculate the circuit current consumption, and if it is too high, can the circuit be modified for lower power consumption?

3. The input impedance is perfect, but as usual I need to drive a 20K load. I *think* the circuit output impedance without the pot is 1200R?? Is this the case and if so could I say use a 4.7K pot for the output, thus looking better for the 20K load without upsetting the circuit or causing problems?

I hope you guys can help coz I think this is about as close as I can get to what I think I need

Cheers
Ray
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Old 7th January 2009, 08:23 PM   #9
mjf is offline mjf  Austria
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[
The input impedance is perfect, but as usual I need to drive a 20K load. I *think* the circuit output impedance without the pot is 1200R?? Is this the case and if so could I say use a 4.7K pot for the output, thus looking better for the 20K load without upsetting the circuit or causing problems?

hello.
the output impedance should be around 1200ohm i think too.
i would not worry about that 10k pot,because some hifi-amps have 20k inputimpedance and use a 10k(alps)pot.
greetings.............
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Old 7th January 2009, 09:06 PM   #10
mjf is offline mjf  Austria
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hello.
i think it will work with a 9v battery.
as seen in the article(two stage fet preamp) the powerconsumption should be around 12ma(it depends on the properties of the fet) with 12v.
with a 9v supply it could go down to(estimated)8ma.
greetings..............
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