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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

What's wrong with this preamp circuit?
What's wrong with this preamp circuit?
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Old 25th September 2019, 11:31 AM   #1
Lysergium is offline Lysergium
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Default What's wrong with this preamp circuit?

I've built a preamp, and it's not working, my amp/speakers do not want this thing in the circuit, bunch of popping, hissing, buzzing.

I have the input, output, DC wired the same as in my previous preamp, which works fine.
ampv2.PNG

I used spare leads from resistors to make some connections in the circuit, could that be a problem? everything seems to be connected correctly.

I used BC547C instead of BC549, but they're basicaly interchangable, as far as i know.
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Old 25th September 2019, 11:35 AM   #2
ubergeeknz is offline ubergeeknz  New Zealand
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BC547C is fine.

Can you take a picture of your construction?

Do your DC voltages match the circuit diagram?
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Old 25th September 2019, 11:55 AM   #3
bobo1on1 is online now bobo1on1  Netherlands
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I don't see any power supply decoupling capacitors, and the circuit doesn't have very high power supply rejection

I suggest adding power supply decoupling capacitors first.
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Old 25th September 2019, 11:58 AM   #4
ubergeeknz is offline ubergeeknz  New Zealand
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Quote:
Originally Posted by bobo1on1 View Post
I don't see any power supply decoupling capacitors, and the circuit doesn't have very high power supply rejection

I suggest adding power supply decoupling capacitors first.
I agree, this looks like a section and not intended to be used as a "complete" preamplifier.
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Old 25th September 2019, 12:01 PM   #5
Mooly is offline Mooly  United Kingdom
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What's wrong with this preamp circuit?
It also has very high gain for a line amp and will not accept typical line level signals without severe clipping and distortion. The gain is around 20db (X10 voltage gain)
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Old 25th September 2019, 12:06 PM   #6
Lysergium is offline Lysergium
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Oh, makes sense i guess. my previous preamp is connected to DC directly as well, but it works. Probably need to fix that anyway.
The cap is on the other side, because i installed the previous cap in the wrong direction, so was worried it's fried.
top.jpg

bott.jpg

supp.jpg
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Old 25th September 2019, 12:13 PM   #7
ubergeeknz is offline ubergeeknz  New Zealand
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Aside from @Mooly comment regarding gain.. you definitely need some local supply decoupling. A 100uF electro and a 100nF ceramic from DC+ to gnd on the board itself, or whatever you have to hand - the exact values don't matter much - might go some way to clearing up the supply noise (and possibly oscillation)
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Old 25th September 2019, 12:15 PM   #8
Lysergium is offline Lysergium
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I was redirected to this design, in a previous thread. was told this would work better.

So the problem is, it has no power coupling capacitors, and it can't accept line input directly form a guitar pickup (humbucker meassuring 7.5kohm)?

So i can fix the power input problem.
Can i fix the signal input problem? different input cap?

Or do you have another good, transistor based pre-amp circuit i could build? (12-18V power supplies preferable, since that's what i got, probably have others as well).
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Old 25th September 2019, 12:16 PM   #9
ubergeeknz is offline ubergeeknz  New Zealand
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Ah. The gain should be okay for a guitar pickup. I don't think you mentioned that.

Add the supply decoupling, and see how it behaves.
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Old 25th September 2019, 12:23 PM   #10
Mooly is offline Mooly  United Kingdom
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What's wrong with this preamp circuit?
The input impedance isn't all that high as it is set by R1 (150k)... I don't know what would be recommended for your pickup though.

The 0.22uF cap is fine for the input.

Maybe if you tell us what preamp does work then we might spot some obvious differences such as gain and input impedance etc.
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