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Old 22nd March 2004, 10:59 PM   #1
androck is offline androck  United States
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Question Help! Preamplifier Component Values

I am an electrical engineering student at the Univeristy of Texas at Austin and I am building a guitar amplifier for my senior design project. For the power amplifier section, I am using a Philips TDA8920 IC and I have built a power supply with a 40VCT transformer which supplies about +/- 28VDC unregulated power to the IC. I found a preamplifier topology in The Ultimate Tone Vol. 2 by Kevin O'Connor that I would like to use to put between the guitar output and the power amplifier input. However, it does not have any values for the components or specific transistors that it is supposed to use. The link below is a scan from the book and I was wondering if anyone could help me determine what components I need to use for this preamplifier. The Philips IC can amplify a signal up to about 1 Vrms before it begins clipping so I think the maximum value that the preamplifier supplies doesn't need to be much higher than that. I would also like to add to this preamp a volume control and a Baxandall two-band equalizer, but I'm not sure where those components should be placed, whether before or after the preamplifier. I understand that the preamplifier will probably need regulated power and I can do that with my existing power supply, though I'm not sure what voltage it needs. Thank you for any help, it is much appreciated.

http://victoly.com/~androck/preamp.jpg
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Old 23rd March 2004, 12:54 AM   #2
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I think I'm right to say the circuit is not a preamplifier, it's the first two stages of a power amp, all thats missing is the output stage. so it would be designed to provide a high voltage gain, which is not what you need, you need a buffer circuit.

If you want to build it so it works I would try these values to start with...

There's are not critical...
R1 = R9 = 10k (sets the input resistance)
R10 = 1k (sets the gain = (R9 + R10)/R10 = 11)
R2 = R3 = R11 = R12 = 100 ohm (degen the input stage)
R4 = R5 = R13 = R14 = 68 ohm (degen the current mirror)
R8 = R17 = 100 ohm (degen the VAS stage)
C4 = C5 = 100uF (Help regulate the Zener voltage)
C3 = 10pF (feed forward capacitor)
C2 = 220uF (NFB DC blocking capacitor)
C1 = 10uF (input DC blocking)

You can play with these starter values...

R7 = R16 = 2.7K (biases the zener diode, try 5mA, (28 - 12) / 2700 = 5.2mA)
ZD1 = ZD2 = 12v (forward bias Q11 & Q12, determine Vdrop across R6 & R15, i.e. 12v - 0.6 vbe = 11.4v)
R6 = R15 = 5.6k (sets the differential tail currents, try 2mA, 11.4/5600 = 2ma)

Q3/Q6 = 2SB649/2SD669 or similar matched pair with adequate Vce rating

All other NPNs try 2N5551
All other PNPs try 2N5401

I notice there is no miller compensation capacitor, you could start with 100pF between the base and collectors of both Q3 & Q6.

You still need an Output Stage though !
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Old 23rd March 2004, 04:37 AM   #3
androck is offline androck  United States
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Thanks for the reply Nick. Maybe I have made a mistake in trying to figure out what I need to build. I know the Philips TDA8920 provides 28dB of gain, so I don't think I need anything in front of it to provide significant voltage boost. I'm not sure I know what a buffer circuit is, but if it allows me to control the volume and have an eq, then yes, i think that's what I need Any schematics you can point me to?
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Old 23rd March 2004, 12:00 PM   #4
LaZarus is offline LaZarus  Netherlands
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Hi, You could look at this page. It has several schematics for preamplifiers, although I didn't look for guitar preamps.
You can also look on this site for great schematics.

Enjoy,
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Old 23rd March 2004, 03:55 PM   #5
azira is offline azira  United States
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I doubt you need a circuit like that in front of your IC. What you'll want is a tone control circuit and a preamp opamp. The opamp is to give you a little more gain from the instrument level signal which is 100's of mV to maybe a 2Vp-p signal. This gives you more headroom overall.
There are a lot of relatively cheap quality opamps you can choose from. Try OPA604 or NE5532. Here's a link to a website who has the circuit for a Fender and Marshall tone control circuit.

http://users.chariot.net.au/~gmarts/ampbasic.htm

You would take the input from the guitar, run it through this circuit, then into your opamp (preamp) and then into the power IC.

Sounds like a fun project.
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Old 23rd March 2004, 09:01 PM   #6
androck is offline androck  United States
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I've been searching through other posts and am starting to think that maybe the best thing would be to build an op-amp based preamp to put in front of the Philips power amplifier. Is it possible to build one circuit and swap out several op-amps to decide which one is best? I've done some searching and besides the OPA604 and NE5532 mentioned, maybe I should try the OPA627, OPA2604, or something else? I'm planning on using this guitar amp for jazz, so I'm looking for what would make the best clean sound while being able to adjust the EQ and volume.
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Old 23rd March 2004, 09:07 PM   #7
androck is offline androck  United States
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What about Jazzpeter65's preamp in this thread:
New JFET guitar preamp project
It looks like it incorporates everything I'd like to have, plus the power supply I've built for the TDA8920/8924 chip could be adjusted pretty easily for the required voltage in Jazzpeter65's circuit.
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