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Mesa Boogie 5-band eq

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7/15/1999 12:35 PM

Mesa Boogie 5-band eq

I'm planning on using the mesa boogie 5band eq as a starting point to designing a multiband eq for my amp. If someone could help me on the corrections, I would be most appreciative! (It's scheme. is in Aspen Pittmans book). So anyway, I see it as basically a differential amp... Q1 just matches the input impedance, Q4 gives extra current output. The base of Q2 is the inverting input and the base of Q3 is the positive input. So, that's how I see it should be! O.K., The LC feedback loop should be connected from the output, not the pos. input, to inverting input. Q2 should have a load resistor to the negative rail... Q4 should have it's emmitter connected between the 3.3k r and the .01 output cap (otherwise Q4 isn't providing output current). I have a great article from Audio Electronics, by Nelson Pass on building class-A, discrete op-amps, which includes building high voltage op-amps! (issue 1 1999). I just noticed, the input polarity in his article seems incorrect- So please someone help!

7/16/1999 1:28 AM
Eddie Van Halen	Eth- So what you're asking is, for someone elses opinion on the mesa- boogie eq scheme. It's not unlikely that mesa purposefully release a non-working schematic. To really know, someone really smart would have to answer this post, but I would guess that in addition to your thaught, a cap to ground from the pos. input(Q3). I don't really know, maybe a cap and a resistor. Good luck- Eddie

7/16/1999 3:02 AM
jason	what was that for? jason

7/16/1999 3:40 AM
Eth	Hey Jason, Thanks for your reply... Eddie Van Halen was actually me- trying to stir interest in answering my post. I was just wondering if anyone could give corrections to the mesa boogie 5 band eq schematic. Supposedly it works well, but the scheme doesn't make sense to me. Does that make sense. I realize my post was pretty erratic. Sorry, any help would be appreciated! -Eth

7/16/1999 12:31 PM

The Mesa schematics in the Pittman's book are very hard to read, and some of them are wrong. If you compare all of them you'll notice that some have a "jumper" where other have a "dot" (connection) near the output of the EQ. One way works, the other don't. I can't remember which, but once you breadboard it it's easy to find out. You can use an opamp instead of the transistors and you can use inductor simulations using opamps or transistors if you can't find the coils.

I'll email you a spice file I used for some simulations. It's not complete (just one band) and it's not the same values of the MESA eq but it can help you to find out the connections and how to use an opamp and simulate the coils with transistors.

The triaxis has an interesting idea - instead of using a pot for each band they use a single knob - when it's down the response is flat, turn it up you've got the "V" shape that evryone uses

7/16/1999 12:34 PM

Well, there's no mail address in your message so here it is:

*

C1 7 8 0.015U

C2 8 9 0.0015U

Q1 5 9 10 2N2222

R3 6 0 100K

R4 0 9 47K

R5 10 8 1K

R6 3 10 10K

R7 6 1 3.3K

R8 2 4 9.9K

R9 0 2 3.3K

V1 6 0 AC 1 SIN (0 1 1e+006)

V2 5 0 4.5

V3 0 3 4.5

X1 1 2 3 4 5 LF351

X2 1 7 2 POT

+PARAMS: POTSIZE=5K PERCENT=50

*

.MODEL 2N2222 NPN (IS=501.657F BF=282.144 NF=1.16176 VAF=100 IKF=1.05431

+ ISE=20.5297F NE=1.36131 BR=2 IKR=988.851 ISC=1.181988E-017 RE=543.714M

+ RC=1.00912U CJE=39.2628P VJE=700M MJE=499.227M CJC=31.2633P VJC=699.997M

+ MJC=499.832M TF=493.812P XTF=499.971M VTF=10 ITF=9.69242M TR=176.624N)

*

.SUBCKT POT PINA PINB PINC

+PARAMS: POTSIZE=0 PERCENT=0

R1 PINB PINA {{POTSIZE}*{PERCENT}/100}

R2 PINC PINB {{POTSIZE}-R(R1)}

.ENDS POT

*

* OPAMP

* PINS: 1=NC+ 2=NC- 3=VEE 4=VO 5=VCC

.SUBCKT LF351 1 2 3 4 5

C1 6 7 2.88675e-012

C2 12 13 1e-011

CE 10 14 1e-019

D1 18 19 D

D2 20 18 D

D3 4 16 D

D4 17 4 D

D5 3 5 D

E1 14 0 POLY(2) 3 0 5 0 0 0.5 0.5

F1 13 14 POLY(5) VS1 VC VE VLP VLN 0 1.59155e+007 -1.59155e+007 1.59155e+007

+ 1.59155e+007 -1.59155e+007

GA 12 0 6 7 0.000251327

GCM 0 12 10 0 2.51327e-009

H1 18 0 VS2 1000

ISS 5 10 0.00013

J1 6 2 8 J1

J2 7 1 9 J2

R2 12 11 100000

RC1 3 6 3978.87

RC2 3 7 3978.87

RE1 8 10 1

RE2 9 10 1

RE 10 14 1.53846e+006

RO2 13 14 25

ROUTAC 15 4 50

RP 3 5 63779.5

VC 5 16 4

VE 17 3 4

VLN 0 20 20

VLP 19 0 20

VS1 11 0 0

VS2 13 15 0

*

.MODEL D D ()

.MODEL J1 PJF (BETA=0.000242944 IS=3.125e-011)

.MODEL J2 PJF (BETA=0.000242944 IS=1.875e-011 VTO=-1.995)

.ENDS LF351

*

*** Parts Count

** Battery 2

** Resistor 9

** Capacitor 2

** NPN 1

** Sine source 1

** Opamp 1

** Macro 1

.END

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