Author Topic: Want to build insane preamp  (Read 5631 times)

Ken W

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Want to build insane preamp
« on: December 21, 2007, 03:27:55 AM »
Hi.  I'm new here, but not to musical electronics (age 9-49), and I want to build an insane solid-state preamp.

The situation is, I was given a busted Marshall IBS MOSfet bass amp.  The pots are nearly all broken or previously broken and badly repaired, but the unit works. I don't need a 200 watt bass amp, let alone one with this fancy EQ.

On the other hand, a 200-watt portable PA/guitar amp would be super.  I have a nice little Mackie mixer, so I don't need more mixer controls, just an external input.  I don't need much volume, I don't play out much, and a 200-watt PA would nicely suit instruments, laptop, and vocals at the scale I do play at.  I am not particularly a purist for tube guitar sound, although playing my new Strat through a solid-state Fender and then my old Zoom 9030 recently made me want to put the Zoom back to work processing drum sounds in bizarre electronica.

Anyways, my (tentative) plan is to do the project in a modular manner as follows.
1. remove existing preamp, leaving power amp input jack on rear
2. build small mixer/fx loop circuit to allow preamp signal, external inputs, and effects loops into power amp.
<at this point, I have a functional 200w power amp>
3. build variation of LXH2 Fender preamp.
4. build LXH2 Fender speaker simulator.
5. using what I've learned so far, build variation(!) of LXH2 Marshall preamp.
6. build LXH2 Marshall speaker simulator
<at this point, I have to decide whether I am REALLY insane>
assuming the answer is "YES", then..
8. drill the 10 more knob holes the chassis has room for
9. design an over-the-top lead channel - I have some ideas here
10. build the third channel

1 & 2 will happen real quickly, as soon as this project has enough priority to take over my mini-shop (i.e. I have a design for the first preamp).  One major change I'm considering is to replace some of the early opamp stages with triode-biased FETs.  I'm thinking if I choose the FET carefully, I can run them from the +/-15v supplies, use higher-pinchoff FETs, and keep them a bit more linear (i.e. cleaner) at lower levels.

Any thoughts?

Ken W

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Re: Want to build insane preamp
« Reply #1 on: January 01, 2008, 09:49:47 PM »
Well, on the other hand...

I was packaging the power amp to work after removing the Marshall preamp completely, and...

although the big power FETs are all good, and run cool, all four of the OTHER active transistors in the amp run so hot you can't touch them - little TO-92 plastic things with no heatsinks.  So I got a schematic, checked the amp against it, found they had indeed used those parts values, and wondered - is it characteristic of the Marshall sound, in whatever form, to run components just barely below destruction?


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Re: Want to build insane preamp
« Reply #2 on: February 08, 2008, 05:51:01 PM »
I personally think some excellent Fets you can use would be the 2N5457 JFets.


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Re: Want to build insane preamp
« Reply #3 on: February 09, 2008, 02:52:21 AM »
Fairly old post, which I was going to answer but somehow forgot... Anyway, I assume we talk about IBS 3520 or 5522?

The concerned power amp runs the voltage amplifier transistor T3 and it’s current source T4 pretty much at their limits (300 mW idle dissipation). No, it won’t affect their sound but their life span is definitely decreased tremendously. On that part it’s just bad design.

If the differential input transistors (T1 & T2) or the current limiter transistors (T5 & T6) are hot then the amp has a fault and you need to fix it. Well, I think one of the differential devices could run quite hot since the differential currents in that design are so out of balance. (200 mW idle dissipation vs. 20 mW in the differential).

You should likely do an overhaul to the circuit. Use the trimmer to adjust the DC offset at output to moderate level if you can: That trimmer can't help much with the poor differential design, the currents are in inbalance along the input impedances so the DC offsets are extreme (they may be several volts at worst). Unfortunately, the lower offset you adjust, the harder the output devices are run. These amps were introduced in the late 80's so values may have drifted. Also, electrolytic capacitors are getting fairly old. No bias adjustment possibility in that amp.


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