Solid State Amplifiers > Preamps and Effects

noisy preamp

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cbg Rick:
A little background, I've been messing around with building chip amps for a few years now. (Guitar amps) Mostly battery powered lm386 amps and a couple tda 2030 amps from kits. I did take a 2 year electronics course about 30 years ago and do remember bits and pieces of what was taught.
Over the last few years I have wanted to build an amp of my own. Nothing outrageous just a simple 20 watt amp. After a couple false starts and becoming overwelmed because I bit off more than I can chew I have a new plan.
I have a TDA 2030 power amp built from a kit that works now I want to put a pre amp in front of it. I have a simple 2 transistor preamp built and according to LTSpice it works.
When I wired it up on the breadboard it does indead work but I get alot of what sounds like 60 cycle hum. The power amp is dead quiet until I put the preamp output into it. I added 2 2200uf caps across the power rails on the breadboard and it does get quieter but is still too noisy.
I scaned a copy of the circuit and attached it since I couldn't figure out how to attach a file from LTSpice.
edit; the resistor values are as calculated in spice, I used the closest values I could.
This is not the finished preamp, just the starting block to build on. I am curious as to why it is so noisy. I'm looking for input on what changes to make and where to go from here.

First thing, C2 polarity is wrong, minus side should go to input (generator). Second thing, input impedance is quite low. To start with, increase R1, R2, R6 and R8 to at least ten times the value shown on the schematic. Lastly, install a resistor (something like 220 or 470 Ohms) between the +24V source and circuit. This will attenuate any hum on the +24V source.

cbg Rick:
Thanks Loudthud, I was thinking about the input impedance. I followed a tutorial on how to bias transistors and thought the values were low, but went with what the math recomended. I have seen circuits with the small resistance in the power rail and wondered what that was for, now I know. I will give these a try and continue forward. Thanks for the help!

cbg Rick:
I have many questions about designing an amp, I'll start with these.....

1. The tutorial I followed in designing this amp started with an Ic of 5ma for the first stage and an Ic of 10ma for the second stage. I looked at the data sheet and found that the hfe for a 2n3904 maxes out at around 10ma  so those numbers make sense to me. Is that the norm for designing a 2 stage amp? Find the max gain for the trany you are using and use that Ic for the final stage and use a lower Ic / lower gain for the first stage? Is this a fair statement?

2. Calculating the resistance for the base resistor was given as use the voltage across the emitter resistor add .7V and this equals the voltage for the base resistor. Divide that by the base current, (calculated as Ic divided by hfe and then multiplied by 5 to 10). 
Using this formula ends up with a small base resistor and a low input impedance for the first stage. Starting with the collector current and 1/2 of the supply voltage at the collector makes sense to me but I'm having trouble seeing how I can get a high input impedance using this method.... I haven't really messed with it yet so I'm sure I'm missing something.

I'll stop with these questions for now.

Easy you need a high impedance buffer.

Look at all those Tube Screamer schematics and note the front end has a simple one transistor buffer, later schematics use an opamp but does the same job. 8|

Personally I'd just use opamps for a preamp. :-X


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