Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: cbg Rick on February 14, 2019, 02:33:45 PM

Title: noisy preamp
Post by: cbg Rick on February 14, 2019, 02:33:45 PM
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.
Title: Re: noisy preamp
Post by: Loudthud on February 15, 2019, 02:33:05 AM
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.
Title: Re: noisy preamp
Post by: cbg Rick on February 15, 2019, 10:36:28 AM
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!
Title: Re: noisy preamp
Post by: cbg Rick on February 15, 2019, 04:04:46 PM
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.
Title: Re: noisy preamp
Post by: phatt on February 16, 2019, 01:14:16 AM
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
Title: Re: noisy preamp
Post by: Loudthud on February 16, 2019, 01:29:26 AM
The 2N3904 is not a good choice for a low level preamp. If you look around at available schematics, you probably won't find a preamp or a distortion pedal made with discrete bipolar transistors since the 1970s. Don't try to reinvent the wheel, use opamps. FET input opamps like TL072 can easily achieve 1 Megohm or more.
Title: Re: noisy preamp
Post by: cbg Rick on February 16, 2019, 10:50:58 AM
Thanks for the help and advice guys  :dbtu: This may seem like a pointless exercise to you guys but I feel the need to make a go at this build just to learn how to make a handfull of components amplify a signal. I had forgotten how much math is involved in designing even a single transistor amp. I have found a few tutorials on You Tube that are very helpful and I will try to use those more going forward.
Using a FET as a buffer is a great tip, I have some FETs in my stash of parts so that will be my next step.