Menu

Show posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

Show posts Menu

Messages - cbg Rick

#1
Preamps and Effects / Re: noisy preamp
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.
#2
Preamps and Effects / Re: noisy preamp
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.
#3
Preamps and Effects / Re: noisy preamp
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!
#4
Preamps and Effects / noisy preamp
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.
#5
Amplifier Discussion / Re: Etching your own boards
January 02, 2019, 09:47:24 PM
I did a test with 3 different pens, a cheap permanent marker, Industrial Sharpie and Sharpie oil. The industrial Sharpie worked best. The etchant ate through the cheap marker and I couldn't control the oil Sharpie, no fine lines with it. If I can figure out how to post pics I'll post the results.
#6
Amplifier Discussion / Re: Etching your own boards
January 02, 2019, 01:53:47 PM
I remember using a Sharpie black magic marker to fill in traces. I have seen recomendations to use oil based pens, do oil based pens work better than the regular permanent markers? I am just trying to gather info before I attempt etching my own. My plan for my first attempt is to draw the traces with a Sharpie and see how well that works before I tackle learning to use a CAD program to draw the circuits. 
#7
Amplifier Discussion / Re: Etching your own boards
January 02, 2019, 10:13:40 AM
Thanks for all the info!  :dbtu:
#8
Amplifier Discussion / Etching your own boards
January 01, 2019, 08:03:01 PM
I'm looking for input on how you guys make your "one off" circuit boards.
I've been messing with a chip amp. I built it on a breadboard then on perfboard, now I want to etch my own circuit board. Back in the  day I used the rub on stencils from Radio Shack to etch my own boards. Searching the internet I see the stencils are still available but the reviews are less than stellar and I could only find 2 different suppliers of the rub on style stencils (both with bad reviews) I did a quick search and found a "how to" on using a printer, transfer paper and heat to put the circuit layout on the copper clad board. Just wondering what options are out there to make a "one off" circuit board. 
#9
No smoke yet, but it was close. I followed the original schematic and breadboarded the circuit. I used 2n3904 and 3906 transistors for Q1,2 & 3 for the output transistors I substituted NTE 152 & 153. With a 35V supply I adjusted R5 for half the supply voltage at the emitters of the output transistors as per the instructions, after about 5-10 seconds the output transistors were to hot to touch. Not wanting to let the smoke but wanting to hear it work I put a 50 ohm transistor between the collector and supply, collector and ground of the output transistors, readjusted for half the supply at the emitters. Everything was cool  8) and the amp does amplify.
So my question is what is the proper way to get the current under control in the output transistors? Would lowering the values of R9 and R10 put less bias voltage across the BE junction of the output transistors and in turn lower the CE current?
#10
I vaguely remember needing to add a small resistance to help clean up crossover distortion, I just don't remember it being called a VBE multiplier. The posts here are helping a bunch and I appreciate you guys taking the time to add your thoughts and sharing your knowledge. I've downloaded LTSPICE and am working on figuring out how to use it. Seems like a great tool.
#11
I'm going into this project knowing that there are better ways to build an amp. I realize the transistors are probably no longer available and substitutions will have to be made. Its a learning exercise.
The points you guys are making about this circuits short comings are not falling on deaf ears, I understand it is a far from perfect amp. I don't think any children or small animals will be harmed if I build it, so I am going to forge ahead. :cheesy:
In case I didn't mention it before, it has been 30 years since I did any real electronics work so I have forgotten a bunch of stuff. It is slowly coming back as I study the replies and google the stuff that I don't understand. I need to look into this VBE multiplier that you mention..... Would it also work to use the BE junction of a TO-220 transistor to replace the diodes and clamp that transistor/diode to the same heatsink? 
#12
Quote from: Roly on May 04, 2015, 11:40:15 PM
I have to say again that the redesign required to turn the original Ge+Si "Lil Tiger" into a workable Si-Si "Runcible Lil Tiger"

I'm sorry but, you keep referring to this amp as having Ge transistors, I checked the transistors in the parts list and I'm not seeing any Ge transistors listed. Am I missing something?
#13
I had issues with motorboating when I ran it through the TDA2030 also. I'm in the process of soldering the preamp onto perfboard now. I think I read the thread you linked to, but I'll read through it again.
#14
Quote from: Loudthud on May 03, 2015, 11:41:51 PM
I have attached my version of a similar circuit. No global AC feedback puts the gain up around 100. The output impedance is not as low as you might think, but it depends on the beta of the output transistors you use. In the one I built, it was around 5 Ohms. Power the preamp from a decoupled supply from the main Voltage rail. Add enough capacitance to stop the low frequency motorboating.

I appreciate you taking the time to do this. I will breadboard this amp and see what happens.
I got a bit sidetracked from this project, I built a Fender Blackface FET preamp  https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CCsQFjAE&url=http%3A%2F%2Fwww.redcircuits.com%2FPage120.htm&ei=ztpHVfmmGMKNNuWVgNAL&usg=AFQjCNG6A16LMLO7h7v6qNx0MGwYvWLn3g&sig2=aGc3cA9fmjTxmGkb-utV_Q&bvm=bv.92291466,d.eXY and in a rush to hear it work used it in front of a TDA 2030 amp I had built from a kit.
The Runcible Lil Tiger or "Loudthud" is moving back to the top of the todo list now. Thanks again.
#15
Quote from: Roly on April 30, 2015, 11:54:35 PM

The base current for the first transistor is derived from the output "half-rail" (emitters of Q4 and Q5).

At power-on Q3 and Q5 start to conduct until the half-rail rises to where Q1 brings the rise under control.

This bias is also therefore a DC (and AC) negative feedback path from the output.  The pot R5 is used to adjust the half-rail so it idles at half the voltage of the supply (and the AC gain ends up being whatever it is at that setting - a runcible arrangement IMO).

I'm  seeing how this might not be the best circuit to learn on. I'm going to forge ahead with it for now.
In the mean time I have built a FET preamp on my breadboard and gotten it to work :dbtu: it's funny how little accomplishments like that can be so inspiring.