Starting my first design project - I am looking for input, comments, and advice - Page 1
Forum section: electronics / projects

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 04, 2012, 10:37:38 pm
content: In summary, my project will function in the audio domain and replace a two NPN transistor amplifier from the 70's with a modern discrete operational amplifier. I am going to design it from scratch and would like to have a professional PCB fabricated and populated as a prototype in the near future.My first question is: What is the best way to interface with this forum? Obviously, I will need to post data like wiring diagrams, etc. What program would you suggest that I draw the schematic out in? I'm a complete noob when it comes to CAD.

author: IanB
author_rank: Super Contributor
timestamp: January 04, 2012, 11:54:51 pm
content: Just post your diagrams as pictures or images. People won't want to download any special files to view things.

author: olsenn
author_rank: Frequent Contributor
timestamp: January 05, 2012, 12:16:56 am
content: Don't feel ashamed to ask around for help or ideas here; we're all still learningAs for your project, what exactly is the goal? You say it's a "function in the audio domain", and that it will involve replacing transistors with an op-amp... is your goal to make a speaker/headphone amplifier to simply boost the volume/power of the source? If that's so, chances are those two npn transistors are actually one npn and a pnp (push-pull), in which case, provided you don't need a whole lot of power, replacing them with an op-amp is a very good idea. Check out the Cmoy amplifier (http://tangentsoft.net/audio/cmoy-tutorial/), it does precisely this. Also make sure to look at the linked articles about which op-amps to use (don't try using a ua741, it will SUCK), which capacitors to use, and how to go about designing a buffered rail splitter (pretty easy).If this was not your intent, please just clarify what you're doing and myself or someone else on this forum will gladly help you I'm sure. As for which CAD software to use, I would start with Eagle, simply because there's a free version available and lots of parts already exist; however, if you want something a little more intuitive to learn on, then I suggest diptrace. As always though, breadboard it first and worry about making a professional pcb later.Good luck!

author: slateraptor
author_rank: Frequent Contributor
timestamp: January 05, 2012, 01:05:08 am
content: Quote from: olsenn on January 05, 2012, 12:16:56 am2 NPN transistor circuit suggests 1st stage common emitter cascaded with 2nd stage emitter follower, in which case, an op-amp based replacement will be a huge improvement...unless it was erroneously assumed that both transistors were NPN.Quote from: olsenn on January 05, 2012, 12:16:56 amHaters gonna hate.I had a lot of respect for that CMoy guy...until I readthis. Qualitatively interesting, but all the sonic this and sonic that nonsense really turned me off with a quickness.

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 05, 2012, 11:24:05 am
content: Quote from: olsenn on January 05, 2012, 12:16:56 amHere is my own hand drawn schematic of the circuit I will be replacing; it is for a very rare Hohner electronic piano.My goal is to replace the schematic I posted with a high fidelity discrete operational amplifier. I want to improve linearity, lower distortion, lower the output impedance (to interface with professional musical equipment), etc.I have a copy of Eagle 5.0. I tried using it but I couldn't find any surface mount footprints in the bundled library:(

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 05, 2012, 01:24:39 pm
content: Here's what I want to do. I have not computed the values or selected components yet so this is basically just a really detailed block diagram.L1 and L2 lower the noise floor. I chose an emitter follower instead of Class AB to eliminate crossover distortion.I'm hoping that the VAS design eliminates the early effect. I might have some trouble computing the MC and dealing with phase issues, so if anybody has some suggestions or links, please let me know. I have to admit that Miller Capacitance, early effect, and phase related issues are my three biggest weak points.I couldn't fit all of the DC blocking and filter components at the input. I was also thinking about putting a servo on the output and eliminating the DC cap but I'm not sure how well it will play with the emitter follower. LTP cascoding?Do you think that I could squeeze all of this stuff on a 2cm by 10cm board with SMT?Does this design look like a good start?

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 06, 2012, 03:13:24 pm
content: The first draft that I posted was garbage. I stripped everything down and here's what I've got now.

author: slateraptor
author_rank: Frequent Contributor
timestamp: January 06, 2012, 07:10:14 pm
content: Either there aren't many old school analog engineers on this forum, or the lack of response is a clear indication that a remedial review of discrete amplifier analysis is in order...myself included.BTW, what program are you using to draw up your schematics?

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 06, 2012, 07:48:34 pm
content: I know that I need to review more about discrete operational amplifiers. I've got several books to consult for biasing once I get an idea together that I am happy with, but they only tackle component blocks instead of the entire op-amp.I am anticipating problems with Miller capacitance and the early effect in the VAS section, as well as phase issues throughout the whole thing. This is partially why I chose to seek input and advice on this project. I can't find any respectable sources that explain how to design an op amp from input to output.The other reason that I chose to seek advice and input is for the CAD phase of my project. I am totally clueless when it comes to circuit board design. I tried Eagle 5.0 and couldn't figure out how to locate a library with SOIC components or SOT -23 stuff - assuming that they are even in there.Circuit Wizard is what I am using to draw the schematics. It is easy and playful but probably worthless for anything else.

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2012, 07:53:05 pm
content: It may help with circuit reviews if you could post diagrams in an image format like gif rather than pdf. To be viewed the pdf has to be downloaded and opened separately. It might not seem much, but it is an extra hurdle that people have to be motivated to overcome.

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 06, 2012, 08:49:28 pm
content: here's a .GIF of my latest version.

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 06, 2012, 08:58:42 pm
content: Here's the circuit I'm replacing:

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2012, 09:01:35 pm
content: Quote from: DaveHardy on January 06, 2012, 08:49:28 pmThat's much better in terms of being able to see your diagrams. You may get a few more contributions with the improved ease of viewing.Unfortunately I can't help you on a technical level as what you are doing is way beyond my expertise. The simple question that comes to mind of course is why discrete? Why not use an IC op amp?

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 06, 2012, 09:15:17 pm
content: Quote from: IanB on January 06, 2012, 09:01:35 pmWell, when I started, I thought that I could incorporate a purely class A design in the output which would avoid crossover artifacts. Unfortunately, I haven't found a design I could study with a good emitter follower though. In theory, a discrete operational amplifier should have less distortion and noise if there are fewer gain stages and higher quality caps. Don't get me wrong, I'm going to try a 5532 and a few others once I get to the breadboard stage.

author: Fox
author_rank: Regular Contributor
timestamp: January 06, 2012, 10:18:03 pm
content: Hi DaveHardy,i haven't looked in much detail in your circuit, but as a general suggestion put a lowpass filter at your input( a 100 or 200kHz or there about).So you get a defined input impedance and you won't pick up RF noise witch can cause all sorts of problems in an amplifier.For amp designs an diy audio stuff in general the diyAudio forumwww.diyaudio.comwould be a good place for research.

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 07, 2012, 12:51:46 pm
content: Quote from: Fox on January 06, 2012, 10:18:03 pmI was going to do that and use ferite.

author: jimmc
author_rank: Frequent Contributor
timestamp: January 07, 2012, 06:01:36 pm
content: A a few quick thoughts ...The collector of Q7 should be connected to the base of Q9 (and not Q8)As shown drive to Q9 is only via (left hand) D7 & D8 - large cross-over distortion.The junction of (right hand) D7 & D8 should not be connected to the output.As shown they shunt the drive to Q8 & 9.The junction of (left hand) D7 & D8 should be connected to the output.Are these to protect Q8 & 9 against possible E-B reverse breakdown with reactive loads?Watch device dissipation of Q14 particularly under overdrive.Jim

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 07, 2012, 10:18:34 pm
content: Quote from: jimmc on January 07, 2012, 06:01:36 pmHey, thanks! I was concentrating on the VAS and just threw up the output section without thinking - I'm glad you caught it. I still want to do class A output but I'm not sure that I'll be able to get the low distortion that I want without the power.Do you have any suggested reading for designing these things? I shouldn't have a problem biasing it section by section, but I am pretty scared of strange phase issues for the amp as a whole as well as early effect, pole frequencies, etc.

author: jimmc
author_rank: Frequent Contributor
timestamp: January 07, 2012, 11:01:43 pm
content: I don't know how I missed it before, but all the diodes in your latest cct are backwards (reversed biased)!Jim

author: DaveHardy
author_rank: Regular Contributor
timestamp: January 11, 2012, 03:08:37 pm
content: Uh, this solves my problem I think. I'm hoping that my source impedance has low current noise.http://www.analog.com/static/imported-files/data_sheets/SSM2019.pdfThanks guys!!