How can you tell when a modification to a schematic will/won't work - Page 1
Forum section: electronics / beginners

author: guitchess
author_rank: Regular Contributor
timestamp: December 27, 2013, 07:00:53 pm
content: Recently, I attempted to builda simple amplifier from a data sheet on an ic that I recycled from a TV. It worked when i removed it. It is a tda7265. This is the data sheet.http://www.st.com/web/en/resource/technical/document/datasheet/CD00000145.pdfI was building the single supply circuit. To simplify, I removed the standby/mute/play section and just applied direct voltage/grounds. Did I make the error of under estimating the section? Or did my noob status cause me to make some other mistake? Any tips to avoid this situation would be appreciated.Thanks

author: Rory
author_rank: Frequent Contributor
timestamp: December 27, 2013, 08:23:26 pm
content: What did you do with the mute/stby terminal?

author: c4757p
author_rank: Super Contributor
timestamp: December 27, 2013, 08:25:36 pm
content: You should probably explain what exactly...Quote from: guitchess on December 27, 2013, 07:00:53 pm...is, because I don't know what the question is supposed to be.

author: Alana
author_rank: Frequent Contributor
timestamp: December 27, 2013, 08:52:37 pm
content: I think this amp may do not like single supply because if was purposely build for +/- power but show us what you did .

author: guitchess
author_rank: Regular Contributor
timestamp: December 27, 2013, 09:21:59 pm
content: Mute/standby (pin 5) went to ground.Sorry, "this situation" is rather vague. I'm merely interested in the tips/ideas/thought processes you more experienced electronics gurus use to determine what is/is not extemporaneous in unknown circuits.Is the design meant for a +- power supply? I too initially thought it was, but after more study, I decided that it meant that pin 5 (standby mute) compared it's input to Vs to determine the ic output state. Please let me know if I completely misunderstood.Thanks

author: madshaman
author_rank: Frequent Contributor
timestamp: December 27, 2013, 09:27:02 pm
content: Honestly? I'd recommended understanding how each circuit works, feasibility of suitable modification or replacements really should be obvious at that point.

author: c4757p
author_rank: Super Contributor
timestamp: December 27, 2013, 09:33:08 pm
content: Look in the datasheet, of course. Under "electrical characteristics", you see:[ref: +Vs]Mute / Play Threshold: -7..-5V, -6V typ.Stand-by / Mute Threshold: -3.5..-1.5, -2.5 typ.A bit confusing, if you don't notice the [ref: +Vs] at the top of the section, but that is saying that the threshold voltage for mute/play is between 5 and 7 volts below V+, and for standby/mute, between 1.5 and 3.5 below V+. You should be able to directly apply those voltages to those pins. The circuits they give as examples just shift digital signals (0-5V) to those levels.

author: guitchess
author_rank: Regular Contributor
timestamp: December 27, 2013, 10:49:52 pm
content: So I am wrong in thinking that pin 5 can go to ground? Ground is less than Vs -6. Or does it mean that at least some voltage must be applied as long as it is less than Vs -6

author: guitchess
author_rank: Regular Contributor
timestamp: December 27, 2013, 11:10:24 pm
content: Quote from: madshaman on December 27, 2013, 09:27:02 pmWhile I completely agree in theory, I don't know how achieving that understanding would be possible without building/modifying circuits. I'm too cheap/impatient to build a countless amount of flashing led circuits till then. In this case I'm simply experimenting with an ic that I salvaged from an old TV.

author: Rory
author_rank: Frequent Contributor
timestamp: December 27, 2013, 11:55:43 pm
content: Quote from: guitchess on December 27, 2013, 10:49:52 pmLook at figures 9 and 10 of the datasheet. It tells you all you need to know about the voltages on pin 5.

author: Rory
author_rank: Frequent Contributor
timestamp: December 27, 2013, 11:57:28 pm
content: Quote from: Alana on December 27, 2013, 08:52:37 pmFigure 2 of the datasheet shows it with a single supply.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 28, 2013, 12:05:09 am
content: QuoteI'm not familiar with the IC, quickly reading the data-sheet it doesn't seem to suggest that pin 5 can't be at 0V and you might expect that to work given that it is below the threshold at which the amp should un-mute.However I notice that the graph which shows the operation of the mute-stand-by cicruit does not show pin 5 going below Vss-10V and none of the example circuits drive pin 5 to 0V.It might be instructive to use a pot as a voltage divider so that you can vary the voltage on pin 5 and verify the operation.Sometimes data-sheets omit stuff or encode it in example circuits without explicitly stating it.

author: Rory
author_rank: Frequent Contributor
timestamp: December 28, 2013, 12:28:27 am
content: Quote from: guitchess on December 27, 2013, 09:21:59 pmCan be used with either split or single supply. Examples of both are in the datasheet.In my experience, the best way to understand any particular device is to study the datasheet thoroughly.

author: Rory
author_rank: Frequent Contributor
timestamp: December 28, 2013, 12:39:45 am
content: Quote from: guitchess on December 27, 2013, 11:10:24 pmHands on experience is good if it helps you understand some fundamentals. Do you understand voltage dividers? How about transistor switches? If you can understand both of these concepts, you will be able to figure out what the mute pin needs to see to make the amplifier work. You can also measure the current draw of the amp chip to determine whether it is in operate, mute or standby modes.

author: guitchess
author_rank: Regular Contributor
timestamp: December 28, 2013, 01:32:17 am
content: I do understand some of the fundamentals, but only on a beginner's level. I get voltage dividers and transistor switches but when you start mixing them together along with caps of various purposes, then lay them out differently in every schematic, things get fuzzy.To further illustrate my inexperience, I hadn't even thought of checking the current to see the state of the ic.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 28, 2013, 04:05:57 am
content: Quote from: guitchess on December 27, 2013, 10:49:52 pmUhh, i don't know if i read it here, but what IS Vs with reference to ground, eg, what power supply ARE you using? I hope atleast 12vAlso, try a small RC to ramp the voltage on the pin down to Vs-6, some devices don't like starting up in odd conditions.Also, i think that pin can only handle around 5ma or so.What are you setting your gain to with what resistors?what is your input?

author: guitchess
author_rank: Regular Contributor
timestamp: December 28, 2013, 05:54:50 am
content: A 12 volt PS is what I am using to power this amp. My next experiment is going to be exactly as you have suggested, to ramp down to the -6v. Gain is set by what is supposed to be 30k resistors according to the datasheet. I only had 33k, so that is what's in there now. The input signal that I'm using for testing is a square wave generator that I built from a plan online, generating a 1khz tone.

author: Unknown
author_rank: Unknown
timestamp: December 28, 2013, 06:23:30 am
content: Uh, I've built a few amps using these things in the past. No need to get fancy, and no need to over-interpret the datasheet - tying pin 5 to ground is fine.edit: check your supply voltage - I seem to remember they needed at least 12v, better 15v, in single supply mode.

author: grumpydoc
author_rank: Super Contributor
timestamp: December 28, 2013, 09:56:09 am
content: QuoteThe data-sheet says min supply is +/-5V which would imply a single ended supply of 10V - not saying you're wrong about the 15V except that sometimes experience does, indeed, trump data-sheets.It would be useful if guitchess could post his exact schematic.

author: guitchess
author_rank: Regular Contributor
timestamp: December 28, 2013, 02:40:51 pm
content: This is the original version with all it's mistakes. I have corrected the Incorrect polarity of the decoupling caps and added the filter cap.I realize that It is not a very elegant layout. This was my first attempt at makinga part in Eagle. It is entirely possible that my issues with the amp are because of a failing in this step. That is why I never really intended for thus to bea help it won't work post. But I'll take any input i can get