Can I power this 6V preamp from 12V? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: October 13, 2019, 03:24:21 pm
content: So I found this preamp circuit on google and it says that it needs a +6 and -6v PSU, Can I power it whit + and -12v cause the amp I’m trying to design needs a hell of a lot of power and a computer PSU is the only powerful enough thing I have. And it has a +12 v and -12v outputs. So can I power it whit that?And yes I know that you can make a virtual ground from resistors and capacitor and yes it would probably work since the only ground in this circuit is for the input and output. The problem is that my amp probably wouldn’t work whit it then because it would share the 12v railSo can I somehow make it work?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 13, 2019, 03:38:23 pm
content: If you're using a computer power supply, simply use a pair of linear regulators (LM7806 and LM7906) to reduce the voltages from 12 to 6V. This will help give you some isolation from the noise of the computer power supply, which you are probably going to need lots of. In particular, the -12V rail tends to be pretty nastily regulated since it's seldom used at all.

author: AVGresponding
author_rank: Super Contributor
timestamp: October 13, 2019, 04:50:31 pm
content: Also carefully check the specs on the current output of the -12v rail, it's probably quite small, less than half an amp.That may not be sufficient to your needs.EDIT: Taking a close look at your circuit, it doesn't use a ground rail, so you're fine to use a 12vdc supply. Since the main decoupling cap is only 470uF, the whole thing probably uses only a couple of hundred milliamps or so.Best way to find out is to build it and see!

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2019, 04:53:57 pm
content: No, that is a simple single supply pre-amp with AC coupled input and output.The supply voltage markings are deceptive, it only requires a single +6V supply. The line marked 6V- is ground, both for the supply and for the input and output.It was almost certainly intended to be battery powered (possibly explaining the markings as in '6V battery +ve and -ve terminals').P.S. Can you please edit the thread title in your OP to be less clickbait - Something like Transistor preamplifier supply, that might help others in the future.

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2019, 09:48:30 pm
content: Quote from: ELS122 on October 13, 2019, 08:51:10 pmSo you built adifferentcircuit to the one you posted and asked about and now expect us totell youguess why your tiny test speaker is burning? Maybe its tiny heart is broken? Anyway I think life is probably too short to care.Maybe clickbait was the wrong word (although yours seems just as bad), but other people seem to manage to think of a sensible topic related titles when they expect help from others! Yours conveys no meaning.

author: mikerj
author_rank: Super Contributor
timestamp: October 14, 2019, 08:27:56 am
content: Quote from: ELS122 on October 13, 2019, 08:51:10 pmClickbait titles deliberately leave out certain details to make people curious. You title leaves out the design which you are asking questions about, imagine if every topic on here was "Please help" or "This doesn't work" or "What's wrong?". It would make the entire forum worthless.You haven't even provided the schematic of this different design that you built, do you expect us all to be mind readers?

author: soldar
author_rank: Super Contributor
timestamp: October 14, 2019, 09:02:42 am
content: Quote from: Gyro on October 13, 2019, 04:53:57 pmAgreed.Quote from: Gyro on October 13, 2019, 04:53:57 pmAgreed. Please use descriptive titles because otherwise the forum becomes unusable, especially because mouse-overing the titles does not give a preview like it does in other forums. Please use descriptive titles.

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2019, 11:50:27 am
content: Quote from: ELS122 on October 14, 2019, 11:41:22 amThe help you received on your posted circuit was polite, as was the request to change the title.Quote from: ELS122 on October 14, 2019, 11:43:06 amAgreed, I think you'd better ask elsewhere. See if you get a better reception for a badly phrased question about some unidentified circuit - most other forums are nowhere near as polite. You seem to have ruined your chances of help here with your bad manners anyway.

author: mariush
author_rank: Super Contributor
timestamp: October 14, 2019, 11:53:58 am
content: Quote from: ELS122 on October 13, 2019, 08:51:10 pmIf it's a DIFFERENT circuit, then how could the people on this forum tell you what's wrong with it without SEEING the circuit schematic?Are they supposed to make random guesses?The -12v on a computer power supply it's often unregulated, because it's only used for serial ports on computers, and that needs less than 50-100mA of power.You can use linear regulators to bring this -12v down to something reasonable, for example 7905 regulators will bring voltage down to -5vIf you don't have negative regulators on hand, simple diodes like 1n400x would drop the voltage by ~ 0.7v ... so you could have around 6-8 diodes in series to drop 4-6v from -12v

author: tooki
author_rank: Super Contributor
timestamp: October 14, 2019, 12:01:15 pm
content: Quote from: ELS122 on October 14, 2019, 11:41:22 amA good thread title (like a good email subject line) is a super shortsummaryof the question/message. For example, a good title for this thread might have been "How do I power this audio preamp circuit?" The key is that it should let the reader know what the post is about. "Would this work?" does not do that in any way, because "this" isn't known to the reader yet! It's not clickbait, but it's still a terrible title.Quote from: ELS122 on October 14, 2019, 11:43:06 amWhat creativity? A thread title with no information isn't creative, it's just a waste of others' time.As for the other circuit, you asked aseparatequestion, while providingnoneof the information needed to answer it. As the one asking for help,youshould be making the effort to make it as painless as possible for people to respond. When you withhold key information, you're wasting others' time.This is one of the fundamental rules of forum etiquette, not just on this forum.(Indeed, it's a good rule of thumb for all questions, regardless of whether online, real life, telephone, etc.)I highly suggest you read the following guide to asking questions online, written by a prominent computer scientist, Eric Raymond. While it's written with computer questions in mind, the general principles apply to any kind of technical venue. -->http://www.catb.org/~esr/faqs/smart-questions.html

author: Simon
author_rank: Unknown
timestamp: October 14, 2019, 12:22:00 pm
content: Just use linear voltage regulators to drop 12v to 6V, this will also clean up switching noise from the SMPS. A PC PSU does not provide much -12V paver.

author: ogden
author_rank: Super Contributor
timestamp: October 14, 2019, 12:41:56 pm
content: Input and output of this amp have DC blocking capacitors, circuit does not have ground connection whatsoever. It means that indeed it can be powered from single, unipolar +12V supply (leaving -12V rail alone). Connect +6 to +12 and -6 to ground, done.

author: tooki
author_rank: Super Contributor
timestamp: October 14, 2019, 05:22:01 pm
content: Quote from: ELS122 on October 14, 2019, 12:05:15 pmHuh? I think someone is a few beers short of a six-pack...

author: Rigolon
author_rank: Regular Contributor
timestamp: October 14, 2019, 08:57:13 pm
content: Quote from: Gyro on October 13, 2019, 04:53:57 pmI'm not very experienced yet, could you help me understand what made you think that? I don't see a problem with being +6V to -6V rails. It looks like a part of a bigger circuit where they had those voltage levels to use. For me I would think that they wanted a 12V potential difference, but didn't have the 12V or didn't want to use the ground rail for some reason... I don't really know how could you be so certain that it's +6V to GND.Quote from: ELS122 on October 14, 2019, 11:41:22 amOk... so maybe you felt people were a little aggressive, or so to say. But they do have a point. I guess it should be better to try and filter the information (the comments that help you understand the community and how optimize the way the forum is used) and ignore the rest (the aggressive behavior). I believe everyone here wants the best for the forum and to help each other.There are a lot of replies here that just don't add anything.That being said, I'm not experienced and don't know how Gyro and Soldar got to know that -6V is actually just ground. But I would believe that the schematics are right, so the potential difference there is 12V... As you do want to use a 12V power supply, I would test the circuit with +12V in the +6V line and ground to the -6V line. I would also try what Gyro and Soldar said about just using ground to the -6V line and see if it works out.Try it out and let us know.Just to empathize, any modification you make try to upload schematics and pictures, it always help to solve the problem faster.

author: soldar
author_rank: Super Contributor
timestamp: October 14, 2019, 10:24:23 pm
content: Quote from: Rigolon on October 14, 2019, 08:57:13 pmNowhere in the schematic does it say "-6V". That's your clue right there. The schematic does not have two points, one labeled "+6V" and the other "-6V", and even then it could be a single 6V supply.The way it is drawn, with a thicker line for negative, clearly indicates that is ground.That schematic is at least 25 years old and I have been reading schematics for decades. I interpret that to be a single 6 V supply. I would bet that was designed for 6V operation and calculating operating currents would confirm it even though it should have a very wide range and probably operates well at 12 V.It also does not look like the best design. Maybe someone would want to model it in LTSpice and see the response.

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2019, 08:23:09 am
content: Yes. The schematic doesn't say +6V and -6V. It has two lines, one saying + and the other saying -, with a '6V' near (below and above) the line. That was a big clue in an old hand drawn schematic. As I said previously, it's the way someone back then would have drawn it if it was going to be connected to a battery.The other clue is looking at the polarity of the electrolytic capacitors on the input and output. Both are oriented expecting the internal circuit nodes to be positive relative to the input and output signals.EDIT: The final clue is the absence of a marked 0V rail as an input and output reference. Without a 0V rail, there can't be separate +ve and -ve supplies.

author: Rigolon
author_rank: Regular Contributor
timestamp: October 15, 2019, 12:49:00 pm
content: To Soldar and Gyro,I see, thank you for teaching me something new. I've misinterpreted the 6V on the lines as +6V and -6V.

author: soldar
author_rank: Super Contributor
timestamp: October 15, 2019, 07:01:54 pm
content: This preamp is very similar. It is taken from a book published in 1970 and mentions it uses silicon transistors which were relatively new at the time.It is not something I would build today except as a learning experience and to analyze it.Note that it is designed for 24 V but it can be redesigned for lower voltages by just changing the values of a few resistors.

author: Zero999
author_rank: Super Contributor
timestamp: October 22, 2019, 07:01:54 pm
content: Quote from: ELS122 on October 22, 2019, 02:14:44 pmThat's because it's an AC coupling, not DC coupling capacitor..If your 1μF was conducting, it was either because it was damaged or connected backwards.