Hello - Page 1
Forum section: electronics / beginners

author: FunnyBunny
author_rank: Newbie
timestamp: April 12, 2014, 11:03:51 pm
content: I'm in my 40s.In college, I was a Mathematics major and took the full selection of lower division physics as well as an EE class on integrated circuits before deciding I didn't want to pursue it any further.Since then, I haven't really touched it. Until now. That means I've forgotten a lot of what I knew and probably much of what I remember isn't reliable. So back to re-learning it all.What peaked my interest - I want a phono pre-amp with some features you don't normally find, and so I started looking at circuits, and I found some circuits that are close to what I want but not exactly. So I'm going to make my own.Basically what I want is flat response (I'll do RIAA EQ in software) with two stages, and a potentiometer between the two stages so I can manually attenuate the gain from the first stage before the second stage.I've got that circuit pretty much figured out, but to power it I want a rail splitter and I probably will do the rail splitter on a separate PCB simply because it is a simple circuit for me to do many times and practice my soldering techniques to get back into things.The rail splitter, I found a pretty good circuit that will meet my needs using a BUF634 which I slightly modified to also use a TLE2426 instead of two resistors and will further modify to add a power switch, a fuse, and two LEDs. Blue LED when it has power and the fuse is not blown, Red LED when it has power but the fuse is blown. That should be pretty easy to add.I know a lot of DIY pre-amps (and even some commercial ones) don't have a power switch, but I like the idea of pushing a button to turn it off when not in use, especially if using a proper power brick that doesn't draw current when there is no load. The fuse is to help avoid frying ICs when I screw up.Both the rail splitter and the pre-amp it will eventually be used with are very simple circuits and good inexpensive circuits for me to start playing with while I re-learn what I forgot. Whether I'll continue in this once I have finished my phono pre-amp and am happy with it, that I don't know.Why I'm here at this forum - KiCad.So hello, pleased to meet all of you, and now I have me a lot of reading to do...

author: kizzap
author_rank: Unknown
timestamp: April 13, 2014, 03:28:02 am
content: Welcome! I for one look forward to reading (and seeing pictures, LOTS of pictures!) about this project.Can you give us any hints as to the features you wanted to implement?As far as the power situation stands: A rail splitter is a fairly simple schematic, but you might need to do a little maths to get the values right.Basically all you need for a rail splitter is two "identical" resistors (I'll return to this later) feeding into the non-inverting input of an op-amp (the + input), with the output of the op-amp feeding back to the inverting input (basically a buffer). the point where you source the level for the inverting input will be your "virtual ground".The variant I have been looking at is this:http://users.tpg.com.au/luce90/RailSplitter/functionality.jpgBasically it is the circuit I am describing, with a couple of extra transistors. You might be wondering why they are there. Basically, they are there so that when the load on one rail, compared to the reference ground is higher then the load on the other, that the op-amp doesn't have to sink or source a high amount of current. Instead it allows one of the transistors to let current through, which ensures that your virtual ground stays at half the voltage supply. Using the two transistors means that the op-amp will not have to work as hard, and you might even be able to use something a little cheaper, even something that is already in your parts bin.As for something a little more technical, remember how I said we would return to the "identical" resistors? Basically the resistors being identical is not too big of a deal. Hell, if you wanted you could have the divider set up to have the rails asymmetric around ground. It really depends on your needs for the circuit. Generally speaking, if you have some margin on voltage rails greater then your signal levels, you will avoid clipping.The power switch? simple, switch where the power comes into the circuit, I would suggest before the rail splitter/conditionerWhere you might need to consider something special is to stop any "squealing" when the device is turned on and off. Sadly I don't know too much about techniques in regards to this.Another thing I am not sure on: using a potentiometer in the signal chain to attenuate the signal. I would think that any potentiometer is going to introduce some kind of noise. Again I am not sure.A final thought I had: is there a Reason to have the rail splitter? if you are not careful, you could introduce noise into the circuit, or worse fry things if you connect multiple grounds together, when they are not all earth referenced. What youcoulddo, is AC couple the signal, then bias the voltage on the signal while you do your conditioning of it. That way you don't need to worry about getting the negative rail.C1, R2 and R3 in this circuit is what I am talking about.Hopefully this helps you out a little-kizzap(oh and in before c4757p ;-) )

author: c4757p
author_rank: Super Contributor
timestamp: April 13, 2014, 03:34:31 am
content: Quote from: kizzap on April 13, 2014, 03:28:02 amNot noticeably, unless there's DC bias across it.Quote from: kizzapBullshit, I'm posting on Saturday night and you're not going to post this until midday Sunday!Quote from: FunnyBunny on April 12, 2014, 11:03:51 pmHoly crap, another math dude! They do exist! Well, Iwasa math major, now I've switched to EE. Hi, FunnyBunny

author: FunnyBunny
author_rank: Newbie
timestamp: April 13, 2014, 06:00:12 am
content: Can you give us any hints as to the features you wanted to implement?Yes -Nutshell, there are some niche recording artists from the 70s and 80s that never had their music released as CD and probably never will because the market would be too small.What I want to do is needledrops, but most pre-amps are designed for playback.I want to record the vinyl to digital and do as much in the digital domain as I can rather than analog domain, e.g. application of the RIAA curve can be done quite precisely with the a filter in SoX so there is no need to even have RIAA EQ in the amplifier. In fact I believe that's what the National Archive does when they preserve vinyl to digital, record with flat response.There is however a need for adjusting the total gain applied because I'm going to go straight to a hardware ADC and I don't want digital clipping, and also there sometimes is channel bias between Left and Right in a photo cartridge.So I want flat response pre-amp that allows manual adjustment of the gain in both channels independently.That will allow me to use a Test LP to calibrate the total gain on each channel manually so they both peak the same even if the cartridge has some bias towards one channel, if my belt driven turn-table isn't exactly perfect (many have small deviation from 33.3 or 45) I can adjust the PCM data digitally before any EQ curves are applied, and any EQ curve that is applied will be quite precise.If I ever decide to needledrop 78s having a flat response pre-amp is also of benefit because RIAA curve is not correct for those, and sometimes you have to play with different curves to get one that is right.Expensive pre-amps exist that do what I want, but I have not seen an inexpensive one that allows independent adjustment of channel gain.-=-The rail splitters I have been looking at are detailed athttp://tangentsoft.net/elec/vgrounds.htmlThe first one I want to try isExcept instead of using two 220k resistors on the left side of the buffer in the diagram, use a TLE2426.I'll probably actually try several different designs, it's not like the parts will break my bank account and trying different ones and see how they perform under load is probably the best way to know what I want to use.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 13, 2014, 07:42:55 am
content: May i enquire as to why a rail splitter rather than 2 distinct supplies? This doesn't seem like it would be a portable device, and thus doesn't seem as if it would be powered by limited choice of supply?

author: SeanB
author_rank: Super Contributor
timestamp: April 13, 2014, 08:01:46 am
content: For 78RPM records I would suggest that you mix the stereo signal down to mono, that way you reduce the noise a little just by the summing. Gain is easy, just take any preamp and simply substitute the RIAA curve networks with a resistor to give a fixed gain. Even the simple USB record turntables probably have a separate preamp to do the first stage gain, so these could also be modified. I should look at my one, and see, never have taken it apart anyway. Google for the John Lindsey Hood record preamp and you get a very low noise and quite good build it yourself preamp, with a simple to replace network so you get flat response. Just be aware that there will be a few stages that combine to give the response, a few are RC circuits on the output of the opamps .http://www.keith-snook.info/wireless-world-magazine/wireless-world-articles.html

author: FunnyBunny
author_rank: Newbie
timestamp: April 13, 2014, 08:47:56 am
content: Quote from: peter.mitchell on April 13, 2014, 07:42:55 amI only want one transformer plugged in to the wall. And it is low current anyway, it's only going to line level.

author: FunnyBunny
author_rank: Newbie
timestamp: April 13, 2014, 08:54:37 am
content: Quote from: SeanB on April 13, 2014, 08:01:46 amThe advantage to doing that in software instead of the amp is that sometimes one side of the groove is more damaged than the other. In software you can just use the good channel when that happens.QuoteIndeed, that's what I am planning to do. A simple first stage with the capacitor to load the cartridge and a simple second stage with a capacitor before output.Both will be fixed gain stages with the attenuation adjustment between them.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 13, 2014, 10:23:39 am
content: Quote from: FunnyBunny on April 13, 2014, 08:47:56 amOk, that still doesn't say why not something like this;

author: FunnyBunny
author_rank: Newbie
timestamp: April 13, 2014, 04:26:29 pm
content: I can't see that image but simple DC transformers with one positive and one negative are everywhere and are cheap, since it is a low current draw application rail splitting is easy to do.EDIT - okay I could see the image by right clicking and opening in new tab, dropbox must have some hotlink stuff.That's more parts than a rail splitter needs. I've got 2 ICs and 2 caps and only 1 of the caps is really needed.

author: FunnyBunny
author_rank: Newbie
timestamp: April 13, 2014, 04:46:08 pm
content: This is basically what I'm thinkingLeft IC is TLE2426Right IC is BUF634Add a switch and fuse.And a couple LEDs - one always lit if there is any voltage (switch turned on). If there is voltage between V+ and V- it's the blue one (Power LED) otherwise red one (Blown fuse light).Blue LEDs are really cool. Gotta have a blue LED.

author: IanB
author_rank: Super Contributor
timestamp: April 13, 2014, 05:37:50 pm
content: I'm not entirely following your power supply intentions.You are drawing a battery symbol in your schematics, but there is no point using a rail splitter with a battery. You can just put two (9 V) batteries back to back.But you say plug into the wall. Do you mean a regular iron core transformer, or a wall-wart switching power supply?If a regular transformer you need rectifying, smoothing and regulation in the circuit that I am not seeing so far.If a switching power supply, there will be switching noise that may not be acceptable for high quality audio applications.But with the transformer approach, it is simple to obtain positive and negative rails with a common center rail without going to the trouble of a rail splitter. A split supply obtained directly from a transformer like this will be much more solid and reliable than manufacturing a center rail after rectification.

author: tony3d
author_rank: Frequent Contributor
timestamp: April 13, 2014, 06:15:04 pm
content: Quote from: FunnyBunny on April 13, 2014, 06:00:12 amIf you want independent gain adjustment for each channel go on eBay, and buy a Crown IC150 preamp. In the back there are left, and right pots for adjusting gain. These preamps are very reliable, and can be found for typically less than $75 in perfect working order. I still have one I bought in 1972, and it still works great!

author: FunnyBunny
author_rank: Newbie
timestamp: April 13, 2014, 06:27:29 pm
content: Thanks!