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<P><FONT color=#000000><FONT size=2><FONT face=arial,sans-serif>Ah,now I understand better. You don't need the pre-amp to feed into a "home" stereo amp, you need it to feed into a car amp. Since car amps are designed to be driven from the output of a car stereo, they need a lot more juice than your typical microphone pre-amp will drive. Using the Cmoy makes a lot more sense to me now. </FONT></FONT></FONT></P>
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<P><FONT color=#000000><FONT size=2><FONT face=arial,sans-serif>Putting the volume control pot in place of the "Rg" in the non-inverting circuit is a good way to do it. If you replace both "Rg" and "Rf" with the pot (connecting the wiper to the non-inverting input), you get much greater amplification range. Of course, for this circuit you don't need or want that. The other way to do it is to just hang the pot right off the output. This works great if the impedance of what you are driving is substantially higher than the pot value. That would be the case for a line input on a home stereo, but I'd bet that the input impedance of the car amp is pretty low, so probably not a good way to do it. Another point, for people who aren't all that familiar with electronics, is that when you turn a pot all the way to one end, it has a resistance of zero - a dead short. Depending on the circuit, this may cause damage. The typical solution is putting a resistor in series with the pot. It is not uncommon to see pots with series resistors on both sides, particularly in feedback loops. </FONT></FONT></FONT></P>
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<P><FONT color=#000000><FONT size=2><FONT face=arial,sans-serif>I consider the skillful use of ferrites to be one of the Black Arts. Another trick with computer supplies is to use them to feed a linear supply circuit. A couple of big ol' caps help to tame the high frequency garbage. Add an adjustable regulator on a heat sink, and you have a handy variable supply. But you will still probably need ferrites!</FONT></FONT></FONT></P>
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<P>My comments about the resistors was more in response to the writeup on the Cmoy. There are a lot of things about that circuit that have a greater effect on the output than the resistor tolerences. You are right on about only needing to be close and inexpensive. I remember the days of carbon resistors. A high quality circuit would use 10 percenters, maybe even a few 5 percenters for critical signals. Everything else was 20 percenters. 1% resistors were usually wire wound, and very expensive (compared to other resistors). The manufacturers would make the carbon resistors, and then measure them to determine the tolerance. That meant if you had a 10% resistor, it was off by more than 5%, otherwise it would have been sold a a 5% resistor. </P>
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<P>With respect to tube amps, I happen to like tube circuits. They glow! Also, they keep you warm on cold days. I can't think of any other real advantages. </P>
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<P><FONT color=#000000><FONT size=2><FONT face=arial,sans-serif>RSJ <BR></P></FONT></FONT></FONT>
<BLOCKQUOTE style="BORDER-LEFT: #0000ff 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 0px">-----Original Message----- <BR>From: Will <STATICPHANTOM@GMAIL.COM><BR>Sent: Nov 13, 2012 4:41 PM <BR>To: Build things for fun! <MAKELEHIGHVALLEY@MAKELEHIGHVALLEY.COM><BR>Subject: Re: [MakeLV] Stereo components <BR><BR>First of all, your response are all great points worth mentioning. What bringing up includes the following.
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<DIV>1) Need, the goal is to use some amplifiers that are cheap and easily attained. In this case, car audio amplifiers. They are robust and make for great shop systems. The problem with these amplifiers is that a computer or CD player just doesn't have enough power to properly drive the amps to acceptable levels. </DIV>
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<DIV>2) I like the Cmoy because it is basic and people can get hands on some fundamental electronics. I'm all for a wall wart... however seeing how the basic design for using car audio uses computer power supplies (of which... we have quite a few at the space). Computer power supplies are also fun because they can bring a whole host of problems to audio. Much of the problems provide a fun learning experience that includes ferrite beads!<BR><BR>I do like the fact that volume controls are separate because it allows for a space to be equalized a little better in if working with a mono signal (it might work as basic PA system but I won't know until we try). I agree with the comment about screwing up input impedance's with putting a pot in the input. Ideally I would recommend putting a POT where Rg is in the inverting configuration described here <A href="http://en.wikipedia.org/wiki/Operational_amplifier" target=_blank>http://en.wikipedia.org/wiki/Operational_amplifier</A>. That is the best way to adjust gain on an amplifier however it will cause distortions. </DIV>
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<DIV>3) I could give a crap about resistors. If people want to sit down and change the resistance with a file to get perfection then have at it. I'm fine as long as the tolerances are close (which are still inexpensive). #1 rule about audio, don't fall down the rat hole of audiophiles that point at everything that isn't perfect to justify dumping money. (See tube amp enthusiasts).</DIV>
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<DIV>4) Power supplies... however for 9 volts I think we may have a problem and would need either a 12 volt power supply line from a power supply or use a regulator to drop down the voltage (we might be able to do something with a resistor network pending some measurements). I know the 741's could take those supply lines but the ones for the Cmoy, no clue and I will have to look at the data sheet. </DIV>
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<DIV>5) As much as cheaper alternatives are there, the nice part about the Cmoy is that the design can be easily taught and constructed. In the end, it's something to be proud of in terms of quality which I think is perfect for the space. </DIV>
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<DIV>As far as end purpose, Jonas will need to comment because I was speaking in terms of something for the class room. I don't know what Jonas wants exactly, he will have to let us know. </DIV>
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<DIV>-Will C. </DIV>
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<DIV class=gmail_quote>On Tue, Nov 13, 2012 at 9:37 AM, <SPAN dir=ltr><<A href="mailto:rsjames@ix.netcom.com" target=_blank>rsjames@ix.netcom.com</A>></SPAN> wrote:<BR>
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<P><FONT color=#000000><FONT size=+0><FONT face=arial,sans-serif>Looks like your basic generic inverting op-amp circuit, run off of batteries. Not sure why you say we need a pre-amp. What signals are we amplifying? Do you want microphone inputs to the amp, or are you playing music?</FONT></FONT></FONT><FONT color=#000000><FONT size=+0><FONT face=arial,sans-serif> <BR>I have a few thoughts about the circuit:</FONT></FONT></FONT></P>
<P>1. If you are using it as a pre-amp, you shouldn't need two batteries for the supply. The output doesn't need to swing that far. If it doesn't need to be portable, use a plug in supply. (Use a wall wart with some extra filtering caps. You could even throw in a voltage regulator, but probably not needed.)</P>
<P>2. There are separate volume controls for each channel, but no common control. Okay if not using it for stereo. Otherwise you have to adjust two controls every time you change the volume.</P>
<P> 2A. Putting the volume control on the input is wrong for a pre-amp. It really messes with the input impedance. </P>
<P>3. In the parts discussion, he talks about using precision resistors. If you want, you can use precision resistors, build the circuit, connect a high quality signal generator to the input, put the correct load on the output, and connect a dual trace oscilloscope on both the output and the input. You can then adjust the volume control pot to get exactly the level you want at the output for a given input. Or, you can just use cheap resistors, and adjust the volume to the level you like. Your money. </P>
<P>4. He also talks about using better resistors to avoid problems with the resistance changing due to temperature. The alternative would be to not build the circuit in a metal box! Also, the output of the 9 volt batteries change with both temperature and age. The power is unregulated, so this will have more of an effect than the resistor tolerances.</P>
<P>5. The recommended op-am is a very nice, rather expensive part. If you are building a true audiophile quality circuit, it would be a good choice. If you are looking for a low cost solution, there are many better choices. Heck, you could even use an audio amp IC, or is that too obvious? </P>
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<P>A few other thoughts:</P>
<P>There are pre-amp kits out there that would probably be cheaper. Most of the cost of a low cost audio circuit is in the connectors!</P>
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<P>If you really want to do this cheaply, hit up thrift stores and yard sales (Goodwill, Salvation Army Thrift Shop, etc.). I was at a yard sale this past weekend, and there was a microphone preamp. It was an a 4 or 8 channel rack mount unit, and I think the guy wanted $25 bucks. I though it was kind of high. You can easily get all sorts of receivers and tape players for cheap. They practically give away VHS and CD players, both of which have usable circuitry.</P>
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<P>R. S. James</P>
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<BLOCKQUOTE style="BORDER-LEFT: #0000ff 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 0px">-----Original Message----- <BR>From: Will <U></U><BR>Sent: Nov 13, 2012 2:01 AM <BR>To: Build things for fun! <U></U><BR>Subject: [MakeLV] Stereo components <BR><BR>Hey all,
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<DIV>Good news! The replacement amplifier has arrived for the space. In the excitement of taking a look at the new amplifier.... I found a small 150 watt Rockford Fosgate stereo amp in the basement. I know Jonas was mentioning need for another speaker system for the class room section. To that.... I have an answer that requires another hack that has been done multiple times (and could use a custom PCB). </DIV>
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<DIV>To use any amplifier a pre-amplier is needed in a similar configuration to my sound system. In high end sound systems a receiver usually performs this function (lower end audio on home stereos have the amplifier and receiver in a single unit) then sends a low noise signal to the amplifiers. To perform the function on of a head unit on my stereo as a receiver a headphone amplifier can be built to act as a receiver. The best headphone amplifier out on the market that audiophiles rave about is the CMOY (<A href="http://tangentsoft.net/audio/cmoy-tutorial/" target=_blank>http://tangentsoft.net/audio/cmoy-tutorial/</A>) can be produced for around $20. </DIV>
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<DIV>I am willing to build a new stereo system when I can, however if someone like to expedite the project by taking ownership feel free to contact me. </DIV>
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<DIV>-Will C. </DIV></BLOCKQUOTE></DIV></DIV></DIV><BR>_______________________________________________<BR>Makelehighvalley mailing list<BR><A href="mailto:Makelehighvalley@makelehighvalley.com" target=_blank>Makelehighvalley@makelehighvalley.com</A><BR><A href="http://makelehighvalley.com/mailman/listinfo/makelehighvalley_makelehighvalley.com" target=_blank>http://makelehighvalley.com/mailman/listinfo/makelehighvalley_makelehighvalley.com</A><BR><BR></BLOCKQUOTE></DIV><BR></DIV></DIV></BLOCKQUOTE></BODY>