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Old 22nd October 2003, 10:15 PM   #1
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Lightbulb School Project

I am doing a final project for school and i have decided on a preamp. The schematic i will be following is available here
LINK

Here is a quick diagram i put together if you don't want to read though the link

Click the image to open in full size.

Now, a few questions that i have:

-What kind of volume controls should i use? i really have no idea what to look for in a variable pot volume or balance control.

-Grounding. I plan on putting in a UV meter and maybe some tone controls. What tips do you guys have to avoid ground noise problems?

-Selector switch. For changing inputs, what should I look for? What works best?

-Internal amps. Do you think putting an amp inside the same case is a good idea? What i mean is, should i have line level and speaker levels in the same box like commercial units?

any help would be great.
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Old 22nd October 2003, 11:43 PM   #2
PRR is offline PRR  United States
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> What kind of volume controls should i use? i really have no idea what to look for in a variable pot volume or balance control. Selector switch. For changing inputs, what should I look for? What works best?

You will do VERY well with the dual-pot and 2P6T switch that Radio Shack sells.

I use their 100K dual audio pot in some "serious" gear, and I have seen an identical pot in some well regarded equipment.

Their cheap rotary switches work well enough. They will get dirty in a decade or so, but for this project I would not worry.

Your plan posted here is different from Rod's because you have left out the volume control. At first it didn't make sense to me: why have 2 chips to get a modest amount of gain? Maybe your simulator doesn't do pots?

There are two fields of study here: electronic theory, and practical soldering and tin-work. Don't get so wrapped-up in one that you neglect the other.

Can you compute the gain of the overall preamp (without peeking at Rod's essay)?

What is that 1p capacitor doing?

Do you really need R6L? Or both of the 1µFd caps at the output? You might or might not need these.... as a student, you should be able to offer some explanation, either from theory or by actual experiment.

> Do you think putting an amp inside the same case is a good idea?

As a general question: It Depends.

In this case: do the preamp and get it working happy, don't tart-up your first project with every frill you can think of.

BTW: VU meters are not simple. They probably need their own amplifier (Rod must have a plan); and real meters are getting very difficult to find, let alone true VU meters. I do like the idea of meters for a student project, but don't let it become a stumbling block.
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Old 23rd October 2003, 11:01 AM   #3
LaZarus is offline LaZarus  Netherlands
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I have built this preamp some time ago and it really isn't critical. You can just build it p2p without problems. Grounding issues haven't been a problem either. The way it put it together and with the components I used the result is actually very good.

I used MKP capacitors, a panasonic pot and a elma rotary switch from www.schuro.de

ps, I omitted the 1pF capacitor.
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Old 23rd October 2003, 11:58 AM   #4
paulb is offline paulb  Canada
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A few questions about your application:
What will you be connecting this to? A preamp isn't much good on its own. Have you got a CD player, power amp, and speakers?
What do you want to do about a power supply?
What kind of box do you want to use?

I may be able to help you out with a few parts, or advise where to buy some others. What school are you at?

I'm building an amp that uses the preamp you've linked to.

Paul
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Old 23rd October 2003, 03:00 PM   #5
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I am going to SAIT in Calgary.

PRR; I didn't put the volume control in the schematic because i was not sure exactly what i was going to use and i wanted to test the op-amp configuration on it's own.
The 1pf cap is for noise. Or so i was told. I don't know if i will put it in the final project yet. Can anyone think of anything negative about putting it in?

paulb; I have all the other equipment that i need for an entire system. The case we will be making ourselves in class. I have not looked at power supplies yet because i don't know what other components i will use so i don't know the final power requirements.

I am looking at this chip amp to add to the preamp. I can use the +15 rail from the opamp to power it, then just use a pair of mc7815 for my - and + rails.
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Old 23rd October 2003, 03:40 PM   #6
PRR is offline PRR  United States
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> The 1pf cap is for noise. Or so i was told.

I don't know if you can even buy a 1pF cap. (5pF used to be the lower limit of readily available caps.)

One reason is that two adjacent pins already have a few pF of stray capacitance, so what is the point of adding one more?

Rod's plan actually shows 100pF here, a more likely value.

On theoretical grounds, I do not like caps across opamp inputs. They always degrade feedback stability. 100pF isn't likely a problem, but 1,000pF across many opamp inputs will turn them into 20MHz oscillators.

As an old radio tech, I think the 100pF should go from U1A "+" input to ground. Then the 1K resistor and 100pFd cap make a 1.7MHz low-pass filter. I might even increase both R and C to make it about 170KHz: high enough to pass audio without any loss at 20KHz (less than 0.5dB) but low enough to begin rejecting the AM broadcast band (often the most troublesome RF source).

That's the kind of thing that a project like this should make you think about.
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Old 23rd October 2003, 05:08 PM   #7
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Quote:
Originally posted by PRR
> The 1pf cap is for noise. Or so i was told. I don't know if you can even buy a 1pF cap. (5pF used to be the lower limit of readily available caps.)
At http://www.elfa.se you can even buy 0.5 pF!
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Old 25th October 2003, 04:00 AM   #8
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ar...that's a weird design... output cap????
wait, you doin this fopr your proj?? pls make sure that your lecturer is not an audiophiles.
good luck!!
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Old 27th October 2003, 12:29 PM   #9
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PRR; since the negative terminal of U1A is a virtual ground, would it not do the same thing? Or should i be safe and move it to a real ground point?

huangyong; What would you add or take off from the design? keep in mind that there is a volume and balance control missing from the schematic.
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Old 27th October 2003, 12:56 PM   #10
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on a note from what I have read:

VU meter is pretty simple with a diode bridge a 1 Microamp movement

selector switches from switchcraft are cheap and have worked in my gear for numerous years

a good wire wound pot has always been faithfull to me and are not exspensive

keep things neat and tidy and P to P works well...thats what I have taught in laymans terms

Cheers!!The DIRT®
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