| scott_jcp |
I am new to DIY audio, but have been working with pro-audio for quite some time now. As my first project, I decided to construct the first pre-amplifier design in Slone's 80 audiophile projects book. However, as I got into the construction of it, I got confused. The design obviously calls for a power supply, but the schematic does not show where it should be brought into the circuit. I thought it would be the Vcc + and - on the op-amp, but the schematic also details a decoupling circuit for those, so the power cannot be applied there. How is this circuit supposed to work?
Thanks in advance,
Scott |
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| Nuuk |
Scott, it depends on which opamp you are using!
Find the datasheet (Google) for the opamp and that should give you the pinout details. Usually the negative rail goes to -Vcc (or -V) and the positive to +Vcc (or +V). The other + and - pins asre for the signal connections.
When you have made that preamp, you may care to try a very simple discrete circuit. You can find details of one on my web site. ;) |
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| scott_jcp |
I managed to figure out enough to look up the datasheet for my op-amp. The reason I am confused is that the schematic shows a pair of decoupling capacitors for the Vcc+ and - terminals, but doesn't show where the power would be connected to this part of the circuit. I assume power needs to be applied across this, but I don't know how to decouple it AND apply power.
Also, where on your site is the gainClone schematic?
Oh, and one more thing. I am currently reading (though slowly) Douglass Self's book on amplifier design, which is very informative, but doesn't address issues beginners may have, like how to determine what components to use to achieve different output wattages, etc.
Thanks a million, this is a great forum. |
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| scott_jcp |
I finished my circuit, and the volume and loudness controls work. However, all is not well. Instead of getting music, I get a continuous thumping noise (about 5 fairly low thuds a second). When I turn off the input source, the sound still continues. Any idea where I need to check for the problem? Its figure 3.1 in Slone's 80 projects. I did not build the phono section, and for the op-amp I used NTE997 with the positive input on the diagram in invert-input and the negative input on the diagram in non-invert-input (of the IC).
Thanks in advance. |
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| Nuuk |
| quote: | | Also, where on your site is the gainClone schematic? |
Go to the Gainclone section.
I'm afraid that I have no expereince of the project that you are building so cannot offer advice. I suspect to get any help, you will need to at least post the circuit diagram here. :att'n: |
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| JojoD818 |
Scott,
I have the same book, but can you be more specific, what page is it? is it the preamp in figure 3.1? I am not too familiar with the opamp you used but is it a single or dual opamp?
Besides that, in the schematic, the "+" in the opamp means you have to connect it in the non-inverting and the "-" in the opamp means it has to be in the inverting pin of the opamp. If I read your post correctly, I belive you have accidentally interchanged the opamp connection. :att'n:
Jojo |
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| JojoD818 |
Oh, and the decoupling caps in Mr. Slone's diagrams means that you put the caps very near the pins of the opamps.
HTH,
Jojo |
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| pinkmouse |
| Sounds like LF oscillation, or "motorboating". From what I have read, this is usually caused by a missing ground reference somewhere. |
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| scott_jcp |
"Motorboating" seems like an accurate description of the noise.
What exactly do you mean by ground reference? Everything on the diagram that is supposed to be grounded I have connected to my power supply ground. Also, I am using RCA jacks, and I have the outside (which I guessed to be ground connected directly to the outside of the jack on the other side of the system. The inside is what I have wired to the preamp. Is this correct?
Another though I had about the problem is that I am using a computer power supply to test the system, but the -12V lead apparently can only output .8A, much less than the amount supplied by the positive lead. Could this cause the motorboating?
Thanks again! |
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| ken.berg |
AS Jojo pointed out, make sure that your feedback resistor (R19) goes (on an NTE 997) from pin3 to pin 1 and pin 4 to pin 6, or pins 12-14 & 10-8. Good practice to short the unused non-inv inputs to ground, and short the unused inverting inputs to the unused outputs.
A feedback network between 2-3, 4-5, 12-13, 9-10 creates positive feedback and builds an oscillator, not an amplifier.
Leaving unused opamps floating is another invitation to instability. |
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| scott_jcp |
oops... discovered I hadn't connected the feedback loop.
Fixed that. However, now I get a much higher frequency motorboating, though the circuit does transmit music clearly if no current is applied to the amp...
What part of the RCA plug needs to be attached to which part of my circuit? And does the other part of the cable just bridge straight through to the other side? |
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| JojoD818 |
Hi Scott,
I downloaded the datasheet for the NTE997 and found that it's a quad opamp (4 opamps in a single package). The "linestage" section in Mr. Slone's book only used a single opamp per channel so you will only need 2 opamps, therefore, you have 2 unused opamps in the NTE997 whose inputs have to be grounded to make them quiet.
I find nothing wrong in using the NTE997 for this purpose (though better opamps exists out there) and Mr. Slone's circuit is quite simple (only 1 opamp per channel). I strongly suggest that you review your connections carefully before applying power.
Oh btw, the outer conductor of the RCA jacks should also be connected to your psu star ground so your preamp circuit's signal could see a ground reference.
Hope this helps,
Jojo |
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| scott_jcp |
Still having problems...
What voltage readings might I be able to take that could help me locate the problem?
I have taken the voltages between my input lines (-12 and 12 respectively), but they read closer to 4V. My ground also gives an odd reading. |
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| ken.berg |
With the (-) lead of you meter firmly connected to ground, and the volume control all the way down, please tell us what DC voltages you read at all the IC pins:
1 inv A V=? 14 inv D V=?
2 ninv A V=? 13 ninv D V=?
3 out A V=? 12 out D V=?
4 out B V=? 11 pos supp V=?
5 ninvB V=? 10 out C V=?
6 inv B V=? 9 ninv C V=?
7 neg supply V=? 8 inv C V=? |
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