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Chip Amps Amplifiers based on integrated circuits

LME49740 preamp not doing so good...
LME49740 preamp not doing so good...
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Old 18th May 2018, 03:19 PM   #1
rip radio shack is offline rip radio shack
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Default LME49740 preamp not doing so good...

Hi everyone, I'm new here and looking for some help with my first "hi-fi" project. Basically, my goal is a super low budget hifi system to prove a point that Crosley record players are garbage. I bought Jamo tower speakers (140 watt capable!) and a Technics direct drive turn table at my local Goodwill for less that $100. I think that was a good start. I found a Radio Shack 175b from the 70's in the trash, though it didn't work so I used it as free components and a platform to build on. (It had a center tapped transformer so that was good. ) Then I hit up ebay for some lm1875 kits, and for $5 dollars a piece I think they're great (just not loud enough but I'll deal with that later on). All that I'm happy with. But the preamp is giving me lots of trouble. I ordered a lme49740 soic ic and mounted it on a little soic to dip pcb, that was $5 for both, and followed the TI schematic. It kinda works, and it sounds like riaa equalization. (At least I think so, its better than running it straight into the amp.) But there is a ton of buzz, 120Hz (I think?) and when I turn up the volume even more than a little bit it oscillates all over the place. Is this common with opamp stuff? I'm using lme7815 and lme7915 regulators to give it the positive and negative supply, and I already made the pin out mistake with the 79 but I fixed it and still have the problems. I'm really not sure what to do, so any help would be appreciated.
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Old 18th May 2018, 03:33 PM   #2
rip radio shack is offline rip radio shack
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Click the image to open in full size.
Click the image to open in full size.
Here are photos, sorry I'm really bad at cable management.
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Old 18th May 2018, 04:46 PM   #3
FauxFrench is offline FauxFrench  France
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Your appendices do not load.

Just to summarize: You have made a RIA pre-amplifier based on the LME49740 IC. Even though you feed it with +/- 15V from two 7815/7915, the pre-amplifier has a lot of hum and seems to oscillate.

Have you used an IC socket for the LME49740?, and do you have another quad-OPAMP with the same pin layout (for instance TL074 or TL084), you can try in the socket for a moment?

A RIA pre-amplifier is working with very small input signals and has an important gain. The more gain, the more the amplifier is sensitive to radiated noise, poor decoupling, long component leads and no shielding in a metallic structure.
LME49740 is a high performance and fast OPAMP. Perhaps the TL074, being more mediocre, could prove that the circuit as such is correct?

Please solve the problem with appendices not loading. We need to see your implementation.

Last edited by FauxFrench; 18th May 2018 at 04:49 PM.
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Old 18th May 2018, 06:31 PM   #4
rip radio shack is offline rip radio shack
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If I just give the links it might be better.

IMG_4534.JPG - Google Drive

IMG_4535.JPG - Google Drive

I didn't use a socket, I guess that would've been smart, but I don't have any opamps laying around so it doesn't make a huge difference. You'll see by the photos my cables are horrible, and I didn't know how much hat mattered. If opamps are super sensitive then I do need to worry about that I guess. Also I should note that all the preamp grounds goes to audio ground, separate from the shield. And I put a 47k resistor and 3.3nF cap on the rca inputs. Should I add coupling/decoupling caps on the +/- supply?
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Old 18th May 2018, 07:05 PM   #5
FauxFrench is offline FauxFrench  France
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Thanks for the photos. As I recall from my past, two types of pre-amplifiers you had to be careful with: RIAA and microphone. The input levels are often some millivolts so you need a large gain. Then starts all the problems and you are for sure not the first to have hum in a RIAA pre-amplifier.

The advantage of your implementation is that at least not on the small board the leads are very long. However, many wires outside and going to the board are quite long. No shielding.

When you make amplifiers, it is important you have (often small: 100nF-1uF) power supply decoupling capacitors with short leads very close to the supply pins of the amplifier/OPAMP. These often small decoupling capacitors prevent rather high frequency noise from disturbing the amplifier and prevent the amplifier from oscillating. On top, you often have bigger decoupling capacitors a bit further away.
Next, the amplifier feed-back path has to be very short (physically) as well to avoid noise and oscillation.

The implementation you show is likely to cause the noise and oscillation effects you observe.

The way ahead is to start "cleaning up" your connections. You may need quite some rework to make it work as you hope. LM49740 is a high quality OPAMP and it deserves a good implementation.

Out of curiosity, what JAMO tower speakers did you buy?

Last edited by FauxFrench; 18th May 2018 at 07:14 PM.
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Old 19th May 2018, 12:16 AM   #6
rip radio shack is offline rip radio shack
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They're Jamo 128's rated for 140w each, peak 200w. They're absolutely massive!

Somewhere along the way of cleaning it up I broke it, or at least I can't get sound out of it right now. It'll still oscillate and buzz though. I'll order another one because they're so inexpensive, as well as some other opamps with the same layout and a socket. Do you think it would be worth trying to make a little metal box to shield it? And decouple with like and electrolytic and a poly film cap as close as possible? Also, should the preamp ground to audio ground or chassis ground? Thanks for the tips, I'll share any progress made on it.
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Old 19th May 2018, 04:53 AM   #7
FauxFrench is offline FauxFrench  France
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I have some Jamo's as well. I believe model 507. Massive, but quite elegant. Jamo never made it to be a top-brand but many of their speakers played very well and were value for money.

In the old days, we immediately reasoned RIAA-> in a shielded box. In a shielded box you will have less problems for sure. Better docoupling and improved layout will all help. I could make RIAAs work in the old days, you can today.

BUT, you can today buy RIAA boards or kits for very modest money. Often with a socket for the ICs. As I have explained, the outline is particularly important for such delicate amplifiers. If you buy such a board, eventually change the initial OPAMPs into the type you like and put it in a metallic box, you quickly and cheap have something that works. A real PCB will be a big help.

I work a lot on Vero-boards because I experiment with the designs. Building a RIAA amplifier is standard and avoiding hum and noise is the biggest challenge - why spend a lot of time and energy here?

Last edited by FauxFrench; 19th May 2018 at 05:00 AM.
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Old 19th May 2018, 05:16 AM   #8
johnego is offline johnego  Indonesia
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Quote:
Originally Posted by rip radio shack View Post
I'll order another one because they're so inexpensive
4x advanced opamps so cheap???
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Old 19th May 2018, 10:29 AM   #9
abraxalito is offline abraxalito  United Kingdom
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LME49740 is essentially a quad version of the LM4562 which is well renowned on DIYA as having very significant issues with RFI. I'd suggest swapping out the opamp to something less RFI sensitive like OPA4134 to see if this fixes up the issues you're having.
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Old 23rd May 2018, 04:10 PM   #10
rip radio shack is offline rip radio shack
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Screenshot 2018-05-23 11.55.17.png - Google Drive

Screenshot 2018-05-23 11.57.50.png - Google Drive

I incorporated the film and electrolytic decoupling caps, they're the 10muF and the 4.7nF right next to the IC, in this design. Is there anything bad or could be done better with this design or should I send it off to get fabricated? I didn't include the socket but I'll get like 10 boards so maybe I'll make a few with different quad opamps and see what happens.
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