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DIY Simple Active Preamp

Thanks for all of the info.

I have no idea what most of it means, however my SSP works very well now with the original chip back in place and a 9v battery and I am extremely pleased.:music:

george
 
Cù Faoil said:
What you have to watch with the LME49990 is the decoupling. The gain bandwidth is so much higher that classic OPAs that you really need to start using RF decoupling techniques.
This is probably why some do and some dont?
All my high gain bandwidth designs are done on double sided PCB with a solid ground plane on top. The IC supplies are decoupled as close to the pin as possible, usually with 22n // 220n // 10u paralleled.
Maybe overkill but nothing has turned into an oscillator yet :nono:
Click to expand...

Sellers response;
What you describe sounds like the op-amp is overheating from oscillation. The LME49990 is a high speed op-amp and requires special attention to local power supply decoupling. It's common to have this kind of problem when swapping a high speed op-amp into a circuit that was designed around a slower op-amp.

I'll give you a couple options here. If you send it back, I can put some decoupling caps directly on the module and send it back to you. There's a decent chance that will fix the problem, but no guarantee.

So is this the solution?

Thanks george
 
gcarlberg said:
Sellers response;
What you describe sounds like the op-amp is overheating from oscillation. The LME49990 is a high speed op-amp and requires special attention to local power supply decoupling. It's common to have this kind of problem when swapping a high speed op-amp into a circuit that was designed around a slower op-amp.

I'll give you a couple options here. If you send it back, I can put some decoupling caps directly on the module and send it back to you. There's a decent chance that will fix the problem, but no guarantee.

So is this the solution?

Thanks george
Click to expand...

Not sure how that would work (solder caps onto the op-amp or the socket?). I would just use an OPA2134PA or an AD823 and call it a day. YMMV.
 
Many members are using LME49990 in SSP and CNC.I am also running CNC with Lme49990.I have set the voltage on +/-10.It gets very hot above that.

Regards,
Sachin
 
gcarlberg said:
Thanks for all of the info.

I have no idea what most of it means, however my SSP works very well now with the original chip back in place and a 9v battery and I am extremely pleased.:music:

george
Click to expand...

Hi George,
Are you using only one battery to run SSP?

Regards,
Sachin
 
gcarlberg said:
Hey Sachin

Yes I'm using a single 9V.

Thanks
george
Click to expand...

Then you are providing a very low voltage to Opamps.That may be the reason for your problem.Try using 2X 9V batteries to get +/-9 V.See pic below,ignore Case GND that is for CNC phonostage.

batteryled.jpg


Regards,
Sachin
 
sachu888 said:
Many members are using LME49990 in SSP and CNC.I am also running CNC with Lme49990.I have set the voltage on +/-10.It gets very hot above that.

Regards,
Sachin
Click to expand...

This still bothers me a bit, Sachin. Just because people are using them doesn't mean that they know the effects. If you push the voltage a smidgen too high, the opamp oscillates rapidly and gets very hot. Turn down the voltage, it cools down but is it possible that it's still oscillating and/or creating ringing, etc.? Besides frying the opamp (relatively cheap, yes), what are the possible negative outcomes? Can one damage the power amp the SSP connects to? I don't know the answer to this and am not qualified to answer, I'm also a worry-wort. For now I'll stick to an OPA2134pa.
 
dodog said:
This still bothers me a bit, Sachin. Just because people are using them doesn't mean that they know the effects. If you push the voltage a smidgen too high, the opamp oscillates rapidly and gets very hot. Turn down the voltage, it cools down but is it possible that it's still oscillating and/or creating ringing, etc.? Besides frying the opamp (relatively cheap, yes), what are the possible negative outcomes? Can one damage the power amp the SSP connects to? I don't know the answer to this and am not qualified to answer, I'm also a worry-wort. For now I'll stick to an OPA2134pa.
Click to expand...

Yes, you can fry the speakers and the amp.
 
It is amazing to me that such a complex circuit (an Op Amp), with its multiple internal P-N junctions and close-coupled circuitry and consequent stray capacitance, can be fed back enough to be both stable and highly linear with fast transient response to outperform a simple complimentary-symmetry pair (PNP and NPN) of discrete components. And especially so to beat out the well-regarded CR Yamaha series. Amazing.
 
omishra said:
You are correct! My suggestion was for the use case without pots and DC blocking caps.
Here suggested schematic in above post is missing that pot. With the use of either 25K pot at input or 1M resistor to refer input to ground, it is possible to eliminate DC offset while excluding that DC blocking cap. Also it does affect sound anyway.

IMO this is only suggestion with my own experience where I excluded pot and DC blocking caps. I measured 0.3mV DC that could be ignored. I wanted highest input impedance possible.
Click to expand...

Has anyone else tried this? I'm using the OPS and not batteries, so I think it would be safe. Anyone have a pic of what this looks like and a procedure for doing this mod on the ssp?
 
sachu888 said:
Many members are using LME49990 in SSP and CNC.I am also running CNC with Lme49990.I have set the voltage on +/-10.It gets very hot above that.

Regards,
Sachin
Click to expand...

I have been thinking about this because when the LME49990 is used one per circuit they each have their own supply rails with filtering caps, therefore each supply input is isolated from each other. This is what happens in the AD797 stage and running at 13v per rail they don't even get warm.

When these are mounted on a dual adapter they share the same supply rails, which could cause oscillation due to variances in current draw?

That's my theory. :scratch2:
 
Phantom said:
Has anyone else tried this? I'm using the OPS and not batteries, so I think it would be safe. Anyone have a pic of what this looks like and a procedure for doing this mod on the ssp?
Click to expand...

Hi,
Omishra was using it with Pass B1 buffer,and this works as a gain stage and volume pot on B1.You can see few pics from RCenteno here:
SSP with B1+CNC and LDR volume control
and also first and second pic of Omishra here
http://www.audiokarma.org/forums/showthread.php?p=6790011

Regards,
Sachin
 
Hi all,

I got 4 chips of AD 797 from Sachin a couple of days back. So soldered a pair onto the stock adapters lying with me, and used this in my SSP. Initial impressions are that this chip is very detailed, and has better HF response and mid-range than 49990. Will have to wait for another 25 hours for this chip to settle down. Will update the thread after that.

Just thought I will post my impression.

N.Murali
 
Ok so the blocking caps stay. I see that some people are using 100uf caps instead of the 10uf in the BOM. Is there any benifit to this if running off of OPS?

That's interesting about the ad797, I look forward to your review, I'm really liking 2134 so far, very neutral and relaxed.

I would like to try the ceramic cap soldered to the power input of the adapter for the 49990, so I could run it at 15v without fears of oscillation, which pins on the adapter do I connect it to?
 
Phantom said:
Ok so the blocking caps stay. I see that some people are using 100uf caps instead of the 10uf in the BOM. Is there any benifit to this if running off of OPS?

That's interesting about the ad797, I look forward to your review, I'm really liking 2134 so far, very neutral and relaxed.

I would like to try the ceramic cap soldered to the power input of the adapter for the 49990, so I could run it at 15v without fears of oscillation, which pins on the adapter do I connect it to?
Click to expand...

On a dual adapter with two LME49990's, one on top and one underneath, you would solder it across pins 4 & 8 on the adapter as they are the power input pins.

This may help, I haven't tried it, you would have to try it and see if the chips still get hot.

CapCompensation_zps36b7f165.jpg
 
Phantom said:
I see that some people are using 100uf caps instead of the 10uf in the BOM. Is there any benifit to this if running off of OPS?
Click to expand...

100uf is the correct value.There was some confusion when designing SSP board and 10uf was printed.Please use 100uf there.

Regards,
Sachin
 
murali_n said:
Hi all,

I got 4 chips of AD 797 from Sachin a couple of days back. So soldered a pair onto the stock adapters lying with me, and used this in my SSP. Initial impressions are that this chip is very detailed, and has better HF response and mid-range than 49990. Will have to wait for another 25 hours for this chip to settle down. Will update the thread after that.

Just thought I will post my impression.

N.Murali
Click to expand...

Initial impression looks good.I think I should also try AD797 in my CNC.

Regards,
Sachin
 
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