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DIY Simple Active Preamp

Pio1980 said:
I don't recall if I responded to this, I built a box in the early 1980s with a Noble 10k stepped attenuator as a 'bypass' tester for line stage pre's and found that in all situations I tried less was indeed more where the source was capable of driving thru the device to the power amp. I also found that the type and quality of the pot really really matters as does the switching in the selector and that all gold plating is not equal or even sonically acceptable. I'm still interested in finding a simple line-level gain stage that will allow the sys to sound 'better' (more alive and transparent) with, using the bypass box as the level control, than without the gain stage. I have yet to try these examples in the thread myself but just cannot overcome deep-rooted skepticism based on the notion of having to 'tune' the thing with cap and chip 'rolling' vs not having to deal at all with the 'sound' of any additional active device in the signal chain of custody.
Click to expand...

You can try Nelson Pass B1.Its a no gain or unity gain buffer with Volume control.It will not colour to the sound. http://www.passdiy.com/pdf/B1 Buffer Preamp.pdf

Regards,
Sachin
 
Pio1980 said:
I don't recall if I responded to this, I built a box in the early 1980s with a Noble 10k stepped attenuator as a 'bypass' tester for line stage pre's and found that in all situations I tried less was indeed more where the source was capable of driving thru the device to the power amp. I also found that the type and quality of the pot really really matters as does the switching in the selector and that all gold plating is not equal or even sonically acceptable. I'm still interested in finding a simple line-level gain stage that will allow the sys to sound 'better' (more alive and transparent) with, using the bypass box as the level control, than without the gain stage. I have yet to try these examples in the thread myself but just cannot overcome deep-rooted skepticism based on the notion of having to 'tune' the thing with cap and chip 'rolling' vs not having to deal at all with the 'sound' of any additional active device in the signal chain of custody.
Click to expand...

Steve I actually used an analog setup where the phono stage was connected directly to the power amp, which has gain controls on it, it sounded good but not musical for want of a better word.

I also tried the Pass B1 which I found too clinical for my liking.

The SSP sounded better with more "warmth" than the B1, I suspect the warmth may be coloration, but in a pleasant way, as in people using carbon resistors.

I think it just depends on what you prefer.
 
Ron,

Renron said:
George and Everyone else.
I humbly apologize for my mild lysdexia (dyslexia ;) ) , I would like to blame it on too many chemicals (at work) I've come in contact with over the years....but, I digress.
I should have named those Opamp chips as LME49990


Ron
Click to expand...

So this is the chip I snagged;

http://www.ebay.com/itm/190762476231?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

It worked great for about 20 minutes, then it sounded as if the tuner was not centered on frequency. Changed stations, put on a cd, no change. Turned the pre off and back on and worked fine for another 20 minutes and same thing all over.

Put the orginal chip back and no problem.

Bad Chip? Wrong Chip?

Thanks
George
 
gcarlberg said:
Ron,



So this is the chip I snagged;

http://www.ebay.com/itm/190762476231?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

It worked great for about 20 minutes, then it sounded as if the tuner was not centered on frequency. Changed stations, put on a cd, no change. Turned the pre off and back on and worked fine for another 20 minutes and same thing all over.

Put the orginal chip back and no problem.

Bad Chip? Wrong Chip?

Thanks
George
Click to expand...
What is dual PS value, it should not increase above +/-10.5V. It gets hot above that.
Btw just now noticed your link shows chips different than mine. I have soldered these many times. Giving link my another post.
http://www.hifivision.com/diy/43219-diy-smd-soldering-soic-lme49990-into-adapter.html

You could have done it yourself cheaply. They are available cheaply in US and from genuine source.
 
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gcarlberg said:
Ron,



So this is the chip I snagged;

http://www.ebay.com/itm/190762476231?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

It worked great for about 20 minutes, then it sounded as if the tuner was not centered on frequency. Changed stations, put on a cd, no change. Turned the pre off and back on and worked fine for another 20 minutes and same thing all over.

Put the orginal chip back and no problem.

Bad Chip? Wrong Chip?

Thanks
George
Click to expand...

Hi George,
Either you are running them on too high voltage or Opamps are fake.There are so many fake parts available on ebay especially Caps and Opamps.

Regards,
Sachin
 
Take a look at the spec sheet:

http://www.ti.com/product/lme49990

It says:

The LME49990 has a wide supply range of ±5V to ±18V.
Click to expand...


but then lists min = 10V and max = 36V. Is this for a dual-chip setup? Either way, why say to keep the power supply +/-10.5V? Shouldn't this chip be able handle up to 18V?

For those using this chip, what voltage are you feeding it?
 
dodog said:
Take a look at the spec sheet:

http://www.ti.com/product/lme49990

It says:




but then lists min = 10V and max = 36V. Is this for a dual-chip setup? Either way, why say to keep the power supply +/-10.5V? Shouldn't this chip be able handle up to 18V?

For those using this chip, what voltage are you feeding it?
Click to expand...

Thats true,but it gets very hot beyond 10.5.You can smell the burning PCB.I am running them on +/-10 V.

Regards,
Sachin
 
sachu888 said:
Thats true,but it gets very hot beyond 10.5.You can smell the burning PCB.I am running them on +/-10 V.

Regards,
Sachin
Click to expand...

Are these not the same opamps used in HT's new AD797 build? Most people are running SLA's or other supplies >12V and I haven't heard of anyone complain of a burning smell yet. Am I missing something? (and I don't doubt that I am. :D )
 
dodog said:
Are these not the same opamps used in HT's new AD797 build? Most people are running SLA's or other supplies >12V and I haven't heard of anyone complain of a burning smell yet. Am I missing something? (and I don't doubt that I am. :D )
Click to expand...

They are the same.I don't know why they run cool on AD797.They get very hot in CNC.You just can't put a finger on them if you run them at 12v.

Regards,
Sachin
 
dodog said:
Phillip, any thoughts?
Click to expand...

It's horses for courses, some chips oscillate and get hot in some circuits, I never tried the LME49990 in that preamp.

I run them at up to 13v per side in the AD797 phono stage and they don't even get warm.
 
HypnoToad said:
It's horses for courses, some chips oscillate and get hot in some circuits, I never tried the LME49990 in that preamp.

I run them at up to 13v per side in the AD797 phono stage and they don't even get warm.
Click to expand...

I'm using them with 13V as well and everything is fine.

I *think* I'm using the trusted OPA2134PA in the SSP but I'd have to crack it open again to see. What is the "go-to" chip for this circuit?
 
dodog said:
I'm using them with 13V as well and everything is fine.

I *think* I'm using the trusted OPA2134PA in the SSP but I'd have to crack it open again to see. What is the "go-to" chip for this circuit?
Click to expand...

I am using the OPA2134 in mine as well.

Tried the AD823 but went back to the OPA2134.
 
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:
 
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...

The AD797 has a double sided PCB with a ground plane on top, with 0.1uf ceramics as close as possible to the supply pins, and 100uf electrolytics as well.

This is recommended in the data sheets and I connected it to my scope with a signal generator to check and no oscillations at all.
 
Very good. The problem is that the 49990 still has gain at 100MHz+. Combined with a generic SO8-DIL8 converter PCB it gets a bit iffy. And you have to consider the self resonance of the decoupling caps at VHF.
 
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