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Which opamps are best?

Hi James

I may, (not sure until later this evening) have a Telequipment D67 scope, see here:

http://www.radiomuseum.org/r/telequip_oscilloscope_d67d_6.html

it is working but the trace on one channel does jump a bit on warm up, after 10-15 minutes it is ok, probbly just a dry joint or dodgy switch, it works all perfectly apart from this it is all good. If my friend who I offered it to last week dosn't want it you can have it if you like, I somehow think he will decide to pass on it so it could well be available, I'll PM you later.

Paul
 
Hi, oz soarer

I have LM6171 in my RCD965 dropped straight in with no problems sound good to. You might want to add some more supplys to the SAA7323 it has its own reg but I added 3 more each section can have its own 5V feed I should have had 4 and fed the each audio stage, its well worth doing.


Pete
 
Anybody have any experience/opinions on these opamps?:


I've just ordered samples of thse from TI, the last one is a prototype (thought it sounded like a successor to the OPA627).
I'm planning to use them in an active crossover, probably.
 
martin clark said:
Yes it is that kind of oscillation, in an electrical sense. It usually means that an amplifer is being driven into producing an uncontrolled output tone(s). This may be way, way above the audio band, so you can't hear it directly, but the side effects are measurable and audible. Typical tell-tales are unusually warm/hot running (check for high current consumption by the opamp, above the quiescent values given in the datasheet) or odd DC offsets at the opamp output that cannot be traced to DC conditions in the circuit. 'Oscillation' means that the circuit is no longer working linearly (the fundamental requirement of audio circuits!) and the unwanted cyclical noise interferes either directly or by intermodulation with the wanted signal. If it's particularly bad it may produce audible effects - the sense that something is 'wrong' tonally; either bass weight missing, or peakiness in the presence range/treble if you have oscillation happening near the upper end of the audio band.

The causes usually come down to inadequate, poorly-chosen or poorly-placed decoupling, or insufficient (electrical) damping; but layout, parts choice, loading, circuit layout and parasitic loads all come into it. The most cogent and thorough single document I'd recommend reading in relation to using opamps is Linear Technologies' Application note AN47 - High Speed Amplifier Techniques

Note oscillation is a risk anywhere there is gain in a system: this includes voltage regulators! Getting this stuff right makes an enormous difference to output quality, and IMO it's 95% of the problem with uninformed* modification.


*as in an uncontrolled / unmeasured /'always-chuck-the-same (boutique) parts at it' approach. Not that things can't work out, but it's often the case one simply gets lucky, or is alwready working on a good pre-existing layout...
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Thank you for the lesson!

I did notice a lack of bass weight when I first installed them. Its a little better now but still a tad lower than It was prior. I think I should look into this further but poking around with a scope is probably beyond my capabilitys, I wouldnt know where to poke? As the problem is likely down to insufficient decoupling I think I should experiment with a few larger values and see what happens. Thing is, ive no idea where the decoupling caps are on the opamps. I thought they were the ones I changed to Blackgate STD'd but that was based on somone elses advice and Im not certain.

I can post a pic of the internals here when I get home. Also have the Schematics for the 640C V2 if anyone knowledgeable enough would care to take a look for me?

Thanks in advance. :)
 
Read the application note Martin pointed to. Read it again.

If it still makes no sense, you probably need to start from a text book - Horowitz and Hill, "The Art Of Electronics" is a bit out of date now, but really clear, and covers everything you need to know (and lots else too, don't bother with chapters on digital stuff). See, for example, http://www.amazon.com/dp/0521370957/?tag=pinkfishmedia-20
 
If you look on page 16 of the schematic the "47N PE 63V" caps (the blue ones on the photo?) are decoupling the psu lines to the opamps. The black gates may well be before these but they're not shown on this page (at a quick glance anyway).

PS, listen to PD and MC before you listen to me... They know what they're doing!
 
James Evans said:
If you look on page 16 of the schematic the "47N PE 63V" caps (the blue ones on the photo?) are decoupling the psu lines to the opamps. The black gates may well be before these but they're not shown on this page (at a quick glance anyway).

PS, listen to PD and MC before you listen to me... They know what they're doing!
Click to expand...

Good, if this is correct then Im ok as they are the caps ive changed to 10uf Blackgate STD's on each opamp. Could I do with a slightly larger value maybe?

PS. Will read up on the above mentioned link.

large
 
I might be off the mark, but the purpose of the low value film cap is to filter HF noise on the supply line, so you may be best sticking the old caps back in. Not sure if a 10uf BG is going to give you the HF performance you need.
 
James Evans said:
I might be off the mark, but the purpose of the low value film cap is to filter HF noise on the supply line, so you may be best sticking the old caps back in. Not sure if a 10uf BG is going to give you the HF performance you need.
Click to expand...


Maybe. Fatmarley (lee) tried the LM's in his 640 but left the film caps standard. He complained of a lack of bass and odd sounding treble? I wouldnt say the sound of mine is quite that bad, but its not perfect.

The guy (Brent on diyaudio.com) who advised the opamp upgrade, and the use of 10uf BG decoupling caps, has also advised that the power rails are dirty and upgrading the regs to low noise ones will improve things. I agree with the idea but I'd like to be sure Im getting optimal prformance out of the LM's first. It seems that the LM's do need a bigger decoupling caps (based on Matts experience) but wheter Electrolytics are ideal I dont know. They are good electrolytics though. I might try a decent film cap there as you mention, a bigger value than the standard though. I will see wht the LM4562 datasheet reccomends.

I tried reading that reference above but its a little to much, I need a easy gradient to learning about this properly so I'll start with some basics as advised by PD.
 
Dont understand it. Can anyone tell me what value/type of decoupling cap National Semicondutor reccomends for the LM4562? I have no idea how to get this opamp working optimally in the 640C.
 
Mike - if you are looking for good material, do try a search here on PFM - or review some of the threads in the Reference area. I suggest you may find this summary post by Andrew Weekes particularly interesting: lots of good stuff there!

[size=-2](James - 'knows what they're doing' haha - true for PD only. If only you could see my personal black museum of Things Gone Wrong ;)[/size]
 
Pete MB&D said:
Hi, oz soarer

I have LM6171 in my RCD965 dropped straight in with no problems sound good to. You might want to add some more supplys to the SAA7323 it has its own reg but I added 3 more each section can have its own 5V feed I should have had 4 and fed the each audio stage, its well worth doing.


Pete
Click to expand...

Hi Pete

The RCD855 is a different beast to the 965. It's a tda1541a machine and very similar/nearly identical to the RCD955.

I will upgrade the original power supply but I certainly plan to add separate regulated PSs to the dac, filter and analog stages as well. I have a Tentlabs XO3 clock fed by an LC Audio XO supply ready to go in too. The PFM Flea was a first choice but the ability to reclock the spdif output via the XO3 appealed to me in case I just want to use the Rotel as a transport.

Dom
 
mikesnowdon said:
Dont understand it. Can anyone tell me what value/type of decoupling cap National Semicondutor reccomends for the LM4562? I have no idea how to get this opamp working optimally in the 640C.
Click to expand...

It doesn't appear to be stated specifically in the datasheet you've linked to Mike but the circuit on pg. 24 gives a clue. It shows a 47uF cap between each of the the supply pins and ground (+Vcc and -Vee). I would guess that's a good starting point at least.
 
oz_soarer said:
It doesn't appear to be stated specifically in the datasheet you've linked to Mike but the circuit on pg. 24 gives a clue. It shows a 47uF cap between each of the the supply pins and ground (+Vcc and -Vee). I would guess that's a good starting point at least.
Click to expand...

Cool.

So what type of cap to use? Film, electrolytic....?
 
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