need advise to optimise my OPamp preamplifier

WhiteRhino

Member
2011-07-09 11:36 am
Hi every one , I got this DIY preamp installed in an old (78) stereo reciever .
Uploded the schema pic and Ltspice file , to be easy to test and verify .
The power source is a single +13VDC , I made a virtual ground as you can see .

I have IC socket so I can change easily to another OPAMP ,

I have here ne5532 , jrc5532 , tl071 , jrc4558 , i tried them all but not sure which one will be the best .

Do you think I should do some changes to the schema ? which opamp do you think will be the best ?

Is it posible to calculate the distorion ?

Late edit , this is original scheam more clearly .
[IMGDEAD]http://s32.postimg.org/3uycsa4f9/2250b_pic.jpg[/IMGDEAD]
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Its a standard text book type circuit. It can't fail ;) All the opamps you mention will work OK. R3 and R4 could be scaled up a little if you plan on using the TL071 to help minimise loading. The R6/R9 reference could usefully be replaced with a clean voltage source.

Its basically fine and will work as intended. Distortion is very low but its not something you can calculate. A basic simulation using basic models won't show the true distortion either.
 

WhiteRhino

Member
2011-07-09 11:36 am
OK , So I see the first advise is to scale up R3 & R4 ,
Or maybe another Opamp will work fine ?
Instead of changing the resistors ? should I change the opamp ?

The R6/R9 reference could usefully be replaced with a clean voltage source.
DO you mean instead of +13VDC I sould make +-13VDC ? and to cancel R6\R9 ? now the power source is like the picture below .

Also R7 is providing rather a lot of attenuation - are you sure its correct? Or maybe R8's value is wrong?
This reciever has a 100k volume pot , in front of it I put a 50K because the gain was too high . do you think i should change it ? maybe a 250K volume pot will be better ?
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
All the opamps you mention will work fine, even as it is now. The 5532 is best suited because of its better drive ability. You've already used 'half' that capability up with the low feedback resistor values. The TL071 is happy with anything over 2k loading. The 5532 is good down to 600 ohms.

If 13 volts is the supply voltage, then you need a stable 6.5 volt bias voltage for the opamp. The two 100k's are workable but basic. Any noise on the supply will be fed into the input of the opamp.

You could replace the two 100k's with say 47k's and add a 47uf cap from the junction of them to ground. Then take a 100k from the junction to the opamp. That's a much cleaner bias arrangement.

Or you could create a stable 6.5 volt reference (say using a zener) and again feed that via a 100k to the opamp.
 
Or maybe another Opamp will work fine ?
Instead of changing the resistors ? should I change the opamp ?

It depends what your aim is here - the 5532 is fine as opamps go, no need to change it. Any other opamp will most likely still benefit from driving an easier load.

This reciever has a 100k volume pot , in front of it I put a 50K because the gain was too high . do you think i should change it ? maybe a 250K volume pot will be better ?
Oh I see so those resistors were just representing a pot. As you were, no problem :) 100k should be fine in this circumstance.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
I have here ne5532 , jrc5532 , tl071 , jrc4558 , i tried them all but not sure which one will be the best.

NE5532 will be the most transparent. Also the lowest in distortion. But I don't like it's sound in long listening (I prefer the JFET input TL071).

The power source is a single +13VDC

JRC4558 is the cheapest or the worst in spec, but at low supply voltage (such as 5V) its performance is subjectively better or comparable.

Do you think I should do some changes to the schema ? which opamp do you think will be the best ?

If the gain of 3k8|1k is too high for you, why not just turn the preamp into a "buffer"? Or you can also lower the gain in the main amplifier.

Noise (or THD+N) is probably more relevant than THD. Higher resistance (the pot and the resistor especially before the gain stage) creates higher noise. The topology itself is a "high noise" type. TL071 will probably be the most noisy.

If you want the best objective performance, pick NE5532. If you want the best "subjective" performance, pick TL071.

I prefer JRC2068 (BJT input) and LF353 (JFET input). LF353 has the musical sound signature of TL071 but with everything else better. JRC2068 is very transparent (lower noise than even a 5532) and sounds at least as musical as JFET-input opamps. For feedback resistance, I use 6k8 for both of these opamps (it was originally selected for LF353).
 
As far as Op Amps are considered, with such a low gain ~5X and easy 100k load all will sound exactly the same, and measurements will be so close as to make microscopic differences irrelevant.

I'm worried at your HUGE input attenuation, you might as well get rid of the Op Amp and drive output from the attenuator.

Or some values are wrong/typos?

If that's really what you need, I question the need to build a preamp at all.
 
OK , So I see the first advise is to scale up R3 & R4 ,
Or maybe another Opamp will work fine ?
Instead of changing the resistors ? should I change the opamp ?


DO you mean instead of +13VDC I sould make +-13VDC ? and to cancel R6\R9 ? now the power source is like the picture below .


This reciever has a 100k volume pot , in front of it I put a 50K because the gain was too high . do you think i should change it ? maybe a 250K volume pot will be better ?
If the gain is too high then reduce the gain of the amplifier. Don't increase the attenuation and then use too much gain after that excessive attenuation.
You have gain equal to {3.8+1}/1 = 4.8times = +13.6dB
reduce that to +0dB (1times) and have a switchable +6dB (2times) for low level sources.
Use a unity gain stable opamp. The 5532 is unity gain stable.
 

WhiteRhino

Member
2011-07-09 11:36 am
I geuss you right I have to reset the gain , I will try first X2 , and scale the feedback resistors a little up. if the gain will be still to high I will set as unity gain .And as dotneck335 mention I will replace R2 150 ohm to 50 ohm .

You could replace the two 100k's with say 47k's and add a 47uf cap from the junction of them to ground. Then take a 100k from the junction to the opamp. That's a much cleaner bias arrangement.

Or you could create a stable 6.5 volt reference (say using a zener) and again feed that via a 100k to the opamp.

Could you Mooly draw me how to do , this two option ?

Another qeustion is ,Do you know if the JRC5532D is the same as TI 5532 ?
 
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Jay

Banned
2003-02-11 9:02 am
Jakarta
I haven't seen Signetics 5532 in the last 25 years (that's 2 years before Signetics was bought by Philips) and have no idea how they sound compared to other 5532s. But RC5532 (which is not necessarily from JRC) has exactly the same characteristics with other 5532s, which I don't like.
 

WhiteRhino

Member
2011-07-09 11:36 am
All the opamps you mention will work fine, even as .


You could replace the two 100k's with say 47k's and add a 47uf cap from the junction of them to ground. Then take a 100k from the junction to the opamp. That's a much cleaner bias arrangement.

Made some changes as you advise me , I tried to implement them with minor changes .
1) The gain is now R3/R4+1 = 3.8/2.5+1 = ~ 2.5 .
2) The volume pot is now 100K
3)I have canceled the 150 ohm resistor at the output , I wanted to put 50 ohm but don't have at home , is critical ?
4)as you advise me moly if i understand you well , did like in the picture .

The preamp working well I did not notice any problem ,
The gain is now fine , not too high , and not too low .

will be glad to your comment .
 

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