Dc nulling Servo amp need help

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Acoustat

Member
Joined 2003
#1
Hello everybody first post here.

I have an old Fulton bravura preamp thats op amp based has lots of dc on the output...varies with volume... its a single stage/single op amp phono circuit and another single op amp for dc servo service I think but i am not sure....i was told that the other single op amp was part of the RIAA what i can tell youthere is 15mf is connected to pin 2 with 100ohm resistor is there a better way to tell if this other op amp is a servo amp?? i have searched for schematics on op amp based servo amps not much comes up ...another question if this is a servo amp to offset DC why did fulton use 70mf in series with its output?? maybe for the line stage??


Help

Lawrence
 
#2
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I replaced the old tlo71cp with LT1360cn8's and now there is more dc on the outputs...
 
#5
LT1360 is a bipolar input opamp with high bias current. If it's not implemented in a way that the bias currents are balanced, and there's an input cap, you'll get a DC-offset problem.

TL071 is a JFET input opamp and therefore has low input bias current.

So you see, when you go from a JFET input to a bipolar input opamp some care has to be taken. If you want a bipolar input opamp there are the modern and excellent LME49720 or AD8599 with low input bias currents. Otherwise stick to JFET input opamps like AD8066, AD8620, AD825, AD823, AD744, OPA2132, OPA627, OPA827 etc.

How much is lots of DC?

I can see there are some NE5534's in there. Are you sure the TL071's aren't the DC-servo while the NE's being the "amp"?
 
#7
nelsonvandal said:
LT1360 is a bipolar input opamp with high bias current. If it's not implemented in a way that the bias currents are balanced, and there's an input cap, you'll get a DC-offset problem.

TL071 is a JFET input opamp and therefore has low input bias current.

So you see, when you go from a JFET input to a bipolar input opamp some care has to be taken. If you want a bipolar input opamp there are the modern and excellent LME49720 or AD8599 with low input bias currents. Otherwise stick to JFET input opamps like AD8066, AD8620, AD825, AD823, AD744, OPA2132, OPA627, OPA827 etc.

How much is lots of DC?

I can see there are some NE5534's in there. Are you sure the TL071's aren't the DC-servo while the NE's being the "amp"?
Click to expand...


right out of the output is about 2.3volts on one channel 3.2 volts on the other....

this LT1360 sounds very real to me...i dont want any euphonic colorations....the tlo71 is good but had dc on the outputs too...so thats why i wanted to see what the LT1360 sounded like i see i need to learn about op amps more....

so what kinda care is needed to make this LT1360 work?

the NE5534's I'm assuming are for voltage regulation ....i dont have a schematic no one i know has one


Lawrence




which op amp would be a better choice
 
#8
If you had large DC offset with the TL071's, I guess there's something wrong with your amp. The amp looks very modded (and to be honest quite ugly), and since you don't have a schematic I think you should draw your own schematic and show it here.
 
#9
actually this was one of Mr Fulton personal preamps...

not modified by anyone only me with the 4 opamps he did weird stuff for sonic reasons serires resistor etc.....i dont question how this looks ..when its right IMMHO cannot be beat

anyways the original question about what to look for or how to tell if its a dc servo amp hasnt been answered maybe you can give some insite on what to look for taltail signs etc...

i appreciate your help...


and yes i think it had some dc on its output with the tlo71cp but i didnt measure it just assumed it because of the noise on the volume pot just like now


Lawrence
 
#10
It's impossible to tell what's wrong without a schematic and some voltage readings.

LT1360 with it's very high input bias current could cause several volts of DC-offset depending on resistor values, pot value and if there are input coupling caps or not (but I assume there are none since the DC varies with the volume).

Either you have to put some JFET inpupt opamps back in the amp (or low input bias bipolar opamps) or you have to balance the resistors from pos input to ground, neg input to ground and feedback resistor and add an input coupling cap.
 
#13
There's a multitude of schematics out there like this one http://sound.westhost.com/project06.htm Google "opamp phono" or something like that. If you want to learn how to implement a bipolar input opamp, do some searching for "AD797 phono" and you'll find circuits like this http://www.euronet.nl/~mgw/diy/phono/uk_phonoclone_2.html

I still think LM4562 = LME49720 could be a good start if you like the sound of bipolar opamps. It's one of the best (many think it's the best), has low input bias current (for a bipolar amp) and it's cheap.

IMO JFET input opamps sound grainier, "dry", has a more focused soundstage - the best are really pin point accurate, adds "texture". In general bipolar input opamps sounds "airier", soundstage is "free floating", softer, clearer treble, "wet". I'm very sure other people here would object to this description or use other adjectives.
 
#14
i am going to try to draw a schematic tonight when i get to work....it wont be anything special...


this is no typical phono thats for sure..weird stuff going on....

if there is anyone else that has thoughts please give us your input...

on another note ...i made the inputs + and - the same resistance ...no more DC!! but that was with 300 ohms to ground what affect would it have if i made the input - say 5.5k the same as the input + ?

Lawrence
 
#20
Put a BlakGate cap on the output and say bye bye to DC !

I would not put a dc null circuit - would affect more the sound than just a very hifi cap. Though i have a pre amp with a dc null but its not a servo : it is a fixed bias.

Btw : very nice looking amp, i just question the huge cap integrity... might just be me , but check it for DC resistance if you can or capacitance.
 
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