Dc nulling Servo amp need help

[Acoustat]

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

[Acoustat]

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I replaced the old tlo71cp with LT1360cn8's and now there is more dc on the outputs...

[sakis]

rail voltage proper both sides ?????

[Acoustat]

16.80 pin 4 17.53 on pin 7

[nelsonvandal]

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"?

[peranders]

The input bias current of the LT1360 is 1 uA which means that offset problems related to this should be 1000 times worse than for a TL071!

[Acoustat]

Originally posted by nelsonvandal
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"?

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

[nelsonvandal]

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.

[Acoustat]

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

[nelsonvandal]

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.