OPA604-again - but the offset is -18v!!!!

[UV101]

Guys,

I have tried but failed to obtain a service manual for my RESTEK Fantasy amplifier.

I wanted to make a few mods to see if I can further push the perfomance of this beast!

I thought i'd look for some straight forward easy gains. OpAmps, decoupling and PSU smoothers.Some of this is straight forward.

It looks like the first stage of gain/buffer is an OPA2604. This should swap out directly with an LME49720(HA) and I see no reason for concern here.

My are of concern is the next stage before main power stage. There is an OPA604 with -18v on both offset trim pins (1&5).

What does the DC offset do? (risk of abuse here I think! )

I assume I shouldn't ignot it and replace with something like LME49710?

any comments muchly appreciated

Thanks

Ian

[destroyer X]

was the operation amplifier connected to nothing , presenting this off set?

Or you have made an entire amplifier, with the op amp into the input and them you had that off set?

Show us the schematic my friend....it looks a natural human mistake..we do that daily...join the club if you did that.

the chip amplifier output lead use to be connected to the amplifier output, and there you should have zero volts... we cannot have big voltage there, as 18 volts for instance, as this will represent 40 watts wast into the amplifier..and if it works, almost half cicle fo the signal will be absent or distorted.

Of set is some residual DC voltage that remains into the output line and you can measure it into the DC milivolts scale...it uses to be smaller than 25 milivolts and people loves to adjust it to less than 3 milivolts.

regards,

Carlos

[analog_sa]

Originally posted by UV101
What does the DC offset do?

The fact that you're asking this question is not very promising. The fact that you're lazy to reverse the circuit, even less so.

Chances something will blow are high but let this not stop you in your quest for perfection.

[analog_sa]

Carlos

You may need to re-read the first post but this time with your glasses on

It's a commercial amp which came with no circuit. There is no offset as such, just the offset-adjust pins are in use and correctly measuring -PS.

[UV101]

Originally posted by analog_sa



The fact that you're asking this question is not very promising. The fact that you're lazy to reverse the circuit, even less so.

Chances something will blow are high but let this not stop you in your quest for perfection.

Hi, Thanks for the info!

Comments are a little harsh I feel! I am in the process of trying to reverse engineer the circuit to see if this sheds any light (but its very complex) . I am asking the question because I don't want to perform what on the surface seems to be a straight forward swap out but in actual fact is a little more comlplex than my current understanding.

I was hoping to find the offset pins n.c. but no such luck!

Does the offset mean the output of the opamp is +/- about the -18v rail? i.e. the effective DC output is at -18v?

I am very familiar with upgrades to multiple CD players and have built my own TDA1541 DAC so I'm not unintelligent. This is the first time I've dared to contemplate messing with this expensive hardware.

Again any help would be very much appreciated

Ian

[analog_sa]

Originally posted by UV101



Does the offset mean the output of the opamp is +/- about the -18v rail? i.e. the effective DC output is at -18v?

No. Most probably the offset is at zero, or close. It is strange you're asking instead of measuring pin6.

In any case the offset of the entire amp will be influenced by the pot connected to pin1,5.

[UV101]

've just reasembled enough to get a measurement on pin 6.

Its 40mv.

[Juergen Knoop]

Originally posted by UV101


It looks like the first stage of gain/buffer is an OPA2604. This should swap out directly with an LME49720(HA) and I see no reason for concern here.

My are of concern is the next stage before main power stage. There is an OPA604 with -18v on both offset trim pins (1&5).

just a comment from the side...with supply voltages >= +/-18V finding alternative audio-opamps to the OPA604 family is difficult.
Your LME is not specified for this.
Regards

[UV101]

Hi Juergen

Is you only concern the operating voltage being right on the limit?

Regards Ian


Analogue_SA, do you have any information or links to such that I could become enlightened on subject matter surrounding the DC offset of OpAmps. I'm sorry if I offended you with my obvious stupidity. I was asking the question in the belief that someone would share their knowledge on such subjects. I thought the idea of the forum was to share and help not to insult. I'm not too lazy to reverse engineer the circuit nor am I so stupid that I can't measure the DC on the output of the OpAmp. Once again I'm sorry to have asked a higher intelligence such a remedial question

[Mr Evil]

Look at the simplified schematic of the OPA604 in its datasheet and you will see why the offset adjust pins are at -18V.

P.S. in my opinion, just about any op-amp is superior to OPA604.