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
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
Was an error, only a human mistake..show us the schematic
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
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
Re: Re: OPA604-again - but the offset is -18v!!!!
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 said:
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
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
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
I presume the offset pins are not connected to anything on the PCB.
The voltages you see are normal. To use the offset adjust facility, a 10K to 100K pot is connected across the offset adjust pins with the wiper connected to V-.
This will allow the o/p offset to be adjusted to zero, or whatever.
Some OAs have the adjustment to V+. It depends on whether the input differential pair ar "NPN" or "PNP"
Any op-amp data sheet will show this.
The voltages you see are normal. To use the offset adjust facility, a 10K to 100K pot is connected across the offset adjust pins with the wiper connected to V-.
This will allow the o/p offset to be adjusted to zero, or whatever.
Some OAs have the adjustment to V+. It depends on whether the input differential pair ar "NPN" or "PNP"
Any op-amp data sheet will show this.
Mr Evil said:
Thanks Mr Evil, Indeed after some further research and much "undoing" to get to the underside of the board, I did find that the pins are "floating" and the -18v is coming from the device itself (albeit I couldn't understand exactly why from the datasheet).
I have replaced the dual OPA2604'2 with some LME49720HA (in TO-99) and replaced the bypass caps with 100uF Rubycon ZA's with stunning results. The rail here is lower (+/-15v).
I'm still contemplating trying some LME47910 to replace the OPA604's now I can see the offset is not used. However the rail is +/-18.5v here and so exceeds the operating voltage for the device. Any suggestions?
Thanks Ian
Look at the one on page 12 because it has some values marked, so you can work it out.UV101 said:
The first thing to notice is that R3/4/8/9 must all have the same voltage across them because they are tied to the emitters of Q1/2/3/4 respectively, which all have their bases, and thus also their emitters, at the same potential because their bases are connected together.
Therefore, if you can work out the voltage across one of the resistors, you know the voltage across all of them. Luckily, the current through R9 is known, because it's just the value of the current source I2, 200μA. So the voltage across R9 is 200μA * 3kΩ = 0.6V
So the voltages at pins 1 and 5 are both only 0.6V above the negative supply rail.
UV101 said:
Exactly!
You don't need to know about what is the voltage on these pins. Use the published circuit for offset trim (Page 12 of the BB data sheet) or leave them unconnected.
There are some clever fiddles using those pins to do special things, but you are not ready for that stuff, yet!
Just stumbled accross this again and was equally offended the 2nd time I read it! Was mistaken over the offset when I posted this the 1st time round. In my lazyness, I now have a daigram (thanks dtes) if you look at the complexity, you will see why I might have been "too lazy" to reverse engineer the circuit. Luckily for me, it doesn't matter that I'm "too lazy" because I now have the circuit.
Anyone have an idea why a Burson discrete opamp wont work here?? The BB604 on the input stage of the main power section
A bit like playing with fire. The output section falls within the opamp feedback loop. Chances the current compensation will work well with the Burson and all the parasitics it brings are not so good. Sorry if this offends you or Burson.
Looking at the circuit I would not recommend using a non-FET input opamp. The Burson is certainly out as it would undoubtedly oscillate.
The opamp is used as a V to I converter in this design, so substituting opamps blindly is a bad idea as the amp is designed around what the opamp does on it's power rails more than what it does on its output. It's similar in arrangement to the Alexander Current Feedback amp (Analog Devices AN-211).
The opamp is used as a V to I converter in this design, so substituting opamps blindly is a bad idea as the amp is designed around what the opamp does on it's power rails more than what it does on its output. It's similar in arrangement to the Alexander Current Feedback amp (Analog Devices AN-211).
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.