Hello!
I have a phono RIIA circuit in my Quad 44 preamp. There are two tantalium caps 2,2uF (C310/311) and 10uF (C304/305).
What is the function of those caps and can I remove any of them to improve the sound or replace them with smth better?
Cheers
Tom
I have a phono RIIA circuit in my Quad 44 preamp. There are two tantalium caps 2,2uF (C310/311) and 10uF (C304/305).
What is the function of those caps and can I remove any of them to improve the sound or replace them with smth better?
An externally hosted image should be here but it was not working when we last tested it.
Cheers
Tom
Hello Tom,
I have modified the RIAA in my QUAD 34.
You should not remove the 10uF (C304/305), as they are
in the feedback path, and provides full feedback on DC-signals
while letting the AC-signals pass to GND.
However you can replace them with better types.
The 2.2uF (C310/C311) prevents DC to pass to the next amplifier stage. I have short circuited these in my preamp, as the DC out of the OpAmps was well below 1 mV.
Meassure the DC voltage on R318/319, and post it here
I have modified the RIAA in my QUAD 34.
You should not remove the 10uF (C304/305), as they are
in the feedback path, and provides full feedback on DC-signals
while letting the AC-signals pass to GND.
However you can replace them with better types.
The 2.2uF (C310/C311) prevents DC to pass to the next amplifier stage. I have short circuited these in my preamp, as the DC out of the OpAmps was well below 1 mV.
Meassure the DC voltage on R318/319, and post it here
Hello Jan!
Thanx for the input, I've shorted C310/311 out and the sound improved. DC voltage was below 1mV.
As for the 10uF, dous it matter if I put in larger ones, like 100uF?
What other modifications did you do to the phono amp? I've put OPA627 instead of the old opamps.
Don't know if I should take out the 47pF compensation caps...
I've put OPA627/637 in the radio cards, I've got "pops" when I turn the volume and sometimes pops when I change the source. It also hums occassionally, and dissapears when I press different source button. What could that be?
Cheers,
Tom
Thanx for the input, I've shorted C310/311 out and the sound improved. DC voltage was below 1mV.
As for the 10uF, dous it matter if I put in larger ones, like 100uF?
What other modifications did you do to the phono amp? I've put OPA627 instead of the old opamps.
Don't know if I should take out the 47pF compensation caps...
I've put OPA627/637 in the radio cards, I've got "pops" when I turn the volume and sometimes pops when I change the source. It also hums occassionally, and dissapears when I press different source button. What could that be?
Cheers,
Tom
Zombie
You will get better results if you short the 10uF caps as in that position any cap screws up the sound and replace the horrid tantals at output with anything reasonable, ie BG-N, polystyrene (if you can make room), etc.
The 47p are not necessary and i suspect the little inductors at the input are best removed.
You will get better results if you short the 10uF caps as in that position any cap screws up the sound and replace the horrid tantals at output with anything reasonable, ie BG-N, polystyrene (if you can make room), etc.
The 47p are not necessary and i suspect the little inductors at the input are best removed.
Hi again,
I also experinced a major improvement in sound, mostly by removing the two DC-caps (C310/311).
You can remove the 47pF caps if you are using OPA627 as it has internal compensation, and no external connections for such caps.
However pin 1 is used for offset (you do not need that).
If you are still using the original TDA Opamps you coyld try to short circuit the 47pF caps, but watch for frequency responce and oscillation.
Others:
Just replace all caps to newer and better ones. There are so many suggestions in this forum, but I still like the "old" 1% polystyrene caps.
Your questions regarding your Radio Card are more tricky...
Did the Pops show up immd. after you have changed to OPA627?
Or is the pops scratch from the volume control??
Your 44 has logic selector circuits using CD4066 analog switches.
Normally they last for ever!!!!!!!!!!!!
Try tapping or cooling some of the components in the radio and selector circuit next time to see if you have a bad solder....
I also experinced a major improvement in sound, mostly by removing the two DC-caps (C310/311).
You can remove the 47pF caps if you are using OPA627 as it has internal compensation, and no external connections for such caps.
However pin 1 is used for offset (you do not need that).
If you are still using the original TDA Opamps you coyld try to short circuit the 47pF caps, but watch for frequency responce and oscillation.
Others:
Just replace all caps to newer and better ones. There are so many suggestions in this forum, but I still like the "old" 1% polystyrene caps.
Your questions regarding your Radio Card are more tricky...
Did the Pops show up immd. after you have changed to OPA627?
Or is the pops scratch from the volume control??
Your 44 has logic selector circuits using CD4066 analog switches.
Normally they last for ever!!!!!!!!!!!!
Try tapping or cooling some of the components in the radio and selector circuit next time to see if you have a bad solder....
It seems like the the input of Opamp is missing either good GND connection, or that the input is floating when no cable is attached.
The input to your opamps in the tuner circuit has a 100nF DC-blocking cap followed by a 1M resistor to ground.
It could be in this circuit you have a problem.....
The input to your opamps in the tuner circuit has a 100nF DC-blocking cap followed by a 1M resistor to ground.
It could be in this circuit you have a problem.....
Attachments
Zombie said:
Tom,
most likely you are experiencing problems with oscillation. The OPA627 is fast and "tricky". The layout of your boards is probably not suitable for the 627. Either use a slower OP amp (like the OPA134 you tried) or try to improve the power supply decoupling on the board (100nF ceramic or foil from positive and negative supply to ground VERY CLOSE TO THE OP AMP).
If the gain is below 5 (if memory serves me correctly), the 637 will oscillate.
Hope this helps,
Thomas
Thank you Thomas!
I have put caps VERY close to the supply pins. In this particular circuit (see pic supplied by ACD) I use the 637 and the spec sheets says it can be used, gain >5.
In fact the amp sounds very good now when I put all OPA627/637, very hard to notice any difference when I feed the power amps straight from the CD or through the 44.
Tom
I have put caps VERY close to the supply pins. In this particular circuit (see pic supplied by ACD) I use the 637 and the spec sheets says it can be used, gain >5.
In fact the amp sounds very good now when I put all OPA627/637, very hard to notice any difference when I feed the power amps straight from the CD or through the 44.
Tom
Thanx guys, I'll rip out the 637 and put them in the phono stage, put 134 instead and 627 from the phono stage to the other radio card (for the CD). Hope it works
Strange tht the amp still sounds good, even if the opamps are clearly unsuitable...
(Sorry for being such an amateur, but when I learnt electronics the 741 was the workhorse and LM series? opamps were the cry of the day. Since then I've left the field for history etc and have long forgotten everything about opamps etc - but I'm still good at soldering
)
Btw, how did you calc the gain in this particular circuit? R204/R202?
Cheers
tom
Strange tht the amp still sounds good, even if the opamps are clearly unsuitable...
(Sorry for being such an amateur, but when I learnt electronics the 741 was the workhorse and LM series? opamps were the cry of the day. Since then I've left the field for history etc and have long forgotten everything about opamps etc - but I'm still good at soldering
)Btw, how did you calc the gain in this particular circuit? R204/R202?
Cheers
tom
Well, at least I know the functions of the different components 
I've read some posts here that have the NE opamps in high regard.
Seems to me that a lot of sound improvement can be achieved only by choosing high quality passive components, especially caps, careful layout, good power supply and so on.
Removing the 100uF tantalium caps from the Quad 44 output gave almost a religgious experience soundwise
Cheers,
tom
I've read some posts here that have the NE opamps in high regard.
Seems to me that a lot of sound improvement can be achieved only by choosing high quality passive components, especially caps, careful layout, good power supply and so on.
Removing the 100uF tantalium caps from the Quad 44 output gave almost a religgious experience soundwise
Cheers,
tom
Zombie said:
1) Closed loop gain:
non-inverting amp, g=(R202/R204)+1
2) gain of the whole stage:
Since the output is parallel to the OP amp's inverting input, the gain of this stage is 1. That's how an OP amp with negative feedback works: It tries to make the voltage at the inverting input equal to the voltage at the non-inverting input.
This circuit is bit strange. They probably tried to get a better immunity against capacitive loads.
Thomas
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