Hi i have one Behringer a500 unused on which i would like to replace all the electrolytics with better parts The stock ones are scaring
The nice things of this unit are balanced inputs and a decently sized toroid (i know nothing about its actual quality)
i am attaching the schematic
I read of some distortion issues for different position of the input attenuators
Looking at the part of the circuit
i see a strange R71 and surprisingly a pot value bigger than the input impedance at the opamp
I always thought that the pot value should be lower of the following stage input impedance
Can i eliminate R71 at least ? i dont like it at all
Thanks a lot
The nice things of this unit are balanced inputs and a decently sized toroid (i know nothing about its actual quality)
i am attaching the schematic
I read of some distortion issues for different position of the input attenuators
Looking at the part of the circuit
i see a strange R71 and surprisingly a pot value bigger than the input impedance at the opamp
I always thought that the pot value should be lower of the following stage input impedance
Can i eliminate R71 at least ? i dont like it at all
Thanks a lot
Hi thank you very much indeed for your kind and valuable advice
This is exactly what I don't want
I am testing some cheap preamps and I think to use this amp because of the attenuators
Other Power amps come without attenuators
I will remove that R71 resistor for sure
Moreover I wonder if I can increase the opamp input impedance by increasing R72 value
I need the attenuators just to silent the output not for volume regulation
Their quality is very low
The amp will be used only for testing noise from the preamps
I hope it will not be noisy by itself
This is exactly what I don't want
I am testing some cheap preamps and I think to use this amp because of the attenuators
Other Power amps come without attenuators
I will remove that R71 resistor for sure
Moreover I wonder if I can increase the opamp input impedance by increasing R72 value
I need the attenuators just to silent the output not for volume regulation
Their quality is very low
The amp will be used only for testing noise from the preamps
I hope it will not be noisy by itself
Oh yes you can do it, and R66 in other channel. Then you'll have the volume regulation curve (orange) like this:
Very gentle in the beginning, very steep at last 25%. If you are using volume pots in two positions (0% - 100%) then it's fine.
Technically you can, but this will cost you DC offset on the output (and some noise).
I see the DC offset could be compensated by adjusting VR6/VR4 (and VR5) but... this is for those who know how to repair power amp.
Hi thanks again Exactly ! the pots are really cheapOh yes you can do it, and R66 in other channel. Then you'll have the volume regulation curve (orange) like this:
Very gentle in the beginning, very steep at last 25%. If you are using volume pots in two positions (0% - 100%) then it's fine.
i think i could replace them with just one double 10klog Alps and so use the amp as an integrated avoiding the need for a preamp
It has both rca and xlr inputs
But i am a preamps lover it's better if it's as good as possible
ok no reason to complicate life I guess that a low Zin should help to keep the noise down ?
but i have decided to sacrifice this amp I need to learn how to desolder and solder parts I find a very cheap source for caps in tme.eu
I have seen good brands caps for almost nothing
and caps quality is very important
i remembered to have this amp after reading of this shocking blind test
https://www.matrixhifi.com/ENG_ppec.htm
this test has also shifted my attention to speakers
It's a good practice to keep resistors around bipolar opamp reasonably low: to minimize offset from input bias current and noise too.
Wish you good luck with modding!
good morning and thank you again sincerely
my dream would be to start from a simple but promising schematic and build an amp from scratch pcb chassis wirings etc.
I have noticed that the pcbs of some kits are much higher in quality than those of commercial amps and this leaves me condused
But I have to start from the simple things And recapping or wiring revision seems to me a useful thing Eliminate if there are possible bottlenecks for the power supply current that reaches the power stages and that which goes from the power stages to the speakers
I have seen amplifiers with diodes that should be undersized for a preamp and output connections made with iron bars Something to vomit
A car audio cable is a hundred times better and costs nothing Sometimes the details have a non-negligible impact
I decided to sacrifice this amp in the name of knowledge
best regards gino
my dream would be to start from a simple but promising schematic and build an amp from scratch pcb chassis wirings etc.
I have noticed that the pcbs of some kits are much higher in quality than those of commercial amps and this leaves me condused
But I have to start from the simple things And recapping or wiring revision seems to me a useful thing Eliminate if there are possible bottlenecks for the power supply current that reaches the power stages and that which goes from the power stages to the speakers
I have seen amplifiers with diodes that should be undersized for a preamp and output connections made with iron bars Something to vomit
A car audio cable is a hundred times better and costs nothing Sometimes the details have a non-negligible impact
I decided to sacrifice this amp in the name of knowledge
best regards gino