DC offset
Wayne,
Yes C26 and C27 are in the circuit. The schematic at the start of this thread says put them in if the op amp is bipolar. I checked the LM833 spec and it says it's bipolar - but I am an op amp newbie so I could be wrong
I checked all the resistors are in the right places (6L6's recommended Vishay ones make that so easy once you figure out the code). I checked the caps too.
Is it worth checking any of the voltages on the op amp pins to get an idea where the problem might be ?
Wayne,
Yes C26 and C27 are in the circuit. The schematic at the start of this thread says put them in if the op amp is bipolar. I checked the LM833 spec and it says it's bipolar - but I am an op amp newbie so I could be wrong
I checked all the resistors are in the right places (6L6's recommended Vishay ones make that so easy once you figure out the code). I checked the caps too.
Is it worth checking any of the voltages on the op amp pins to get an idea where the problem might be ?
BOM
Batty,
If you use the BOM in #5 of this thread be aware that R26 appears twice ! The second time it should be R36. I found this out last night when I was checking the resistor values in my build. I have an Excel version with my notes and Mouser codes you are welcome to, but there may be a another version somewhere in this thread someone else may be able to point you to.
Batty,
If you use the BOM in #5 of this thread be aware that R26 appears twice ! The second time it should be R36. I found this out last night when I was checking the resistor values in my build. I have an Excel version with my notes and Mouser codes you are welcome to, but there may be a another version somewhere in this thread someone else may be able to point you to.
dc offset
see post 388 of this thread for the details of what I did. I originally had 220pF in parallel with the 4.64K feedback resistors. Getting rid of that capacitor got rid of the oscillation. As I recall, the apparent offset was in excess of 15 mV. Of course you don't really know because your DVM is responding in some fashion to that oscillation. The schematic shown in post 388 is what I have with the exception that the as-built op amp is an OPA2604 instead of the OPA1612 I started out with. Given that I am using surface mount components in this area of the circuit I didn't think the board would withstand yet another round of rework.
see post 388 of this thread for the details of what I did. I originally had 220pF in parallel with the 4.64K feedback resistors. Getting rid of that capacitor got rid of the oscillation. As I recall, the apparent offset was in excess of 15 mV. Of course you don't really know because your DVM is responding in some fashion to that oscillation. The schematic shown in post 388 is what I have with the exception that the as-built op amp is an OPA2604 instead of the OPA1612 I started out with. Given that I am using surface mount components in this area of the circuit I didn't think the board would withstand yet another round of rework.
The 15 Volt works fine, runs a little cooler also. I had a stash of the 22 volters to use.Wayne, Thank you for the help and the design.
If you don't mind, I'm going to dig up some things from my AC circuits class that was required in school, even though I'm a mechanical engineer.
In the power supply, the voltage out of a bridge is 1.4X the voltage input. I think we can assume that there is sufficient capacitance to use this value at the output of the filter cap bank. So we get 1.4X15V = 21V into the regulator, which is set to 15 + 1.8 (LED)=16.8V feeding the outputs. Ignoring the limits of the output transistors, for right now, the 15V transformer will provide enough voltage to keep the system fed since 21V > 16.8V.
If we up the transformer to 18V and keep the regulator the same, we get 1.4X18V=25.2, which seems to me to be also sufficient, but would cause greater heat dissipation in the regulator, since our output is still the same.
I guess the real question is: Given equal availability and cost, is there any reason to go with a transformer greater than 15V? Which leads to a related question: If there is a desire run the output devices at a higher voltage by changing the regulator circuit, how much and by which method would be preferred to do so? Post #2 lists several methods of setting the regulator output voltage with multiple voltages listed.
Thank you for any answer you have. I'm going to try and understand the output device data sheet.
Rob
6l6,
I went through your build guide on the prototype site. I looked for comments and couldn't find any. It seems there might be some folks here who might have the same problem.
THE CASE Someone found one. I think for me and other's the next major issue is having them cut/stampped/drilled for the connections. Does anyone know of or can the DIYfolks also provide Links to shopes that will make the holes for the connections and sell it as 0part of a kit.
If folks are lookin for a kit, and see picktures of it, then only see a board... and parts, no chassis, that is a bitch. I've not found a D cutter for the Jacks to keep them from spinning around in face or back plates. I've looked for years.
So how to we/us/community set them up to make the hassle of doing all the chassis work simpler for everyone around?
Yes, shame on you, placing the IEC right next to the transformer, thats when it hit me, there should be a build off with a group of folks to hammer out the design chassis and wiring to get it right. Then sell them the kit pre finished pre cut chassis where everything should be etc.
They are my comments. Hope there is soemthing that will help the new buyer's experience be even better.
A really nice job you did people are going to have fun doin it.
All the best for you and your crew.
Cheers,
I went through your build guide on the prototype site. I looked for comments and couldn't find any. It seems there might be some folks here who might have the same problem.
THE CASE Someone found one. I think for me and other's the next major issue is having them cut/stampped/drilled for the connections. Does anyone know of or can the DIYfolks also provide Links to shopes that will make the holes for the connections and sell it as 0part of a kit.
If folks are lookin for a kit, and see picktures of it, then only see a board... and parts, no chassis, that is a bitch. I've not found a D cutter for the Jacks to keep them from spinning around in face or back plates. I've looked for years.
So how to we/us/community set them up to make the hassle of doing all the chassis work simpler for everyone around?
Yes, shame on you, placing the IEC right next to the transformer, thats when it hit me, there should be a build off with a group of folks to hammer out the design chassis and wiring to get it right. Then sell them the kit pre finished pre cut chassis where everything should be etc.
They are my comments. Hope there is soemthing that will help the new buyer's experience be even better.
A really nice job you did people are going to have fun doin it.
All the best for you and your crew.
Cheers,
LM 833 DC offset
Wayne,
I get the following DC voltages on the LM 833 pins with no inputs connected and no headphones plugged in :
Pin 1 O/P1 -0.38V
Pin 2 I/P 0.3V
Pin 3 I/P 0.3V
Pin 4 VEE -16.63V
Pin 5 I/P 0.3V
Pin 6 I/P 0.3V
Pin 7 O/P2 - 0.36V
Pin 8 VCC 16.47V
Do they explain why I get 24mV DC offset on both channels ? If so what do I do about it please ?
Someone else on the other Whammy thread got 17mV DC offset with an LM833 opamp which disappeeared when they changed to an OPA2134. Is there a problem with the LM833 opamp in this amp in its 'standard' configuration ?
Wayne,
Yes C26 and C27 are in the circuit. The schematic at the start of this thread says put them in if the op amp is bipolar. I checked the LM833 spec and it says it's bipolar - but I am an op amp newbie so I could be wrong
I checked all the resistors are in the right places (6L6's recommended Vishay ones make that so easy once you figure out the code). I checked the caps too.
Is it worth checking any of the voltages on the op amp pins to get an idea where the problem might be ?
Wayne,
I get the following DC voltages on the LM 833 pins with no inputs connected and no headphones plugged in :
Pin 1 O/P1 -0.38V
Pin 2 I/P 0.3V
Pin 3 I/P 0.3V
Pin 4 VEE -16.63V
Pin 5 I/P 0.3V
Pin 6 I/P 0.3V
Pin 7 O/P2 - 0.36V
Pin 8 VCC 16.47V
Do they explain why I get 24mV DC offset on both channels ? If so what do I do about it please ?
Someone else on the other Whammy thread got 17mV DC offset with an LM833 opamp which disappeeared when they changed to an OPA2134. Is there a problem with the LM833 opamp in this amp in its 'standard' configuration ?
Most of the DC-offset probably comes from the opamp (due to something called input bias current, as explained here) The LM833 is bipolar input whereas the OP2134 is JFET and as JFET opamps have much lower input bias currents, they also tend to have a much lower output offset.
Your voltages looks spot on to me, so if you are concerned with the offset either switch to a FET-input opamp or consider going through the thread for suggestions on how to trim the output stage to null the offset.
Your voltages looks spot on to me, so if you are concerned with the offset either switch to a FET-input opamp or consider going through the thread for suggestions on how to trim the output stage to null the offset.
LM833 DC Offset
Wayne,
It is in a socket, but I don't have another one. RS has them for 49p (63cents) so I will buy a few more and report back. Thanks for the pointer on the resistors. I will get some of those at the same time.
I am not sure what is implied by 'Three uA through R3 will give .3 Volts.' I understand V=IR, but I am not sure what the objective is. Is it to reduce the 0.3V on the I/P pins to zero ?
Wayne,
It is in a socket, but I don't have another one. RS has them for 49p (63cents) so I will buy a few more and report back. Thanks for the pointer on the resistors. I will get some of those at the same time.
I am not sure what is implied by 'Three uA through R3 will give .3 Volts.' I understand V=IR, but I am not sure what the objective is. Is it to reduce the 0.3V on the I/P pins to zero ?
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