Have a preamp I'm messing with (attached partial schematic). I want to just flat get rid of the output capacitor. I'm hoping ya'all can help me here...
Root problem is...if the amp has been off for a while, on powerup I get a strange 'scratchy' sound at the output of one channel. It's almost like a dirty volume control would sound, but kinda fluttery and mainly in the frequency range of about 80-120Hz. Lasts perhaps 1 minute or so.
I suspect that the large 100uf polarized cap at the output is the problem. It only does this in one channel. Afterwords, the amp sounds and behaves just fine. When the amp was set up and running in my system, I swear it was the right channel. After disassembly, and changing a few resistors (needed to increase gain a bit), and checking the opamp socket (a nice gold plated Amp socket, not a machined socket, but a decent one nevertheless), now the right is fine and wierd startup 'flutter' has moved to the left.
Yes, it is possible that I had channels reversed somewhere down the line and it was left channel all along, soooo....
With the circuit as it is:
On startup, the opamp output on each channel first swings to about +7 Volts, then drops to about -2 Volts, finally settling at about +1mV on left channel, and about +2mV on the right. I suspect that, if the output was DC coupled, it would settle much faster. I also suspect that the startup 'flutter' is related to the reverse-biasing of this polarized electrolytic, as well as its large size (original size was 3.3uf, but I added pre-out/main-in jacks, and with 3.3uf I couldn't drive a very low impedence at output).
Wierd thing is that the 'flutter' is intermittent. I suspected the tone control switch (S6 in drawing), and have cleaned the hell out of it with DeOxit and Pro Gold. Made no difference. Eight times out of ten I can turn it on and no flutter is there. The other two it will flutter for about 1 min in the left channel (used to be the right...??) and finally calm down.
Since the DC offset of the opamp is so low (1mV and 2mV), why can't the output cap just come out? Is there a reason to have it there? The next amp stage has a 4.7uf cap at it's input (it's a emitter-follower in a Szilaki configuration, so input impedence is high). And most devices that the preout would connect to would have a cap at the input (notable exception: I use an active crossover with this preamp, and the input stage is a DC coupled buffer, feeding an AC coupled filter...seems the initial DC swings shouldn't bother it too much...) Tone controls (which don't get used much) are basic NFB stuff.
As a test, I'd like to simply short across the 100uf cap and see what happens with the wierd startup flutter. Can I do that? Or should I take the cap out before shorting??
Is removing the cap going to be a problem since then both the 120K feedback resistor and the 820K resistor /5pf cap will then both be DC coupled to the opamp output?
I'd really like to toss the 100uf cap...can I?
Root problem is...if the amp has been off for a while, on powerup I get a strange 'scratchy' sound at the output of one channel. It's almost like a dirty volume control would sound, but kinda fluttery and mainly in the frequency range of about 80-120Hz. Lasts perhaps 1 minute or so.
I suspect that the large 100uf polarized cap at the output is the problem. It only does this in one channel. Afterwords, the amp sounds and behaves just fine. When the amp was set up and running in my system, I swear it was the right channel. After disassembly, and changing a few resistors (needed to increase gain a bit), and checking the opamp socket (a nice gold plated Amp socket, not a machined socket, but a decent one nevertheless), now the right is fine and wierd startup 'flutter' has moved to the left.
Yes, it is possible that I had channels reversed somewhere down the line and it was left channel all along, soooo....
With the circuit as it is:
On startup, the opamp output on each channel first swings to about +7 Volts, then drops to about -2 Volts, finally settling at about +1mV on left channel, and about +2mV on the right. I suspect that, if the output was DC coupled, it would settle much faster. I also suspect that the startup 'flutter' is related to the reverse-biasing of this polarized electrolytic, as well as its large size (original size was 3.3uf, but I added pre-out/main-in jacks, and with 3.3uf I couldn't drive a very low impedence at output).
Wierd thing is that the 'flutter' is intermittent. I suspected the tone control switch (S6 in drawing), and have cleaned the hell out of it with DeOxit and Pro Gold. Made no difference. Eight times out of ten I can turn it on and no flutter is there. The other two it will flutter for about 1 min in the left channel (used to be the right...??) and finally calm down.
Since the DC offset of the opamp is so low (1mV and 2mV), why can't the output cap just come out? Is there a reason to have it there? The next amp stage has a 4.7uf cap at it's input (it's a emitter-follower in a Szilaki configuration, so input impedence is high). And most devices that the preout would connect to would have a cap at the input (notable exception: I use an active crossover with this preamp, and the input stage is a DC coupled buffer, feeding an AC coupled filter...seems the initial DC swings shouldn't bother it too much...) Tone controls (which don't get used much) are basic NFB stuff.
As a test, I'd like to simply short across the 100uf cap and see what happens with the wierd startup flutter. Can I do that? Or should I take the cap out before shorting??
Is removing the cap going to be a problem since then both the 120K feedback resistor and the 820K resistor /5pf cap will then both be DC coupled to the opamp output?
I'd really like to toss the 100uf cap...can I?
Attachments
EchoWars said:
Root problem is...if the amp has been off for a while, on powerup I get a strange 'scratchy' sound at the output of one channel. It's almost like a dirty volume control would sound, but kinda fluttery and mainly in the frequency range of about 80-120Hz. Lasts perhaps 1 minute or so.
I suspect that the large 100uf polarized cap at the output is the problem. It only does this in one channel. Afterwords, the amp sounds and behaves just fine. When the amp was set up and running in my system, I swear it was the right channel. After disassembly, and changing a few resistors (needed to increase gain a bit), and checking the opamp socket (a nice gold plated Amp socket, not a machined socket, but a decent one nevertheless), now the right is fine and wierd startup 'flutter' has moved to the left.
Yes, it is possible that I had channels reversed somewhere down the line and it was left channel all along, soooo....
With the circuit as it is:
On startup, the opamp output on each channel first swings to about +7 Volts, then drops to about -2 Volts, finally settling at about +1mV on left channel, and about +2mV on the right. I suspect that, if the output was DC coupled, it would settle much faster. I also suspect that the startup 'flutter' is related to the reverse-biasing of this polarized electrolytic, as well as its large size (original size was 3.3uf, but I added pre-out/main-in jacks, and with 3.3uf I couldn't drive a very low impedence at output).
Wierd thing is that the 'flutter' is intermittent. I suspected the tone control switch (S6 in drawing), and have cleaned the hell out of it with DeOxit and Pro Gold. Made no difference. Eight times out of ten I can turn it on and no flutter is there. The other two it will flutter for about 1 min in the left channel (used to be the right...??) and finally calm down.
Since the DC offset of the opamp is so low (1mV and 2mV), why can't the output cap just come out? Is there a reason to have it there? The next amp stage has a 4.7uf cap at it's input (it's a emitter-follower in a Szilaki configuration, so input impedence is high). And most devices that the preout would connect to would have a cap at the input (notable exception: I use an active crossover with this preamp, and the input stage is a DC coupled buffer, feeding an AC coupled filter...seems the initial DC swings shouldn't bother it too much...) Tone controls (which don't get used much) are basic NFB stuff.
As a test, I'd like to simply short across the 100uf cap and see what happens with the wierd startup flutter. Can I do that? Or should I take the cap out before shorting??
Is removing the cap going to be a problem since then both the 120K feedback resistor and the 820K resistor /5pf cap will then both be DC coupled to the opamp output?
I'd really like to toss the 100uf cap...can I?
You certainly can and should toss the output cap away. Particularly if you have a cap in the next stage.
In fact most times it's better to pull out all output caps, particularly the last cap before the power amp, and put a very good quality cap there (some prefer film types, some BG).
In this case just use a wire to short the output cap. Check if you have DC at the chip input and also short the input cap.
But I don't think what you are telling is related with output cap failure. The IC sockets might be a reason, and you should change them to machined types. As the chip is a dual type, have you tried another dual to see what happens?
The intermittent problem might be related to a cold solder too, so if you didn't yet go and use your solder pen on all pads.
The next thing you should do is bypass all the pots you have (volume, balance, treble, bass, etc.) and see how the signal gets through. Perhaps even bypass this whole stage, feeding a CD signal direct to the power amp, and see if the problem vanishes.
In fact this is where I would start trying: bypassing this stage.
Another thing: when you say the amp was off for a while, how long is that? Where was it kept?
Carlos
I really doubt the socket is the problem. Again, it is a very good Amp part. I did not have any machined parts at the time.
I have also changed the 2604 with another, and receive the same results with DC offset (close anyway) and also have the same problem with the wierd flutter.
I have resoldered many connections looking for a bad solder joint, and have even closely examined the board with a 10x maginifying glass to see if I might see a poor joint. They all look good, and I reflowed quite a few anyway.
The problem originates here in this stage. The volume control is before this stage, and changing the volume setting (or balance for that matter) makes no difference in the level of the flutter. This opamp stage is the first active stage in the amp. and the flutter goes away (in the speakers anyway) if I pull the pre/main jumper (as it should if it was coming from the preamp). Not only that, but when I can get the flutter to appear, I can see it on the scope at the opamp output.
The 4.7uf cap in the schematic and the 47uf cap in the feedback loop are both BG non-polar. The cap at the next stage is also a 4.7uf BG. The 100uf cap was a simple Panasonic FC polarized, since there is not much room to work with here. Peter Daniel sent me a couple of Panasonic HFQ's (thanks Peter) that I think sound a bit better and certainly fit a lot better, but the problem with the flutter persists and I believe if I use no cap at all then the problem will go away.
By 'off for a while', I simply mean an hour or so. This amp has been in near continuous use for 25 years.
So, dumping the output cap is a reasonable thing to do...and the 820K resistor and 5pf cap won't affect anything?
I have also changed the 2604 with another, and receive the same results with DC offset (close anyway) and also have the same problem with the wierd flutter.
I have resoldered many connections looking for a bad solder joint, and have even closely examined the board with a 10x maginifying glass to see if I might see a poor joint. They all look good, and I reflowed quite a few anyway.
The problem originates here in this stage. The volume control is before this stage, and changing the volume setting (or balance for that matter) makes no difference in the level of the flutter. This opamp stage is the first active stage in the amp. and the flutter goes away (in the speakers anyway) if I pull the pre/main jumper (as it should if it was coming from the preamp). Not only that, but when I can get the flutter to appear, I can see it on the scope at the opamp output.
The 4.7uf cap in the schematic and the 47uf cap in the feedback loop are both BG non-polar. The cap at the next stage is also a 4.7uf BG. The 100uf cap was a simple Panasonic FC polarized, since there is not much room to work with here. Peter Daniel sent me a couple of Panasonic HFQ's (thanks Peter) that I think sound a bit better and certainly fit a lot better, but the problem with the flutter persists and I believe if I use no cap at all then the problem will go away.
By 'off for a while', I simply mean an hour or so. This amp has been in near continuous use for 25 years.
So, dumping the output cap is a reasonable thing to do...and the 820K resistor and 5pf cap won't affect anything?
EchoWars said:
I have done that many times on CD players, shorting the output cap. You can put a series resistor in the vacant place, something like 220 ohms or so.
It will work great.
Carlos
So...what might be the pros or cons of either simply putting in a jumper wire, or adding a 220 ohm resistor?
Anyone?
Anyone?
EchoWars said:
Not many much cons for using a 220 ohm resistor, and many pros. The most important pro is that the resistor will protect the preamp output stage by limiting the current.
Most CD players already have those resistors.
Though I must say this is more useful with headphone amps.
Carlos
lets dump the flutter
Sounds to me that at startup you have an oscillation burst. Did you change the opamps from the original ones?
Try with C133 & C134 47pF or so.
Jan Didden
Sounds to me that at startup you have an oscillation burst. Did you change the opamps from the original ones?
Try with C133 & C134 47pF or so.
Jan Didden
Capacitor leakage current can cause exactly this problem, although it may not be the problem in this case. Larger capacitors exhibit the problem more (leakage is proportional to value), higher voltages are best.
It's a common problem in old AC-coupled preamps, in the inter-stage DC blocks (e.g. Naim).
Andy.
If you look, there is a 665 Ohm resistor at the pre-out jack for current limiting. Adding the 220 Ohm will simply raise output impedence slightly. I guess what I don't understand is the relationship of the 120K feedback resistor, and the 820K/5pf capacitor (there to limit gain at HF, I assume)...and whether it is better to have them separated by a cap, a small resistance, jumper wire, or perhaps it makes no difference at all.
janneman,
The 2604 replaces a very old 7-pin inline opamp from the mid 70's, a specialty part made for Kenwood, so yes, it is an upgrade (on a small custom PC board with the eight leads running to the original Kenwood board, leads perhaps 2.5cm long, power bypassed with .18µf film cap) ). Would seem strange then an oscillation burst would show itself as mainly low-to-mid frequency noise. Regardless, I have a few 39pf caps around, and may replace the 5pf cap with one.
Right now I have temporarly jumpered over the 100µf cap, and I cannot get the turn on flutter to return. I will have to experiment some more and ensure that this is a true fix.
You see, just by listening I am almost convinced this is a problem with leakage, or a problem with using a polarized cap in an environment where it is getting no bias. The 100µf/50V cap at the output has had a Panasonic FC, a Panasonic HFQ, and I even stole a Black Gate 100µf/50V K series cap from another piece of equipment as a test, and all show the same problem with the wierd turn on flutter. All these caps are polarized, and here I've got a whole 1 or 2mV on them for bias. They do not seem to like this, and I can't get a decent bi-polar small enough to fit where the polar comes out, which is why I want to toss the cap out completely.ALW said:
Again, I have the cap jumpered across for now, and I'll have to continue experimenting to see if the flutter is gone. But still want to know if there should be a small resistance separating the 820K/5pf loop, and the 120K feedback resistor...
Hi,
I favour Jan's diagnosis here, and I wouldn't be surprised if the problem is related to some sporadic instability, which as things warm up, disappears when it settles down.
Stray inductances and capacitances can cause problems like this, and it could be the result of the particular layout of the components involved.
I know how hard it is to describe sonic effects, but I am a little puzzled by the apparent conflict of terms you use here, with 'scratchy' indicating much higher frequencies to me than say 80 to 120 Hz, which are more like the LF 'motorboating' effects (due to instability) which are more common in valve circuits.
Also, whilst I would always recommend removing any *unnecessary* signal path coupling caps to improve the sound, I would be concerned that you say you see 7volts (albeit temporarily) at the outputs.
If such a high DC voltage is even temporarily fed to a subsequent DC coupled amp, or whatever, I reckon the result will be quite a loud unwanted noise, and this will possibly 'bottom' your speakers, which is not a good thing!!!
If you do adopt this suggestion, I would certainly make sure you always switch the preamp on *before* any subsequent amplifier.
Do you have access to a 'scope, perhaps, which should help in seeing the shape of the 'interference' bursts, and this might throw some more light on the problem?
Regards, 🙂
Edit: R13 and R15, merely determine the amount or degree of feedback, so I don't see any need for any other resistance here, if this is your question in your later posts. The very small (bypass) cap, is just for stability purposes.
I favour Jan's diagnosis here, and I wouldn't be surprised if the problem is related to some sporadic instability, which as things warm up, disappears when it settles down.
Stray inductances and capacitances can cause problems like this, and it could be the result of the particular layout of the components involved.
I know how hard it is to describe sonic effects, but I am a little puzzled by the apparent conflict of terms you use here, with 'scratchy' indicating much higher frequencies to me than say 80 to 120 Hz, which are more like the LF 'motorboating' effects (due to instability) which are more common in valve circuits.
Also, whilst I would always recommend removing any *unnecessary* signal path coupling caps to improve the sound, I would be concerned that you say you see 7volts (albeit temporarily) at the outputs.
If such a high DC voltage is even temporarily fed to a subsequent DC coupled amp, or whatever, I reckon the result will be quite a loud unwanted noise, and this will possibly 'bottom' your speakers, which is not a good thing!!!
If you do adopt this suggestion, I would certainly make sure you always switch the preamp on *before* any subsequent amplifier.
Do you have access to a 'scope, perhaps, which should help in seeing the shape of the 'interference' bursts, and this might throw some more light on the problem?
Regards, 🙂
Edit: R13 and R15, merely determine the amount or degree of feedback, so I don't see any need for any other resistance here, if this is your question in your later posts. The very small (bypass) cap, is just for stability purposes.
Hi Bobken...
Yes, I have a scope on it, but since jumpering the output cap I cannot get the output flutter to return.
In my first description, 'scratchy' was perhaps inaccurate, 'flutter' is probably more like it. I have heard amps motorboating, and this is not it. When it does it, it might go for 5 or 6 seconds, stop for a few seconds, then return for a few, eventually disappearing after about 60 seconds. If your were sitting here, I'd make the sound 'phft' and say that is sorta close.
I did manage to have the scope on it once when the flutter was present, and there were many components present, but HF components were pretty low level...the majority of the noise seemed to be in a band between 100Hz-1KHz. Since it didn't hang around long enough to lend itself to a detailed analysis, that's about the best I can do.
BTW, with the cap jumpered, the wierd DC swings at powerup are drastically lessened. Now it varies a bit, sometimes no DC swing at all, and others it swings +/- about 1.5v and quickly settles. Seems to depend on how long it had been off...the longer it had been off, the less the DC startup swing (after 15 minutes off or so, DC startup swing is almost nil).
On a related note, my amps are on a delayed turn on. When preamp is powered up, amps power up about 20 seconds later...
Yes, I have a scope on it, but since jumpering the output cap I cannot get the output flutter to return.
In my first description, 'scratchy' was perhaps inaccurate, 'flutter' is probably more like it. I have heard amps motorboating, and this is not it. When it does it, it might go for 5 or 6 seconds, stop for a few seconds, then return for a few, eventually disappearing after about 60 seconds. If your were sitting here, I'd make the sound 'phft' and say that is sorta close.
I did manage to have the scope on it once when the flutter was present, and there were many components present, but HF components were pretty low level...the majority of the noise seemed to be in a band between 100Hz-1KHz. Since it didn't hang around long enough to lend itself to a detailed analysis, that's about the best I can do.
BTW, with the cap jumpered, the wierd DC swings at powerup are drastically lessened. Now it varies a bit, sometimes no DC swing at all, and others it swings +/- about 1.5v and quickly settles. Seems to depend on how long it had been off...the longer it had been off, the less the DC startup swing (after 15 minutes off or so, DC startup swing is almost nil).
On a related note, my amps are on a delayed turn on. When preamp is powered up, amps power up about 20 seconds later...
Hi,
This being the case, I reckon it must be due to some thermal effects, as this is about all that remains which could be happening in the time scales you now mention.
If it was the result of a cap *changing* in some way as it charged up, it would normally be all over much sooner with the components you have in this circuit, as all of those caps will be charged up almost instantaneously I would guess.
These intermittent problems are a pig to find sometimes, but if you really wish to get to the bottom of it, you need to find a way to force the problem to occur, 'to order' as it were. Then you can do some worthwhile investigative work, to track the problem down.
I have done quite a bit of fault-finding over the years, and some very strange things can happen with components.
One of the worst to track down was a resistor which was fine when 'cold' but, after a few seconds of current passing thro it, the internal heating effect made the resistor increase in value by a factor of several hundred times, although the resistor didn't ever feel hot!
It could well be something like this which is the cause of your problem, but this kind of situation is very unusual.
Since it is seemingly a 'warmup' problem, this is good start (better than with completely sporadic faults), and with all stereo gear, you have the advantage of two channels to make comparisons/measurements etc.
I have fixed a lot of faults without having any clue as to what any of the parts did (no circuit diagrams available), simply by doing this, and where 'warmup' problems are apparent, a good old hair dryer and a can of 'freezer' with a fine nozzle will work wonders to force the fault to remain for long enough to discover it.
Luckily, you have so few components in this circuit, that with a bit of patience (when you have found a way of *ensuring* the fault can be produced) you can merely swap components from one channel to the other (one at a time) until the fault changes to the other channel.
I would start with the freezer can though, and freeze initially just part of the circuit, and then see if the fault remains for a longer time at startup. Then try other areas, in succession, until there is a change in the characteristics or time-span of the fault.
Generally, I have found it is quite quick to locate the approximate area, and then by warming the whole area up with a hair dryer, but just freezing one single component at a time, you can home in on the dodgy component (or perhaps bad joint).
I hope that this helps.🙂
Regards,
This being the case, I reckon it must be due to some thermal effects, as this is about all that remains which could be happening in the time scales you now mention.
If it was the result of a cap *changing* in some way as it charged up, it would normally be all over much sooner with the components you have in this circuit, as all of those caps will be charged up almost instantaneously I would guess.
These intermittent problems are a pig to find sometimes, but if you really wish to get to the bottom of it, you need to find a way to force the problem to occur, 'to order' as it were. Then you can do some worthwhile investigative work, to track the problem down.
I have done quite a bit of fault-finding over the years, and some very strange things can happen with components.
One of the worst to track down was a resistor which was fine when 'cold' but, after a few seconds of current passing thro it, the internal heating effect made the resistor increase in value by a factor of several hundred times, although the resistor didn't ever feel hot!
It could well be something like this which is the cause of your problem, but this kind of situation is very unusual.
Since it is seemingly a 'warmup' problem, this is good start (better than with completely sporadic faults), and with all stereo gear, you have the advantage of two channels to make comparisons/measurements etc.
I have fixed a lot of faults without having any clue as to what any of the parts did (no circuit diagrams available), simply by doing this, and where 'warmup' problems are apparent, a good old hair dryer and a can of 'freezer' with a fine nozzle will work wonders to force the fault to remain for long enough to discover it.
Luckily, you have so few components in this circuit, that with a bit of patience (when you have found a way of *ensuring* the fault can be produced) you can merely swap components from one channel to the other (one at a time) until the fault changes to the other channel.
I would start with the freezer can though, and freeze initially just part of the circuit, and then see if the fault remains for a longer time at startup. Then try other areas, in succession, until there is a change in the characteristics or time-span of the fault.
Generally, I have found it is quite quick to locate the approximate area, and then by warming the whole area up with a hair dryer, but just freezing one single component at a time, you can home in on the dodgy component (or perhaps bad joint).
I hope that this helps.🙂
Regards,
These are some good ideas, and I intend on trying them. The next two days, however, I am doing 12 to 14 hour days at work, and won't feel much like screwing with the amp by the time I get home.
I have another preamp exactly like this one here that I used as a testbed for ideas, and I changed out the opamp from the stock to the 2604 on it first. Layout is stock with the exception of the piggyback opamp board and the piggyback board for the Szilaki configuration CEF. My 'proof of concept' amp is identical with the exception of most cap values and the lack of the pre-out/main-in that was added to the second, and this testbed amp functions perfectly.
Please do me a favor and check the thread in the next day or so. First thing I'm going to do is pull out the opamp output cap and replace with a jumper wire. Then I'm going to freeze spray the critter and see if I can make the fault return. If I cannot, then I must assume that the noise was perhaps related to the output cap reverse biasing / under biasing.
Thanks guys...your assistance is appreciated.
I have another preamp exactly like this one here that I used as a testbed for ideas, and I changed out the opamp from the stock to the 2604 on it first. Layout is stock with the exception of the piggyback opamp board and the piggyback board for the Szilaki configuration CEF. My 'proof of concept' amp is identical with the exception of most cap values and the lack of the pre-out/main-in that was added to the second, and this testbed amp functions perfectly.
Please do me a favor and check the thread in the next day or so. First thing I'm going to do is pull out the opamp output cap and replace with a jumper wire. Then I'm going to freeze spray the critter and see if I can make the fault return. If I cannot, then I must assume that the noise was perhaps related to the output cap reverse biasing / under biasing.
Thanks guys...your assistance is appreciated.
Carlmart (is your name Carl??), you were right and I was wrong. Found two bad solder joints on the feedback resistors of each channel. Odd as hell that it happened to be the same resistor on both channels (what are the odds??), but explains why the 'flutter' started out in the right channel, and after pulling it apart, moved to the left...probably from banging the resistor around while connecting the scope probe.
Yes, I'm admitting this in public (coulda lied and said that I found a bad component). Been soldering since I was 10 years old, and I'm pretty damn good at it. Still, goes to show that assumptions when troubleshooting are a bad thing. All in all, still only took me about 2 hours to find them.
I removed the output caps and replaced them with a jumper regardless of my findings since I still have yet to find a capacitor that sounds as good as a straight piece of wire. DC offset is .4mV on one channel and 2mV on the other. The sound of the preamp is much improved (and it truly wasn't bad to begin with either). I really, really like the OPA2604...glad I ignored those who said it sucked.
EchoWars said:Carlmart (is your name Carl??), you were right and I was wrong. Found two bad solder joints on the feedback resistors of each channel. Odd as hell that it happened to be the same resistor on both channels (what are the odds??), but explains why the 'flutter' started out in the right channel, and after pulling it apart, moved to the left...probably from banging the resistor around while connecting the scope probe.
Yes, I'm admitting this in public (coulda lied and said that I found a bad component). Been soldering since I was 10 years old, and I'm pretty damn good at it. Still, goes to show that assumptions when troubleshooting are a bad thing. All in all, still only took me about 2 hours to find them.
I removed the output caps and replaced them with a jumper regardless of my findings since I still have yet to find a capacitor that sounds as good as a straight piece of wire. DC offset is .4mV on one channel and 2mV on the other. The sound of the preamp is much improved (and it truly wasn't bad to begin with either). I really, really like the OPA2604...glad I ignored those who said it sucked.
My name is Carlos, and I am very happy my suggestions could help you.
Mistakes with soldering are a very common thing, even in commercial products, so there's nothing to be ashamed of.
Sometimes we are afraid we are heating a part too much and try do it faster, or use a solder pen which is not hot enough.
Shorting the output cap is also a very good step, as long as you have a very good cap before the power amp or a DC sensor. The best cap is no cap, and if you did not do so already, short the one at your CD player's output.
Carlos
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