Can I increase capacitance on these TONE BOARD caps?
- Thread starter roger2
- Start date
I want to thank everyone for the awesome discussion and for all of your input :thmbsp: This has been a very informative thread.
For anyone reading this thread wondering why I would be so concerned about single opamp... I have put a lot of work into this amp trying to make it something special: all electolytics new, poly caps in key spots, metal film resistors in key spots, new bias trimmers, new speaker relay and relay driver transistor, new differential input transistors, approximately 95% of solder joints reflowed. Not done yet, but all zeners will be replaced as well.
It has been, and continues to be, a huge learning process. Unfortunately, I learned recently that this 4558 is always in the signal path, essentially functioning as the pre-amp (if I understand correctly). And that the 4558 is noisy and generally disliked.
@ecluser
I understand your basic idea for modding a socket to fit into my board. Not 100% clear on the details yet. But before going further I think it best to decide IF I am even going to do an opamp swap.
Values of Ci15 and Ci16 will remain unchanged.
No scope. But I do have a DMM and know how to use Ohm's Law. If it is required or preferable to keep supply voltage at +/- 9v by adjusting Ri27 & Ri28 that is something I can do.
For anyone reading this thread wondering why I would be so concerned about single opamp... I have put a lot of work into this amp trying to make it something special: all electolytics new, poly caps in key spots, metal film resistors in key spots, new bias trimmers, new speaker relay and relay driver transistor, new differential input transistors, approximately 95% of solder joints reflowed. Not done yet, but all zeners will be replaced as well.
It has been, and continues to be, a huge learning process. Unfortunately, I learned recently that this 4558 is always in the signal path, essentially functioning as the pre-amp (if I understand correctly). And that the 4558 is noisy and generally disliked.
@ecluser
I understand your basic idea for modding a socket to fit into my board. Not 100% clear on the details yet. But before going further I think it best to decide IF I am even going to do an opamp swap.
Values of Ci15 and Ci16 will remain unchanged.
No scope. But I do have a DMM and know how to use Ohm's Law. If it is required or preferable to keep supply voltage at +/- 9v by adjusting Ri27 & Ri28 that is something I can do.
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Can any of these be made to work safely in my tone board as a replacement for the 4558? I can do additional decoupling and adjust input resistors as required.
Tx Inst TL072 Would this opamp be stable in the circuit? It was recommended elsewhere as a very stable and predictable opamp. High output impedance was noted. Also suggested was 4.7pF across Ri7 and Ri8.
-notably faster slew rate than the 4558 but the bandwidth is the same as the 4558 at 3mHz
-from mfgr description: "...ideally suited for high-fidelity and audio preamplifier applications..."
-is a JFET input-stage design
-Note in the spreadsheet that Input Bias Current is specified in pA not nA like all the others
Tx Inst RC4559 slew rate and bandwidth modestly higher than 4558. Could this one possibly offer some slight audio performance gain with a minimum of (or no) risk?
Tx Inst RC4558 this one appears to have a good noise spec. Would this 4558 be quieter than my original 4558? I would assume that this would be the safest possible swap, is that correct? This 4558 has 1.7v/uS slew rate, slightly faster than what is probably in my amp originally.
Texas Instruments TL072 .pdf specsheet is too large to attach. Here is a link to the DigiKey page that has link to the specsheet.
EDIT: Input Offset Voltage (typical) for the NE5532 is .5 mV, not .05 mV as shown in the spreadsheet
Tx Inst TL072 Would this opamp be stable in the circuit? It was recommended elsewhere as a very stable and predictable opamp. High output impedance was noted. Also suggested was 4.7pF across Ri7 and Ri8.
-notably faster slew rate than the 4558 but the bandwidth is the same as the 4558 at 3mHz
-from mfgr description: "...ideally suited for high-fidelity and audio preamplifier applications..."
-is a JFET input-stage design
-Note in the spreadsheet that Input Bias Current is specified in pA not nA like all the others
Tx Inst RC4559 slew rate and bandwidth modestly higher than 4558. Could this one possibly offer some slight audio performance gain with a minimum of (or no) risk?
Tx Inst RC4558 this one appears to have a good noise spec. Would this 4558 be quieter than my original 4558? I would assume that this would be the safest possible swap, is that correct? This 4558 has 1.7v/uS slew rate, slightly faster than what is probably in my amp originally.
Texas Instruments TL072 .pdf specsheet is too large to attach. Here is a link to the DigiKey page that has link to the specsheet.
EDIT: Input Offset Voltage (typical) for the NE5532 is .5 mV, not .05 mV as shown in the spreadsheet
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BinaryMike
Pelagic EE
The TL072's low input bias current offers the possibility of modifying the circuits to eliminate electrolytic input coupling caps in favor of films, but that's already a strong possibility in the tone amp. Of these choices, I would definitely go for the 5532 because of its low noise and superior load driving capability. The phono preamp chip has to drive 560R at high frequencies due to RIAA equalization, which is just barely acceptable even for op-amps with plenty of grunt like this one. And I would boost both supply rails to 15~18V in order to maximize phono preamp headroom.
ecluser
Super Member
If you use the 5532, keep in mind that the operating current of the 5532 is larger (7mA vs 3.5mA for the 4558). The voltage drop in the resistors on the supply line will be larger and you will need to change the zener for a larger voltage. I would use 20V zeners, and capacitors rated to 25V.
back on the project 
Apologies for the long interval since last posting here. Was kept busy with a new job that required considerable commute time, so time to work on the project was non-existent for several months.
I did continue briefly after my last post. Basically I had decided to play it safe and leave the old 4558 in place. This decision based mostly on EchoWars' warnings and my lack of oscilloscope (hardware and expertise). Also: the multiple wires coming off the PCB, my assumption that the design of this PCB, with a fairly new-at-the-time technology, might not be laid out well for faster (or much faster) opamps, the fact that other design decisions on this amp have been brought into question in this very thread, etc, etc.
All of which became a moot point when I decided, as one final tweak, to reflow the solder on the tone board. Apparently I fried the old 4558.
So a new opamp went from being an option to a necessity. I re-read ecluser's ideas on improvising a socket. And I dug into Digikey and Mouser to see if there were any existing sockets that might be modified to work, and ordered a few candidates. Then I put the amp on the shelf for a few months.
Apologies for the long interval since last posting here. Was kept busy with a new job that required considerable commute time, so time to work on the project was non-existent for several months.
I did continue briefly after my last post. Basically I had decided to play it safe and leave the old 4558 in place. This decision based mostly on EchoWars' warnings and my lack of oscilloscope (hardware and expertise). Also: the multiple wires coming off the PCB, my assumption that the design of this PCB, with a fairly new-at-the-time technology, might not be laid out well for faster (or much faster) opamps, the fact that other design decisions on this amp have been brought into question in this very thread, etc, etc.
All of which became a moot point when I decided, as one final tweak, to reflow the solder on the tone board. Apparently I fried the old 4558.
So a new opamp went from being an option to a necessity. I re-read ecluser's ideas on improvising a socket. And I dug into Digikey and Mouser to see if there were any existing sockets that might be modified to work, and ordered a few candidates. Then I put the amp on the shelf for a few months.
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First pic shows my socket candidates, beginning with the wirewrap type (Digi-Key ED4308-ND) suggested by Ken Kantor. The legs on this were clearly long enough. The only problem was that the legs were too fat to fit through the holes in the PCB. This PCB has very flimsy traces and small pads that will flake off for any or no good reason. I did not want to risk drilling into it.
Next up is a right angle socket (Digi-Key ED58432-ND) which I slightly modified by spreading the rows of legs from their original .1" spacing. Again, this could have worked here. But close proximity to resistors Ri7,8 would have prevented removal of the opamp after installation and would have required soldering the socket with opamp in place.
The winner was what Mill-Max calls a "Long Solder Tail" socket (Digi-Key ED90113-ND). The tails are .170" long - enough to work where the shorter tails on a generic Radio Shack socket (earlier in this thread) did not.
Second pic shows the tone board in its current state. I did (finally) get 16v caps into positions Ci15,16 (power supply decoupling).
Third pic shows .22uF ECQ-V bypass caps on Ci15,16 as well as Ci5,6 (the bi-polars on the outputs of the opamp).
<schematic of this board appears in posts 1 and 10 on page 1 of this thread>
Next up is a right angle socket (Digi-Key ED58432-ND) which I slightly modified by spreading the rows of legs from their original .1" spacing. Again, this could have worked here. But close proximity to resistors Ri7,8 would have prevented removal of the opamp after installation and would have required soldering the socket with opamp in place.
The winner was what Mill-Max calls a "Long Solder Tail" socket (Digi-Key ED90113-ND). The tails are .170" long - enough to work where the shorter tails on a generic Radio Shack socket (earlier in this thread) did not.
Second pic shows the tone board in its current state. I did (finally) get 16v caps into positions Ci15,16 (power supply decoupling).
Third pic shows .22uF ECQ-V bypass caps on Ci15,16 as well as Ci5,6 (the bi-polars on the outputs of the opamp).
<schematic of this board appears in posts 1 and 10 on page 1 of this thread>
Attachments
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BinaryMike
Pelagic EE
Thanks for documenting this fix. I couldn't count the number of times a Mill-Max product has saved my bacon...
ken kantor
Fowl Humor
Nice. What opamp/manufacturer did you select for the replacement?
Working right and sounding good?
-k
Working right and sounding good?
-k
Opa2134
I am going to order an OPA2134 to try in there now that I have a socket - just to see what happens and whether I can tell a difference in sound versus the NJM4558's and 4559's that I have been playing with.
I will add .1uF directly across pins 4 and 8.
What type of cap should I use for the .1uF? Does a ceramic have better performance (less inductance for example) than say a stacked polyester ECQ-V?
If ceramic would be better, what type and voltage?
I am going to order an OPA2134 to try in there now that I have a socket - just to see what happens and whether I can tell a difference in sound versus the NJM4558's and 4559's that I have been playing with.
I will add .1uF directly across pins 4 and 8.
What type of cap should I use for the .1uF? Does a ceramic have better performance (less inductance for example) than say a stacked polyester ECQ-V?
If ceramic would be better, what type and voltage?
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@ Ken Kantor
Seems to be working OK with either NJM4558 or 4559. But the sound seems sterile or maybe even a bit harsh at higher volume.
However, in addition to using the new opamps I also changed out several of the old carbon film (I assume) resistors to metal film. Some key ones most likely and this is the preamp. So I do not know whether to attribute (my perceived) change in sound to the different opamps or the different resistors.
I am getting together a Digi-Key order for the OPA2134, an additional decoupling cap (referred to in last post) and new carbon film resistors.
Seems to be working OK with either NJM4558 or 4559. But the sound seems sterile or maybe even a bit harsh at higher volume.
However, in addition to using the new opamps I also changed out several of the old carbon film (I assume) resistors to metal film. Some key ones most likely and this is the preamp. So I do not know whether to attribute (my perceived) change in sound to the different opamps or the different resistors.
I am getting together a Digi-Key order for the OPA2134, an additional decoupling cap (referred to in last post) and new carbon film resistors.
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BinaryMike
Pelagic EE
Some op-amps, such as the 5532, must have a 100nF (0.1uf) ceramic directly across the power pins to assure stability. Some of them behave better with ceramics or other types of decoupling caps from both supply pins to a high-quality ground system instead. Doug Self observed that performance issues related to unsatisfactory power supply decoupling can be quite subtle. A distortion analyzer is sometimes necessary to confirm the cure, but the op-amp's datasheet should always be consulted for guidance.
The PS decoupling caps have been bypassed with .22uF film, as shown in the last pic in post #67. These caps are not directly connected to pins 4 and 8 but they are pretty close to the opamp. The OPA2134 datasheet says to bypass power pins with 10nF ceramics, but the closest ground point on this board is these already-bypassed PS decoupling caps. I would be reluctant to solder a third cap to these locations. The OPA2134 may not necessarily be permanent.
I just though I would add a cap directly across pins 4 and 8 of the socket as an additional, generic preventive measure since I want to experiment with a more modern (than the original) opamp.
I just placed a Digi-Key order which includes 4 options for a decoupling cap across pins 4 and 8:
- .1uF Panasonic ECQ-V
- .1uF COG 50v ceramic
- .047uF COG ceramic
- .01uF COG ceramic
Which of these would be best?
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ken kantor
Fowl Humor
There is nothing about the variations in opamp performance that could conceivably influence things like "sterility" or "harshness." Anyway, the audio signal has already passed through dozens, perhaps >100, opamps on the way to your ears. It is extraordinarily unlikey that making small changes to one at random, (eg- your 4558 > 2134), will be at all audible. In your circuit, I would expect perhaps some tiny difference in noise level, that's all.
-k
Wouldn't the OPA2134 offer the same possibility as the TL072, both having FET inputs and low input bias current? (even lower on the 2134 than theTL072)
Would I have been incorrect to assume that films could be used here regardless of the opamp in use? In what situation (physical size notwithstanding) would electrolytic be preferable film?
@BinaryMike
I always value your opinions and appreciate you posting here. And I think I get part of what you are saying about the 5532. It is an industry standard, a workhorse, and is superior to the 4558 in many respects.
But for me right now, upgrading an opamp, while avoiding the common pitfalls, is about as much of a challenge as I am up for. And I need to learn a lot more to make that happen. But changing rail voltages of two boards to accommodate the 5532's higher operating current seems like an avoidable complication. I don't want to mess with the gain structure of the amp. And honestly, I am not concerned about the phono section. If I raise the Vf of the zeners on the PS board that would require adjustment of the phono board resistors. I did put a socket and a new NJM4558 in the phono section and am happy to leave it alone now.
If possible, what I would like to do is find an opamp, to replace the 4558 on the tone board, that would require a minimum of component changes and that could operate at the original +/-9v. Just to keep things simple for a rookie. Maybe that is not possible I don't know, but right now it seems like a reasonable initial goal.
By the way, I did pick up Doug Self's Small Signal Audio Design. It is a great resource as you have noted previously. I went straight to the opamp chapter - where first I got the idea to experiment with the OPA2134
The OPA2134's operating (quiescent) current is similar to the NJM4558. I do not know how its other parameters would fit with the existing circuits in the KA-4006 tone board. But I am here to learn :yes:
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BinaryMike
Pelagic EE
Essentially any FET-input op-amp offers an extremely low input bias current specification. The major gotcha is that FET bias current rises exponentially with temperature, but that's not usually a problem in audio applications.
Lytics are never preferable to films when performance is the sole criterion. They're only used because they offer much smaller size and lower cost in large capacitance values, and they can be made to work acceptably well by careful circuit design.
BinaryMike
Pelagic EE
The reason for increasing power supply voltage is to improve dynamic range, not to support higher op-amp supply current (but this may also be necessary). The 5532 and other good audio chips operate just fine on +/-9V supplies -- their performance just isn't optimized. You could reserve this mod for another day.
Regarding gain structure: There's not really anything to worry about. Circuit gain is almost purely a function of feedback networks, which is the beauty of designing with op-amps. And all of the op-amps under discussion here are unity-gain stable, which means that oscillation won't be a problem unless power supply decoupling goes seriously awry.
Thank you, BinaryMike, for clarification on those points.
I was thinking of using 1uF Wimas for the input caps (Ci1, Ci2) if they will fit.
Your explanation of opamp gain is helpful. I have been reading from several sources but your comments really helped to make this point sink in.
I was thinking of using 1uF Wimas for the input caps (Ci1, Ci2) if they will fit.
Your explanation of opamp gain is helpful. I have been reading from several sources but your comments really helped to make this point sink in.
Just wanted to clarify, to say that I am not concerned about the phono section is not true. In addition to the opamp socket and new 4558, the phono board has been recapped and Wima film caps were used in the input and output positions.
I don't have a TT now but have tested the phono board with a signal generator.
The main thing is that, unlike the tone/preamp board, the phono board in this amp is easily accessible. If a TT comes in to the picture it will be easy to give it more attention if needed.