Phono board - looks pretty good design and implementation:
What would I do with it?
Cartridge loading capacitors:
MM input C1 is only 100 pF - my personal preference is 220 pF as a starting point, Martin MS uses also Shure cartridge which needs 400.500 pF according to user manual;
MC input C10 is 680 pF - may be too large for a starting point, needs to be adjusted according to cartridge?
MM/MC switch is semi open type - definitely needs cleaning and some "work" since it is very rarely used.
The utmost solution would be to replace it with a relay having an operating swtch on the back panel - so switching between MM/MC inputs would not require opening the cover.
Signal path caps:
C5 Rubycon orange 47 uF
C6 Rubycon Black gate 100 uF
C9 Rubycon blue 1 uF
C14 Rubycon blue 1 uF
I would replace these with either film capacitors or bipolar lytics (according to brand preference):
Power rail caps:
C11 Rubycon blue 1000 uF
C13 Rubycon blue 1000 uF
C205 Rubycon Black gate 2,2 uF
C208 Rubycon Black gate 2,2 uF
I would replace with new good quality lytics, also I would increase capacitance of the smaller ones.
In any case I would solder some samples out and measure their condition.
Opamps are LM627 (input stage) and NE5534 - If everything else were fixed in the amp then I might think of replacing the 627 ones but that would reuire listening tests etc..
What would I do with it?
Cartridge loading capacitors:
MM input C1 is only 100 pF - my personal preference is 220 pF as a starting point, Martin MS uses also Shure cartridge which needs 400.500 pF according to user manual;
MC input C10 is 680 pF - may be too large for a starting point, needs to be adjusted according to cartridge?
MM/MC switch is semi open type - definitely needs cleaning and some "work" since it is very rarely used.
The utmost solution would be to replace it with a relay having an operating swtch on the back panel - so switching between MM/MC inputs would not require opening the cover.
Signal path caps:
C5 Rubycon orange 47 uF
C6 Rubycon Black gate 100 uF
C9 Rubycon blue 1 uF
C14 Rubycon blue 1 uF
I would replace these with either film capacitors or bipolar lytics (according to brand preference):
Power rail caps:
C11 Rubycon blue 1000 uF
C13 Rubycon blue 1000 uF
C205 Rubycon Black gate 2,2 uF
C208 Rubycon Black gate 2,2 uF
I would replace with new good quality lytics, also I would increase capacitance of the smaller ones.
In any case I would solder some samples out and measure their condition.
Opamps are LM627 (input stage) and NE5534 - If everything else were fixed in the amp then I might think of replacing the 627 ones but that would reuire listening tests etc..
Definitely indeed... and I would say to the point of linking it out if its only ever used in one position.
Continuing along the signal path - TL072 is used as line out output buffer:
, while NE5534 is used as DC servo:
Leaving the justification of that output buffer aside I would swap these IC-s.
, while NE5534 is used as DC servo:
Leaving the justification of that output buffer aside I would swap these IC-s.
Does this mean that the amp may become unstable in case DC in amp output is "too good" i.e. near zero?
The opamp might oscillate at few MHz. The DC servo accuracy might be degraded somewhat as a consequence, but the opamp should be compensated for unity-gain stability since high frequency gain is 1. See a 5534 data sheet for details.
The TL072 is a dual, so it's not directly swappable. The NE5532 is a dual is a dual.
The TL072 is a dual, so it's not directly swappable. The NE5532 is a dual is a dual.
Correct you are 🙂
Must check on the board that it is actually two 5534 chips and not one 5532.
Edit:
Checked it out - there are actually 2 NE5534 chips on the board (as servos) so the replacement would not be direct but still necessary.
Last edited:
The 5534 is an unlikely choice for a DC servo but to be fair it is configured correctly and will give minimal DC offset. There is no possibility of oscillation with it configured as it is as an integrator. The TL072... I've always rated the original TL0's when used properly and again this one is configured correctly.
Hi Mooly,
Sorry, but I must respectfully disagree.
The servo shown in the final pane of post 43 is indeed correctly drawn from a global perspective, but it needs a 22pF cap between pins 5 and 8 to ensure local stability.
Unfortunately, the TI data-sheet application figures don't show the cap for unity gain, but its text notes the opamp is stable for (noise) gains of 3 or more. From TI data sheet: "These operational amplifiers are compensated internally for a gain equal to or greater than three. Optimization of the frequency response for various applications can be obtained by use of an external compensation capacitor between COMP and COMP/BAL."
The best illustration of this subtlety I could find is Bob Cordell's oscillator from his THD analyzer, shown below. (https://www.cordellaudio.com/instrumentation/thd_analyzer.shtml)
Note in the integrator stages (eg. IC2), he uses C2=22pF. But in IC1 where the noise gain is about 2.27, he uses C1=15pF. At IC8, noise gain is 3.2 and he doesn't use any compensation cap at all. In contrast, at high frequencies integrators have noise gain of 1, thus needing the compensation cap.
Best regards,
Steve
Sorry, but I must respectfully disagree.
The servo shown in the final pane of post 43 is indeed correctly drawn from a global perspective, but it needs a 22pF cap between pins 5 and 8 to ensure local stability.
Unfortunately, the TI data-sheet application figures don't show the cap for unity gain, but its text notes the opamp is stable for (noise) gains of 3 or more. From TI data sheet: "These operational amplifiers are compensated internally for a gain equal to or greater than three. Optimization of the frequency response for various applications can be obtained by use of an external compensation capacitor between COMP and COMP/BAL."
The best illustration of this subtlety I could find is Bob Cordell's oscillator from his THD analyzer, shown below. (https://www.cordellaudio.com/instrumentation/thd_analyzer.shtml)
Note in the integrator stages (eg. IC2), he uses C2=22pF. But in IC1 where the noise gain is about 2.27, he uses C1=15pF. At IC8, noise gain is 3.2 and he doesn't use any compensation cap at all. In contrast, at high frequencies integrators have noise gain of 1, thus needing the compensation cap.
Best regards,
Steve
Since replacement of the opamp has turned out to becomplicated (finding a proper substituted) I would add that under the board - ceramic cap is small and there should be enough room for it. X7R would be good enough for the purpose?
Next thing - tone control:
Bass MIN and MAX positions:
Treble MIN and MAX positions:
Both regultaors in MIN and MAX positions:
Looking at these results - I cannot say I like them - treble regulation is a bit weak and not looking the same in min and max directions, bass in max positions reduces treble adjustment.
If the amp would stay mine then I would spend some time on LTspice simulations - probably better result can be obtained by some component replacements and changes.
Bass MIN and MAX positions:
Treble MIN and MAX positions:
Both regultaors in MIN and MAX positions:
Looking at these results - I cannot say I like them - treble regulation is a bit weak and not looking the same in min and max directions, bass in max positions reduces treble adjustment.
If the amp would stay mine then I would spend some time on LTspice simulations - probably better result can be obtained by some component replacements and changes.
No problem 🙂
I do get what you are saying, and I'm looking at it from what happens in a real world use as it is shown in the amp. Compensation is mandatory for normal gain applications less then 5 (is it 5 or 3? for the 5534)
In the servo there is no direct DC feedback, the opamp has become part of the loop as a whole and its operation more like a comparator but of course with the large integrating cap slapped directly from output to input... that's not explaining it very well is it.
The 5534 will leap into oscillation if not compensated when the gain is configured to a low value but does it here? I suspect not and I personally think you would see anything on a wideband scope... but it would be great to test the theory 🙂
While measuring the tone controls I switched the power amplifier to external input mode (switch on the board that also needs cleaning (due to lack of use) and grounded the power in inputs with grounded RCA plugs.
And then I accidentally discovered that activating the speaker relays (pushing the buttons on the front panel) with no load on speaker terminals sent the amp temporarily to speaker protect mode (power led turned orange).
This did not happen when testing the amp in "Normal" mode i.e. feeding it via internal preamp.
So what could be the cause - perhaps the fact that the RCA sockets are not connected directly to the power amp input as one would expect but via a 6k8/1k8 voltage divider?
The amp has till its covers off so all sort of interference can be picked up.
This behaviour would need to be sorted out. (replace R53/153 with e.g. 100k and short R52/152 as a starting point?)...
And then I accidentally discovered that activating the speaker relays (pushing the buttons on the front panel) with no load on speaker terminals sent the amp temporarily to speaker protect mode (power led turned orange).
This did not happen when testing the amp in "Normal" mode i.e. feeding it via internal preamp.
So what could be the cause - perhaps the fact that the RCA sockets are not connected directly to the power amp input as one would expect but via a 6k8/1k8 voltage divider?
The amp has till its covers off so all sort of interference can be picked up.
This behaviour would need to be sorted out. (replace R53/153 with e.g. 100k and short R52/152 as a starting point?)...
Preamplifier section.
I would start from adding local power reservoir capacitors - C13/14 of 10 pF seems a bit "smallish" in the end of the rail tracks.
47 uF lytics in parallel with e.g. 100 nF ceramics would be more like my preference:
Balance pot I would rewire so that it is effective also in direct mode - if it is actually in use (i.e. not in neutral position) then there is no reason to assume that in direct mode the adjustment is not required.
The output buffer stage (TL072, IC 203) of pre out I would jump.
And - as usual - signal path lytics (C15/115, Rubycon blue) I would replace with film caps or new lytics.
The operation mode (Normal / External input) switch needs cleaning - it is very rarely used if ever.
I would start from adding local power reservoir capacitors - C13/14 of 10 pF seems a bit "smallish" in the end of the rail tracks.
47 uF lytics in parallel with e.g. 100 nF ceramics would be more like my preference:
Balance pot I would rewire so that it is effective also in direct mode - if it is actually in use (i.e. not in neutral position) then there is no reason to assume that in direct mode the adjustment is not required.
The output buffer stage (TL072, IC 203) of pre out I would jump.
And - as usual - signal path lytics (C15/115, Rubycon blue) I would replace with film caps or new lytics.
The operation mode (Normal / External input) switch needs cleaning - it is very rarely used if ever.
After all that I would look at the board and scratch my head - what is the purpose of not replacing all other smaller lytics that have been left there...
The big reservoirs (10000 uF, 63V) I would leave as they are.
Well, that's all folks about this small nice device 😉
The big reservoirs (10000 uF, 63V) I would leave as they are.
Well, that's all folks about this small nice device 😉
Well, seems that it was not "all" - forgot the power amplifier section and 15V regulators.
15 V regulators - the lytics I would replace for good new ones, the output somoothing caps can be left at their designed size if additional capacity will be installed at the consuming IC-s 8described earlier).
Power amplifier - the inüut cap I would replace with a film or good new lytic and C11 I usually like to be around 100 pF (i have found that difference of this capacitance can be audible in some amplifiers).
Rest of the lytics in that sectin I would just replace with new ones.
15 V regulators - the lytics I would replace for good new ones, the output somoothing caps can be left at their designed size if additional capacity will be installed at the consuming IC-s 8described earlier).
Power amplifier - the inüut cap I would replace with a film or good new lytic and C11 I usually like to be around 100 pF (i have found that difference of this capacitance can be audible in some amplifiers).
Rest of the lytics in that sectin I would just replace with new ones.
Finally some scope measurements.
Power rails at powerup seem pretty nice:
At poweroff the positive and negative voltages drop slightly differently and final settling takes half a dozen seconds so I would count to ten before switching it on again after switching it off. Most probably nothing will happen when switched on faster but the service manual shows a record of chnages caused by amp instabilty so why push your luck...
Power rails at powerup seem pretty nice:
At poweroff the positive and negative voltages drop slightly differently and final settling takes half a dozen seconds so I would count to ten before switching it on again after switching it off. Most probably nothing will happen when switched on faster but the service manual shows a record of chnages caused by amp instabilty so why push your luck...