Maintaining a phono preamp over time

[mk2]

I have a Project "The Phono Box" preamp which I have been using for 20+ years.

I'm capable of soldering, and have a little understanding of electronics. What parts should I be looking to refresh?

It has been making a muffled crackle when switched on from cold, for many years. Because of this, I've just replaced the two 25V 470uF capacitor and I'll see what happens. I assumed (perhaps incorrectly!) these are smoothing of the DC power input.

But now it's drawn my attention to other things. The sound has always been 'soft' to my tastes, but I do seem to EQ to a very bright and detailed sound. Also I think a little distortion may have appeared over the years as I'm noticing the 'warm' fuzz around vocals more and more that I don't hear on CDs.

However, these are just subjective judgements, I could be noticing what I didn't before.

So what is the actual science here? ie. which of the parts of the preamp have the most profound impact on the sound, and which will wear or age?

My understanding is the RIAA curve is defined by a specific set of capacitors; if so, the accuracy of the equalisation from new should never really be in question. But if these capacitors could age, could this cause distortion, or softness? Are these the electrolytic capacitors I can see (what about the others?) and should I replace all of them?

And the op-amps. Do these suffer any kind of 'ageing' or is it just a matter of the quality/cost of them? The small chips I can see are MAL TL061CN 90A806 and 458D JRC 11776.

I'm not an electronics person, but this is a good opportunity to learn a little -- many thanks.

 

[MaxwellsEq]

Electronics is an engineering discipline, which makes it pretty much in the design -> measure -> refine -> manufacture-at-scale business (followed by diagnose/repair hopefully a long time later). Most electronic items are "designed to a cost", which means that compromises are always required if a device is to be profitable. In the boutique parts of the industry, it's possible to make money selling a few very expensive items a year - at the other end, there needs to be a reasonable volume and a reasonable profit.

So for you to answer your own question - you need to measure the device. These days. you can do this fairly cheaply with sound files, a DAC, an ADC and software. Or you could do it the old fashioned way with a signal generator, scopes and meters.

If you find yourself thinking: "but I don't know what to measure, and I don't know what good looks like", you will have started your electronics learning! At the very least, you will need to compare this particular device against one in perfect condition, or against the specs in the workshop manual for the device. With the latter, you will need to be able to trace circuit diagrams vs the real built PCB. You will then need to measure things such as the DC values (which usually set the operating parameters) and small signal behaviour (including noise and distortion) - you can then compare these against the manual's nominal values.

In general, for older devices - movable things such as trimmers and potentiometers can become noisy and unreliable. Capacitors can age, but if they are operated inside their temperature and voltage ranges, they may still be within tolerance (you can take them off the board and measure them). Resistors (if operated inside their temperature and power ranges) should last well. Op-amps can go wrong (but usually are OK if again specified correctly). Dry solder joints can become highly resistive over time, especially if there are a lot of physical or temperature/humidity challenges. Connectors can become oxidised.

 

[Ricardus]

Do you at the very least have a schematic for this one?

 

[LTig]

I have a Project "The Phono Box" preamp which I have been using for 20+ years.

I'm capable of soldering, and have a little understanding of electronics. What parts should I be looking to refresh?

It has been making a muffled crackle when switched on from cold, for many years. Because of this, I've just replaced the two 25V 470uF capacitor and I'll see what happens. I assumed (perhaps incorrectly!) these are smoothing of the DC power input.

But now it's drawn my attention to other things. The sound has always been 'soft' to my tastes, but I do seem to EQ to a very bright and detailed sound. Also I think a little distortion may have appeared over the years as I'm noticing the 'warm' fuzz around vocals more and more that I don't hear on CDs.

However, these are just subjective judgements, I could be noticing what I didn't before.

So what is the actual science here? ie. which of the parts of the preamp have the most profound impact on the sound, and which will wear or age?

My understanding is the RIAA curve is defined by a specific set of capacitors; if so, the accuracy of the equalisation from new should never really be in question. But if these capacitors could age, could this cause distortion, or softness? Are these the electrolytic capacitors I can see (what about the others?) and should I replace all of them?

And the op-amps. Do these suffer any kind of 'ageing' or is it just a matter of the quality/cost of them? The small chips I can see are MAL TL061CN 90A806 and 458D JRC 11776.

I'm not an electronics person, but this is a good opportunity to learn a little -- many thanks.

The caps in the RIAA deemphasis are too small for electrolytics so they should not age. The same is true for opamps and resistors.

However don't underestimate the influence of the input capacitance on frequeny response. Add its value to the cable capacitance and if those combined are higher than the capacitance load recommended by the maker of the pickup it makes sense to reduce the input capacitor accordingly.

 

[JeffS7444]

I'm capable of soldering, and have a little understanding of electronics. What parts should I be looking to refresh?

Records and styli wear out; phono preamps, not so much.

What's your turntable setup like?

 

[DVDdoug]

It has been making a muffled crackle when switched on from cold, for many years.

It's not unusual for an amp or preamp make a pop or click during turn-on. Higher power amps usually have some kind of "soft turn-on" so the speakers don't get damaged but it less common in preamps and sometimes the preamp needs to turn-on first.

Are you sure it's not just something you didn't notice before?

Because of this, I've just replaced the two 25V 470uF capacitor and I'll see what happens. I assumed (perhaps incorrectly!) these are smoothing of the DC power input.

Most likely those are power supply filters but they are mostly for filtering out hum or noise from a switching power supply. Electrolytic capacitors do tend to age and the capacitance drops. But as power supply filters you just need a minimum capacitance and something like 200uF might be enough, or if there is an external power supply it has capacitors and the internal caps may be not even be "necessary".

But now it's drawn my attention to other things. The sound has always been 'soft' to my tastes, but I do seem to EQ to a very bright and detailed sound. Also I think a little distortion may have appeared over the years as I'm noticing the 'warm' fuzz around vocals more and more that I don't hear on CDs.

However, these are just subjective judgements, I could be noticing what I didn't before.

A lot of older records were a little "dull" sounding and distortion on records is not unusual and the sound quality varied a lot. And they do "develop" noise and possibly the distortion gets worse over time, especially if there is mis-tracking. The cartridge also affects frequency response.

Your brain has probably adapted to better digital sound. I assume newer records have better and more consistent frequency response but I don't have any experience with modern records.

So what is the actual science here? ie. which of the parts of the preamp have the most profound impact on the sound, and which will wear or age?

Solid state electronics tend to "last forever" and then fail "randomly". As MaxwellsEQ says, moving pots can get noisy and electrolytic capacitors can degrade. Other types of capacitors tend to be stable. I doubt there are any internal trim-pots. Connectors can sometimes corrode and become intermittent.

​

My understanding is the RIAA curve is defined by a specific set of capacitors; if so, the accuracy of the equalisation from new should never really be in question. But if these capacitors could age, could this cause distortion, or softness? Are these the electrolytic capacitors I can see (what about the others?) and should I replace all of them?

The RIAA EQ circuit probably doesn't use electrolytic capacitors so they should be stable.

Electrolytic capacitors are used as power supply filters and sometimes as input/output* "DC filtering" capacitors. DC is zero-Hz, so these are high-pass filters and if they age you could loose bass, but usually they are designed to cut-off below the audio range so they can age without affecting the sound.

And the op-amps. Do these suffer any kind of 'ageing' or is it just a matter of the quality/cost of them? The small chips I can see are MAL TL061CN 90A806 and 458D JRC 11776.

If the op-amps fail, they are likely to fail badly rather than to slightly deteriorate, and the left & right channels are unlikely to fail or deteriorate equally.

I'm not an electronics person, but this is a good opportunity to learn a little -- many thanks.

Sometimes it's easier to build your own circuit than to understand and repair and unknown design, especially if you don't have a schematic. Of course, choose an op-amp design to keep it simple (and with predictable performance).

I found this one which is pretty simple, but it's not the simplest because it's switchable for moving magnet and moving coil cartridges.

With op-amps the critical things are that you want a low-noise op-amp and you should use precision (1% tolerance) in the feedback circuit, which determines gain and RIAA EQ compliance. You also need a low-noise power supply and/or additional power supply regulation. Linear voltage regulator chips just require one or two capacitors. Most op-amps use dual (positive and negative) power supplies, so that's two voltage regulators. A soft turn-on circuit is more complicated. (But dual power supplies tend to have quieter turn-on than a signal power supply.)



* Because of the high input impedance, the input caps can be lower value and are less likely to be electrolytics.

 

[mk2]

Just returned home to the wealth of information in these replies. Thanks very much for these explanations. I'll read them carefully and reply to some of the questions, hopefully tomorrow.

 

[Jim Hagerman]

It has been making a muffled crackle

That is the sound of a poor electrical connection. Perhaps a broken wire or solder joint, internal cable, or loose RCA jack? Look for something like that. It likely won't be an issue with opamps or capacitors or resistors, especially if the unit has been in service all this time.

 

[restorer-john]

This unit? This one looks to be their first, from perhaps the mid 90s.

9 electrolytics, 1% resistors and a couple of linear regulators. Check the three terminal regulators solder joints as a) they are not secured and b) they may run a little warm enough to potentially have a dry joint which could cause the issue, just long enough before the warmth expands the leg/s in the solder joint 'fixing' the issue.

 

[mk2]

Thanks for all your replies. Yes, the spirit of my question was more general, so the replies explaining the major components and common failure modes are very insightful and exactly what I was hoping to understand, thanks.

Yes, I could measure this specific device but it's unlikely I could obtain officially the specification or schematic, especially for this older revision. Even with that, it would also not be clear baseline, whether it originally met (or exceeded) that specification when I got it, and what is over time.

What's your turntable setup like?

It's a Technics SL-1200mk2, with Ortofon Pro S and a new-ish stylus. Everything balanced and set up properly (I hope)

Tracking force recommended is 4g, but that is with DJ use in mind. I wouldn't normally use this as I find ~1.8g works fine for listening and I presume causes less wear on the records. But if I'm testing or diagnosing anything, I use the recommended force to be sure.

Other specs:

Internal impedance, DC resistance - 750 Ohm
Internal inductance - 450 mH
Recommended load resistance - 47 kOhm
Recommended load capacitance - 200-600 pF

It's not unusual for an amp or preamp make a pop or click during turn-on. Higher power amps usually have some kind of "soft turn-on" so the speakers don't get damaged but it less common in preamps and sometimes the preamp needs to turn-on first.

Are you sure it's not just something you didn't notice before?

Quite possibly, yes. Though replacing the 24V 470uF seems to have fixed it in the main. There's still an expected click on power-on, but it's no longer followed by ~1 second of rustling sound.

That is the sound of a poor electrical connection. Perhaps a broken wire or solder joint, internal cable, or loose RCA jack?

I don't think so, however it is frustrating from a practical point of view that the sockets on this unit just aren't very tight and easily dislodge when things are moved around.

 

[SSS]

I have a Project "The Phono Box" preamp which I have been using for 20+ years.

I'm capable of soldering, and have a little understanding of electronics. What parts should I be looking to refresh?

It has been making a muffled crackle when switched on from cold, for many years. Because of this, I've just replaced the two 25V 470uF capacitor and I'll see what happens. I assumed (perhaps incorrectly!) these are smoothing of the DC power input.

But now it's drawn my attention to other things. The sound has always been 'soft' to my tastes, but I do seem to EQ to a very bright and detailed sound. Also I think a little distortion may have appeared over the years as I'm noticing the 'warm' fuzz around vocals more and more that I don't hear on CDs.

However, these are just subjective judgements, I could be noticing what I didn't before.

So what is the actual science here? ie. which of the parts of the preamp have the most profound impact on the sound, and which will wear or age?

My understanding is the RIAA curve is defined by a specific set of capacitors; if so, the accuracy of the equalisation from new should never really be in question. But if these capacitors could age, could this cause distortion, or softness? Are these the electrolytic capacitors I can see (what about the others?) and should I replace all of them?

And the op-amps. Do these suffer any kind of 'ageing' or is it just a matter of the quality/cost of them? The small chips I can see are MAL TL061CN 90A806 and 458D JRC 11776.

I'm not an electronics person, but this is a good opportunity to learn a little -- many thanks.

From my experience nothing really age. Even low voltage electrolytics stay good. Sometimes one may have a reduced capacity. All other parts are in general stable. If there is no difference between the two channels then I would do nothing. I like and use vintage solid state amps and these are around 30 years old without degradation. Something else are vacuum tube amps, there due to high voltages ageing happens.

 

[mk2]

This unit? This one looks to be their first, from perhaps the mid 90s.

9 electrolytics, 1% resistors and a couple of linear regulators. Check the three terminal regulators solder joints as a) they are not secured and b) they may run a little warm enough to potentially have a dry joint which could cause the issue, just long enough before the warmth expands the leg/s in the solder joint 'fixing' the issue.

It's interesting. This unit in your photograph has a large voltage regulator. If you check my original photo, mine has something looking more like a transistor -- I assume it's still a voltage regulator but smaller and lower rated? My unit would be dated around ~2000, so perhaps a newer design than a mid-90s one.

I may re-visit the box to check for dry joints (in general). Because I think there may be some intermittency to what I'm experiencing. Here's why:

I remembered I had another preamp in the house: a Pioneer DJM 500 mixer. It may not be typical HiFi but it is useful to compare. However it is also ~20 years old.

In a simple test (with Tracy Chapman's "Fast Car") I'd be unlikely to tell a difference between the Project and the Pioneer; certainly the tone and distortion levels were similar. But it was clear to me that in both cases the vocals were not as distorted as I had experienced with the Project unit the other day.

 

[raindance]

Tracking too light causes exactly the vocal symptom you are describing. That stylus is designed for 4g tracking weight, so use it as designed.

 

[mk2]

Sometimes it's easier to build your own circuit than to understand and repair and unknown design, especially if you don't have a schematic. Of course, choose an op-amp design to keep it simple (and with predictable performance).

I found this one which is pretty simple, but it's not the simplest because it's switchable for moving magnet and moving coil cartridges.

Now I love the idea of this. Mainly because I can think (perhaps ill-advisedly, in my ill-informed newbie) of a few ways it could be fun to do things differently.

Mainly that I'm surprised that the first thing a preamp does is to ground one side of the turntable input, which I assume is naturally balanced. And since I have a balanced audio downstream, it could be fun to actually build a balanced->balaned preamp which provides higher levels too.

Also, since I have DSP downstream, the possible benefits to doing the RIAA equalisation there, thus simplifying the (analogue) preamp. Or whether it's even possible to ADC the phono directly.

I'm more of a digital audio guy so to work on the analogue electronics would be really interesting. However I probably don't have time for more projects!

 

[mk2]

Tracking too light causes exactly the vocal symptom you are describing. That stylus is designed for 4g tracking weight, so use it as designed.

It's true, and I'm completely aware of this... hence the comparison. However in my simple tests the difference is not pronounced enough at these weights. However, yes you're right, I am not introducing this extra variable when trying to diagnose a difference or problem.

it would take some actual science to convince me that the Ortofon 5S (Hifi) and Pro S (DJ) are not manufactered identically -- and the model 'range' comes from recommending a different tracking weight for DJ vs. Hifi and measuring a slight difference in performance. This is just my speculation.

 

[computer-audiophile]

This unit? This one looks to be their first, from perhaps the mid 90s.
...

9 electrolytics, 1% resistors and a couple of linear regulators. Check the three terminal regulators solder joints as a) they are not secured and b) they may run a little warm enough to potentially have a dry joint which could cause the issue, just long enough before the warmth expands the leg/s in the solder joint 'fixing' the issue.

Good point! Sometimes re-soldering helps.

... Something else are vacuum tube amps, there due to high voltages ageing happens.

Electron tubes in amplifiers age primarily due to the wear, thermal stress and degradation of their internal components, such as the cathode, anode, and filament. This process is often accelerated by factors like heat and the flow of electrons, which can cause materials to break down over time. Nevertheless, tubes in small-signal amplifiers can last for many years if they are operated within their specifications.

 

[SSS]

Good point! Sometimes re-soldering helps.



Electron tubes in amplifiers age primarily due to the wear, thermal stress and degradation of their internal components, such as the cathode, anode, and filament. This process is often accelerated by factors like heat and the flow of electrons, which can cause materials to break down over time.

Of course this is the general ageing of electron vacuum tubes whereas in non-power application like input stages the tubes can last decades what my Fender guitar amp shows for instance. But I mean also ageing of stressed components due to voltage and elevated temperature. Especially the electrolytics wear out as well as the plate resistors where the resistors may cause a krackling noise.

 

[computer-audiophile]

Dies ist natürlich auf die allgemeine Alterung von Elektronenvakuumröhren zurückzuführen, während die Röhren bei Nicht-Stromanwendungen wie Eingangsstufen Jahrzehnte halten können, was zum Beispiel bei meinem Fener-Gitarrenverstärker der Fall ist. Damit meine ich aber auch die Alterung beanspruchter Bauteile durch Spannung und erhöhte Temperatur. Insbesondere verschleißen die Elektrolyte und die Plattenwiderstände, wobei die Widerstände ein knisterndes Geräusch verursachen können.

Yes, sorry. I had edited my post to say something the same. (Your quote/answer came too fast)

 

[SSS]

Nette Übersetzung, eigentlich OK. Aber Plattenwiderstände? Welches Übersetzerprogramm?

 

[computer-audiophile]

By the way:

Of course, it's nice to keep such old electronics in operation, if only for environmental reasons, or if you like to tinker. Otherwise, you can sometimes find phono preamplifiers for ridiculously small amounts that are closer to SOTA in terms of their construction. Not too long ago, for example, I bought such an example from AliExpress, which I think cost 25 euros and worked quite well, sounded good and had a decent cabinet finish.