Neptune mixer model 611 - Works for a few minutes then fades out and distorts

[Zonzi]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

I think I see what you mean, that's in cases like when you have a manually switchable button for voltage adaptation, isn't it ? We have an old Akai AP001C at home that has something like this.

In my case it seems like it's not the same thing, because there's no switch that I can see. My best guess is that the previous owner replaced the power cord and the transformer with the closest he was able to find, which delivers a bit too much. But I might be wrong of course, and I' m inclined to believe what you say.

So, what you try to warn me about is that, if I go the regulator way but overvolt them with twice the voltage, it's gonna go down the nasty way, right ? But if my voltage is indeed stable, albeit too high, then a regulator is a reasonable choice. Am I correct in these assumptions ?

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

For going to a different line voltage, getting the right transformer is the correct solution. I'm still not quite sure what was done for the conversion - was a transformer swapped? So it's now also the wrong voltage in a 120v area?
A regulator is specifically meant to be used when the voltage is not stable at all, also depending on loading. An analog circuit like what you have here does not vary its load much and thus does not require a regulator.
Really what you're doing is bandaging a voltage mismatch issue and only that. It's not the right solution for a new device or design, but fixes your specific problem.

[petehall347]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

can a bucking transformer be added before the main transformer ?

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

diode dropping is another solution... lots of ugly solutions.

[Zonzi]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

I see, thanks. Yeah I guess now it would be unoperable with 120V because it gives me a reasonable 15-20V voltage with 220V primary. I could be wrong, but since the power cord was replaced to EU plig, the transformer would have been replaced as well, otherwise it'd all have burnt up isn't it ?

The voltage delivered by the power stage is indeed very stable (perhaps noisy or ripple-y but I don't have a scope) so I'm still convinced a better-suited transformer would be best.
That said, the primary issue was apparently fixed by putting back the correct value resistors around the 1458 IC. I too would prefer to have clean and well-suited voltage for the board, but if finding the right transformer is difficult, I'm already happy it works now.

There's also the issue with the first 2 channels which I described earlier but is it even relevant to this voltage problem ?

Regarding the last question, what's a bucking transformer ? Is it like a buck regulator on a motherboard ? There's not much space on the power side of the board but who knows, if it's something tiny, it could fit ?

[petehall347]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

I see, thanks. Yeah I guess now it would be unoperable with 120V because it gives me a reasonable 15-20V voltage with 220V primary. I could be wrong, but since the power cord was replaced to EU plig, the transformer would have been replaced as well, otherwise it'd all have burnt up isn't it ?

The voltage delivered by the power stage is indeed very stable (perhaps noisy or ripple-y but I don't have a scope) so I'm still convinced a better-suited transformer would be best.
That said, the primary issue was apparently fixed by putting back the correct value resistors around the 1458 IC. I too would prefer to have clean and well-suited voltage for the board, but if finding the right transformer is difficult, I'm already happy it works now.

There's also the issue with the first 2 channels which I described earlier but is it even relevant to this voltage problem ?

Regarding the last question, what's a bucking transformer ? Is it like a buck regulator on a motherboard ? There's not much space on the power side of the board but who knows, if it's something tiny, it could fit ?

hope this helps ..https://sound-au.com/articles/buck-xfmr.htm

[Zonzi]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

Really interesting thanks ! So what you suggest is that maybe the right transformer was used, but at the time maybe the mains voltage was 220 whereas it's now 230-240V, which would in turn increase the secondary voltage.

I will try to get a reading of my mains voltage in order to have a better idea of what I'm having to live with.

Now I hope the article doesn't imply you have to build your own bucking transformer, because I don't really have any tools in that regard

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

bucking transformer is a clever use of an off the shelf transformer to reduce AC voltage.
It's expensive however, well, anything that involves buying a line frequency transformer these days I'd call expensive.

I'm currently fearing that "EU conversion" the "technician" did was simply replace the plug... It may well still work at 120V as long as the voltage swings of the audio signal do not clip.

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

Another possibility is if you have a center tapped transformer already in the device which I can't tell by looking at it - is rewiring the full bridge rectifier to a half bridge full wave rectifier. Then you'll get about half the voltage output, and be back where you started from when the input voltage got doubled?

[stj]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

mains voltage is very variable, regulators are best.
in the states you could have 110v or you could have 127v depending on cable run to your building and total load.
in europe the swing is even worse - 210-250v

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

Again, not an issue with an analog device that has solely high PSRR op amps and does not need to have a reference. When thinking about voltage fluctuations, it's digital circuits that really suffer.

[Zonzi]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

Another possibility is if you have a center tapped transformer already in the device which I can't tell by looking at it - is rewiring the full bridge rectifier to a half bridge full wave rectifier. Then you'll get about half the voltage output, and be back where you started from when the input voltage got doubled?

I don't know if it's center tapped, but I would assume it is, since it has a GND wire.

This idea sounds really intriguing, does it only involve rewiring the diodes ? I was assuming that my mixer already had a full wave rectifier, given the four diodes and the two smoothing caps ?

Edit : by the way, I tested my mains voltage, and I'm at almost spot-on 230V RMS. So at least one thing appears to be normal in my setup

[stj]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

thats actually interesting,
most of europe didnt change, they stayed at 220v or 240v
they got away with it because it's allowed to fluctuate by upto 10% and 220v + 10% is 244v

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

oh if you're already using the center tap, then nope, cant do this idea then. This was if you were not using the center tap.

[redwire]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

Mains voltage fluctuations will cause slow DC shifts that cause woofers to move around. A problem if you have large PA/club amplifiers on the mixer's output.
I saw it, someone turns on a large load like stage lighting and the woofers are moving. That mixer the guy used a 12.6-0-12.6V transformer with +/-15V regulators IC's but it was no good at the venue which had a bit low mains voltage. So the 15V regulators did not have enough coming in (<17VDC) and dropped out whenever the fridge at the restaurant started up.
Fix was a new power transformer 14-0-14V.

The op-amp IC's bias current fluctuates with VCC fluctuations and electrolytic coupling capacitors charging/discharging causes it to get amplified. The mic preamp is likely the worst for this.
This is why a regulated power supply is used with audio gear, as well as the reduced hum.

I would just add a regulator board, but if you only have raw +/-16V then I would downgrade to 12V regulators, or change the power transformer as well.
The regulator IC's need extra 2V coming it.

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

That was not necessarily a problem with regulation, that was a regulator inducing problem into the design! Remember an ideal power supply can supply infinite current. A regulator with dropout will change its internal resistance a lot when running below dropout voltage, causing severe droop problems that go beyond the 10%, say 50-60% droop - if the droop went so far that the audio signal could no longer be faithfully reproduced, well, there's your distortion.

Also if your input voltage can't be held because of overloading your power cables and infrastructure, is it the amplifier's fault?

As said amplifiers have traditionally been designed with good PSRR. Look at all those 1970-1980s and earlier radios and power amps that which none have regulators in them and yet they still sound fine. It's all this digital crap and poor design these days that require regulators.

[redwire]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

Yes, for +/-15VDC regulators, they have around a 2V drop. If you undervolt their input I find they do not collapse as much as the PSRR goes to zero and they begin to pass-through ripple and hum, which certainly wakes up the crowd and band lol. Once the internal ref starts to have ripple appearing in it, it likely amplifies that too.

Musicians play in all sorts of different locations and venues, so mains voltage and power quality can be poor. Stage lighting is a big dynamic load, many kW switched on/off. The best time for gear to crap out when the lights hit lol.
If you design equipment for say -10% low line, it can end up overheating at nominal, and then damaging IC's if high line say +10% with no regulators.
Small power transformers put out up to 15-30% more voltage (than rated) at light loads. I have some new power transformers spec'd for 115V/230V primary and others 120V/240V so another 4% right there. What you'll end up is not easy at all, I find I have to actually build the PSU circuits and manually test to see what I'll get. I think OP's got +/-16V raw with 230V input.

Mains voltages have gone up since the 70-80's. All my old gear sees more voltage than the day, around 10%.
Old school audio mixer designs run the op-amps near their max. for greatest headroom and S/N.

[eccerr0r]

Re: Neptune mixer model 611 - Works for a few minutes then fades out and distorts

small power transformers have high impedance outputs so it's expected they droop more at load. If you overspec the transformer you won't see the droop. Also if you're depending on power using Vpeak versus Vrms of transformers, you are asking for problems.

I also would hope that circuit designers would not be putting in zero tolerance for voltages. 10-20% is mandatory for high reliability circuitry. If you're designing for 0% that's a recipe for low yield and a lot of RMAs.

Also people should be testing their systems with power. Note that if the voltage power droops, the IR drop is happening somewhere, and you may have more problems than just audio problems... namely, fire problems...