From observation they only add noise and degrade regulation.
Prudent circuit design should involve enough PSRR (power supply rejection) to make this inconsequential.
JR
Transistors on regulator ground pin
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Prudent circuit design should involve enough PSRR (power supply rejection) to make this inconsequential.
JR
moamps
Well-known member
So, how would you solve the bias of 7815 to get 18V output voltage?
I wouldn't do that.... IMO it's basically an attempt to look good on paper... another dB or two of signal swing does not make a significant difference, except to the potential customers reading data sheet specifications.
JR
PS; To answer your hypothetical perhaps with a zener diode in series with the ground leg, with a capacitor across the zener. Now that LEDs are cheap and efficient you could get a quick 2V or so by putting a LED in series, again with the cap to filter out noise and stiffen it up at low current. Of course if I was actually going to do this (I wouldn't) I would have to put it on my test bench and measure for unintended consequences.
i wish somebody would make a 6.3 volt reg that could stand startup heater currents,
Newmarket
Well-known member
I'll suggest replacing them with LM317 / 337 regulators used as variable Voltage Regs . Just dial up the voltage you need with better noise characteristics.
with maybe a pass transistor on the tail end for more current,
Newmarket
Well-known member
I own one of these, the 24-channel version, and the PSU gets HOT indeed. I've heard people refer to the PSU as a weak link in these. Even though it's nothing fancy pantsy, I've really come to love mine, especially after a lot of extensive modifications. But I'm not too fond of electronics running hot. Might get around to undo this "cheap trick" in the supply...
Dropping down the regulator voltage a couple volts will reduce heat in the channel electronics but may put more heat into the regulator. The heat in the regulator is related to the difference between the unregulated input voltage and the regulated output voltage times the current draw. Dropping down the regulated output will actually increase the voltage across the regulators and power dissipation.
JR
That's true, thanks for pointing that out. So I guess a better solution would be to undo the cheat and then get a transformer that puts out a tad less voltage? The +48v supply (which is just stepped up DC from the same rectifier responsible for the +/- rails) will get robbed of some juice, but that shouldn't be all that difficult to fix.
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I am reluctant to suggest how to redesign a product's power supply, like this casually.
Is the whole mixer hot, or just the regulators? Do they have sufficient heatsinks?
JR
The PSU is generating a lot of heat, channel cassettes just a bit more than most other opamps I've touched (tenderly, in the shadows of a moonlit meadow), but that might not be a problem. I mean, it's worked this far, and hasn't gotten hotter.
The regulators have big heatsinks mounted to the chassis. Regulators get hotter than most other regulators I've touched (in a strictly platonic way). They don't burn you immediately, but holding your finger there for a few seconds isn't comfortable.
On the other hand, once again: it's worked well all these years, so maybe modifying this part of the mixer is just unnecessary work. I don't know, sometimes it just bugs me to know it gets so hot. I really like keeping things cool.
Heat is bad (MTBF "mean time between failure" doubles for every 10 degree C temperature reduction), but if it's been working for years it might not need to be fixed.
From my experience with 3 terminal regulators they are pretty robust. They will even shut themselves down in thermal protection mode before self-destructing. I have seen 3 terminal regulators in products that ran so hot for so long that they crystallized the solder connections to their PCB pads. Touching up those cold or dry solder joints brought them back to life. Ironic calling them cold solder joints when fault was caused by excess heat.
JR
From my experience with 3 terminal regulators they are pretty robust. They will even shut themselves down in thermal protection mode before self-destructing. I have seen 3 terminal regulators in products that ran so hot for so long that they crystallized the solder connections to their PCB pads. Touching up those cold or dry solder joints brought them back to life. Ironic calling them cold solder joints when fault was caused by excess heat.
JR
Newmarket
Well-known member
Yes - those three term 7xxx regs are pretty tough - thermal protection etc.
Although I recently had a faulty 7915 on an old A&H PSU.
What is the thermal interface between the package and heatsink ?
Thermal Grease / Pad. Either way I'd look to replace it.
Existing grease may have lost effectiveness over time. If replacing note that Some thermal compounds are much better than standard.
But I think I'd go for a pad - from experience TGARD500 material is excellent performance and also robust in case heatsinks are not super smooth.
https://www.laird.com/sites/default/files/2018-11/THS-DS-TGARD500_03_20_17.pdf
Although I recently had a faulty 7915 on an old A&H PSU.
What is the thermal interface between the package and heatsink ?
Thermal Grease / Pad. Either way I'd look to replace it.
Existing grease may have lost effectiveness over time. If replacing note that Some thermal compounds are much better than standard.
But I think I'd go for a pad - from experience TGARD500 material is excellent performance and also robust in case heatsinks are not super smooth.
https://www.laird.com/sites/default/files/2018-11/THS-DS-TGARD500_03_20_17.pdf
vintageampfixer
Member
One approach you might try could be to add a single diode in series with the input to each regulator circuit - Silicon diodes have a forward voltage drop typically of around 0.7V and this might be enough to reduce the power dissipation without impacting the output voltage.
That lowers your drop out voltage a like amount.
Perhaps not obvious you could use a smaller reservoir cap, that would generate higher ripple voltage but lower average unregulated voltage and less dissipation. Of course too small of a reservoir cap would also impact drop out voltage.
JR
vintageampfixer
Member
yes indeed, however if the issue is too much voltage being dropped across the regulators (leading to excessive power dissipation and heat), then if feels unlikely the approach would be problematic, although I'm sure a weather eye on the voltage rails when the desk is running at full load would be sage.
I hadn't thought of that ...
... but isn't it also likely to induce ripple noise as well as reducing the MTBF for the smoothing capacitors because they're now seeing far higher ripple voltages and thus greater ripple current? You correctly note it'd also reduce the drop out voltage but by how much is probably harder to calculate whereas the diode is quite predictable from its forward volt drop. Great example of different circuit designs which achieve the same result though!
vintageampfixer
Member
I just saw the circuit! 78xx series regulators were rated at 1A so with 4,400uF total smoothing caps before the regulator, and using the rule of thumb which guides (guided?) linear PSU design to use "1000uF per Amp" of load, I think they may have struggled with noise and were doing their best to minimise it.
The broadcast specs at that time were quite stringent and while I can't immediately remember the target noise figure for "broadcast" desks, it was very, very low. The premise was that with many tens of equipment items in the signal path, if each added <n dB of noise, then the resultant S/N ratio would have still have been acceptable at the station output.
Bizarrely, I was at that time also working for one of the "other" broadcast manufacturers - in my case I was employed by Audix Ltd which was later acquired by Tannoy Goodmans International before ultimately being split up, sold off and now I think pretty much wound up. Stranger still ... I now live near Burgess Hill ... although I can't find any record Alice Broadcast and I don't recall ever having heard of that company in the past.
vintageampfixer
Member
Sorry I didn't see this earllier. It would be very interesting to know what the AC voltages are between:
J1-4 and J1-5
J1-4 and J1-6
With these AC voltages known, it's possible to calculate the DC voltages we should expect across C5 and C7. Please also measure these DC voltages:
Across C5
Across C7
Lastly the output voltage of the two regulators REG1 and REG2:
Across C11
Across C12
If you'd measure these and reply, we'll see what we can come up with that might help
