console power supply with +/-15v linear for audio and +/-24v SMPS for logic?

[zzzzz]

I have been tasked with rebuilding a PSU for an early 1970s Auditronics Grandson 110 for a friend. Service manual power requirements state "bipolar +/-15 VDC 2A regulated for Amplifier Circuits" and "bipolar +/-24 VDC 4A unregulated for LED's, Logic, Lamps, and Relays" The 15v supply also has a "sense" circuit. Everything goes to the console via a barrier strip on the back.

Anyone who knows about Auditronics knows the stock power supplies are weak and crap out. Tons of documentation of owners replacing them with beefier Acopians, Power One / International Power linear modules, etc. It is a very old design using an opamp regulating scheme with LM709s (predecessor to LM741...) and 2N3055 (or later, 2N3771) for current boosting. Attached schematics for the original Grandson 110 PSU and the later, slightly improved & updated "Son of Grand" 501 PSU.

Instead throwing money at recapping or rebuilding the stock supply (again), I got a really great deal on a brand new huge Power One HDD-15-5-A linear PSU module (+/-15vdc at 5A and has the built-in sense circuit). My initial plan was to keep using the old PSU rack for the +/-24v and disregard the 15v supply, but my friend would like to house the whole PSU in one enclosure for convenience sake.

Since there is no audio on the +/-24v Logic rails, I figure SMPS would be adequate for that purpose -- I plan to "daisy chain" two Meanwell LRS-100-24 modules (rated 4.5A each) with a CLC scheme I have successfully used with other, albeit smaller, projects. Based on Auditronics ancient and prob inaccurate data of 4A draw for the 24v rails, LRS-100-24 seems to be a good fit.

I've attached a schematic of how I *think* everything should get hooked up in one box. Any thoughts before I move forward? Is the R.GND / C.GND scheme to tie the 24v and 15v rails to the chassis "star" ground necessary? Or is there a better way to connect all of the common/grounds/0v etc. etc.


Should note that the console does not have a direct connection to the PSU chassis ground. See attached “interconnects” doc for a diagram.

Also regarding external fuses on the VAC lines, Meanwell has a note about external fuses on their site: https://meanwellpowersupplies.com/f...-suggestion-for-using-an-external-input-fuse/ came up with the 2A value based on the AC current being rated 1.9A but I’m assuming should prob go a little higher value…

SOME NOTES:

On the original PSU scheme, the respective “common” for both the 24V and 15V rails were tied together on the PSU’s barrier strip and also meet at the power xformer center tap.

On the functional physical PSU’s barrier strip, all respective “common” ref pins have jumper wire linking them all together to Chassis ground. Both the 24V and 15V rails (pin 2 and pin 8) have been jumped to Chassis Ground (pin 17 and/or 18 on the barrier strip), but someone ALSO jumped the “sense” common (pin 5) to these other ground connections, but shouldn't the "sense" common NOT connect to anything other than the "sense" circuit?

Also: The board never had the phantom power option operational, so the owner does not care to add phantom power at this point. If he ever wants to add phantom, I'd probably just get one of the little Meanwell RS-15-48 modules as other people have done here on the forum.

 

[zzzzz]

Update on the Meanwell LRS-100-24

Despite what the datasheet says, these pull 2.1A AC current. Seems to be a 4A slo blo fuse on board. I do realize external fusing is redundant, but is there any harm to put another 4A external fuse in series with the live VAC line? or is that totally useless?

Also, already seems to be a CLC network on the output of the module. 470u/35v to 0v / V-, can’t read the inductor, and 470u/35v to 0v / V-.

Just quick visual inspection — take with a grain of salt.

 

[zzzzz]

UPDATE: new power supply built-out and working as it should with the console as of three days ago.

"Final" schematic attached.

Dramatically lowered noise floor and hum across the whole board (recorded measurements -- have to actually evaluate) and a huge improvement in low-end tightness / response vs the stock PSU. The 24v SMPS modules seem to be doing exactly what they should without being underloaded or overloaded.

One of the engineers at the studio told me with the old, potentially dying PSU, he noticed the console would sound a bit more "mushy" at the end of a session versus the start. He said that is no longer an issue with this new PSU.

 

[JohnRoberts]

Congrats...

I would ground the PS chassis using a 3 wire line cord. I hope that doesn't cause new hum problems.

JR

 

[zzzzz]

Congrats...

I would ground the PS chassis using a 3 wire line cord. I hope that doesn't cause new hum problems.

JR

Something different than the earth pin of the IEC inlet to a single chassis bolt?

Currently have that, and then a large copper bus wire going to the chassis bolt.

The 24v and 15v commons are then tied to the chassis ground bolt through a “loop breaker” network which is the 10R resistor in parallel with 100nF X2 safety cap.

Did not notice a huge difference in 60Hz hum when tying the 24v and 15v commons straight to the chassis ground bolt versus the RC, but left it as it was because it was working and the studio typically has some grounding issues.

 

[Mihobro]

I have been tasked with rebuilding a PSU for an early 1970s Auditronics Grandson 110 for a friend. Service manual power requirements state "bipolar +/-15 VDC 2A regulated for Amplifier Circuits" and "bipolar +/-24 VDC 4A unregulated for LED's, Logic, Lamps, and Relays" The 15v supply also has a "sense" circuit. Everything goes to the console via a barrier strip on the back.

Anyone who knows about Auditronics knows the stock power supplies are weak and crap out. Tons of documentation of owners replacing them with beefier Acopians, Power One / International Power linear modules, etc. It is a very old design using an opamp regulating scheme with LM709s (predecessor to LM741...) and 2N3055 (or later, 2N3771) for current boosting. Attached schematics for the original Grandson 110 PSU and the later, slightly improved & updated "Son of Grand" 501 PSU.

Instead throwing money at recapping or rebuilding the stock supply (again), I got a really great deal on a brand new huge Power One HDD-15-5-A linear PSU module (+/-15vdc at 5A and has the built-in sense circuit). My initial plan was to keep using the old PSU rack for the +/-24v and disregard the 15v supply, but my friend would like to house the whole PSU in one enclosure for convenience sake.

Since there is no audio on the +/-24v Logic rails, I figure SMPS would be adequate for that purpose -- I plan to "daisy chain" two Meanwell LRS-100-24 modules (rated 4.5A each) with a CLC scheme I have successfully used with other, albeit smaller, projects. Based on Auditronics ancient and prob inaccurate data of 4A draw for the 24v rails, LRS-100-24 seems to be a good fit.

I've attached a schematic of how I *think* everything should get hooked up in one box. Any thoughts before I move forward? Is the R.GND / C.GND scheme to tie the 24v and 15v rails to the chassis "star" ground necessary? Or is there a better way to connect all of the common/grounds/0v etc. etc.


Should note that the console does not have a direct connection to the PSU chassis ground. See attached “interconnects” doc for a diagram.
View attachment 136390
Also regarding external fuses on the VAC lines, Meanwell has a note about external fuses on their site: https://meanwellpowersupplies.com/f...-suggestion-for-using-an-external-input-fuse/ came up with the 2A value based on the AC current being rated 1.9A but I’m assuming should prob go a little higher value…

SOME NOTES:

On the original PSU scheme, the respective “common” for both the 24V and 15V rails were tied together on the PSU’s barrier strip and also meet at the power xformer center tap.

On the functional physical PSU’s barrier strip, all respective “common” ref pins have jumper wire linking them all together to Chassis ground. Both the 24V and 15V rails (pin 2 and pin 8) have been jumped to Chassis Ground (pin 17 and/or 18 on the barrier strip), but someone ALSO jumped the “sense” common (pin 5) to these other ground connections, but shouldn't the "sense" common NOT connect to anything other than the "sense" circuit?

Also: The board never had the phantom power option operational, so the owner does not care to add phantom power at this point. If he ever wants to add phantom, I'd probably just get one of the little Meanwell RS-15-48 modules as other people have done here on the forum.

For the +/- 24DC supply, see if you can get a single two output switching supply as it will be a safer than having two single ones. The second is that if you do use two separate supplies, I don't think it is a good idea to run the AC supply inputs in series. These supplies are designed to run from 64 to 264 VAC or so so it would be better if they were run in parallel, at 120VAC( or 220VAC). The inputs are isolated so you should not have any problems there. But you could also have issues isolating chassis, if you tie the negative OP to the positive OP of the other supply. So just check for continuity between the chassis ground and the DC output terminals. Finally in general it is good to exceed the current ratings of a conventional supply when using a switching supply by 1.25 to 1.5 times as switching supplies can get cranky when their current capacities are exceeded. Conventional supply designs tend to be more forgiving when their limits are reached.

 

[zzzzz]

For the +/- 24DC supply, see if you can get a single two output switching supply as it will be a safer than having two single ones. The second is that if you do use two separate supplies, I don't think it is a good idea to run the AC supply inputs in series. These supplies are designed to run from 64 to 264 VAC or so so it would be better if they were run in parallel, at 120VAC( or 220VAC). The inputs are isolated so you should not have any problems there. But you could also have issues isolating chassis, if you tie the negative OP to the positive OP of the other supply. So just check for continuity between the chassis ground and the DC output terminals. Finally in general it is good to exceed the current ratings of a conventional supply when using a switching supply by 1.25 to 1.5 times as switching supplies can get cranky when their current capacities are exceeded. Conventional supply designs tend to be more forgiving when their limits are reached.

Yup! Took all of those things into account — sorry if my earlier posts read a little confusing.

I actually did not wire the VAC inputs to the two meanwell modules in series, just looks to be that way in the schematic.

PSU still going strong with no issues at all, so I will definitely be making one of these for myself and likewise will consider this path for future projects.

I’m interested in running the 15v audio supply as a switch mode as well, but I have another brand new Power One HDD-15-5-A so that’ll be my +/-15v .