With 0.5A load current so 22W load about 10W is lost to heat. I suspect current so also power to heat losses in reality are much higher. With classic dinosaur regulators stuff will be uncomfortably hot. Someone will suggest using a fan 😉
Got the impression the Krell has separate supply voltages per channel. That would require 2 of the suggested boards. I suppose the casing is not large enough for those with the transformer added but I may be wrong.
It just screams for an intelligent lower losses design.
Got the impression the Krell has separate supply voltages per channel. That would require 2 of the suggested boards. I suppose the casing is not large enough for those with the transformer added but I may be wrong.
It just screams for an intelligent lower losses design.
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Well, if we're going to throw random solutions at it, then I vote for THIS
FlexReg Single and Dual Rail Regulated Power Supply using 78xx/79xx OR 317/337
I am getting excellent results for a watt-sucking Class A pre.
FlexReg Single and Dual Rail Regulated Power Supply using 78xx/79xx OR 317/337
I am getting excellent results for a watt-sucking Class A pre.
Attachments
The class A pre may be Watt-sucking but should the PSU do the same?
BTW it is not random but should be well thought out. Better than before seems a good starting point.
BTW it is not random but should be well thought out. Better than before seems a good starting point.
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The laziest and an insult to intelligence is to copy the original design with its imperfections and today's energy prices/safety demands. There is no law against making stuff better. Maybe unusual but I do this regularly. Manufacturers do make mistakes and hobbyists can make better designs certainly after decades of developments. No need for any "name" or anyone and be the helpless poor consumer, just take matters in own hands and do it yourself. Start with voltage and current demands and design a symmetric uLDO regulator PCB with separate regulators for the preamp fitting on the mounting holes of the old board. Up to the OP.
An analog devices quality is determined by the quality of its PSU. There are also no devices that perform less good with a better PSU.
An analog devices quality is determined by the quality of its PSU. There are also no devices that perform less good with a better PSU.
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I guess the solution is an 80 pound power supply or switch mode.
There can be no other options to this diabolical nightmare....
There can be no other options to this diabolical nightmare....
Diabolical nightmare? My hands are itching, this is done in an evening. I get flak regularly for having cheap Chinese non DIY stuff as I think it is silly to DIY for 10x the cost but linear PSUs are THE devices to DIY. Very rewarding too.
80 pound? Why? an 2 x 18V 80 VA transformer, rectifier bridges with Schotky diodes, CLC and 2 positive and 2 negative regulators. First lesson electronics but then with uLDOs like 2 x TPS7A4700 and 2 x TPS7A3301.
Challenge... make it better... creativity... you know the stuff everyone does in DIY audio. Not just doing something but by careful calculation (the main parameters are known if you read carefully) and then making something nice.
80 pound? Why? an 2 x 18V 80 VA transformer, rectifier bridges with Schotky diodes, CLC and 2 positive and 2 negative regulators. First lesson electronics but then with uLDOs like 2 x TPS7A4700 and 2 x TPS7A3301.
Challenge... make it better... creativity... you know the stuff everyone does in DIY audio. Not just doing something but by careful calculation (the main parameters are known if you read carefully) and then making something nice.
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First of all (if this hasn´t been done already), make sure the pre is working properly and the see, how
much power/current, it actually draws. I doubt, the VRDN is up to the task, but that´s just a guess.
After esatblished current draw, then make a solution, that doensn´t overheat, when suppying the neccesay current.
All the guessing is just a waste of time. (IMHO)
If it was me, I would just forget about "regulated" supply. 10A bridge rectifiers, ample transformer with 4 x 15volts secondaries,
and a decent bank of electrolytics. (gives in the ballpark of 21,5volt)
much power/current, it actually draws. I doubt, the VRDN is up to the task, but that´s just a guess.
After esatblished current draw, then make a solution, that doensn´t overheat, when suppying the neccesay current.
All the guessing is just a waste of time. (IMHO)
If it was me, I would just forget about "regulated" supply. 10A bridge rectifiers, ample transformer with 4 x 15volts secondaries,
and a decent bank of electrolytics. (gives in the ballpark of 21,5volt)
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Make the PSU less good than the original? A preamp susceptible to mains voltage fluctuations and mains noise? That is guessing and a waste of time allright.
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Folks, thank you for the lively conversation and diverse opinions, they are much appreciated!
I have a bench PS on the way. It will power the preamp for testing purposes, no need to wait for the Krell PS to be fixed.
I think a little perspective might be helpful, at least to some. Asking money for KSP-7B is typically $1700 - $2000, so selling is probably $1250 - $1500. It is not hard to see that "tossing it" is not going to happen, and I assume those who suggested doing so were speaking metaphorically 🙂
Another thing I'm not likely to do is go to a switching power supply. Noise issues but mostly... inappropriateness. Would I rather have an inefficient, power-hungry, heat-spewing, objectionably designed and poorly ventilated linear supply? Yes, for this particular breed of preamp I would.
More thoughts
I take resale value into account because this preamp may not be in my system forever, so I would rather not do anything that might hinder a future sale. I'm also a big fan of originality... generally speaking.
Excellent suggestions have been made to improve upon Krell's design and correct their most glaring misses. All points are well taken, so thank you! And @jean-paul your frankness is as always much appreciated 🙂
I also agree that passive pres make a lot of sense. Right now I'm running a DAC straight into my amps and it sounds great, but it's temporary because I need analog inputs. The Krell has plenty, and a phono stage too that's both highly configurable and very well regarded.
Sorry this ran a little long 🙂
I have a bench PS on the way. It will power the preamp for testing purposes, no need to wait for the Krell PS to be fixed.
I think a little perspective might be helpful, at least to some. Asking money for KSP-7B is typically $1700 - $2000, so selling is probably $1250 - $1500. It is not hard to see that "tossing it" is not going to happen, and I assume those who suggested doing so were speaking metaphorically 🙂
Another thing I'm not likely to do is go to a switching power supply. Noise issues but mostly... inappropriateness. Would I rather have an inefficient, power-hungry, heat-spewing, objectionably designed and poorly ventilated linear supply? Yes, for this particular breed of preamp I would.
More thoughts
- On durability: I reckon my preamp was built around 1991. So it lasted 34 years. If rebuilt exactly to factory spec, warts and all, it would likely be good for another 34 - maybe more, since modern PCB materials are better than the stuff Krell used way back when. Granted, whoever owns my preamp in the future might be looking at another rebuild right around 2060, but honestly I'm not going to worry about that.
- Mods and resale value: Many audiophiles love mods, but only when they come from the original designer, cost $3000, and come with an official factory upgrade sticker and a paper certificate and a little logo affixed to the case. But DIY mods? Forget it, they won't have that. That's the reality, I'm afraid.
I take resale value into account because this preamp may not be in my system forever, so I would rather not do anything that might hinder a future sale. I'm also a big fan of originality... generally speaking.
Excellent suggestions have been made to improve upon Krell's design and correct their most glaring misses. All points are well taken, so thank you! And @jean-paul your frankness is as always much appreciated 🙂
I also agree that passive pres make a lot of sense. Right now I'm running a DAC straight into my amps and it sounds great, but it's temporary because I need analog inputs. The Krell has plenty, and a phono stage too that's both highly configurable and very well regarded.
Sorry this ran a little long 🙂
"Would I rather have an inefficient, power-hungry, heat-spewing, objectionably designed and poorly ventilated linear supply? Yes, for this particular breed of preamp I would."
Why exactly? The device is not original with a copied or self designed PCB anyway. I for sure would feel like a donkey copying old design errors in. Like forgetting ventilations slots. If resale value is key parameter then an original new PSU board is the only way to go, nothing DIY.
Repair effort/time/costs may be pretty close to that resale value 😉 Tell me, I practically only do one offs.
Why exactly? The device is not original with a copied or self designed PCB anyway. I for sure would feel like a donkey copying old design errors in. Like forgetting ventilations slots. If resale value is key parameter then an original new PSU board is the only way to go, nothing DIY.
Repair effort/time/costs may be pretty close to that resale value 😉 Tell me, I practically only do one offs.
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That's a grey area.
The odds of finding a NOS PCB are right around zero. The Krell factory has been shuttered since last summer, and whether it will ever reopen is in debate.
If you 3D-print an unobtainium part for your '60s Jaguar, and it looks and feels and operates like the original part, is that permissible? I say yes..
As far as my time, I am not charging myself for it 🙂 I'm chalking it up to learning, and that has value too
Your time is worth $40 per hour or so, minimum.
Collectively we have spent more than resale value of the device
in just analysing your problem.
I would sell the thing, build a better pre amp for my use, go on in my life.
I use mostly digital files, have not touched my cassettes in 10 years, and my meagre collection of LPs (less than 5) was converted to cassette long back...and later I found much cleaner digital versions of the same material on line.
So phono stage ability is really useless today, you are using a DAC to play files.
And most recordings from the LP days are today available as digital files, so really the turntable, scratchy records etc., are not something I need, the dynamic range and clarity are much superior on digital media.
It is your choice, whether you want to listen to music, or entertain yourself with a project like this.
Collectively we have spent more than resale value of the device
in just analysing your problem.
I would sell the thing, build a better pre amp for my use, go on in my life.
I use mostly digital files, have not touched my cassettes in 10 years, and my meagre collection of LPs (less than 5) was converted to cassette long back...and later I found much cleaner digital versions of the same material on line.
So phono stage ability is really useless today, you are using a DAC to play files.
And most recordings from the LP days are today available as digital files, so really the turntable, scratchy records etc., are not something I need, the dynamic range and clarity are much superior on digital media.
It is your choice, whether you want to listen to music, or entertain yourself with a project like this.
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