I’m working on an amp platform similar to the concept of the NPXP by Anand (poseidonsvoice) https://www.diyaudio.com/community/threads/nelson-pass-experimenters-platform-npxp.373417/
Mine will also be a single chassis dual mono design, using 2 of the xrk SLB dual-rail power supply boards https://www.diyaudio.com/community/...ct-crc-cap-mx-class-a-power-supply-gb.336479/ , and also a single custom Toroidy power transformer with quad secondaries.
I too plan to use this for many of the PassDIY / FW clone amps, which of course are designed to use a +/-24VDC PS. But, I’d also like to use this with the Alpha Nirvana (standard 8ohm version) https://www.diyaudio.com/community/threads/alpha-nirvana-39w-8ohm-class-a-amp.344540/ which prefers a +/-27or 28VDC PS. I’d prefer not to run the AN at +/-24vDC, and I don’t want to use a Variac to boost the primary voltage either. Too easy to accidentally bump the voltage up without realizing, plus a semi-permanent Variac in the living room would never fly with the interior design committee [wife].
After some discussions with Toroidy, I've discovered that they can build a single transformer that can do multiple secondary voltages, as long as you only use one voltage at a time.
So my question for you all (especially those familiar with Toroidy transformers or XRK's SLB) is: what secondary voltages should I get? 20 and 24? 21 & 23? Something else?
For the low voltage, if I go by Anand's experience, I should order 20V secondaries to get +/-24VDC (though his was 800VA, and I'm considering one that's 600VA. Not sure what the price difference is between 600 and 800VA, but I could check if you all think it's worth spending more for the extra 200VA).
But if I go by this https://www.diyaudio.com/community/...x-class-a-power-supply-gb.336479/post-5759699 I need 22V secondaries
And similarly, to yield +/-27VDC for the AN, do I want 23V secondaries? 25? Something else?
My wall voltage is usually 120VAC. It sometimes will drop to 118 during the midsummer peak cooling season, but I'll likely not be using this amp during those hottest summer months anyway, so let's just assume I always have 120V coming into the primaries.
Mine will also be a single chassis dual mono design, using 2 of the xrk SLB dual-rail power supply boards https://www.diyaudio.com/community/...ct-crc-cap-mx-class-a-power-supply-gb.336479/ , and also a single custom Toroidy power transformer with quad secondaries.
I too plan to use this for many of the PassDIY / FW clone amps, which of course are designed to use a +/-24VDC PS. But, I’d also like to use this with the Alpha Nirvana (standard 8ohm version) https://www.diyaudio.com/community/threads/alpha-nirvana-39w-8ohm-class-a-amp.344540/ which prefers a +/-27or 28VDC PS. I’d prefer not to run the AN at +/-24vDC, and I don’t want to use a Variac to boost the primary voltage either. Too easy to accidentally bump the voltage up without realizing, plus a semi-permanent Variac in the living room would never fly with the interior design committee [wife].
After some discussions with Toroidy, I've discovered that they can build a single transformer that can do multiple secondary voltages, as long as you only use one voltage at a time.
So my question for you all (especially those familiar with Toroidy transformers or XRK's SLB) is: what secondary voltages should I get? 20 and 24? 21 & 23? Something else?
For the low voltage, if I go by Anand's experience, I should order 20V secondaries to get +/-24VDC (though his was 800VA, and I'm considering one that's 600VA. Not sure what the price difference is between 600 and 800VA, but I could check if you all think it's worth spending more for the extra 200VA).
But if I go by this https://www.diyaudio.com/community/...x-class-a-power-supply-gb.336479/post-5759699 I need 22V secondaries
And similarly, to yield +/-27VDC for the AN, do I want 23V secondaries? 25? Something else?
My wall voltage is usually 120VAC. It sometimes will drop to 118 during the midsummer peak cooling season, but I'll likely not be using this amp during those hottest summer months anyway, so let's just assume I always have 120V coming into the primaries.
Can you ask Toroidy if they can put following windings: 22, 4, 2, 1V. By connecting these in series you can go from 22 to 29V in 1V steps. For my valve projects I got some toroidy s with 80, 40, 20, 10 and 5V - I can increment in 5V steps, and this is fed into a voltage doubler.
OOOoo yeah, I like that idea. Maybe at 18, 4, 2,1, then I think I could even build the 4ohm version of the AN, which uses +/-21VDC, and get as high as I'd need to for the 8ohm AN. And of course cover all the Pass/FW amps in between.
PF4E,
With dual SLB’s on a 120V primary and 800VA Toroidy:
20V secondaries will give you approximately 23-24V DC loaded.
22V secondaries will give you approximately 27-28V DC loaded.
24V secondaries may too much, with dual SLB’s and you’ll hit about 29-30V DC. That isn’t a bad thing, if you’re heatsinks can handle it , but given that your Pass experiments are going to run between 1.3A to 1.8A bias, the dissipation may exceed the capabilities of your heatsink. My 5U/400 chassis will handle 30V DC secondaries and 1.7A bias just fine but will run warm (102 watts dissipation). I don’t think you can easily string secondaries together without compromising the max current capabilities of a transformer or of a particular secondary. You either use one set of secondaries, or another. Toroidy should provide this level of detail for you.
I used 800VA because 1) the physical size is the same as a 600VA, it runs cool (35 degC max with the top on) 2) I NEVER have to worry about if I am overloading the toroid since it literally acts like a dual mono 400VA in my experience especially when each channel is drawing the same amount of bias current and 3) One can physically center a single transformer much more easily than dual toroids unless they are non encapsulated (but… the Toroidy Supremes are).
The cost difference between 800VA and 600VA for me was immaterial but for others, it is material. Up to you. I am living on this planet once, so I went big. I would look at the cost differences of adding on extra secondaries and also ask yourself if you are truly going to be implementing multiple designs in that chassis. Of course, I’ve already proven that since the AN is the 5th amp module to be tried in the NPXP. You may listen to the Alpha Nirvana (AN), and say to yourself…I’m done.
BTW, the AN works just fine at +/-24V DC, just lower power (30 watts @ 8 ohms, clipping), so don’t overthink this. If you have speakers of low sensitivity, a large room, loud SPL requirements, etc…then the AN may not work for you. But if you are already contemplating small sized Pass amplifier experiments (i.e. FirstWatt), then you are talking about <1 dB difference between 30 watts vs 39 watts. I have 96dB sensitive speakers. No, I am not clipping the AN at 100dB peaks at a listening position of 12 feet away. And this is in a room 26 feet deep, 10 feet tall and 16 feet wide.
I would also do an impedance/phase curve measurement of your speakers from 20Hz to 20khz before building the right version of the AN. I would not go by the manufacturer’s ‘nominal impedance specification’ which is usually published by the marketing division. You want to know where the impedance curve range is in the area where most music lies. In other words, between 50Hz to about 5khz.
Best,
Anand.
With dual SLB’s on a 120V primary and 800VA Toroidy:
20V secondaries will give you approximately 23-24V DC loaded.
22V secondaries will give you approximately 27-28V DC loaded.
24V secondaries may too much, with dual SLB’s and you’ll hit about 29-30V DC. That isn’t a bad thing, if you’re heatsinks can handle it , but given that your Pass experiments are going to run between 1.3A to 1.8A bias, the dissipation may exceed the capabilities of your heatsink. My 5U/400 chassis will handle 30V DC secondaries and 1.7A bias just fine but will run warm (102 watts dissipation). I don’t think you can easily string secondaries together without compromising the max current capabilities of a transformer or of a particular secondary. You either use one set of secondaries, or another. Toroidy should provide this level of detail for you.
I used 800VA because 1) the physical size is the same as a 600VA, it runs cool (35 degC max with the top on) 2) I NEVER have to worry about if I am overloading the toroid since it literally acts like a dual mono 400VA in my experience especially when each channel is drawing the same amount of bias current and 3) One can physically center a single transformer much more easily than dual toroids unless they are non encapsulated (but… the Toroidy Supremes are).
The cost difference between 800VA and 600VA for me was immaterial but for others, it is material. Up to you. I am living on this planet once, so I went big. I would look at the cost differences of adding on extra secondaries and also ask yourself if you are truly going to be implementing multiple designs in that chassis. Of course, I’ve already proven that since the AN is the 5th amp module to be tried in the NPXP. You may listen to the Alpha Nirvana (AN), and say to yourself…I’m done.
BTW, the AN works just fine at +/-24V DC, just lower power (30 watts @ 8 ohms, clipping), so don’t overthink this. If you have speakers of low sensitivity, a large room, loud SPL requirements, etc…then the AN may not work for you. But if you are already contemplating small sized Pass amplifier experiments (i.e. FirstWatt), then you are talking about <1 dB difference between 30 watts vs 39 watts. I have 96dB sensitive speakers. No, I am not clipping the AN at 100dB peaks at a listening position of 12 feet away. And this is in a room 26 feet deep, 10 feet tall and 16 feet wide.
I would also do an impedance/phase curve measurement of your speakers from 20Hz to 20khz before building the right version of the AN. I would not go by the manufacturer’s ‘nominal impedance specification’ which is usually published by the marketing division. You want to know where the impedance curve range is in the area where most music lies. In other words, between 50Hz to about 5khz.
Best,
Anand.
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Awesome, thanks so much for your comprehensive detailed response, Anand. I really appreciate it.
My default formula for power amp transformer supplies is DC at 1.3 times the secondary AC, assuming about a 50% draw on the VA rating of the transformer. Example: 36 V split AC secondary from 300 VA Antec into 150 watt DC draw gives +/-23 VDC.
Always great to hear it from the man himself. To clarify, that is only for the typical full-wave bridge rectifier + CRC power supply, correct?My default formula for power amp transformer supplies is DC at 1.3 times the secondary AC, assuming about a 50% draw on the VA rating of the transformer. Example: 36 V split AC secondary from 300 VA Antec into 150 watt DC draw gives +/-23 VDC.
I know xrk always says his SLB PS (using drops 3V more than the usual rectifier + CRC PS.
How do you do this? I don't have a scope, just a few DMMs, and I know those are not going to help. Not sure if a scope is even the right tool for that measurement...
This is the simplest I have seen which is what I use: https://www.parts-express.com/Dayto...ter-Based-Audio-Component-Test-System-390-807
Ask around with your local audio friends, there might be one who has one and can do the measurement for you.
There are other ways of course, such as REW, etc…but the above item is ready to go and easy to run on a PC/laptop.
Best,
Anand.