A continuous 300mAdc would require a 600mAac transformer as the minimum.
I would run the transformer at ~50% of it's maximum rating, thus requiring a 1.2Aac transformer.
The peak current demand of the opamp is ~40mA. with 18 duals that comes to an absolute maximum transient peak current demand of ~1.5Apk.
Most of this will come from the decoupling adjacent to each opamp and the remainder from the little bit of smoothing after the regs.
The smoothing before the regs sees virtually none of the peak demand, thus the 1200mAac transformer does not need to be rated for that worst case 1.5Apk demand.
1.2Aac on a 18+18Vac transformer is ~43VA. Just pick any 50VA to 60VA transformer for this 18 dual opamp circuit.
But, are there any other current loads?
I would run the transformer at ~50% of it's maximum rating, thus requiring a 1.2Aac transformer.
The peak current demand of the opamp is ~40mA. with 18 duals that comes to an absolute maximum transient peak current demand of ~1.5Apk.
Most of this will come from the decoupling adjacent to each opamp and the remainder from the little bit of smoothing after the regs.
The smoothing before the regs sees virtually none of the peak demand, thus the 1200mAac transformer does not need to be rated for that worst case 1.5Apk demand.
1.2Aac on a 18+18Vac transformer is ~43VA. Just pick any 50VA to 60VA transformer for this 18 dual opamp circuit.
But, are there any other current loads?
I also concluded that non-polarized capacitors create much less distortion that orinary polarized capacitors. I covered this in my book, "designing Audio Power Amplifiers" in Section 13.8.There I show a plot of distortion vs frequency for a polarized 100uF, 16V electrolytic and a 100uF 100V non-polarized capacitor.
Distortion for the NP cap is about 20 times smaller. The NP capacitor was an ordinary NP capacitor normally intended for loudspeaker crossovers. At a current of 4mA RMS and a frequency of 20Hz, distortion in the voltage across the capacitor was 0.0005% for the NP. Under these conditions the voltage across the capacitor was about 300mV RMS.
Bear in mind that this is the distortion of the voltage across the capacitor; the resulting amplifier distortion due to the capacitor will be smaller. Note that a typical 100W amplifier with a 1k/19k feedback network creates only 1.5mA RMS in the feedback capacitor at full power.
Cheers,
Bob
I hope it does
Can you hint at what area(s) have been tweaked ?
Noise has been reduced by making the 10K pots 5K, and using different opamps.
Distortion has been reduced by using all polypropylene caps.
Cost has been reduced by using smaller caps where that does not compromise anything else.
So you can see it's very much a Mark 2 version.
While we're on the subject, I would like to say that the tone-cancel facility in the published preamp was added by Elektor.
Thanks Andrew. It sounds like there will be at least a part 2 to Doug's project - the MC/MM phono stage pre-amp. Article here. Seems like this circuit alone would require another 300mA+ (worse case). Unless you build 2 separate PSUs, the PSU will need to accommodate the current demands of this circuit too. I'm assuming placement of the phono stage in the same enclosure - albeit well-shielded. That may not be an option for some.
WRT the caps in the feedback network, it's important to remember the distortion issue only arises at LF where the cap impedance increases. So even if you are using lower value feedback resistors, as long as you select the cap value based on the same -3dB corner frequency, your distortion should beno worse than a cap sized for a higher feedback resistance network.
If I were you I don't think that I would be so tolerant of a magazine arbitrarily changing my design. After all your name is on the article, not the party making the changes. All that is published is attributed (blamed??) upon you. I guess that means that you have 'gentler soul' than what I do.
Thanks Andrew. It sounds like there will be at least a part 2 to Doug's project - the MC/MM phono stage pre-amp. Article here. Seems like this circuit alone would require another 300mA+ (worse case). Unless you build 2 separate PSUs, the PSU will need to accommodate the current demands of this circuit too. I'm assuming placement of the phono stage in the same enclosure - albeit well-shielded. That may not be an option for some.
According to the website the June issue has part 3 which includes the PSU:
Preamplifier 2012 (3) - ELEKTOR.com | Electronics: Microcontrollers Embedded Audio Digital Analogue Test Measurement
Note from the parts list they specify a 2 x 18V @ 50VA transformer
Hi Bob,
I own your excellent book also, thank you. I found the section on SPICE simulation particularly helpful BTW.
I did read your findings on NP capacitors used in the feedback network, and also recall a referrence somewhere to the input coupling capacitor of a power amp needing to be of the highest possible quality (apologies if I'm misquoting, don't have it handy).
Given that advice, what do you think about the importance of capacitor type in this application (large coupling caps between stages in a preamp)? Are polarised electros harmless provided they are large enough?
Cheers!
Thanks a lot owdeo. Just what I needed tonight... Sending Newark another $200+ parts order.According to the website the June issue has part 3 which includes the PSU:
Preamplifier 2012 (3) - ELEKTOR.com | Electronics: Microcontrollers Embedded Audio Digital Analogue Test Measurement
Note from the parts list they specify a 2 x 18V @ 50VA transformer
Might as well go ahead and order the PCBs and get a head start. Now if only Elektor would offer a snazzy aluminum enclosure - with pre-punched front and rear panels. But that just might be asking too much! If I were you I don't think that I would be so tolerant of a magazine arbitrarily changing my design. After all your name is on the article, not the party making the changes. All that is published is attributed (blamed??) upon you. I guess that means that you have 'gentler soul' than what I do.
Guess that's a compromise that has to be accepted when publishing in a mag that presents practical projects to build as opposed to theoretical articles like EW&WW?
But I agree in so far as that when I first read the article I thought some aspects were at odds with his previous design philosophy (such as avoiding exotic components), and although we now know otherwise it does appear as though the author is responsible for every aspect of the design and practical implentation. This is different to the 5532 Opamplifier articles where it was made clear that the design was his and the implementation was Elektor/Giesberts.
I must say I miss the extra space that probably would have been devoted to design insights etc had it been in EW (the way EW used to be anyway)... eg analysis of design choices such as to what degree the improvement made in noise output from the Baxandall tone stage by using 1k pots justifies the extra cost of the additional opamps and large caps.
Thanks a lot owdeo. Just what I needed tonight... Sending Newark another $200+ parts order.
Yep, the enclosure is always a problem...PS I'm sure you know this already but if I were you I wouldn't order the transformer from Newark/Farnell...their markup is incredible! RS is much cheaper here and they have a better quality brand, but better still I like to use a local manufacturer that makes them to order (there's one in my suburb!) to try and support local industry before it all dissapears completely to you-know-where (not that my hobby requirements are likely to help them much in this regard I suppose...
)I was planning on sourcing the xformer from a different supplier - where I've bought some others. They're down the road in Jersey. I can say this project has materialized into more than I originally thought. Not that it's complicated or anything. But it does now appear that you will need all the PCBs for a totally integrated solution. It will NOT be inexpensive either!
PS I'm sure you know this already but if I were you I wouldn't order the transformer from Newark/Farnell...their markup is incredible! RS is much cheaper here and they have a better quality brand, but better still I like to use a local manufacturer that makes them to order (there's one in my suburb!) to try and support local industry before it all dissapears completely to you-know-where (not that my hobby requirements are likely to help them much in this regard I suppose...
I etched my own boards in the 70-80s, but alas I gave that up years ago in favor of simply buying one-off boards, that are of much higher quality. Although all the bare PCBs for this project alone will be ~$200USD. Ouch!Has anyone created a BOM for all these parts we could simply upload to our favorite supplier?
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I down loaded the 1st part of the article and had a quick read.
Very nicely thought out and looking at the specs and the level of the components used, I think this is a damn fine design. Its great to see a preamp design with (decent Baxandall) tone controls also, since most high end pre-amps dropped these years under the pretext that they damaged the sound, which is of course absolute bunk. Having not heard this preamp, I cannot comment on the sound of course.
The only really 'exotic' components are the pots, but you are always going to have a few parts in any worthwhile design that will be a bit difficult to get hold of. If a custom enclosure is on its way, I may consider building one myself . . .
Naaah!! I feel an X-Altra Mini II coming on!!!
Very nicely thought out and looking at the specs and the level of the components used, I think this is a damn fine design. Its great to see a preamp design with (decent Baxandall) tone controls also, since most high end pre-amps dropped these years under the pretext that they damaged the sound, which is of course absolute bunk. Having not heard this preamp, I cannot comment on the sound of course.
The only really 'exotic' components are the pots, but you are always going to have a few parts in any worthwhile design that will be a bit difficult to get hold of. If a custom enclosure is on its way, I may consider building one myself . . .
Naaah!! I feel an X-Altra Mini II coming on!!!
I did a number of measurements of various capacitors on Clio and on an AP1 and came to the conclusion that most electrolytics of any promise were being unjustifiably maligned in audio. And near the top of the list came the fairly standard but good ALCAPs. In fact, on the impedance side, there were good arguments to use them instead of PP.
I tried various MKP4 and MKP10s against Oscons, ZAs, ZLs and various NPs, all at approx 10uF for direct comparison. What was interesting was that it harder to quantify the losses trigonometrically on the film/foils because the inductance of the winding started to kick in, inside the audio band.
Nor were the losses on the electrolytics obviously crappy, either. One 10mF reservoirs I was able to get 8 mOhms only. This is achievable with computer grade caps at £20 a piece, but in this instance was a very standard part at <£2.50.
For signal path type values the angles were, as an example, 85.3 degrees at 20k for 16uF and an overall Z of 0.51 where it should be 0.4973 ohms (so it's not actually 16uF!) but it's still pretty titchy at around 40mOhms.
I agree, too often folks mix and match test conditions to prove their point.
I've always thought that tone control was a great piece of design. When I first read it I thought "That's the way it should be done", quickly followed by "No audiophile can possibly complain about this!" Though I bet someone has!
I seem to remember, though I don't have it in front of me, that it might have been possible to simplify it. Whether I thought you could perhaps remove a buffer or two I can't remember, but I seem to recall there was a way of doing a cut-down version. Now I'll have to find it, but was wondering if (other than the component changes you've listed) that was one of the things you might have done?
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