Hello Guys
I am starting to build a pair of sc480 amps and i have a couple of questions i would like to ask you guys.
as i am building stereo i would like to get away with one Transformer/Toroidal i believe i need 300va and i have seen a couple of people from here go with a 30v 300va Toroidal, would this one be suitable? Altronics - Your One Stop Audio Visual & Electronics Supplier
also i would like to have volume control from what i understand i can get this two ways, either do a pre-amp kit or a 50k Ω dual-gang log potentiometer (as stated from silicon chip website) if i used the potentiometer would i put that on the input or output i am a little confused? or would a pre-amp kit be better and could someone point me in the right direction.
sorry for all the questions hope it makes sense and thanks in advanced.
I am starting to build a pair of sc480 amps and i have a couple of questions i would like to ask you guys.
as i am building stereo i would like to get away with one Transformer/Toroidal i believe i need 300va and i have seen a couple of people from here go with a 30v 300va Toroidal, would this one be suitable? Altronics - Your One Stop Audio Visual & Electronics Supplier
also i would like to have volume control from what i understand i can get this two ways, either do a pre-amp kit or a 50k Ω dual-gang log potentiometer (as stated from silicon chip website) if i used the potentiometer would i put that on the input or output i am a little confused? or would a pre-amp kit be better and could someone point me in the right direction.
sorry for all the questions hope it makes sense and thanks in advanced.
Whilst it only looks a small margin over the specified 28V AC supplies, a 30V supply allows the rails to sit at +/- 43V rather than 40 nominal, It's the difference between high and low failure risk for untested TIP3055/2955 output devices and the VAS transistors.
Though you can select devices by testing Vce breakdown voltage with a suitable very low current voltage source and DMM, it does not protect the output devices from their rated SOA excursions and failure. OTOH, as the SC480's use low cost parts, perhaps you can afford the odd burnout or simply substitute the outputs for any of the much better, modern TO 247 types like NJW 0281/03O2 etc.
IMHO, these make a huge improvement in figures and subjective sound and are worth the effort. Given the unavailability of stock 28V transformers here, maybe these are the only options. Initially, use the PTC devices in the outputs to save a lot of grief at test stage, at least.
It is possible to add VI current limiting, but conservative use may be just as effective.
Preamps are a matter of subjective opinion, but you need something to drive the amp with a suitably low impedance, so a device with 2 buffers, one before and after the pot. is usual. This is the same as used by SC in their Studio preamp, class A and ULD amps where premium Op-amps are the low-gain buffers in simple, very effective circuits.
Using just the pot is also possible, but the input will be dependent on level setting for performance. With a high input impedance, like 47k, it may sound fine at modest levels.
Though you can select devices by testing Vce breakdown voltage with a suitable very low current voltage source and DMM, it does not protect the output devices from their rated SOA excursions and failure. OTOH, as the SC480's use low cost parts, perhaps you can afford the odd burnout or simply substitute the outputs for any of the much better, modern TO 247 types like NJW 0281/03O2 etc.
IMHO, these make a huge improvement in figures and subjective sound and are worth the effort. Given the unavailability of stock 28V transformers here, maybe these are the only options. Initially, use the PTC devices in the outputs to save a lot of grief at test stage, at least.
It is possible to add VI current limiting, but conservative use may be just as effective.
Preamps are a matter of subjective opinion, but you need something to drive the amp with a suitably low impedance, so a device with 2 buffers, one before and after the pot. is usual. This is the same as used by SC in their Studio preamp, class A and ULD amps where premium Op-amps are the low-gain buffers in simple, very effective circuits.
Using just the pot is also possible, but the input will be dependent on level setting for performance. With a high input impedance, like 47k, it may sound fine at modest levels.
Sorry i am still a little lost on what ya mean there, i don't really want to stuff around with replacing the output's yet until i am confident with that sorta stuff, really wanted to know if i could get away with it , maybe if i downgraded to a 25v toroidal i know i would loss some power but it would be a safe bet, or go with two 28v transformers the only thing that upset me with that was the weight it will add.
You have already found the simplest safe solution. A standard 25-0-25 transformer will do fine, with really, only a loss of dynamic headroom. However, in most capital cities there are transformer winders like Harbuch who will wind a correct 28-0-28V toroidal or E-I type for not too much more than than the imported ones stocked by the retailers. This is what I now do to cover the dwindling ranges available.
The choice is yours, though if opting to use 2 x 28V transformers, one assumes you mean the 28-0-28V E-I type transformers from Jaycar, specified by Silicon Chip. Building a basic amp like this as a dual mono amplifier is actually a good idea for stereo separation, if that interests you.
The choice is yours, though if opting to use 2 x 28V transformers, one assumes you mean the 28-0-28V E-I type transformers from Jaycar, specified by Silicon Chip. Building a basic amp like this as a dual mono amplifier is actually a good idea for stereo separation, if that interests you.
hi phoenix455,
I have build a couple of these. The first ones were on special for $38 then a few years later I was able to get them for $12.50 each.
I have always built them as dual mono using Harbuch transformers. Details below:
http://users.tpg.com.au/gerskine/greg/silicon chip sc480.htm
If you are in Sydney, get the Harbuch toroids.
Manufacturer of toroidal TRANSFORMERS-Home page
regards
I have build a couple of these. The first ones were on special for $38 then a few years later I was able to get them for $12.50 each.
I have always built them as dual mono using Harbuch transformers. Details below:
http://users.tpg.com.au/gerskine/greg/silicon chip sc480.htm
If you are in Sydney, get the Harbuch toroids.
Manufacturer of toroidal TRANSFORMERS-Home page
regards
Thanks Ian and Greg,
I am in Melbourne, i presume i could call Harbuch and order them?.
I like your build Greg, my kit is version one but apparently they are very similar,
Its a bit hard to see in the picture but did you cut a whole were the heat-sinks are?
Just to clarify volume control again if i wont with just using a put , do i put it in on the input just before the amp or on the output just before the speakers?
Thank you,
I am in Melbourne, i presume i could call Harbuch and order them?.
I like your build Greg, my kit is version one but apparently they are very similar,
Its a bit hard to see in the picture but did you cut a whole were the heat-sinks are?
Just to clarify volume control again if i wont with just using a put , do i put it in on the input just before the amp or on the output just before the speakers?
Thank you,
hi phoenix455,
There is a cutout in the back panel, its a little smaller than the heatsink.
The pot goes on the input. Most of the pots on the market are under a couple of watts.
I have found a picture of another one. The toriods in this one are not Harbuch, probably from Jaycar.
regards
Attachments
Hi Greg
Thank you for the info and new pic.
Have you put a volume control on any of your sc480's ?
Ian had mentioned in his first post that a pot may be ok, just trying to weigh up preamp or pot, considering cost i may just go a pot i hope its sufficient enough, would a double gang pot would be suitable for duel mono/stereo?
Thank you
Thank you for the info and new pic.
Have you put a volume control on any of your sc480's ?
Ian had mentioned in his first post that a pot may be ok, just trying to weigh up preamp or pot, considering cost i may just go a pot i hope its sufficient enough, would a double gang pot would be suitable for duel mono/stereo?
Thank you
Hi phoenix455
In Melbourne, you have Tortech and Dyne Industries producing toroidal transformers. They may be more expensive, but that could be offset by the high freight cost from Sydney. Note; some have on-line ordering to avoid lengthy discussions or misunderstandings about details like dimension options, lead types etc.
Regarding connecting the pot direct to the amp inputs, I think Silicon Chip's SC480 articles illustrated the simple circuit with the last published article. Presumably, kit suppliers duplicated that with the kit instructions, as they usually do.
In Melbourne, you have Tortech and Dyne Industries producing toroidal transformers. They may be more expensive, but that could be offset by the high freight cost from Sydney. Note; some have on-line ordering to avoid lengthy discussions or misunderstandings about details like dimension options, lead types etc.
Regarding connecting the pot direct to the amp inputs, I think Silicon Chip's SC480 articles illustrated the simple circuit with the last published article. Presumably, kit suppliers duplicated that with the kit instructions, as they usually do.
Hello Ian
Thanks for that information, i have sent an email to both to get a quote and i will decide from there.
Thanks for that information, i have sent an email to both to get a quote and i will decide from there.
since the money you are going to spent is more or less te same and since the real cost is the trafo sinks and banks you might as well think to built something better sound wise
another option might be to find original pairs of MJ 15003 -4 and replace the 3055-2955 with them ... you will gain stabillity durability and better performance any way ..
be aware the in this family there is a lot fakes world wide so obviously you need to be carefull where you buy from ..( both for the 3055 and 15003 )
as about the preamp all in a hands full of info is like that :
-----passive pot in the input will be as clean you never listened before in your life
but there is going to be an obvious absense of dynamics in low levels of power
passive pots generaly will also be some question of choise depenting on impendance of input and output
----- option 2 might be an active buffer like the Nelson Pass B1 works very well easy to built and to my opinion is the cleanest available and very next to a passive pot and without impendance issues ...still there is also obvious absense of dynamics in low levels ( B1 has no gain ) and also to my opinion the B1 tends to round up things in the ends
------ option 3 the DOZ ...easy to make ...nothing weird with it and sounds very well with none of the above situations enough gain and very clean if made propelry
off topic
my friend ian !!! how are you ? long time no seen ... too busy latest months for staying in the diy the time i used to in the past
kind regards
sakis
another option might be to find original pairs of MJ 15003 -4 and replace the 3055-2955 with them ... you will gain stabillity durability and better performance any way ..
be aware the in this family there is a lot fakes world wide so obviously you need to be carefull where you buy from ..( both for the 3055 and 15003 )
as about the preamp all in a hands full of info is like that :
-----passive pot in the input will be as clean you never listened before in your life
but there is going to be an obvious absense of dynamics in low levels of power
passive pots generaly will also be some question of choise depenting on impendance of input and output
----- option 2 might be an active buffer like the Nelson Pass B1 works very well easy to built and to my opinion is the cleanest available and very next to a passive pot and without impendance issues ...still there is also obvious absense of dynamics in low levels ( B1 has no gain ) and also to my opinion the B1 tends to round up things in the ends
------ option 3 the DOZ ...easy to make ...nothing weird with it and sounds very well with none of the above situations enough gain and very clean if made propelry
off topic
my friend ian !!! how are you ? long time no seen ... too busy latest months for staying in the diy the time i used to in the past
kind regards
sakis
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ok so i put in a quote for a 28-0-28 300va toroidal transformer, would this be correct? for powering of both amps
Hi Sakis
could you give me some links on option 2 and 3 please, are these available in kits or do i need to buy pcb and components individually?
Hi Sakis
could you give me some links on option 2 and 3 please, are these available in kits or do i need to buy pcb and components individually?
Hi phoenix455,
I don't put volume controls on my amps. I use the preamp or just use the volume control on my source. I am sure this amp will work OK in most circumstances with just a pot.
Jaycar pot: 10k 50k or 100k
50K 9mm Square Potentiometer Log Double Gang (A) - Jaycar Electronics
You will see one in the last picture (unconnected). I haven't used one yet, but if you want to give one a go PM me, I have lots of them plus a little PCB.
regards
Hi sakis,
This amps comes in 2 types one with TO3s and the other using TO3Ps. Mine use TO3s but phoenix45 is using TO3Ps.
Soundwise, these things are OK. I regularly compare with AKSAs, P3A, gainclones etc and some people actually prefer my standard SC480.
In theory the TO3P version should sound better than my TO3 version.
I do have a DOZ with 2N3773 outputs which I haven't fired up for many years. As with all Class A amps it runs hot and IMHO a little scary for first time builders.
regards
HI, i have some problems on the SC 480 Amp. < I tried to DIY one sides of the amp and it works. So far i tried to change the BC640 into 2N4033, and it works normally. Meanwhile, i take a risk and changed the BC639 into 2N3019 luckily it works. And after a few test it goes down, cause i have series a 200W light bulb which when i saw the light turns brightly so i close the power supply.
And i tried to put back the same BC639,640 and test. This time the light bulb turn really bright which means the current gain is too high and by testing the out put voltage the voltage Sec is only +5.0 & -5.0 <? so what is the problem?
I tried to change some of the components BD139, MJE340,MJE350, BC557 & both BC639,640< which is still not in function.
Could any one help?
And i tried to put back the same BC639,640 and test. This time the light bulb turn really bright which means the current gain is too high and by testing the out put voltage the voltage Sec is only +5.0 & -5.0 <? so what is the problem?
I tried to change some of the components BD139, MJE340,MJE350, BC557 & both BC639,640< which is still not in function.
Could any one help?
Hi Briansune
There are many possible faults not covered in your post but consider these issues:
First, there were building notes in SC magazine which followed some issues after the first couple of construction articles. These covered more details for a complete amplifier set-up and fault finding. Make sure you have all the details.
Second, the set-up is with resistors, not the light bulb which may result in too low a rating to allow working rail voltages to test with 5V rails, or anything less than about 12V, often will force any amp into a non-functional or fault condition which can't be tested or set up. If the rail voltage falls that far with a huge 200W lightbulb in any case, you have major faults like toasted, incorrectly connected or simply under-rated semis. Check for solder bridges and the correct lead arrangement for the different types of parts. Check that the bias setting is in minimum and not maximum position to start with. Also, a semiconductor fault may not simply be read with a DMM showing Vbe as wrong. It's a guide to common failure, but there can be other faults which only show up when significant current is flowing.
I guess you are also trying to run the amp. with higher rails than originally specified or you probably wouldn't be concerned about VAS types. That's your choice, but you should use higher rated VAS parts for dissipation and voltage rating. If you look at Greg's website (see previous posts), you'll find a selection guide for transistors but few good parts are available from our local suppliers whose catalogues don't seem to have changed much in 40 years! WES, RS or Element14 will have better options here in Oz.
There are many possible faults not covered in your post but consider these issues:
First, there were building notes in SC magazine which followed some issues after the first couple of construction articles. These covered more details for a complete amplifier set-up and fault finding. Make sure you have all the details.
Second, the set-up is with resistors, not the light bulb which may result in too low a rating to allow working rail voltages to test with 5V rails, or anything less than about 12V, often will force any amp into a non-functional or fault condition which can't be tested or set up. If the rail voltage falls that far with a huge 200W lightbulb in any case, you have major faults like toasted, incorrectly connected or simply under-rated semis. Check for solder bridges and the correct lead arrangement for the different types of parts. Check that the bias setting is in minimum and not maximum position to start with. Also, a semiconductor fault may not simply be read with a DMM showing Vbe as wrong. It's a guide to common failure, but there can be other faults which only show up when significant current is flowing.
I guess you are also trying to run the amp. with higher rails than originally specified or you probably wouldn't be concerned about VAS types. That's your choice, but you should use higher rated VAS parts for dissipation and voltage rating. If you look at Greg's website (see previous posts), you'll find a selection guide for transistors but few good parts are available from our local suppliers whose catalogues don't seem to have changed much in 40 years! WES, RS or Element14 will have better options here in Oz.
Sorry for the English!
Ya, Really thanks for pointing out the possibilities, and i think you have miss-understand the use of my light bulb< the series is with the transformer but not the amp output, this is because the current deliver to the transformer will be easily observed by the light bulb. The point of this setting is to prevent current over flow.
Ya, Really thanks for pointing out the possibilities, and i think you have miss-understand the use of my light bulb< the series is with the transformer but not the amp output, this is because the current deliver to the transformer will be easily observed by the light bulb. The point of this setting is to prevent current over flow.
No misunderstanding with how to use common bulb testing there. Usually, in quiescent (no signal or load) mode, amplifiers like this one draw only 25-40 mA. With a total 60 VAC supply, that's only a couple of watts load. Typically, a 60W bulb is more than adequate to supply much higher fault current and still be only partly lit.
If you can light up a 200W bulb with your load, you obviously have a much larger load than it should be and have had little or no protection from the bulb. Logically there is a short or failed or incorrect component(s).
BTW, is this using original PCB and overlay or a DIY construction?
If you can light up a 200W bulb with your load, you obviously have a much larger load than it should be and have had little or no protection from the bulb. Logically there is a short or failed or incorrect component(s).
BTW, is this using original PCB and overlay or a DIY construction?
THANKS AGAIN
IC <i am using a DIY construction, and after few checking i changed the 2n3055 &MJ2955 it works again but this time, the Vrms +ve sides & Vrms -ve sides drop from 30V to 18V what happen? and the current across the light bulb is about 0.5A.
IC <i am using a DIY construction, and after few checking i changed the 2n3055 &MJ2955 it works again but this time, the Vrms +ve sides & Vrms -ve sides drop from 30V to 18V what happen? and the current across the light bulb is about 0.5A.
Hi Briansune
Assuming your previous figure iof 5V rails was correct, Ohm's law says that less current (no shorted output transistors) means higher voltage, so you now have 18V rails. but they should be, according to your last post, 30V. So you still have significant error(s),
Think about it, 0.5A is about 10 times the expected current for a correctly biased amp. on a 220V mains supply. With zero bias, it will be only a fraction of this small amount.
Now. it is likely you have a bias error and you must set bias to lowest or zero before further problems leave you with a pile of more toasted parts. Determine if and how the generator works and set it at zero as a priority.
Assuming your previous figure iof 5V rails was correct, Ohm's law says that less current (no shorted output transistors) means higher voltage, so you now have 18V rails. but they should be, according to your last post, 30V. So you still have significant error(s),
Think about it, 0.5A is about 10 times the expected current for a correctly biased amp. on a 220V mains supply. With zero bias, it will be only a fraction of this small amount.
Now. it is likely you have a bias error and you must set bias to lowest or zero before further problems leave you with a pile of more toasted parts. Determine if and how the generator works and set it at zero as a priority.
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