SC480 Amp Questions

[phoenix455]

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.

[Ian Finch]

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.

[phoenix455]

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.

[Ian Finch]

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.

[Greg Erskine]

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/gre...ip%20sc480.htm

If you are in Sydney, get the Harbuch toroids.

Manufacturer of toroidal TRANSFORMERS-Home page

regards

[phoenix455]

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,

[Greg Erskine]

Originally Posted by phoenix455

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?

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

[phoenix455]

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

[Ian Finch]

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.

[phoenix455]

Hello Ian

Thanks for that information, i have sent an email to both to get a quote and i will decide from there.