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Old 14th May 2003, 11:44 PM   #1
JZatopa is offline JZatopa  United States
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Default trasformer help (I have no clue)

I have never made anything electrical before from scratch so I am looking at the gainclone because it is cheap and looks like the easiest way for me to start. Because of this I am hopping some of you can help guide me along the way as I collect parts (I am a broke student so any help would greatly be appreciated )

Would either of these transformers work work?

http://www.apexjr.com/Clearance.htm

the Primary 115V Secondary 24VCT & 14VCT looks like it would work but I am not sure.

Also I was wondering if I were to put a whole lot more V into the opamp what would happen? If I could find a way to keep it cool would it work at a higher level or do they have a limit?

Also has anyone tried powering one of these with a (I may not get this name right) fallowing down convertor? Like the one in the sunfire amp.
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Old 15th May 2003, 08:03 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Hi DJNUBZ, you have picked the right project to start your hi-fi building career and there is no shortage of good advice on this forum.

I won't comment on the transformers as they are not available in the UK. As regards other parts, there isn't a great deal of difference between using something decent and something horrible so don't rush out and buy the cheapest bits that you can find. I also have to build on a budget due to financial constraints but the cost of a couple of decent caps and resistors is nothing compared to what you can spend on the case etc!
If you are a complete beginner, then you will find my site, which is dedicated to helping people start out in DIY hi-fi, useful. Just click on the small WWW button below. There are lots of money saving tricks too.
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Old 15th May 2003, 02:24 PM   #3
elizard is offline elizard  Canada
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as far as "what would happen if you were to feed it a lot of V" if you put too much voltage to the chip, you will blow it up

maximum supply voltage for the lm3875 (the most common one used) is 84V
which means the rails have to be +/- 42V (at the absolute most), so the secondaries of the transformer should be no more than 30V

although 30V for the secondaries would still be too much for the chip, so its recommended to keep them under 24V (my transformer, which hasn't arrived yet, is 22V secondaries (taken as advice from variuos people from here)

also you won't need much cooling, the chips mounted to the case, or a small heatsink will do just fine
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Old 15th May 2003, 02:30 PM   #4
paulb is offline paulb  Canada
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The ApexJr Signal transformer would work but it will only give you 12-0-12 VAC, or about +/- 16 VDC (12*1.4). This will limit your power output (look in the datasheet for the LM3875 for some graphs on expected power output). The transformer is meant for 240VAC but I assume you have 120VAC line voltage (in the US).
If you're using 4-ohm speakers, this might be okay.
The Fender transformer has too high a voltage.
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Old 15th May 2003, 02:34 PM   #5
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Hi,as you are in amaerica, the transformer listed there will give only 12 - 0 - 12 rating (24V centre tapped), which will give you around +/-16V supply for the chip, which is just about enough (not much power from it though, only about 10 or 11 WPC from a LM3875). On the pluss side, it would definately allow enough current to run a few of these devices at thet voltage.
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Old 15th May 2003, 04:27 PM   #6
dhaen is offline dhaen  Europe
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Is "Signal" a US brand name?
If it is, that's pretty confusing for us over here.
We have a safety policy that draws a thick line between "signal" and "power".
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Old 15th May 2003, 06:59 PM   #7
paulb is offline paulb  Canada
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Quote:
Originally posted by dhaen
Is "Signal" a US brand name?
Yes.
And yes, it is confusing. I unfortunately propogated the confusion by trying to distinguish it from the Fender transformer. Oops, that's confusing too.
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Old 15th May 2003, 08:28 PM   #8
JZatopa is offline JZatopa  United States
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ok I am a little confused on something.

The site says this

(Signal 220V input BL 1752)
* Primary 230V Secondary 48VCT 7A & 28VCT 3A
or Primary 115V Secondary 24VCT & 14VCT

I understand that these are 220v and I will be putting in less V then that but I am confused on where it says Primary. What is with the 2 different versions (230V,115V)? Arnt these the rated input voltages? What am I missing?

If I use this here I will have 24V. Does that mean I have a 12+ and a 12- or will I have a 24+ and 24-? From what you are telling me I would have 12+/12-. So why can't I use the 55V fender? Wouldn't that give me 27.5+/27.5-?
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Old 15th May 2003, 08:48 PM   #9
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From what it says one there with it being a dual 55V, I think you are more likely to find it being a 55 - 0 - 55 transformer, which would give a fairly high +/-78V supply after rectification and smoothing. As for the Signal, I do believe that you would get +12 and -12 from the transformer, which would rise to about +16V and -16V after you had rectified the output.
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Old 15th May 2003, 08:50 PM   #10
elizard is offline elizard  Canada
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the 115V/230V is the input voltage .. 115 if you're in north america, 230 if you're in europe

24VCT means 24V center tapped, +12/-12

55VCT is +/- 27.5V .. that's too much for most chips, since that needs to be converted to dc ..
and when converted it comes to ABOUT 27.5*1.4=38.5V
LM3875 has a max voltage of 84V, which is 42V rails .. 38.5 is kinda getting close to its max rated voltage ..

if you could drop the rails 2-3V then it should be allright ..

but even with the 27.5 it will most likely be ok, ALTHOUGH the sound might suffer

EDIT: just checked the apexjr site .. its dual 55V secondaries, 110VCT!!!
so no, you can't use the fender one .. sorry
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