The transformer is undersized (I've seen recommendation of 1.5x expected output power) but what does "undersize" actually mean?
Is it undersized on high volume only or undersized for any volume level? (I believed if the amp is class A then I would expect to have problems)
Thank you.
eDIT1: or when there's a low frequency tune (lots of bass), it'll be undersized even at low volume?
EDit2: 2x 4A 25VAC (for a transformer rated 100VA) for the secondaries and using unregulated supply. (just a rectifier bridge + some capacitors)
Is it undersized on high volume only or undersized for any volume level? (I believed if the amp is class A then I would expect to have problems)
Thank you.
eDIT1: or when there's a low frequency tune (lots of bass), it'll be undersized even at low volume?
EDit2: 2x 4A 25VAC (for a transformer rated 100VA) for the secondaries and using unregulated supply. (just a rectifier bridge + some capacitors)
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@mrjayviper
datasheet suggest +-35 on 8 ohm loads. ( if you use 4 ohm speakers on that rail voltage use big heatsink maybe even a fan and don't push it to hard. Check how hot it gets . )
( I ran tda7294 100w ic on +-40v on 4ohm, would get very hot )
What impedance are your speakers?.
4a on 25vac is fine. You'll get around +- 35Vdc , 25 × 1.41 = 35.25v, in reality it will be less probably and rails will sag a bit under load.
You'll be fine for 2x 40w or so , either 4 or 8 ohm loads.
Go for it but check the transformer if it overheats.
For example : You can run a 400w amp on 50w transformer, but you can't get the full power, you could get more than the 50w transformer rating most likely but it will overheat and die , if you push it.
datasheet suggest +-35 on 8 ohm loads. ( if you use 4 ohm speakers on that rail voltage use big heatsink maybe even a fan and don't push it to hard. Check how hot it gets . )
( I ran tda7294 100w ic on +-40v on 4ohm, would get very hot )
What impedance are your speakers?.
4a on 25vac is fine. You'll get around +- 35Vdc , 25 × 1.41 = 35.25v, in reality it will be less probably and rails will sag a bit under load.
You'll be fine for 2x 40w or so , either 4 or 8 ohm loads.
Go for it but check the transformer if it overheats.
For example : You can run a 400w amp on 50w transformer, but you can't get the full power, you could get more than the 50w transformer rating most likely but it will overheat and die , if you push it.
Attachments
It should be fine for normal domestic listening levels.
I've built many A/B amps with a 1:1 ratio and found the transformers barely warm up at all.
Undersized means the transformer will run too hot and the supply output voltage will sag causing the amp to clip.
The transfomer data sheet may have a spec for allowable temperature rise, which is usually in the 40C - 50C range for a toroid.
As long as it operates below that temp after running for an a hour or so at your levels, it will be fine.
I've built many A/B amps with a 1:1 ratio and found the transformers barely warm up at all.
Undersized means the transformer will run too hot and the supply output voltage will sag causing the amp to clip.
The transfomer data sheet may have a spec for allowable temperature rise, which is usually in the 40C - 50C range for a toroid.
As long as it operates below that temp after running for an a hour or so at your levels, it will be fine.
@mrjayviper like you said "
Is it undersized on high volume only or undersized for any volume level? (I believed if the amp is class A then I would expect to have problems)" Class A would be a problem for sure as you constantly draw rhat amount of power. Class AB, depends how hard you push the amp, quality of transformer, some even get hot doing nothing. You can use that transformer for sure, but check how hot it gets, and depends for how long and how loud you play your music. Hey it might not even get hot at high powers , you have to try to find out.
Yes higher power then needed is always better, cooler operation, not a fire hazard.
Use fuses ,for ac outlet and also rails.
Some Chinese amps use on the limit or undersized transformers and they work just fine. Reliable? No. But works.
One Pioneer amp that I have has 130w per channel, 2 channels. So 260w and the transformer rating , power consumption rating is 700w. Of course I didn't see more then 300-400w power draw at the outlet.
But it runs cool even if you listen whole day and the transformer can supply lots of power when needed , peak current draw will be higher then rated power draw.
Is it undersized on high volume only or undersized for any volume level? (I believed if the amp is class A then I would expect to have problems)" Class A would be a problem for sure as you constantly draw rhat amount of power. Class AB, depends how hard you push the amp, quality of transformer, some even get hot doing nothing. You can use that transformer for sure, but check how hot it gets, and depends for how long and how loud you play your music. Hey it might not even get hot at high powers , you have to try to find out.
Yes higher power then needed is always better, cooler operation, not a fire hazard.
Use fuses ,for ac outlet and also rails.
Some Chinese amps use on the limit or undersized transformers and they work just fine. Reliable? No. But works.
One Pioneer amp that I have has 130w per channel, 2 channels. So 260w and the transformer rating , power consumption rating is 700w. Of course I didn't see more then 300-400w power draw at the outlet.
But it runs cool even if you listen whole day and the transformer can supply lots of power when needed , peak current draw will be higher then rated power draw.
- Conclusion, it will work, but you have to test, to see how much power that transformer can supply, how hot it would get doing so.
- always use fuses for mains and rail voltages. Better to pop a fuse then burn the transformer or house .
@mrjay more caps in parallel would be better than one big cap, use small ones as well. Example : 4x 2200uF per rail better then one big 10.000uF.
Also add 1x 100uf + 1x 100nF per rail in addition to the big caps.
High capacitance per rails will help transformer provide peak current draws .
Even 2x 2200uf per rail is good enough.
But if you have them, use more.
More then 15.000 is also not beneficial as cost / performance imo.
Also add 1x 100uf + 1x 100nF per rail in addition to the big caps.
High capacitance per rails will help transformer provide peak current draws .
Even 2x 2200uf per rail is good enough.
But if you have them, use more.
More then 15.000 is also not beneficial as cost / performance imo.
I
1. do you mean a fuse on the secondaries?
2. how would I determine the fuse rating?
Thanks again
1. do you mean a fuse on the secondaries?
2. how would I determine the fuse rating?
Thanks again
@mrjayviper Yes on secondaries, one fuse on each rail , after bridge rectifier, caps.
if the amp outputs 50W on +- 35v rails, using ohm's law : ( P = V x I ( Power in W = V in voltage x I in current Amps ) , 50w / 35v =1.42A ) it will draw 0.7A on rail (0.71A to be exact) (25w + 25w ) , each rail being half of the waveform , assuming class AB 60-75% efficiency , 50W x 0.7 (70% efficiency) = 35W, another 35w wasted as heat, 17.5W per rail, 17.5w / 35 = 0.5A, 0.5A+ 0.71A = 1.21A.
So 1.21A per rail , if stereo , 2.4A per rail. But with more current draw on peaks, you can increase that value.
3A fuse per rail would be good.
if the amp outputs 50W on +- 35v rails, using ohm's law : ( P = V x I ( Power in W = V in voltage x I in current Amps ) , 50w / 35v =1.42A ) it will draw 0.7A on rail (0.71A to be exact) (25w + 25w ) , each rail being half of the waveform , assuming class AB 60-75% efficiency , 50W x 0.7 (70% efficiency) = 35W, another 35w wasted as heat, 17.5W per rail, 17.5w / 35 = 0.5A, 0.5A+ 0.71A = 1.21A.
So 1.21A per rail , if stereo , 2.4A per rail. But with more current draw on peaks, you can increase that value.
3A fuse per rail would be good.
1. Fuses in each DC rail between power supply output and the connections to the amp boards. Many commercial amps do not have these but they are good idea for DIY projects because accidents do happen. Fuse rating can be about 3 A, derived from 100W /35V = 2.86 A for one fuse feeding both channels, Or use two 1.5 A fuses with one for each channel. Fuse type here should be "fast acting".
2. Mains fuse rating is determined by 100 VA / 240 V* = .42 A. Practically speaking, I would round this up to 1 A and this fuse type should be a time delay or slow-blow.
* or your mains voltage.