Hey guys I am still learning and would like your input on transformers. I'm looking at LJM amp boards and found the L12-2. I want to use just 1 to power a passive Sub at 4ohm. I have an old transformer from a Kenwood kr-950b stereo receiver. It had similar if not identical specs to what the L12-2 requires. I have been trying to find out a ballpark on what that transformer is actually rated to handle with no luck. I attached a screenshot of the L12-2 specs if that helps. The transformer I have has 2 sets of secondary wires, they are a center tapped 40 0 40 as well as 18v/0v. Im pretty sure that the back of the Kenwood it came from said 3.5amps AC and 250 or 300w but I can't remember and the case is long gone. Can anyone tell about what size VA rating that transformer might be? Thanks
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Ditto. Is that mono/stereo? One tfmr or two? Cheap or more hi spec? Not enough data. To get a rough idea you need to start at the basics, ohms law or you could do an internet search see what other builders have used. You need to do more research.
First measure your transformer no load voltage with a true rms meter. This is important becauee 40-0-40 is a bit on the high side.
Next would be to measure accurately the wire size, and, if EI core the exat outside measurers of the iron core. Weigth also helps
Next would be to measure accurately the wire size, and, if EI core the exat outside measurers of the iron core. Weigth also helps
I've done research and have found absolutely nothing other than the part # of the transformer and that yielded nothing. It was a pretty popular high end stereo receiver in the early/mid 80s I believe. I found the service manual with the wiring diagrams and parts list but again no specs for transformer. It is only one transformer bigger than my fist and weighs approximately 4 pounds maybe more. I'm a complete newbe to amplifiers so I don't know what classification it is. I posted here because I saw the L12 was referred to as a tube amp. The Kenwood was class a/b and used the exact same SK 2837 and 1186 "tubes" or whatever they're called. It had 1 or 2 pair per channel and was rated as 85 wpc rms or above was exactly the wording.
I can get measurements when I get home from work tonight. Weight will be tough I don't have a scale to weigh that. It does test out to be 39v-42ish vac using a rms meter. The other pair is 18v. I will post a picture of the transformer as well. After running the 40v through a full bridge rectifier and the 2 10,000 UF caps I get 55-58vdc. That's unregulated just measuring at the caps. Ive tested the 2 40v wires before and always get a minimum of 110vac depending on what the primary voltage is at the time sometimes 115vac.
Gorgon53 are you talking about the L12-2? I don't have it I was thinking of buying it and was wondering if the transformer I already have would power it. I guess what your saying is it's most likely too powerful? Those are unloaded and unregulated measurements so would it be safe? I know once you play music the voltage will sag but I have no idea by how much because I have never used a linear power supply.
You could use the L12-2 amp into 4ohm speaker, with a 35-0-35V 400VA transformer.
You say the transformer you have, measures up to 42Vrms unloaded. This is very close to an unloaded 60Vdc and you need an amp that can handle at least +-60Vdc.
But such an amp, to be able to work into a 4 ohm load would also need to be fed from a powerfull enough transformer, a 500VA would be ok, anything smaller will propable not burn up, but the sag will be higher.
You say the transformer you have, measures up to 42Vrms unloaded. This is very close to an unloaded 60Vdc and you need an amp that can handle at least +-60Vdc.
But such an amp, to be able to work into a 4 ohm load would also need to be fed from a powerfull enough transformer, a 500VA would be ok, anything smaller will propable not burn up, but the sag will be higher.
Sorry it's been a long week! I finally got to take another look at the transformer and measure the diameter of the secondary wires. The wire measured 1.0mm or 0.04" in diameter. That worked out to be what I think is 18 guage without the enamel. The leads connected to the secondary are all 18g and there's 5 total(2 for 18v and 3 for +-40). The weight is possibly more than I originally said and could be closer to 6 or 7 pounds. The body measures 3" tall, 3.5" wide, and 2.1" thick. That's the measurements of the center section of plates. The windings stick out about 3 3/4" wide x 2 3/8".
For the final assembly, avoid the diodes in the air like that, you literally create an emitter antenna.
You need some sort of heat sink mounted rectifier anyway. Mounting them up in the air like that is good for getting the heat out of the diodes, but not really good enough here. And one of the bigger “block” type rectifiers can handle the turn on surges from charging the capacitors. Those will work for a while, but eventually die.
”Tubes”, huh? Sounds more like a mosfet amp to me.
”Tubes”, huh? Sounds more like a mosfet amp to me.
The Kenwood kr-950b receiver your transformer came out of is rated as,
Power output: 80 watts per channel into 8Ω (stereo)
Speaker load impedance: 4Ω (minimum)
seems to be an okay match for a single channel 4-ohm nominal load use, although perhaps a bit under powered for driving a sub.
Power output: 80 watts per channel into 8Ω (stereo)
Speaker load impedance: 4Ω (minimum)
seems to be an okay match for a single channel 4-ohm nominal load use, although perhaps a bit under powered for driving a sub.
Judjing from the ca.2kg core weigth and your secondary winding wire size, the current rating would be 2Arms, 2 x 37.5V, so 150VA, 7% regulation, that is a bit anemic for a 4ohmload but if you can liv with the then quite substantial sag it should be ok under normal use, it wont burn up
Thanks guys you have helped answer my question I've been wondering about for awhile now. I wasn't going to leave the diodes like that I just had them arranged for testing because I've never experimented with linear power supplies before. I had the 2 caps zip tied to a bracket attached to the transformer. I wanted a guesstimate on what VA/amperage it might be capable of so I could determine if it was strong enough to handle a sub. Now I know I will abandon that idea and maybe use it for a 2 ch. Amp in the future.
A slightly anemic transformer is easier on the diodes. One good thing that happens with a transformer that sags a lot is increased conduction angle in the rectifier. And less initial surge at switch-on, due to the same mechanisms.
The problem with an anemic transformer is it makes life tougher for the output,driver, and VAS transistors. In comparison to one that puts out the same voltage at full load, but less the rest of the time. SOA needs to be sized for unloaded (max) voltage, not what it sags to.
The problem with an anemic transformer is it makes life tougher for the output,driver, and VAS transistors. In comparison to one that puts out the same voltage at full load, but less the rest of the time. SOA needs to be sized for unloaded (max) voltage, not what it sags to.