Hello all,
I've got a Avel Lindberg 67530 transformer that I would like to use for a discrete class-B amplifier (not chip amp). I bought it through Apex Jr years ago.
2x25V @ 2A, Link to transformer datasheet:
http://www.appliedcarbon.org/powersupply/AvelLindberg_67530_sc2_toroid_transformer.pdf
Looks like a 100 VA unit, I wonder what size amp you would feel confident to build around this transformer?
Maybe 30Wx2 @ 8 ohms? Maybe some VI limiting would be needed? What do you think?
-Dan
I've got a Avel Lindberg 67530 transformer that I would like to use for a discrete class-B amplifier (not chip amp). I bought it through Apex Jr years ago.
2x25V @ 2A, Link to transformer datasheet:
http://www.appliedcarbon.org/powersupply/AvelLindberg_67530_sc2_toroid_transformer.pdf
Looks like a 100 VA unit, I wonder what size amp you would feel confident to build around this transformer?
Maybe 30Wx2 @ 8 ohms? Maybe some VI limiting would be needed? What do you think?
-Dan
Hi Dan
Unless it is constantly drawing current like a Class A circuit does, the efficiency of the lower conduction Class types will assist. 100VA is 4 amps at 25 volts Power is E x I , however there are inefficiencies such as the diode conduction. A LT4320 can assist partnered with low Rds on N channel mosfets likeIRFB4310 to make the diode bridge. Others may have comment but it suggests to me a 40 watt per channel design if done this way using each tap and using Class B
Hope this helps.
Unless it is constantly drawing current like a Class A circuit does, the efficiency of the lower conduction Class types will assist. 100VA is 4 amps at 25 volts Power is E x I , however there are inefficiencies such as the diode conduction. A LT4320 can assist partnered with low Rds on N channel mosfets likeIRFB4310 to make the diode bridge. Others may have comment but it suggests to me a 40 watt per channel design if done this way using each tap and using Class B
Hope this helps.
Interesting, I've never used a bridge rectifier controller before. I'm not sure its worth the money and complexity in my case to save 1 V or so of voltage drop. Especially since I have some 25 A bridges sitting in a drawer...
Thanks for your input.
With Class B it all depends on duty cycle, which depends on the type of music you like. A 100VA transformer can provide perhaps 40W of continuous DC but up to 100W of occasional DC. Ideal Class B has peak efficiency of 78% but real Class B will be somewhat less. 60W peak (i.e. 30W+30W) would seem to be the maximum, provided that your music has quiet parts between the loud parts. If you listen to modern highly compressed continuous noise music then 25W (i.e. 12W+12W) may be better.
A conservative rule of thumb for stereo power is the transformer's VA rating divided by 4, in Watts per channel. So a 2 x 75W amplifier could use a common 300VA transformer. I've also seen commercial amplifiers fitted with smaller transformers (particularly the E-I laminated types) that are as low in VA rating as the total maximum 8 Ohm RMS output power but these depended on the pronounced voltage sag of that type, to limit overloads. Overall, that's not such a bad idea in a domestic amplifier and Technics once did a lot this cost-cutting and weight saving in their amplifiers.
Because my DIY builds seldom get used outside homes and even then, only played at polite levels, there seems little point in matching power input to the maximum continuous power output plus losses. I have found VA rating divided by 3 to be just as acceptable in practice. As DF raised the issue, that's on a varied musical duty cycle rather than continuous high level pounding bass content.
If anyone is into organ music, or if you otherwise like ponderous, house shaking bass, this may require a re-think or looking at Disco PA amplifier design and supply ratings.
Because my DIY builds seldom get used outside homes and even then, only played at polite levels, there seems little point in matching power input to the maximum continuous power output plus losses. I have found VA rating divided by 3 to be just as acceptable in practice. As DF raised the issue, that's on a varied musical duty cycle rather than continuous high level pounding bass content.
If anyone is into organ music, or if you otherwise like ponderous, house shaking bass, this may require a re-think or looking at Disco PA amplifier design and supply ratings.
100VA will work satisfactorily powering a range of amplifier from 50W of total maximum power to 100W of total maximum power for all domestic duty sensible listening. That excludes drunken parties.
The downside to using low VA transformer is their high regulation. That's why I recommend a minimum of 160VA toroid for power amplifier duty.
The voltage droops more for low power demand and droops even worse for high power demand.
The effect of this high regulation is that the supply rail voltage to the amplifier drops a lot when a sustained high power output is attempted. The transformer/amplifier combination protects itself !
The 25Vac only suits 8ohms loading. It will give roughly 50W+50W into 8ohms+8ohms. But that will drop as the duration of the signal extends beyond half a second or so.
Do not use 4ohms speakers. The amplifiers would attempt to deliver 100W into 4ohms. That will serverely overload the transfomer and it will start getting hot, if you persist in approaching that level for a couple of minutes.
The downside to using low VA transformer is their high regulation. That's why I recommend a minimum of 160VA toroid for power amplifier duty.
The voltage droops more for low power demand and droops even worse for high power demand.
The effect of this high regulation is that the supply rail voltage to the amplifier drops a lot when a sustained high power output is attempted. The transformer/amplifier combination protects itself !
The 25Vac only suits 8ohms loading. It will give roughly 50W+50W into 8ohms+8ohms. But that will drop as the duration of the signal extends beyond half a second or so.
Do not use 4ohms speakers. The amplifiers would attempt to deliver 100W into 4ohms. That will serverely overload the transfomer and it will start getting hot, if you persist in approaching that level for a couple of minutes.
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Don't believe the HYPE that you need a massive tranformer that is rated for multiples of the max output power. If what you are listening to can be considered "music", then there is no reason to overspec the transformer. By music I mean NOT highly compressed pop music played that is at max volume. The reason is that "audiophile" music will have a "crest factor" of at least 10dB, which is the average ratio of peak signal to the average signal level. You may have heard that most modestly efficient loudspeaker only require a couple of Watts to reproduce normal listening levels. That is is "average" power level. But music has peaks and transients that require 10 times this much power (e.g. 10dB crest factor) but that is still only 10-20 Watts of power for short term peaks. On top of the signal related demand of the amp there are some dissipation and losses to consider, so apply a factor of 50% efficiency as a rough guide if you want. You are still barely approaching ONE TIMES the amp power.
Your 100VA transformer will do just fine driving a pair of 8 Ohm loudspeakers at regular listening levels.
If on the other hand you want to really drive your speakers hard, listening to compressed music programme, etc. Then you might consider choosing a transformer that is 2x the rated max power of the amp.
As an example, I have used an 80VA 42VCT transformer for a single channel LM3886 amplifier into 4 Ohms (rated for 68W max power into that load) without the slightest problem with music signals. This setup will look bad under full power sine wave testing, but who listens to that?
Your 100VA transformer will do just fine driving a pair of 8 Ohm loudspeakers at regular listening levels.
If on the other hand you want to really drive your speakers hard, listening to compressed music programme, etc. Then you might consider choosing a transformer that is 2x the rated max power of the amp.
As an example, I have used an 80VA 42VCT transformer for a single channel LM3886 amplifier into 4 Ohms (rated for 68W max power into that load) without the slightest problem with music signals. This setup will look bad under full power sine wave testing, but who listens to that?
Find any big PA amplifier, even the ones rated for 5k continuous watts and designed for touring, and note the VA consumption rating on the back panel. Much less than the stated power. Normally, it is the power consumption at 1/8 power pink noise. The cheap ones, at 4 ohms, the expensive ones at 2 ohms - but nonetheless not sine wave. You will also find that the VA rating of the transformer to be just the next standard size up. A standard 1.5KVA toroid will take a 10kw load for 15 minutes before reaching the 130C that the thermal switch opens at. The speaker will have given up the ghost long before, and you will have flipped the breaker at the panel a couple of times already. Smaller transformers, however, will thermal more rapidly and on the better amps they will up size them a bit. But still not enough for full signal sine wave. They used to in the 70's for commercial rated amps (Crown, Peavey, BGW, etc) but not anymore. Home stereo equipment never did except for freakish high end stuff costing thousands and weighing in at 100 pounds for 200 watts.
If this is typical of professional equipment, you can expect much less from consumer grade HT recievers.
Just size your transformer up 25-50% over what is 'typical' for what you're building and it will be way better than what you buy in the store.
If this is typical of professional equipment, you can expect much less from consumer grade HT recievers.
Just size your transformer up 25-50% over what is 'typical' for what you're building and it will be way better than what you buy in the store.
Thanks for the awesome replies folks. Makes me feel better about moving forward with this transformer for my project. I will use the amp for desk/computer near field speakers with a listening distance of a meter or less. Now sure that I'll be able to achieve an adequate SPL in that situation.
I've got the G Randy Slone book in front of me where he spec's a 432 VA transformer for a 70W+70W domestic amplifier... So I thought it couldn't hurt to get some opinions!
- Dan
I've got the G Randy Slone book in front of me where he spec's a 432 VA transformer for a 70W+70W domestic amplifier... So I thought it couldn't hurt to get some opinions!
- Dan
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