I am thinking of building an amp for a pair of speakers using these two kits

Two of these amps http://www.futurekit.com/2009/manual/fu ... 658E-2.pdf

Power supply is +35 -35 Vdc and at least 2A current for one amp. What transformer would I need to properly run both amps?

I am figuring on at least 6 amps minimum current rating of the transformer to give plenty of headroom so the supply voltage don't drop under load. My plan is to use a center tap transformer with a bridge rectifier to supply the + and - volts. I will be using a minimum of 10,000 uF of capacitance for each of the positive and negative power supplies. I am thinking one of those toroidal power transformers would work good for the power supply.

The amp has a 1 volt input sensitivity. Is there a simple circuit that I can use to boost a line level signal to 1 volt output while increasing the input impedance so that I can use the nice 100K dual gang Alps pot I have? the input impedance of the amp is 15K. I am thinking maybe a single transistor or FET would work. The amp has +15 and -15 Vdc outputs for powering a tone control board.

I am thinking this amp will sound very good as it is of the DC coupled design except for the input coupling capacitor.

Price is $19.95 for a stereo kit (two separate kits) not including shipping.

Claims 35 watts into 4 or 8 ohms, but it will more than likely be approx 17.5 watts for 8 ohm loads and approx 35 watts for 4 ohm loads.

I do have a Sony TAE 5450 preamp that would work to drive the amp if I can find a good quality replacement 10K dual gang pot for it.

Their specs are a bit ambiguous. In one place they say +- 35V, "more than 2A," but then they place "2A" by the transformer in the suggested power supply schematic. The problem is, transformer AC amps are not DC amps and for a dual complimetary rectifier supply the DC to AC amps ratio is about 1.8. That's because transformer loading comes in capactitor charging spikes (increased heat load) and not a sine (RMS).

If we take the 35W number into 8 ohms we do come up with right around 2A (2.09) RMS, which is DC 2 A. So their transformer rating would seem to be too low, depending on what you 'expect', which gets back to 'music' because you won't ever have 35 Watt 'continuous', unless you simply 'measure' it that way with a tone.

Btw, their 24VAC number will give you slightly less than 35V (loaded).

If you really wanted to support 35W 'continuous' you'd need roughly 3.8A (per channel) to keep the transformer from overheating but, in reality, their 2A would suffice since average music power should be no more than 4 Watt (-10 dB peak) and 2 A AC, divided by 1.8, will give, not counting other losses, roughly 9+ Watt continuous (-6 dB peak).

Btw, it's 'worse' with 4 ohms because that's more current at lower volts and it's current load that's heating the power transformer. It's even worse for the output transistors because they have both the increased current and more voltage drop.
Line level is 2V so 1V sensitivity will drive it to clipping with the input -6 dB. That is, it will if the source is normalized to 0 dBfs but sometimes a source is 'weak'. like people who under record MP3s, etc. and whether 6 dB is 'enough' to compensate is debatable. Personally, I like to have around 316mV sensitivity which, in this case, would be about 10 dB preamp gain.

A single transistor is a bad idea and they've given you the power for opamps. All you need is a decent audio quality dual with a couple of resistors to set gain but if you want to be really sexy get one of the remote control switching preamps off ebay for 40 bucks or so. It's the whole front end with 5 inputs, 10 dB preamp and, of course, the remote control.

Looks like I'll need a 25-0-25 volt 10 A transformer. If I have to get a transformer with two secondaries that would equate to 5 amps per secondary, right?

I figured on having a 500 mV input sensitivity as I am not sure exactly what output levels all my devices have. Will primarily be using with a cellphone and laptop computer, but may also use it occasionally with a FM tuner made in the 80's.

I have thought about those remote control preamps, but am not too sure if I really want to go that route right now as I can get the necessary variable resistor for the Sony for less than what the remote volume control boards sell for or I can just use a car audio preamp/eq I have.

That's an awful lot of bucks for power you don't really need, 500VA for 70 Watts of audio?, and you could probably pick up a decent 'vintage' 50W-100W receiver on Ebay for the same or less.

4A should be enough to 'pass' the 'continuous' test, which is to 'precondition' the amplifier at 1/3 power for 10 minutes and then test max power at onset of clipping, defined as less than 2% total distortion. Technically speaking 11.7 Watt is a smidgen over the 4 A 'continuous' rating and, of course, 35 Watt certainly is, but there's probably enough thermal mass to absorb it, assuming you have good case ventilation. Whether their heatsink can handle it is something I can't answer.

As for the "25V," 35 Watts into 8 ohms is 23.6 V peak so I'm not sure where they come up with 35 Watts on 35V rails. I suppose it 'might' need 11V bias headroom but that seems rather high so the 35 Watt 'might' be a heatsink limitation. More likely, I'm guessing since they 'spec' a 24V transformer, it might be denoting the 'unloaded', or at idle, power rail as 35V, which sounds about right for a transformer 'regulation factor' at 30V loaded, leaving 6V bias headroom.
I'm not sure what you mean by "two secondaries." If overdoing it with 10 Amps that's 48VCT 10 Amps or two 24V, each 10 Amps. Or two 48VCT, both 5 amps, which amounts to like 2 transformers.
Oh, I'm glad you mentioned cellphones. I don't know what they consider those things to be but the output is very small. They work fine on my 316mV inputs but I'm not sure 500mV would be enough. Might not seem like much but that's 4 dB.

There's a bit of 'voodoo' involved because people generally expect to hear things 'distort' at full CW rotation of the volume knob, so they 'know' they can reach full power, but if it serendipitously falls right at CW max they're never sure if they could have gotten 'more' if only the knob could go further. So, for a 'confidence factor' you need to do more than is technically 'necessary'. On the other hand, too much and the volume knob barely cracks at normal listening levels, which doesn't feel good either.
Like I said, a decent audio dual opamp and a few resistors is all you'd need for a 'volume control' preamp.

I meant two secondaries not center tapped like most toroidal transformers are made.

If they are expensive speakers, think of adding a fuse in both + and -power supply
leads or in series with R24 and R26.

For $20.00 they might have skimped on transistors.

They are a small pair of Olson speakers that are not expensive, but do sound good.

I could get one of those speaker protection circuit kits and use that.

Speaker protection circuit.

http://store.qkits.com/moreinfo.cfm?Product_ID=989

I still have to come up with a power transformer though.

I'll need a 25-0-25 volt 10 A transformer or a dual secondary 25 volt (not center tapped) 10 A transformer.

The power transformer is gonna be the hard one to find.

You can do it with one transformer, center-tapped:



You will need about 40-0-40 to give 35 V DC out, however.

A full wave rectifier yields about 0.9 x the AC rms.

http://www.hammondmfg.com/pdf/5c007.pdf

Rich

I had thought about that, but am going with a toroidal transformer which has two identical secondaries that can be put in series.

I was about to say no when I realized, oops, I made a boo-boo. The split rails will each do half the current so the 10 amps is 5 amps each and not 10 amps each as I had previously said. The same applies to the other current calculations.

That got me to rechecking and RMS is also a boo-boo because we're not looking for the 'equivalent heating power' but average current the power supply has to provide. For a pure sine RMS is .707 of peak but average is .637. It's not a huge difference, .901 less, but, well, might as well be correct.

This means their "more than 2 amp" spec is about right for including the 4 Ohm speaker load case.

So, for 35 Watts into 4 ohms we have 2.96 Arms total with each winding doing half for 1.48Arms and converting that to average we get 1.332 A. Depending on the 'rectifier factor', Hammond liking 1.613 and my source liking 1.8, you have a transformer AC current load of between 2.15 A and 2.4 A. Remember, that is heat loading and I don't care how much 'bass' you think there is the amplifier is not going to be running a 35 Watt tone continuously for hours upon hours so the fractional portion over 2 A is of no real consequence.

As for 'sag', we don't have a complex filter after the bridge so it's predominately transformer regulation factor and .85 is pretty common. So, at full load on the 24VAC transformer we should have about (24x1.25)-2(diode)=28V (Hammond's factor does not comport with what I've seen) and unloaded it will be up to 40V (24/.85 times 1.414 worst case [minus diode drop]). Split the difference for a swag estimate of 'nominal load' and it's around 35V. Also remember that worst case current load is with 4 Ohms where we only need 17V peak, plus driver bias, for 35 Watt.

I also ran a quick and dirty sim to see what the power transistors had to dissipate and with the 28V 'droop' it came to 20 Watt at 35 Watt out into 4 Ohms. Hold the supply firm at 35V and it increases to 30 Watt, which I'm not sure the heatsink can handle. But, then, if you're playing 'music' it won't ever be doing 35 Watt continuous.

Everyone seems to have their own 'rule of thumb' numbers but I wish they'd explain how they arrive at them. The ones I used came from ATC_Frost, Magnetics Division.

I've seen the Hammond numbers but their .9 factor is lower than what I usually see on a full wave bridge output and they also don't seem to care about diode drops. Or they're trying to 'include' it in the .9 factor but there'll be a huge difference depending on how large a voltage you're talking about. I mean, if you're making a 5 V supply then a couple volts of diode drop is a 'big deal' but it's virtually negligible on a 450V tube supply.

Since the manufacturer spec's a 48VCT transformer I'm convinced their "35 V" number is 'nominal', meaning what their 2 A transformer does at some nominal load significantly less than 'full power'.

Thanks for the info. Not sure what the heatsink can handle, but I may install a small 12 volt fan to provide cooling for the amps since this will more than likely be built in an enclosure. I figure one 3" fan and drilling holes in the cabinet directly under the heatsinks should work unless I get a cabinet that has vents in it then I may not need a fan.

Manufacturers generally spec a transformer output voltage at its rated current. At less than rated current, the output voltage will be higher. If you check a wall wart rated for 12 V, say at 500 mA, the no load output might be over 16 V.

You are correct that for low voltage supplies, it is critical to have enough AC voltage to allow for diode drops. Mostly, I tend to err on the high side and then use a 3-terminal regulator to get down to the actual voltage I need. I think Hammond does NOT include diode drops in their 0.9 factor:



Rich

I like how Antek does their transformer ratings. They list pretty much all the necessary specs for all operating conditions of the transformer.

So which transformer would be what I need?

25V 10A http://www.antekinc.com/details.php?p=98

25V 8A http://www.antekinc.com/details.php?p=78

25V 6A http://www.antekinc.com/details.php?p=689

25V 5A http://www.antekinc.com/details.php?p=53

One thing I noticed is that the higher current transformers seem to have better voltage regulation from no load to fully loaded.
Keep in mind I want minimal voltage sag at the rated output power of the amplifiers.

I guess I am a bit of a fanatic about stable power supplies.

I could go with the 24 volt version of those transformers if need be.

I suppose that I could use a center tapped power transformer and use a variac to set it at 25 Vac then use a bridge rectifier and two filter caps just to see what the unloaded dc voltage will be.

High current transformers use heavier wire for the windings, so the copper loss is reduced. Think of the copper loss as an internal resistor in series with your load. If the current goes up, the voltage drop across this "copper" resistor will cause your output voltage to sag.




I thought in your original post you wanted dual outputs at 35 VDC? None of these transformers will do that. You will get a little less than 25 V DC from a 25 V rms transformer.

Rich

From what I understand the dc voltage output will be the peak of the ac waveform so if I am correct 25 volts will get me right at 35 Vdc unloaded.

Yes.... unloaded a 25 v rms will give you 35 V, minus about 2 V for diode drop.

This will drop to about 23 volts DC under load. If you can live with that, then a 25-0-25 is a reasonable choice.

Power supplies are usually designed to give the desired voltage at the rated load.

Rich

Depends on the load and how much current the power transformer can supply.

For my Sylvania RM-300X radio I used a 22 volt 5A toroidal transformer to produce 30 Vdc and under load the voltage only drops to 28 Vdc, but that is only with about a 1 amp load.

That only holds for the unloaded case.

As I mentioned for the heating calculation, the capacitor is charged only during the AC peak but to do so at the absolute, 0mS duration, 'peak' would take infinite current, which is clearly impossible. Current is limited by the winding DCR (and AC line impedance) so it 'takes time' and the AC peak is 'chopped off' due to IR drop. This is one reason why the 'rule of thumb' factors vary because a low regulation factor implies higher losses so there will be more peak clipping during the charge period, resulting in a lower charge peak to the filter capacitor.

The amount of current needed when unloaded is just leakage and is so small that it charges up to essentially peak.

Yes. By "manufacturer" I meant the amp kit manufacturer and was refering to their stating "35 V" on the power rails. I don't think it's meant to imply you 'need' 35 V at 35 Watt to get 35 Watt. I think they're giving the builder a number that represents something akin to what they 'will see' when it's powered up if you use the suggested 48VCT 2 A transformer.
Ah, yes. The 'note'.
Actually, we hadn't gotten around to considering AC line tolerances. I was just calculating current loading.