looking for a lab/bench power supply for amplifier testing

[tomchr]

A good lab supply is well worth the cost, though they are expensive. I enjoy using my HP 6643A supplies. They're programmable (0-35V, 0-6A) 210 W supplies. They can be configured in series or parallel for higher output voltage and current, respectively. The HP 665x-series is the 500 W version.

Whatever you do, stay away from the el-cheapo supplies on eBay/Amazon/Alibaba/whatever. I made the mistake of buying a pair thinking I could save a buck before buying the HP supplies. FAIL! The model I bought used a discrete R-2R DAC and simple micro controller for controlling the output voltage. The bandwidth of that control loop was well below audio frequencies, so for 1 kHz testing, the supply struggled to regulate and put out several volt of bounce. It was pathetic. Back to Amazon it went.

A variac + isolation transformer can work. Get a good variac as some of them have rather high wiper resistance.

An alternative - if you only have a few voltages to perform your tests at - is to have a power supply built up and a set of power transformers that you connect it to.

Tom

 

[cakyol]

I have a 3% regulation 1000VA toroidal at 120/240 volts primary and 2 x 50 volts secondary. I need a dual symmetric dc supply so I MUST have the variac FIRST in the circuit so I can use the dual output of the toroidal transformer to be able to get dual output.

I am planning on getting a US made Staco 10 amp 0 - 140V variac (120V input).
I will connect the variac output (140V AC max) to the toroidal input. I know I will only

So the max swing of the toroidal output will be 140 / 120 x 50 = 58 volt AC.

This would give me dual symmetric dc output at 82 volts per rail.

I have all the components except just the variac. And I found that on ebay Staco model 1010.

 

[SyncTronX]

Am I correct in stating that (if I recall some have posted here) that we can use two step down transformers back to back?

For example:

120VAC to 24VAC [coupled to] 24VAC to 120VAC.

Can we use each leg with a .1uf X2/X3 or 1uf X2/X3 rated caps to ground? Also a mains fuse. Does it matter where the mains fuse is? That is before the first transformer on the main line. Between the transformer nodes maybe smaller secondary fuses?

Then I found Mr. Carlsons Lab YouTube: LINK

I am asking because I don't know.

Cheers,

 

[rayma]

we can use two step down transformers back to back?

Can we use each leg with a .1uf X2/X3 or 1uf X2/X3 rated caps to
ground? Also a mains fuse. Does it matter where the mains fuse is?

Yes, back to back transformers are ok. Caps are ok.
Fuse should be in the hot AC line (black for US).

 

[cakyol]

One piece of bad news.. I have been informed by the manufacturer of my toroidal txformer that it will sustain the power transfer properly as long as the input voltage is no less than about 75% its rated value. In other words, he says that for a 120 volt primary, the transformer will work ok up to about 90 volts AC. He mentioned that if the primary drops below that, even tho the voltage ratio is maintained at the secondary, unfortunately the secondary will start to collapse once current starts being drawn from it.

This poses a problem when I want to use this power supply generating dc of about 54 volts (ac about 38 volts).

So it means this supply will be useful only between about 54 - 80+ volts DC. And it may have the negative property where, if a load is connected, the output will not linearly respond to the variac dial but will jump up suddenly when the 90 volts is reached.

So, does anyone know of a "more linear" transformer type which maintains its power transfer function a bit more linearly, ie can sustain the VI relationship even with low primary voltages ?

Does a standard "isolation transformer" deal better with this issue ?

I guess it probably comes down to the generated magnetic flux in the transformer given the primary voltage. And it seems like the flux collapses below a certain primary. It is as if a material type is needed which can be magnetised to full capacity very quickly with very little current and does not get saturated even at very hi levels of current.
Magnetic unobtanium maybe ?

Thanks

 

[FauxFrench]

Honestly, it sounds strange.

Evidently, the output power of a transformer reduces with reduced primary voltage simply because the output voltage as well is reduced.
Could it be that the core magnetic remanence is so high that below a certain primary induced field the magnetic flux in the core is simply not coupled to the secondary? "Rayma" is normally very good with transformers, and "Elvee".

Did you speak to a technician or a salesman?

 

[cakyol]

FauxFrench, I understand that the ouput voltage is reduced. That is actually what I want. I want the transformer to maintain the voltage relationship based on its turns ratio. But also, I want to be able to draw some reasonable current from it when operated at lower primary voltages. It seems like the relationship completely breaks down at primary voltages at less than 75% of the rated input. Basically I am looking for a dual ouput txformer which can properly operate with 10% of the primary voltage.

 

[FauxFrench]

I understand your intentions and my argument with the reduced voltage/power transfer relate to if this may be what a sales-person has misunderstood. And, like you, I would expect the input/output relationship to remain much further down in voltage. If it was a person you spoke to who knows his transformers well (a technician) we must trust his words. If it was a salesman....?
The only reason I can logically imagine for a power collapse below this threshold is if the magnetic flux is not conducted well due to remanence. If so, I guess most transformer manufacturers use more or less the same core material. Personally I have not tried my net-transformers much below nominal primary voltage.

 

[amplidude]

Psu

Schematics for a nice dual power supply of let's say +/- 60 volts10 amps and current limiter would be most welcome, a psu with the high voltage/high current for amp outputs and some +/-5 to 40 for driver side volts for.. an "all in one" audio tester psu.. inputs & outputs are most welcome..

 

[cakyol]

Here is what I have done:

- Used a toroid 110/220 -> 2x50 V at the input with an inrush current limiter.
- The above satisfied the requirement of mains isolation.
- Fed the outputs of the toroid (2 x 50V) into this beast I bought off ebay: POWERSTAT 116CU-40-2 DUAL VARIAC 40V 25AMP | eBay

- It was perfect that the variac was dual gang (so I can make a SYMMETRIC supply) and that it was 0 - 40 volts (fed by 0 - 50Volts) which it can easily handle.
- Feed the symmetric outputs of the variac into a rectifier & smoothing caps.

And voila, I have 0 to + and - 75 volts DC symmetric supply. I already had the toroid (1000VA) and the rectifier and the smoothing caps. So, it only cost me the variac an a box. I obviously have to add 2 voltmeters & 2 ammeters as accessories.

 

[amplidude]

I to use that solution, and power resistors to limit current in initial power up repair etc. But would be nice with a solid state psu, I actually have a psu that does 60v 10amp, (delta elektronika) but it's single supply and way to expensive to buy one more.