advice regarding PSU

[spendorspain]

Hi all


I’ll probably buy one of the “entry level” Borbely Headphone Amplifiers (the solid-state EB-602/210), but if I can use it with a regulated power supply that I already have, because I think it is a high quality one and to avoid the high cost of the Borberly super PSUs. The 210 HP amp is very hungry, specially if used with low impedance cans. According Mr. Borbely, it needs a regulated supply that outputs +15/0/-15v at 160mA or +24V/0/-24v at 100mA each. It is also recommended to feed it from separate supplies (dual mono supply for a dual mono amp).

The power supply I already have is a White Noise Cascode PSU (PSUCAS), a kit from a company that closed a couple of years ago, so I can’t ask for advice to them. I’ve also asked Mr. Borberly about its use with the EB-210 HP amp, but as he’s unfamiliar with this PSU he couldn’t give me any specific recommendation. I hope somebody in the PSU forum at DIYAudio can help me.

The designer of my PSU said in the construction manual: “The PSUCAS uses adjustable regulators (LM1086) for maximum performance, the ripple rejection has been improved still further, and the output noise has been lowered. There are now two completely independent, floating regulated outputs, which can be used as such, or series connected to give a split rail power supply. The PSUCAS low current (< 1A) regulated power supply uses innovative circuitry to provide superb performance without any of the drawbacks inherent in conventional regulated power supplies. The PSUCAS is normally configured to give jumper selectable outputs of either 15V (shorting jumper installed) or 24Vdc (no jumper), but the output can be set to any figure from 3Vdc to 35Vdc by altering the value of two resistors (…) If your application will draw more than about 100mA from the cascode power supply at 15Vdc, or more than about 200mA at 24Vdc, then you will need to fit larger heatsinks than those supplied with the kit, to prevent the regulators from entering thermal shutdown. Alternatively if you only need 15V dc you can use an 18Vac rather than a 25Vac transformer. Similar considerations of heatsink size apply if you use a transformer with a higher VA rating than that supplied to obtain more output current. With sufficiently generous heatsinks (5ºC/W) on the regulators, the 30VA transformer supplied is capable of sourcing 350mA at either 15Vdc or 24Vdc”.

The PSUCAS I have uses a 30VA toroidal transformer with 230Vac primary and two 25Vac@0.6A secondaries, fast soft recovery rectifier diodes SBYV27-100 and I’ve fitted two Nichicon Gold Tune 4700uF/35V caps, bypassed with 4.7uF and 0.1uF film caps, and Elna cerafine for the rest of capacitors. According the construction manual, this PSU outputs around 100mA on each rail at 15Vdc, or 200mA on each rail at 24V dc, with the transformer and the small (very small) heatsinks supplied.
I think I could use my PSUCAS to power the EB-210 HP amp but using a bigger transformer and only if I could mount the regulators on big heatsinks. As the space on PCB is very limited (see image attached) I’ll need to mount them off-board, with its three pins connected to the PCB with short isolated wires soldered to the IC legs. I’ve seen this in some equipment (Meridian bolted to the chassis a 7805 regulator in its 206 CD player, connecting it to the PCB pads with a several cm 3-conductor cable). This way, I could use the recommended 5º (or less) C/W heatsink, mounting each pair of isolated regulators on one, for example at each of the sides of PCB. Do you think this small amount of extra wire could impair the performance of the regulators?

If all the above is right and the PSUCAS I have could supply the 100(24V) / 160mA(15V) needed, I think I have two options here.

Option 1
I can use my PSUCAS as one split rail power supply to give +15V/0V/-15V (shorting jumper installed and 50VA transformer with 2 x 18Vac secondaries) or +24V/0/-24V (no jumpers and 50VA, 2x 25Vac transformer). Its two halves must be connected in series, with the two terminals in the 0V block on the PCB joined together with a short length of wire. Then the +V DC terminal of one half is the positive output, the -V DC terminal on the other half is the negative output and the 0V output can be taken from either of the 0V terminals. With 24V and 100mA the power available at amplifier output is 135mW (50 ohms) and 87mW (32 ohms), but with 15V and 160mA the power into low-impedance loads increases to 300mW (50 Ohm) and 219mW (32 Ohm). My Grado HP-2 are low impedance headphones, with a nearly flat impedance of 40 ohms, so perhaps I should operate the amplifier at ±15V with 160mA.
However, if I operate it at 24V/100mA, the demand the amp puts on the PSUCAS transformer and regulators is lessened and they will run cooler (no need for big heatsinks), but then perhaps the EB-210 HP will be less comfortable driving the 40-ohms Grado HP-2 and with reduced sound quality.

Option 2
I mention this because Borberly recommends to feed its HP amp from separate supplies (dual mono supply) and I have one spare dual rail splitter PCB and parts, yet unpopulated (from a WNA MkIII HP amplifier). It is possible that it would be better to configure my PSUCAS as two independent single 30Vdc outputs (using the appopriate resistors) and transform these in two independent +15/0/-15Vdc (one per channel) with the added splitters. Thus, the power supply for the EB-602/210 HP amplifier would be dual mono (I could even buy one 30VA transformer for each channel). According the rail splitter building instructions, when feed from the 24Vdc 500mA switched-mode PSU supplied with the kit, the rail splitter outputs +12/0/-12Vdc at 250mA, so I think they could be used with the EB-210.
However, the PSUCAS manual says “the raw DC inputs to the cascode regulator (1.4 x ac outputs from transformer) should be at least 7Vdc greater than the highest regulated output required”. With transformer secondaries of 25Vac, there would be only 35Vdc (25Vac x 1.4) at voltage regulator inputs and the regulated output to the rail splitters would be only 28Vdc (35v – 7v). Therefore, the dc supplied to the EB-602/210 would be, in theory, just +14V/0/-14Vdc per channel (not 15Vdc) and perhaps this one less volt would be a problem (I hope not).


Would this WNA PSUCAS (in one of the configurations described above) be suitable for powering the EB-602/210 HP amplifier I’d like to buy? Is it preferable to have two independent +14/0/-14Vdc (not 15Vdc) supplies for each channel or only a single +15/0/-15Vdc connected in parallel to both channels? Or is it perhaps sufficient to use the single +24/0/-24V@100mA option (the approx. 130mW at 24V/100mA are probably already deafening). What do you think?

Thanks in advance for your help.

Jose

[spendorspain]

This is the schematic for the option 1 (one dual rail PSU)

[spendorspain]

And this is the schematic for the option 2 (two single rail converted to two dual rail by a stereo rail splitter)

Thanks again for your help