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bikejog 19th January 2004 12:50 AM

Hafler DH-220 Transformer
I brought som NOS hafler DH-220 drivers boards and I'm looking for information on the voltage output of the power transformer. I've got conflicting information from Some say it's 42V ct. and others say it's 45V ct. Any body knows for sure?
The troubleshooting section in the schematic lists a max DC voltage of 65V DC so that puts it more towards 45V ct catagory. I just wanted to be sure.


Duo 19th January 2004 02:02 AM

In this case I doubt a 3V+/- difference will cause a problem with the driver board.

Many amplifiers will work fine over a wide range of power supply voltages.

EDIT: Just don't go too far over or you'll fry it, and too far down will sound bad. Too far as in, more than 10V over if you're not sure.

GRollins 19th January 2004 02:31 AM

As Duo says, I don't think you're going to have a problem either way. As I recall, the DC voltage going to the boards is about +-60V or so. You'll drop a volt or so going through the rectifier, and the bias will draw the voltage down another whisker. I'd say 45-0-45, i.e. 90VCT.
(A 45V center tap transformer would give you 22.5-0-22.5 and yield rails on the order of 30V or so. Not necessarily what you want, unless you intend to rebias for class A...)


bikejog 19th January 2004 03:13 AM

Sorry. I meant 90V ct. I have a spare 80V ct laying around. Can I use that to test the driver board? I mean will it cause damage?
Another question is I see that there's a small by-pass cap (C-401) at the gate-source pins of one of the N-Channel Fets (only one), but nothing on the other parallel FET and nothing on the two P-Channel Fets. Question is what's that cap for and how do I know which FET to connect this cap to.

GRollins 20th January 2004 01:39 AM

I'm at work and don't have any Hafler schematics with me. My computer situation at home still isn't back to normal. Soon, I hope.
Offhand, I don't see that an 80VCT transformer would do any harm.


SY 20th January 2004 02:17 AM

Go for the lower voltage. At worst, you'll lose a couple of watts, pretty insignificant.

I don't have the schematic, but from your description, that cap was used in an attempt to make up for the non-complementary nature of the MOSFETs used in this circuit (Early Hitachis, IIRC). My guess would be that the cap is something like 200pF or so?

bikejog 20th January 2004 12:53 PM

The cap is 680pF. It's on only one of the N-Channel Fets. Here's the link to the schematic.

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