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-   -   Aleph 5 High DC Offset (http://www.diyaudio.com/forums/pass-labs/148086-aleph-5-high-dc-offset.html)

john65b 26th July 2009 06:05 PM

Aleph 5 High DC Offset
 
Hey all,

Just finally finished my Aleph 5 and it sounds very nice so far. I have an issue with DC Offset at 3.7V on both channels. I have checked it with and without preamp connected, and consistent at 3.7V. I have not tried shorting the input and reading the DC Offset..

I measure across the R14 (400 ohm) and get 4.75V, so this is OK...bias is at .666V around .91 ohm source resistors (didn't have the 1 ohm), so this kinda checks out a little high at .733A. I am drawing 4.65A per rail, 34.6VDC rails on single power supply.

Trannie is a Piltron 600VA 29-0-29V single secondary toroidal, single 25A 400V rectifier, 56kuf - .215ohm - 56kuf power supply..

I am using Brian's boards, and had followed the Aleph 5 schematic, but the front end is an Aleph 60 type, with the higher input impedance. R21 is 400 ohm instead of 453 ohm... R19 is 220K...

Like I said, came up on the variac nice and smooth, no hiss or hum, and does not seem to be overheating or having any other issues.

Should I throw a pot into R14 and trim the DC offset down, or check something else? Could it be the AC Gain resistor R21 at 400 instead of 453?

Thanks in advance for the help.

Bill Fuss 27th July 2009 12:27 AM

I built pretty much the same amp and it works perfectly, less than 10mv offset, so it will work fine. Put a 500R multiturn 1/2W pot in for R11 to adjust the offset, but set it at 220 before you turn it on.

Are you sure you have a 29-0-29 trafo, you should have 40V rails with that.

Best, Bill

john65b 27th July 2009 02:02 AM

Hello Bill - good to hear from you.

The trannie is a 29-0-29, but I have a CRC filter (56kuf/R215/56kuf) - I ran it on PSU designer and came out right at 35VDC rails loaded at 4 A...

R11 on the Brian Bell board is R10 on the Aleph 5 schematic - they are 221ohm. Is this the placement of the trimpot you are referring to?

Isn't R14 (R18 on BB board) the DC offset resistor?

Bill Fuss 27th July 2009 02:09 AM

I have the A5 schematic, it is R11. Feeds both fets.

john65b 27th July 2009 02:29 AM

R11 on the Aleph 5 schematic is R8 on the BB board (confusing that they are not same numbers)

I thught R8 (R11) was the DC Offset resistor, but then read below from a post:

"On brians Pcb R12 is AC current gain, R13 Bias, R18 Dc offset.
I have one channel up and running, with standard values I get 20mv Dc on output, 4.58v across Q4 and 5, 0.588v across R35-46, Q1-2 are @ 60 C and my heatsinks are 24 C above ambient,
So yes the parts list that I posted works and I did not fit the A5 C6 that has no location (please don't mix this up with the C6 that is on brians pcb that is for a zenner bypass cap)"

Anyway, looks like the poster got R18 confused with R8 maybe?...

Bill Fuss 27th July 2009 02:41 AM

Sorry for the curt reply before, I was in the middle of something. What is R18 on the board in relation to the A5 schematic. I dont have the board schematic handy so lets stick with the A5 schematic.

john65b 27th July 2009 02:45 AM

2 Attachment(s)
Not a problem...R18 on BB board is at the bottom of the A5 schematic - shown as R14.

It is between the Q1 drain to negative rail, and is 392 ohm.

I found the spreadsheet that that poster posted (wow, that was complicated)

It is attached...

Bill Fuss 27th July 2009 03:22 AM

Well, if you're down under 20mv now you are home free.

Kudos to NP for these single ended wonders, A5 and AJ.

Have fun John.:cool:

john65b 27th July 2009 11:51 AM

Sorry Bill, don't understand you last post. I am still at 3.7V DC offset, not at 20mv.

That attachment I pulled seems incorrect.

I will check that R11, and if 220 ohms checks out, I will put the trip pot in and dial it down.

I will reply with my results as soon as I am able.

Thanks

Bill Fuss 27th July 2009 12:24 PM

It was me that was confused, didn't realize the 20mv was part of the quote.


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