I too have had some issues w/DC offset. One channel is steady...I am running 1.2V bias and <5mv offset. The other channel I've never been able to get below 50mv offset. Bias holds steady but offset swings wildly with adjust of the pot. Also, this channel's offset is very temp sensitive. Even holding my hand on the heatsink effects DC offset. Inputs were shorted when testing.
Jfets are matched BL grade...mosfets are unmatched fairchilds. I've read some other people have had this problem. I don't know if it's the mosfet poor match or what. Sounds fine though but I've been thinking about getting a set of toshiba mosfets to try.
P3 adjustment is DRAMATIC. I've got mine set up now at equal measurements across R3 and R4...9ohms I think. But I've tried a few different things. Basically I've found that adjusting P3 changes the width and height of the soundstage and focus. One direction or turn of the pots makes for a wide, spread out soundstage which places the focus farther back. Other direction makes the soundstage tall and narrow with focus more forward. If I get some time I am going to try out some software-based scope testing to really dial it in. Right now I am just guessing.
Jfets are matched BL grade...mosfets are unmatched fairchilds. I've read some other people have had this problem. I don't know if it's the mosfet poor match or what. Sounds fine though but I've been thinking about getting a set of toshiba mosfets to try.
P3 adjustment is DRAMATIC. I've got mine set up now at equal measurements across R3 and R4...9ohms I think. But I've tried a few different things. Basically I've found that adjusting P3 changes the width and height of the soundstage and focus. One direction or turn of the pots makes for a wide, spread out soundstage which places the focus farther back. Other direction makes the soundstage tall and narrow with focus more forward. If I get some time I am going to try out some software-based scope testing to really dial it in. Right now I am just guessing.
I made an adjustment in the wiring and the bias and off-set settled. When I plugged it in to a small amp and speakers, I only hear 60hz hum.
Problem could be a few things:
-Volume pot wiring issue. I have a P.E.C. pot to throw in to see if it helps.
-Signal ground going direct to earth ground without NTC thermistors.
-Transformer shield in the wrong place. Wired it to earth-ground without checking docs.
-Unshielded thin cables too close to power source. When I touched the wires at star-ground point, the hum got better and worse by wiggling them.
-Insufficient voltage to circuit. I saw it was only 15v+/- at the PS output. Not sure why since it read 24+/- before wiring it to the circuit.
Hikari1- That's interesting about P3 adjustment and sound stage changes. I have the bits and pieces to make a distortion analyzer, but should probably get the preamp working first!
Anyone else has any ideas where to look, I'd like to hear it.
Vince
Problem could be a few things:
-Volume pot wiring issue. I have a P.E.C. pot to throw in to see if it helps.
-Signal ground going direct to earth ground without NTC thermistors.
-Transformer shield in the wrong place. Wired it to earth-ground without checking docs.
-Unshielded thin cables too close to power source. When I touched the wires at star-ground point, the hum got better and worse by wiggling them.
-Insufficient voltage to circuit. I saw it was only 15v+/- at the PS output. Not sure why since it read 24+/- before wiring it to the circuit.
Hikari1- That's interesting about P3 adjustment and sound stage changes. I have the bits and pieces to make a distortion analyzer, but should probably get the preamp working first!
Anyone else has any ideas where to look, I'd like to hear it.
Vince
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first solve all what you wrote
That's a good hint, because it would be easy to just try one thing.
I do have another question. I sent all ground points ( Ins, Outs, circuit, volume control) directly to the power supply ground as a "star-ground".
Is there a benefit of just making a bus ground for all Input grounds and the same for Output grounds? It's definitely simpler and a more clean wiring scheme.
thx.
Vince
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Yes, solved it. It was a grounding issue in both channels, so the problem was exactly the same on both sides.
When adjusting for bias and offset, it settles quickly but it is sensitive to small turns of the pots. Got bias nailed at 1v and offset to ~.03v DC.
Sound? Can't tell just yet, but in a few minutes it lullabied me to sleep.
It's very smooth and is full bodied. Very relaxed sound. At the moment, it appears to be to rolled off but I have a tendency to like a sharper sound. I don't want to judge it just yet. Would have to experiment with P3 as well.
Also have a new DAC that's only 4 days old running.
Wouldn't be fair. One thing I will say is it drives the L'amp CCS amp better than the B1.
Best thing I did was install 6 inputs and two outputs! Last two preamps i made had only two inputs and one output set.
Thanks NP!
Vince
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Here's a slightly better photo of the inside.
One other thing. I picked up a new line filter.
It's just a matter of plugging it in.
Debating whether or not to use it.
Was also going to try a x2 cap on the switch.
Definitely going to let it burn-in for a bit of time before making adjustments.
Interesting enough, the 20th anniversary first Zen article was mentioned in another thread.
That same issue of Audio Amateur featured an article about transformer tuning. Just involved placing a single specific sized cap across the bridge.
Any thoughts on line filtering?
Vince
One other thing. I picked up a new line filter.
It's just a matter of plugging it in.
Debating whether or not to use it.
Was also going to try a x2 cap on the switch.
Definitely going to let it burn-in for a bit of time before making adjustments.
Interesting enough, the 20th anniversary first Zen article was mentioned in another thread.
That same issue of Audio Amateur featured an article about transformer tuning. Just involved placing a single specific sized cap across the bridge.
Any thoughts on line filtering?
Vince
Attachments
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http://www.diyaudio.com/forums/powe...sformer-snubber-using-quasimodo-test-jig.html
what counts is resistor (too) , in fact - not cap alone
what counts is resistor (too) , in fact - not cap alone