BA283 Debugging!

dalmaproductions · Jul 11, 2011

Hi guys, I'm completely stuck on this, have tried almost everything. I'm using one of Igor's BA283 neve output boards. Transformers are Carnhill VTB9049 (LO1166) and Carnhill VTB9046 (31267). I have nothing connected to the board insert. Schematic:

http://i251.photobucket.com/albums/gg291/diy33609/ba283_mkup_sch.png

What I get at the out is a huge high end roll off. Its completely cut off around 600~700 Hz. If I inject a signal at J13-J14 after input transformer the problem manifests. If I inject a signal at the output transformer primary, I still get the problem, BUT, as soon as I disconnect the output transformers primaries from circuit, I can inject a signal to the output transformer and final output is perfect.

I've changed the bc184's, changed all electrolytic caps, reflowed all solder joints. Tested the output trafo in different circuit, works fine. Any Ideas?

Thanks!

tommypiper · Jul 11, 2011

That's a weird looking schematic... why not go by one of the original and proven Neve schematics?

Do you get the problem injecting tone at ouput transformer primary when power to the circuit is off? (but transformer is wired into circuit)

Have you checked with a scope for oscillations?

Are you using a choke?

Show us pictures of your build... maybe we'll see something.

Have you asked Igor?

Is that his schematic?

dalmaproductions · Jul 11, 2011

Hi! Yes thats igors schematic. I do find it hard to read, specially with my limited electronics knowledge.

I think you asked an important question: "Do you get the problem injecting tone at ouput transformer primary when power to the circuit is off? (but transformer is wired into circuit)"

- No, the problem goes away when power is off. The transformer passes signal normally in this case. I have listened to this, as I power off, the full bandwidth slowly creeps through and begins to sound normal after the unit powers down completely.

No choke. Haven't check for oscillations. I haven't asked igor because he doesn't answer lately due to being extremely busy.

tommypiper · Jul 13, 2011

OK. well.... First, have you checked your PS voltage? Make sure it's clean 24 v DC. Second, have you checked your grounding? Probe around within the circuit with your meter continuity setting and make sure grounds are connected throughout the circuit;, and then turn on your power and test for DC, make sure you're getting clean 24v DC where it's supposed to go.

Then, it's a question of checking your signal as it passes through the circuit, front to back. Find out where it loses its HF. That will reveal the problem.

If you don't have a scope use some other method, like a probe connected to an amp so you can hear the signal at different stages. (Be sure to use sacrficial speakers or headphones, in case they get damaged by unexpected noise. I use old recycled car speakers and have blown one of them this way.)

It sounds like your output transformer is fine. You have a problem in the active circuit.

dalmaproductions · Jul 13, 2011

Ok just about solved. Now I need someone to explain what the hell this means 8)

I followed the signal through the board, turns out the signal loses all the high end right after R"1. Before R"1, its fine.

I measured R"1, ohm value is correct and no bad contacts.

Then I decided to take C2" out of circuit, and voila, perfectly flat frequency response at the output. What does this mean? And what is this cap supposed to be doing? I think I have the right value but i will double check, could be wrong.

dmp · Jul 13, 2011

Was C2" = 220pF?
That is what it is supposed to be.
If you have 220uF, it will filter out all the highs on the transistor output.

0dbfs · Jul 13, 2011

Looking at this original schematic:
283-SCHEMATIC

Shows C1 to be 220pF and R1 to be 2k2.

C1 is a negative feedback path and helps prevent high frequency oscillations in that gain stage. If you have a lower value cap it will probably roll off in the highs in the audio band..

-jb

dmp · Jul 13, 2011

lower value cap

Wrong (I think), the lower the value cap the higher the frequencies that roll off. A higher value cap would cause a roll off in the audio band.

0dbfs · Jul 13, 2011

Well, I meant lower as in larger-value (less-zeroes after the decimal)...

Oops!

Like you said in your previous post... The value should be smaller to make the filter-frequency higher.

Hope that clears it up!

Cheers,
jb

Igor · Aug 3, 2011

Hi people, please check this for "debugging" as well))
http://www.groupdiy.com/index.php?topic=43413.msg564079#msg564079

BA283 Debugging!

Help Support GroupDIY Audio Forum:

dalmaproductions

Well-known member

tommypiper

Well-known member

dalmaproductions

Well-known member

tommypiper

Well-known member

dalmaproductions

Well-known member

dmp

Well-known member

Attachments

0dbfs

Well-known member

dmp

Well-known member

0dbfs

Well-known member

Igor

Well-known member

Similar threads

Latest posts