Author Topic: AD-3208 Reconstruction filter headaches  (Read 650 times)

Kirby

AD-3208 Reconstruction filter headaches
« on: February 27, 2020, 08:41:30 AM »
Hi everyone!
Just rebuilding the GGG AD-3208 Analog delay on a bread board but with V3205 BBD's and and my signal from the seems die as it enters the reconstruction filter..
I'm at a bit loss why this happens as I've built other analog delays and haven't had a issue with the LPF's.
If i route the output of the second BBD straight into the compander it gives nice delays and repeats.
As soon as it passes into the fist stage of the reconstruction filter it kills the signal to the point that it will only give a really poor degraded single repeat.
I have rebuilt this part of the circuit several times in case I've made a mistake and checked all the components and every thing appears correct. If i remove the 2n5088 transistor all together it gives about the same results. Has anyone else experienced a similar issue with this delay. I'm using a SA751 as a compander with 2 x MN3205's with MN3102 clock.
Please help I think i'm starting to go a bit stir crazy trying to understand why it wont pass though the last LPF before the expandor!

j_flanders

Re: AD-3208 Reconstruction filter headaches
« Reply #1 on: February 27, 2020, 10:41:45 AM »
So, your problem is located here: ?

There's no output at the emitter of the first transistor?
What about the points right before the transistor, after each of those 10k resistors?
Transistor and pin out correctly oriented?
What about the second transistor? If you lift the left leg of the very first 10k resistor and go from bbd output to the second part of that filter, the 10k resistor after the first transistor?

If any of those caps are faulty and/or there's direct contact to ground it's like turning a volume pot to 0.

« Last Edit: February 27, 2020, 10:51:26 AM by j_flanders »

Kirby

Re: AD-3208 Reconstruction filter headaches
« Reply #2 on: February 29, 2020, 05:29:00 PM »
Thank you for the reply j_flanders :)
That is indeed the section that is/was causing trouble.Everything seemed to be wired as it should and checking all the components values and transistors showed no issues.
Testing it all on the scope with a 400hz sine wave, i found the output from the BBDs was around 3 volts and cleaned the signal up when passing through the first 10k/2nf juction.signal dropped to around 2.6v at the 10k/33nf junction (ok not so bad) by the time it hits the Emitter junction the signal is cut down to about 1.8v. If i took out the transistor completely it was pretty much the same signal.If i measured all the way to the end of the output of the second transistor it came out at about 950mv. I noticed when connecting it into the the expandor it loaded the signal all the way down to 200mv and distorted!
What is strange is when the whole circuit is fired up the signal at the emitters of the transistors shows full signal and slowly degrades down to 950mv. Is there something else on my bread board that could be dragging it all down when the transistors are connected? Couldn’t really find any.
I did go to the effort to take sound samples and scope pictures of the problem. I wasn’t able to log into the site for a few days as it seemed to be down or hacked?? So in the mean time i decided to build a op-amp filter instead and the result was amazingly good! Cleaned the the signal up beautifully and no loading to the expandor and much louder. So much signal that i have pad it back down before the summing amp and increase the repeats feedback resistor back to 100k. So although i was not able to sold the mystery of the transistors what i did use worked alot better after all.  :D
I can still post the scope and sound samples if you wish to see them.
In the mean time! Thanks for your response.

Rob Strand

Re: AD-3208 Reconstruction filter headaches
« Reply #3 on: February 29, 2020, 06:07:35 PM »
Check your DC voltages throughout that part of the circuit.
Plopping around the pot since an early age.