New API 525 build thread.

[whomper]

Hi,

Started my build and am in the process of collecting needed parts.

A quick question on part replacement; Can I replace the BC550C with BC547C or BC560C with BC556C? Can I use the BCxxxB instead of the BCxxxC?

 

[peter purpose]

No, no and no. Use the parts listed.

 

[noisywill]

Folks,

Got hold of most of the bits....just need the time now!

Quick couple of questions.....

Are 1/8w CF resistors ok for the switch board?  Difficult to find MF ones the correct physical size... I'm guessing yes, but just wanted to check before I fired in the component order.

Also, anyone have a source for 1M PRV6s?

Ta,

Will

 

[peter purpose]

1M prv6 are as rare as rocking horse poo.... use a trimmer.
The 1/8W resistors will be fine.

 

[noisywill]

Ta Peter,

Thought that was the case about prv6s.

Cheers for the info/answering of idiot questions!

Will

 

[whomper]

Hi,

Have a couple:
1. As I have two boards, the link thing eludes me. Have read somewhere that one should use a 120 ohm resistor, so one unit with it and the other with just a link? Also, does one need to switch both modules to stereo for this?
2. Have used the classicapi trafos. Any specifics on wiring?

 

[peter purpose]

Link info....

Total of 120R and both switched.

API 2623 info. Clearly marked on pcb.

 

[peter purpose]

Set up info....

 

[whomper]

Hi,

In the last mile of building my two units. Am stuck with finding the 1m trimmer.
Any mouser or somewhere else alternative that one can recommend?

 

[peter purpose]

Any Bourns 3310 type should do you.

 

[whomper]

Just a quick note on build progress. While not completely done (need to source the bourns trimmers, have used alpha pots for now), have undergone calibration of the two units and they sound superb!

Great project and sound quality of the final product. Well done.

Looking forward for the EQN...

(This 500 format is addicting  )

 

[noisywill]

These might do...

http://www.ebay.co.uk/itm/Panel-Mount-Sealed-Potentiometer-0-5W-Screwdriver-Adjust-Pot-Various-Values-/120805820485?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item1c20968845


Cheap enough if they don't.

Will

 

[901]

I finally found the mistake I made with mine. I was tricked by the wiring of the transformers. The transformer in the sidechain has labels on the board saying "- + -_+". I assumed that one of the secondary coils should go to the first "+ -" pair and the other one to the second. While this is true for the output transformer, the sidechain transformer has its middle ground point on the side. The extra "_" actually means ground.

Maybe others have overlooked this, too. The unit actually kind of works, but not really good.

And I had the opportunity to test mine side by side with a production API 525. It behaved quite differently. I figured this is because of the extra ATK pot. And indeed, now that I shorted the pot altogether, it´s really close to the API.

 

[haima]

Thanks for the report on your build 901. I need to get onto making mine one day soon!

now that I shorted the pot altogether, it´s really close to the API.

I wonder how it would work if you change the attack pot to a variable resistor, placed in series with the sidechain CV, before C3 - that way at fully counterclockwise you would effectively have the stock API circuit where the sidechain CV hits C3/R5 directly.

According to Peter's schem, C3 is wired to the wiper of the attack pot in a way that there's always an extra 1M of resistance in series with the sidechain signal compared to the stock API design - no matter how you set the attack pot.

Perhaps the series resistance option has been tried and doesn't work?

 

[901]

I wonder how it would work if you change the attack pot to a variable resistor, placed in series with the sidechain CV, before C3 -

Do you mean like this?

 

[haima]

Yes, that's what I was thinking...

 

[emrr]

the attack still looks whack.  See here in the old thread:

http://www.groupdiy.com/index.php?topic=22699.msg474572#msg474572

 

[haima]

Thanks EMRR - there's lots of good info in the previous version thread!

But, isn't dmp's schematic (below), the same as what 901 has drawn above?

 

[haima]

Hmmm, I must be missing something...  ??? quite likely! ;D

To me it looks like both images describe the same circuit:

- sidechain CV enters the scene and connects to one side of a 1M variable resistor
- the other end of the 1M variable resistor connects to the junction of C3 and R5.
- C3 (0.1uF) terminates to circuit common/ground.
- R5 (30k) takes the CV on home to the gain reduction FET gate.

Where am I going wrong here? Thanks for the help!

 

[901]

No, the R's are in different order, and affect the time constant differently.

If the arrow in dmp's picture is the gate of the FET then it´s only drawn right to left instead of left to right.