LA-2A blend control (wet/dry)

Hello all...

A friend has asked me if it would be possible to add a blend control to an LA-2A clone for him. He wants the ability to do parallel compression in one box (similar to the wet/dry control on the Chandler germanium comp).

It seems like a reasonable mod to me. Has anyone out there attempted something like this?

For reference, the schematic can be found here: http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_1968.jpg

As always, your comments are greatly appreciated!!

search crush and blend and you will find what you seek... if you really wanna be slick you'll build it as a stand alone so it will work on any compressor he has...

Thanks. I've looked at the crush and blend circuit - it's great design, but I'm looking for something that would be internal and wouldn't require an additional power supply.

+10 on building it as a stand alone box.  I can see // comp  being more useful on super-crush units than on an LA-2A.

Trying to do it within the LA-2 would be tricky.  Likely be some compromises in the stock performance.

I've had plans to do a dedicated box like this.  Could be sonically invisible or make a statement.

Could also be used as FX blender.


Done passively, a 2 split and recombine would incur losses - depending on design could be 6db or more.

earthsled said:

I'm looking for something that would be internal and wouldn't require an additional power supply.

Click to expand...

not gonna happen in any reasonable manner or with any relative ease. 

Isn't R7 doing something alike? How about adding a pot there? Wouldn't work in limit mode though. Or adding a volume pot after the input tranny, going to the first grid via a decoupling resistor (one would need to be added after R1 as well)? With that you could dial in some direct signal. I'm no tube designer, so don't shoot me  : ???

I don't know how well an external crushnblend circuit would work here because there will be some phase response from the LA2A's trannys. Don't know how well this blends?

Michael

earthsled said:

Hello all...

A friend has asked me if it would be possible to add a blend control to an LA-2A clone for him. He wants the ability to do parallel compression in one box (similar to the wet/dry control on the Chandler germanium comp).

It seems like a reasonable mod to me. Has anyone out there attempted something like this?

For reference, the schematic can be found here: http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_1968.jpg

As always, your comments are greatly appreciated!!

Click to expand...

I reckon it is possible.
You need to take the input signal (junction of xfmr secondary and R6) to CCW side of a 220k-1Meg pot (Lin is acceptable, S-law would be better), via a resistor that you will need to adjust later (you can omit it at first and install it later); let's call it Rx.
Take the CW side of the pot to wiper of Gain pot (R1). Disconnect grid of V1 from wiper of R1 and connect it to wiper of new pot; this is your blend pot.
The total value of blend pot plus Rx must be high in order not to tamper with the compression characteristics, but not too high since it's in the grid circuit of V1; Miller effect may impair HF frequency response.
For adjustment of Rx, you may fit a trimmer or wire a pot and set it as to have no level variation relative to the Peak Reduction control when the  blend pot is at full CCW.

You need to take the input signal (junction of xfmr secondary and R6) to CCW side of a 220k-1Meg pot (Lin is acceptable, S-law would be better), via a resistor that you will need to adjust later (you can omit it at first and install it later); let's call it Rx. Take the CW side of the pot to wiper of Gain pot (R1). Disconnect grid of V1 from wiper of R1 and connect it to wiper of new pot; this is your blend pot.

Click to expand...

Here's a pic to illustrate Abbey's suggestion...



I'm wondering about the s-law taper pot. Are these typically stocked? Could the law of a linear pot be altered to more of an s-curve shape by adding an additional resistor between the CW terminal and wiper of the blend pot?



By the way, Abbey - I owe you a round of drinks - look me up if you're ever in the Nashville area.