Diy Drip LA-2A pot modifications

[dirty1_1garry]

Hello!


Recently I finished my DIY Drip LA-2A Opto4 optical compressor.

More info and photos could be find on drip forum:
http://www.dripelectronics.com/index.php?option=com_kunena&func=view&catid=6&id=5852&Itemid=41

I take my friend's UA LA-2A reissue and made some comparisons. Results you could check in my Gearslutz tread. There you could find audio examples and audio comparisons.
http://www.gearslutz.com/board/gear-shoot-outs-sound-file-comparisons-audio-tests/840823-diy-drip-la-2a-modern-la-2a-reissue.html#post9077096

One bad thing that I found in drip LA is a very low operating range of potentiometers. Yes, I know that original LA-2A is very loud device. For tuning output gain into operation level you should turn up a knob in millimetres! I have AD converter with +14-16dBu in which I connect my LA, so my output gain pot turn not more then on second numbers from zero!. That fact makes LA not usable.

BUT I find that reissue LA have much more usable potentiometers range than my drip LA!! Operation area of reissue pots is about 12 o'clock.
In that fact I tried to find out the reasons why it's SO. I opened my friend's LA reissue devise and research all resistors and capacitors value. What I've got. All values is the same accept marked. (attached file *schematic)

If understand right LA is so loud devise because it has really hot output stage on 12BH7 tube that give about 40dB of gain.

Do you have ideas why LA reissue NOT so loud?
My idea:
- Gain of output stage could be less then in original but schematic sad that there no any changes.
- Output transformer(which is a replica of vintage one) could have less output.

I  heard from D-LA2A tread that pretty nice opportunity to make LA not so loud is using 25K log pot with 75K resistor in series. Will try.

 

[fazeka]

Stupid question here: Is the taper of the pots between the two the same?

 

[alexc]

I don't think the BH7 is where the gain is at - it is a dc coupled white cathode follower and so it's gain is <1.

The gain comes mostly from the AX7 V1a and V1b - both are high mu and high plate resistors, so gain is .. high. There is feedback from the output transformer primary, so that tames it some, but still quite high gain. One way to reduce might be to alter the feedback amount.

Also, I don't know what the re-issue uses, but the step-down ratio of the output transformer A24 is 15K:600 so 5:1 step down. If a higher ratio is used, as is sometimes done, then that would also lower the output level.

There may also be a simple voltage division reduction of level at the Gain pot by using some series resistance.

And terminating the output transformer secondary with a parallel 600R resistor also would probably reduce the output somewhat. This is commonly done when interfacing 600R outputs to modern equipment inputs.

My diy builds usually have output at around 8'oclock or '2 out of 10' for my Motu inputs which clip at around 10dBu.

Taper of pot is log law but they can differ quite a bit.

Cheers

 

[CJ]

12AY7/6072 has better sound and a u of 44, and it uses the same base as a 12AX7a,

so two ways to win, the compressor becomes more user friendly and it sounds better,

 

[Biasrocks]

Pad the input as suggested in the LA2A manual. I made it switchable on my build.

Based on your sound examples I'd say your build sounds better than the reissue. It sounds flat and boring compared to yours.

Regards.
Mark

 

[dirty1_1garry]

12AY7/6072 has better sound and a u of 44, and it uses the same base as a 12AX7a,

so two ways to win, the compressor becomes more user friendly and it sounds better,

You mean use 12AY7 instead of 12AX7 (V1) or instead of 12BH7 (V2)?

Will try!

 

[dirty1_1garry]

Pad the input as suggested in the LA2A manual. I made it switchable on my build.
Based on your sound examples I'd say your build sounds better than the reissue. It sounds flat and boring compared to yours.

Nice to hear your comments)

About input pad. I don't think that it's a correct method how to make LA sound quieter. That's like if you make a CD player quieter instead of making quieter out amplifier in which it united. Input section will not work in correct mode and you will pick up internal noises. The best way - decrease make up of output stage. But you should know how to do this correctly)) CJ offer good idea about using 12AY7 tube.

 

[alexc]

12AY7/6072 has better sound and a u of 44, and it uses the same base as a 12AX7a,

I agree with that for sure.

I've been designing in AY7 as a moderately high gain stage in alot of my most recent preamp stages instead of an AX7 and have been very happy with the result. I've used it in the front end of guitar amps and studio preamps, as well as eq makeup and my one and only tube mic mod.

I have some maintenance to do on my earliest la2a build - a combined fender pre + la2a unit. I think I will try the ay7 there too.

PS - I don't think there is any problem with padding the input providing you can still achieve the gain reduction at the t4b that you want. But true, there are many ways to go about optimising your gain structure the way you want it.

 

[Kingston]

About input pad. I don't think that it's a correct method how to make LA sound quieter.

It's one of the ways to do it right. The original input is designed for fifties signal level (quiet). You are blasting a ridiculous (by fifties standards) +14-16dBu into an input stage that is simply not designed for it.

If you only change the make up gain stage, you are still blasting ridiculous (by fifties standards) levels into the opto (sidechain) stage. Admittedly this is a lesser issue.


Besides an input pad you could use a lower step up or even 1:1 input transformer, the Jensen "mod". (the original was 1:9, which is not justified today)

The solution that worked best for me was a custom taper rotary switch for both the R1 and R2 potentiometers. 25K log pot with 75K resistor in series works ok, but only if you are willing to sacrifice the high gain of the make up gain stage completely.

 

[Biasrocks]

Input section will not work in correct mode and you will pick up internal noises.

I'm not sure why your drawing conclusions apparently without trying any of the suggestions. I'd much rather pad down an input that mess with a proven design and sound. Extra noise, is a non-issue in my experience as we're dealing with line level signals here. It also gets the pots working in the middle of their travel much like the re-issue you were comparing.

Below is the relevant section from the LA2A manual.

I hope you getting working the way you want.

Regards,
Mark

 

[emrr]

Noise is constant in an LA-2A, so pad is fine. 

I assume the gain control taper is different on the UA.  There are no circuit differences that would account.