LA-2A projects

Known Issue: GR Gain reduction isn't working, GR Meter doesn't work

• the most information is often “change the V4 tube” or “optocell”. BUT did you tried to turn stereo adjust Potentiometer - its important for DIY Builds (first testing)

• in 99% of all DIY Builds is a 33K resistor on the meter pcb fine, replace R25 as described in the T4B optocell install guide with a 50K trimmer.. (ask google)

• longterm issue with Meter/GR function at all: the NE- filament lamp (at left side of the pcb) acts like voltage regulator and degrade over the time → check the DC Voltage across the NE lamp. furthermore its possible and better in longterm to use a 62V Zenerdiode (1Watt or more) instead of the NE-lamp, which gives you a stable voltage and less noise.

• more important for old devices is degradation of the Optocell, it makes so sense to buy old used “Original” optocells.

Improvements for DIY Builds

1. use true PCB Version DIY Projects - cheaper, more safety, faster and easier to build and repair, less problems/issues/risks to build it, less hum/noise (better grounding) less bleeds (https://www.dripelectronics.com LA2A Limiter DIY PCB+kit - HoneyBadger Audio ) chassis are available worldwide for the pcb versions too (don-audio, frontpanels.de, diy-racked and others)

2. start with a star grounding to avoid known issues with noise or non functional operation

3. use High Quality resistors parts - there's no reason to use 5% overpriced carbon resistors - use 1% Metal film as much you can mouser: RN55 or RN65 series for example (less thermal noise)

4. use High Quality film capacitors: don't use the yellow/orange ceramic caps which are with the Z5U Dialectricum - it inserts a microphone effect and Distortion. more on my page: capacitors capacitor types the best what you can install are Polypropylene caps for example Vishay, Panasonic ECW series or silver mica for the 510-2nF caps.

5. install decoupling cap at the NE lamp/zener diode with at minimum 470pF or more - check with a scope the ripple across the NE lamp or zener diode and change the cap value.

6. Shielded cables : as described in the vintage vibes guide or other guides, shielded cables are important or you get bleeds, hum and hiss.

7. the chassis/case is often painted or anodized, eloxal - which means a ground connection from just a screw and nut isn't enough, measure the resistance from Earth connection on the IEC connector to the groundpad/screw - it must be less 0.6-1Ohm, the best value should be 0.1-0.2R. (depends on your DMM and DMM cables too), which brings us a solution, use tooth washers and/or scratch carefully the coating behind the washer. but from my experience a good tooth washer is enough. or go with 2. star grounding.

8. Optocell: the response time is affected by the optocell, a standard a good Optocell system is the T4B cell. there are others like the IGS cell and with different “speed times” https://www.dripelectronics.com

9. Powersupply ripple: the LA-2A Design is by default noisy, but can be improved with a solid ground as described above. but more important is the PSU Ripple, use High Quality Powersupply electrolyte caps with low ESR, measure the ripple with a scope under load and install additional decoupling caps for DC only , at the Rectifier CR1/CR2 or across C7-A C7-B, make sure to install 400V ratings.

10. last but not least : C5 10uF/200V capacitor: this affect the sound at the output, you can choose from cheap 1€ caps thru snake oil. A problem is the availably worldwide.. What do we need: it's basically an audio grad cap , bipolar is fine , power rating MUST BE more than 200V for safety reasons. the measured voltage is 115V but with headroom for long llifecylce, we need 200V rating. Mundorf caps, Jantzen caps should be used. take care on the physical dimensions or look at Number 11. A good choice are Mundorf supreme caps or look here: Auf der Suche nach Film- oder Folien-Kondensatoren?

11. You can mount the C5 cap with additional holders (drill holes in the chassis/frame and use cable tie holders (screwed to the chassis))