Zener Limiter Build

[anvl]

" both channels sound scratchy and poppy, unusably so if the unit is driven" ... probably related to matching zener diodes (in my opinion)

 

[Cqwet Dbdfte]

Boards are finished, lets hope the layout is ok. Circuit breaks up pretty well into input, output and side chain. Power supply will be remote. Only a couple of slight component changes over the original circuit - have adjusted the HF boost and the pad at the input. Chassis arrived yesterday but the wings/ears were bent during shipping.

Using perf boards for a simple circuit is OK , but for these circuits it is faster and cheaper to order PCB's from JLC, GO-GO etc for a few bucks.
Too many things to go wrong in those type boards and trouble shooting is harder without a CAD program to do error checking.
Pain has thought me.

 

[Ó Draighnáin]

I have matched zeners now. I used the first test jig I built (absolute - the one posted a few comments back) to group similar zeners and then the second jig (relative) to match pairs. Without the initial grouping matching pairs would be quite tedious.

 

[slipperyflax]

Did swapping zeners fix your noise issue? I've had problems with noisy zeners in the past but swapped them and everything was fine after that. Can't remember any details though

 

[Cqwet Dbdfte]

Zeners in clipping are classic noise sources.

 

[Ó Draighnáin]

I have reduced it considerably but have not eliminated it yet. Swapping out the zeners definitely had an impact. I have made a lot of progress but I consider this unit unfinished as of yet.

There a few things to do in addition to more zener swapping, e.g., adjust my meter shunt resistors slightly (needles should be bang in the middle/vertical for an unmodded version with original hold control), review all published voltages (there are quite a few on the schematic) against my voltages and compare, also look into the AOT resistors and distortion patterns a bit more critically.

However, taking a break from electronics at the moment, spare time is currently allocated to other projects (e.g. practicing to get a recently composed finger picking piece to performance level proficiency, building a parlour guitar form and managing my little private viticultural research project (it is spring here)).

I will get back to it anon.

 

[deleteduser]

super cool project man

 

[Ó Draighnáin]

After making some headway with other projects I came back to this. I read up on some of the earlier build threads to avail myself of some trouble shooting tips, not so much there for someone building from the schematic.

I re-checked the input and side chain circuits against the schematic, everything seemed in order. I have used BC179s and BC549s in the circuit (except for a couple of BC559s), because this is what I had that looked suitable. Output was 2N4919G/2N4922G mounted on heat sinks.

All rail voltages were pretty close, the meters needles were rising to about the midpoint, the hold control had the correct voltage and was moving the meter needle. I was definitely getting compression, I could see that the recovery switch was working (if you adjust the hold control and then return it back to center it will move quickly to where you adjust to but the rate of return to center will depend on the recovery setting. However, distortion was pretty high at most hold settings and I was not getting any meter swing. I swapped positions 1 and 3 on the hold pot as they were the wrong way around (As I understand it, clockwise rotation of the hold should send the meter needle to the left).

I spent time adjusting the AOT resistors. I used 620ohm for AOT1. I adjusted AOT7/9 until I had the VT5/6 voltages within about 0.02V of each other, which is within about 0.3%. This was using just a single 3k6 resistor (same value worked for both channels). I will try and get closer by dropping in a trim pot to find the exact resistor(s) value for even closer voltage balance. AOT10 was standard 10ohm and AOT8 was unused. The compression/limiter AOT values currently in place are just the standard ones for the moment. Working on the supplies for VT5 and VT6 made a big difference.

I'm happy to start trying to work out how to use it now.