Zener Limiter Build

[Ó Draighnáin]

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.

 

[thomasdf]

Impressive ! What transformers are these ? Keep us posted

 

[Ó Draighnáin]

Thanks!

Input transformers are BBC LG/61ASC (1.75:1). It doesn't look like they were Sowter manufactured, might be Gardners. Not sure what to expect here. They were used on the AM6/3 limiter and as a valve (line to grid) input transformer.

Output transformers are model number 31740, manufactured in the USA by Leightner Electronics, which came from an Autogram IC-10 broadcast console. Can be wired 1:2. They look really well made.

 

[Ó Draighnáin]

It looks like Brian Sowter wrote the following on another forum:

"An LG/61ASC is not in my stores catalogue, however an LG/60 is

1:2.68 ratio, 600:1600 Impedance, 10.6 henries primary Inductance, internal cost in 1980 £3.75

An LG/63 had a ratio of 1:2.78 so the 61 is likely to be similar."

 

[thomasdf]

Thanks ! Interesting
What schematic did you follow ?

 

[Ó Draighnáin]

I used the original schematic (the transformerless version). The difference I implemented is with the 9db pad and HF boost at the front end. I tuned the pad and the HF boost to take into account the voltage drop from the input transformer windings to still maintain a drop of about 9dB. Nothing very scientific, just spending some time plugging different resistor capacitor combinations into a spice model.

I rhe read somewhere that the input transformer I am using is designed for a secondary of 4k3? This seems like a good match.

I read that when using 1N5231 diodes there is about a 6dB signal drop, so that is why I went I:2 on the output.

Otherwise, the hold, recovery and switching is all going to follow the original. I am not trying to build a distortion box. Looks like it will take some time to tune.

 

[alphasnk]

Impressive! I built two zeners from Friphom's pcbs with 1:2 api output transformers, they work great. Let us know how the tuning goes!

 

[Script]

That must have taken some time
Inspiring. Thanks for posting.
Hope it all works on first fire up.

 

[thomasdf]

I used the original schematic (the transformerless version). The difference I implemented is with the 9db pad and HF boost at the front end. I tuned the pad and the HF boost to take into account the voltage drop from the input transformer windings to still maintain a drop of about 9dB. Nothing very scientific, just spending some time plugging different resistor capacitor combinations into a spice model.

I rhe read somewhere that the input transformer I am using is designed for a secondary of 4k3? This seems like a good match.

I read that when using 1N5231 diodes there is about a 6dB signal drop, so that is why I went I:2 on the output.

Otherwise, the hold, recovery and switching is all going to follow the original. I am not trying to build a distortion box. Looks like it will take some time to tune.

Amazing ! Can't wait to see the rest of it Keep us posted !

 

[slipperyflax]

@Ó Draighnáin Any progress?

 

[Ó Draighnáin]

Hi, I've been side tracked by progressing a couple of diy U67-type mics (I posted elsewhere about these). I finished the first mic only yesterday. I am wavering between finishing the second mic while I am in that zone or taking a break to progress the zener. I need to finish them all off relatively soon though. These mics and the limiter might well be my last builds before I transition completely to just building acoustic guitars.

Next big job on the zener is metal work. Once the chassis is cut and drilled (and external power supply for this one) it all seems to feel more downwind sailing.

I have all the components at hand for the limiter and I will certainly put up a progress pic after all the chassis cutting and drilling has subsided, and the boards, meters and controls are in place (before the final wire up).

Cheers.

 

[Ó Draighnáin]

A lot of the metal work is now out of the way. Not sure how to orient the boards best yet but the project is starting to take shape. Power supply will obviously be external.

 

[slipperyflax]

Inspiring!

 

[thomasdf]

Looks promising

 

[Ó Draighnáin]

Looks promising

I fear (the appearance of) this compressor will disappoint from here. These days I am a no frills with the finishing, so this compressor will never look bling. In the past I would have used all Grayhill switches but that was when they were $20/piece.

As long as it works (fingers crossed).

Perhaps building it as I have, and getting it to work, will be of some assistance to others considering a scratch build.

In the end, I decided to arrange the controls as if one of the original limiters was lain down flat then mirrored, with the meters obviously rotated 90 degrees.

That means that the output is far bottom left and far bottom right for the two channels.

 

[Ó Draighnáin]

A few mistakes as usual.

No signal. Bit it of head scratching until I realised I wired the XLR outputs incorrectly yet again. Signal straight to ground. I must have wired hundred of these sockets. Nevermind.

Just trouble shooting in bypass mode to start with. Attenuator not working, volume drops to nothing after about 10 steps. Turns out I accidentally used a 820k resistor in the output control switch instead of a 820r resistor. Apologies to all the curses directed towards the switch manufacturer.

Voltages off one of the side chain boards are all wrong. After some forensics, one of the resistors has a broken leg. Fixed that and you can see in front panel picture that the meters are sitting about right (used a 1.0mA meter with a shunt).

Voltages are ok but will continue to be tuned I suspect. The schematic actually has quite a few DC voltage references. A few of the voltages on my board are a little high (e.g. output of C1 base of VT1, voltage other side of R4 also running a little high 6.8V instead of 6.4V) perhaps indicating lower current draw from the transistors I used? However, same the transistors in different locations provide very close to specification voltages (e.g. collectors of VT1 and VT4). Power supply was delivering 27.99 VDC nicely, and dialing in the +/-20VDC to within less than about 100mV was easy enough. for a first pass The hold voltage is about 9.9VDC so all the basics seem to be about there.

Both channels of the unit are nice and quiet in bypass mode (which is just the signal going through the input and output transformers). The unit works in compression and limit modes, however, both channels sound scratchy and poppy, unusably so if the unit is driven. It is not hum. One channel is worse than the other. Could be the zeners, I thought they were pretty close but I might need to review that.

First thing to do is re-flow all the solder joints on the boards and check all the terminal board connections and interconnect wires.

Not there yet but I am not unhappy with the progress so far.

 

[anvl]

I love your project !!! Congratulations.
Are you sure you used matched zener diodes??
It's very important to have mathed zeners as follows:
Matched D1 & D3
Matched D2 & D4

 

[Ó Draighnáin]

I am far from certain they are closely matched enough. I read somewhere that they need to be within about 3mV and I couldn't get that resolution with the first test jig I tried. They were only matched to about +/-10mV (the issue is with my multimeter which has a low overload threshold for mV readings). I am going to build another test jig which will allow me to take mV readings.

 

[gyraf]

As you're looking for relative measures, not absolute ones, couldn't you just use a variable voltage source (psu) as reference offset?

 

[anvl]