IJ Research 33609 Neve clone - meter tracking issue

[beatnik]

Hi everyone,

I have built a Neve 33609 compressor clone back a few years ago using pcbs by Igor, may he rest in peace.

I have some issues with the metering circuit and unfortunately Igor disappeared from the forum before I could finish the build and ultimately passed away so I couldn't get support from him.

The compressor is working well and the two channels are very well matched compressing within +/- 0.1dB across the whole gain reduction range but I could never get the meters to track properly.

I am following the test procedure from the original 33609 manual. I have attached the manual page along with schematics

The problem is with step 4.2 : I set RV3 to display 20dB gain reduction on the meter, but when I vary the external CV signal the meters are not tracking correctly, showing more gain reduction than what's actually going on. The manual says a tolerance of +/- 1dB is to be expected but I am experiencing way more than that.

To make things worse, the two meters follow a different "curve" meaning at certain CV settings they don't display matched readings - even though ongoing compression is always equal between the two channels.
This really is the most problematic thing - I could probably live with the meter scales being equally offset, but having them out of balance between each other can be very confusing.

I have used 1mA meters from Hairball. These meters are normally sold for the GSSL compressor but they were listed in the 33609 BOM as a suitable alternative - (I believe the original 33609 used PPM meters ?)
I am wondering if the Hairball meters might need an extra adjustment to be used in the 33609 circuit, however I did not find reports of other builders having problems with these meters.

DC voltages at the schematic test points are spot on. I have double checked all component values and I did even match the BC184 - BC214 transistors hfe between the two channels (probably overkill, but I had a lot of these). I did not match the other transistors.

Would be great to know if you have any suggestions

 

[beatnik]

Thinking the problem might lie with the panel meters I have tried measuring the signal coming to the meters using an external ammeter.

I set CV signal to obtain a level reduction of 20dB and adjusted RV3 meter tracking trimpot to show 1mA on the external ammeter.
When sweeping the CV signal to obtain from 0dB to 20dB gain reduction the meter signals are very consistent between the two channels - within +/- 0.1mA

While keeping the setting at 20dB gain reduction and 1mA reading on ammeter for both channels I reconnect the internal panel meters and the needle jumps around 12dB on one channel and 14dB on the other.

Unless I am missing something this proves there is a problem with the meters, first of all the 1mA signal should place the needle on 20dB on the scale and secondly the two meters should be in the same spot, well the manual says within +/- 1dB

Do you reckon it's possible to make these meters work, perhaps adding extra trimpots ?

 

[beatnik]

I have been experimenting with potentiometers in place of R32 and R35 and with that I can adjust the meter to read 20dB at 1mA, but at other settings it's off the scale.

Just randomly turning trimpots is not going to provide a solution, I am wondering if someone can understand better the schematics and suggest a more thoughtful approach.

 

[TwentyTrees]

This might not help hugely, but the issues you describe are very similar to my experience of the Hairball 8027B 1mA meter. I've a pair in a dual 2254 clone, and I've never been able to get them to track gain reduction accurately across their whole range. I always assumed it was an error somewhere on my part. Never checked how close they are to each other, mind...

 

[beatnik]

Thanks for letting me know.

In my case there are two issues
1) the two meters have a different response ie give a different reading (for the same given signal) and
2) the gain reduction dB scale on the meters does not match what is happening in the compressor, regardless of calibration settings

I have been studying photos of original vintage 33609 same model as what Igor's clone was based on and the meters were ppm with a custom non-linear scale.
So it looks like the Hairball meter with the stock scale is not gonna give an accurate reading across the whole range - a custom scale will have to be made. This would solve problem 2)

But in my case I also need a solution for problem 1) that the two meters are way off between each other, and I am afraid this is due to poor tolerances in the Hairball meters...

Do you also have a different response between the two meters or it's just the scale that is not lining up ?

 

[beatnik]

I have tried using a pair of big Sifam vu meters, the tracking between the two channels is much better within 0.5dB however the scale is off range.

They probably require some scaling resistors on the output of the meter driver, is there a formula to calculate the required resistances ?

 

[Brian Roth]

Try swapping the meters between channels and see if the tracking wrror moves.

Bri

 

[radardoug]

Put the 2 meters in series, and a third known accurate milliamp meter.Add 10K in series. Connect the whole string across a variable supply.
Wind up the supply till you get 1mA on the accurate meter. Then what do the other two read? Should be just full scale. If they are over full scale, add a resistor across each meter to pull them individually to full scale.

 

[beatnik]

Try swapping the meters between channels and see if the tracking wrror moves.

Bri

Done that and the error definitely follows the meter. Also I have measured the signals coming off the meter drivers with a dmm and the two channels track consistently within +/- 0.1mA across the whole range from 0dB to 20dB compression.
The problem is definitely with the meters.

Put the 2 meters in series, and a third known accurate milliamp meter.Add 10K in series. Connect the whole string across a variable supply.
Wind up the supply till you get 1mA on the accurate meter. Then what do the other two read? Should be just full scale. If they are over full scale, add a resistor across each meter to pull them individually to full scale.

Thanks for the tip. I am gonna try this today and report back

 

[beatnik]

I have done some further testing and the Hairball meters definitely have poor matching.

I will try get hold of a pair of Sifam AL-19WF , it seems these would fit the existing front panel cutout and hopefully they have closer tolerances