DIY 500 series STUDER 169 Recording Channel (Mic Pre + EQ) Documents Published!

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iprovlek said:
What is value of L1!?

Can I use 10ohms in place of L1!?

if L1 on audiox's schematics is the same as L1 in studer's schematics then the 169 had these:

yes, original L where Philips p/n 62.01.0115. they can be changed with TDK p/n B82114-R-A4.
in italy, they can be found at Distrelec with code 350214.

I´d like to add a "EQ In/out" switch in this circuit... In attachement is scheme... I want to use preamp separately without EQ "affecting".
Are my thougs proper? Make it sense?



Edit: Schematic modified...


  • EQ in modiffication.jpg
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Vac11 said:
I´d like to add a "EQ In/out" switch in this circuit... In attachement is scheme... I want to use preamp separately without EQ "affecting". Are my thougs proper? Make it sense?

No, it doesn't work. In your drawing the EQ (U2) output will short the mic pre output signal.

The additional switch should select between the mic pre output and the EQ output signals.
I have some news about my construction of this chanellstrip ...

1) No smoke when first time turn on.
2) Preamp works great - a ton of gain.
3) On the doas sockets is proper voltage +-16V.
4) The device is powered with about 0.04A with all DOAs in sockets.
5) Device works great with NE5534 and also with my DOAs (SGA-SOA1).
6) The preamp, mids and bass are working properly and sounds realy great to my ears.

I have troubles with HIghs. They dont work correctly. When I turn the HIghs pot to the O (center) position or to the CW end I get some radio high frequency oscilation, distortion and small loss in gain... When I turn this pot CCW (to -15dB) oscilation is gone and preamp and Bass and Mids on EQ works correct.

I try swap L1 ferrite for another... Without any changes. I used this ferrite:

I desoldered and after then measured R21, R22, R23 and R24, R25 (correct values) and I reheated C16, C17, C18 in filter... Without any changes...

I double check HIghs potentiometer wiring - everything is OK. I measured potentiometer value. It is OK (2x10K lin).

Please can you help me and guide in the right direction? Can it cause a bad choice of ferrite L1? Any advice would be very helpful...

Thanks Audiox for this amazing project!





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Hi Audiox,

Wondering what was the idea of the HPF option empty space on PCB, i guess building a simple CR filter there but just to confirm.


Reviving this thread for a couple of questions.

For those that have built one of these, would an H-Pad or a T-Pad be more optimal in this circuit? I am currently building the PCBs I have laying around and and the first one I built with an H-Pad, but I would like to get more attenuation, but I don't know exactly what resistance values to use for source and destination with the calculators that are available online. Since the pad will be when used in conjunction line level output devices, I used 600ohm for the source and 200 for the destination as I am using a NTM4 transformer on these builds. I used the values given, but I am getting around 15dB instead of the desired 30dB or something close to unity. I can follow along with formulas, but I get lost trying to figure which values are what I need to use.

The other thing is the HPF. I guessed and used 100nF in the spot that says HPF, but I don't hear anything happen when I make the connection between the adjacent OPT pads. I didn't think anything else was required to make this work. I might have the wrong capacitor value for this circuit.

Other than that, I've managed to cram all the features in one 500 series unit. Selectable frequencies for Low and High, Pad, Polarity, EQ In/Out, and +48. The mic preamp is quiet and sounds really good as does the EQ section. I'll show what I did to make it all fit after I figure out the Pad and HPF.



EDIT: EQ Bypass scheme doesn't quite work right as described or as I had previous drawn. See below
It's all been figured out!

I wound up doing all the options on this guy:

EQ In/Out
Selectable HF/LF

I tried to attach the .fpd file that I am using, for some reason that file extension is not allowed. I thought I was able to do that in the past. Hmmm... I can email it on request. It's a modified version of the one that was posted earlier on this thread

The EQ In/Out selector placement is the trickiest one. I had to trim the center facing legs of the MF select and HF gain and then file down the edges of a tiny SPDT switch and wedge in between the pots. I still need to add a little super glue to it to hold it to the board, but once the pots are soldered in around it the switch doesn't move. The holes to attach the panel to the PCB or to an L-bracket have been left. The hole to the left of the +48V switch is for a phantom LED indicator.

Right now I have one circuit fully working, but I still have to install it into the front panel. That will have to wait as I leave town this weekend. My goal was to just get the board working before I left. Then I have 5 more to do when I get back.

I'll send pics when I get back. In the meantime, I would happy to answer any questions if someone is building one and would like to add some of the extra options.

It's a shame these boards aren't readily available.



EDIT: EQ Bypass scheme doesn't quite work right as described or as I had previous drawn. See below.
Wanted to share my front panel design file that I'm using. It's a modified version of the layout posted earlier on this thread. The ones I actually have have errors on them because I'm an idiot. This file has been corrected. Just change the extension to .fpd.

I'll post all the info tonight on how to implement hopefully just in case there are still some people out there holding on to boards and are waiting to start on this project.




  • Studer 169 Recording Channel all options.brd
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Okay, here we go...

For the EQ In/Out switch, I followed the provided schematic earlier in this post and came up with this. The VERY first thing I would recommend is removing the Link at the top of DOA 2. If you forget to do this then test the switch, in the bypass position you will generate painful, squealing feedback at weaponized levels. So do this first. Seriously.

To avoid a bunch of cutting traces and cut down on the number of jumpers being used, I moved C15 to the two solder points above R34 that are not connected to anything. You will have to cut the trace that connects those two points. Positive side connects to R34 as shown and the negative goes to the pole of the SPDT for EQ bypass. I ran this wire through the unused DOA socket hole to connect directly to the capacitor.

The connection for the Bypass position of the switch depends on how you want the HPF to function. If connected to position 1, the HPF will still function when the EQ is bypassed. In Position 2, the HPF is bypassed with the EQ. Buyer's choice.

For the EQ In position of the SPDT, I connected it to the unused output connection of U2 (pin 6) since I'm using DOA and these points are tied together on the board.

The Link that is directly under the DOA 1 outline needs to be removed otherwise the HPF will be bypassed all the time.

For the HPF capacitor I wound up using a 1uF. Seems to roll off around 80Hz. Some might prefer 820nF to get just the HP frequency a little higher.

The HF and LF selection modification was taken from the 169 EQ page:

I thought about using some different frequencies, but I haven't bothered trying to calculate those values, so I went the lazy route and used other people's work.

The Pad is -30dB which is unity gain for this circuit. I used an H-Pad with the following values:
                          -Input (primary/secondary) - 390R, Bridge - 6.65R, Output (primary/secondary) - 82R-
This should be right at -30dB @1kHz.

The +48V indicator LED positive leg connects to the On position of the phantom power switch with a 2k4 resistor as the LED used is 20mA. There is a solder pad directly below R17 on the side towards the gain potentiometer which connects to the ground plane. I used this for the negative led of the LED.

Frank was kind enough to make me L-Brackets for this project, but that in addition to the faceplate allows only the minimum length of bushing on the pots to be exposed to fasten a nut to it. I had to flip some of the nuts over or use different ones to accommodate where the threads actually ended on the bushing. That might not make sense, but it will become very apparent if you decide to use an L-Bracket.

5mm spacers seem to be the correct length for attaching this board to an L-bracket.

All of the switches I used (except for the first one I built) are sub miniature C&K, which are very expensive for DPDT in this size. You can use the slightly larger miniature ones which will save you money, but the labels on the faceplate will get covered up by the mounting nuts and depending on which pots you use, the bottom corners of the switches will need to be sanded to fit. The holes drilled on the faceplate are to accommodate sub miniature toggles with bushings up to 5.20mm.

EDIT: EQ Bypass scheme doesn't quite work right as described or as I had previous drawn. See below.


  • 169 Board Edits.pdf
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The EQ In/Out switch used were Grayhill SPDT's as they were the smallest ones I could find that would fit between P3 and P4. Even then, the inside terminal connections of P3 and P4 (I used the Omeg pots as specified) have to be trimmed to make enough room. To avoid destroying your pots like I did, trim the as much material as you can with some quality small side cutters and then make any final adjustments with a Dremel or similar tool. I tried to just do it all with the Dremel and ruined the connection points on the side I was trimming. I had to replace nearly all of the those pots. Solder leads onto the SPDT before installing it between P3 and P4. It will lay on the PCB. I applied some super glue to switch and adhered it to the board. I put the faceplate on and then used a small screw driver to push is as forward into the EQ in/Out hole on the faceplate as it would go and then let the glue set.

EDIT: EQ Bypass scheme doesn't quite work right as described or as I had previous drawn. See below.


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Here's a pic of the finished board. I'm just waiting on knobs and these are done. I do have a pile of OA10's to build to put in the 6 units I built. There are lots of off board connections so it gets a little challenging to keep it neat.

So there you go. It can be done, but does require some patience and paying attention.

Thanks a million to audiox for this project.




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audiox said:
The Neumann OA10 could be one alternative:

Do you think the DOA be replaced by a NE5532 ? Wouldn't be an easier build?  Do you think it will sound too differently?
I dug these out to put them to use when I realized my initial EQ bypass scheme is not actually correct. It can create a low end popping sound when switching in/out. I'm trying to find a better way to do this within the parameters of the 500 series panels I have, but haven't found a way yet. The way I posted above will bypass the EQ as I confirmed it will pretty much work, but I don't believe things are being loaded properly. I'll keep poking around and when I find something I'll post it here.



EDIT: EQ Bypass scheme doesn't quite work right as described or as I had previous drawn. See Below
So it was pointed out to me that the EQ Bypass scheme doesn't quite work right as described or as I had previous drawn. And it can't be done simply by breaking the connection at C15 and jumping into R32 as the impedance isn't matched correctly at that point and there is a drop in gain. To do this correctly would require another op amp scheme to go directly from the preamp to a balanced output with the proper gain and impedance at the output stage. So I've abandoned needing to use an EQ bypass as the center detents for the frequency gain is near spot on, at least for me.

However, I do have one small thing that I have been unable solve. All but one of the six modules I've built have something slightly off when I sweep the mid frequencies while boosting the mid band. On the unit that is fully working, I can see the frequency selected being boosted with proportional bandwidth and that consistent when sweeping the full range. For the other 5 modules, when I sweep to the middle of the range (approximately 800-3k) the bandwidth is widened and is about 4 dB less gain than the working unit, but at the ends of the frequency range the EQ behaves exactly as is expected. The difference in how it sounds is more dramatic than how it looks on an analyzer. Gain, level, HF and LF all seem to be matched.

I've done the swapping of op amps replacing the pots in the mid band and have been making comparisons between the fully operation and slightly less operational units. Aside from a missing connection or a misplaced part which has yet to be seen, the only thing that remotely makes any sense is that there is some sort of phase cancellation going on in those specified ranges, but I wouldn't know where. The Mid section of the schematic is isolated from everything else, and there aren't many things to check, so I've been going around in circles hoping to find something new. It did occur to me that the unit that seems to have a fully working mid band could be the faulty one, but that would only make sense if the natural characteristic for this circuit is that the bandwidth widens and lessens in gain in the mid section. I've attached the portion of the schematic that has the gain, EQ, and output sections. As is, they all are working fine and I could put them back together and not think about the slight differences, but when doing listening tests the mid band seem more useful with the consistent gain and bandwidth as the fully working one does and I would like all of them to function accordingly. I could certainly use some fresh ideas where to do testing.




  • 169 Channel Gain:EQ Schematic .pdf
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