Studer 928 Story...

sweetbeats

Reel deep thoughts...
So I’m almost done comprehensively testing everything including the master section.

I also tested all the audio connections on the six 39-pin Siemens connectors on the back. That all works!

The group modules are 100% too. :)

What I *have* discovered though is that my issue with the Master A left meter is not just the signal to the meter...the Master A left channel output is maybe 12dB quieter than the right channel. That’s not good. The issue exists whether you route program, talkback or tone generator signal to the output. The signal is good in the CUE buss, monitor buss, and at the insert, so that will help to narrow it down.

The stereo correlator meter is also not behaving right, but I’m not sure if that has anything to do with the Master A meter module not being installed. I’ll have to deal with that one later. But I really want that one to work.

But the transformer outputs on the group outs and master outs really do sound a cut above the non-transformer outputs...not that those sound bad...it all sounds good. I just wasn’t expecting such a distinct difference. That will make for some nice options during tracking and overdubs. I remain impressed and pleased with how so much of the console is not only working, but really solid. And NONE of the pots (except for the aforementioned mic trims) have any hint of skritchy...and they all feel really nice. And the noise floor is so low.

Gotta get that Master A left channel working right...[sweetbeats starts flipping through schematics...]
 

sweetbeats

Reel deep thoughts...
[MENTION=201181]michael1991[/MENTION]

So your monitor meters...what buss do they monitor? The Control Room buss? And if so how are the gain reduction meters utilized?
 

michael1991

New member
They monitor the Control Room bus, the gain reduction meter seems to have no function.

I the meantime I have made some quick measurements.
The Master A is equipped with new caps + foil bypass.
Channel 40 is original state. Testsignal 1 kHz square wave.
F397ECD6-1BD1-45A1-874F-449E78B670F8.jpg

Channel 35 is equipped with new caps + foil bypass and replaced opamps
D398F360-55F9-4E2D-BE45-2897C128A98E.jpg

Channel 40:
IMG_3334.JPG


Channel 35:
IMG_3335.JPG

The y-axis span is only 1 dB...

Have to compare old state in channel + master, and new caps in channel and master, and new caps + opamps seperately, but I can state, that the new opamps sound even more transparent, clearer, more punchy.

Greetings from Germany
 

sweetbeats

Reel deep thoughts...
[MENTION=201181]michael1991[/MENTION]

Is there any way you can do me a huge favor, and pull the bottom panel off and post a pic of where that monitor meter plugs into the motherboard?

Also, what frequency square wave tone were you analyzing? And it looks like, while the changes reduced the frequency response variance by about 0.1dB, the distortion increased. Have you done any formal before and after distortion analysis?
 
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michael1991

New member
IMG_3393.JPG
Hope this is what you were asking for.

it was a 1kHz square wave.

I have not made distortion or noise analysis so far, but i think i will next week
 

michael1991

New member
Hi, i think i figured it out.
all the master section meters are connected seperately to a "riser board", where wired go to the mainboard. i think it are the blu/white twisted cables with label 31 and 32 that correspond to the l / r monitor meter

IMG_3420.JPGIMG_3419.JPGIMG_3421.JPG
 

sweetbeats

Reel deep thoughts...
[MENTION=201181]michael1991[/MENTION] thank you SO much for taking the time to dig up that information. I really appreciate it!

The reason I’m asking is because I scored a couple meter modules on eBay, one of them being a “MONITOR” module, the other, which appears to be an identical configured module is labeled “SERVICE”. They are both “tested...working”:

5E131EEB-4EF6-4520-9AD5-2F28A1ED2AC1.png

7FAE5353-0444-4A66-A167-0C48FD0E4CBB.png

That’s right...I got the first one for $3.25USD and the second for $15.00USD. The second one was originally listed with a starting price of $5.00USD, but I think the seller bumped it up after the first one went for so cheap. That’s really cheap. I mostly see Studer 70mm meter modules selling for $100-200USD. The shipping was $15.00USD each, but the seller agreed to combine the shipping. The “SERVICE” module I’ll just keep as spares, but I’d like to use the “MONITOR” module on the control room buss.

I cleaned up one of the group modules. It’s hard to tell since overall the console is in pretty good shape, but it did shine up a bit...the one with the orange fader knob is the one I cleaned up:

79C8C0C4-A147-4CB8-A62B-5A5C56EF5B50.jpeg

Another little update if you will has to do with some A/B listening tests I was doing between the transformer outs and non-transformer outs. I’ve just been using headphones for function testing, but I dragged my studio monitors out to the shop the other night...hooked them up as well as my MacBook and MOTU 8M interface so I could reproduce different material from a good quality source and also simultaneously track material output from both the opamp-based direct outputs on the mono input modules and from the transformer outputs on a Master module.

In the headphones I thought I was hearing a distinct difference between the two; that the transformer coupled outs sounded “better” (wider soundstage, better hi-mid clarity, maybe warmer/more present LF?). But...with the monitors hooked up, both outputs sound f***ing *awesome* to my ears. Like, I was shocked by what I was hearing compared to everything else I’ve had those monitors hooked up to...big...wide...huge...awesome...so that means either it’s really good (the Studer), or I’ve just not had the monitors hooked up to anything decent all these years. So, both output types sound awesome. I was exploring this to see if I was going to find some way to convert the non-transformer direct outs to transformer, but I don’t think so...like, I can still hear some subtle difference, like the bottom end has maybe a bit more presence with the transformer outputs, and the soundstage seems a bit wider or deeper with the transformer outputs, but I suspect that is more likely due to the transformer outs (in this case) being from a stereo VCA module vs the opamp-only output being from a pair of mono VCA modules...like there might be better stereo imaging from the stereo module. And actually that could effect the perceived LF response if there are any minor phase issues between the two mono modules vs the stereo master module.

But I just listened to different styles of familiar music at different levels for, like, two hours. I’ve been so busy for so many years it seems like it’s been a long time since I just sat down and listened to music. My monitors have never sounded so good. It’s remarkable. And what else was remarkable was how easy it was to hear elements I would consider as “issues” with even the professional mixes I was listening to. Not all of them, but a few of them. The good bad and ugly were all out there to be heard. Again, both output amps sounded great. I was A/B-ing the source throughout my listening session. I wouldn’t consider the transformer outs really “better” at this point, just a little different flavor...and maybe better stereo image, but, again, I think that may be related to one being a stereo module and the other being two mono modules.

The sales brochure for the 928 refers to all inputs and all outputs equipped with Studer transformers. I have confirmed all the inputs can be configured with transformers...the mic inputs are standard with transformers, and there are lands on all the mono and stereo input module PCBs for input transformers for the line inputs. But there is no such accommodation for output transformers except for those that are standard on the group and master modules. And physically I don’t see a way to incorporate them on the mono modules anyway...there just isn’t room. And I tipped the frame up and removed the bottom panels so I could see the motherboard and there’s no accommodation for them there, not really any room to incorporate them there. Same story on the jack PCBs. There’s probably room behind the jack PCBs, but here again as I’m taking a look, this thing is a piece of manufacturing/engineering artwork...there’s no way I could match the quality, at least not without a great deal of effort and some significant expense, were I to try and modify the direct outs with transformers, at least not the Studer circuit, which is what I’d want to do. I’m going to chalk the language in the brochure up to over-zealous marketing folks, or that it refers to trafos on the summing outputs only, and assume transformers on the direct outs never were. And the direct outs sound comparable to the transformer outs anyway to my ears using the monitors and with some varied listening. I think this is another “leave it alone”. I can’t wait until I get to the point I can mix on this thing.
 

michael1991

New member
It looks like you made a really good deal on those meters!

I have to recap one more stereo channel before I can move on with the monitoring section (For A/B comparison i leave chanel 40 with old caps and channel 39 with only new electrolytics without foils).
yesterday I recapped channel 32 and found out that this channel has no input transformers, i think that also the first stereo channel 31 is transformerless. Will find this out soon. Soundwise it comparess to the transformer coupled input like a bit more transperent but i find myself digging the transformer sound more, it sounds a bit more gritty, more lifefull. but not a huge difference at all.

With the sound on monitors I understand what you are talking. The HS8 i have connected are a solid but not an incredible pair of monitors. And it is setup in an horible way, too near at the walls. But this setup still sounds great.
IMG_3430.JPG

If you are into electronics i can encourage you to try some things in one or two (stereo)channels.
I would definetly recommend to try some bypass foil condensators over the electrolytics. (i use Panasonic ECH-U1H104JX9 and really like the result)
As little background info:
electrolytic caps are polar components, and the schematics look like there is no dc offset in the circuits intended. This means that the neagtive parts of the signal go through he caps in the "false" direction. Since the signals are not large, this will not destroy the caps, but this will introduce nonlinear distortions. Because of that i choosed higher voltage ratings on the electrolytics. I do not know how the parallel foil caps behave in this combination in theory. All I can state so far is, that I really like what the do soundwise.

And try some other opamps on one channel ( I used TI LM4562 as replacement for MC33078 and AD OP275 for TL 072)
For testing you don't have to considere anything, if you decide to upgrade your whole desk, you have to use more powerful power supplys, since these opamps draw approx. double the current :D:laughings::eek:

Will do some more measurements tomorrow.

Just found out, that the faderstart only works on my stereo channels. Have to check the manual if this feature is jumperconfigurable...

Keep on rocking:guitar:
Michael
 

ecc83

Well-known member
Can I just caution others reading this (facinating!) thread? Swapping out op amps ad hoc can lead to problems. The increase in supply current has rightly been mentioned but there is the matter of the offset polarity of inputs and outputs. The NE5534 will have both pins NEGATIVE of zero volts which is the opposite of most other op amps. I do not know how the LM4562 fairs in this regard but it would be very wise to check.

The offset is but a few tens of mV and will cause modern electro caps no immediate problems but long term they could start to pass a very low current and make pots and switches noisy. Likely their very long term life will be impared but SO long that none of us here need worry!

As ever, I refer gentle readers to Self, he has the full, very well tested dope on all things IC and Capacitor in Small Signal Audio Design.

As yer were.
Dave.
 

michael1991

New member
Thank you for your critical hints.
I only swapped the heavily used MC33078 and TL072. The replacement recommendations were from an experienced engineer who worls for a small company who design some parts for RME as their flagship M32pro, so I trust the recommondations. Second I the offset characteristics are the same but smaller in value. I checked for oszillation even in the most extreme settings i could achieve, and no problems have shown, so I think that this is a valid replacement.

I do not recommend to replace an opamp with any other just because it is "in general a great opamp", so I am glad you pointed this out.

And I am always happy for new inut and knowlage, so thanks again! And if some other experienced person want to participate in this thread, this would be great :)
 

ecc83

Well-known member
Thank you for your critical hints.
I only swapped the heavily used MC33078 and TL072. The replacement recommendations were from an experienced engineer who worls for a small company who design some parts for RME as their flagship M32pro, so I trust the recommondations. Second I the offset characteristics are the same but smaller in value. I checked for oszillation even in the most extreme settings i could achieve, and no problems have shown, so I think that this is a valid replacement.

I do not recommend to replace an opamp with any other just because it is "in general a great opamp", so I am glad you pointed this out.

And I am always happy for new inut and knowlage, so thanks again! And if some other experienced person want to participate in this thread, this would be great :)

That is fine, my comments were not aimed at your project in particular but for anyone proposing op amp "up grades". The offset polarity flip is one to be aware of.

I would also like to say that many experts in the audio field do not consider that modern electrolytic capacitors need bypassing in signal circuits but, if you hear it, you hear it! OP amps shoud in any case have 100nF twixt +&- pins and as close the chip as possible but that is nothing to do with any supposed shortcomings in the electrolytics.

Power supply caps are rather different, Silicon rectifers do generate switching pulses at MHz frequencies.

Duggy tells all!
 

michael1991

New member
OP amps shoud in any case have 100nF twixt +&- pins and as close the chip as possible but that is nothing to do with any supposed shortcomings in the electrolytics.

The cap between signal input +/- or power +/- ?

I will thest your recommodations and if the difference is "huge enough" i will implement it, even if it leads to even more work:D
 

ecc83

Well-known member
The cap between signal input +/- or power +/- ?

I will thest your recommodations and if the difference is "huge enough" i will implement it, even if it leads to even more work:D

No! POWER plus and neg and it is just to ensure stability, it will have no effect on sound quality since, 1) I am sure the local decoupling is beyond reproach and 2) you would have "seen" a thickening of signal traces (the actual RF waveform is hard to resolve. If it ain't broke, FCS don't try to fix it!

One effect of oscillation is a reduction of expected headroom but you need some VERY good kit to diagnose THD around 5532s and even better for the LM4562. Tape replay will be at least 10,000 time more distorted that that latter chip delivers.

I repeat: These caveats are GENERAL observations. I am sure the Studer gear is really as good as it gets.

Dave.
 

sweetbeats

Reel deep thoughts...
The cap between signal input +/- or power +/- ?

I will thest your recommodations and if the difference is "huge enough" i will implement it, even if it leads to even more work:D

I have a lot of input I want to offer, but little time at the moment.

The main point I want to offer is to slow down...consider not incorporating a modification until you understand what it is. Every time, and I mean EVERY time you execute a modification to the original design you invite:

A. Risk of damage
B. Unintended consequences
C. Side effects that have to be managed with additional modifications that;
D. May impact performance.

Every time. Period.

Know and understand what you are modifying and why you are modifying it before doing it. Do not just go with what somebody else is telling you you should do. Your gear is too precious to go changing it up like somebody with dysmorphia who is on plastic surgery #10.

Please don’t take offense. But you are incorporating all these modifications based on advice from people you trust, but the screenshots you posted from the software analyzer don’t show any significant improvement, potentially an increase in distortion. And those analyzer tests don’t actually say anything about how it *sounds*.

At this point in my life I don’t modify something unless there is clear evidence the mod is necessary to maintain spec or protect the device, or there is clear evidence the mod enhances the character of the device, AND I’m finding the device is limited and I need the barrier removed in order to utilize the device.

If I ignore these tenants I’m at risk of:

A. Damaging my gear
B. Changing my gear into something generic devoid of the personality it had,
C. Spending money and time on mods that castrate or damage my gear.

Do you know how much boutique lust-worthy gear is based around the “lowly” TL072?

And the 33078 is, like, a TL072 on steroids. But that boutique gear does NOT sound like it does because it is full of the fastest and “better” opamps...no...people love it because of its sonic character or functionality or both.

The Studer has some of the most thoughtful signal path circuitry I’ve ever seen. And to my ears it delivers the fruit of all those little thoughtful elements. I’m not going to say your advisor(s) is/are wrong, or your ears...I just feel you are jumping at every possible thing that purports to make it “better”. I have a good number of advisors around me too...people that have designed and built well-known gear too. More importantly these are also people that have used gear in real-world situations for decades and know more than I’ll ever know about all this stuff. Their typical advice? Leave it alone, particularly something like the Studer, or at least proceed with care and caution and to test each change to verify it met the goal and didn’t also bring unintended consequences. And more importantly they have taught me to stop and ask “why” when I want to go changing something; thoughtful suggestions after analysis as opposed to blanket “always do this...it makes it better.” And after years and years of listening but not really hearing, I’m finally heeding this advice. Maybe you have experience with some really incredible gear for the Studer to sound so poor, and if that’s the case then okay. But my amateur ears just keep wanting to listen to anything through my stock 928 desk.

Again, no offense...

And what ecc83 is talking about is placing a small value cap between each power rail and ground close to each opamp. This can reduce induced noise in the power rails at the opamp, but this doesn’t make it sound “better”...it may reduce HF interference, but most opamp’s CMRR is well good enough to fend off this interference. What the bypass caps are good for is to stabilize the opamp and prevent oscillation. Look at the schematics. The bypass caps may already be there. So ecc83’s suggestion is essential if you are determined to put “fast” opamps in there and here aren’t any bypass caps already. Otherwise, leave it alone...?

More later.
 

ecc83

Well-known member
I echo Sweetbeats warnings.

I sispect the whole "re capping" issue was partly started by a single range of computer MOBOS that had faulty capacitors and these died horribly after a few short years. There are highly professional refurbishing companies that undertake this work and they will often fit 105C caps in place of 85C jobs because densely populated devices like big mixers are not cooled as well as one would like and they want to guarantee their work for 10 years at least. The 105C caps have NO impact on sound quality (neither does fitting caps of unecessarily higher voltage rating)

My advice, such as it is, for the home refurbisher is to first MEASURE the frequency response and distortion levels of the gear and compare that with the book specifications. ONLY if there is considerable difference, greater than 2dB say or the given tolerance, should you even consider recapping or other changes. If you don't have access to test equipment that is good enough to meet those tests for such people I say leave it TF alone OR if you think something is awry, get pro help.

And finally, as with Mr Beats? No offense meant in the slightest degree.

Dave.
 

michael1991

New member
I don't take your answers as offense. So there everything is fine:)

I agree your approach in general, but I think my constaints are a bit different to yours.
I did not buy this desk because I needed it, or because i would absolutely start to mix with it in the shortest time possible. I bought it because it looked like a absolute crazy project to get this desk to work again. So I personally do not care at all if I modify the desk apart of the original specs, of if it meets the original specs again after replacing the dried out caps.
Because of this I do not care so much if I might damage some part. The good thing in this level analog technics is, that (besides some mechanically destroyed or burned pcbs etc.) nothing is really irreparable, so I can try out things in the knowing, that it will be possible to get in original state.

The tl072 and mc33078 are widely used because they are quite long available and good enough for the specs they wanted to meet. There is nothing wrong with liking the results that are achieved with that technology. Today there are some other products in the market that have improved properties over them. So why not figure out how they change the performance of a device that is never intended to be super critical in heavy production environment. If the result is liked over the original, this is great. If it will result in better specs, it would be even better.
(the colleague I pointed out earlier who did an intership at studer told me the developers stated that they have chosen these opamps because they were not much inferior to the ne5532/34 but consume less power. not because of the overall great performance of them)

What you refered as distortion, I think that is noise. I don't think that it is comparable, because the channels are all different. Every channel has its own noise performance, so comparing different channels against each others is not the best approach for this task. This desk is not calibrated at all, so it is hardly valid to point the different noise properties to changed opamps. I will compare the noise performance of a channel before and after replaced parts, to get some insight into this topic.
But I think the deterministic part of it (the changed Frequency response is valid to compare. I think the lower deviation on the upgraded parts (even if it is a small change) is a hint for "better performance" even if this does not show all aspects of good sound.

All measurements are and will be done with an Rhode&Schwarz UPV.

For the effects of proper power decoupling i want to point out to the Analog Devices tutorial on "decoupling techniques". There are some interesting cases described.

Looking forward to some more fruitful conversation:)
Best wishes
Michael
 
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