Simple active summing mixer.

[radiance]

Waiting for the pcb's to arrive......I'll keep posting my progres here..(if any :shock: )

 

[capnspoony]

I'm curious as to how this is panning out as well .. especially after building NYD's active mixer

best
Richard

 

[radiance]

Well, there's some progress...

PCB's are in and look good. Some minor mistakes which are easy to fix.
They are huge...aproximately 20cm X 40cm and another of 16cm X 40cm.
Will post some pics soon.

Short describtion: 20 channel active summing mixer with 3 busses (two sub groups and one master). Channels are stereo and only have a bus selection switch (Lorlin) which also acts as a mute switch and another toggle switch that switches between stereo or dual mono. All switching is done with relais (small smt ones). Master and group busses have some bells and wistles but more about that later. Inputs have THAT line receivers and summing opamps are either DOA's (2520 footprint) or dip 8 (for example opa604). Summing opamps can run on 24V or 15V. The rest of all IC's runs on 15V. Relais have their own ground bus so no switching noises will enter the audio (hopefully).

So far I've partly stuffed one side of the master bus just to see how it's passing signal and so far it's working as planned.
Right now I'm waiting for some THAT line drivers/receivers to arrive.
I really have to take this slow, one step at the time. The scale of this project makes it very expensive and since this is my first pcb design I want to get everything absolutely right before proceeding to the next step. That means I'll test every part on it's own before connecting everything together....O and there's an nice headphone amp in it as well

 

[radiance]

Here some pics from my test setup. It's passing a square wave fine.
That's a balanced signal going into a THAT1243 line receiver > 990 DOA summing opamp > opa604 buffer > THAT1646 line driver into my scope
No listening tests yet. Will do that later to determine whether I like caps or a servo circuit or bipolar caps or maybe fancy audio fool caps or whatever.

 

[Phazinhead]

Wow !

very nice work Radiance !

I would like to ask you something:

I'm currently wondering of building a summing box or a little mixing console. I'm wondering of using some 2520 DOA and why not some JFET amp (this one : http://www.groupdiy.com/index.php?topic=1184&postdays=0&postorder=asc&start=0
a control of the second harmonics level can be added on them). I'm also wondering of using input and output transformers.
The main question is actually : is it really necessary to spend lot of money in DOA and transformers so as to have a "good" sound.. or do some OPA IC and THAT line driver could have a good sound too ?
Have you found some answer to that kind of technical questions when you designed your summing box ?
thanks for your advice, and good luck for your work, it looks very clean !

best

Phaz'

 

[radiance]

[quote author="Phazinhead"]
The main question is actually : is it really necessary to spend lot of money in DOA and transformers so as to have a "good" sound.. or do some OPA IC and THAT line driver could have a good sound too ?
Have you found some answer to that kind of technical questions when you designed your summing box ?
[/quote]

The only part where it really matters is the summing amps. That's why I used a 990 DOA for summing. I don't think that line drivers or line receivers or buffer stages are going to make a huge difference to the total sound.
I've inserts on both the masters and groups so if I want "color" I can always insert some kind of "coloring" device which has tubes and iron in it.

And believe me I will....

 

[Phazinhead]

The only part where it really matters is the summing amps. That's why I used a 990 DOA for summing. I don't think that line drivers or line receivers or buffer stages are going to make a huge difference to the total sound.
I've inserts on both the masters and groups so if I want "color" I can always insert some kind of "coloring" device which has tubes and iron in it.

okay ! ... and could it be better to have only DOA everywhere ? :green:
DOA are cheap to make.. around 5 euros for a 2520 DOA for example if I remeber right.. I think it is as expensive as a good Op amp IC.

 

[radiance]

[quote author="Phazinhead"]

okay ! ... and could it be better to have only DOA everywhere ? :green:
DOA are cheap to make.. around 5 euros for a 2520 DOA for example if I remeber right.. I think it is as expensive as a good Op amp IC.[/quote]

Seems a bit like overkill to me but sure, go ahead. :thumb:

 

[Tubemooley]

First of all, your project looks great. Really good work. Second, where are you getting 2520-style DOA's for 5 EUR? I believe that you are pretty close in that estimate for the parts alone. 5 EUR may be a bit low but not by that much. However, how much do you pay yourself for working on your own projects? Probably not much. I have that same problem. Yes, the parts for any hand-built DOA are not that expensive. But the assembly, matching and testing time should be considered. Lastly, concerning DOA's versus monolithic opamps and line drivers, etc. We have done a fair amount of testing in this area. While it is true that the "good" monolithic opamps (OPA604, OPA134, etc.) are basically indispensable, I find that they are not the same as a true DOA. The monolithic IC's have quite a few transistors in there (20 to 30 in some cases) and integrated capacitors and resistors too. I have found that the monolithic opamps usually always end up "filtering" or "veiling" the sound. Yes, I have many, many opamps in my gear. They are hard to not use. A good circuit implementation will minimize these veiling effects. Then, when it comes to the THAT drivers and receivers, go read the datasheets on those. They usually say "transformer-like performance". These are good chips. I have used them. I used the SSM-2142 way back in 1993. But they are not the same as a true transformer-balanced output amplifier. We used to own a Dangerous 2Bus LT summing box. It was a very good box. We have since sold it because we were able to upgrade to a better discrete summing mixer. However, if you have the opportunity, open up a Dangerous 2Bus and look inside. The receivers are Burr-Brown INA134 (I think) balanced receivers. The opamps are all OPA134 (or 2134 or 4134). And the outputs are all DRV134's. All very good Burr-Brown devices. The Dangerous box is pretty close to transparent. It does not impart very much of the veiling effect. It is very transparent and clean. However, it does not have the oomph and balls of our discrete summing mixer. A properly designed discrete mixer will have really solid low-end and ultra silky smooth high-end. It is difficult to get all of that with just monolithic opamps. The internal design of the monolithic opamps is too "stomped on" to give good specifications but that ends up impacting sonic performance. That's what I think anyways. And transformers are big and expensive. So if you're spinning circuit boards, you may want to leave a few hooks in your design to make changes later. DW.

 

[radiance]

[quote author="Tubemooley"]First of all, your project looks great. Really good work. [/quote]

Thanks :grin:

[quote author="Tubemooley"]Second, where are you getting 2520-style DOA's for 5 EUR?[/quote]

Dunno, maybe Phazinhead knows :wink:



[quote author="Tubemooley"] Then, when it comes to the THAT drivers and receivers, go read the datasheets on those. They usually say "transformer-like performance". These are good chips. I have used them. I used the SSM-2142 way back in 1993. But they are not the same as a true transformer-balanced output amplifier. [/quote]

I just used those THAT drivers/receivers cause they're supposed to be a bit better than the Burr-Brown equivalents. I'm not after transformer like performance. The outputs of my Lynx Aurora go straight into my summing mixer. Also, the line receivers I'm using (THAT1243) don't even have transformer like performance. You're talking about the THAT1203. Man, I know those data sheets by heard now.

[quote author="Tubemooley"]We used to own a Dangerous 2Bus LT summing box. It was a very good box. We have since sold it because we were able to upgrade to a better discrete summing mixer. However, if you have the opportunity, open up a Dangerous 2Bus and look inside. The receivers are Burr-Brown INA134 (I think) balanced receivers. The opamps are all OPA134 (or 2134 or 4134). And the outputs are all DRV134's. All very good Burr-Brown devices. The Dangerous box is pretty close to transparent. It does not impart very much of the veiling effect. It is very transparent and clean. However, it does not have the oomph and balls of our discrete summing mixer. A properly designed discrete mixer will have really solid low-end and ultra silky smooth high-end. [/quote]

I know the dangerous and how it's build. It's quite simple actually.
It uses an OPA 134 as summing amp. My design is quite similar only it uses an DOA 990 as summing amp.
Sure, I would love to have a fully discrete summing mixer, but if you're into the process of building something this big, you WILL simplify along the way. I decided to only use DOA's where it matters the most and AFAIK that's summing.

 

[Tubemooley]

That's cool. You're building something you need and something you want the way you want it. I can respect that. You and I should agree to disagree. You're building something..... something cool. That's the important part. Rock on brother. DW.

 

[Tubemooley]

I definitely don't want to be a douche-bag here even if that's how it sounds. Ultimately, if possible, you would want to open up the Aurora and upgrade the opamps in there. It is THOSE opamps which will have the most significant impact on your sound. It is THOSE opamps which impart the veiling effect first. After that, you cannot get rid of the veiling effect because it's in there already. The first amplifier which the DAC's see is the most important. After that, it's all down-hill. I've spent a lot of time dissecting and modifying our signal path, especially right after the DAC's. A 23-cent opamp just outside the DAC will give your entire signal path a 23-cent flavor. Been there, done that. The converter manufacturers don't know squat about real audio quality. Or, if they do, they cheap out and make marketing decisions which decrease the overall audio quality of their boxes. There are close to zero readily-available converter boxes out there worth buying. It's the same on the ADC side. In fact, it's a bit worse. The ADC's are even more sensitive to what they get fed than the DAC's are to what they feed. I've been in there. Open up your Aurora and see what the opamps are. DW.

 

[radiance]

[quote author="Tubemooley"]I definitely don't want to be a douche-bag here even if that's how it sounds.[/quote]

No, no..all feedback is welcome. I can take it :thumb:


[quote author="Tubemooley"] Ultimately, if possible, you would want to open up the Aurora and upgrade the opamps in there. It is THOSE opamps which will have the most significant impact on your sound. It is THOSE opamps which impart the veiling effect first.[/quote]

I opened up the Aurora before I started on this project. Output stage of the Aurora consists of: DA> OPA2227 > OP275 > OPA2134.
You're absolutely right when you say that the first opamps have the most impact on the sound.
I had 30 Lundahl transformers + 30 DOA 990 kits laying around for this project (the summing mixer, not for upgrading the Aurora LOL) but it would not had made any sense after the Aurora. I decided to use similar quality opamps in my summing mixer as found in the Aurora and only go for the DOA 990 in the summing stages.
Also, I don't feel confident enough to upgrade those smd opamps in my Aurora, especially since I just bought this new.

 

[Tubemooley]

OK. Cool. You did the investigative work on the Aurora. That's good. They are using better opamps in there than I had figured. We had a Lynx One or Two on trial once about three years ago. We fired it up and listened to it and it was horrible. So we just returned it to the dealer. You've made the right decision now that you know what the Aurora is doing. I did heavy mods to our Creamware A16 box. It's all SMD in there. I do SMD re-work at my dayjob so I can do that stuff in my sleep. But even I get the willie's when I'm working on an expensive box which I purchased. The Creamware box is now the back-up box and hardly gets used anymore. But the mods I did in there were well worth it. DW.

 

[radiance]

[quote author="McCroskey42"]Since you're already using THAT 1646's for the main outs, why not try mediatechnology's 1646 headphone driver? The posts here have been nuked, but it's pretty well documented over in the Pico Compressor forum. Sounds awesome, too! (listening to Dark Side of the Moon through it right now, actually...)[/quote]

Build it and it sounds great :thumb:
here some pics

 

[ruairioflaherty]

Great work Roland! Keep it up :thumb:

You must be getting thirsty, seeing as we have the best Guinness here in Ireland - :guinness: :guinness: :guinness:

Cheers,
Ruairi

 

[radiance]

Thanks mate :thumb:
Indeed, I'll need some draft along the way to cool the brain.

Still so many steps before this project is finished.
So far everything works as planned. That's such a good feeling :grin:
Today, I've tested the mono switch in the monitor section and again succes.

But then again, this is a very straight forward project, no rocket since at all.....

 

[radiance]

Here another test pic...

 

[dagoose]

:shock: Nice!!
Maybe i will also do a summing mixer one day if i just had the time..

 

[syn]

Looks good!!! What are the rotary swtch. you use? :thumb: