new monitor controller - inspired by NewYorkDave's design

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matthias

Well-known member
Joined
Jun 4, 2004
Messages
768
Location
germany / frankfurt
hello,

here's the design of my new monitor controller.
It is based on a schematic by NewYorkDave.

monitorcontrol2008.pdf

I added a headphone amp, individual switching for inputs and outputs
and I slightly adjusted some resistors to more common values.

do you think it's ok to connect the headphone amp directly after the stepped attenuator?

any ideas for improvements?

...all suggestions are welcome...


thanks,
matthias
 
There's a few points I found :
1°) I don't know how an output on a 5K attenuator can react on a 600 ohm load... Most of the monitors and amps have 10K input impedance and don't pump current but...
2°) Buffer the headphone amp input... Your 5K output attenuators isn't 5K with it. It is : attenuation value in parralel with a 1K.
3°) I would to solve these problems, attenuate the signal before the OP275.

I hope it helps. :wink:
 
Some more thoughts:
* R20 and R27 are pointless with the opamp outputs AC-coupled, skip them.
* As shown, the source selection doesn't make sense to me--you either mix both sources or have them off both.

I would to solve these problems by attenuating the signal before the OP275.
Look again--the opamps are used for the mono switch only.

Samuel
 
Look again--the opamps are used for the mono switch only.
Samuel
oooops ! I didn't see ! :oops:
In fact, I didn't look a long time because the concept of a passive monitor controller isn't clear for me. What's the goal ? I mean the hugest and best desk always have an active monitor control. To switch between two different monitors at a different acoustic level isn't revelant IMO neither to switch between 2 sources without matching levels. If it's to save money I understand... If it's not to deteriorate the signal, you've got to make T pads with a constant impedance (input and output).
 
this came up before in

http://www.groupdiy.com/index.php?topic=21032

hey, look at this-

[quote author="NewYorkDave"]In my opinion, the Prime Directive for any monitor controller is that it must not disturb or degrade the signals being monitored. Passive controllers may be fashionable at the moment but if you agree with that Prime Directive, it's hard to deny the advantages of high-quality active circuitry--especially if any kind of mixing (i.e., a mono mode) is to take place in the monitor controller. [/quote]

:wink:
 
Sorry, this threat on the goodness or badness of a passive control is already done and I didn't want to start a discussion about this...

So we've got to stop talking about this and help matthias... if we can... :wink:

Anyway, the link is really interresting... :thumb:
 
thanks for all the answers...


for me this is not about fashion..
I don't have a mixer. just a computer and some converters....
without a monitor controller the output level is definately too loud! even when I tame down the input control of my monitor speakers.


I made some corrections to the first design...

monitorcontroller2008-2.pdf

- skipped R20 and R27
- decoupled the headphone amp with some opamps

do you think I need R23 and R29 ? and I also can't find a reason for R21 and R28

I adapted these directly from nyd's schematic http://twin-x.com/groupdiy/albums/userpics/MonCtrl1.pdf



@ lolo-m: the input imedance of my monitor speakers is 10k so I think the 5k attenuator is ok...

@ Samuel: the source and destination switching may look a it strange, but I want to use some of
these eao illuminated pushbutton-switches that I've bought long time ago for this project.
so I can switch every source and dest. on and off this also meets the function of a mute-button
 
For me this is not about fashion...
Without a monitor controller the output level is definately too loud!
We're not questioning the need for a monitor controller. We're questioning the current trend about those beeing passive rather than active.

Do you think I need R23 and R29?
Yes, they keep the caps at 0 V which prevents pops when you hit the mono switch.

And I also can't find a reason for R21 and R28.
They decouple stray capacity from the opamp output. Probably not needed if connections are short, but good practice.

You don't show PSU decoupling for the opamps--absolutely needed.

Samuel
 
It seam to be OK like this but you can save 2 OP275 making the differencial sum with only one and enterring assymetric in the headphone amp. Notice that, because you have a buffer there, you can had a stereo pot to have an extra attenuator for the headphone amp. It can be usefull...
 
We're not questioning the need for a monitor controller. We're questioning the current trend about those beeing passive rather than active.
ahh... ok... I bought a passive relais attenuator kit from dantimax some time ago so I never thought about an active solution...

You don't show PSU decoupling for the opamps--absolutely needed.
yes, you're right... I excluded the whole psu part for more clarity...

Do you think I need R23 and R29?
-
Yes, they keep the caps at 0 V which prevents pops when you hit the mono switch.

but what about the caps?
my converter already has 22µF caps at the input...
so could I omit all other dc-blocking caps in the circuit...
(except for the headphone section, there I would normally place them at the output before the 10R Resistors )
 
[quote author="matthias"]
but what about the caps?
my converter already has 22µF caps at the input...
so could I omit all other dc-blocking caps in the circuit...
(except for the headphone section, there I would normally place them at the output before the 10R Resistors )[/quote]
The opamps may have dc at the output and that's why everyone put dc-blocking caps in the circuit. But looking at different schematics, my question is what is the way when they are polarized ? On my DDA desk, the caps are all in the same polarity during all the signal path !!! the dc-blocking caps can only block one kind of dc (don't remember if it was + or -). Is there any reason ?
 
I bought a passive relais attenuator kit from dantimax some time ago so I never thought about an active solution...
There's no reason why you couldn't use your attenuator kit for an active monitor controller. If you are interested I might find time to draw a sketch.

The DC-blocking caps can only block one kind of DC.
No no no--if the offset is well below reverse bias (i.e. < 1 V) you can orientate polarised caps either way.

Except for the headphone section, there I would normally place them at the output before the 10 Ohm resistors
You would need very large caps to get good low-frequency response and low distortion with low-impedance headphones. Better let the output DC coupled, a few mV offset shouldn't bother the headphones.

Samuel
 
No no no--if the offset is well below reverse bias (i.e. < 1 V) you can orientate polarised caps either way
I trust you but I don't understand... Of course Caps won't burn with dc <1V reverse... But I don't think they'll block anything reverse... My questions are :
- What are they supposed to do ?
- What's the good dc-blocking caps polarity on the opamps output ? And WHY ???

I'm really sorry to ask this, but this question remained unanswered to me during the passed 15 years !!! :green:
I hope I'm clear...
 
But I don't think they'll block anything reverse...
Sure they do!

What are they supposed to do?
You answered that yourself a few post above..? They block DC which would otherwise make pots scratchy and switches plop.

What's the good DC-blocking caps polarity on the opamps output?
Usually the output offset can have either polarity, so there is no preferred polarity. There are some special circumstances where one polarity is preferred, e.g. at inputs and outputs where phantom power might be applied or in case the offset is dominated by input bias currents with known polarity.

For more details try a search, things were discussed before.

Samuel
 
so... here's hopefully the final version of my monitor controller design....


monitor controller2008v3.pdf

- separated the mono circuit with relais to prevent any crosstalk...

- added trimpots to adjust the volume range of the 64-step attenuator

...

what do you think ? any major faults :wink: ?
 
Hi Matthias,

omit the 'k' in your trimmers, 100R should do, if needed at all in this place.
Still not shure if the attenuator's connection to gnd is needed. Without connection to gnd it would stay floating ballanced when not switched to mono. The trimmers placed in series with R17 and/or R24 might be a better place to catch imballance in case switched to mono. As always YMMV.
 
hi,

omit the 'k' in your trimmers, 100R should do, if needed at all in this place.

the trimmers (should be 47k, precision type with 25 turns) are not for "imbalance correction", the output level from my converters is very high, so I want to tame the input level of the stepped attenuator (master volume control) down a bit... then the useable range of my stepped attenuator will be much more wider...

Still not shure if the attenuator's connection to gnd is needed.
for the configuration as a voltage divider (as shown here) you need the connection to ground. I know there are other alternatives but I have already built a relais attenuator for this purpose.
 
I'm pretty sure these trimmers are the wrong solution for various reasons. What is the attenuation range you really need?

For the configuration as a voltage divider (as shown here) you need the connection to ground.
No, not with differential signals. There are both advantages and disadvantages to each way, but likely not enough difference in practice to care.

Samuel
 
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