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Old 28th April 2013, 07:39 PM   #11
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Quote:
Originally Posted by mickeymoose View Post
Put a 100 ohm resistor on the outputs of A301 and A401.
Hi again Mickeymoose,

Thanks for all your help - this is my last question I promise!!

I have added the 100 ohm resistors (R308 and R408) and attached a revised schematic. Have I placed these correctly or should they be placed after DC drain resistors R306/R406 and before the switches (so as to isolate shunt capacitance)?

I have also decided to keep things simple and use a log pot for VR301 and VR401.

Thanks a lot once again for your assistance.

Dufflefan.

Hi Speedskater,

Iím very sorry but I donít know exactly which elements of the schematic you are referring to. It is quite likely I might have drawn something poorly!
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File Type: png 20130428 4 Channel Mixer Full Schematic v1.03.png (108.2 KB, 63 views)
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Old 28th April 2013, 08:54 PM   #12
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Dufflefan:

You missunderstood me. The switch should place a short between the 100R and the 10kR to ground, otherwise you still have the level shift when an input is muted.
When all channels are active the 10kR are in parallel and set the gain of A501 with R501 (10k:4) if one channel is off it is 10kR:3.

Your ground symbols are OK. It is just the way your program does it and identifies the circuit nodes.

Overall gain is also affected by the settings of VR301 and 401. Maybe they are better placed at the inputs of A301 and 401 or go with the scheme of the M267.

And: keep asking! E
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Old 29th April 2013, 01:18 AM   #13
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Hi again Mickeymoose,

My apologies for the misunderstanding!

I have moved the volume controls to the inputs of A301 and A401 as suggested.

I have also relocated the 100R resistors (R308/R408) to the end of the outputs before the 10K resistors. Between these I have inserted the switches which divert the signal to ground when muted.

I have attached a new schematic showing these changes. I hope this is correct!

Quote:
Originally Posted by mickeymoose View Post
And: keep asking! E
Thanks again for the guidance, it is very much appreciated!
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File Type: png 20130428 4 Channel Mixer Full Schematic v1.04.png (107.8 KB, 57 views)
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Old 29th April 2013, 02:15 PM   #14
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The 4 Ground symbols under the input IN+ and IN- connections.
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Old 29th April 2013, 02:23 PM   #15
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Dufflefan:

Your current swithing (muting) arrangement will cause gain changes when the switch is activated.
What I meant: Connect the 100R to the 10kR. The switch onto this junction, the other side to ground. Muted when the switch is closed. This is not the best, but the cheapest way. I prefer a semiconductor solution. E
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Old 29th April 2013, 07:33 PM   #16
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Quote:
Originally Posted by mickeymoose View Post
Dufflefan:

Your current swithing (muting) arrangement will cause gain changes when the switch is activated.
What I meant: Connect the 100R to the 10kR. The switch onto this junction, the other side to ground. Muted when the switch is closed. This is not the best, but the cheapest way. I prefer a semiconductor solution. E
Hi Mickeymoose,

I've got it now! Sorry for the misunderstandings.

I think I'll stick with the switches for muting purposes but next time I'll read up on semiconductor muting. I haven't read enough on the subject as yet but I will certainly be considering it for my next project once I've read more.

Thanks again for all the assistance. When you're learning with these things having such help is a huge benefit. It is greatly appreciated.

Dufflefan.

Hi Speedskater,

If I've understood correctly, I think you're referring to parts FB101, FB102, FB201 and FB202. If so, these are ferrite beads rather than ground symbols. I'll work on making the symbol a little clearer for next time!
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Old 29th April 2013, 11:41 PM   #17
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No it's the circuit ground symbols to which I refer. I fear that you plan to connect them to the XLR jack's pin #1.
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Old 30th April 2013, 12:17 PM   #18
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I'd like to disagree with Mickeymoose on the change of gain with channel volumes and mute switches; on a high z mixing bus, this would be true, but on a virtual earth ultra low z bus like this there will be no change. Just a tiny change in noise levels, insignificant.

As a user (rather than designer) of mixers since op amps were not good enough for quality audio (my first mixers were designed round vacuum tubes), I've got certain habits, like separating the channel pot from the input gain, and putting a master volume so that different sensitivity power amps could be used with reasonable levels on the input pots (if A501 is compensated for zero gain, increase R 501 to 47k and put a 47k potentiometer – no, I suppose that's a rheostat, technically - round it in parallel).

Warning (not that you can do anything about it); once upon a time one could assume that mixing the two sides of the stereo into mono would give an acceptable balance. Older mixing engineers like myself always checked the mix in mono, in case it went across AM radio or televisions.
Newer producers, knowing their products are going to be distributed in mp3 and listened to on phones, find it more impressive to put in antiphase information, ignoring compatibility entirely, which can make for some pretty weird balances in mono. As I say, nothing you can do about it.
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Old 30th April 2013, 11:26 PM   #19
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Quote:
Originally Posted by Speedskater View Post
No it's the circuit ground symbols to which I refer. I fear that you plan to connect them to the XLR jack's pin #1.
Hi Speedskater,

Apologies for the confusion! I understand now. If I am correct this brings up the ďPin 1 ProblemĒ which I have read about on the forum and elsewhere. I have heard differing opinions on whether or not to connect pin 1 to chassis ground or audio ground.

I had indeed intended to connect pin 1 of the XLRs (channels 1 and 2) to audio signal ground. My idea was to have audio signal ground for channels 1, 2, 3, 4 and the output jack all connected with only the power supply virtual ground connected to the chassis. However, thanks to your warning I have now altered the grounding system as shown in the attached schematic.

My design is now as follows:

Channels 1 and 2: Pin 1 of XLR connections to chassis ground.

Channels 3 and 4: Sleeve of TRS jacks to audio ground.

Output Jack (Mono/TS socket) to audio ground.

Virtual Ground of Power Supply to chassis ground.

Would this work ok? I canít see that I have connected any ground loops with this setup but then again I am learning all the time!

Thanks very much for the help, I really appreciate it.

Dufflefan.

Quote:
Originally Posted by chrispenycate View Post
I'd like to disagree with Mickeymoose on the change of gain with channel volumes and mute switches; on a high z mixing bus, this would be true, but on a virtual earth ultra low z bus like this there will be no change. Just a tiny change in noise levels, insignificant.
Thanks for looking at the schematic. Having made the changes as per the recommendations of Mickeymoose and Speedskater, I am confident the design has been improved. It is satisfying to know however that my first design wasnít too far off being ok. Something I canít say expected so thanks for the comments!

Quote:
Originally Posted by chrispenycate View Post
As a user (rather than designer) of mixers since op amps were not good enough for quality audio (my first mixers were designed round vacuum tubes), I've got certain habits, like separating the channel pot from the input gain, and putting a master volume so that different sensitivity power amps could be used with reasonable levels on the input pots (if A501 is compensated for zero gain, increase R 501 to 47k and put a 47k potentiometer Ė no, I suppose that's a rheostat, technically - round it in parallel).
This is very interesting Ė a long time in the future, when I am more experienced, I hope to design valve amplifiers so this is useful to know. As I know which devices will be plugged into this mixer (and it is very unlikely these will change), I will retain the volume controls as per the current design to keep things simple.

Quote:
Originally Posted by chrispenycate View Post
Warning (not that you can do anything about it); once upon a time one could assume that mixing the two sides of the stereo into mono would give an acceptable balance. Older mixing engineers like myself always checked the mix in mono, in case it went across AM radio or televisions. Newer producers, knowing their products are going to be distributed in mp3 and listened to on phones, find it more impressive to put in antiphase information, ignoring compatibility entirely, which can make for some pretty weird balances in mono. As I say, nothing you can do about it.
Thanks for the information about mono balance and anti-phasing regarding modern recordings. Although there is nothing I can do, it is still very interesting to learn!
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File Type: png 20130430 4 Channel Mixer Full Schematic v1.06.png (111.5 KB, 37 views)
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Old 1st May 2013, 03:30 PM   #20
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Great, you are getting there! I would still change the virtual ground op-amp to an inexpensive 071 or TIs TLE2426.
Cheers, E
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