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Need help understanding Neve summing bus with PM2000 mixer
Old 25th November 2019
  #1
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Need help understanding Neve summing bus with PM2000 mixer

I wonder if I can get some thoughts on how possible this is. The PM2000 has 33K summing resistors going into the "program" bussing network. I've attached a pic of the schematic of where the input strip enters the main busses. And also the schematic of the PM2K master channel. And the BA283 schematic (w/ 2 amplifier stages)

Is this as simple as isolating a pair of program bus rails and plugging them into 2 ch's of Carnhill VTB9046? (I believe is the correct line input transformer part) attached.

The total resistance of 32 channels of 33K in parallel is 1031.25 ohms. Is this the source impedance going into the mix bus? Should present no problem being loaded with that Carnhill at 10K:600 input correct? (I think that's the value Neve usually had for line in?)

The BA283 schematic doesn't have a line input transformer on it. Is the spot marked "U" the secondary of a line input transformer?

(Or perhaps I don't need a line in transformer at all but I have a feeling it gets more complicated without)

Anyway, final question is, are 2 channels of BA283 going to represent a problem for one rail of my mixer's power supply? (It's +/-24V) Current draw wouldn't be increased that much, but I don't know if there might be other implications that could warrant a special +24V supply for BA283's.
Attached Thumbnails
Need help understanding Neve summing bus with PM2000 mixer-screen-shot-2019-11-25-5.52.42-am.png   Need help understanding Neve summing bus with PM2000 mixer-screen-shot-2019-11-25-6.13.43-am.png   Need help understanding Neve summing bus with PM2000 mixer-pm2kmixbus.jpg  
Attached Files
File Type: pdf 283 schem(1).pdf (601.2 KB, 14 views)

Last edited by Jonk; 2nd December 2019 at 01:30 AM..
Old 26th November 2019
  #2
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The question you have to ask is - would the results actually be any better than the standard PM2000 mix bus? Probably not. So a lot of effort for a very small return. Just use it and make music with it. Thousands of shows have been mixed through PM2000s, a lot of them very good.
Old 26th November 2019
  #3
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After many years of obsessively modding gear and trying to squeeze out every bit of quality... I finally see radar Doug’s point ! Recording better music has a much higher return on investment that modding things every which way. Most people don’t spend enough time listening to music anymore to appreciate the difference I think, not that that should be the driving force for anything though.

I think (without being a math mod impedance expert) that what you are proposing is reasonanble, kind of like an insert for the mix bus. In theory you could use that to use any mic preamp as your summing amp, which could open a lot of options for colour etc. It’s something I’ve i often considered but never went for.

Let us know how you progress please!
Old 27th November 2019
  #4
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Let me explain a little more. First, the 2 buss is important. How many stages is this back from your monitors? If we're going backwards, common scenario might be monitors<DA<AD<mixer2 bus. I know from having installed the API 2 bus from CAPI that it makes a dramatic difference on the sound. And it's treating everything. My reasons for going back from the CAPI to the PM2K's original busses had to do with gain, although in hindsight I could have altered the gain of the opamp I believe to have attained a better spot.

Anyway, the PM2k busses do sound pretty good, especially my tweaked pair with 990C's and UTC output transformers. But good enough for a live show and good enough for the 2 buss are different. I've noticed a very slight roll off for years, (regardless of opamp or transformer) which always has to be made up for with a little lift here and there or on the mix itself.

I have 8 of my input strips with direct outs wired to 1:1 transformer. Recently I swept these channels and found that they were perfectly flat out to 22K. I then swept with the PM2K's master bus in the signal path and found that there is a very slight rolloff. Whether people can hear this or not I'm sure this being gearslutz there will be some sort of argument, but anyway, around 3-5K the the rolloff starts. By 5K it is .5dB down. By 16K it is 1dB down and by 18K, it is down 2dB. Perhaps these figures are acceptable but this rolloff seems to confirm my suspicions that there it a very slight darkening of things run through this board via the full signal path.

I am simultaneously building 4 EZ1290 (DIY 1073's w/out the eq) and I've already installed one master bus in this console . I was more focused on putting it in internally, but actually a simple 2 bus send would be cool, then I could just patch it into a pair of the 1290's either with a switched line in transformer and just the BA283 part of the circuit (Is this at the point "U" on the Neve schematic?), or pad down the summed L R master even more before going into a mic preamp. Although I'm not sure the latter makes sense if I'm at about 1K output impedance for both busses and I start adding resistance just to make it up again with more amplification, am I not adversely affecting the loading of the bus stage? That's why I asked is it so simple as to think I can just plug this stage directly into a 10K:600 line in transformer, like Neve used for line ins and let Neve do all the output lifting?

I just don't know exactly where I am with gain at the L and R bus rails (prior to master buss) For the CAPI summing bus I felt like that point needed to be higher going into the CAPI 2 buss even when I got help calculating for 33K summing resistors. I just had to boost the CAPI buss so much to get to the input levels for final mix that I gave up and went back to stock.
Old 27th November 2019
  #5
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Why dont you just look for the components in the PM2000 bus amp that are rolling it off? An hours investigation could solve your problem.
Old 27th November 2019
  #6
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Quote:
Originally Posted by Radardoug View Post
Why dont you just look for the components in the PM2000 bus amp that are rolling it off? An hours investigation could solve your problem.
this!

actually part of the charm - if there is any?

(still got two pm3500's somewhere at the warehouse - but pm2000? must be another 10 years older...)
Old 28th November 2019
  #7
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Quote:
Originally Posted by Radardoug View Post
Why dont you just look for the components in the PM2000 bus amp that are rolling it off? An hours investigation could solve your problem.
Thanks Doug. I appreciate your trying to save me time and expense. Do you see anything in the circuit that would lead to that kind of rolloff? Is the screen shot I posted of the PM2K master bus schemo legible? The path goes through an (inductor?) then two IC chips, then there is an insert which returns to the fader and then finally amplified one last time by the DOA and on the output transformer. This behavior as I mentioned happens regardless of transformer (Yammy, Carnhill 1:1, or UTC A20) or discrete opamp (Yammy 80100 or 990C) IC1 and IC2 for the main L&R I changed to OPA604, but once again, the stock bus channels with the Hitachi chips exhibit the same behavior. I also changed the output caps to 470uF but I don't think that would do it? Otherwise stock circuit.
Old 28th November 2019
  #8
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Check the master out in the insert for the roll off. That will help isolate it! The master out right after the first two opamps in the master section.
Old 28th November 2019
  #9
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Radardoug's Avatar
 

Look at the network on IC1 and 2 pin 5, also the series inductor at the input.
Old 29th November 2019
  #10
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Thanks guys.

I'll try sweeping out of the master sends. Just haven't had time to get back to it yet.

Doug,

What is the purpose of the inductor at the input? Would the rolloff have to do with the value of the cap in series right after the inductor?

And the cap and resistor in series from pin 5 to ground? Would I experiment with the little pF cap values. (lower values? Higher?)
Old 30th November 2019
  #11
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The inductor is there to keep rf out of the mix amp. The cap is there to decouple any d.c. You need to learn the classic inductance and capacitance formulas for reactance. The components on pin 5 look like they are a stability network, but first try removing them and see what happens. Look at the data sheet for the opamp.
Old 1st December 2019
  #12
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Okay, I swept out of the master sends (prior to the DOA output section) and the problem remains. So we can focus on PM2K master out circuit prior to the fader.

I also removed the cap and resistor from pin 5 to ground on both IC1 and IC2, same result.

Also, here's the datasheet for the HA1457 https://www.datasheetarchive.com/pdf...chi%2520HA1457

In the notes (their test example) it says that the caps C106, 107, and 108 are responsible for frequency response. If I'm reading correctly this translates (on the PM2K schematic) to the 33p and 15K in parallel between the input (pin 7 and the output pin 1) right? Is this the next place to look?
Old 1st December 2019
  #13
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The 33pF and 15K in parallel are not your problem. The network on pin 5 goes to another pin as well. Disconnect that as well.
Old 2nd December 2019
  #14
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You mean the 220pF and 330pF caps connected to pin 1? I removed those too. ???
Old 2nd December 2019
  #15
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Remove the 100 pf across the first opamps input terminals.
It would be worth checking that the series inductor is actually 100uH as the schematic says. Try shorting it out.
Old 2nd December 2019
  #16
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Hmmm. I pulled that 100pF cap and shorted the inductor and no change.
Old 2nd December 2019
  #17
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What are you using for testing? Do you have probes? If so just poke around with one and see where the change happens. Like before the inductor is no roll off. After? Then do the next component etc. You will find it quick. Like before the opamp is no roll off, after? Etc.
Old 2nd December 2019
  #18
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Are you sure the rolloff is not happening in the channel?
One way to check is send signal from your oscillator through a resistor straight to the bus.
Use the value of the mix resistor, 33K?
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