PM1000 mix buss. Reccommend me something cool.

Recording Techniques, People Skills, Gear, Recording Spaces, Computers, and DIY

Moderators: drumsound, tomb

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

PM1000 mix buss. Reccommend me something cool.

Post by honkyjonk » Tue May 06, 2014 3:41 pm

Hey folks,

A little blurb before I ask a couple questions: I have 8 PM1K strips that I FINALLY (it's been years) have up and working. I have a huge power one supply to power them all. It's in a big orange ammo box. Right now I have a modified (cut WAY down) PM1K chassis that is holding four of them plus one master channel. Since I do so much 4 and 8 tracking, I really like the idea of building a couple 4X1 mixers for summing things on the way in. And I want something I can travel with. They're not going to be extremely small, but hopefully, smaller. And then of course, I want to be able to link them for a simple 8X2. My plan is to ditch the wooden console chassis, and rack these up in two 4U-5U 19" chassis'.

But what I'm looking for is ideas concerning the mix buss, and keeping that big punchy sound the PM1K's have when wired to go out of the "board out" straight into the output transformer. The PM1000 mix buss is just not so stellar. They're great as preamps, but sending stuff out of the main mix busses, from my experience sort of crummifies stuff in a bad way.

Anyway, I'm REALLY tempted to use something like this: http://classicapi.com/catalog/product_i ... cts_id=403 This is a clone of an API master buss, and I used it in my PM2K. It sounds amazing. However, it needs a bipolar power supply, which I suppose I could build (I've already done it once, but then that's another peripheral thingy to deal with) And also, I want 2 4X1 mixers, so I'd have to either cut one of those boards in half or order two or them, I'm thinking I might be complicating things a bit. Perhaps there is a much easier way to mix 4 channels into 1.
So I'm looking for ideas, preferably something that can tap off of the +44(48V) (Even if I have to drop it down somehow)
I have a bunch of 600:600 ohm output transformers around, so I'd like to use a pair of those if possible.

Any ideas? I wonder if I could build out a Neve style buss amp and split 48V in half? I wouldn't have to use Neve transformers but I bet it would still be pretty cool. I'm sort of looking for a cheap, discrete solution to maintain the mojo the input strips already have.

OR. . . the PM1K master buss amps are just the input strips' amp circuit padded down I'm aware. Maybe someone has a suggestion as far as how to make them sound good? (I do have 2 of them)
Stilgar, we've got wormsign the likes of which God has never seen!

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

Post by honkyjonk » Tue May 06, 2014 4:02 pm

And theoretically speaking, is there an easy way to make +48VDC into +/-24DC?

That would be rad, because I have a few Yammie opamps around as well. Maybe I could get away with using that API buss amp clone with my Yammie opamps and Yammy transformers. Hmmmmmm

BTW, the power supply I have has all the power at 44-48V I would need. It's a 3A supply. Total overkill.
Stilgar, we've got wormsign the likes of which God has never seen!

User avatar
Scodiddly
genitals didn't survive the freeze
Posts: 3957
Joined: Wed Dec 10, 2003 6:38 am
Location: Mundelein, IL, USA
Contact:

Post by Scodiddly » Wed May 07, 2014 4:29 pm

Passive summing and then a couple more channel strips to bring the results back up to line level?

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

Post by honkyjonk » Thu May 08, 2014 10:42 am

Thanks. This would be the easiest and most cost effective thing to do. The master module seems to be a close repeat of the input strips anyway. And I have a couple master modules, so this would be a great solution. They just don't sound very good as is. I attached some schematics. (Sorry about the links to the purple site, but that's where some easy handles for the pics are)

Anyway, the input strips appear to have 33K summing resistors built in. The question is where to go into the master modules. If you look at the master schematic, you'll notice the "combine amp input" section conveniently circled by the g.s. poster as "A." The part of the amp that most resembles the input strips' amp is setions "B" and "C" which are duplicates of one another. I'm wondering if I should try going directly into the "line amp in" which is right after the master fader. Don't know if there's going to be any impedance issues though in regards to the inputs strips' output. Also, changing the input cap (C7 on the master schematic) to 3.3uF like in the input strips? There are some other differences in resistor values as well in this section. So, ahem . . . this is where I start to get lost. I admittedly don't know enough about what these component values do.

http://www.gearslutz.com/board/attachme ... m-copy.jpg (input strip)

http://www.gearslutz.com/board/attachme ... beled-.jpg (master)
Stilgar, we've got wormsign the likes of which God has never seen!

User avatar
floid
buyin' a studio
Posts: 983
Joined: Tue Jan 03, 2006 1:39 pm
Location: in exile

Post by floid » Thu May 08, 2014 2:32 pm

the input strips function as line amps at the lowest gain position. IIRC the 4p11t gain switch tweaks various resistor values in the first gain stage, perhaps replicate these?
Village Idiot.

User avatar
floid
buyin' a studio
Posts: 983
Joined: Tue Jan 03, 2006 1:39 pm
Location: in exile

Post by floid » Thu May 08, 2014 2:51 pm

a while back i came across a tube mixer design that used a utc LS-141 to sum four inputs via two center tapped primaries. a dual 1.2k ct:600 interstage tranny could be just the thing?
Village Idiot.

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri May 09, 2014 5:45 pm

The PM1000 mix buss is just not so stellar.
They just don't sound very good as is.
Can you qualify or quantify either of these statements?

If you really want to change the character of the thing, using more input strips as summing amps may not get you there - it'll just repeat the original design, just with more effort to get there.

The PM1000 is already a passive mix bus. As such, if you start losing channels, you mess up the load impedance of the busses, and need to compensate by dropping the gain of the summing amps (that's what the 16/24 switch inside the master modules does).

As you identify, there are really only a couple of amplifier topologies in the thing, just tweaked for the specifics. The mic pre, fader buffer and line amp are all nearly the same, as are the summing amps and aux drivers. The tantalum cap at the input to every one of those is a definite weak point.

If you really want to change things up, maybe try one of Doug Self's hybrid amps? Cheap and cheerful, at least.
"What fer?"
"Cat fur, to make kitten britches."

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

Post by honkyjonk » Fri May 09, 2014 6:40 pm

Scum,

If there was one thing, I'd say increased distortion was what was bugging me the most. Kind of nasty distortion. Perhaps poky distortion might be the term. I'm just going from memory right now. I haven't tested the 4>1 summing that much.

I actually DON'T want to change the sound. I like the sound of the input strips. If they sounded the same being summed, that would be rad. That's exactly what I want. So far I've tried summing through the master strip like normal (even with a 3.3uF input cap I believe) and out of it into the output transformer.

Then I also wired a master channel to go from the the "combine amp output" directly into the output transformer. Less gain, but I had remembered being surprised that I liked that unbalanced out when I had a full PM1K board. That was a long time ago though and this time it was feeding a transformer. So, might have been an impedance issue.

I'm guessing perhaps what we're dealing with here is what you said about an impedance issue with the master buss not presenting the right load for such few input strips. Hmmmm.

It's a pretty far fetched comparison, because I've tested this 4X1 thingy so little, and it's been so long since I had a full board, but perhaps I do remember the master buss sounding better on the full board because there were so many channels.
Stilgar, we've got wormsign the likes of which God has never seen!

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Thu May 15, 2014 4:25 pm

So there are a couple things that might contribute here.

Here's the Fred Forssell whitepaper on summing busses.
Read it, think about it, understand it. It's at the heart of the system here. Impedance is not voodoo, its a predictable, repeatable phenemenon.
www.forsselltech.com/media/attachments/summing_buss.pdf

The PM1000 uses passive summing. Nearly everything more modern uses active summing.

From that paper - the big takeaway should be:
The ACN topology also allow the console designer to switch channels on and off the
buss.
...whereas the passive design does not. If you remove channels, then there's less voltage drop in the summing network. The voltage on the buss will be higher.

The amplifier following it was designed with certain parameters in mind, a certain range of acceptable voltages coming into it.

So there's the possibility that you're simply overloading the summing amp, causing it to clip.

Another possibility is that the input the the summing amp isn't clipping, but it's got more gain than is required for only 4 channels, so it's output is clipping.

And there's also the class-A elephant in the room - class-A transistor circuits have an intrinsic distortion. The tops of the waveform will be more amplified more than the bottoms. But if it's not clipping, it's a significantly more subtle-sounding distortion.

You can eliminate the first two causes, but the class-A thing is intrinsic in those summing amps.

So calculate a load resistor that's equivalent the the source impedance of all of the missing channels, and strap it on the summing busses, to see if that helps with the distortion. Halve that value if you're still not happy.

And if you're still not happy, try a different topology altogether.
Then I also wired a master channel to go from the the "combine amp output" directly into the output transformer. Less gain, but I had remembered being surprised that I liked that unbalanced out when I had a full PM1K board. That was a long time ago though and this time it was feeding a transformer. So, might have been an impedance issue.
That's almost certainly risky from an impedance point of view. The two extra transistors on the 4-transistor amp are there to help drive the output transformers. There's not much drive in the 2-transistor circuit.

One other option might be to use the 4 matrix monitor inputs on each master strip as a 4 channel summer. It's already there and functional.
"What fer?"
"Cat fur, to make kitten britches."

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

Post by honkyjonk » Fri May 16, 2014 8:30 am

Thanks Scum

That is all very helpful information. A really great link there too.

So, let me know if what I'm thinking here sounds right. If in a 16 channel PM1K, the source impedance of the buss (unloaded by the buss itself) is 33K/16, which is 2.06K, then for a 4 channel mixer to achieve 2K, we'd need 8.25K summing resistors correct?

And then changing R3 in the master for the internal gain switch to about 1.5K? I'm guessing this isn't that crucial to get exact because the switch for both 32 and 24 channel versions uses the same 22K resistor.

This however, is all for a 4X1 mixer, but I have enough channels for two 4X1's, which I want to be able to use independently and also link together for an 8X2. I could probably use the same internal gain switch in the master, to accommodate for these two options I would imagine, switching between roughly 3K (for 8) and 1.5K for 4? These are just estimations judging from the difference between 22K and 8.2K for that gain switch.

But, if I'm summing 8 channels, then I would need 16.5K summing resistors. Or I could split the difference I suppose.

Anyway, am I completely off track here?

Btw, would you raise the value of both C1 and C7 in the master buss? Or just C1?
Stilgar, we've got wormsign the likes of which God has never seen!

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri May 16, 2014 11:46 am

You're not quite there in your thinking about the summing busses.

A passive summing buss forms a voltage divider.

Consider if you have 10 channels summed through 10K resistors. You have a signal on only one of the channels - the others are all silent, and driven from low impedance sources. So you have a voltage divider of the active channel's feed resistor, over the parallel combination of the other resistors, which are effectively grounded.

So 9x 10K in parallel = 1/(9 * (1/10K))) = 1111 Ohms.

A divider of 10K over 1111
= Vin * (Rbottom/(Rbottom+Rtop))
= Vin * 1111/11111
= Vin * 0.09999

So the voltage from the channel is attenuated by a factor of 0.0999, or about 20 dB.

For the output to be the same level as the signal coming from the one driven channel, you need to follow it with 20 dB gain.

This is a ratiometric thing - as long as all the resistors are the same value, the attenuation will be the same. There will be a sweet spot, though. If the resistors are too small (say 10 Ohms), they are a heavier load for the channels to drive. If they're too large (like 10 Meg), they'll contribute noise.

So back to the problem at hand: the PM-1000 summing busses attenuate about 24 dB when there are 16 channels present, and the summing amp then restores that attenuation. More channels means more attenuation, requiring more gain. I don't have a simple figure for the relationship between the feedback resistor and summing amp gain...

Or, conversely, fewer channels means less attenuation. If you want to use the summing amp as-is, increase the attenuation of the buss. A single resistor from the buss to ground will do the job. The value would be the parallel equivalent of all the channels that are missing...or two resistors and a switch, to account for your 4 and 8 input versions.

Or move to an active buss, and then adding channels doesn't matter nearly as much.
"What fer?"
"Cat fur, to make kitten britches."

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

Post by honkyjonk » Fri May 16, 2014 2:56 pm

Awesome. Thanks Scum. You always answer my questions. I'm beginning to think of you as my personal tech. (Ruh roh!)

I'm gonna go look at the Fred Forsell link again and try to understand this:

"A divider of 10K over 1111
= Vin * (Rbottom/(Rbottom+Rtop))
= Vin * 1111/11111
= Vin * 0.09999"

This though, is an oh-duh moment: " The value would be the parallel equivalent of all the channels that are missing.."

Of course. I was thinking about it all wrong.
I'm gonna wire in a switch on the master buss for between 396K (or closest thereabout) and 264K (or closest thereabout) and ground and see how that works.
Stilgar, we've got wormsign the likes of which God has never seen!

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri May 16, 2014 4:13 pm

396K (or closest thereabout) and 264K (or closest thereabout)
Can you show your work as to how you arrived at those values? I'm not following...
"What fer?"
"Cat fur, to make kitten britches."

honkyjonk
dead but not forgotten
Posts: 2182
Joined: Wed May 07, 2003 10:50 pm
Location: Portland

Post by honkyjonk » Fri May 16, 2014 4:59 pm

33K X 12(channels absent in a 4X1)=396K
33K X 8 (channels absent in 8X2)= 264K

???
Stilgar, we've got wormsign the likes of which God has never seen!

The Scum
moves faders with mind
Posts: 2745
Joined: Thu Jul 03, 2003 11:26 pm
Location: Denver, CO
Contact:

Post by The Scum » Fri May 16, 2014 10:42 pm

But parallel resistors aren't exactly additive.
"What fer?"
"Cat fur, to make kitten britches."

Post Reply

Who is online

Users browsing this forum: No registered users and 82 guests