Allen and Heath Saber Modifications

I own a 24x16 Allen and Heath Saber, and I've been considering modding a channel, or possibly a pair of busses, to see if I can get better performance... or should I simply say just sound better.

I'd like you guys to do some possible brain storming to help me decide if these would even be worthwhile modifications. Here's a few things I've considered:

1. IC replacement... The Saber has TL072CP and 5532 ICs in it. Haven't searched what to replace with, possibly a BB or Analog Devices?

2. Transistor replacement... I think that the main gain stage is based around BC214C transistors. Once again, not sure what to replace with?

3. Change frequency range on EQ... the reason I've thought of this is because is has a fixed HPF at 70Hz and 140Hz. The LM adjustment only goes down to 200Hz, so I can't get into the LF with a peak EQ. If possible, I'd like to get the LM down into the 100Hz range or lower.

That's all I can think of at the moment... please comment and advise. I can scan a schematic tonight when I get home if anyone would like to see one.

Greg - I have the exact same board and would be interested in hearing of any changes anyone suggests. I would also be very interested in the channel schematic. I have a copy of the main bus module if you would like a copy (I could put it up on my site) but I never seen the one for the channel.

Thanks
Kevin.

WE switched out the high and low corner frequencies. Can't remember the specifics... it was easy enough to do.

The other biggie was to solder FETs onto the jumper pins for +4dB/-10dB switching. they all had their gates taken to +18V or -1V through 1M resistors so that a single SPDT switch would change the whole console operating level.

I like it as it is sonically... wouldn't get too obsessive about changing it! Try a couple of channels by all means, but never be so proud that you don't want to change back! :wink:

Keith

The poor old 5532... Why does it get dissed so much? Everyone wants to replace the old guy :wink:

--Dave, who uses 5532s and ain't ashamed of it either :thumb:

[quote author="NewYorkDave"]The poor old 5532... Why does it get dissed so much? Everyone wants to replace the old guy :wink:

--Dave, who uses 5532s and ain't ashamed of it either :thumb:[/quote]

same here...it´s my most often used audio opamp..good sound, excellent price

Kev does too,
a good work horse and so to is the LM833.

In a console like that, decent sounding but not earth shattering, I always thought the place to do the most impactful mod would be in the master section. I guess if you use the input amps for tracking, then there is a logic to mod the input channels, but if you just use it for monitor and mixing, personally, I would replace the output amps with something discrete and add a transformer and then work back from that to upgrade the summing IC's on all the busses that pass audio to the main two output. I used to have a ghost and if for some reason I would have been forced to keep it, thats what I would have done. You can mod the hell out of an input channnel, which if you take a direct out to tape could rule, but you still have to pass through the summing to get to the master and then whatever is happening there. If you can get the summing amps kicking and a nice solid output happening, perhaps then the 5532's will sound in a whole new light. I liked the sound of the ghost, but had the bussing been faster sounding and the master had some depth to it, that could have been a seriously awesome console. The softness of the input channels combined with a stiffer more accurate bussing system really would have solved all the mush issues I had with that board. I was glad to see it go. I have a whole new world of problems now...

dave

The summing amp is the one that works at the highest signal gain with everything assigned, and therefore has the lowest negative feedback gain margin, specially at higher frequencies.

If I were looking into a mod, it would be there. The saber amps have a gain of about x100 when everything is assigned, depending on the frame size. a simple op-amp swap might not yield as much as a true redesign...

Keith

Hmmm... you've got me thinking. In the case of a simple single-opamp inverting stage, you could use a dual opamp configured as one inverting followed by one non-inverting stage, enclosed within one feedback loop. Then you'd have metric shitloads of open-loop gain to give you plenty of feedback margin.

Im finally starting to get some aspects of this, thats kind of scary...

dave

[quote author="longsoughtfor"]Greg - I have the exact same board and would be interested in hearing of any changes anyone suggests. I would also be very interested in the channel schematic. I have a copy of the main bus module if you would like a copy (I could put it up on my site) but I never seen the one for the channel.

Thanks
Kevin.[/quote]

I have a complete manual including all schematics and PCB layouts (channels, busses, all logic circuits)... however, the schematic for the master module is missing. It seems to have been torn out. Is this the one you have? If so, I'd like to get my hand on it. Also, I'll post a channel and buss schematic for all to see tomorrow.

To everyone else, thanks for the comments and suggestions, maybe I will start with the master module and work backwards. I rarely use the preamps for tracking in the Saber. I just use it for monitoring and mixing. I need to get my hands on that master module schematic though.

Keep the ideas rolling !!!

greg-

I used to routinely run the outputs of my ghost into a pair of 1271's, which did a lot for the depth issues I had with the board. You could always start with something just that simple before you hack it apart to get a flavor for where you can go with it. The difference I noticed from running the unmodified console into that line amp inspired me to want to rip the whole thing apart but sold it long before it ever got to that point. If you are using it everyday and dont want to take it out of commission that might be an easy bandaid.

dave

[quote author="NewYorkDave"]Hmmm... you've got me thinking. In the case of a simple single-opamp inverting stage, you could use a dual opamp configured as one inverting followed by one non-inverting stage, enclosed within one feedback loop. Then you'd have metric shitloads of open-loop gain to give you plenty of feedback margin.[/quote]

But you may run into stability issues. Check out the composite opamp paper here:
Burr Brown SBOA002.pdf

I prototyped a few of these that were amazing 3-30MhZ oscillators.

> you could use a dual opamp configured as one inverting followed by one non-inverting stage, enclosed within one feedback loop. Then you'd have metric shitloads of open-loop gain to give you plenty of feedback margin.

Try it. Two optimized op-amps in cascade will give you a -12dB/Oct open-loop roll-off and 180 degree phase shift. It will oscillate. (I see Brad beat me to this point.)

If you KNOW the summing op-amp will always work at high noise gain, decompensate it. If you have 12 inputs *always* connected, you can compensate for gain of 10. If you don't have a decompensatable op-amp, you can try a second amp inside the main loop with its own feedback to set its gain a little less than the main loop noise gain. This is about what the paper Brad points to says. This also allows use of a low-noise weak-output amp for input, with a hissy but beefy amp for output. (It also sells more chips, which has to be good for Burr-Brown/TI.)

I am neglecting bus capacitance. In many real cases, this sucks up even more phase margin. You may think you can cancel it with a feedback resistor. Yes, if the bus capacitance is stable, but watch that your op-amp does not strain driving the feedback capacitance.

Yerss.... and there is the conundrun I'm afraid!

-I habitally de-select any unused channels (no point having 64 buss feeds with the faders down if you're only using 8 channels...) so for my porpoises (or purposes) the summing amp gain will vary dramatically from session to session.

A friend of mine who is rather clever at this sort of stuff recently modded a console to allow a seperate set of switched passively-summed resistive feeds from some unused poles on the buss routing switches. He took the passively-summed outputs into a pair of Neve mic pres, but can also run them through APIs and the like. He says it's magic. I don;t usually argue with him.

Why not try it? -either run a parallel set of buss feeds or just disconnect your summing amplifier and see what happens. You'll need to adjust gain to compensate for the losses with for more or less signals feeding the buss, but the sonic advantages might be worth the extra 'calibration' step at mixdown.

Keith

Keith

I've got some scans of the Saber channel and buss modules. I'm also going to scan in the master module tonight.

Allen and Heath Saber M310 Channel Module Schematic

Allen and Heath M330 Buss Module Schematic

I still haven't decided what to modify. I didn't have much time last week to work on this project. If I were to do some IC replacing (which I haven't decided upon yet), what would be recommended to replace the TL072 ICs and 5532 ICs with?

Please look at the schematics and let me know if you see anything that would be worth modding. And I'm really not that familiar with mixer/console design, so where are these summing amps that have been discussed so far. Are these the amps/ICs that do all the summing from the channels and busses to the main outputs? If so, where would these be on the schematics I have... I have the whole service manual.

Greg - sorry for not getting back to you sooner. I had been away last week and these were here at my office:

Schematic:
http://www.lsfsound.com/Public/LRcct.pdf

Layout 1:
http://www.lsfsound.com/Public/LRlyt1.pdf

Layout 2:
http://www.lsfsound.com/Public/LRlyt2.pdf

Kevin.

Here's what I would consider:

In the channel, C12 - the 330uF blocking DC from the gain pot - seems to be very small. at max. gain is's looking into 22R, so you'll loose low-end at high gain. Normally you see 2200-4700uF here. And it should preferably be very good quality, possibly shunted with a small polyester cap.

I don't really like that they use the mic preamp for tape return - first attenuating with R10-11-15, and then gaining up again. Couldn't this be taken directly to the insert switch in stead?

Loose C4-5-6-7, the electrolytics in line/tape inputs. I'll bet thet your source outputs already has these.

C18 (at HPF OFF) could be substituted for a 470nF polyester.

Bypass C44 with a small polyester cap. This drives all output from the channel.

Jakob E.

One thing the bothers me about this board is that the direct outs are un-balanced. Any suggestions on making those outputs balanced?

Thanks
Kevin.

[quote author="longsoughtfor"]One thing the bothers me about this board is that the direct outs are un-balanced. Any suggestions on making those outputs balanced?

Thanks
Kevin.[/quote]

There are some balancing circuits on Kev and SSLTech's sites. I don't know how much current those circuits draw, but as long as the power supply has enough juice so to speak, I'm thinking that would do the job. I rarely use the direct outs so I've actually never considered that. I just use the Saber for mixing/monitoring.