Allen and Heath Saber Modifications

[Greg]

[quote author="gyraf"]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.[/quote]
What's considered a small cap... 100n? 100p?

[quote author="gyraf"]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?[/quote]
I was also very suprised by this. My last board, which was a piece of crap, didn't use the mic preamp. I was under the impression that most boards were designed as my last one was. Let me ask you a question about this circuit. It looks like there's discrete differential front end that is debalancing the signal, then the 1st half of IC1 is a just a fixed gain stage, correct? If I were bypass this gain stage, could I still use the debalancing circuitry or would I need an additional circuit (or tranny) to do the job?

[quote author="gyraf"]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.[/quote]
Thanks and thanks.

[quote author="gyraf"]Bypass C44 with a small polyester cap. This drives all output from the channel.[/quote]
When you say bypass, do you mean shunt it or replace it. I'm not exactly sure what you mean. Small - 100n? 100p?

Thanks for all the advise Gyraf it's definitely appreciated.

 

[StephenGiles]

All great stuff - and you havn't played a note to record yet!
Stephen

 

[Greg]

[quote author="StephenGiles"]All great stuff - and you havn't played a note to record yet!
Stephen[/quote]

Wait what?!?

 

[gyraf]

[quote author="Greg"]What's considered a small cap... 100n? 100p?[/quote]


anything from 10nF to 220nF, depending on how much room is in there. Probably easiest to mount on the pcb track side.

It looks like there's discrete differential front end that is debalancing the signal, then the 1st half of IC1 is a just a fixed gain stage, correct? If I were bypass this gain stage, could I still use the debalancing circuitry or would I need an additional circuit (or tranny) to do the job?

IC1 is an integral part of the mic pre, and can not be "extracted" on its own. You could either debalance your tape input passively or use an input transformer (does it really need to be balanced?)

Then use the "tape" switch to send it directly to the insert point.

When you say bypass, do you mean shunt it or replace it. I'm not exactly sure what you mean. Small - 100n? 100p?

Shunt it with anything from 10nF to 220nF, depending on how much room is in there. Probably easiest to mount on the pcb track side.

Jakob E.

 

[Greg]

I decided to remove caps C-4,5,6,7 and see if I could hear a difference. I first tried the line input, and it sounded a bit clearer than a channel with the caps in. What I mean it that it was a little less muddy, but it was subtle. Could something as simple as removing a couple caps from the signal path improve quality? I'm going to give things a listen again today to see if it really makes a difference, and that it didn't just sound better b/c I wanted it to sound better, hah. And even if the change in subtle, I'm thinking that if you do that to ALL 24 channels, the end result could be worth while. I have some parts on order for some of the other mods Gyraf suggested. I also want to try a couple different ICs in the preamps and maybe in that last gain stage. Can a Burr Brown OPA2604 be subsitituted for a TL072? Is there an Analog Devices or Linear Technology substitute?

As other members hear have suggested, swapping ICs might not be worth the trouble, but I'd like to give it a shot anyway (at least on one or two channels). Also, regarding the summing amps: if a buss isn't used (ie I just assign the channel to the L-R mix, where would the summing amps be located that do that? If a buss is used, I'm assuming the summing amps are in the buss module, correct?

As I stated before, I'm not all that familiar with mixer topology.

Thanks.

 

[gyraf]

[quote author="Greg"]Could something as simple as removing a couple caps from the signal path improve quality? [/quote]

Yes. Specially if they are worn.

But take care where you remove DC blocking caps.

Only remove caps where you know you're DC-free, or you will get very scratchy switches and pots.

To find your mixbus amplifier, look around in the area of the master fader.

Jakob E.

 

[Greg]

[quote author="gyraf"]Specially if they are worn.

But take care where you remove DC blocking caps.

Only remove caps where you know you're DC-free, or you will get very scratchy switches and pots.[/quote]

I'm more concerned with the tape inputs over the line input. I record and playback from a Tascam ATR60-16, and I sometimes use an ADAT XT-20 for additional tracks. I'll check to make sure that ATR has DC blocking caps on the outputs. If I were to measure the DC at the inputs of the console, what is acceptable? Or do I want 0VDC?

I know I've been asking alot of questions but I've learned alot from you guys here at the forum. I'm a degreed EE and I'll be starting grad school next week as an EE. School is nice and all, but the amount of useful, practical information I get from this forum pertaining to electronics is so far above and beyond what I learned in college.

 

[longsoughtfor]

Greg the LR summing is done on the module with the master fader and the mute panel. When you pull that module (350?) out it is two or three PCB's on one face plate... to get to them you need to remove the knobs and the nuts on each pot. Not a big deal really but a nut driver makes quicker work of it.

Kevin.

 

[Greg]

Thanks Kevin -

I had to remove the master module when I originally bought the console to replace a bad pot. When I bought the board, it had a couple bad pots and a bad mute switch. Luckily, the guy I bought it from tallied up all the parts needed to fix everything and gave that to me with the board. He was too scared to get in there and fix it. So I got the board for a little less. I also had to replace ALL the bulbs in the VUs. I'm considering changing over to LEDs, but I think I'll take one thing at a time.

 

[longsoughtfor]

Greg - you wouldn't happen to have the schemo for the midi/mute stuff would you? I get intermittent mute events with ch23&24 on the board. I'm guessing it's noise but would like to investigate it better.

Thanks
Kevin.

 

[longsoughtfor]

Whoa - I'm all the way up to 10 posts

lol.

 

[Greg]

[quote author="longsoughtfor"]Greg - you wouldn't happen to have the schemo for the midi/mute stuff would you? I get intermittent mute events with ch23&24 on the board. I'm guessing it's noise but would like to investigate it better.

Thanks
Kevin.[/quote]

I think so. I've got the whole service manual and if I remember correctly, all the midi mute info was in there. Give me the weekend to dig through it. Do you actually use the mute automation? I just scrounged up an old computer to be used as a midi sequencer, but haven't gone beyond that. If you do use the midi mute automation, any advise on how to get it up and running.

And I ordered some sample Burr Brown ICs from TI yesterday. I'm going to do a couple swaps. Hopefully I won't run into any instability/oscillating issues.

 

[longsoughtfor]

I have experimented with the automated mute stuff but don't use it regularly. I striped a tape with SMPTE code and ran it through a Motu Midi Express & Sonar, recorded some mute events to Sonar and played the whole mess back again. That worked OK.

The problem I am having is that if I press the mute button on ch24, sometimes, ch23 mutes as well and often, I cannot then un-mute either without a power cycle. So I am guessing the mute processor or it's associated logic is latching up somehow.

I use ch24 for talkback mostly, doing so allows hands free talkback rather then pushing the button on the board to talk.

Kevin.

 

[Greg]

I have a problem with the mutes on aux returns 1 and 3... I cannot get either of them to mute. I haven't even attempted to fix the problem. I'll see if I can find what you're looking for this weekend.

 

[Svart]

I took the advice that Gyraf gave earlier in the thread and adapted it to my X2 console. I shunted the tape i/o coupling caps because I know for sure my Mx-2424 has both input and output couplers, I bypassed the channel output coupling caps with small polypropylene caps, I also bypassed the insert coupling caps with polypro on the channels that use non-coupled gear and on my busses I shunted the caps because i only use the GSSLs on the busses. I can tell a big difference! it's subtle when listening to one channel but the more you add to the mix, the more apparent it is. I did the same with the busses and master section. Now, It seems that some of these coupling caps are very small, the inserts were 47uf, shouldn't they be something more like 500-1000uf? where will higher quality caps make the most difference?

 

[Greg]

What effect does shunting an electrolytic (usually for DC coupling) with a polyester have on the signal. Gyraf suggested this to me in this thread and I was just hoping to get a little more knowledge on the matter. I'm asking because there are additional electrolytics (on the output of the EQ) and I'm wondering if a polyester would be benefitial there as well.

 

[Svart]

I have since changed the 'lytic caps to panasonic FC but kept the polypro shunts in also. to my ears this makes the higher frequencies more clear and less mushy. this may be different for each person's console too though, but as a general rule they tend to clear up the highs a bit. I just stuck a .1uf polypro in there wherever the signal goes through a lytic coupler like before the channel faders and across the phantom blockers.

in total it made my desk sound much better once i had done all mono channels and the main channels.

 

[gyraf]

Greg,

The paralleling polyester capacitor's function is simply to shunt (and thus lower) the electrolytic's generic poor impedance (ESR) at high frequencies.

Jakob E.

 

[Greg]

Thanks for the replies. I made the changes yesterday that Gyraf suggested to me; however, I have not listened to the channel. I changed the 330u to a 2200u Panasonic FC, and shunted it with a 100n polyester. I also changed the 10u lytic to a 470n polyester that is part of the HPF (as he suggested), as well as shunting the output cap with 100n polyester. And I removed the lytics on the tape and line inputs.

I noticed that there are also some additional 10u lytics in the signal path (one blocking DC at the wiper to the fader, and one at the inverting input of the output drive stage). Could these also be replaced with a 470u polyester, or possibly a better quality lytic? Jamicon lytics are stock.

There are also additional 47u lytics (one at the preamp, HPF, and EQ output). Any suggestions here?

I'm asking because I just don't want to start replacing caps if it's not in a place that could potentially improve response. I'll definitely experiment with this, but I'd like to get some suggestions first.

 

[gyraf]

The fader DC blocking cap shouldn't be considerably less than 10uF for low-end response. Parallel the electrolytic with a polyester or polypropylene cap (easiest mounted on PCB reverse side). You may be able to fit in a Wima MKS2 10uF/50V polyester cap - but they're a bit expensive.

The same goes for other electrolytics in signal path.

Experiment on a couple of channels to see if you have improvement that makes it worth the trouble.

Jakob E.