Tascam M512 / M520 Upgrades

[sweetbeats]

Well, there are a few of us around these parts that have an M512 or M520. Many of you know I have an M520 that I'm going through and cleaning up.

Many of you have likely read my thread in this forum asking How do you know its time to recap, of which the M520 became a subject. In it dementedchord said:

as to what chips to use... looks like most of whats in there are 4556's tl072's and such... there are way better chips available but before i would recommend some specifics i'ld want to see an actuall schematic... board layouts and parts lists dont work for me... i need to see what that one is used for... ya dont want to just add say the fastest op-amp available if there's a chance that it's used as say a line driver where current handling is more important than speed... get my drift??? also if/when you change out these op-amps ya should consider adding extra decoupling caps at each opamp...easiest way is to jumper a cap between pins 4+8 on dual opamps...

Many of you may have read some or all of my Tascam M520 Story over in the Analog Only Forum and I inquired therein about potential upgrades to the the M520. At that time Somnium7 said:

Well I can attempt a recommendation or two if I had the schematics.

Well, here are the schematics. They were scanned at 300dpi from copies. I think there is enough detail to read everything. Apologies in advance for some of the larger sheets which had to span 2 or more pages...the largest scanner bed I had available was 11" x 17". There is, however, sufficient overlap page-to-page for the multiple page diagrams.

Again, with that M520 I have 8 spare input cards and a complete spare master and monitor section, so I've got stuff I can screw up on.

Hoping for some solid advice on conservative upgrades that would likely improve the performance of the board...noise/distortion, etc. Not looking for a lumpy high-lift race cam here, just a good street cam...does that make sense?

 

[sweetbeats]

I'm divided...

I'm starting to delve into this a bit...gearing up to start recapping, at least some of the channel cards. The board seems noisier than I'd expect, especially if I route a channel or two to all 8 busses and then monitor those busses...its a roar. Somebody said somewhere that can be a side effect of aged caps.

But I'm divided on where to post this stuff. I have my fairly sizeable thread "Tascam M520 Story..." over in the Analog Only forum...this is all part of that story, and yet the subject matter is very DIY forum-ish, so I dunno.

Plus there are a number of M-520 related questions over in the aforementioned recapping thread. Just getting spread around a bit.

 

[dementedchord]

i suspect that the noise you're experiencing now is the agregate of all those decoupling caps being dried out... as to the chips maybe i'll get a chance to do a search for you later today... i ahvent done much repair work in the last few years and will need to find the current offerings...

 

[sweetbeats]

That would be great. I'll take whatever you have to offer suggestion-wise.

Thanks also for the confirmation from your vanatagpoint about my suspicion reagarding the noise and the caps. Funny things, those electrolytic caps.

 

[sweetbeats]

I keep hearing over the years about Burr-Brown opamps...what is the stink about them? Are they really that good/worth the price difference?

 

[dementedchord]

burr-brown chips are cool... the opa2134 would be good one for you to use... the thing is all the major manu's have cool op-amps available... when i started out doing tech work my boss for some reason got off on national chips and consequently i have had a tendency to stay with them... kinda like car brand loyalty i guess... so if i was gonna try to do this with just one type of chip i'ld probably use the LM6172 good frq response/slew rate handles enough current to be used as a line driver... if you're looking at linear technology's line the LT1358 looks like a winner as well...

 

[evm1024]

Nice thing about the 2134 is that it does not need any special bypassing to prevent oscillating like some of the faster chips might, plus it has good drive.

In your M520 I would look at swapping out U1 if you use the onboard mic pres, U2 for line in and if you use phono U3.... U5,U6 and U7 can be swapped out. Don't worry about U4 as that it only drives the overload LED.



--Ethan

 

[sweetbeats]

dementedchord and Ethan, thanks for the replies and the suggestions. This is really exciting.

when i started out doing tech work my boss for some reason got off on national chips and consequently i have had a tendency to stay with them... kinda like car brand loyalty i guess

Or recording gear......I can't seem to get away from Tascam vintage gear...

the thing is all the major manu's have cool op-amps available

Right...okay. That's good perspective...I suppose cost may become a factor too as I start looking closer at this, and what I'm reading into your statement is that there are lots of options on the market that outperform the stock IC's spec'ed a quarter-century ago...yes?

Nice thing about the 2134 is that it does not need any special bypassing to prevent oscillating like some of the faster chips might, plus it has good drive.

You guys are nuts......How do you know that stuff??? I guess that's my way at the moment of paying my respects. Thanks for sharing that tidbit...that's really interesting. How does that work anyway? The answer is probably beyond me, but the fact is important, because adding bypass caps will involve time, expense and risk. If the opa2134 is in range cost-wise all things weighed that may be a very good option for me.

In your M520 I would look at swapping out U1 if you use the onboard mic pres, U2 for line in and if you use phono U3

So, two questions:

• I can use the same chip for U1 ~ 7 (excluding 4) even though the stock chips vary (TL072's for U1, 5 & 6, NJM4556 for U2 & 7, and NJM2041 for U3)?
• It looks to me like U2 is for the TAPE input/2 TRACK A/B inputs, and U3 is for the PHONO inputs on channels 3 & 4 and the LINE inputs on 5 ~ 20...Am I looking at that right?

U5,U6 and U7 can be swapped out.

Again, with the same chip that I select for U1 ~ 3? U5 ~ 7 is for the eq section...that's cool...this is COOL!

So...hm...we're talking close to $200 for all the input cards...maybe I could just start with 8 or something...and then would I use the same chip for the buss amp and monitor amp PCB's

 

[dementedchord]

Nice thing about the 2134 is that it does not need any special bypassing to prevent oscillating like some of the faster chips might, plus it has good drive.--Ethan

i personally cant verify this... if i were you i'ld probably put a .1mf cap from pin 4-8... or at the very least check it with a good oscilloscope after doing the work cause if it's oscillating you aint gonna hear it...

 

[evm1024]

i personally cant verify this... if i were you i'ld probably put a .1mf cap from pin 4-8... or at the very least check it with a good oscilloscope after doing the work cause if it's oscillating you aint gonna hear it...

good advise in general. He does have a tek 465 he can check it out with. The opa2134 can be made to oscillate but it is not common. I was making more of a comment about how tricky keeping a 100 MHz opamp from oscillating is.

I've never had an opa2134 oscillate in any of the Tasscam equipment that I've put them in. (Mostly 32/34/38 channel cards)

--Ethan

 

[evm1024]

• I can use the same chip for U1 ~ 7 (excluding 4) even though the stock chips vary (TL072's for U1, 5 & 6, NJM4556 for U2 & 7, and NJM2041 for U3)?
• It looks to me like U2 is for the TAPE input/2 TRACK A/B inputs, and U3 is for the PHONO inputs on channels 3 & 4 and the LINE inputs on 5 ~ 20...Am I looking at that right?

Again, with the same chip that I select for U1 ~ 3? U5 ~ 7 is for the eq section...that's cool...this is COOL!

So...hm...we're talking close to $200 for all the input cards...maybe I could just start with 8 or something...and then would I use the same chip for the buss amp and monitor amp PCB's

I did not look the entire schematic over. What I did notice is that the very inexpensive TL072 were used where the signal was in the -10 dBv (300 mv) region and the NJM4556 was used where better low level signal performance was needed. As I recall the NJM2041 was used in circuits where higher drive was needed like balanced line outs. But don't quote me on that.

Price is important as well as how much current the replacement opamps take. You can change opamps and end up overtaxing your PS (not in this case I think).

--Ethan

 

[sweetbeats]

If you don't mind, I'll post up the cliff notes version of what opamps are used for what throughout the M-500 console, and then await comment.

So actually, with the whole bypass cap thing, it may be cheap insurance yes? I haven't looked at prices yet but I'm imagining that a 0.1mF cap can't cost too much...and then I just stuff the tails of the cap in with the appropriate tails of the opamp and solder away right?

And then the issue of current draw...I also recall that it probably wouldn't be an issue for the PS-520, but to be sure I look at the spec sheets for the original opamps and then the new ones, add up the total opamps in question on each rail and then do the math to calculate the total difference between the old and proposed new opamps right?

 

[evm1024]

Yup on the current draw.

The bypass caps should have their leads as short as possible. So solder them to the bottom side of the PCB. For some cranky opamps it may help to solder them directly onto the opamp leads But I suggest you stay away from demanding opamps unless you have time....

Oscillating opamps can make any audio sound like mud, distortion and other weird things. And when you go poking around with a scope the probe may chang things enough so that it stops oscillating.

I think that I posted this link before but here it is again. The Tangentsoft folks are into headphone amps and hear many more differences than I do - must be the younger ears and fewer explosions. Anyway, thay do have good information that does not read like a scientific paper.

http://www.tangentsoft.net/audio/hs-opamp.html

Price for bypass caps - cents. 0.1 or 0.01 uF will do the trick. PS rail to PS rail and/or each rail to ground.

--Ethan

 

[sweetbeats]

Yup on the current draw.

I know I'll have a question about how to ascertain my power supply's capacity once I get the calculations complete...first select proposed opamps...then calculate...then see if she can handle it...

solder them to the bottom side of the PCB

So maybe stuff the opamp and solder the non-bypassed tails and then stuff the cap in and solder the remaining opamp tails with the pre-trimmed cap tails on the underside?

I suggest you stay away from demanding opamps unless you have time....

Philisophically what I'm wanting to do is akin to putting a hotter cam in a car, but not so much lift and angular differential from stock spec that the car won't idle and requires airplane fuel to run...I'd like it to run on regular (though maybe it runs a little nicer on super) but to enjoy the advancements in cam grind processes and design technology that have taken place since the car was made in the mid-80's...I don't want to retro-fit variable valve timing...

I think that I posted this link before but here it is again.

You probably have posted it, but somehow I missed it. Thanks for putting it up here. I read the preamble and it looks like a good one.

 

[sweetbeats]

If you don't mind, I'll post up the cliff notes version of what opamps are used for what throughout the M-500 console, and then await comment.

So here goes, with the caveat that I am stumbling with the right terminology to describe these opamp functions, BUT this has been a good exercise to help me better understand what these things do...little signal boosters in these kinds of applications...often two stages, one where an input source (either a jack or the output of a channel or buss) is amplified and from there it goes to a variable floodgate (a variable resistor aka pot or fader) the output of which goes to another stage where there is another booster to amplify the signal, I'm assuming, up to a nominal operating level. Sometimes one dual opamp will handle both stages, while at other times, depending on the complexity or number of sources in the circuit, multiple opamps handle input and output functions to the circuit. Cool...as always, correct me if I'm wrong anybody.

I'll reference the schematic (to which there is a link to a zip of pdf's of the schematics in post #1) and then list the opamps, their part type and functions (as far as I can understand) of each:

BUSS AMPL. PCB

• U101, 201,...801 NJM4556D These drive the input stage of the 8 BUSS MASTER faders
• U102, 202,...802 NJM4556D These drive the output stage of the BUSS MASTER faders to the monitor buss and the PGM OUT jacks

TALKBACK PCB

• U1 µPC4558C Drives the OSC OUT jacks
• U2 µPC4558C Drives the SLATE buss
• U3 µPC4558C Drives the output of the talkback mic

METER AMPL. PCB

• U101 LM339N Drives the VU meter PEAK LED's
• U102 µPC4558C Drives the VU meters (are these actually passing audio, or does the audio branch off to drive the meter? I'm guessing it is the latter, but I'm just being lazy and asking instead of chasing from one schematic to the next)

IN/OUT AMPL. PCB

• U021 ~ 028 NJM2041DD These drive the 8 balanced out jacks of the nifty eight-channel BALANCE AMP

MOTHER (B) PCB

• U1 NJM2041DD This is listed in the Parts List as an NJM4041DD, but I believe that to be a typo and the schematic supports that...anyway, it drives the output of the INST input jacks on channels 1 & 2

MONITOR AMPL. (B) PCB (this was the hairiest to study...)

• U1, U2 TL072CP These drive the AUX MASTER faders, their source being everything routed to the AUX MASTER busses
• U3, U4 TL072CP These are stage 1 of the output side of the AUX MASTER circuit...these drive the MON switch in the MONITOR SELECT switch rack in the master section, their source being the AUX MASTER faders
• U5, U6 NJM4556D These are stage 2 of the output side of the AUX MASTER circuit...these drive the AUX OUT jacks.
• U7, U8 NJM2041DD These also drive the MON switch in the MONITOR SELECT switch rack, their source being the SUB IN jacks
• U9 NJM2041DD This drives the SPARE switch in the MONITOR SELECT switch rack, its source being the SPARE input jacks
• U10, U11 NJM2041DD These drive the output of the MONITOR SELECT switch rack which is U15, and the HEADPHONE level pot
• U12 TL072CP This drives the SOLO master pot, its source being everything routed to the SOLO buss
• U13, U14 NJM386D These drive the HEADPHONES jack, their source being the HEADPHONE level pot
• U15 NJM2041DD This is like a pre-first stage for the STEREO MASTER busses A & B...I'm a bit thrown by this one, but its source is U11 and U12 and it outputs to transistor laden circuits in the STEREO MASTER busses...the circuit splits and part of it goes to U13 & U14, and the other part goes to U16 & U17
• U16, U17 NJM2041DD These drive the STEREO MASTER faders A & B, their source being U15
• U18, U19 NJM2041DD These drive the STEREO output jacks A & B, their source being the STEREO MASTER faders A & B
• U20 TC4049BP Good gracious...I have no what this does...

 

[sweetbeats]

Anybody have a chance to look at this?

Sorry. I realize it is a bit more than a simple little issue...

My big question for right now is really going back to the input cards, and whether to just use OPA2134 opamps for all 6 audio-related opamps on each input card. If I understand correctly its not a question of performance right? In other words, the OPA2134 is entirely appropriate for each of the 6 applications on the input card...it will perform well...it is rather a question of whether or not the PSU can handle it right?

If that is the case, can somebody help me understand which parameter(s) I refer to on the spec sheet to determine the current draw of an OPA2134? Is it the "output current"?

 

[sweetbeats]

Still looking for advisement...can anybody help?

From previous posts:

So maybe stuff the opamp and solder the non-bypassed tails and then stuff the cap in and solder the remaining opamp tails with the pre-trimmed cap tails on the underside?

and

My big question for right now is really going back to the input cards, and whether to just use OPA2134 opamps for all 6 audio-related opamps on each input card. If I understand correctly its not a question of performance right? In other words, the OPA2134 is entirely appropriate for each of the 6 applications on the input card...it will perform well...it is rather a question of whether or not the PSU can handle it right?

If that is the case, can somebody help me understand which parameter(s) I refer to on the spec sheet to determine the current draw of an OPA2134? Is it the "output current"?

*~*~*~*~*~*~*~*~*~*~*~*~*~*

Now on this one...

Price for bypass caps - cents. 0.1 or 0.01 uF will do the trick. PS rail to PS rail and/or each rail to ground.

Are we talking electrolytics here? And what voltage rating? Would it be 25V since these would be sitting in 15-18V rails?

 

[evm1024]

Bypass caps are typically ceramic caps. Not electrolytics. Electrolytics are used because of their size to capacitance ratio. A 10 uF mylar cap is huge!

Looking at the opa2132 spec sheet I see that it can provide 40 mA drive which is just about typical for modern opamps and gives us a maximum current draw per amp (2 per package) This number is only important for the output stages that have to drive loads like tape head amps (40 ma is more than enough) and balanced lines (ditto)

no load current is a good starting point. That the quiescent current in the datasheet and is listed as +- 4 ma per amp. So each package draws 8 ma from the + rail and 8 ma from the - rail.

The TL072 lists around 1.4 mA per amp per rail. Call it 3 mA per package per rail.

The opa draws about 2.5 times as much current as the opa.

Look at that another way. The PS uses 7815 and 7915 for the +- 15 rails. They can supply about 1.5 amps. Lets keep 0.5 amps for "headroom", the 1 amp can power 125 opa2132....

Reards, Ethan

 

[sweetbeats]

Bypass caps are typically ceramic caps. Not electrolytics.

Ohhhhhkay...cool. Thank you.

The opa draws about 2.5 times as much current as the opa.

Look at that another way. The PS uses 7815 and 7915 for the +- 15 rails. They can supply about 1.5 amps. Lets keep 0.5 amps for "headroom", the 1 amp can power 125 opa2132....

I assume you mean opa2134 right?

Hmmm...so now I need to figure out what the 15V rails are powering...I'm assuming it is not just channel cards but monitor amp stuff and more...So it sounds like there is a possibility the PS will have to be modded if I went with all opa2134's since that would be 120 opamps just for the cannel cards, or select some lower draw opamps for less critical paths...

 

[dementedchord]

remember 2 things... he figured the draw against 66% of the supply leaving "headroom"... plus your adding some extra capacetence to the supply so that should help as well so the difference would seem to be marginal... as to the decoupling caps i use polyproplene and seems i remember you asking as to the voltage... go with 50V... from rail to rail.. total diff in the rails is between 30-36V so 50's should be fine...