Tascam M512 / M520 Upgrades

So I'm recapping a set of 4 channel cards...

Because I'm having signal reliability and integrity issues on a current project on which the M-520 is being utilized, I figgered it was high time to recap and cleanup (clean and exercise pots and switches) a set of 4 cards and swap out a 4-channel module. I also figured it'd be a good time to test some different opamps. Because there were some question marks about U2, U3 and U7 above I'm just going to focus on U1 (mic preamp), and U5 & U6 (EQ section). My plan is to buy an assortment of 3 ~ 4 kinds of opamps, and outfit 2 of the cards with DIP sockets for U1, 5 & 6...and then try them out.


  1. For bypass caps, would these work well? And if so...
  2. Are they polar?
  3. If I'm using sockets, do I just jumper the appropriate pins of the socket assuming that the opamps that go in there will have the same circuit configuration?
  4. And here's where the rubber meets the road...do I put the bypass cap between pins 4 and 8??[/


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[*]For bypass caps, would these work well? And if so...

Too small, find a 0.1uF poly film cap instead.

[*]Are they polar?


[*]If I'm using sockets, do I just jumper the appropriate pins of the socket assuming that the opamps that go in there will have the same circuit configuration?

You can. If the opamps don't have the same pinout, you have much bigger problems! Although that would be very unusual if the originals are TL072.

[*]And here's where the rubber meets the road...do I put the bypass cap between pins 4 and 8??


Another thing you should consider is a small cap in the feedback loop (if there isn't already one there). This is to keep the opamp from oscillating at ultrasonic frequencies. OPA2134 shouldn't be too much trouble, but if you are trying out lots of different opamps, it could happen with a very fast opamp. 22pF ought to do.
Thanks, guys...

Ethan, thanks for the link for the poly bypass cap...that'll be easy to add to my cart. :)

Quick change of topic (but related), I'm going with Nichicon KT caps for pretty much anything on on an audio-related PCB. I've been tearing my hair out trying to figure which caps to use...one cap would be available in a certain series and another not...KT's have a wider range of values (i.e. I'd be able to meet my needs with just that series), but they are "only" rated at 1000hrs...I finally decided to quit freaking out about it...The KT caps are very affordable, are spec'ed for audio (whatever that means), and have the higher -55 ~ 105 temp range. I figure they've got to perform better than what was put in at the factory, and those have lasted this long...I'll be an old man by the time this thing needs to be recapped again so why knock myself out trying to find everything in a 2000hr, 3000hr or more cap? And then it looks like I'm gravitating toward the Nichicon PW series for power supply applications...same temp range but 3000hr rating...How does that sound?

So anyway, my reason for leading into that is Mouser has the the KT series cataloged and DigiKey does not...plus Mouser has the Molex power supply connector, and now they have my bypass caps. ;):D

So, msh, Ethan, feedback resistor cap...I need some instruction. Is the feedback loop the one that typically goes from the output of an opamp back to the negative input? And then the feedback resistor cap...where does that link? Between ground and input side of the feedback resistor?? Any other tips?

And any recommendations on a selection for the feedback resistor cap?
I tend to use PW for everything, mostly because that series has every value imaginable, also in really small packages. But KT looks OK to me. Don't worry too much about the life rating; that's at maximum temperature (105C = 221F) and maximum voltage. If you are operating at 25C, you should get (in theory) more like 256,000 hours.

The feedback cap goes from output to - input, in parallel with the feedback resistor.
Cool. Thanks for the "input" (pun intended).

I understand the feedback cap in parallel...I can't remember who it was but I remember now somebody saying that it is easier to replace the feedback resistors when you put the caps in so that you can stuff, solder and trim both tails at the same time instead of having to mess around with what is already there. Am I recalling correctly on that?

The feedback cap goes from output to - input, in parallel with the feedback resistor.

Huh! Okay...applying that, look below at the schematic section for U1 (mic pre opamp) in the M-520...I don't see a feedback resistor for the output of "U1 (2/2)"...unless it is R14 feeding the input of "U1 (1/2)"?? R15 is the feedback resistor for "U1 (1/2)" right?


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What IC

In the input section r7 and r13 are the feedback resistors and already have a cap. U5/1 is a unity gain buffer (gain of one) and has a wire for the feed back loop. U5/2 has r54 for its feedback resistor.

Next question: What IC are you goinng to use (I assume the opa2134) and how many are you going to buy?

The reason for my asking is that I'm looking at a bulk buy of something like the LME49720 to get the price down in the $2.25 each range. Of course you need to buy a thousand.....
Is U6 the same setup as U5 then regarding the feedback loops? (sorry...don't have the schematic handy), and does U5/2 have a cap in place like the loops in U1?

I haven't placed my order yet, and I'm going back and forth between just getting 6 2134's (to do a couple cards), or maybe 3 2134's and 3 2132's to see if I can hear any difference that is likeable.

That's all I'm doing for now...my order is pushing $250 and its almost all entirely caps... :eek:

What's the story with the 49720?
u5 has no resistor inthe feedback... straight wire... that's because it's the current driver for the eq section that's coming next... r15 is feedback for U1 1/2... U1 2/2 is a balanced input amp r13 c9 is yhe feedback... though it's also going through Q2 as well... Q2 looks to me as if it's being used here in a limiter function... r7c5 doesnot appear to be feed back... though ya cant see where it goes... perhaps an overload led????
R7,C5 goes...um...it goes...to...um...picture below. Following it to the right it does indeed lead to U4 which is the comparator for the overload LED.

I wish I understood how to read transformers in schematics better. I don't understand the flow through them...can anybody help me or point me to a resource or two?

So demented, you are saying that U1/2 does not have a cap in the feedback loop (R15), Ethan you are saying that it does (R7,C5). I'm just trying to figure out what caps I need. Demented, Ethan has the advantage of the full schematics which are linked at the beginning of this thread. It does look to me though as if R15 is the feedback resistor for U1/2...

Is R54 the feedback resistor for U5/2?


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OK (I dont see any transformers.....)

So Q1, Q2, U1/2, U1/1 form a balanced input amp. Being balanced and using transistors makes it a little harder to understand. R14/R15 = 1 (10k/10k) so the gain of that opamp is 1. It is an inverter with a high input impdence so as not to load U1/2. R7/C5 is the feedback path for U1/2 and R13/C9 are the feedback path for U1/1. Being balanced they are inverts of each other. Which goes to the Phase switch S2 which provided Mic phase selection. Fro S2 the signal goes to access send jack , through the jumper and back in the access receive jack. From there it goes through C25 and U5/1 and on to gain setting and EQ or not EQ and so on....

U4 just picks off the signal for the peak led as you noted.


PS because U1/1 is gain 1 no cap is used.
Hahaha...what an IDIOT!

Where I said "transformer" I meant "transistor". :o

Yeah...THAT'LL confuse the issue...

You're talking over my head, but I'll study what you said...I understand that R15 is not a feedback resistor as it is working with R14, and no cap is needed there. R7/C5 and R13/C9 form the feedback loops for U1/1 and U1/2...caps are already present. What about U5 and U6 though? One stage is gain setting, so no feedback resistor, but the other stage?
My big question at this point, since I want to get my Mouser order going, is what value caps will be needed if U5 and U6 don't have caps in the feedback loop already? Will it depend on the circuit? Probably...I see that the feedback loops for U1/1 and U1/2 are 68pF...would that hold true for others as well?

I'm ordering a surplus of the 0.1µF poly caps (the ones recommended for bypassing the dual opamp)...would those work? Probably not...that's, like...100,000pF...a little different than what's in the feddback loops for U1. :confused:

Don't use 0.1uF. Doubling the cap value halves the frequency. So a 0.1 could roll off everything above 300 Hz for example (don't quote me on that number I did not calculate it). You want the roll off to be in the 100 KHz plus range.

68 pF would work, 33 would work. Just make them good quality.

For right now I would leave off the bypass myself.

Nah...that was my bad for leaving so much out in the first one...that's like saying "solve this mathematical problem but we're not going to show you the values or functions..." :D

Thanks for your help.
Two urgent questions!

I wouldn't put urgent, but I have a tracking session Tuesday eve at which I'm hoping to use the two channel cards on which I upgraded U1, 5 and 6, and replaced 6 caps (others are on backorder from Mouser). I need to get the cards in ASAP and accomplish a couple more things before then.

I'm ready to put the cards in, but I want to confirm two things:

1. poly caps are bipolar, yes?

2. caps C1 and C2 on the M-520 input PCB have no tolerance listed, yet the cans are yellow just like a 10% cap on my Tascam 58...I replaced them with 20% caps since no tolerance was listed, but I keep wondering...don't know how to tell from the can what the tolerance is...can anybody help me sort that out? I want to know if I'm okay having put in 20% caps in those position. Schematics are linked on the first page of this thread.

Here's a shot of the component-side of one of the modded PCB's...probably won't mean much except maybe to fellow M-500 owner's, but it is technically my first recap job/opamp swap, so...

You can see the caps that were replaced...they are pretty colors, and the three OPA2134PA's in sockets. I was so excited to open the box from Mouser and see those metallic jade green bipolar caps...that's my favorite color...hey...minds, simple pleasures. :D

Here is a shot of the trace-side...you can see the three bypass caps...they were tricky to get in as the leads were just barely long enough...

Here is a shot of that yellow cap I mentioned above.


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Took a closer look at the schematic

and I'm pretty sure those yellow caps (C1, and C2) are just 20% caps or greater. Those two caps are the first capacitors the mic input hits. Its kind of fortunate but at least the caps that are on backorder from Mouser aren't key to the mic pre.

Still seeking confirmation that poly caps are bipolar...

Got the cards installed this eve...everything is hooked up but I'm awaiting confirmation on the above inquiry before hitting the power switch...cleaned the pots, switches, contacts and LED sockets.