M-208 opamps

Discussion in 'TASCAM DIY Repairs and Mods' started by dukeofearl, Jan 30, 2021.

  1. dukeofearl

    dukeofearl New Member

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    Hello,

    I just got on of these mixers and was thinking of swapping out the opamps for NE5532's. Are these a drop-in replacement?

    I'd like to lower the noise. I plan to replace the electrolytic caps in the PSU as well.

    Thanks
  2. -mjk-

    -mjk- Moderator Staff Member

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    If you know both the chips' numbers you can download the data sheet for each and see if they are interchangeable.
  3. SkywaveTDR

    SkywaveTDR Well-Known Member

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    There is nothing bad about the Signetics 5532 but what people do is they only look as far as how many pin are on that black box. The fact of the matter is that there is a lot more that go into the selection of the op amp characteristics. In Engineering the circuit characteristics have different Poles and Zeros in the frequency domain while the older Op Amps exist in a circuit meant for them. You can go to the trouble of installing the NE5532 but you will also have to bypass the power supply pins with a film capacitor to keep them from Oscillating and burning themselves up. At Teac we often had DIY types bring in units with 5532 that burned out due to their lack of understanding of Engineering and then we repaired their units to working putting the same parts as were in there in the first place. In addition in a case where the Op Amps were changed successfully these, over-all noise did not change due to one component- there are other things that are noisy down the line and again they do not even consider this. There are IC's that are even more quiet than 5532 IC now but a lot of changes might be needed to get a true overall result. You are therefore not wise to listen to those selling snake oil like mods. I earned my BSEE while working at Teac and graduated in 1985. I would only modify a mixer if the results were 6dB or better in change and I don't think that will happen with these.
    Last edited: Feb 5, 2021
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  4. dukeofearl

    dukeofearl New Member

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    thanks for the response. so you'd recommend leaving it stock?
  5. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Yes, lessen the noise from where it comes from. Many times the mixer is blamed for bad practices. I have used these Tascam mixers for a long time with good results. I have M50, M1516 and M520 mixers.
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  6. Denis Petrov

    Denis Petrov New Member

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    NE5532 are pin compatible with NJM4560 and parameters are close enough. Probably won't make much difference but you can certainly give it a shot.

    A more promising swap would be to replace input transistors Q101 and Q102 with low-noise parts like 2N5089 or a more modern equivalent, I'd have to research a bit. Don't know off the top of my head if the pins are in the same sequence.
  7. SkywaveTDR

    SkywaveTDR Well-Known Member

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    The design of the circuit affect the results from the transistor- the results from changing transistors from the 2SC1844F to another will be a waste of time. The NJM4560 is a good selection for this application and a 5532 at a slew rate of 9V/uS will have to be compensated for as it is twice the slew rate of 4560 at 4V/uS but why do you need such an upgrade that will only be measured by instruments- people go crazy with this stuff and at the same time they have no Engeering degrees to even understand the technical figures.
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  8. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Why does no one go after the LME49720 with only 2.7nV noise figure and even a NJM4580 is .8uV.
    The LME49720 has a slew rate of 20V/uS but You will not like the cost. It is best to just leave things as they are. The people that have to go into making changes like this either have time on their hands and a lot of money to waste as getting involved with this is going to take a lot of money to change all this- If you have such money just buy a Neve console. Why the lowest Tascam line of mixers?

    You can nit pic at the circuit design all day long- why not have NASA design one for you with liquid nitrogen gas cooling?
    Last edited: Feb 3, 2021
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  9. dukeofearl

    dukeofearl New Member

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    Well the cost to swap out some opamps is under $100 and a Neve console is 50k+ so that's silly. And for a straight opamp swap I don't think I need to get the engineers from NASA involved. I think these tascam mixers sound good, but there is a lot of hiss/noise, so was looking if there was a way to lower that a bit. And the noise is coming from the mixer, not the source.

    So I was looking for people who have done an opamp replacement that didn't require any other modifications - and to see if it succeeded in lowering the noise. There seems to be some discussion online about doing this, but not a lot of results posted.
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  10. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Exactly. When a person brings me a unit reel deck or mixer and say they want it modified I ask what mod and what is the resultant enhancement? They never come up with an answer as most these guys that come up with this stuff are selling snake oil solutions and do not have sufficient technical knowledge. The fact is that if you use an Op Amp with a greatly higher slew rate or higher bandwidth which these higher grade op amps do, then you will need to limit the response of the bandwidth in the feedback section of the circuit and or bypass the power supply rails. Say you do put a new op amp in and go looking for better noise and hear greater hiss due to the op amp oscillating at 1MHz and getting hot at the same time- will you have an enhancement? I say no. Even if the item is put in properly is the lower noise of one item going to make a large difference when mixed with all the 4560 down the line? The weaker link comes into play here.
    The reason no results are posted is that they do not have the ability to measure it as it requires a faraday cage to get ambient noise down to lower than -70dB. Even at Teac being in an industrial building there was no way to measure a deck doing -80dB or anywhere near -100dB.
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  11. SkywaveTDR

    SkywaveTDR Well-Known Member

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    One guy who does come up with tape deck enhancements is Alex Nikitin but he is an actual tape deck engineer and when sent question about his changes he has good and valid answers as I look them over. I do some mute transistor mods that he has come up with and he has a number of head enhancements using FET front ends to the heads. He is located in the UK. He also produces the upgraded 4066 IC that can enhance the performance of a deck.
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  12. Denis Petrov

    Denis Petrov New Member

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    If I may ask, @dukeofearl - what type of source and how is it connected? What is your setup? And is the unacceptable noise level based on comparison with a different console?
  13. Denis Petrov

    Denis Petrov New Member

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    I've had my Tascam M-216 waiting for repair so I thought it'd be a good time to get it done and maybe run a few experiments. I pulled out the channel I've been working on and measured the noise floor at max trim - minimum trim noise is below that of my 16-bit sound card ADC and I don't have a 24-bit card handy.

    Noise in the equalizer and the output stage is also below my sound card input, so essentially all of the noise in the channel comes from the front end stage.

    A couple of findings of interest so far:
    1) Engaging the -30dB pad lowers the noise floor only by ~14dB at the same trim level, which looks like a loss of 16dB of dynamic range. The pad attenuates input and keeps front end gain intact, lower noise is due to the reduction of front end source impedance;
    2) Replacing front stage transistors resulted in an increase in noise floor (+1dB at 0dB pad and +4dB at -30dB pad). The collector current is probably optimized for noise performance of the stock transistors and would have to be adjusted for a different type, so it's not a straight swap.

    Swapping in LM833N had zero impact on front end noise, as expected.

    Interestingly, Tascam manual recommends engaging -30dB pad before adjusting trim on channel overload; based on the observation #1 above, to maximize noise performance trim should be attempted first.

    I'll have it on the bench for a couple more days so I'm open to trying more ideas. But it seems to me that swapping in chips is going to be a wash.
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  14. SkywaveTDR

    SkywaveTDR Well-Known Member

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    Well just as a test did you try any BiFet amps or the 10MHz one or even that expensive LME49720? Some swear by the OPA op amps OPA2134. I think all these do is extend the bandwidth that then goes way outside the audible range but what is the point to that? Noise may also be due to common carbon resistors where lower noise would be Metal film. A guy at Shure Brothers in the research team told me wire wound resistors were the quietest.
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  15. Denis Petrov

    Denis Petrov New Member

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    I didn't have any of these handy to try, sorry. I have some old TL072 parts but it's too much work to get these in.

    As I understand it, the basic design choice of using op amps with 10+MHz unity bandwidth for audio is to reduce distortion and noise at mid and high frequencies while also keeping output impedance low. High slew rate is actually not really important for audio applications (driving 20kHz sine wave 10V p-p only requires 200 mV/usec) and is a byproduct of high unity gain bandwidth.

    1MHz unity bandwidth op amps may have a high open-loop gain of 100dB+ but that starts to roll off starting at 10 Hz at 20dB/decade. At 1000 Hz open loop gain will fall to only 60dB, at 10kHz to 40dB, and at 20kHz to only 34dB. If you need to provide any amplification in your circuit, you may not enough gain left at high frequencies to allow you to achieve your noise, distortion and output impedance goals.

    10 MHz unity bandwidth amp gives you 20dB more gain at mid to high frequencies.

    Metal film resistors in the front end stage feedback loop and input signal path could be potentially helpful, but I doubt there will be any drastic difference.
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  16. SkywaveTDR

    SkywaveTDR Well-Known Member

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    I think the gain required for the frequencies needed is sufficient otherwise they would have used another op amp. These things were built for the musician that could not afford the real item of a reel deck and that so they are made with cheap parts. No one is expecting high grade of performance from these and that is why putting in $10 op amp intro a very cheap plastic Porta Studio to me makes no sense. Yes there are much better op amps but in view this one part is not the whole deck but a combination of many cheap parts- you might as well buy the reel deck and go with a higher end set up in the first place rather than changing parts on something of plastic parts that will not last. Some of the head base studs are breaking off the parts and it take extraordinary work to try and figure out a solution. Like dual cassette decks these were considered throw away.
  17. Denis Petrov

    Denis Petrov New Member

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    Looking at Tascam M-2516/2524 and M-3500 schematics I see that the front end is identical to M-2xx. Sure they upgraded op amps a bit but I think the driver for that is marketing, not sound quality. They use the same exact transistors in the front end, which is the deciding factor for noise level. M-300 also has the same front end with a 5532 op amp. I could not find M-106/108 schematic but won't be surprised to see little difference.

    Most of the price and target market differentiation appears to be in the feature set, not electronics or build quality. There are only so many ways one can put a pot or a pcb or a case together, and Tascam is using off-the-shelf parts for all its tiers. 2xx is a barebones console one can drag around for live performances and such, the build is solid and it looks like it performs just as well as M-3500. Nothing particularly cheap about it.

    Upgrading Portastudios makes little sense in general - people using them in 2021 are typically intent on using their low fidelity for creative purposes.
  18. Old School Boffin

    Old School Boffin New Member

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    Hi guys,
    This is my first post as a new member.
    An interesting discussion!
    Good points have been raised my everyone.
    I agree that it's probably not good scientifically to simply replace ICs willy-nilly, hoping for a substantial sound improvement, although I have done this with several channels and the master section of my 488 MkII, changing the original ICs (4560?) for 5532s. It seems to have been successful with no issues a couple of years later, and sounded subjectively "clearer". I did not attempt to measure for an improvement in noise levels. At the same time, I upgraded all the electrolytics associated with the ICs, bypassing them each with a 1uF polypropylene cap (as suggested by a veteran technician on a forum similar to this one). This may have actually delivered some improvement in sound quality over the decades-old original caps. Doing this was a bit of an experiment, and does not mean that I recommend it, especially for those without many hours to spare or an abundance of dexterity!
    Now that I have upgraded to a 238 connected to an M-208, the 488 will sit idle most of the time.
    I definitely think that a lot of people consider chip-swapping a shortcut to improved noise specs or a better sound, but in most cases it will probably not yield a lot of improvement alone.
    As mentioned, matching the impedance of the source to the mixer will have an influence on the SNR, and these are approaches that should probably be considered before "IC-rolling".
    My M-208 is pretty quiet to the ear, except when connecting a valve mic preamp and a low output ribbon mic. In this scenario, the trim is set so high that I am hearing a little hum and hiss. I will be re-capping the power supply and then will look at a sensible way to reduce any remaining spurious noise. I am sure the mixer sounded fantastic when new, so getting it to work as new is a better plan than swapping parts for a possibly unrealised improvement, based on some ill-informed amateur comments in forums.
  19. -mjk-

    -mjk- Moderator Staff Member

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    This is probably the strangest comment on the Portastudio line that I've ever seen. Yeah, I bought mine because it's low fidelity. :confused:
  20. -mjk-

    -mjk- Moderator Staff Member

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    @Old School Boffin welcome to the Tascam Forums.

    Grady Motes has been doing that since the late 70s. He and I rebuilt several large frame recording consoles, and he bypassed every single electrolytic capacitor in the audio path with a film. That completely eliminated the midrange smear. One studio owner was a little bit upset because he said that he would have to go back and re-record everything because now he could hear how bad the smear was.
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