Beginner attempt at building pedal from scratch (Tascam 246 Input Assembly)

[meldrop]

Hey yall!

This is my first time on this forum, but I am attempting to take a build that I know is far beyond my abilities. I am an electrical engineering student currently, and have built just some basic guitar pedals, and done a bit of prototyping circuits. Hoping to get some questions answered about this more advanced looking circuit im trying to understand.

My general idea is to recreate the input pcb of the Tascam 246 Portastudio. Similar to the Mid Fi Demo Tape Fuzz also inspired from the Tascam 246. However, while diy pedals like this already exist, I was hoping to more accurately copy the input pcb assembly of the portastudio.

Currently im just trying to understand the path of the audio signal, and general understanding of how everything is working. I also included another image with drawings and questions.

I guess, if i try prototyping this pedal, would it be okay to combine the two different circuits into one for the pedal, or are the audio paths at differently levels, or is the entire thing going to be loud enough? Would it be possible to run this circuit at 9V as a proper guitar pedal, or should I stick with 12v.

Sorry for all the questions, but I am new and eager to learn more but have no idea where to start. I feel like this would be a cool project to really get into the specifics of audio circuits.

[ElectricDruid]

Welcome Meldrop!

You've started your project with a nice clear schematic, so that helps a lot!

The routing of the signals is not entirely obvious to me, since there's a lot of switches and stuff going back and forth (line in, send/return insert, mic in, buss outs, etcetc). I guess I should have a look at the original machine to get a clearer idea.
That said, the actual circuit blocks are pretty clear. There's a mic pre-amp bottom-left, in the red circle. That's followed by an overload LED, driven by a comparator circuit, in the blue box. Next up, we come to an EQ section in the green box, and finally there's simple gain stage providing a bit of a boost and a pan control (in the yellow box).



The whole thing runs on a +/-12V bipolar power supply. Changing that to a single-ended 9V supply like a typical pedal would be a significant change. It's possible though.

[antonis]

we come to an EQ section in the green box

With special pots..

[meldrop]

Thanks for all the info! I think im understanding more, Ive also included more photos of the schematic sheet for clarification.

Firstly, I believe that for my purposes, I can consider the switch to always be set to line/mic, and the trim controls provide a gain from -60db -> -10db. The other options are for the tape recording aspects of the device.

Secondly, if im trying to prototype this, I would run the signal from the mic-preamp section to the EQ section, then just the beginning part of the yellow box, as I dont think pan would be a useful feature on the a guitar pedal. Hopefully, that would get me back up to -10db which I assume is a good range for sending out to amps.

It seems like it would be challenging to switch the +/-12V power supply to 9V power. Would there be any resources where I should look towards to find methods of reworking the bipolar power circuit to a unipolar power supply?

[antonis]

It seems like it would be challenging to switch the +/-12V power supply to 9V power.

I'm affraid you'll have to locate ALL bias points (GND for op-amps and JFETs differential pair) and connect them to a +4.5V point..
(you might need to recalculate some items, mainly resistors, values..)

Of course, you'll lower signal headroom more than 3 times..
(for real-world op-amps..)

[ElectricDruid]

+1 what Antonis said. If you do a search on here, you'll find an absolute ton of stuff on converting to single-ended/unipolar power. It's not *hard*, but you'll have to look at and think about each stage. Perhaps we could go through one part on its own so you can see what's involved and see whether you think it's worth it?

The loss of headroom is a good point too. If you go from a 24V supply to a 9V supply, that's inevitable.

Those routing/level diagrams are incredibly useful - well done Tascam! Why has no-one else ever done anything like that?!? Or not often enough, certainly!

[antonis]

Perhaps we could go through one part on its own so you can see what's involved and see whether you think it's worth it?

Just a sample of what Tom proposed..



P.S.1
C111 & C113 could also be wired to Vref (as far as there is a relatively big cap there..) but, IMHO, it should be better to not put "garbage" into Vref..

Also, R132 could be wired to GND but with a series cap for DC gain rolling-off to unity..

P.S.2
It should be more straightforward to wire all the above points to Vref (except op-amp Vee, of course) but it shouldn't be considered good design practice..
(in the mean of avoiding any "unnecessary" connections there..)
You see, Vref (via its stabilizing capacitor) can act as AC ground so signal doesn't mind at all..

Circuits Collection
Single-Supply Op Amp
Single-Supply op-amp design
(special focus on filtering and supply decoupling..)

[ElectricDruid]

Thanks Antonis.   That's *exactly* what I'm talking about!!

[meldrop]

hanks Antonis.   That's *exactly* what I'm talking about!!

Just a sample of what Tom proposed..

Thanks Tom and Antonis! All this info is super useful and thanks so much for taking the time to lay out all these diagrams/post and resources.

This stuff is definitely reaching the limit for my current understanding of circuits, but im glad that I can really delve into it all. Im gonna take a day or two to read through the single-supply op-amp design documents Atonis sent over and see if I can better understand that diagram. And eventually take on the other sections of the circuit.

I guess my main question before moving forward with the 9v conversion, would it be possible to maintain the proper -10db output of the circuit even after the power supply change? After some other research I did today, it seems like the op-amps might behave differently and change the sound of the circuit as well. And if the headroom of signal decreases about 3x, could that be remedied by changing resistor values or would the gain stage have to be "beefed up" to compensate for the difference in power?

Sorry if this isnt making that much sense, im trying to understand myself while writing out these posts.

Would it easier and more faithful to the design and to run it off +/- 9v (by combining two 9v supplies). Or even +/-12 v like the original (given the popularity of pedalpower units that have 12v outputs.). I understand that the design would still have to be reworked in any case, as the power would be unipolar 18v or 24v, and you would have to split that in have for an artificial +/-9v or +/-12v?

I guess its up to personal preference but im having trouble weighing the pros and cons of both, being new to this all. Thanks for all the help so far, I really appreciate everything and super excited to continue fleshing out this idea.

[PRR]

routing/level diagrams are incredibly useful - well done Tascam! Why has no-one else ever done anything like that?!?

I held them up to an author as a shining example. It's not even novel: you find similar diagrams for broadcast mixers from the 1950s.

"Langevin-console.gif is much older, passive (lossy) mixing. Three stages of amplifier, two in each input with fader between, mixer, master, output amp. The block diagram and level diagram don't line-up so I added arrows."
http://www.uneeda-audio.com/kearney/kearneys_mixer.pdf  pg 25-26

[antonis]

Would it easier and more faithful to the design and to run it off +/- 9v (by combining two 9v supplies). Or even +/-12 v like the original (given the popularity of pedalpower units that have 12v outputs.). I understand that the design would still have to be reworked in any case, as the power would be unipolar 18v or 24v, and you would have to split that in have for an artificial +/-9v or +/-12v?

First of all come the differences between single and dual (symmetrical) supply circuits and supplies thereselves..

e.g. dual supplies audio circuits are more plain (no need for DC blocking capacitors and Vref circuitry) where single supply power circuits are more plain (no need for center tapped transformer and dublicated smoothing/regulating circuitry for negative supply..)

[ElectricDruid]

From the copy you uploaded of that Level Diagram, I can't read what the actual units are. It looks like it could be dBU? There are lots of potential reference levels:

https://en.wikipedia.org/wiki/Decibel#Suffixes_and_reference_values

Just "-10dB" on its own without any further unit is a measure of gain or attentuation (attentuation in the case where the number is neagtive) and doesn't tell you anything about the level. +10dB is the same as x3.16 gain. -10dB is the same as x0.32.

If you reduced the headroom, you *might* have to tweak the input gain a bit because it's possible the existing circuit has enough gain to create a 20Vpp signal from whatever standard input level it's expecting - that would certainly clip on a 9V supply!
Such a tweak to reduce the maximum input gain to something more sensible for the new supply voltage wouldn't be a huge change.

[PRR]

easier and more faithful to the design and to run it off +/- 9v (by combining two 9v supplies). Or even +/-12 v

It is a classic and NON-trivial amplifier. _I_ would stick strictly to the script: +/-15V +/-12V. You can get little modules to do this.

[ElectricDruid]

It is a classic and NON-trivial amplifier. _I_ would stick strictly to the script: +/-15V. You can get little modules to do this.

The original script for this is definitely +/-12V (check the schematic at the top), but you can get little modules for that too!

[PRR]

> definitely +/-12V

You are correct. The same company and designer did a LOT of +/-15V work, and I assumed.

[ElectricDruid]

> definitely +/-12V

You are correct. The same company and designer did a LOT of +/-15V work, and I assumed.

In many ways, it's odd that it's *not* +/-15V.

[meldrop]

Still haven't been able to jump into the op-amp research fully, have finals to finish these next two weeks. Did a little looking around, and I think it would be possible to boost the 9v to around 24V with the mt3608 module? Would it be acceptable to set 0v as the virtual ground, and 24v as the VCC? I expect the amp is not drawing any massive current, so the module would be a feasible idea to use in the build? I was also reading that op-amps do not play well with signals that are close in voltage to the power rails they use? It causes some non linear gain issues? So since a guitar level signal is pretty close to 0V around 50-100mV on the low end, would take not work, and it would be better to somehow change the power to +/-12V. (Was reading a bit on this, but it seems id have to use a negative rail generator, but the build I was looking into couldnt handle the current needed for the circuit. Only around 4mA, but i guess op-amps would draw current from the negative rail?)

Also, I actually was able to get ahold of a Tascam 244 (with the same input amp) from Craigslist for a good price as the cassette drive is broken. The circuit sounds amazing, I spent just a few hours playing around with the crunchy tones, and the dual pot design allow for some fun phaser-ish sweeps.

[drdn0]

Still haven't been able to jump into the op-amp research fully, have finals to finish these next two weeks. Did a little looking around, and I think it would be possible to boost the 9v to around 24V with the mt3608 module? Would it be acceptable to set 0v as the virtual ground, and 24v as the VCC? I expect the amp is not drawing any massive current, so the module would be a feasible idea to use in the build? I was also reading that op-amps do not play well with signals that are close in voltage to the power rails they use? It causes some non linear gain issues? So since a guitar level signal is pretty close to 0V around 50-100mV on the low end, would take not work, and it would be better to somehow change the power to +/-12V. (Was reading a bit on this, but it seems id have to use a negative rail generator, but the build I was looking into couldnt handle the current needed for the circuit. Only around 4mA, but i guess op-amps would draw current from the negative rail?)

Also, I actually was able to get ahold of a Tascam 244 (with the same input amp) from Craigslist for a good price as the cassette drive is broken. The circuit sounds amazing, I spent just a few hours playing around with the crunchy tones, and the dual pot design allow for some fun phaser-ish sweeps.

I don't know whether that specific module will work, but I can't see why not - switching frequency is high, it can definitely output enough current and it's a small enough form factor. Here's the deal though - you don't just need a high supply voltage, but you need a +/- voltage. If you generate 24v with that, your need to set your vref/virtual ground as 12v (which can be done through a voltage divider - a pair of equal series resistors, with your vref connected to the junction between them).

Using a LT1054 inverter would be just as easy, giving you 12v and ground, and then a separate -12v rail to connect.

[antonis]

Implementation of either DC-DC step up or switched capacitors voltage converters isn't recommended for audio circuits, unless you are well familiazrized with items population placement and board traces & ground layout techiques..

Much more easy and effective (at least, for testing purposes) should be 2 X 9V batteries configuration..
(connected in series with 0V taken from their junction..)

P.S.
It's not difficult to get +/- 12V either via a center-tapped full-wave or bridge rectifier..

[meldrop]

Okay, finished up my finals today. I think I will go along with this idea for testing purposes ->

Much more easy and effective (at least, for testing purposes) should be 2 X 9V batteries configuration..
(connected in series with 0V taken from their junction..)

However, I will keep in mind all this about

Using a LT1054 inverter

.

Just before I start ordering the parts Id like to know where to route the audio signal out of the circuit. And ive created a BOM for the build as well. Haven't checked the availability of all the parts so much post about that later.



I have some questions on the image i included, mainly is it okay to remove the back end of the circuit that handles the effects and panning on the audio signal? By omitting it from the design would it break anything before or is it hard to know without testing?
Second, if it is okay to omit, do I route the signal out to the jack as I depicted in the image?
Third, the circuit that handles the overload light is connected to the BUSS that is labeled OV(D). What would I connect to for testing?
Forth, N.F. resistors are near the +/-12v ins. These are just non flammable resistors?