208i - do line inputs bypass preamps?

[Kane Williams]

Hello,

First post here.

As the title asks, do any of the line inputs bypass the preamps on the 208i?

Thanks

 

[cripple9]

Going by the block diagram for the 208i (p.37 in the Reference Manual), it seems all the inputs of the combo jacks run through the preamps. Unfortunately, the diagrams have often proven to be improperly drawn. For example- the 208i's diagram shows the 48V phantom power is applied to the line/inst inputs of the combo jacks, as well as the XLR - before going to the preamp, but that would be dangerous if true.

Incorrect block diagrams for multiple interfaces (not just Tascam) have been pointed out in other threads here as well, so good luck.

 

[Kane Williams]

Weird. Got an email from Tascam today and they actually told me the diagram on P37 shows a True Bypass mode which does bypass the preamps. I can see the MIXER True bypass, which goes straight from the ADC to the USB.

 

[cripple9]

I suppose I could just be getting old & have no idea how to read these diagrams anymore, but I'm pretty sure that signal flow is completely illogical, lol. Maybe someone else here with access to a 208i can chime in and clarify it for you.

I've never used the 208i, but I do have a US-16x08. It also has True Mixer Bypass, but it's not shown in the block diagram because it wasn't implemented in the software until after the Ref Manual was written (they still haven't updated it since 2015 *cough*). Unless I'm reading it wrong, the 16x08's diagram shows all the inputs pass through a preamp too. It doesn't use combo jacks, but ch's 9 & 10 are 1/4" inputs that can switch between line/inst level (with gain knobs). Ch's 11-16 are 1/4" line-level inputs with a PAD switch and no gain knobs. True Mixer Bypass does bypass the mixer effects as it should, but I hear no change to any of my dry (pre-mixer) inputs at all. The gain knobs on ch's 1-10 still function normally as well. Call me ignorant, but that doesn't seem like it's bypassing the preamp to me...

 

[Mark Richards]

Think it through. What does a preamp do to a mic signal, and would it be necessary to do that to a line level signal?

 

[Arjan P]

@Mark Richards, I'm pretty sure you're not defending the shown schematic in post #2, because that is pretty useless indeed, with phantom power applied to the line input. And Tascam saying there is a mode called True Bypass that bypasses the preamp: the schematic should show at least a little sign of it.. (ofcourse, no signal will ever 'bypass' all preamps, but just saying, they're sending nonsense emails)

 

[Mark Richards]

I'm pretty sure you're not defending the shown schematic that is pretty useless

Arjan, I'm suggesting the Block Diagram isn't a circuit schematic and shouldn't be taken literally.

Sending a line-level signal through a pre-amp serving the mic circuit makes little sense to me from a design perspective. To do that, TASCAM would have to step down the 1.23V line signal back to mic level mV so the line signal doesn't overload the mic pre-amp; and then step the line signal back up to 1.23V for internal processing. Maybe that's how TASCAM designs its lower end audio gear.

But IMO it makes more sense from an audio signal perspective to design separate mic and line electrical circuits, with the line circuit going directly to the Gain pre-amp. Phantom power is isolated on the LowZ XLR circuit; the HiZ TRS line circuit stays separate from the mic circuit; and both signals go at line level to the Gain pre-amp.

 

[cripple9]

I'll try to be a little more straight-forward.
I understand the emails from Tascam could sometimes be handled by someone that may not always know what they're talking about, but the block diagrams should definitely be more accurate and obvious. I can't imagine how many people may have damaged gear as a result of poor documentation. As I stated before, it's not just Tascam with bad diagrams.
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The 208i's inputs are ALL combo jacks. There are no standalone Line-INs. AFAIK, combo jacks don't bypass the preamp on MOST interfaces, (someone please correct me if I'm wrong). There are some out there that can bypass the preamps with the use of inserts, etc., but the 208i doesn't have inserts and I don't see anything about the ability to bypass the preamps in the 208i's manual. As far as I can tell, the only way to bypass the preamp would be to use the "OPTICAL IN" with some other external gear.

With that being said, the Ultra HDDA pres are probably transparent enough for the average user. As long as the levels are kept reasonable, you shouldn't notice much of a difference.

Here's an article with some decent advice:
https://geekmusician.com/how-to-bypass-audio-interface-preamp/


And this is quoted from a support article of another manufacturer (brand names omitted):

"Do the line inputs on my interface bypass the Mic Preamps?

Applies to: All Products

When connecting to the combo XLR/jack inputs using a 1/4 inch jack cable on a *name omitted* interface the signal cannot 'bypass the mic preamp'.
The signal goes through the variable gain section of the preamp circuit, allowing you to set the input level of line-level audio sources. Due to the transparent nature of our preamps, however, this does not colour the signal.
On *names omitted* interfaces, we would recommend switching 'Air' mode off if you wish to get the most transparent signal possible.
There is some attenuation and gain available on the line signal, which may benefit users if they are using a unit that sends a very loud or very quiet signal for their liking.
To work out where you need to set the gain level, please see the following article which applies to both line-level and mic-level inputs: *link omitted*

How high should I set the gain control on my interface?​

Some of our interfaces also include 1/4 inch jack (TRS) line inputs that do not pass through a variable gain circuit. These inputs instead pass through a fixed-gain line amplifier circuit."
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@Mark Richards , What you're saying about separating mic/gain pre's does seem to make sense to me. Can you show any conformation of this actually being the case in regards to combo jacks in general, or at least with the 208i? The most common stance I've seen on the subject is similar to what I show above. Most forums and manufacturer support info I've seen says combo jacks don't bypass preamps. It would be great to know for sure. I'd love to learn something new. (I'm definitely not trying to be disrespectful BTW)

 

[Mark Richards]

any conformation of this actually being the case in regards to combo jacks in general, or at least with the 208i?

The only way to know for sure is to get hands on the 208i service manual and trace the circuits.

 

[-mjk-]

The combo jacks have separate audio paths:

https://www.neutrik.com/media/8030/download/combo-circuits.pdf?v=1

So it depends upon what the designer did with those paths. But the 6.5mm and XLR paths are not connected inside the jack itself.

 

[cripple9]

Now I'm just lost, lol. This is from my old Tascam US-2000. Channels 7/8 are MIC/INST combo jacks (NO Line-level inputs in those) on the front panel. There are INSERTS routed to them on the back panel. What exactly is going on here and what's the difference between the amp symbols?

 

[Kane Williams]

On my TC Impact Twin, the combo jacks don't bypass the preamps, but the line inputs on the back do. If I connect my external mic preamps etc to those, there are no gain nobs for those inputs. That is what I was hoping from the 208i, that I would turn the gain knobs for those combo inputs and nothing would happen. I was hoping the unit senses the impedance and line level signal and thus re-routes.

However, I am also guessing that the combo jacks on the Tascam won't bypass the preamps, but I'm hoping this is not the case as I have just bought one (used) and will be selling it on straight away if not.

I agree that the preamps will be very nice, but I want to use my others. I doubt I will ever use the mic preamps. I wanted to replace the TC mainly for more headroom because I want to use my external compressors etc with my DAW and the TC is offers nowhere near the +20dBu / +24dBu of the Tascam. I also assumed the converters in general would be a step up, as the TC (although great) is a bit long in the tooth.

I got another email from Tascam btw, when I asked them if they are sure the line inputs bypass the preamps. They directed me to P16 and this:

3 THRU button Click a THRU button to light it and send the input signal

for that channel directly to the computer without passing through the input signal mixer.
Since the signal does not pass through the mixer, phase, equalizer and compressor settings will have no effect.

 

[Arjan P]

Arjan, I'm suggesting the Block Diagram isn't a circuit schematic and shouldn't be taken literally.

That was my point: if it's not correct, then don't show the amount of detail they do. The diagram in post #11 (like the one in my DM3200 documentation) has the same level of detail but this one is realistic - that's useful.

 

[Mark Richards]

...The 208i's inputs are ALL combo jacks...What you're saying about separating mic/gain pre's does seem to make sense to me...The most common stance I've seen on the subject is...combo jacks don't bypass preamps. It would be great to know for sure.

I've been searching the internet for combo jack schematics without much luck.

I did get the input circuit schematic for the current TASCAM portastudios (which use combo jacks). As Phil Tipping pointed out to me, although the XLR and TRS circuits are separate initially, the signals become combined at the entry point to a shared pre-amp (the trim knob circuit + op-amp).

 

[Kane Williams]

I just plugged an external mic preamp into the combo jack using a TRS cable and the gain knob did still indeed add gain, so unless I'm missing something in the software, the preamps are not bypassed simply by using TRS cables.

That's a shame, I'll have to sell this on. I'm sure the amps are reasonable transparent, but I'm a purist, I want all the sound coming from the external gear. Really nice build quality etc though.

 

[-mjk-]

@Mark Richards the link I posted above shows the connections of the combo jacks made by Neutrik.

I don't get what all the fuss is about. It's a single amplifier with an attenuated input for the Line level. If the circuit is clean, does it matter? I use an external mic pre (actually I use a vintage console) on the Line inputs. The Tascam circuit is clean and doesn't add color that I am capable of hearing. All the coloration comes from the console mic pres. In the end it's about how it sounds. I've used the Tascam mic inputs too and I was very surprised how good they sound. I'm used to API, Neve, MCI and SSL consoles as well as my Soundcraft Ghost and X32 mic pres. I think the Tascam mic pres hold their own.

 

[cripple9]

I got another email from Tascam btw, when I asked them if they are sure the line inputs bypass the preamps. They directed me to P16 and this:

3 THRU button Click a THRU button to light it and send the input signal for that channel directly to the computer without passing through the input signal mixer. Since the signal does not pass through the mixer, phase, equalizer and compressor settings will have no effect.

Those emails are ridiculous, lol.

That is what I was hoping from the 208i, that I would turn the gain knobs for those combo inputs and nothing would happen...

However, I am also guessing that the combo jacks on the Tascam won't bypass the preamps, but I'm hoping this is not the case as I have just bought one (used) and will be selling it on straight away if not...

I assumed that's what you wanted, but I'm guessing the only way to bypass the 208i's preamps would be to send your external preamp through the OPTICAL IN on the back panel (if your standalone preamps have optical outs).

-------------------------

The combo jacks have separate audio paths:

https://www.neutrik.com/media/8030/download/combo-circuits.pdf?v=1

So it depends upon what the designer did with those paths. But the 6.5mm and XLR paths are not connected inside the jack itself.

I did get the input circuit schematic for the current TASCAM portastudios (which use combo jacks). As Phil Tipping pointed out to me, although the XLR and TRS circuits are separate initially, the signals become combined at the entry point to a shared pre-amp (the trim knob circuit + op-amp).

Thanks for that. I'm gonna slip into an OCD coma now, lol.

-------------------------

@-mjk- I've had some pretty awful sounding interfaces in the past, but I can't hear any coloring with the Tascam gear I've used either. I can understand OP's view though.

 

[Kane Williams]

I'm not rushing to sell it yet, I'm still considering the situation. I'm going to try the unit for processing busses from my DAW (Logic) with my external compressors (ADR Compex etc). If I can get this to work, then maybe I'll try it for a while.

One thing that does confuse me is this info.

Rated input level:

-54 dBu (0.0015 Vrms, GAIN knob at maximum)

+4 dBu (1.228 Vrms, GAIN knob at minimum) Maximum input level: +24 dBu (12.282 Vrms) Gain range: 58 dB

I'm sure this is just me being thick, but isn't the minimum and maximum the wrong way round?

I'm also wondering if it would be possible to get someone to mod this thing so 2 of the channels bypass the preamps. I'd be happy with them ending up line inputs only (no 46v etc).