dual Vs single opamp for stereo mic preamp

[spaceludwig]

Once it's inside that shielded box, the LT1128 should be the quietest. See earlier posts.

Paul,

Just received what was supposed to be 1128 but the individual sent me 1028 instead. Apparently these are stable at gains of -1. Am I going to have problems with this at low gain (i.e. between 15 - 20db)? Or is this like the OPA637 which is only stable at gains of 5 or more?

 

[spaceludwig]

Hmm, can't do the 1Khz signal measurement as the circuit is now behaving bonky. When I hook it to a PSU I get around 28V at one pin and -2V at the other. I've removed the opamp so it can't be that. The power supply is 5A 60V, it is reading correctly when the preamp is not plugged in. I have done continuity check on all the traces and there doesn't see to be a short anywhere... I am stumped! What's strange is that it was working fine before I took out the transformer.

 

[spaceludwig]

OK, got circuit running again.

Question: If I take the signal straight from the output to a recorder do I need to put a cap in the signal path to block DC? If so what value would be best? Would 1uF be OK?

 

[gemini86]

...depends what the input impedance of the recorder is. If it's 10k, 1uF would corner at about 16Hz.

 

[spaceludwig]

Hmm, problem is I don't know what is standard input Z for portable recorders. Perhaps 10uF would be the safest bet?

 

[gemini86]

What model is it? can probably find a manual on line if you don't have it. You should know all the impedances of your gear.

 

[spaceludwig]

Hmm, the Tascam dr-100mkii has different input z for their 2 line-in channels. One is 2.5k and the other is 10k. This seems quite silly.

 

[pstamler]

Hmm, the Tascam dr-100mkii has different input z for their 2 line-in channels. One is 2.5k and the other is 10k. This seems quite silly.

It's not silly; these aren't the two channels (L&R) but two different "line" inputs. One comes in on the XLR jack and has the standard pro nominal level of +4dBu; its input impedance is 2.5k. The other comes in on a 1/8" jack and has the semi-pro nominal level of -10dBV, or about -8dBu. It has an input impedance of 22k. I don't know the circuit details of the two inputs but there's probably some kind of valid reason for the difference in impedance.

Peace,
Paul

 

[JohnRoberts]

+1..  the 2.5k input is actually a little low for a normal line input (10k is more typical for dedicated line inputs) and is probably a mic input being used as dual purpose.

JR

 

[spaceludwig]

I know there is some very sound (ptp) reasons for Tascam engineers to do this. It just ends up being a silly situation if you are trying to use a stereo mic and have to use inputs with different impedances. My understanding is that both XLR line inputs have different input Z. Where does that leave someone wanting to amplify a stereo signal?

Of course, I am free to use a different portable recorder.

 

[pstamler]

I know there is some very sound (ptp) reasons for Tascam engineers to do this. It just ends up being a silly situation if you are trying to use a stereo mic and have to use inputs with different impedances. My understanding is that both XLR line inputs have different input Z. Where does that leave someone wanting to amplify a stereo signal?

I would be very surprised if that were true. Have you measured the impedances?

Peace,
Paul

 

[spaceludwig]

No, it's just the way I interpreted the specs in the manual, probably incorrectly. I will have a chance to try it out in a couple of weeks.

But to answer the original question I guess 10uF would be the safest value?

 

[JohnRoberts]

I know there is some very sound (ptp) reasons for Tascam engineers to do this. It just ends up being a silly situation if you are trying to use a stereo mic and have to use inputs with different impedances. My understanding is that both XLR line inputs have different input Z. Where does that leave someone wanting to amplify a stereo signal?

Of course, I am free to use a different portable recorder.

Whoa.. are you saying you have two different input impedances on mic inputs?  Or are you talking about sticking a mic into a line input?

JR

 

[spaceludwig]

Whoa.. are you saying you have two different input impedances on mic inputs?  Or are you talking about sticking a mic into a line input?

JR

Original question: Should I put Dc blocking cap at the output of the preamp this whole thread is about (since I am not longer using balancing transformer), and if so, what value. I was asked what the input Z of the recorder. From the manual of the Tascam DR-100 it would appear as though both XLR's have different line-input Z. I am most probably mistaken in my interpretation of the specs but that's how I read it. Perhaps you could confirm?

 

[pstamler]

Original question: Should I put Dc blocking cap at the output of the preamp this whole thread is about (since I am not longer using balancing transformer), and if so, what value. I was asked what the input Z of the recorder. From the manual of the Tascam DR-100 it would appear as though both XLR's have different line-input Z. I am most probably mistaken in my interpretation of the specs but that's how I read it. Perhaps you could confirm?

I looked at the specs and that's not at all how I read it. One pair of line inputs is +4dBu balanced; the other pair is -10dBV unbalanced. They have different impedances. A 10uF blocking capacitor would work for either.

Peace,
Paul

 

[spaceludwig]

I looked at the specs and that's not at all how I read it. One pair of line inputs is +4dBu balanced; the other pair is -10dBV unbalanced. They have different impedances. A 10uF blocking capacitor would work for either.

Ok then, issue resolved.

I'l be posting a schematic of a PSU for this unit shortly. Perhaps I'll start another thread in case it's of interest for other applications.

 

[spaceludwig]

Question about loading: If i use a 5K resistor as was suggested previously do I simply substitute the 20K resitor (R8) at the input of the opamp? Or I add that in addition to the 20K?

Also, are the two zeners necessary at the input in this application? Is it for protection due to phantom power? And if so, is it necessary seeing that there is a transformer between the 48V and the input? (asked another way, would the zeners contribute any noise that could otherwise be avoided)

 

[ricardo]

If i use a 5K resistor as was suggested previously do I simply substitute the 20K resitor (R8) at the input of the opamp?

Yes

Also, are the two zeners necessary at the input in this application?

No.  You don't need the Zeners with your transformer

But your biggest problem will be your 2000x gain range.

 

[spaceludwig]

Thanks for the reply, Ricardo.

I removed the zeners and after testing adjusted the 1.6K in the zobel for best square wave when loading with 5K resistor.

As for the 2000X gain I will just change R5 to 20 or 30R.

 

[PRR]

Use the mini-jack (with a GOOD plug) so you don't need heroic line-drivers or massive gain which is only padded-down internally.

In MIC the XLR is 2.1K.

In Line the XLR is 2.5K.

Two Phantom resistors series across a 2.5K input is 2.111K.

The Line input IS the mike amp, but Phantom disconnected.

I really think the internal level is -10dBV, slightly buffered to the ADC.

The XLR Line input is padded or buffered and gained-up and padded-down to get to the same point as the -10dBV side jack. It is not "better" unless you are in a hostile environment (or large studios which must assume hostility). It is "worse" in that there are more parts in the way to taint the precious audio.

The mini-jack input is "worse" in that at-best a mini-jack is not a 99.99% connection. We all have WalkMen and earbuds where we have to jiggle the connector.