Recommended implementation for THAT1512

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tubegeek
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Re: Recommended implementation for THAT1512

Post by tubegeek » Tue Mar 26, 2013 7:55 pm

So in looking at the existing circuit (LINEMIXERSchematic.jpg) I see a couple of things that interest me.

1. The buffer ahead of the existing clip indicator (IC1B) looks like a good place to pull off a pre-fader, pre-pan signal - if I roll my own clip indicator I can repurpose that send as a pre-fader send. Nice.

2. If I wanted to leave well enough alone and just use the existing indicators - what do you think of the existing circuit? And how would I be able to tweak the indicator level to suit my new gain structure? I don't really understand which of the voltage dividers does what.

(I have a feeling that my usual practice of redrawing the circuit so it makes sense to ME will be a good step towards understanding it here - this is not my favorite schematic drawing ever.)

3. If I roll my own circuit, at least I'll have a red/green LED already available and mounted up for my circuit to use (no metalwork, yay!)

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JR.
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Re: Recommended implementation for THAT1512

Post by JR. » Wed Mar 27, 2013 11:58 am

tubegeek wrote:So in looking at the existing circuit (LINEMIXERSchematic.jpg) I see a couple of things that interest me.

1. The buffer ahead of the existing clip indicator (IC1B) looks like a good place to pull off a pre-fader, pre-pan signal - if I roll my own clip indicator I can repurpose that send as a pre-fader send. Nice.

2. If I wanted to leave well enough alone and just use the existing indicators - what do you think of the existing circuit? And how would I be able to tweak the indicator level to suit my new gain structure? I don't really understand which of the voltage dividers does what.
I'm too lazy to look, do the math.. resistors form simple voltage dividers, relate that voltage to 0VU for your system...
(I have a feeling that my usual practice of redrawing the circuit so it makes sense to ME will be a good step towards understanding it here - this is not my favorite schematic drawing ever.)
Yes, design engineers often have a way of looking at circuits that makes sense to them. Schematics are often re-drawn by people that don't understand them, so we get what we get. Including errors in old schematics.
3. If I roll my own circuit, at least I'll have a red/green LED already available and mounted up for my circuit to use (no metalwork, yay!)
yup

In many cases bi-color LEDs is completely adequate to answer is signal happening and not clipping. I have designed consoles with only a handful of meters, but bi-color on tens of audio paths.

JR

tubegeek
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Re: Recommended implementation for THAT1512

Post by tubegeek » Sat Apr 13, 2013 9:07 pm

I've got a single mic preamp channel almost finished on perf board. I've built this first one as a standalone mic pre stage to feed a line mixer via XLRs, running off a battery supply (+/- 18V rails.) I want to hear what it sounds like before I build up a bunch of channels.

It's in three little boxes: the mic pre with its transformer, the +/- 18V power supply, and (to come, possibly) a floating, battery phantom supply (probably +27 or +36 for now, so I can do a test or two with a condenser mic.) I've drawn a simplified schematic showing the interconnections beween the sections but omitting just a few parts of the op amp circuit, which is taken straight from Figure 4 of the app notes. Those parts are indeed in there.

With all these boxes and their various floats and references I just want to make sure I'm making the correct connections for ground and chassis before I plug the op amp into their socket. Is pin 3 of the unbalanced output the correct ground reference for the power supply and the rest of the opamp circuit? I'm doing this quick and dirty for now so no balanced output chip or even output buffer, I'll just keep the cable really short. (Should I expect to need a resistor between the op amp and the output XLR for stability?)

Is there anyone who'd like to double check my connection scheme please? Also, I think I have the RF suppression caps arranged correctly (on the op amp side of the input transformer, bypassing to the chassis ground at the input jack) but I wanted to double check that too.

What value cap should I use to bypass the phantom power at the entry to the box? Typical 100 nano ceramic like an op amp bypass, or what?

Thanks!

-j
that setup.jpg
THAT 1510/1512 battery powered mic preamp
that setup.jpg (361.95 KiB) Viewed 6835 times

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JR.
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Re: Recommended implementation for THAT1512

Post by JR. » Sun Apr 14, 2013 9:58 am

I would be inclined to use a build out resistor in series with pin 3, and not connect the power supply through the audio low or reference.

JR

tubegeek
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Re: Recommended implementation for THAT1512

Post by tubegeek » Sun Apr 14, 2013 4:53 pm

JR:

My drawing may be misleading. The connections at the XLR out jack are shown as if they are made on the XLR connector but (+, –, E and G) will actually be made by their own 4-pin connector, either a Molex type or possibly an RJ-11 or RJ-45 snap-in type to save some room. Depends on what I have at hand.

E and G will be connected inside the box to XLR 3 and XLR 1. Better? Those connections DO have to be made, though, right?

Build out R: got it. I'll use the suggested value, I think it's 200 ohms? Pin 3? I thought it should go on Pin 2?

Thanks for taking a look.

-j

tubegeek
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Re: Recommended implementation for THAT1512

Post by tubegeek » Tue Apr 16, 2013 5:30 pm

It works. The gain adjustment is odd but I was expecting that, I don't have a pot of the appropriate value. I tested it, I'm getting a maximum of 48 dB of gain, which is less than I expected with a step-up transformer. I haven't measured the transformers yet, I may have a smaller step up than I expected, or else there is something wrong I haven't figured out. Bandwidth: flat below 10 Hz and - 3dB is at 75kHz. MOL is just about exactly +24 dBm with the +/- 18V battery supply.

So I guess I better listen to it now!

Thanks for the help. I'll report in when I get more info about the gain situation.

NOON
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Re: Recommended implementation for THAT1512

Post by NOON » Sat Jun 10, 2017 1:56 am

I figure this one is the probably the most relevant thread to continue this discussion. I've got some THAT 1512s that I will be making into some preamps for myself and a couple of friends and wanted to try and summarize the current 'best practice' implementation for this chip based on several threads on here and elsewhere. I've attached my schem so far.
THAT1512 PREAMP SCHEM.pdf
(59.35 KiB) Downloaded 85 times
(It's currently split across a couple of small boards so 8 can be front panel mounted in 1U, hence the internal connectors etc. )
Only main deviation from schems so far is the addition of an input impedance switch between ~600R, ~2K and 'high' (~10K) and the bumping up of the T bias resistors to 10k, for higher max input impedance.

A few questions I'm hoping the brains trust can help with...
1) Is this pretty much the best-case circuit for a 1512? Any further tweaks or suggestions?

2) Have I implemented the servo properly? (it was based on various bits of discussions on here rather than a complete circuit. Left off the fine tuning trimmer for simplicity)

3) is the OPA2188 a suitable low cost opamp for the servo? Specs looked pretty comparable to the OP07, there was a thread here that started off about the OPAX188 series for servos that branched off into general servo discussions without explicitly saying they were good enough, but it was implied. Dual package makes layout considerably easier than using OP07.

Thanks.

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mediatechnology
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Re: Recommended implementation for THAT1512

Post by mediatechnology » Sat Jun 10, 2017 7:50 am

Thank you for joining us and posting NOON.

Your schematic looks good.
I would do the following.

1) Add a differentially-connected capacitor across the input network for RFI. 200 pF-1nF?
2) The peak current at turn-on in R9 and the phantom switch is 480 mA or limited by the short-circuit current of the phantom supply. Is R4/C9 really necessary? I've drawn a few circuits with them myself and never knew why. Once I looked at it I thought "why?"

I would eliminate R4 and C9. a) Phantom turn on current is limited by the phantom resistors. b) The phantom supply should be quiet enough without C9 being on each module. c) Any noise on the phantom is in common mode and rejected. d) You have to manage huge current transients at turn-on. e) One less capacitor to discharge at turn-off.

3) "Back ground" S1 when phantom is off to discharge the input caps through the phantom resistors.
4) C3/C5 seem low in value. Are you trying to use film caps here?
5) L1/L2 add DC resistance so account for that at high gain when selecting your stop value resistors. I like having L1/L2 but do be aware that they pickup noise and do better when shielded or screened.
6) The OPA2277 is a good DC servo op amp. There's no passive input pole in the servo which is something JR has educated me about. Take a look at the Deboo integrator viewtopic.php?f=12&t=866&start=2 and then follow the links to DC-coupled preamp.
7) Check for infrasonic resonance in the servo: http://www.thatcorp.com/datashts/AES919 ... ifiers.pdf (Thanks Gary and Olaf.)

8) I've also found - and I don't know why - that a small value of resistance, 4R7 to 10R, between the bridge rectifier and the input pins improve HF CMRR. Rather than have a pair of 10R phantom protection resistors, split the two 10R into four 4R7 and have the bridge connections in the middle. You don't add additional resistance that way. I also found that any differentially-connected capacitor should be on the left-hand side of the final two resistors. The HF CMRR improvement from the added R is the complete opposite of what I would expect. I've confirmed this in two INA designs now one using a 1570 and the other op amps.
9) I see that you are using a monolithic bridge for the protection diodes. I found that the "P" and "N" type diodes in a DB104 have different junction capacitance (and Vf). This reduces HF CMRR. In one design I went back to discrete diodes (glass-passivated 1N4004G or S1D/B) but before I did I found that suggestion 8 above reduced the effect.
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JR.
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Re: Recommended implementation for THAT1512

Post by JR. » Sat Jun 10, 2017 10:46 am

+1 to waynes comments

Not sure why you need 2 inductors in gain leg, is the gain pot mounted at some distance in a noisy environment? In the past I have driven the gain pot cable shield to reduce capacitance effects.

The THAT chip probably has clamp diodes inside so added external ones (D1,D6) are at best redundant.

It requires one more cap but making the first stage of the servo non-inverting will be better behaved (IMO).

10uf for phantom blocking are borderline small... if using 10uF might be worth trying to match capacitance for better LF CMR, likewise 10 Ohm in series with inputs should be close (1%) for HF CMR (I've seen trims there in old designs).

I don't see any Cs to ground at input... high RF and input rectification could be an issue, what does THAT recommend in their app notes (I'm too lazy to look)?

JR

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mediatechnology
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Re: Recommended implementation for THAT1512

Post by mediatechnology » Sat Jun 10, 2017 12:52 pm

Not sure why you need 2 inductors in gain leg, is the gain pot mounted at some distance in a noisy environment? In the past I have driven the gain pot cable shield to reduce capacitance effects.
To provide gain rolloff at AM RF frequencies is why I've tried them.
The THAT chip probably has clamp diodes inside so added external ones (D1,D6) are at best redundant.
The internal diodes are not "strong" enough. That is what drove me to blow up lots of THAT1510s: http://www.thatcorp.com/datashts/AES790 ... eturns.pdf
I don't see any Cs to ground at input... high RF and input rectification could be an issue, what does THAT recommend in their app notes (I'm too lazy to look)?
I believe the current thinking is a delta-connected three capacitor network. Previously it was wye. (Or is it Wye?)
I would definitely put locations on the PC board for three RF caps.

I've had pretty good results in a modestly high AM RF environment with the flat phono preamp with a differential C (which also happens to be Cload) and input series resistors to prevent CM RF rectification.
Read Damore v. Google and ask yourself the following questions:
Are Googlers really the people you want in charge of your search results, browser, data or operating system?
Would you be comfortable riding in a Google self-driving car knowing that Googlers might want you dead just because they don't like you?
Would you trust a Googler with your children?

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