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Preamp questions._
Preamp questions._
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Old 12th July 2016, 12:31 AM   #1
Leek is offline Leek
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Default Preamp questions._

I have recently began building a preamp. I'm using a Balanced line transmitter/receiver to balance my I/O. My preamp stage is a NE5532 op amp style circuit. I have 0.1uf Wima caps between each +/- power input on each op amp. I'm using all Wima (Film) 1% caps for the entire circuit. Resistors are all Vishay metal film 1%. My power supply is supplied using a Hammond 161G28 Transformer, .5% caps/resistors and 7812/7912 regulators. Decoupling is provided by Wima 0.1uf caps in entire circuit.

Balanced line transmitter/receiver schematic (Balanced Line Driver & Receiver)
The only exception is I'm using the OPA2134 in the transmitter circuit for it's better quality.


NE5532 preamp schematic (> circuits > Pre MIC microphone preamplifier 2 CH by IC NE5532 or LF353 l40881 - Next.gr)
Identical using all Wima/Vishay 1% components.


PSU schematic (12 Volt Symmetric Power Supply Circuit)
Identical using .5% components.

My question is, what kind of quality can I expect from this. My soldering is neat and etc. Further more, how would I go about adding I/O transformers? This is a learning process for me. The circuit may not need transformers, but I WANT to add them.

Last edited by Leek; 12th July 2016 at 12:34 AM.
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Old 12th July 2016, 01:04 AM   #2
anatech is offline anatech  Canada
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Preamp questions._
Hi Leek,
This thread is closely related to your existing thread on the same subject. Do you want them combined?

-Chris
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Old 12th July 2016, 02:06 AM   #3
Leek is offline Leek
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It's not really related? One is transformer related. This is aimed at individuals who have built similiar designs and feedback.
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Old 12th July 2016, 02:11 AM   #4
abraxalito is offline abraxalito  United Kingdom
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With opamps the sound quality is largely a function of how much signal-modulated power supply noise you have on your rails.

If you're going over to transformers I'd suggest going discrete for your gain stages rather than using opamps, it will allow you to build pure classA circuits.
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Old 12th July 2016, 02:21 AM   #5
Leek is offline Leek
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I can adapt the preamp stage for a discrete class A build. But, as of now I'm curious of the TL071, NE5532 and OPA2134 i'm using in my build. What should I expect? A good sounding preamp? Garbage?

As for transformers, I'm looking at Jensen. They seem to be big and clean? The Trident 80b console used a input transformer followed by a TL071. So, there's got to be some ideas for a transformer balanced input/output for my circuit.

Last edited by Leek; 12th July 2016 at 02:25 AM.
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Old 12th July 2016, 02:25 AM   #6
abraxalito is offline abraxalito  United Kingdom
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The results you get will depend on your source - if you're using a regular S-D kind of DAC with opamp output stage then the additional harm another handful of opamps is going to do is limited (so long as you don't mess up the layout details). So if you're used to S-D DACs as a source your preamp will sound pretty good to your ears.

The fact that you're talking transformers might just be an indication you're after something that's a tad more transparent than the run of the mill stuff though?
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Old 12th July 2016, 02:35 AM   #7
Leek is offline Leek
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Well, I plan to use standard recording mics with the circuit. Eg: SM57,C414 and MD 421. I have a phantom supply for the C414 as well will add one to the circuit eventually. Transparency, no. I'm just experimenting with designs and components. I would like the additional tonal variation of a bypassable mic input transformer and line output transformer. As well as the knowledge gained from installing the transformers.

Im used to preamps in cheap audio interfaces. I'm hoping this would be an improvement over something like a digidesign 002 preamp?

Last edited by Leek; 12th July 2016 at 02:46 AM.
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Old 12th July 2016, 11:29 AM   #8
sgrossklass is offline sgrossklass  Germany
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If your input xfmr is supposed to be "bypassable", you better use a balanced mic input (Rod Elliott has a project for one, too, other than that a classic instrumentation amplifier job e.g. using THAT1512 or SSM2019 would get the job done as well). The preamp shown would not make a good combination with the balanced line driver anyway, as it has output impedance in excess of 50 kOhms and something closer to a voltage source would be required. Output would still be balanced but voltage would not be symmetrical, not to mention less-than-ideal distortion (classic JFET-input opamps, when run noninverting, should be fed from lowish source impedance).

I would add some extra gain after the mic preamp as well, maybe 14-20 dB after a level control pot (10-50k, and only a few hundred ohms of series R). A noninverting amplifier using NE5532 would actually work well for this. While CMRR benefits from more gain up front, distortion inevitably goes up.

The ouput from the second gain circuit could then be mixed using an inverting adder circuit (either NE5532 or OPA213x would be fine for one), run through a main volume pot, and buffered with a unity-gain buffer before run into the line driver.

Mind your XLR wiring - pin 1 should connect straight to chassis, which connects to circuit ground only at the central group point (at least for audio frequencies - the odd 10 nF cap may turn out to be useful at RF).
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Old 12th July 2016, 01:45 PM   #9
anatech is offline anatech  Canada
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Preamp questions._
Hi Leek,
The NE5532 and 5534 are both excellent op amps. The OPA2134 is an extremely good J-FET type op amp, but it also has really high CMRR - making the non-inverting case less of a problem than many other FET input op amps suffer from.

The TL072 family of op amps sound terrible next to the OPA2134. In addition, the drive capacity of the TL072 is very low, try not to use those except for housekeeping duties. That means for applications outside of the signal path.

With what you are designing, and using the NE553x and OPA2134 parts, should sound excellent. That's as long as you are keeping your end of the bargain up with the design, layout and soldering. It would really help you to read the data sheets carefully for those parts (including the applications section) as well as any applicable application notes that most manufacturers publish. If you haven't read Doug Self's book on signal stages (including mixing consoles), I suggest that you at least read the sections that apply to what you are doing now. Read the entire book as soon as you can.

Back to your power supply. Having too high a voltage does give you an opportunity to do some extra C-R-C type filtering and shedding the high voltage at the same time. This will increase your isolation from the HF garbage on the mains.

Lastly, pay attention to the current specs (listed per amplifier in a multi-amplifier package), and do not forget to include some headroom for signal current that goes into driving some outboard gear and the amplifier(s). You can regulate things like your mic amplifier and future phono section individually to isolate the influence of the output stage and any send amplifiers. Low level inputs are susceptible to things like this.

-Chris
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