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-   -   Solid-State Balanced Microphone Preamplifier (https://www.diyaudio.com/forums/pa-systems/337494-solid-balanced-microphone-preamplifier.html)

H713 5th May 2019 04:36 AM

Solid-State Balanced Microphone Preamplifier
 
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Not really sure if this is the right category to post this in- let me know if it isn't.

I'm currently modifying (redesigning) a Sony MXP-2900 mixing console. This is really my first try at designing a microphone preamplifier, so I'd like to get some feedback on my schematic. This is designed with studio usage in mind, and will most likely run off of +/- 18V rails, though I would like to make it usable from +/- 16V (500 series rack) up to +/- 24V (what this console will ultimately use for power rails when it's complete).

Notes:

The input transformer is just a 1:1. I don't see any reason why a JT11P-1 (line input transformer) couldn't be used for a microphone input. Technically this is not necessary, but it improves the CMRR and immunity to RFI. Output transformer is optional as well.

-When I prototype the circuit, I'll try both the BC550/BC560 transistors as well as the KSC1845/KSA992 transistors and see which results in lower noise and distortion.

I'd really appreciate it if I could get some feedback, as I've never designed a mic pre before, and it is certainly a bit of a design challenge.

basreflex 5th May 2019 07:14 AM

a mic input transformer needs proper shielding and be of humbucking design such that external magnetic fields are cancelled in the dual winding structure of primary and secondary. and R-C series load on the secondary reduces ringing of the leakage induction.
the 50k resistor is closing a loop of 2 inverting stages-> instability ?

MarcelvdG 5th May 2019 07:26 AM

R13 and R16 provide positive feedback rather than negative feedback, is that intended?

Due to the thermal noise of R9 and R10, with everything else noise-free, you already have a 4.5 dB noise figure for a 200 ohm source. No big deal when people are shouting into condenser microphones from a short distance, but it could be an issue with soft sounds and dynamic microphones.

If you intended negative feedback, maybe you could take the feedback to the emitters and replace R9 and R10 with lower values, or with lossy ferrite beads.

I would definitely add anti-parallel diodes across the base-emitter junctions of the transistors, especially the input transistors. A large transient, like you could get from connecting a microphone with the phantom supply on, could cause avalanche breakdown in the base-emitter junction, which tends to cause a large increase in 1/f noise.

The output transformer may need AC coupling, right now any offset at the amplifier's output will saturate the transformer's core.

basreflex 5th May 2019 07:38 AM

luckily the transformer takes up large common mode jumps like phantom disconnects. protecting the weak BE diode is always good.

MarcelvdG 5th May 2019 07:48 AM

I'm not sure it does protect the transistors when, for example, pin 2 of the microphone's XLR connector makes contact a few milliseconds earlier than pin 3.

basreflex 5th May 2019 09:05 AM

klopt, goed punt

mchambin 5th May 2019 09:28 AM

Use ZTX851, they are the best available for ultra low noise.
The trend in microphone preamps is going transformer less.
This paper http://www.thatcorp.com/datashts/AES...ic_Preamps.pdf
is great, it explains the reasons for the front end circuitry and explains 3 typical and good designs.
_Simple Mic Preamppage 27. It is easy and give good results.
_Complementary Feedback Pair Mic Preamp page 35. Is better.
_Current Feedback Instrumentation Amp, page 45. My favorite. There is no need for the two constant current source. About this topology you can find details looking at "Monte Generoso Samuel Groner" and "Double balanced Graeme Cohen".

basreflex 5th May 2019 11:21 AM

for low source impedances a transistor with low base resistance is best. traditionally PNP transistors with 'N' base material have a lower base resistance.
yet transistors engineered for HF or high currents have also low base resistance. the ZTX851 has a base resistance of 15milliohms according to the spice model.. seems a bit unrealistic to me.
for example a HF transistor like 2sc5065 of my favourite brand performs very well.

mchambin 5th May 2019 12:32 PM

The ZTX851 noise has been measured and compared with other transistors. This is far more dependable than Spice models.
Ultra low noise amplifiers
ZTX951 a PNP is lower noise but lower gain. Could be used instead of ZTX851, but not worth it.

H713 5th May 2019 05:55 PM

Quote:

Originally Posted by MarcelvdG (https://www.diyaudio.com/forums/pa-systems/337494-solid-balanced-microphone-preamplifier-post5781067.html#post5781067)
R13 and R16 provide positive feedback rather than negative feedback, is that intended?

Due to the thermal noise of R9 and R10, with everything else noise-free, you already have a 4.5 dB noise figure for a 200 ohm source. No big deal when people are shouting into condenser microphones from a short distance, but it could be an issue with soft sounds and dynamic microphones.

If you intended negative feedback, maybe you could take the feedback to the emitters and replace R9 and R10 with lower values, or with lossy ferrite beads.

I would definitely add anti-parallel diodes across the base-emitter junctions of the transistors, especially the input transistors. A large transient, like you could get from connecting a microphone with the phantom supply on, could cause avalanche breakdown in the base-emitter junction, which tends to cause a large increase in 1/f noise.

The output transformer may need AC coupling, right now any offset at the amplifier's output will saturate the transformer's core.

Yes, negative feedback was intended. I'll have to look at the spice simulation, but it looks like R16 was actually supposed to be connected back to Q1 rather than Q2.

I didn't even think about the noise from R9 and R10, but you bring up a good point. I'll try connecting to the emitters and see how it simulates. I'll play around and see what I can come up with.

Also, I'm sure there are better transistors out there, but I'm going to start with the BC550/BC560 and KSC1845/KSA992 because I have them in stock. If the noise performance isn't where I want it, then I can proceed from there.

As a side note, I have some Jensen JE-MB-C bridging transformers that I will probably use at least for the first couple preamps, as well as some JT11P-1 that I can try as well to see what happens. I also have about 16 transformers out of a Sescom mic splitter (1:1 ratio) that could work as well, though given what they came out of they're probably nothing special. I'm fully aware that the current trend is transformerless (especially due to cost), but I think I want to stick with a transformer coupled design for my own purposes.


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