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Author Topic: S/N in big consoles  (Read 13414 times)

Jim Williams

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Re: S/N in big consoles
« Reply #45 on: February 02, 2011, 10:54:51 am »

Geoff Emerick de Fake wrote on Mon, 31 January 2011 10:58

Jim Williams wrote on Sat, 29 January 2011 11:14

Actually less of them if you use the right parts. There are very low noise jfets available. Linear Systems makes the selected low noise 1 nv 2SK170.
That's exactly what I said; you need to parallel 100 of them to achieve 0.1nv.
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 NXP (Phillips) makes the BF862 at .7 nv voltage noise and minimal current noise.
Then you only need 50 of them.
Quote:

 That also allows one to set the input impedance very high to minimize loading.
The actual input impedance is more dependant on the topology and design choices than by the devices. The  typical Cohen preamp with LM394's has an input impedance >100k.


Which is still not high impedance like you can do with the jfets, I'm talking meg ohms here.  100k bias resistors used alone with  a LM394 will cause a lot more resistor source noise than that part is designed for, a low input impedance to match it's noise and current noise specs. Once the 6.81k phantom resistors are fitted, that sets it low again anyway. With a phantom switching scheme that switches out the phantom resistors and input blocking caps you can raise that input impedance pretty high with the jfets without source impedance noise problems that occur with bipolar transistors.
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Jim Williams
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Geoff Emerick de Fake

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Re: S/N in big consoles
« Reply #46 on: February 02, 2011, 02:44:28 pm »

Jim Williams wrote on Wed, 02 February 2011 09:54

 
Which is still not high impedance like you can do with the jfets, I'm talking meg ohms here.
What's the use?
Quote:

 100k bias resistors used alone with  a LM394 will cause a lot more resistor source noise than that part is designed for, a low input impedance to match it's noise and current noise specs.
This is irrelevant. Once the source impedance is connected, the impedance the input devices see is much lower. 200 ohms in parallels with 200kohms makes 200ohms.
Quote:

 Once the 6.81k phantom resistors are fitted, that sets it low again anyway. With a phantom switching scheme that switches out the phantom resistors and input blocking caps you can raise that input impedance pretty high with the jfets without source impedance noise problems that occur with bipolar transistors.
What are "source impedance noise problems"? What's the difference between 100k in parallels with 200r and 100Meg in paralles with 200r? The only effect on noise the input impedance has is the loading effect. 0.018dB difference. Nothing to write home about.
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #47 on: February 02, 2011, 06:23:16 pm »

Indeed.. I don't follow what Jim is concerned about.

I used to terminate all with 2k (bridging to 200 ohm) terminations, but have since read some mic designers designed for somewhat higher, but not 100k high. .

I suspect some preamp designers use high Z terminations, just so they'll sound different. "you can' t sound better if you sound the same as everybody else".   Rolling Eyes

---

Back to the ribbons, I am curious about very low noise JFETs mostly for academic reasons not for any imagined practical advantage, especially with ribbons.

JR

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Jim Williams

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Re: S/N in big consoles
« Reply #48 on: February 04, 2011, 11:30:47 am »

No concern, it's just another way to skin a cat. Sonically an all jfet low noise high input impedance mic preamp design does sound different. As I mentioned before, the higher input impedance allows the designer to use smaller value, higher quality film caps to block 48 volts in place of common larger valued electrolytic capacitors. It does allow one to inject high impedance sources without a DI box, but that's not even needed.  Besides an all jfet design, I also have some with all transimpedance amplifiers, those are very fast and once again, different sounding. They run at over 100 mhz and sound very good.

Which ever topology I use, they all sound a lot better to me than the commercial stuff you can buy.
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Jim Williams
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #49 on: February 04, 2011, 05:32:08 pm »

Jim Williams wrote on Fri, 04 February 2011 10:30

No concern, it's just another way to skin a cat. Sonically an all jfet low noise high input impedance mic preamp design does sound different. As I mentioned before, the higher input impedance allows the designer to use smaller value, higher quality film caps to block 48 volts in place of common larger valued electrolytic capacitors. It does allow one to inject high impedance sources without a DI box, but that's not even needed.  Besides an all jfet design, I also have some with all transimpedance amplifiers, those are very fast and once again, different sounding. They run at over 100 mhz and sound very good.

Which ever topology I use, they all sound a lot better to me than the commercial stuff you can buy.


Speaking of other ways to skin the cat, I like the concept of just letting a DC coupled mic preamp float up to the phantom voltage and cap couple with higher impedance film caps after it's up at line level.  One could fly an A/D converter up there with the preamp and literally do a capacitor free audio path...  While I am still inclined to go with conventional (circa 2k) resistive terminations. Of course non-standard terminations could be an obscure feature for esoteric variants.

JR

PS: I think my designs sound good too...  Laughing  Doesn't everybody think they have pretty (or smart)  children?
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