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Author Topic: S/N in big consoles  (Read 13650 times)

JGreenslade

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Re: S/N in big consoles
« Reply #30 on: January 28, 2011, 08:05:55 am »

There were a handful of transistors supposedly made specifically for MC carts - I believe up until the early eighties, but you might be right (the audiophile boom seemed to peak around the late '70s from what I've seen). Sanyo and Hitachi both offered devices that were claimed to be developed for this purpose. IIRC, Mackie used the Sanyo parts in one of their consoles. Well, say what you want about Mackie, but their consoles have never been noisy!

There's a Self-designed MC preamp that parallels the SB737 at the front end.

2SA872 - I seem to remember that off top of head.

Justin
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #31 on: January 28, 2011, 10:38:15 am »

JGreenslade wrote on Fri, 28 January 2011 07:05

There were a handful of transistors supposedly made specifically for MC carts - I believe up until the early eighties, but you might be right (the audiophile boom seemed to peak around the late '70s from what I've seen). Sanyo and Hitachi both offered devices that were claimed to be developed for this purpose. IIRC, Mackie used the Sanyo parts in one of their consoles. Well, say what you want about Mackie, but their consoles have never been noisy!

There's a Self-designed MC preamp that parallels the SB737 at the front end.

2SA872 - I seem to remember that off top of head.

Justin


I was not familiar with the 2SA872. That looks like a true low noise part for mic preamps or perhaps input stage of a DOA, but Rbb (as speculated from constant NF curves) looks larger than 737/786 pair that were optimal for a decade lower source impedance (tens of ohms, not hundreds).

As I recall the 737/786 parts were developed by a small Japanese company that was then bought by ROHM who distributed them in the US. I don't recall any other parts that were really close to them. I used a lot of these in mic preamps where they were probably lower Rbb than needed, and lower than modern replacements since they went obsolete, and IC based preamps that are now widely used.  

JR
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Jim Williams

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Re: S/N in big consoles
« Reply #32 on: January 28, 2011, 11:18:00 am »

The 2SB and 2SD devices were the slow stuff, the 2SA and 2SC are the fast ones.
Besides those slower parts, there were/are faster varients of them.

The Hitachi 2SA1083 and 2SC2545 series were the quicker parts. Toshiba also makes similar parts, the 2SA1316 and 2SC3329.

Renesas also now makes the 2SA1084.
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Geoff_T

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Re: S/N in big consoles
« Reply #33 on: January 28, 2011, 11:20:36 am »

JGreenslade wrote on Thu, 27 January 2011 15:04



This reminds me of a chat I had a while back with a certain pipe-smoker who looks after a gaggle of vintage Neves here in the UK. A client had remarked that, whilst their facility could live with the console's noise floor, they were interested in bringing it below that of the new-fangled hard disk recording devices. Said engineer decided to try an experiment, by equipping a selection of bus amp cards with uber-low noise Japanese epitaxial types (the type developed in the eighties for low noise MC cartridge preamps), putting these in place of the original cards that contained BC184s and seeing what difference this made to the broadband noise trace on the FFT...

...suffice to say, there was no difference at all. The transistors themselves are by far one of the least dominant noise sources - as I'm sure the Neve stalwarts here will be fully aware. I just thought it would make an interesting interjection to relay the tale. I guess he wished in his heart that he could walk away from the studio feeling smug that he'd taken 12dB off the console's floor... No cigar that day!

Justin


Hi

In the era of the 8038/8048 the production manager at the time (Niall MacDermot nick-named Flying Officer Kite) came up with a wonderful idea that the B- bus system should be a welded/brazed solid section of 1/4" copper, all the way back to the EP5 input connector. The solid bus bar structure was fitted in the metal shop at the time of console frame assembly and, once in, could never be removed.

The console was duly assembled and tested and the noise floor was no better (I recall it may have been a dB or so worse) and the idea was scrapped.

So there is one console out there somewhere with this wonderful welded bus bar...

Smile

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John Roberts {JR}

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Re: S/N in big consoles
« Reply #34 on: January 28, 2011, 11:53:50 am »

Jim Williams wrote on Fri, 28 January 2011 10:18

The 2SB and 2SD devices were the slow stuff, the 2SA and 2SC are the fast ones.
Besides those slower parts, there were/are faster varients of them.

The Hitachi 2SA1083 and 2SC2545 series were the quicker parts. Toshiba also makes similar parts, the 2SA1316 and 2SC3329.

Renesas also now makes the 2SA1084.


Yup,  the larger base-emitter structure to get base spreading resistance down so low (typically 2 ohms in 737) has other consequences (higher capacitance? etc). I am inclined to say those others are not the same parts just faster, but instead tweaked differently with smaller geometry, so arguably different parts.

Since the very low noise parts are lower noise voltage than they need to be for most  current applications, RIP.  

While I haven't been keeping up with the new ribbon mics. Aren't they pretty low source impedance?

JR
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bruno putzeys

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Re: S/N in big consoles
« Reply #35 on: January 28, 2011, 01:23:51 pm »

Yes, but even lower than that. I've not yet managed to make a transformerless pre for a ribbon with sufficiently low noise.
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #36 on: January 28, 2011, 03:21:41 pm »

bruno putzeys wrote on Fri, 28 January 2011 12:23

Yes, but even lower than that. I've not yet managed to make a transformerless pre for a ribbon with sufficiently low noise.


Lower than 2 ohms? Interesting, you'd need a transformer just to get it up to a typical mic impedance, and the transformer windings have to get significant down in single digit ohms..

Transformers were popular for MC preamps too. I got some favorable responses to my transformer less MC amp back in early '80s. .  

JR
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Geoff Emerick de Fake

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Re: S/N in big consoles
« Reply #37 on: January 28, 2011, 04:46:00 pm »

A ribbon has a typical impedance of about 1/10th of an ohm. The necessary transformer uses a few turns of large-gauge wire at the primary.
Even with these precautions, most of the reflected impedance at the secondary is resistive.
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #38 on: January 28, 2011, 05:57:51 pm »

Geoff Emerick de Fake wrote on Fri, 28 January 2011 15:46

A ribbon has a typical impedance of about 1/10th of an ohm. The necessary transformer uses a few turns of large-gauge wire at the primary.
Even with these precautions, most of the reflected impedance at the secondary is resistive.


Thanks, that does sound like it wants to be a transformer input...  I guess you could parallel a silly number of these very low noise transistors, but they're obsolete now anyhow.

I've seen some circa 1 nV JFETs, I wonder if they are better candidates to parallel? While noise current into .1 ohm can't be that much of an issue.  

Perhaps 6 of one, half dozen the other, or  maybe six of one "and" a half dozen the other.   Laughing

JR

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Geoff Emerick de Fake

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Re: S/N in big consoles
« Reply #39 on: January 28, 2011, 06:09:01 pm »

As Bruno mentioned, there have been some experiments. The essence of the problem is that, in order to achieve a good ratio of En to In, the input transistor compound would have to operate at several amps quiescent.
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #40 on: January 28, 2011, 08:37:55 pm »

Geoff Emerick de Fake wrote on Fri, 28 January 2011 17:09

As Bruno mentioned, there have been some experiments. The essence of the problem is that, in order to achieve a good ratio of En to In, the input transistor compound would have to operate at several amps quiescent.


The low noise parts already need a couple mA for MC amps.

That's why I mentioned the JFETs, while nominall starting at higher En, their In should be much lower than the bipolar, but still either way that In is only working into < 1 ohm so will generate modest noise voltage.

I'm more concerned about a suitable cascode/bootstrap topology to deal with all the device capacitance from a bunch o' parts in parallel.

But not worried enough to kill brain cells over this... unless ribbon mics get real popular.

JR
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Geoff Emerick de Fake

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Re: S/N in big consoles
« Reply #41 on: January 29, 2011, 06:22:07 am »

With a <1ohm source, you need noise voltage <0.1nV/sqrootHz, which is theoretically achievable by running the input transistor compound at about 1 amp.
With FET's, you would need to parallel 100 of them to achieve that, with about the same quiescent current.
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Jim Williams

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Re: S/N in big consoles
« Reply #42 on: January 29, 2011, 12:14:24 pm »

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. NXP (Phillips) makes the BF862 at .7 nv voltage noise and minimal current noise. That part does work well in mic preamp front ends without the noise trade-off of most other jfets. That also allows one to set the input impedance very high to minimize loading. It also allows you to reduce the value of the input blocking caps letting you select quality film caps in place of traditional electrolytic caps to block 48 volts.
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John Roberts {JR}

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Re: S/N in big consoles
« Reply #43 on: January 29, 2011, 01:26:03 pm »

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. NXP (Phillips) makes the BF862 at .7 nv voltage noise and minimal current noise. That part does work well in mic preamp front ends without the noise trade-off of most other jfets. That also allows one to set the input impedance very high to minimize loading. It also allows you to reduce the value of the input blocking caps letting you select quality film caps in place of traditional electrolytic caps to block 48 volts.


Yup, my recollection is that some of these low noise JFET have Idss well into the tens of mA, so when biased up for lowest noise, you can get up into amps long before 100 parts. Of course you will need to manage package dissipation, etc.  

Somewhere I saved some data sheets for a silly low noise JFET but I am not sure those were audio bandwidth parts.  I used a 2sk117 back in the '80s in a phono preamp and that was a sweet part. No doubt they've improved on that in 30 years.


JR

PS: I also made a simple phantom powered DI for bass guitar (or whatever)  out of a pair of 2sK117, that sounded OK.
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Geoff Emerick de Fake

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Re: S/N in big consoles
« Reply #44 on: January 31, 2011, 01:58:33 pm »

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.
Quote:

 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.
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