The JFet/Tube Input Transfoless Mic Pre

[analag]

I have designed and built the FET/tube cascode input mic pre. After extensive listening to this device I wonder why designs like this isn't MUCH more popular. If speed, clarity, resolution and detailed sound is your thing, then this is it. And no it is every bit a quiet as a transformer input pre even some SS ones too.

Active devices:
(4) LSK170B
(1) 6922
(1) 5751
(2) 5687

Fully differential throughout, no feedback around the cascode input stage.
To PRR...it is astoundingly quiet.

Analag

 

[JustinS]

So.... do we get some photos & a look at the full circuit??

Justin

 

[analag]

Photos... yes.

 

[chrissugar]

Yes, show us some pics. :grin:

chrissugar

 

[featherpillow]

Is the LSK170 a good choice for HV FET applications like this? I'd suppose it is...how much voltage can these handle? I see from the data sheet that breakdown is roughly 40v. What did you use for plate voltage on this, Analag? I'm curious to see how you implemented that cascode (I'm assuming it was source to plate?)...

 

[tk@halmi]

Sounds like a great project. Can you post the final schematics?
Yes, it is interesting that you do not see this FET/Tube topology used more by cost driven gear manufactures. Expecially that it can help eliminate espensive transformers.
I am curious how gain control works on this thing.

Thank You,
Tamas

 

[roberflas]

Oh, yes, please schematics and photos.
Thank you

 

[analag]

This is a two stage preamp with gain reduction between the cascode and the output amp stage. The input can handle hot mic signals without clipping. Gain reduction is via a 12 step rotory switch. Plenty of gain and headroom.

1V p-p input and it takes as much as 4V p-p before it starts to distort significantly. This is the input stage (FET/tube cascode).

Simulators predicted frequency response

Pics coming soon.

Analag

 

[Learner]

[quote author="analag"]
Simulators predicted frequency response

Pics coming soon.

Analag[/quote]

man....... you know have to be a tease don't cvha... :green:

 

[analag]

I need a digital camera

not too clear huh

the transistors are behind the two caps on the left

and that's all

Analag

 

[chrissugar]

Looks nice.
You should buy a better camera. :wink:
It looks like you have some serious PSU for this preamp.

chrissugar

 

[tk@halmi]

Looks fantastic anyhow! I love Solen caps.

 

[PRR]

> I need a digital camera

Try more light. On my JamCam and BarbieCam, full sunlight was too much, but anything better will usually benefit from stronger light. Try between 10am and 2pm in the shade of a sunny day; direct sunlight is often harsh on equipment photos, diffuse north-sky (south-sky for Kev and chums) is nice light and usually ample.

Full sun: 5,000 foot-candles
North-Sky: 1,000 f-c
Living room: 5 f-c
100 Watt lamp at 4 feet: 50 f-c

 

[Svart]

PRR and a Barbiecam? :shock:

yes please, finished schemos! i am itching to build this thing..

 

[PRR]

 

[edanderson]

in response to tamas's comment, electro harmonix has just recently released a tube/fet hybrid mic preamp. completely transformerless 12ay7 input 12au7 output with fet "current balancing" and a discrete buffered aux out. runs off a 12VAC wall-wart, no doubt with another in reverse inside the box.

EH "12ay7 mic preamp"

ed

 

[tk@halmi]

Nuts. Is there anything Barbie does not have?

 

[analag]

What is "FET current balancing" it's just hype for the electronically ignorant. I build my own boxes to get away from all the hype and tricks/short cuts, prosumer manufacturers depend on to make a profit.

Analag

 

[adrianh]

analog;

2 things

Get those damn 6922 out of the circuit if they are Russian.
That is a real poor sounding tube.
NOS 7308 will be better. 6NP1 will sound best.

Put transformers in there if you want it to sound better.

 

[analag]

The tubes are JJ ECC88 they are also called 6922. I have much tubes and I interchange them also according to what sound I may need for a particular song, I may be working on. I spend my spare time developing circuits to get away from the use of transformers...the myth that tubes sound best with transformers don't impress me at all....
By the way my first job was as a transformer winder for 8 years. You name it I wound it.

Analag