Mic Pre

[analag]

I hacked the REDD pre and came up with something so nice, I am tempted to share it with my loved ones. Featuring octals, B- / B+ power. It is a marriage between the detail and speed of solid state and the hugeness and depth of tubes. And quiet!

 

[pucho812]

This exceeds the limits of my medication

 

[Phrazemaster]

This exceeds the limits of my medication

Omg what would we do without you Pucho!!! Freaking hilarious!

Analag - sounds cool! Do tell...

 

[shabtek]

we are in the right sub-fora

 

[Ricardus]

TAKE MY MONEY!!

 

[ruffrecords]

Speed is not all it is cracked up to be.

Cheers

Ian

 

[Tubetec]

Ive always wanted to take a closer look at +/- Ht rails ,
Ive seen it done on some designs ,but I dont fully understand it .
Give us a look

 

[ruffrecords]

Ive always wanted to take a closer look at +/- Ht rails ,
Ive seen it done on some designs ,but I dont fully understand it .
Give us a look

It opens up the possibility of directly  coupled stages and, more importantly, NFB down to dc. The latter allows gain to be varied over a wide range without compromising stability, something which is very hard to do with regular tube circuits because of the very large differences in the dc voltages of outputs and inputs.

Cheers

Ian

 

[iomegaman]

I've been wanting to insert my 2 channel NYDEQ into my two channel Orange 86 but haven't found the courage or the time...I keep buying crap that consumes my time and money...

This Amerimex console and the Roberts 770 make me feel like I've gotten trapped in a time loop back to the early 80's...

(To be fair I love them both the Amerimex console is a dream)

 

[abbey road d enfer]

Ive always wanted to take a closer look at +/- Ht rails ,
Ive seen it done on some designs ,but I dont fully understand it .

As Ian hinted at, multiple rails are useful when DC response is needed, that's why they were mandatory in oscilloscopes. However, one must always remember that voltages are potential differences, so the nuance between +150/300v and +/-150V is just where you put the reference.

 

[analag]

In this case it's B+ 250/ B- 40

 

[abbey road d enfer]

Ideally, the rails should be about symmetrical and the DC input and output voltages should be close to zero, but since zero-volt is an abstract concept, one could consider shifting all voltages by about half the single rail. there would be issues with lifting the heaters, though. And the absence of a complementary tube (what a PNP is to an NPN) doesn't make things easy, but it's doable, how instrumentation and high-power amps have shown.

 

[ruffrecords]

Here is a screenshot of the schematic of a dc couple tube mic pre I designed a while back. If you simulate this you will find the input grid is at exactly 100V and the plate of the output tube is at 98V. So you could conceivably replace the 200V PSU with plus/minus 100V ones and connect the input grid straight to 0V via the 470K resistor.

Cheers

ian

Edit: Oops forgot the pic!

 

[Tubetec]

Cascode Af Amplifier , L.B. Hedge came to mind ,

http://douglas-self.com/ampins/wwarchive/Cascode%20amplifier%20Jun56%20p1.jpg

So in this case the negative rail allows the grids to remain at ground potential and also allows a high value resistor on the long tailed pair without requiring a crazy HT voltage .

 

[clintrubber]

Ideally, the rails should be about symmetrical and the DC input and output voltages should be close to zero, but since zero-volt is an abstract concept, one could consider shifting all voltages by about half the single rail.
...
, how instrumentation and high-power amps have shown.

FWIW, the Tektronix Type 122 comes to mind: +135V / -90V supply  (so for instance employing (5) 45V battery units)

http://w140.com/tekwiki/images/f/f7/Tek_122_schem.png

 

[abbey road d enfer]

FWIW, the Tektronix Type 122 comes to mind: +135V / -90V supply  (so for instance employing (5) 45V battery units)

They didn't that for DC performance, though. I would think the motivation was getting the most Common Mode impedance.

 

[clintrubber]

They didn't that for DC performance, though. I would think the motivation was getting the most Common Mode impedance.

In- & outputs are DC-coupled, but internally 2 coupling caps in the signal path, so this is indeed not about DC-amplification.

The -90V indeed convenient for resistor headroom for the LTPs, but for the last stage (CF) one could argue they could have AC-coupled that as well and skip the DC-level control.

OTOH, could also imagine there's a use-case where it'd be convenient to DC-shift the AC-signal.

 

[clintrubber]

Something else, we're now around 18 posts in this thread... about time TS spills the beans and posts a schematic, right ?  ;D

 

[Tubetec]

The Tek 122 looks like its not far off making a half decent mic amp ,
there not too expensive , and build is nasa grade  ,

heres one on ebay with good photos  ,

https://www.ebay.com/itm/Tektronix-Low-Lever-Preamplifier-Type-RM122-Preamp-RM-122-for-512-Oscilloscope/323649764349?epid=4028076788&hash=item4b5b077bfd:g:gikAAOSw851cPSUU

schematic here

http://w140.com/tekwiki/images/f/f7/Tek_122_schem.png

Output stage of HP200cd oscillator has some interesting stuff going on with +/-Ht rails and feedback .

I'm also slightly intrigued by early tube op amps , Philbrick , again the +/- rails are used

 

[clintrubber]

The Tek 122 looks like its not far off making a half decent mic amp ,
there not too expensive , and build is nasa grade

Having scored a pair of these Type 122 units on a flea market, was in fact the reason I went searching for info and the very reason I found (the precursor of) this great forum long ago    :-*

(the Type 122 is the non-rackmount version of the RM 122 of that eBay auction)

I recall we've been discussing the possible modification of this circuit back then by adding for instance overall feedback (with adaptations to the present signal path, since the BW-switching inside the loop is then obviously undesirable).

Also recall another forum member actually employed these circuits as mic-preamps, IIRC for ribbon mics. Don't recall which mods he did, possibly using an input-TX.

Note there's a low-noise mod suggested here, which may come in handy for mic-preamp use:

http://w140.com/brophy_122_mod.pdf

The Type 125 is the supply unit, which I unfortunately didn't find back then.