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Old 15th August 2008, 04:43 PM   #11
kevinkr is offline kevinkr  United States
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I would be a bit concerned about pentode partition noise particularly with very low output microphones where a lot of gain is required. Almost all vintage commercial microphone amplifiers I have seen use triodes for this reason.
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Old 15th August 2008, 05:39 PM   #12
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This design is almost 40 years old and I think 1000's of amplifiers were made of this and sold.
Still working on different places.
Look for EL6435, EL6415, EL6425 on the internet.
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Old 19th November 2008, 03:31 PM   #13
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ok back to this thread

i saw the pultec microphone preamp it looks like tha bomb.
as for schematics they also look great but for me reading schematic
is like explaining to a newborn why the DOW jones is at it's all time low

i gave up on a full kit cause there is no site that actually will sell you one without you taking a mortgage.

i saw a thread here by neoific

First DIY job: Dual Tube Recording Preamp

which is really helpful, he also talked about making his own wiki
about the subject....
but i get 404 when i press on his links.

i need an advice where to begin on a simple tube microphone preamp that will start my trip in tube wonderland.
links on where i can get specific parts like blank PCB's, transformers,chasis and tubes would be great!! hey i would buy you a beer for it if it was possible online
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Old 19th November 2008, 05:43 PM   #14
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Quote:
Originally posted by Thebrain


ok back to this thread

i saw the pultec microphone preamp it looks like tha bomb.
as for schematics they also look great but for me reading schematic
is like explaining to a newborn why the DOW jones is at it's all time low

i gave up on a full kit cause there is no site that actually will sell you one without you taking a mortgage.

i saw a thread here by neoific

First DIY job: Dual Tube Recording Preamp

which is really helpful, he also talked about making his own wiki
about the subject....
but i get 404 when i press on his links.

i need an advice where to begin on a simple tube microphone preamp that will start my trip in tube wonderland.
links on where i can get specific parts like blank PCB's, transformers,chasis and tubes would be great!! hey i would buy you a beer for it if it was possible online

Hi Thebrain,

I just saw this, and I looked at the discussion in this thread and the
one you reference, and my first reaction is like wow, what a spread
of opinion. Also there are a lot of funky designs for micpres out
there. Not that they won't work but there are some well known
circuits that have developed over the years for various applications.

Various because while there are general purpose micpres that will
work with a wide range of mics, that is a big challenge to do well.
It would be good to decide what kind of mics you will be using mostly
because the best micpre for ribbon mics will be different from the
best micpre for condenser mics and vice versa.

Are you going to use this micpre in a recording studio, interfaced
to balanced +4dbu line inputs? What types of mics do you need
this for? (dynamic, ribbon, tube condenser, electrets?) the input
impedance and gain required varies greatly.

If you want to build a good micpre for recording studio use that
will work well with a range of mics, I haven't seen anything nearly
as good as the G9:

http://www.gyraf.dk/gy_pd/g9/g9pd.htm

This is a complete design, well thought out, and very popular. You
can probably get in on a group buy of PCBs or even start one.

If this is too much, then you will probably need to clarify what it is
you want to do i.e. mic types, gain, impedance, etc. , look for a
simpler, more specialized design, and be prepared to do more
research.

There is some DIY activity around the Pultec preamp you might
tap into, I can't find any links right now but I ran across it a few
weeks ago. It might meet your needs just fine.

A few tips to help filter the good stuff out:

Micpres for professional use are nothing about high impedance.
Zout of microphones is from a few ohms to about 200 ohms for the
most part. Typical input impedance of a micpre is 200-1600 ohms
(vintage gear) and higher for some modern and pro-sumer gear.

Gain needed is about 40db for condenser mics with about 1600
ohms Zin. How the gain stages respond to overload is very
important for condenser mics because some can put out over 10V
peak-peak when driven by a bass drum or even a screamer up close.

You need more gain, up to 80db,
for dynamic mics and ribbon mics. Luckily, ribboon mics have very
low Zout so it's a little easier to build a low noise high gain preamp,
similar to a MC phono preamp in a lot of ways.

So one size can be made to fit all, but not optimally without going
to some expense like variable input impedance and switchable
gain stages, feedback control like the G9 does, etc.

Studying some classic micpre schematics like the Pultec, UA,
Telefunken V72, V76, RCA BA-2, and others will reveal a lot more
than looking at modern schematics. Modern designs are afflicted
with severe component-itis (with notable exceptions by Hafler, Fearn,
and some others).

Probably doesn't qualify me for the beer, but I'll toast to your project
this evening anyway!

Cheers!

Michael
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Old 19th November 2008, 10:59 PM   #15
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thanks for the for the detailed reply!

you surely deserve a pint

i am a sound engineer (well i was... there is no money in that so i've gone to the hi tec industry ....)

my main use for this preamp will be home recording.
will be using dynamic and condenser mics.

i had my taste recording with professional preamps such as avalon,neve,TLA and telefunken.
i'm looking for something simple, more for the fun of it and especially
for learning.

i did a line tube preamp kit and had very much fun doing it.
problem is i don't have really something useful to do with it

so you catch my drift... i wanna do something fun,useful and pretty simple...


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Old 19th April 2009, 03:53 PM   #16
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Location: Holt, Norfolk
Quote:
Originally posted by mississippi
Here a schematic diagram of a mic_amp with the EF86.
That's an interesting circuit but it looks like only part of a much bigger one. Do you have the rest of the circuit?

Cheers

ian
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Old 28th February 2012, 05:04 PM   #17
migs is offline migs  United Kingdom
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Default Late reply

Hey Ruffrecords, I know this is a bit late as a reply (only 3 years), but better later than never. The pre-amp is part of the EL6425, valve mixette/amp produced by Philips in the 50/60's. I have the EL 6415 at home, which is almost exactly the same, but less output power on the amp side. I think they were made to work with 50 Ohms type of mics at the time, at least the one I have with me. I'm trying to work on this at the moment, some help/guidance would be appreciated.
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