Tuesday, July 28, 2015

Microphone preamps transistor and vintage tube style

One of over 30 stereo mike preamps I made for Tape-Op customers 
 Preamps have Tamuri transformers, phantom, pad, and Di in jacks
60Db gain max, over 22DB levels into 600 ohms

The PM1000 board is center, one gain stage per side, In and out transformers are back right and left sides
There is a diode coming from the DC in jack, (middle back), to prevent reverse polarity
I used camera tape to “stick” all the wires down, as Cam tape not gooey

The level control, (upper right front) a 600 ohm pot, is feeding the 600/600 ohm line out transformer through a two watt, 10 ohm, carbon resistor. The slowness of the carbon resistor adds some warmth. 

The gain stages have a push-pull output stage, that can drive approx 22 volts into 67 ohms RMS, so the 300 ohm load not a problem. The shiny TO-5 transistors are the new ECG NPN/PNP outputs. I used good audio quality Xicon “Reds”, aluminum electroylitics in the signal path. They were the best a few years ago, now the ELNA Silmic and Cerafine caps are way above any of these for audio quality

This is a old Altec transister PA amp being turned into a Mike Preamp and power supply for five other  mike preamps. I had the engraved labels made to get a 'Pultec look' for the panel.

               Here is the new inside, the PM1000 board with two gain stages mounted to the front. 
    The yellow inter-stage transformer is in the center front, the in transformer is at upper left on the back of the front panel. This transformer adds 'smooth' to the sound and more gain without noise.
               Power supply, feeding to the barrier strip, is upper middle, on the back, and right. 
     The power supply in the Altec provided +44 for the preamps, and +48 for the phantom power

  Here you can see the VU at "O" VU which is set for "O" VU = +4 level out
I test every preamp at +12DB levels out to see that the line out stage 
has headroom over +12db into 600 ohms. 
Here are some pics of some basic steps to 'racking' two Yamaha PM1000 
or any mixing console 'channel strips'
Step one is getting everything lined up and where doe it go? questions answered
Making sure the front and back panels are perfect most important
If a hole is just a bit to far off on the front, it can not be fixed

A trick way to mark the holes on the front side

One channel ready to be mounted
The one pot will be replaced with a EQ - In/Out switch

Punching holes with a chassis punch once pilot holes are drilled

Two channels mounted with fresh screws from the back
 Now that all the holes have been drilled or punched, all the parts can be mounted 
with no worries as to proper placement

I built a whole series of thee 19" by 3-1/2" rack mount
chassis stereo preamps with the talkback yam boards. These boards
have two 'Gain Stages' on each board, so you just need a input transformer
 gain, level, phantom power controls, DI input and a output transformer

This is one of a series of 19" by 1-3/4" single - space rack mount stereo preamps
I built with a new board I designed that has a Modded PM1000 'Gain Stage'.

This board has the basic setup with, Left to Right
Mike balanced in, Pad-off, -12, -20db, DI-input, Level, Phantom power on/off
Gain, and a sweet vintage SONY 600 ohm line out transformer

 The board has a +48 volt power supply filter, phantom power feed, 
Balanced input stage on the top left, phantom and pad on the lower right
and the 'Line Gain Stage' on the lower left.

The four transistor 'Gain Stage' has been modded by; 
a. changing the two output transistors to lower gain/higher power versions
that increases the big warm tight sound, raises the maximum output level, 
and lowers the noise. b. using metal film resistors for lower noise, c. the board 
has pads for radial electrolytic caps or metallized polyester caps.
The metallized polyester caps have a clearer, 'Tube Amp" sound 

A board under test, the VU is set for "O" VU  +4 level out
This new board can drive down to 56 ohms and can drive a 
step-up line out transformer for +28DB out levels, and tons
of NEVE sounding headroom.

The two round 'metal - can' transistors are NTE great big and warm
sounding transistors. They are idling at almost one-half watt each.
This gives the 'Gain Stage' a big tight bass and a strong clean sound.
The 'Gain Stage' is also noiseless.

This is the true - balanced in stage that is super quiet
It has a unbalanced out to the level control, or line in
selector switch, and about 20DB gain.

The phantom power, two stage pad, and click filter is shown
here on the lower left corner. The balanced mike in goes through
the pad/phantom circuit, and then to a mike in transformer, or the 
transformerless balanced in preamp

This stereo preamp has a switch, Pr/Mix on the lower right, 
that selects the mike/Di inputs, or two RCA jacks on the back that
allow you to hear your mix on your headphones. 
The  'Gain Stages' make great headphone amps for higher 
impedance headphones

There is a +4 red LED for each channel


The preamp below is one of a series of "Classic 3-stage tube mike preamps
I built using all vintage parts including vintage tubes.

  This classic circuit is given more headroom by using two #76 triodes 
in parallel to drive the 15k/600 ohm Altec 'Red' line out transformer
This gave these preamps a comfortable +15DB out without clipping

One of a pair of custom tube mike preamps built for a customer
Unit has Vintage or NOS preamp tubes, wide range gain control
Russian paper-oil caps, all vintage parts including the knobs

These preamps (above/below) have a vintage 39/44 tube or a rare EF40 tube
 which is equal to a EF86, as the pre stage
 You can switch tubes without clicks while the preamp is active

One 6sn7 as a Class-A driver is coupled to a large Philips line out transformer 
on the right side of the chassis. The gray box is the line out transformer
The big 1930's 39/44 tube is by the Altec 'green' input transformer on the left
This preamp has all paper-oil caps, and three gain controls that give a 
great range of gain and overall sound of the preamp