Pief

Hi Guys,

I.m having some difficultys.  I.ve been building a dual channel mic preamp quite some time now.   Had some hum problems,  plain mistakes you name it.
It,s working now but i,m having problems balancing the signal.  because there is still some noise present and i,m searching for ways to turn the noise down. as you can tell the schematic is not finished but it shows what i,m doing.

As soon as i ground the Center-tap of the O.t. on the primairy side,  the signal get crackely and drops the gain significantly.
If i ground the centertap on the secondairy side of the Ot the residual noise drops to acceptable levels, no crackles, but with a loss in some high end frequentys.  Still the signal seems o.k.  but less bright,

Is the latter situation normal? What is commen practice? 
i guess i never really understood.... 

Thanks in advance,

Pief

P.s.: on theoutput xlr  i.ve led the ground terminal to the chassis right next to it. Maybe the ground on the xlr is suppose to go to the ct on the secondairy?
« Last Edit: May 21, 2018, 09:16:50 AM by Pief »


Pief

Re: Olmsted BRDCSTR build, Help me understand use of Ct @ OT
« Reply #1 on: May 26, 2018, 05:52:13 PM »
No one here with some insight in this?  or is my question stupid in some way?

Anyways: I did some testing on my good interface instead of the cheapo on the workbench.
This thing is hot. Must be boasting around 75 dbs of gain.  I can hear my neighbours fart on the sofa. So i guess at that level it,s normal for some hiss to come in to play, but the residual hum is not acceptable. 

furthermore tried and tested to add some negative feedback to the Cathode of the ef86. Variable between 10 - 60 K. 
Residual hum goes away upon engaging negative feedback, but comes back gradually within about 5 seconds.

Starting to think the amplification factor of EF86 i.c.w, stepup is just to high already.

Help !


DaveP

Re: Olmsted BRDCSTR build, Help me understand use of Ct @ OT
« Reply #2 on: May 29, 2018, 04:00:44 PM »
OK,

First off, there is way too much gain for most practical applications.  You need to decide what type of mic is going to be used with this?  If it's a condenser, then 40dB will do, or 50-60dB for a dynamic.  Only a low output ribbon at a distance from the source would ever need 75dB.

To start with, I would change the EF86 grid resistor for something 10x lower, like 51k or even 22k.  The noise of the tube is equivalent to about 25k, so the 500k resistor is contributing 20 times more noise than the EF86!

You don't need to ground the CT's of the OPT.  If you ground the CT of the primary you screw the ratio and make the load too heavy.  The CT of the secondary should not be grounded and be left at 600 ohms.

You really don't need the gain stage of the  ECC81 either, it's great as a cathode follower, but non linear as a gain stage.
Lets look at the gain situation in more detail:

Your IPT is giving you about 23dB,  your OPT is losing 12dB so your net gain is 11dB plus the gain of the amp.  The EF86 will give you around 45dB with the values chosen, so that makes 56dB as is. Perfect!

You should not follow a pentode with a triode normally, as the Miller capacitance of the triode will lose your top end.  A bootstrapped CF has a very high input impedance so the coupling cap only needs to be around 22nF, not 100nF.

So re-wire the ECC81 doubled up as a cathode follower (CF) or use one half for each channel if stereo.
Ditch the gain control as it will still contribute some noise and use the bootstrap CF as described here:-
http://www.valvewizard.co.uk/accf.html

Read everything this guy has to say to fill in the gaps in your knowledge.

The gain of the CF will be around 0.9 something so it will leave you with a fine 55dB mic pre.

I have found from experience that the input tube will give less hum if you provide a DC heater supply.

I hope there is enough advice there to sort out your problems.

DaveP
Soundcloud: Delayed Action.

Pief

Re: Olmsted BRDCSTR build, Help me understand use of Ct @ OT
« Reply #3 on: May 29, 2018, 05:50:25 PM »
Thank you very much Dave for taking the time to write this up.

It is certainly more advice than i hoped for.

Let me get started right away!

Grtz.

Pief


 

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