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craigmorris74

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Sep 9, 2005
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benton, ky
I am building a PEQ with the mid cut of the MEQ added in, similar to what was discussed in an older thread:
http://www.groupdiy.com/index.php?topic=2534&highlight=pultec+filter

My design is based on this layout from CJ.
http://www.angelfire.com/ky3/craigorris74/meq_peq.jpg
The output of the filter section hits an output transformer, which takes the impedence to 150 ohms. I use my mic pres for makeup gain.

I'm using Edcor input and output transformers. They're both center tapped. What do a do with the center-tap connections? :oops:

Thanks for your help, more questions to follow,

Craig
 
Ok my next question is help troubleshooting. It's passing signal but getting a funny frequency response, even if bypassed. When I pass a sine wave through, the signal increases almost exponentially from the low frequencies to the highs. Anybody ever had a similar problem when building an EQ?

Craig
 
Do you mean that it is passing exponentially more signal as frequency increases?

If you have plugged the schematic listed above directly into an output transformer before going into your mic pre, this is the problem. The circuit cannot drive the output transformer passively.

Why? I can't be as authoritative as some, but in general passive EQ circuits have "insertion loss" which is loss of gain for any signal passing through the circuit, regardless of the settings. Also, I don't know which transformer you're using for the output, but generally output transformers need to be driven by an amplifier stage as their input impedance is fairly low. Low impedance can drive high impedance (1:10 ratio here is favorable) Think 5534, 2520, 12AX7... some sort of amp. there are many, many examples on this forum.

In the meantime you could try bypassing the output transformer completely and take the output and ground wires of your eq and plug them directly into your mic pre's pins 2 and 3 respectively. That should allow you to hear something, perhaps sounding a bit more healthy. Start with your gain LOW. Expect some noise.

let us know how it goes!

Frank :guinness:
 
My transformer in this case was functioning essentially the same as the interstage transformer in a Pultec, but in my case taking the impededance down to 150 Ohms rather than whatever the interstage in the Pultec takes the impedence down to before the make-up gain amp.

I'll try it without the output transformer, then report back. Thanks for the help.
 
So, I looked back at the original schematic, and the transformer in question has a 10k input impedance - so as long as yours is also 10k input impedance, then my previous post could be irrelevant and the problem elsewhere...

do you have a scope?
 
You'll also need a function generator (for a square wave) because one way to move forward after having ruled out wiring error is to pass a square wave through the circuit (500hz or so) and probe different points along the signal path to observe where the problem is.

For some backround on what you might see:
http://www.tangible-technology.com/audiobasics/upgrades/upgrade1_opamp_cap.htm

Did you try bypassing the output transformer? The sound you described is similar to the sound of a balanced line missing one side, or one side of a secondary not grounded, etc.
 

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