Lukpio

How to turn a Pultec into oscillator.
« on: May 13, 2005, 10:03:10 AM »
Hello,

I'd like to share a simple, passive circuit that turns a Pultec-type equalizer into oscillator. The purpose of it is to measure the peak frequency of the high frequency boost.
When we connect the output with the input (with proper phase) through simple attenuator, the positive feedback occurs resulting in oscillation at the peak frequency which can be measured with a frequency counter.



The 2200 resistor isolates the input of the frequency counter from the output of the equalizer. Diodes (silicon) limit the input voltage, so that the equalizer works in its optimum range.

If the circuit doesn't excite itself check the phase or adjust the value of the resistor that is paralleled with diodes.

The output wave is almost perfect sine wave at several volts.
To excite oscillations the knobs have to be set adequately. All pots should be set fully counter-clockwise, except the peak boost knob, which has to be set at full. The bandwidth of course should be set at the sharpest setting (high Q).

This is the perfect tool for pre-aging inductors and the make-up amplifier. Changes of the resonant frequencies can be observed at the beggining, but after a few days they settle down.
Lukasz Piotrowski
www.amtecaudio.com


NewYorkDave

How to turn a Pultec into oscillator.
« Reply #1 on: May 13, 2005, 10:40:57 AM »
Welcome to the board, and thanks for sharing your circuit. The concept is brilliant in its simplicity: use the device as its own test instrument! Most of us probably would have reached for an external oscillator without pausing long enough to consider that the Pultec can do that job itself. Well done.

matthias

How to turn a Pultec into oscillator.
« Reply #2 on: May 13, 2005, 10:51:08 AM »
Quote
This is the perfect tool for pre-aging inductors and the make-up amplifier. Changes of the resonant frequencies can be observed at the beggining, but after a few days they settle down.



that's very interesting...  
I didn't know that inductors change its res freq over some time...

but what's happening there on the technical side...???
does the inductor change its mh value, or dc resistance??

what kind of inductor do you use in your pultec ??

thanks,
mat

Re: How to turn a Pultec into oscillator.
« Reply #3 on: May 13, 2005, 01:40:37 PM »
Active methods of testing some circuits
can be elegant, simple, and sometimes only usable method.
There is method to test amplifier distortion
with wien type  network and bulb for stabilization.
If measured amp have distortion orders of lower than
any low distortion oscillator, this is only method  how to measure amp distortion.
The only what you need is bridged-T first harmonic reject at the output
and any recording equipment and computer...

                          xvlk

matthias

How to turn a Pultec into oscillator.
« Reply #4 on: May 15, 2005, 05:34:09 AM »
@ Lukasz:  

your pultec looks so damn nice !!   :thumb:     http://www.amtecaudio.com/peq-1a.html

from where do you get that nice knobs for boost and cut???

thanks,
mat

Lukpio

How to turn a Pultec into oscillator.
« Reply #5 on: May 15, 2005, 11:43:46 AM »
Hello,

The circuit works not only with Pultec-type EQs, but also with all equalizers which have enough boost (cut doesn't work) and sufficiently narrow (sharp) bandwidth (i.e. high Q).

Ferrite inductors change their inductance over time, because of changes in permeability of the core. The effect is the most noticeable at the beginning. The similiar effect (but much less apparent) occurs in capacitors and I'm not speaking of electrolytic caps, but the ones with plastic dielectric. This causes frequency drift in the LC circuits.

In the PEQ-1A we use ferrite pot core and OKW knobs.
Lukasz Piotrowski
www.amtecaudio.com

CJ

How to turn a Pultec into oscillator.
« Reply #6 on: May 16, 2005, 02:22:00 PM »
Cool idea.
It would be interesting to compare a simulated answer with this method.
This method takes everything into account, the response of the input iron, the filter, and the output amp and iron.

Note that the inductor might drift back to it's original state if your eq sits for many months, so maybe wake it up for a few hours before you use it.
If I can't fix it, I can fix it so nobody else can!
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