API 553 Gain Trim Value

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Fablab

Well-known member
Joined
Nov 12, 2008
Messages
347
Location
Italy
Hello, I am building a clone of API 553 Equalizer
In the schematic there is the possibility to put a trimmer for the gain control, but there is no indication about the value.
As I'd like to have a little more gain than the standard value, someone knows what value must have this trimmer?
Thank you in advance
 
I've looked at all my data sheets and schematics, and nothing is mentioned .
Looking at the circuit, if you short Pin 6 to Pin 7 then R13 an R14 will act as a pad between the hot a cold resistors R1 and R2, giving a balanced U-pad.
Using a potentiometer will give you varying attenuation. (But don't ask me numbers! ;D)
 
Hi Kevin,
in my circuit the gain trim  is placed as you say between pin 6 and pin 7 but I don't know the right value  :)


 
Hi Fab

In the 5003 version of this EQ, I left it out, seemed to work OK. Search here for 5003 for more info.

I have 500 series PCBs for this, let me know if you need any.

Peter
 
Hi Peter,
I have bought the pcb some time ago ... now I am searching for the inductors
Is it possible you have two pair for sale?
Fab
 
At first I did not understand how this EQ worked; I am always puzzled by circuits that have components connected between the + and - inputs of an op amp - I just can't see what is going on.  I have spent some time studying it and now I think I do understand it and it is surprisingly simple. With the pot wiper at the + input end, the input signal is simply attenuated by the pot divider formed by the 27K input resistor and whatever network is connected between the wiper and 0V. With the pot wiper at the - input end, the gain is increased by the parallel combination of the 27K from the - input to ground and the network connected between the wiper and 0V. Because the input resistor is the same value as the one from the - input to ground, the boost and cut are symmetrical. When the pot is at the mid point you get a bit of cut and an equal bit of boost producing an overall flat response.

Once I understood what was going on I realised you could do this with any op amp, including the EZTubeMixer line amp. So I did some simulations and sure enough it works. This means I could do a tube version of the 553 EQ........

Since the EZTubeMixer line amp needs a 2:1 output transformer it needs 6dB gain when everything is flat, and the API circuit has a gain of -8.6dB when I simulated it. I soon realised that you need to change the gain in two parts. First increase the gain in the op amp and second reduce the attenuation in the input. This is necessary to ensure the mid point boost and cut remain the same so the overall response remains flat with the pots centred. So the bottom line is that you need to change both the 27K resistors to trim the gain and they need to be changed by the same amount.

I simulated the trim control across pins 6 and 7 and all this does is reduce the gain still further and only by a couple of dB.

Cheers

Ian
 
I ran another simulation. If you change both 27K resistors to 10K then the unit has unity gain.

Edit:

To be clear, with 27K resisotrs the gain is -8.6dB. With 10K resistors the gain is 0dB

Cheers

Ian
 
ruffrecords said:
Edit:

To be clear, with 27K resisotrs the gain is -8.6dB. With 10K resistors the gain is 0dB

Cheers

Ian

How odd, both my data sheets say the unit should have 0 insertion loss. I'll try and pull one out of the desk and check for circuit differences.
 
Walrus said:
How odd, both my data sheets say the unit should have 0 insertion loss. I'll try and pull one out of the desk and check for circuit differences.

The figures I gave are for the op amp circuit alone so you need to add whatever gain/loss there is in the output transformer.

Cheers

Ian
 
hi Ian, sorry i’m a bit new to analyzing these wonderful circuits. do you know what the symbol is in the middle of the schematic?
 

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