LED resistor problem

Moderators: George Geddes, David Martin, dave robinson, Iain Purdon, ecca

LED resistor problem

Postby davejriley » Wed Feb 19, 2014 11:52 am

I bought the kit about a year ago but am only now building it. I am confused re 'three pairs of resistors' to test as LED resistors. Apart from two apparently spare 10k res. I also have two each of 1k2, 12k 22k and 33k. Which are intended as LED resistors and any idea why I have the other two ( + the 10k spares ) - have I missed something somewhere ?



Re: LED resistor problem

Postby ecca » Wed Feb 19, 2014 12:25 pm

1K2 resistor for LEDs, the others are various values to bias the FETs.
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Re: LED resistor problem

Postby davejriley » Wed Feb 26, 2014 2:57 pm

I'm more confused now. I can see nothing in the instructions to guide which resistors are needed to bias the FETs and nothing about how to bias them whereas the instructions indicate that there are three pairs of resistors in the kit , though not which resistors, needed to determine the brightness of the LEDs . The instructions also state that the FET resistors are both 6k8 ?!


Re: LED resistor problem

Postby scmitche » Sun Mar 02, 2014 9:54 am

Hi Dave,
Sorry I've not been on the site for a few days and therefore have only just seen this.

Since you bought the kit a year ago we have produced extra building instructions in the form of Workshop Bulletins, which can be found on Piet's Blog. I assume you are building the standard, manually operated, unit not the Arduino Automated one. Of particular relevance to you are Bulletins 1,2 and 5. If you have already fitted resistors R22 and R26 (100k) in the FET circuit please remove them as they cause an uneccessary DC offset into the following operational amplifiers (Bulletin 1). Download Bulletins 1, 2 and 5, which will guide you through the necessary steps to optimise your eTap2hw particularly in regard to FET biasing and reduction of noise for best sound and signal headroom.

Regarding the FET bias resistors, do not use the 6K8/2k2 combination of drain/source resistors before reading bulletins 2 & 5. If you run into problems I can help providing you were to supply me with 3 voltages and the existing bias resistor values as I have a program written to calculate resistor combinations to get you very close to optimum bias.

Look here for the Bulletins:

Any problems you have please PM me and then we could perhaps communicate by email to get you going.

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