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rborer 22nd August 2001 08:23 PM

I am working on a MOSFET power amp design that needs source degeneration resistors. These must be non-inductive.
My initial choice was to parallel 4 carbon-film, 1-Watt resistors. I took this idea from an Elektor design from the 80s. ("1KW Power Audio Amplifier", anyone tried that one?)

Since the PCB real estate is expensive, I switched to 5-Watts wirewounds, but non-inductive are hard to find.

I would like to hear your considerations about sources for non-inductive resistors. I know that Caddock has a line, but other options would be nice. I head about RGA's MPC71 but don't know where to buy them.

Hoffmeyer 22nd August 2001 08:34 PM

Hi there.
Why do you want non-inductive source-resistors?
Usualy the small inductance in the source/emitter-resistors help to 'protect' the outputstage against a capacitive load.

Eric 22nd August 2001 08:38 PM

Check out the Mills wirewound resistors from <a href="">Michael Percy</a>. They run about $3 each. They are non-inductive, 1%, 50ppm resistors.

rborer 22nd August 2001 09:11 PM


I included a damped inductor before the output, to protect against capacitive loads.
I just want to avoid dealing with unkown inductances. I had bad experiences in the past.


Thanks for the tip.

Hoffmeyer 23rd August 2001 06:27 AM


fair enough.

Samuel Jayaraj 23rd August 2001 09:58 AM


Non-inductive 5watt resistors in a ceramic pack can be sourced from RS Components (I think they are from Megitt, but can't quite remember).

Yes, I made the 1KW amplifier from Elektor. I faced some problems with the powersupply to the input section where the 7808 and 7908 regulators have been lifted to 80volts using zeners. They seemed to be a bit unstable. I have four of these boards assembled but not used.

Any ideas on alternate power supplies (plus-minus 80 and 72 volts) for the input section of the Elektor amp.

rborer 23rd August 2001 02:40 PM

Couldn`t find the mentioned parts at RS, except for thick films, but they don't go lower than 1 ohm... Will keep looking.

About the powersupply, I would go for a non-feedback discrete regulator, zener/resistor plus BJT at 80V and then step down with another zener/resistor to 72V.

My main concern with the Elektor design is the use of zeners to protect the small signal transistors from the high voltages. With 500uA of bias current, they should produce a lot on noise... Your experiences?

tvi 28th August 2001 04:17 PM

KOA make almost the same resistor as the Fukushima(RGA?) MPC71, it the KOA BPR58

<a href="">MPC71</a>
<a href=""> KOA BPR.PDF</a> <font size="-2">(261k)</font>

I have both as pulls from some PSUs and the MPC71s are Magnetic the BPRs aren't.

MCM have dual emitter resistor from noble 2,3 & 5 Watt
Part# 28-0500, 0505, 0510, 0520, 0525, 0530, 0540, 0545 0550, 0555
As to their constuction and magnetic properties MCM doesn't
have much info.

<center><font face="graphos">Regards

Samuel Jayaraj 1st September 2001 09:59 AM


When my Elektor 1KW amp worked briefly before I realised the instability with the power supply to the input stage, I did not notice the zeners producing noise. In other words, the sound quality was very good and there was no masking effect if the noise were present. Thanks for your tip on zener/resistor/bjt power supply. I had made a PCB for this quite sometime ago but did not use it or even test it. I will, one of these days.

I re-checked and found that RS does indeed have 5 watt non-inductive resistors in a ceramic package and they go from 0.22E upward. Please check the RS catalogue.

Wire-wound resistors with some inductance at the output stage may not be a good idea since it pushes some amplifiers into oscillation.

If the PCB area permits, use 1E 1W MFRs or CFRs in parallel to obtain the required value (change to 2W if higher wattage is required). This has an added advantage that the output current is distributed over a larger area as opposed to a single solder point, as is the case when a single resistor is used. Infact, this practice may be obligatory when very high currents are circulating, eg., the output stage of an amplifier meant for ribbon speakers. Another advantage is that if you can measure low resistance, you can obtain almost equal resistances for each of the output devices without having to spend on high tolerance high wattage resistors.

rborer 3rd September 2001 09:47 PM

Sam and tvi

I found the little guys at RS. Will have to go with them or the KOA's because my PCB is crowded, but thanks for the tip.
Will post again when I get some serious sound flowing out.


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