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2Bak 23rd September 2002 06:45 PM

Zen 4 resistors
From Nelsons partlist:
R0, R14, R15: 0,47R metaloxid, 3 watt.
R1: 1R metaloxid, 3 watt.
R10: 100R metaloxid, 3 watt.
R20: 22R metaloxid, 3 watt.

I was planning to use:
R0, R14, R15: 1R metalfilm, 3 watt, in parallel.
R1: 1R metalfilm, 3 watt.
R10: 100R metalfilm, 3 watt.
R20: 27R metalfilm, 3 watt.

What will happen if R0, R14, R15 are 0,5R and R20 is 27R?
A couple of watts output decrease?


Nelson Pass 23rd September 2002 09:20 PM

R14 and 15 are already .5 ohm (well .47 actually),
so this is just fine.

If R0 is .5 ohm, then the bias is about 1.3 amps, which
is enough for 30 watts into 8 ohms. Adding 1 ohm parallel
to that (R1) gives .33 ohms, which with a circuit that sets the
voltage across these at .66 volts, makes 2 amps bias.

This runs the amp hotter, but also allows 30 watts into 4 ohms.


2Bak 23rd September 2002 10:38 PM

Thanks Nelson, I'll stick to R0 0,5R and leave out R1 for my Zen. My heatsinks are on the small side already.
30 watt/8 ohm will be a fine amp for my speakers, no doubt. :yummy:
Should R16 be 1K if R1 is left out?

Could I put a few additional parts questions here:

1. The CL-60 thermistor would it prevent my 10A fuses in my old apartment to blow at turn-on? I use one 800VA, 20A, 36volt trafo for both sides. Or does this part perform some kind of thermal protection and closes down the amp, if it gets to hot?
I was thinking to place a softstart relay circuit before the trafo, but if the thermistor does the job, then I don't need it, do I...?

2. I use a 20K trim pot instead of the recommended 25K. Would that be fine?

3. What would be the best distance from the pcb to the heatsinks, i.e. how much room does the FETS need?
8 mm OK?


jh6you 24th September 2002 12:29 AM

[list=1][*][*]The 20k pot is OK. I could replace it even with a set of 10k pot + 5k resistor in series, which provides us with better possibility for precise adjustment.[*]If your PCB likes summer season, then as close as possible. If likes winter season, then as far as possible. If doesn’t mind the season, the distance doesn’t matter.[/list=1]

AudioFreak 24th September 2002 01:20 AM

Basically keep the FETs as close to the PCB as possible so that you can keep all the leads short and hence minimize their effect. As far as heat goes, as long as the PCB doesnt touch the FET's casing, thermal effects wont be too bad, just keep the PCB about an inch away from the cases and heatsink.

miguel2 24th September 2002 09:04 AM


Is there any advantadge of increasing the voltage on the regulated PSU instead of increasing current? I got a 750VA 45-0-45 transformer which will give about 54V unregulated. Should I use about 45V regulated or is it better to just burn it on the pass transistor?:)


Nelson Pass 24th September 2002 11:09 PM

I would bump up the regulated supply a few volts. It
will sound slightly better, as the Mosfets do like some


miguel2 25th September 2002 08:31 AM

Thanks Nelson. :)

I will use an additional zener to raise it some 4-5 volts, all other things remain equal.

I got this transformer used by 10 Euros, including two 35A / 400V bridges, 8 x 1000microF/250V caps and a lot of TIP3055's which I don't know what to do with.

Yesterday I was testing the transformer and with anything connected it powered up OK, but when I used the test leads of the multimeter on it (in voltage mode) the switchboard went down several times! Any ideas why?

I just guess that the 8000 microF will not be enough but I am going to experiment with those before I can get my hands on some more caps... I will just have some more ripple - Microcap simulation gave around 10V on unregulated and 0.01V on regulated.


2Bak 13th October 2002 01:52 PM

Power down
I will use my Zen 4 (not build yet) also for cinema and background music listening.

I've started to worry a bit about my electricity bill :2c: and was thinking of making some sort of power down switch to lower BIAS.

My idea was the following. In both positions I leave out R1 cause my sinks are already on the small side.

Normal power:
0.5R at R0

Low power:
1R at R0 (or more?)

It's OK for me that output power drops from say 25 watt to 10 watts when I watch a movie.

Do I get what I want if I make such a resistor switch?
Did anyone try such a power down facility?

AudioFreak 15th October 2002 09:01 AM

Probably the easiest and safest way to achieve that would be to use 2 parallel resistors for full bias and then switch 1 of them out of the circuit when you want the low bias setting.

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