Bias info with regard to Denon Amps - diyAudio
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Old 28th December 2004, 10:13 AM   #1
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Default Bias info with regard to Denon Amps

Hi there folks

I'm a newbie electroniciker type chap and looking at playing with my old denon amp (which I can't find any diagrams for).
I'm a mechanical engineer and fairly handy with my hands (no pun intended) and want to start looking at the biasing of the amp and replacing the preset pots etc.

I was reading some stuff regading biasing/checking/adjusting the bias settings on the output stages but after having a look inside my amp I'm getting a bit confused. Let me lay it down:

DENON 480r 50 W amp A/B design (I think). Not a particularly good amp in it's day, and hence a good start for my endevours.

Now looking at the output stage, we have (for each channel):

2SC4208a small transistors that are positioned in a hole drilled in the heatsink (not physically attached). The only preset pots in the whole amp seem to be attached to the collector of these transistors and the current thus measured is miniscule (microamps) with no input signal. Also seems to be a bit different for each side.

Then we have 1 each of 2SA1490 and 2SC3854 power transistors each side screwed onto the heatsink.

There is not any pot for DC offset control, guess it is done elctronically. (offset is negligable anyway, so I guess it is working as well).

So I guess my questions would be what to look for/do when adjusting the bias?
Should the current through the pots for each small transistor 2SC4208a be identical? Or has it been set this way to finetune another part of the circuit?

When adjusting the bias for the small transistors via the pots, am I only adjusting their outputs or does it cascade down the line to the Power transistors? At present the power transistors are both showing very identical idle voltages for each channel.

Any other advice or assistance much, much appreciato.

Cheers

Matt
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Old 28th December 2004, 12:25 PM   #2
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If you can give me the full model no & approx age I'll see if I can spring some service info for you when the distributer opens after new year.
If its was sold in kiwi land ist brobably an aust model.
cheers
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Old 28th December 2004, 01:39 PM   #3
anatech is offline anatech  Canada
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Hi Matt,
Actually, the 480r receiver was better than most mid range units. The small transistors are bias (heat sensing) transistors. The pots set the current through the output transistors. Do not replace the bias controls unless they are bad. The bias current can be measured by looking at the voltage across the emitter resistors. Do not run the amps above the suggested bias current. It will not buy you anything and the heatsink is only so big. Just check and correct the bias if you must and enjoy the unit. Be very careful not to slip with your probes.
The current is not equal through the controls except by chance and the DC offset is corrected by a servo circuit. Given that you measure idle currents that are close in each channel, the bias currents are most likely within spec. for that unit.
-Chris
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Old 28th December 2004, 05:25 PM   #4
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Thanks guys

Yes, the amp was bought in NZ around 1994 and I'll get more info off the back of it when I get back from an oil platform in Spain!
Apart from that the model is PMA-480R.

Cheers

Matt
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Old 6th January 2005, 10:30 AM   #5
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chased up on the service manual, a photo copy is available but it would cost.
If you are keen let me know & I'll tell you where from.
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Old 13th April 2005, 03:18 PM   #6
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Thanks for the reply Lowlevel - sorry for my long hiatus.
Yes, still keen - wherabouts?

Also I've found out that the L R channels (at speaker outputs) are slightly different in voltage levels when the amp is supplied with a constant frequency input. This could account for my little imaging problem. Tried to adjust the pots but it didn't make any difference to the levels.

Matt
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Old 13th April 2005, 03:27 PM   #7
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Did you read anatech's post? It sounds like you've just messed up your bias currents...
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Old 14th April 2005, 01:36 AM   #8
anatech is offline anatech  Canada
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With the amp off..... Gently turn the pots with a metal screwdriver. You may be able to feel the dents in the carbon track. Centre the pot in the dent and you will be close to the current before you turned the controls. Then, adjust following the manufacturer's proceedure.

Many an amp has come my way after being at the "twiddlers" house first. This trick works well but no alchohol, no excessive tea, chocholate or coffee. You need to feel the motion gently.

Hope this helps you out.

-Chris
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Old 14th April 2005, 03:24 AM   #9
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audio products australia are the distributers for denon.
I am not sure if they have an office in NZ.
check the phone book.
get back to me if you need the details of the aust office.
cheers
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Old 14th April 2005, 02:28 PM   #10
K-amps is offline K-amps  United States
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Quote:
Originally posted by anatech
With the amp off..... Gently turn the pots with a metal screwdriver. You may be able to feel the dents in the carbon track. Centre the pot in the dent and you will be close to the current before you turned the controls. Then, adjust following the manufacturer's proceedure.

Many an amp has come my way after being at the "twiddlers" house first. This trick works well but no alchohol, no excessive tea, chocholate or coffee. You need to feel the motion gently.

Hope this helps you out.

-Chris

Lamp in series with the amp also works nicely if you have no clue where the bias pot should be. For a class-AB amplifier, choose a bulb thats 1/2 rated the one channel of your amplifier. So for a 50wpc amplifier choose a 25 watt bulb.

First you need to turn down the bias off, i.e. the side where the bulb is not lit . Then begin turning up each channels bias such that turning it up will make the bulb go a very dull (barely visible) dull amber with both channels biased.

Needless to say, You also want to make a note of the bias pots for reference before you twiddle...

So you will need to nudge each channel's bias pot a little till you get this level.

This is not the optimal level but will get you close so that when you turn on your amp, you do not blow the OP devices. As always choose the manufacurer's specs on what bias level is optimal.

PS: I do not agree with Chris's assertion that cranking up the bias buys you nothing... but thats me. Chris is trying to make sure you have a reliable amplifier. You can "usually" crank up the bias till the heatsinks idle at no more than 43C-45C or 25C above ambient. With the amp playing at normal levels, the heatsinks should not go over 55C or 60C in short bursts.
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