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Bias adjustment C-audio RA501

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mmuetst View Drop Down
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    Posted: 01 July 2020 at 7:43pm
Hello Forum,

I'am looking for a friend information so I can adjust his amp: C-audio RA501
I can only find other types on hifiengine (RA1001).
The amp is not broken, I only want to check the bias settings.
That is why I look for a service manual or someone who can give me the information about setting the bias.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 01 July 2020 at 7:58pm
For the older RA-500 it's 50ma, BUT the newer RA-501 a is completley different pre-driver circuit and maybe different.

Maybe measure what you got (switch on amplifier and leave for 10 minutes then measure) and see if they are the same or if they are near the older RA-500 bias reading.

Hope this helps.


Edited by simonp1100 - 01 July 2020 at 8:01pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmuetst Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2020 at 4:30pm
Originally posted by simonp1100 simonp1100 wrote:

For the older RA-500 it's 50ma, BUT the newer RA-501 a is completley different pre-driver circuit and maybe different.

Maybe measure what you got (switch on amplifier and leave for 10 minutes then measure) and see if they are the same or if they are near the older RA-500 bias reading.

Hope this helps.
If I look at the schema on  https://www.hifiengine.com/manual_library/c-audio/ra1001.s - https://www.hifiengine.com/manual_library/c-audio/ra1001.shtml
I see two measure points A and B on the Gate after the 560 ohm resistors
When I measure on these points I get 102 volt not 300mV, so you are probably that this circuit is different. 
I cannot find any schema of this amp....




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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2020 at 5:42pm
The circuit you have is for the RA1001,RA2001 & RA3001, this is completley different to the RA500 / RA501, so do NOT use this a your reference circuit. Why do you need to measure the bias current anyway ?, this only needs to be done after a repair to any of the amplifier driver / output stage ?.

If you have over 100 volts on the gate resistors of the FET's then there is a serious issue with the amplifier and would need repairing.


Edited by simonp1100 - 02 July 2020 at 5:43pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmuetst Quote  Post ReplyReply Direct Link To This Post Posted: 03 July 2020 at 8:06am
Originally posted by simonp1100 simonp1100 wrote:

The circuit you have is for the RA1001,RA2001 & RA3001, this is completley different to the RA500 / RA501, so do NOT use this a your reference circuit. Why do you need to measure the bias current anyway ?, this only needs to be done after a repair to any of the amplifier driver / output stage ?.

If you have over 100 volts on the gate resistors of the FET's then there is a serious issue with the amplifier and would need repairing.
The amp is not defect. But I want to check its settings. 
The 100V is between point A and B. I hope some one does have a copy of a manual, or can tell how to check the bias.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote service dept Steve Quote  Post ReplyReply Direct Link To This Post Posted: 03 July 2020 at 5:14pm
On that drawing the test points are for quiescent voltage which is not what you need, if you are checking the bias current it is a different procedure which on a c audio is very easy. warm it up for a while, Remove one of the low tension fuses and measure the current across the fuse holder. 50ma should be about right, or you can use a scope to look for crossover distortion but as was pointed out earlier unless there has been work done you won't need to adjust the bias. mosfets sound a bit like valves but they don't drift much.The indicators are mushy highs = too low, easy over heating even without music= too high
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Monitech Quote  Post ReplyReply Direct Link To This Post Posted: 03 July 2020 at 8:52pm
An RA500 was a 3rd generation rebadge
RA501 was a C Audio design and was different cct
To the RA 1,2,3000
The bias will be set by measuring the voltage between the two base's of TR 1 and TR 3. THE Iq will be very low. I have the cct if you require it. I would set up with a triangle wave and look at the crossover distortion at 1 KHz and 10 KHz 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmuetst Quote  Post ReplyReply Direct Link To This Post Posted: 08 July 2020 at 3:24pm
Originally posted by Monitech Monitech wrote:

An RA500 was a 3rd generation rebadge
RA501 was a C Audio design and was different cct
To the RA 1,2,3000
The bias will be set by measuring the voltage between the two base's of TR 1 and TR 3. THE Iq will be very low. I have the cct if you require it. I would set up with a triangle wave and look at the crossover distortion at 1 KHz and 10 KHz 
Great answer. There are two potentiometers whitch must I use? And what is the voltage between TR1 base and TR3 base?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmuetst Quote  Post ReplyReply Direct Link To This Post Posted: 08 July 2020 at 3:26pm
Originally posted by service dept Steve service dept Steve wrote:

On that drawing the test points are for quiescent voltage which is not what you need, if you are checking the bias current it is a different procedure which on a c audio is very easy. warm it up for a while, Remove one of the low tension fuses and measure the current across the fuse holder. 50ma should be about right, or you can use a scope to look for crossover distortion but as was pointed out earlier unless there has been work done you won't need to adjust the bias. mosfets sound a bit like valves but they don't drift much.The indicators are mushy highs = too low, easy over heating even without music= too high
Thanks, I get the answer from Monitech: The bias will be set by measuring the voltage between the two base's of TR 1 and TR 3.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote service dept Steve Quote  Post ReplyReply Direct Link To This Post Posted: 08 July 2020 at 3:59pm

[/QUOTE]Thanks, I get the answer from Monitech: The bias will be set by measuring the voltage between the two base's of TR 1 and TR 3.[/QUOTE]

Good luck with that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Monitech Quote  Post ReplyReply Direct Link To This Post Posted: 10 July 2020 at 12:39am
Rv1 is the cmrr on the input
Rv2 is the bias
I have no notes and it's been 25 years since I looked at one
The output stage is a compound configuration 
R11 & 12 are connected to the base's of tr1/3
The output stage will essentially be off with no signal. 
You will need a 4 ohm 200 watt load minimum
A signal generator & a scope
To set up the amp. 
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