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C-Audio SR-707 Protect Issue

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simonp1100 View Drop Down
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    Posted: 13 March 2019 at 5:49pm
Originally posted by qwerp qwerp wrote:


Thank you to all that helped. especially simonp1100 : )


No worries, glad to help.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 4:21pm
The final issue i had is pretty much solved

i simply increased the input level.

I have 3 settings on my soundcard 

using a 1khz sine wave  at 0.1db from the software altering the attenuation options (small relays on the card i think)
the multimeter readings i can have are:

1. .39v ,  loud output.

2. 1.12v  potentially very loud as one would expect  (no distortion)

3. 2.27v  would be very very loud if i turned it  up (with 2.27v sine wave input and the main volume dial just slightly up I can hear a very slight distortion, I presume this is from the tl071 opamp struggling with the input level)

so that's pretty cool! 

Thank you to all that helped. especially simonp1100 : )












Edited by qwerp - 13 March 2019 at 4:21pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RoadRunnersDust Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 2:45pm
Originally posted by Xoc1 Xoc1 wrote:

Originally posted by RoadRunnersDust RoadRunnersDust wrote:

Don't suppose anyone on this thread has the schematics for the SR404, SR606/707 and the XR3801? Embarrassed

Edit: That they'd be willing to share?

https://www.schematicsunlimited.com/c/c-audio


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Don't know how I've ever missed this googling?!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Xoc1 Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2019 at 1:10pm
Originally posted by RoadRunnersDust RoadRunnersDust wrote:

Don't suppose anyone on this thread has the schematics for the SR404, SR606/707 and the XR3801? Embarrassed

Edit: That they'd be willing to share?

https://www.schematicsunlimited.com/c/c-audio
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RoadRunnersDust Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2019 at 9:50pm
Don't suppose anyone on this thread has the schematics for the SR404, SR606/707 and the XR3801? Embarrassed

Edit: That they'd be willing to share?

Edited by RoadRunnersDust - 12 March 2019 at 9:50pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2019 at 11:12am
Originally posted by qwerp qwerp wrote:

I have it connected to a 100w 8 ohm full range studio speaker,  using an unbalanced media player to unbalanced in. The output is quite loud ( loud enough I worry what my neighbours are thinking), it is full range. flat.  but not so loud i have to worry about hurting my speaker. i know  the speaker could be pushed more.

I can only imagine its been tuned to a lower volume i have noticed there is a sticker on the back saying LF amp 3.top layer so i presume this was from an array of speakers.. hence i feel there might be a pot to turn or a resistor to change to make it a standalone amp?

it is full range and sounds nice. just the level issue.

thanks for any help you can offer.

Can you accurately measure AC voltage in vs voltage out? That will tell you if the amp still has all the gain it should. Then you could do a test into some sort of dummy load to see if it will sustain high power. For a quick test, three to five 3000 watt electric kettles in parallel will give you a load of about 4 ohms when hot. Or, a couple of old voice coils in a bucket of water works too :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2019 at 10:57am
Originally posted by simonp1100 simonp1100 wrote:

The bias current should go up and down as VR1 is turned, if it's not then check the preset pot, this maybe the reason it will not adjust.

DO NOT ADJUST VR2 this is the common mode rejection pot for the input stage and is set at C-Audio and only needs setting up if some components on the input stage have been changed / replaced. Dead

Changed out VR1 too....I can measure the resistance changing when I turn it. I've also checked the two driver transistors just after the bias stage, and they measure fine. 
On the subject of leaky transistors. My tester sometimes shows a leakage result (ICEO) of 1uA, which then normally disappears if I repeat the test. 
Is 1uA considered a lot of leakage, would this result indicate I should replace the device? I note that on datasheets, leakage is normally quoted in nA so 1uA would be 100x more...


Edited by studio45 - 12 March 2019 at 11:00am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2019 at 10:47am
Originally posted by simonp1100 simonp1100 wrote:

Originally posted by studio45 studio45 wrote:

Glad yours is working too qwerp :)

I also can't get the bias set right on ch B. I am reading 396mV between the drives, which seems too high. Although it does pass audio, sounds fine and doesn't seem to be running too hot.
Adjusting VR1 does not affect the reading, in either direction. I have already checked out the BF422 transistor, 1.5k and 2.7k resistors and they all measure as good. 
I even added 1k in series with VR1 in an attempt to gain more downward adjustment range, but I still just get that 396mV figure, no matter where VR1 is set.

Just tried substituting an MPSA42 for the BF422, in case my transistor tester is lying to me, but no change, still settles at 396mV.

Any ideas? 


The bias current should go up and down as VR1 is turned, if it's not then check the preset pot, this maybe the reason it will not adjust.

DO NOT ADJUST VR2 this is the common mode rejection pot for the input stage and is set at C-Audio and only needs setting up if some components on the input stage have been changed / replaced. Dead



I have it connected to a 100w 8 ohm full range studio speaker,  using an unbalanced media player to unbalanced in. The output is quite loud ( loud enough I worry what my neighbours are thinking), it is full range. flat.  but not so loud i have to worry about hurting my speaker. i know  the speaker could be pushed more.

I can only imagine its been tuned to a lower volume i have noticed there is a sticker on the back saying LF amp 3.top layer so i presume this was from an array of speakers.. hence i feel there might be a pot to turn or a resistor to change to make it a standalone amp?

it is full range and sounds nice. just the level issue.

thanks for any help you can offer.








Edited by qwerp - 12 March 2019 at 10:52am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 11 March 2019 at 9:25pm
Originally posted by studio45 studio45 wrote:

Glad yours is working too qwerp :)

I also can't get the bias set right on ch B. I am reading 396mV between the drives, which seems too high. Although it does pass audio, sounds fine and doesn't seem to be running too hot.
Adjusting VR1 does not affect the reading, in either direction. I have already checked out the BF422 transistor, 1.5k and 2.7k resistors and they all measure as good. 
I even added 1k in series with VR1 in an attempt to gain more downward adjustment range, but I still just get that 396mV figure, no matter where VR1 is set.

Just tried substituting an MPSA42 for the BF422, in case my transistor tester is lying to me, but no change, still settles at 396mV.

Any ideas? 


The bias current should go up and down as VR1 is turned, if it's not then check the preset pot, this maybe the reason it will not adjust.

DO NOT ADJUST VR2 this is the common mode rejection pot for the input stage and is set at C-Audio and only needs setting up if some components on the input stage have been changed / replaced. Dead


Edited by simonp1100 - 11 March 2019 at 10:34pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 11 March 2019 at 7:48pm

Studio45 on another subject, Have you altered vr2? Do you know if it is for dc offset or is it a gain trim?

thank you


Edited by qwerp - 11 March 2019 at 7:55pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote qwerp Quote  Post ReplyReply Direct Link To This Post Posted: 11 March 2019 at 7:41pm
Originally posted by studio45 studio45 wrote:

Glad yours is working too qwerp :)

I also can't get the bias set right on ch B. I am reading 396mV between the drives, which seems too high. Although it does pass audio, sounds fine and doesn't seem to be running too hot.
Adjusting VR1 does not affect the reading, in either direction. I have already checked out the BF422 transistor, 1.5k and 2.7k resistors and they all measure as good. 
I even added 1k in series with VR1 in an attempt to gain more downward adjustment range, but I still just get that 396mV figure, no matter where VR1 is set.

Just tried substituting an MPSA42 for the BF422, in case my transistor tester is lying to me, but no change, still settles at 396mV.

Any ideas? 


hi are you measuring between the orange and pink wire (crocodile clip to the actual wires) on the rear of the mosfet circuit board? with it set to dc on multimeter? Thats how i read on a sr707. 

good idea adding a resistor.   i was considering changing out the vr1 to a different value pot to allow me to get 350mv. But is it really that necessary to hit 350mv exactly?

Did you manage to  change the fan?

I am baffled why my amp seems not so powerful. i do not have a high watt speaker to test, but i thought it would distort this cheap naf old speaker I have here.  maybe i do not fully understand the science?  Maybe with a powerful high watt speaker this amp will show its true power ...or maybe someone changed a resistor somewhere near the opamp to reduce the level so to prevent someone blowing up a set of speakers at its last installation (  gain reduction of some type) ?


( to clarify....the diagram above is wrong the green circles were just a guess.  (the points to probe are the pink and orange wire sold points on the rear of the mosfet pcb)










Edited by qwerp - 11 March 2019 at 7:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 11 March 2019 at 6:39pm
Glad yours is working too qwerp :)

I also can't get the bias set right on ch B. I am reading 396mV between the drives, which seems too high. Although it does pass audio, sounds fine and doesn't seem to be running too hot.
Adjusting VR1 does not affect the reading, in either direction. I have already checked out the BF422 transistor, 1.5k and 2.7k resistors and they all measure as good. 
I even added 1k in series with VR1 in an attempt to gain more downward adjustment range, but I still just get that 396mV figure, no matter where VR1 is set.

Just tried substituting an MPSA42 for the BF422, in case my transistor tester is lying to me, but no change, still settles at 396mV.

Any ideas? 
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