C-Audio SR-707 Protect Issue |
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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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Posted: 13 March 2019 at 5:49pm |
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No worries, glad to help. |
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qwerp
Registered User Joined: 01 March 2019 Status: Offline Points: 41 |
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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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RoadRunnersDust
Young Croc Joined: 03 December 2013 Location: United Kingdom Status: Offline Points: 560 |
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Thanks! Don't know how I've ever missed this googling?! |
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Xoc1
Registered User Joined: 15 October 2012 Location: Devon UK Status: Offline Points: 397 |
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https://www.schematicsunlimited.com/c/c-audio
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RoadRunnersDust
Young Croc Joined: 03 December 2013 Location: United Kingdom Status: Offline Points: 560 |
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Don't suppose anyone on this thread has the schematics for the SR404, SR606/707 and the XR3801?
Edit: That they'd be willing to share? Edited by RoadRunnersDust - 12 March 2019 at 9:50pm |
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studio45
Old Croc Joined: 16 October 2007 Location: United Kingdom Status: Offline Points: 3863 |
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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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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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studio45
Old Croc Joined: 16 October 2007 Location: United Kingdom Status: Offline Points: 3863 |
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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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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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qwerp
Registered User Joined: 01 March 2019 Status: Offline Points: 41 |
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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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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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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.
Edited by simonp1100 - 11 March 2019 at 10:34pm |
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qwerp
Registered User Joined: 01 March 2019 Status: Offline Points: 41 |
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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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qwerp
Registered User Joined: 01 March 2019 Status: Offline Points: 41 |
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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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studio45
Old Croc Joined: 16 October 2007 Location: United Kingdom Status: Offline Points: 3863 |
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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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Studio45 - Repairs & Building Commotion Soundsystem -Mobile PA
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