hello : )
I have a C-Audio sr-707 and managed to make it work by replacing a few resistors and it now passes audio ( full range and sounds very good)
It is meant to have around 800w- 1000w per channel?
I have a C-Audio sr-707 and managed to make it work by replacing a few resistors and it now passes audio ( full range and sounds very good)
It is meant to have around 800w- 1000w per channel?
I connected up a 100w speaker to test it with a strong line level signal into the 1/4inch unbalanced input.The output level of the speaker was quite loud, but not so much I was worried i would harm my speaker. This is the same on both channels. I presumed i would have to be careful turning up the volume, and maybe only need to turn it up a 1/4 of a turn. but i had it up to max.
The rear of the amp states "LF amp 3.top layer." it came from a venue, i believe it was an inhouse system. So it might have been tuned for a PA system for a specific room?
I wonder if this makes any sense to anyone?
Thank you for any help you may offer. : )
schematic for the amp (and other variations)
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What is your definition of "a strong line level signal" (full-scale Vrms, program level in dBFS)?
This amp should have a voltage gain of 33.6 dB when turned all the way up, enough for quite a racket when confronted with CD player levels (2 Vrms, -10 dBFS average = 115 W average).
What did your cabling look like when testing? Feeding a stereo signal into a balanced input would definitely not yield the desired result, for one... If that's not the problem, possibly the input stage has been modified for lower than unity gain or has a problem. (TL071 on +/-18 V, right on the edge of its ratings... someone was evidently feeling lucky.)
Speaking of inputs, that circuitry is a bit quirky.
1. With those resistor values, how is the CMRR trimming supposed to work? (As shown, R54 + VR2 + R61 is always greater than R60.) Maybe R54 actually is a 9k1 rather than a 10k?
2. Even so, CMRR is not likely to hold up too well across the audible range since there is no counterpart to C34 (10p) in parallel to R60.
3. There is what appears to be a drafting error in XLR wiring - it is drawn mirrored, with pins 1 and 2 switched. Probably not the case in the real thing.
3. Also, due to its age, the unit does not follow AES48-2005 conventions for wiring yet. These mandate that XLR pin 1 (shield) be connected to chassis ground as directly as possible, with no direct connection to local circuit ground. Here the pin 1s plus TRS shield run over the shielded balanced cable to local input circuit ground, which is not ideal as large shield currents may contaminate the ground, limiting CMRR to less than it could be.
I suggest that you
a) add wires for a connection to chassis (case) at every input jack (I hope you have a decently large soldering iron as well as some sandpaper and flux) and
b) disconnect the existing shield connection at the cable and insert a smallish capacitor in series (maybe 10n or so).
This amp should have a voltage gain of 33.6 dB when turned all the way up, enough for quite a racket when confronted with CD player levels (2 Vrms, -10 dBFS average = 115 W average).
What did your cabling look like when testing? Feeding a stereo signal into a balanced input would definitely not yield the desired result, for one... If that's not the problem, possibly the input stage has been modified for lower than unity gain or has a problem. (TL071 on +/-18 V, right on the edge of its ratings... someone was evidently feeling lucky.)
Speaking of inputs, that circuitry is a bit quirky.
1. With those resistor values, how is the CMRR trimming supposed to work? (As shown, R54 + VR2 + R61 is always greater than R60.) Maybe R54 actually is a 9k1 rather than a 10k?
2. Even so, CMRR is not likely to hold up too well across the audible range since there is no counterpart to C34 (10p) in parallel to R60.
3. There is what appears to be a drafting error in XLR wiring - it is drawn mirrored, with pins 1 and 2 switched. Probably not the case in the real thing.
3. Also, due to its age, the unit does not follow AES48-2005 conventions for wiring yet. These mandate that XLR pin 1 (shield) be connected to chassis ground as directly as possible, with no direct connection to local circuit ground. Here the pin 1s plus TRS shield run over the shielded balanced cable to local input circuit ground, which is not ideal as large shield currents may contaminate the ground, limiting CMRR to less than it could be.
I suggest that you
a) add wires for a connection to chassis (case) at every input jack (I hope you have a decently large soldering iron as well as some sandpaper and flux) and
b) disconnect the existing shield connection at the cable and insert a smallish capacitor in series (maybe 10n or so).
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sgrossklass Thank you very much !!!
I was so flustered with the amp circuit boards and repairing the protect circuit, I did not put a multimeter to the signal input.
My soundcard (source for the test) has 3 output attenuation levels.
I just ran a test with a 1khz sine wave (0.1db)
results for line level multimeter readings and 100w speaker connected below:
setting 3. = .39v ( no distortion and quite loud)
setting 2. = 1.12v ( no distortion and lots of potential level)
setting 1. = 2.27v (with the front volume dial down there is a very slight amount of distortion from i presume the Tl071 opamp,
but even more level available)
So maybe 2v would be perfect? setting 1 with a small inline pad prior to the input so it does not distort.
i unfortunately I do not have an oscilloscope to see the waveform, that would be cool to see.
But this definitely fixed the issue i was having.
my soldering iron is a weller (95watt). I shall have to see how easy it is to access the rear panel.
Thank you sgrossklass you are awesome : P
I was so flustered with the amp circuit boards and repairing the protect circuit, I did not put a multimeter to the signal input.
My soundcard (source for the test) has 3 output attenuation levels.
I just ran a test with a 1khz sine wave (0.1db)
results for line level multimeter readings and 100w speaker connected below:
setting 3. = .39v ( no distortion and quite loud)
setting 2. = 1.12v ( no distortion and lots of potential level)
setting 1. = 2.27v (with the front volume dial down there is a very slight amount of distortion from i presume the Tl071 opamp,
but even more level available)
So maybe 2v would be perfect? setting 1 with a small inline pad prior to the input so it does not distort.
i unfortunately I do not have an oscilloscope to see the waveform, that would be cool to see.
But this definitely fixed the issue i was having.
my soldering iron is a weller (95watt). I shall have to see how easy it is to access the rear panel.
Thank you sgrossklass you are awesome : P
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So these are input levels if I understand you correctly?
The input section should support fairly substantial levels (possibly 9+ Vrms), though distortion may well be less than ideal with an unbalanced source, as TL07x have fairly high common-mode distortion and would prefer output loading closer to 3x what it is here.
I suggest you use setting 3 and then determine what you get at the output (with level controls all the way up), in parallel to the speaker (though that should make little difference). Should be something close to 18.7 Vrms.
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