Quote:
Originally Posted by AudioIdiot
So I promised to share my findings.
Audio Research LS28 before and after gain modification:
The measurements are all taken with the balanced I/O and unless otherwise noted the load impedance to the output is 100k using an Audio Precision System 2 Cascade.
Some static measurements:
Residual noise 20-22k
Before modification 18,77dB gain = -97 dBV L / -98 dBV R
After modification 12,95dB gain = 101 dBV L / 104 dBV R
This is the output noise with the volume at 103 and referes to an output voltage of 1V.
The 12V trigger has an open voltage of 12,45V and with a load of 7mA it dropped to 12V. This calculates to an output impedance of 63 Ohms.
The voltage seems to be protected by a PTC
The basic Schematic of the LS28. A very clever zero feedback balanced design. The two mirror imaged sections are the amplifiers for the hot and cold signal. The input stage is biased by a single current source that allows unbalanced to balanced conversion.
Channel mismatch is lower than 0,1 dB > 100k and lower than 0,005 dB from 20k to 22kHz
The modified gain has a slightly higher bandwidth with -1,75dB @ 200k versus -2,2 dB @ 200k for the original gain.
Remarks:
The output capacitor of 10µ/50V is under specified with 70V across the cap !!! It should at least be a 100V model. I think this is a liability and could cause quite some problems.
The 2A Slow blow fuse is to big for a 50VA transformer-based design. The total power dissipation sits around 45W. I would advise a for 115V mains : 0.8A 5x20mm IEC127 high I²t-value. For 230V mains: 0.4A 5x20mm IEC127 high I²t-value.
I also noticed that the voltage feeding the 180V regulator is 315V. That is quite a waste and unnecessary power dissipation. The transformer can be used from 100V to 250V AC so a large margin is not needed. I expect this comes from earlier tube rectifier-based designs where the voltage drop over the rectifier is much higher.
I can't add a PDF here or pictures that are not on the web, but I can tell you the unit measures absolutely fabulous. Very high bandwidth that is slightly influenced by the volume control position. Very low harmonic distortion up to 4V RMS into 100k Ohm with 2nd harmonics below -100dB. Excellent.
It is a pity that some choices are a bit strange. The fuse rating is absolutely wrong. The 50V output caps are dangerous. and I don't understand the ridiculous high voltage that is feeding the 180V regulator.
So far an update on the unit. Still waiting for the transformer.
Peter
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I've owned older (70's- 80's) AR gear and found many of their choices seem to be poor engineering or compromises. Under-rated cap voltages, excessive input voltages to regulator stages, too high (excessive) tube bias etc. I'm not surprised by what you have mentioned. During my 33yr career in military electronics reliability has been of utmost importance.