stereofun
Super Member
A few month ago I restored a very nice condition AU-999 including the pre-amp and bass mod. I have since tweaked around a bit and wanted to share. My main focus has been the driver board - the feedback loop and the differential pair. This would not have been possible without direct or indirect help from Leestereo, Kevzep and Trnsfrmr - Thanks guys!
During the original restoration The only deviations from original besides the pre-amp and bass eq mod, was a new rectifier and 2 x 100v/12K filter caps. Then a polypropylene input coupling cap as well as a higher voltage feedback cap. The output transistors were replaced with MJ21194's as only two originals remained - the two substitutes were significant slower than the MJ's, which are almost on par with the originals. The amp sounded very good.
Mod#1
A 2.2 uf Polypropylene at the C801 driver board position instead of the 1.0 uf stock mylar.
Result: No appreciable difference until the amp gets cranked up to moderate or high output. Here the 2.2 shows better performance in the highest notes, but only gives little to no benefit in the low end.
Mod#2
Replacing the C807 bipolar feedback cap to Nichicon Muse 100uf/50v. Replacing feedback resistor R819 with a .5w metal film type for better temperature coefficient and replacing it's paralleled ceramic cap on the back with a CoG type as well as the low pass ceramic C803. (thanks Leestereo)
Mouser part number for the ceramic low noise caps: 581-SR211A560JAR, 581-SR211A150JAR Here a link to a post about the benefits of C0G ceramic caps.
Result: Noticeable - cleaner sound.
Mod#3
The goal here is to get the current through the differential input pair within 1%. The mod works best if you upfront replace differential input pair, TR801/803 with a low-noise, high-gain, high band-with set of transistors matched to within 1% hFE - a suitable candidate could be ZTX795A's, but other would work too (check specs). If you want to keep the original transistors then that is ok, but you will likely spend time swapping resistors until you get even current. We will get to that.
As an option you can begin by replacing the current source resistor R845 with a 1% .5w metal film - again the goal is to have the most stable and low noise resistors in these critical positions - but the main components here are the diff. pairs emitter resistors, R811/813. Assuming you have already gain matched TR801/803, you now replace R811/813 with 1% .5W metal film types that you have matched even closer out of a batch.That's it - as a check you can measure how close you got the currents (see below)
Now, If you kept the original transistors you need to probe mV across the resistors and convert to mA (Amp on, no signal) to see if these two currents are within 1% - If it is not you can calculate which resistance you need to replace in one side to match the other. For this operation it will be a huge help to measure your batch beforehand and document them so you know the resistance of those that went in as well as the candidates for swap if currents needs adjustment (see picture below).
Result: Cleaner and more spatial sound. It is like the amp has gone audiophile. Prepare to hear more heavily breathing jazz musicians on old recordings
The difference in current through the resistors, converted from their voltage drop and their actual measured value, caused a 10% variation on the left side and 3% on the right. After the new resistors were in, the change in current was within 1% on each differential pair and also within 1% channel to channel.
For instance - Left channel, before mod.:
R811: Voltage drop 65mV / value 56.3 ohm = current 1.15 mA
R813: Voltage drop 64mV / value 56.8 ohm = current 1.27 mA
After new resistors in place the numbers were: 1.116 /1.121 for the currents
R845 is connected to the + rail, then pulls the current (EF) up through the emitter resistors R811/13 of the diff. pair Tr801/3
Here you can also follow the feedback going through R819 and the bipolar C807 to the base of diff. pair Tr803.
Here an interesting post where AK member trnsfrmr discusses the idea of balancing the diff. input pair. He suggests a more sophisticated method by using a constant current source in place of the current source resistor R845.
If you give it a shot, let me know what you think.
Cheers,
P.S for MOD#3 You really need to have these probes with a tiny hook at the end (here shown on a different board of course)
During the original restoration The only deviations from original besides the pre-amp and bass eq mod, was a new rectifier and 2 x 100v/12K filter caps. Then a polypropylene input coupling cap as well as a higher voltage feedback cap. The output transistors were replaced with MJ21194's as only two originals remained - the two substitutes were significant slower than the MJ's, which are almost on par with the originals. The amp sounded very good.
Mod#1
A 2.2 uf Polypropylene at the C801 driver board position instead of the 1.0 uf stock mylar.
Result: No appreciable difference until the amp gets cranked up to moderate or high output. Here the 2.2 shows better performance in the highest notes, but only gives little to no benefit in the low end.
Mod#2
Replacing the C807 bipolar feedback cap to Nichicon Muse 100uf/50v. Replacing feedback resistor R819 with a .5w metal film type for better temperature coefficient and replacing it's paralleled ceramic cap on the back with a CoG type as well as the low pass ceramic C803. (thanks Leestereo)
Mouser part number for the ceramic low noise caps: 581-SR211A560JAR, 581-SR211A150JAR Here a link to a post about the benefits of C0G ceramic caps.
Result: Noticeable - cleaner sound.
Mod#3
The goal here is to get the current through the differential input pair within 1%. The mod works best if you upfront replace differential input pair, TR801/803 with a low-noise, high-gain, high band-with set of transistors matched to within 1% hFE - a suitable candidate could be ZTX795A's, but other would work too (check specs). If you want to keep the original transistors then that is ok, but you will likely spend time swapping resistors until you get even current. We will get to that.
As an option you can begin by replacing the current source resistor R845 with a 1% .5w metal film - again the goal is to have the most stable and low noise resistors in these critical positions - but the main components here are the diff. pairs emitter resistors, R811/813. Assuming you have already gain matched TR801/803, you now replace R811/813 with 1% .5W metal film types that you have matched even closer out of a batch.That's it - as a check you can measure how close you got the currents (see below)
Now, If you kept the original transistors you need to probe mV across the resistors and convert to mA (Amp on, no signal) to see if these two currents are within 1% - If it is not you can calculate which resistance you need to replace in one side to match the other. For this operation it will be a huge help to measure your batch beforehand and document them so you know the resistance of those that went in as well as the candidates for swap if currents needs adjustment (see picture below).
Result: Cleaner and more spatial sound. It is like the amp has gone audiophile. Prepare to hear more heavily breathing jazz musicians on old recordings
The difference in current through the resistors, converted from their voltage drop and their actual measured value, caused a 10% variation on the left side and 3% on the right. After the new resistors were in, the change in current was within 1% on each differential pair and also within 1% channel to channel.
For instance - Left channel, before mod.:
R811: Voltage drop 65mV / value 56.3 ohm = current 1.15 mA
R813: Voltage drop 64mV / value 56.8 ohm = current 1.27 mA
After new resistors in place the numbers were: 1.116 /1.121 for the currents
R845 is connected to the + rail, then pulls the current (EF) up through the emitter resistors R811/13 of the diff. pair Tr801/3
Here you can also follow the feedback going through R819 and the bipolar C807 to the base of diff. pair Tr803.
Here an interesting post where AK member trnsfrmr discusses the idea of balancing the diff. input pair. He suggests a more sophisticated method by using a constant current source in place of the current source resistor R845.
If you give it a shot, let me know what you think.
Cheers,
P.S for MOD#3 You really need to have these probes with a tiny hook at the end (here shown on a different board of course)
Attachments
Last edited: