kevin gilmore Posted May 26, 2014 Report Posted May 26, 2014 its not the pot, but when you get it and take it apart, take a picture of the bottom of the board.
nemomec Posted May 26, 2014 Report Posted May 26, 2014 Here are the pictures: http://www.pic-upload.de/view-23329928/KGSS_1.jpg.html http://www.pic-upload.de/view-23329900/KGSS_2.jpg.html http://www.pic-upload.de/view-23329932/innen.jpg.html http://www.pic-upload.de/view-23329931/innen_revision.jpg.html http://www.pic-upload.de/view-23329937/innen_poti.jpg.html http://www.pic-upload.de/view-23329907/poti_unten_1.jpg.html http://www.pic-upload.de/view-23329908/poti_unten2.jpg.html Its the pot or not, good question. I measured now the resitors of the pot on the bottom of the board and they are increasing linear from 4 to 23K ohms over the entire pot rage. But the pot has only a audible range from 6 to 8 hour and is then extremly loud with massive distortion with a 1,6V cd-player source!? I use the KGSS now after a premap and it sound very good but this can´t be the final solution. 1
spritzer Posted May 26, 2014 Report Posted May 26, 2014 New PCB too with output resistors. Still a mess though and those heatsinks should be soldered down and grounded. As for the pot, it is a linear unit (says 50KB on the back) with resistors to fake the log curve. This never works correctly so I for one will just remove it and put in something else.
Audiojunkie Posted May 26, 2014 Report Posted May 26, 2014 New PCB too with output resistors. Still a mess though and those heatsinks should be soldered down and grounded. As for the pot, it is a linear unit (says 50KB on the back) with resistors to fake the log curve. This never works correctly so I for one will just remove it and put in something else. At least the soldering is so much better than the LL you "fixed".
justin Posted May 26, 2014 Report Posted May 26, 2014 Hello, my chinese KGSS is now received. Some positive things: - it has a voltage selector for 230V and 115V - it has a teflon stax plug - the bias voltage is adjustable over the pot TVR1 (you need a high impedance voltmeter) - well built But I have one big problem, the gain is extremly high and i can´t adjust the volume pot over 8 hour without distortion on different sources. Where i can adjust the gain or which resitors must be checked? Edit: I think the cheap volume pot is defective i measured one of these and it was nearly impossible to adjust the pot to get 100Vrms on the output with a 1.5Vrms input. It was like trying to listen with IEMs on a high gain amplifier. Also, there was considerably channel imbalance at this level (off about 25%)
kevin gilmore Posted May 26, 2014 Report Posted May 26, 2014 (edited) real numbers. (single ended input) measured single stator to ground kgsshv with 450v power supplies and 2 x 100k feedback resistors with 572 millivolts rms input produces 800 volts peak to peak output (not clipping) Chinese kgss with 350v power supplies and 2 x 154k feedback resistors with 320 millivolts rms input produces 700 volts peak to peak output (just barely clipping) so with a 1.5v rms input the pot if it was linear would have to be at 20% or less to avoid clipping. and the matching on the pot is going to be lousy at 20%. if you change the 154k resistors out to 100k resistors, the input at clipping would be at 500 millivolts rms. Which was the original design spec. nothing is wrong with the pot. (other than its a cheap piece of shit and used poorly) you probably need a 10db input pad. and you need to cut the gain by changing the feedback resistors back to the original design spec. Edited May 26, 2014 by kevin gilmore
nemomec Posted May 27, 2014 Report Posted May 27, 2014 Ok, thanks Kevin, i think i change the pot and replace the resitors R73-R76 with 100K. I heard yesterday the chinese KGSS some hours and they sounds very very good with huge dynamic and strong bass extension (testet with Lambda Nova Signature). But the pot is horrible, i heard also a little imbalance and i full agree with justin that said "It was like trying to listen with IEMs on a high gain amplifier".
rx79ez08 Posted May 27, 2014 Report Posted May 27, 2014 I also got my Chinese KGSS. Similar problem to nemomec, with a few odd ones: 1) The socket is not an original Stax one like the item description 2) One of the screw from top of the amplifer is missing 3) There was a plastic spacer caught at the bottom plate of the amplifer, causing the bottom plate to not close properly 4) RCA/XLR selection switch inject a lot of noise into the amp when RCA is selected. When XLR is selected no noise is injected, and funny enough RCA input also work in XLR mode. The pot and gain setting are terrible, there seem to be 3 volume setting, when you turn it to minimum you get an intermediate volume, turn it up a bit you get no output, further up to about 9 o'clock you get full volume. That doesn't hughly bother me though, as I always have a pre-amp (to remotely control the volume) between the dac and the amp, so I treat the amp like a power amp and adjust the pre-amp. I do like how it sounds with a 404LE, pretty much as nemomec described it, good dynamic and bass. Will have to try it with other headphones I have.
kevin gilmore Posted May 27, 2014 Report Posted May 27, 2014 rca and + input of xlr wired in parallel. the switch switches the - input of the xlr to ground for rca. guess they could not buy more stax jacks
spritzer Posted May 27, 2014 Report Posted May 27, 2014 The ad is indeed rather misleading when it comes to the inputs. Only one should be used at a time. For the volume control, I've no luck finding a replacement which would work on the PCB outside of a custom order. Alpha has a nice 9mm option but that would take 500-1000 units... The TKD 4CP-601 might fit but it's an odd size so the best bet is probably to move the pot to the front panel and run 9 wires back to the holes for the old pot. I'll post pics of this when I'll get my amp. Also, some components really need to be replaced as they are overloaded as it is. Using a RN60D where we used a 3W resistors in the stock KGSS can't be good.
nemomec Posted May 27, 2014 Report Posted May 27, 2014 "4) RCA/XLR selection switch inject a lot of noise into the amp when RCA is selected. When XLR is selected no noise is injected, and funny enough RCA input also work in XLR mode." Same problem with my KGSS, RCA works also in XLR Mode, but the noise is gone after connect to my Stax SRA-12S preamp. The feedback resitors now changed to 100K but with no huge effect on the gain (grazy soldering on this manufactured board ). I have now ordered a ALPS RK27114 pot and hope they are works fine in this amp and solve the gain problem. The sound of this amp is very good and i hope we can solve the issues!
kevin gilmore Posted May 27, 2014 Report Posted May 27, 2014 sure looks to me that the pot is wired wrong. input goes to the wiper, output goes to the amp... ??
spritzer Posted May 27, 2014 Report Posted May 27, 2014 sure looks to me that the pot is wired wrong. input goes to the wiper, output goes to the amp... ?? Nice catch, it's indeed reversed. Do you have any pics off the bottom of my amp? It has some crazy stuff going on there...
rx79ez08 Posted May 28, 2014 Report Posted May 28, 2014 Had a rather interesting phone conversation with the seller, he don't speak or understand much spoken English and I don't speak much Mandarin which make for an interesting combination..... He confirmed a few things: 1) The gain was intentionally set to a very high value, he said if you can lower your DAC gain by 20-30db that should be fine, then the volume control should work fine. I don't quite understand his explaination of why he did that. 2) As Kevin and spritzer said, the RCA and XLR cannot be connected together. 3) Apparently because of patent issue he can no longer install original Stax headphones jack in his amp. He thinks the replacement is just as good, if not better than the original one. He gave a detail explanation which I did not catch very well. After some pushing, he will send me some extra screws and fuses, so I guess that is a good outcome for me.
nemomec Posted May 28, 2014 Report Posted May 28, 2014 He confirmed a few things: 1) The gain was intentionally set to a very high value, he said if you can lower your DAC gain by 20-30db that should be fine, then the volume control should work fine. I don't quite understand his explaination of why he did that. I don´t understand/believe this, if i use a preamp to reduce the input gain the volume control doesn´t work correct! I think the pot is a false model or wrong wired!?
kevin gilmore Posted May 28, 2014 Report Posted May 28, 2014 (edited) the pot is definitely wired wrong. what he probably did is turn the pot into a variable resistance for current output dacs. This would then require all the extra voltage gain. the whole input section needs to be re-wired. will draw up what I think it should be when I get home. the 4 resistors to ground by the xlr input jacks. 2 of them are 49.9k ?? . R40,R47 What are the other 2.R37,R42 the 4 resistors from the pot to the input fets are 2.2k?R9,R10,R11,R12 the 4 resistors on the bottom of the board, what is the value? the schematic I did showed only one channel. If you replace the 154k resistors with 100k, you have to replace a total of 8 resistors, 4 for each channel. Edited May 28, 2014 by kevin gilmore
Tyll Hertsens Posted May 28, 2014 Report Posted May 28, 2014 Can you measure the stax in-ears ? Sent from my iPhone using Tapatalk I'll look into it. I'd like to get some measured. Do you know what you have lined up and what you are missing? I could conceivably send the following: SR-Omega SR-007BL SR-4070 SR-X Mk3 Pro Thanks Al, I'll keep it in mind as things get rolling. Not sure what we're missing yet.
nemomec Posted May 28, 2014 Report Posted May 28, 2014 the pot is definitely wired wrong. what he probably did is turn the pot into a variable resistance for current output dacs. This would then require all the extra voltage gain. the whole input section needs to be re-wired. will draw up what I think it should be when I get home. the 4 resistors to ground by the xlr input jacks. 2 of them are 49.9k ?? . R40,R47 What are the other 2.R37,R42 the 4 resistors from the pot to the input fets are 2.2k?R9,R10,R11,R12 the 4 resistors on the bottom of the board, what is the value? the schematic I did showed only one channel. If you replace the 154k resistors with 100k, you have to replace a total of 8 resistors, 4 for each channel. Ok here are some infos: R40/R47 40K R37/R42 510 ohms R9-R12 2,2K 4 resitors on the bottom are at 23K with full volume setting. I have changed the 8 resitors for the two channels R73/74/75/76 and R35/36/38/39 to 100K. Crazy stuff, but sounds extremly good
spritzer Posted May 28, 2014 Report Posted May 28, 2014 Thanks Al, I'll keep it in mind as things get rolling. Not sure what we're missing yet. The SR-X Mk3 Pro is an interesting one as it is the rarest of all Stax headphones (well save for the SR-2 which doesn't actually exist...) and sounds stunning. The 4070 is the closed Pro model so a bit special too.
kevin gilmore Posted May 28, 2014 Report Posted May 28, 2014 this is how it should be wired, but the pot is actually not wired like that if the footprint for the alps is correct in their documentation http://gilmore.chem.northwestern.edu/chinesekgss.pdf
rx79ez08 Posted May 28, 2014 Report Posted May 28, 2014 Do you mean the PCB is wrong or the pot is mounted the wrong way?
kevin gilmore Posted May 28, 2014 Report Posted May 28, 2014 the pcb is wrong. but you need to check the pot to be sure the pins on the pot should be ground,out,in
spritzer Posted May 28, 2014 Report Posted May 28, 2014 I've never seen a pot that doesn't have the wiper in the middle. The PCB has to be wrong.
nemomec Posted May 29, 2014 Report Posted May 29, 2014 In my knowledge a normal Alps pot has ground/output/input, this pot on the PCB is wired with the input in the middle!? I think the best way is to unsolder this cheap pot and take a new one on the front with cables to the PCB.
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