A75 grounding

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I am new to this forum and to DIY audio.
I am currently in the final stages of completing the A75 and the balanced line stage projects. This is after one year of planning, building and testing and of course much patience from my family.
My final problem, (so he says) one that has given me a few more grey hairs, so here goes.
The A75 appears to test fine, that is I have low DC offset <20mv, biased at 170 mv, with 6VAC at the output, this is using the prescribed test procedure from the article. The problem is when I ground the -IN using S-2 (unbalanced) the bias voltage drops to approximately 20mv the AC output goes to 30V which results in a very unhappy R31 with about a half a volt across it.
I have double checked the switch wiring and even removed it, in addition I have doubled and tripled checked the front end board for incorrect components.
Any suggestions on tracing this problem would be greatly appreciated.

Thanks, BBB
I built the A75 about 5 years ago and have not experienced any problems from assembly to current. I'm currently using the balanced Zen preamp, but have also ran the Zen unbalanced preamp.
First I would take a look with an O-scope at the output to see if you are experiencing oscillations balanced or unbalanced. Make sure your input source has no dc component. You say that you tested as per the the article. Did you test first with balanced then tried unbalanced?
With the amp not powered (that is unplugged from the AC and no preamp to the input) what are the input resistances to ground? You can use the speaker ground terminal or pin 1 of the XLR which is ground. The measurements should be as follows: Balanced pin 2 (+ input) to ground 1.8k ohms, R2 plus R29, pin 3 (- input) to ground 1.8k ohms R4 plus R28. Unbalanced pin 2 to ground 75k ohms, pin 3 to ground 0.0 ohms. If you were to take a measurement from the other side of R2 and R4 to the input pins 2 and 3 then you would be measuring 475 ohms for the balanced mode. For unbalanced pin 2 would also be 475 ohms, but pin 3 would be slightly less at 350 ohms, R2 in parallel with R29. The measurements from R2 and R4 to ground in balanced mode would be 1.35k ohms. In unbalanced mode R2 to ground would be 75k ohms and R4 to ground would be 350 ohms.
With so little information to go by and not knowing your level of expertise, this might be a good start. Hope this helps. By the way, the amp sounds great and has been educational in all its variations, enjoy.
Thank you, for the response to my question. First , expert? probably not it seems that the older you get you understand less of what you thought you already knew.
I have not connected the line stage with the amp yet .
As I was reviewing the project last night
I realized that I may have done some of the tuning with the -IN connection ungrounded so I will revisit the setup with it grounded.
Source has no DC component.
Tonight I will double check the resistances as you have called out with any luck they will be similar.
I am hoping to try the Balanced Line stage this weekend.
By the way the amp seems to be stable when it warms up this is of course with S-2 in the balanced position.
Thanks again for all of the information I will let you know the findings tomorrow.

A75 Oscillations when input unterminated

After many years of faithful service from my A75 amp I decided to install some unbalanced RCA inputs because I have built a pair of Aleph mono blocks and could now free the A75 up for others to enjoy. I wanted others who don't have balanced sources to be able to listen to this amp.
I previously posted that I hadn't run into any problems with this amp. While testing, after installing the RCA's, I discovered that the amp was oscillating when the input was unterminated and balanced to unbalanced switch was in the unbalanced position. If the switch is in the balanced position and no source then the amp is stable.
The frequency of oscillation is about 25KHz and goes rail to rail. If I run the front end board by itself it is fine. If I remove power from the outputs by pulling the fuses from the power supply the amp is fine. I put some pots across the 75K ohm input resistors and when I adjust to about 20K ohms the amp stopped oscillating. It appears that others have experienced this same problem and was wondering if someone might be able to shed some light on the subject. I'm content to always have a source connected to this amp but as a DIYer I would like to gain some knowledge in this area. Any knowledge would be much appreciated.

Mine did the same thing but at a lower freq. When I first heard it, it freaked me out. Just as you said, when the input was connected or grounded it was fine. I always suspected it had something to do with the different input setups but never really investigated it.:dead:
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