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60 WPC Amplifier for DIY Turntable Motor Drive
60 WPC Amplifier for DIY Turntable Motor Drive
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Old 12th April 2018, 05:34 PM   #291
Pyramid is offline Pyramid  United States
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With the SG4 at standby, there should be no AC output (the outputs will idle at 2.5VDC so make sure the input to the amp is AC coupled). If you are reading an AC voltage on either the SG4 output or the amp output, something may be oscillating. Easier to trouble shoot if you have a scope.

What are you using to power the amplifier? Could the noise be coming from there?
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Old 12th April 2018, 06:19 PM   #292
alexkosha is offline alexkosha  United States
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60 WPC Amplifier for DIY Turntable Motor Drive
Quote:
Originally Posted by Pyramid View Post
With the SG4 at standby, there should be no AC output (the outputs will idle at 2.5VDC so make sure the input to the amp is AC coupled). If you are reading an AC voltage on either the SG4 output or the amp output, something may be oscillating. Easier to trouble shoot if you have a scope.

What are you using to power the amplifier? Could the noise be coming from there?
Let me check with my scope and DVM tonight and update you. I use advised LP-40-600 transformer for TDA7293 amp. Will see its datasheet for signal coupling. Thank you a lot for direction.
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Old 13th April 2018, 07:34 PM   #293
alexkosha is offline alexkosha  United States
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60 WPC Amplifier for DIY Turntable Motor Drive
Thank you for troubleshooting tips.
I don’t see any abnormality during troubleshooting and mentioned by Pyramid setup is seems as it should be.
a. Amp is AC coupled and no DC is going through it. I see 1.5 VDC during STBY on input and “0” VCD on output.
b. AC going through the AMP and DC component of it is “0”.
c. Output signal looks clean
d. No GND loop.
I attached images with correspondent titles that might give you some info.
Hiss is still there and my last resort is to check output transformers connections for 12V parallel setup. Maybe I messed up right there…, but I checked that prior install.
Maybe I need to go back and recheck that.
Any other ideas?
Attached Images
File Type: jpg Amp_IN_DC_STBY.jpg (591.8 KB, 63 views)
File Type: jpg Amp_OUT_DC_STBY.jpg (456.5 KB, 63 views)
File Type: jpg Amp_OUT_AC.jpg (433.2 KB, 62 views)
File Type: jpg SG4_output_910mVAC.jpg (433.8 KB, 58 views)
File Type: jpg Final output 100VAC.jpg (543.0 KB, 57 views)
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Old 13th April 2018, 08:13 PM   #294
alexkosha is offline alexkosha  United States
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60 WPC Amplifier for DIY Turntable Motor Drive
Quote:
Originally Posted by Pyramid View Post
What are you using to power the amplifier? Could the noise be coming from there?
If trust my subjective ears, the noise is coming from 2 output transformers. Not from amps or correspondent feeding transformer. The pitch of noise is changing when I touch output transformers plastic enclosures.
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Old 15th April 2018, 12:33 AM   #295
alexkosha is offline alexkosha  United States
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60 WPC Amplifier for DIY Turntable Motor Drive
I fond out that my hiss is coming from 2 (simi-toroids) transformers. Pyramid, as always, was right. One is Stancor LB type 12v and set to feed SG4 via LM317 and second is Signal LP-40-600 24VA with 20-0-20 for for my amps. It is not loud and barely noticeable when my lid is closed. I can live with it, but please let me know if you have some knowledge about such hiss attenuations. My mains might contain some small DC and I’ll try to use DC blocker. What else to try?

Last edited by alexkosha; 15th April 2018 at 12:44 AM.
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