That's a good point. But I just find some sort of allure with the passive voltage gain of the transformer. Distortion worsens on the bottom end with the transformer but I only use my SissySIT above 80Hz. Currently I'm just really happy with the synergy I'm getting. And the buffer delete just turned the resolution up another click, without any drawbacks IMO.
I’m in complete agreement with your findings. Driving VAS transformer directly with preamplifier has advantages. However, low output impedance preamplifier like BA3, ACP+, DCG3 or similar is required.
Small reminder, if there is anyone not already aware of benefits:
https://www.diyaudio.com/community/threads/ludef.369990/post-6845895
Finally back to checking out any potential ground loops in my build.
I did find that disconnecting/reconnecting my transformer shield/static wire to the chassis makes no audible difference in the hum level. I would've expected there'd be at least a little bit more hum with this not connected, or is this meant to be only a safety ground?
I did find that disconnecting/reconnecting my transformer shield/static wire to the chassis makes no audible difference in the hum level. I would've expected there'd be at least a little bit more hum with this not connected, or is this meant to be only a safety ground?
Tried the donut in multiple configurations (vertical against plate as well as rotated).
Zero impact on hum level. Same with moving the mains cables to different positions.
So, there must be something going on with either my grounding config or my PSUs. The hum was in both channels pre-monoblocks, and also goes away immediately after powering off -- even tho the caps are still providing juice for a couple seconds afterwards. Maybe I need to revisit the PSUs.
Zero impact on hum level. Same with moving the mains cables to different positions.
So, there must be something going on with either my grounding config or my PSUs. The hum was in both channels pre-monoblocks, and also goes away immediately after powering off -- even tho the caps are still providing juice for a couple seconds afterwards. Maybe I need to revisit the PSUs.
I'll give that a shot as well when I go and try to re-orient the PSU 90 degrees, thanks for the suggestion @william2001!
No change to hum level when shorting RCA input to gnd.
Try taking the star ground off the power supply board. You could put it between the amplifier pcb and power supply board.
This is one of Andrew T's many valuable tips:
"The star should be located to minimise the LOOP AREA of the circuits that circulate current into and out of that star.
This requires the star to be on the PSU to AMP twisted triplet.
When you go to a stereo build you will find it is better to have the star very close to both channel amplifier PCBs.
In a mono block build the star ends up ON the Amp PCB."
Andrew T.
This is one of Andrew T's many valuable tips:
"The star should be located to minimise the LOOP AREA of the circuits that circulate current into and out of that star.
This requires the star to be on the PSU to AMP twisted triplet.
When you go to a stereo build you will find it is better to have the star very close to both channel amplifier PCBs.
In a mono block build the star ends up ON the Amp PCB."
Andrew T.
Have you measured ripple at the PS output and do you have efficient loudspeakers?
It looks like you have 33 mF capacitors and calculated output ripple with 33 mF/0.11R/33 mF and 1.8 A load is about 28 mV rms. If I am to believe LTSpice, amplifier has Sissy PSRR of 30 – 35 dB. That would leak 0.5 to 1 mV 120 Hz signal from rails to output, even with grounded input. With 90 dB/1W loudspeakers, 1 mV will produce about 20 dB sound level and that can be heard in conditions you describe.
Lot of assumptions and theory, but it's better to check before tearing apart complete amplifier or using sledge hammer for a corrective action.
Edit: Measuring or estimating weather hum is 60 or 120 Hz, gives indication do you deal with EMI from transformer or with PS ripple.
It looks like you have 33 mF capacitors and calculated output ripple with 33 mF/0.11R/33 mF and 1.8 A load is about 28 mV rms. If I am to believe LTSpice, amplifier has Sissy PSRR of 30 – 35 dB. That would leak 0.5 to 1 mV 120 Hz signal from rails to output, even with grounded input. With 90 dB/1W loudspeakers, 1 mV will produce about 20 dB sound level and that can be heard in conditions you describe.
Lot of assumptions and theory, but it's better to check before tearing apart complete amplifier or using sledge hammer for a corrective action.
Edit: Measuring or estimating weather hum is 60 or 120 Hz, gives indication do you deal with EMI from transformer or with PS ripple.
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First monoblock shown here just before being buttoned back up, and a shot of the back. Installed tombo's input stage (https://www.diyaudio.com/community/...t-and-similar-amplifiers.382345/#post-6922424) for Cinemag Tx direct drive. Verified correct operation of the IPS at my workplace where I have access to test equipment. Gain was exactly 4x, as expected. Haven't placed back into system yet, but should work great.
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Oh man... This is promising!
I went ahead and swapped in the SLB PSU and Neurochrome goodies, and from a quick test tonight I could not tell if the amp was on or off when pressing my ear to the woofer.
Now, to find time to work on the second channel. Mono is fun and all, but I don't own any mono records so..
I went ahead and swapped in the SLB PSU and Neurochrome goodies, and from a quick test tonight I could not tell if the amp was on or off when pressing my ear to the woofer.
Now, to find time to work on the second channel. Mono is fun and all, but I don't own any mono records so..