Do you have DC on Pass B1 o/p

[kroyin]

DC on preamp output is not desirable specially when used with power amps like Pass F5
which do not have DC blocking caps.
I have 32mV on left out while right out is ok (it takes 15 mins to reach stable values).

Any suggestions pls

 

[doors666]

how much is there at the amp output, thats what matters. standard test is with input shorted.

 

[kroyin]

Below 20mV on both channels, so I'm good there

 

[baswamin]

Doesn't Pass B1 have a DC blocking capacitor on the output? I checked the schematic and looks like it does. Are you asking if you will still see some DC on the output?

I need to check on my Pass B1 output if there is any DC. I am assuming with the DC blocking capacitor, it should be minimal.

Thanks

 

[doors666]

<20mv. it doesnt get much better than that, does it

 

[kroyin]

Yup that is with i/p shorted with b1 connected one channel goes to 127mV, the other stays below 20...guess I'll have to check the B1

 

[2chFreak]

anything above 20mV is not acceptable, typically over time, it should settle to below 10mV.

must check to see suspecting semicon, swap & see again

 

[doors666]

anything above 20mV is not acceptable, typically over time, it should settle to below 10mV.

must check to see suspecting semicon, swap & see again

below 20mv is excellent. upto 100mv is acceptable. lots of people say that its not even worth debugging if its below 50

 

[omishra]

I could not measure any voltage on pass B1 output, it could not be there as it has DC blocking cap at output. For f5 output offset please short the input of F5 to the ground (center of rca to outer shell) and then measure it rather than input wires/RCA unterminated. If your JFETS and MOSFETs are not matched then this could happen.

 

[2chFreak]

reliable source, quality & matched semicon costs more but saves all the headaches in the end ..

OP is recommended to swap out the semicons for better quality & matched ones

 

[kroyin]

Om

B1 standalone (nothing connected) gives me 32mV on left out.
F5 standalone (i/p shorted o/p open) offset ~10mV on both

B1 connected to F5 (no source connected to B1, F5 output open) gives 88mV on left channel (F5 is voltage multiplier so any DC will be amplified).
Think I'll desolder the 11uF cap on left channel and check capacitance value with multimeter

 

[2chFreak]

Get a matched set (w/ low offsets) of Transistors and substitute the same on the B1. All your problems should get over.

Source your SK170 toshibas from a good, reputed source. 11uF cap is def not the prob

Unmatched transistors with poor offset on the Left channel is the culprit

 

[sachu888]

Get a matched set (w/ low offsets) of Transistors and substitute the same on the B1. All your problems should get over.

Source your SK170 toshibas from a good, reputed source. 11uF cap is def not the prob

Unmatched transistors with poor offset on the Left channel is the culprit

Toshiba SK170 are matched pairs bought from Spencer(Fet Audio).He is a reputed member at DIY audio.I have bought many Toshiba's matched Jfets from him and never had a problem.

Regards,
Sachin

 

[2chFreak]

yeah! Spencer is known to me but the OP continues to face high offset for ONE ch.

Should swap out the SKs and see if the problem eliminates. It should as there's nothing else in that ckt.

 

[kroyin]

Unfortunately I had ordered 4 matched sk170 from Spencer...would need to get more..meanwhile will swap the jfets across channels to see what happens....but this is a sudden issue ....with b1 connected to F5 I had measured 10mV across F5 before

 

[2chFreak]

Om

B1 standalone (nothing connected) gives me 32mV on left out.
F5 standalone (i/p shorted o/p open) offset ~10mV on both

B1 connected to F5 (no source connected to B1, F5 output open) gives 88mV on left channel (F5 is voltage multiplier so any DC will be amplified).

Move the SKs of the Left ch to the right & the right to the Left. And see if the high DC offset swings now to the RIGHT ch.

This way you will pinpoint bangon where the problem lies. If it does, replace the transistors of the high offset with another fresh set.

 

[kroyin]

Thanks to everyone ...found out the issue with DC offset on the left channel
.It was the 11uF Axon cap, though it measures well it the multimeter it has somehow lost its DC blocking capacity (switched L/R channels caps and offset switched to right)

Have ordered some K73-16 10uF 63V off eBay (I hold these Russian caps in very high regard after using K71-4 in the MYREF)

 

[sachu888]

Thanks to everyone ...found out the issue with DC offset on the left channel
.It was the 11uF Axon cap, though it measures well it the multimeter it has somehow lost its DC blocking capacity (switched L/R channels caps and offset switched to right)

Have ordered some K73-16 10uF 63V off eBay (I hold these Russian caps in very high regard after using K71-4 in the MYREF)

Hi,
One of my friend on Audiokarma had sent me K73-16 10uF 63V and 1 UF caps.I was very happy with the sound until yesterday I found loud thud on right channel.Omishra suggested me to measure Dc and it was 11.8MV on right output and none on left.I changed then to stock Axon caps and no DC,no thud.Few pics

By sachu888 at 2012-08-29

By sachu888 at 2012-08-29

Original post: Pass B1 BOM - Page 2 - AudioKarma.org Home Audio Stereo Discussion Forums

Regards,
Sachin

 

[kroyin]

Sachin 11.8V .... my F5 would have blown

 

[sachu888]

Sachin 11.8V .... my F5 would have blown

Yes hyeah:
It was 11.8 MV sorry for typo
Regards,
Sachin