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SOHA II Builders Thread - Page 112

post #1666 of 1678

Hi all,


It seems that I have solved my DC events, that triggered the e12 from time to time.


I tried to see what happened if using only one channel. So I removed the tube of one channel at a time. I tried all permutations of channel tube removed and tube. As a conclusion: one channel at a time was always working. But two channels at the same time generated those DC events. It could comme from a load problem on the power supplies.


So I went again through the initial setup sequence to check the voltages and idle currents. Not changed a lot, but had to adjust slightly the tail current of the input staage of one channel.


And now seems to work wothout DC events. Let's see over what happens the weeks...





post #1667 of 1678

I am working on building my SOHA II and I have the BOM at mouser. Problem is two of the transistors are no longer avaiable, the fairchild MPSA42RA and the Fairchild MPSA14_D74Z.  Any ideas on replacements for these, the MPSA14_D74Z is Q1p and q2p in the power supply and the MPSA42RA is q3e on the epsilon 12. thanks

post #1668 of 1678
Originally Posted by scottie4442 View Post

I am working on building my SOHA II and I have the BOM at mouser. Problem is two of the transistors are no longer avaiable, the fairchild MPSA42RA and the Fairchild MPSA14_D74Z.  Any ideas on replacements for these, the MPSA14_D74Z is Q1p and q2p in the power supply and the MPSA42RA is q3e on the epsilon 12. thanks


I think you've swapped the descriptions, the MPSA14 is part of the e12 and the MPSA42 are in the power supply


Try these


512-MPSA42_J18Z, you may need to bend the leads slightly, also EOL but they're still in stock now


863-MPSA14G, works perfectly in a build I recently did that cloned the simplified e12 in the SOHA II

post #1669 of 1678

thanks, I will add them to the BOM at mouser.  I am also going to try 610-MPSA14 and 512-MPSA42, this is what mouser says are substitues for the original.  I will try both of the replacements for each of these transistors and post on here what I findout

post #1670 of 1678

Running into problems with the initial setup/start-up and I can't find the cause. 


Amp has all the correct (very close) voltages before strapping jumpers J1 J2 and J3


TL+ = +15.14

TL- = -15.20

TH = -12.36

TB+ = 128.8


After strapping J1 and J2 I don't have any voltage on R10L or R10R and there is no current flowing across ether. I do hear the e12 relay click after a few seconds.


The buffers are installed and I have the correct +15v and -15v at the buffer (OPA134).


I am using the current Mouser BOM with no mods installed at this time. I have double checked for bad solder joints and did not find any, I have checked almost every point on the board with the DMM and getting readings that I would expect everywhere I have checked.


After a few hours on this, I have replaced Q5/Q8 (BC550C) and Q6/Q7 (BD139) and even ICL/ICR (OPA134) and I still have the same problems.


Anyone have any ideas what I might check next?


Thanks in advance for any help!

post #1671 of 1678
Hi, I'm midway through my build.

Can anyone give me advice on how to install Q6R/L and Q7R/L?

From data sheets it seems like the BD 139 part should be installed like in the standard ECB layout with the 3-dot side facing away from the heatsink,
BD139: http://www.mouser.com/Search/ProductDetail.aspx?qs=ljbEvF4DwONoQGCKnqafCQ%3D%3D
ECB layout diagram: http://cavalliaudio.com/diy/soha%20ii/images/SOHAIINormalOPMounting.png

Am I reading this all correctly?

I am pretty confident I have the ECB order correct, since there are plenty of data sheets available for the part in the TO-126 package. However, I don't know if I have to flip it around so the heatsink is mounted to one side or the other. I'm sure a bunch of you can easily answer this.
post #1672 of 1678

In the event that anyone else reads this post - I found the problem.


Continued checking all components and found that I had installed 4.99K resisters in R12L, R12R instead of 4.99Ω as needed. D'OH!


After correcting my mistake everything is now working as it should and the amp sounds great!

post #1673 of 1678

Crazy Sam,


If you are installing them on the top-side of the board - then yes - the printed side with the three dots will be facing away from the heatsink and you will use the mounting position towards the front of the amp (towards the tubes).

Edited by GT Den - 12/12/14 at 7:07am
post #1674 of 1678
Hi All,
I'm getting good start-up sequence voltages but no e12 trip. e12 trips without tlo81 installed, however.
If anyone is able to help I can give you more details, but in the event that this thread is pretty much dead in terms of people who could help troubleshoot I don't want to waist my time typing up a bunch of voltages. Thanks in advance if you see this and can help!
post #1675 of 1678

Hi Crazy Sam,


Have you looked at the voltage (DC) on R1E and R2E?  If the DC offset is high, the relay will not trigger.


If the relay is clicking without ICE (TL081) installed I would think that voltages to the relay are good from LV+E.


What voltage to you have on v- of the TL081? (Pin 4)



You and I might be the only ones reading this thread so I'll do my best to help were I can.

post #1676 of 1678
Hi GT Den,
During initial start up a few days back, I fried r10L with a short on Q7L. I think this fried some silicon, since the DC offset at R8L is ~12V while the R8R reading is nearly 0. If I disconnect the left side from the e12 by removing I think R1E, the relay clicks and the right side functions. I ordered new transistors from mouser and I will redo the left channel when I get a chance.

I hope this works - the right side as a mono preamp sounded very good.
post #1677 of 1678



I have finished (well, I thought so..) building my SOHA II.

I have done the setup and it makes music.

But I have a problem with that music...I have no bass...and not a lot of mid also, as I can hardly hear the male voices...

In the upper, everything is perfect....

Where can it come from ??? It sounds like I would have a high pass filter... 

I have check all the capacitors values and it seems OK to me.

Any idea where it could come from ?


I think I have to mention that I am not spot on all the values given in the setup, but from what I read it shouldn't have been a problem.

- TL+ and TL- ok at +15 and -15 V

- TH 12,96 instead of 12,6 V

- TB+ at 117V instead of 125V without the tubes (107V instead of 103V with the tubes)

- I had no problems to get the desired voltages with the different trimpots and had 60V at TB+L and TB+R during set up


After everything done and tubes installed, TB+L and TB+R are at 59,7 where it is said it should be at least 60V...

On one channel I have my 19V difference with TPL but only 15V for the other chanel. I guess it comes from the tubes that are not paired (but it was said it was not necessary).


So then, why no bass/mids ?


Thank you for your help !

post #1678 of 1678

HAHAHA !!!!! 

Working now !!!!

It was just my jack plug that was going too deep and so I guess the ground was linked to one of the channel ! By just pulling it a bit out, everything works perfect !

Thank you for my help ;-))

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