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SOHA II Builders Thread - Page 40

post #586 of 1686
Hello all, I am new to this place. I am very interested in building the SOHA II. I have been calling and emailing Jeff at Glass Jar Audio and have no response from him. Any idea?
Keep up the great work!
Stan
post #587 of 1686
I was wondering if this would be a good second build after a Starving Student? Or should I do something simpler with perf board to get used to using a board?

Quote:
Originally Posted by ssoaos View Post
Hello all, I am new to this place. I am very interested in building the SOHA II. I have been calling and emailing Jeff at Glass Jar Audio and have no response from him. Any idea?
Keep up the great work!
Stan
I got a response within 24 hours using the contact page on his web site.
post #588 of 1686
keep a clear head, don't be afraid to ask and follow the start up / test directions on cavalli audio and it should all go smoothly..dB
post #589 of 1686
Quote:
Originally Posted by ssoaos View Post
Hello all, I am new to this place. I am very interested in building the SOHA II. I have been calling and emailing Jeff at Glass Jar Audio and have no response from him. Any idea?
Keep up the great work!
Stan

Great to here of your interest, and welcome! The SOHAII is a great amp, you won't be disappointed should you choose to build one.

Jeff can be a very busy guy - give him some time to respond, up to a week if he's really swamped.

-Bill
post #590 of 1686
Hi Bill and Scompton,
Thank you for your quick response. I am very interested in building it. Do you have tried it against the Little Dot MK III? I am debating between the two.

Thanks!
Stan
post #591 of 1686
I have a question about some of the small signal transistors. ie Q5...

I'm using the BC550Cs. Normally, I would trust the silkscreen on a board for the orientation of the part, but I'm confused. On the silkscreen, it shows the flat side of the transistor facing the rear of the board. This is also what it appears other builders did in their pictures. But being paranoid, I look at the data sheet. Based on the numbers labeled on the silkscreen vs the data sheet, it seems to me the flat part should face the front of the amp.

Am I missing something? I have socketed them all so it's easy enough to switch them around, but I am not going to power up the amp until I am sure.

Here is the data sheet Alex links on the site
http://www.cavalliaudio.com/soha%20ii/docs/BC550.pdf

Here is the data sheet from Mouser
http://www.onsemi.com/pub_link/Collateral/BC549B-D.PDF

They both show pin 1 on the left if the flat face of the transistor is towards you.

So do I trust the picture of the part on the silkscreen or the pin labels?

I'm going to look at the schematic to see if I can figure this out, but I think I must be looking at something wrong if nobody else has asked this question.

Edit:
OK I think I figured it out. It looks like the pin 1 on the silkscreen is the emitter, which is pin 3 on the transistor. So the shape on the silkscreen is correct, the pin labels are wrong.

I'm pretty confident I'm reading all this right, but I'm not going to apply any power until someone confirms my findings.
post #592 of 1686
The SOHA II PCB silkscreen and the data sheet pin labels are both correct.

Pin 1: collector
Pin 2: base
Pin 3: emitter

When recommended transistors are used (BC550/560), just place the transistor as depicted on the PCB silkscreen and you are fine.
This kind of things were pointed out and verified already.

In one of my build, I have used different type transistors (MPSA42/92) for the current mirror.
The placement of the mentioned transistors are 180 degrees rotated... but there is a good reason for that.
post #593 of 1686
Quote:
Originally Posted by Ferrari View Post
The SOHA II PCB silkscreen and the data sheet pin labels are both correct.

Pin 1: collector
Pin 2: base
Pin 3: emitter

When recommended transistors are used (BC550/560), just place the transistor as depicted on the PCB silkscreen and you are fine.
This kind of things were pointed out and verified already.

In one of my build, I have used different type transistors (MPSA42/92) for the current mirror.
The placement of the mentioned transistors are 180 degrees rotated... but there is a good reason for that.
Thanks Ferrari

I did use the silkscreen shape. I basically just double checked where the base, emmiter, collector were connected in the circuit. I actually powered it up before I read this, but I do appreciate the reply!

The amp has passed the first check
Well except TB+ was 120V instead of 103V, but I think that may be due to using the larger transformer? I'm going to study the circuit and make sure I know why that is before I move on.

I'll keep everyone posted. I should have went to bed awhile ago, but I'm too excited to hear this amp

Edit:
Search is my friend
Found Alex's post on this
http://www.head-fi.org/forums/5113583-post256.html

Time to move on, or should I sleep? hmmm.....

Edit2:
Didn't sleep, but I will now.
I haven't heard the amp yet. I couldn't pass the last test
My TPL and TP+L are about the same. Not a little off from 19V, it's less then 1V difference. One channel is pretty much identical. I have a zener in R4, so is that it? I can't tell right now. Thinking isn't my strong suit this tired lol. I'm gonna quit now before I start frying parts.

Good news is the tubes lit up fine and the relay is clicking. Wasn't brave enough to hookup the inputs yet.
I think I'm doing something wrong, so I should probably go to bed almost 5AM here.
post #594 of 1686
I couldn't figure out why I wasn't getting the 19V difference between TB+ and TP. I wondered if I have the heater voltages wrong, so I tried the other position. Upon turning on the amp, I started to smell something burning and saw a tiny amount of smoke, but I had already cut power about the time I saw the wisp of smoke.

I double checked the switch. It seemed to be in the right position, but I was still getting the same reading at TB+L and TPL (~60V). So my TB+L and TB+R are already a little low. The tubes were both lighting up though, so I went ahead and connected an input.

I only have sound out of the right channel

While I worked on my BOM, I thought I had a good handle on how this amp worked, but now that I'm having trouble, I'm really at a loss for where to start.


In regards to the transistors and the silkscreen. I still think the pin labels are wrong. According to the datasheet for the BC550's, with the flat side facing you, pin 1 is on the left. All the transistors on the board have pin 1 on the right of the flat side. I know Ferrari says it's correct, so what am I missing?


Update: It is a bad tube :x
Moved tubes and the problem switched channels. I got others, so I'll do more testing later. So while I'm glad I figured out the dead channel, I'm bummed about the bad tube. Now to figure out why my voltages are weird.

btw, the one channel does sound nice


Update2:
It's not a bad tube. I ordered 6 different pairs of tubes. I've tried 5 now, and in all of them, only one channel. I have figured out that there is some sound in the bad channel, but it's faint and distorted. In fact, the good channel is distorted as well.

What's really weird is that swapping the tubes makes the problem switch channels. When it follows the tube, makes me think the tube is the problem, but this behavior has me stumped.

I'm gonna walk away for awhile and hope someone has an idea what's going on.
post #595 of 1686
Quote:
Originally Posted by Hayduke View Post
I couldn't figure out why I wasn't getting the 19V difference between TB+ and TP. ...

I double checked the switch. It seemed to be in the right position, but I was still getting the same reading at TB+L and TPL (~60V). So my TB+L and TB+R are already a little low. The tubes were both lighting up though, so I went ahead and connected an input.
If you have less than 1V difference and are using a Zener at R4, is it possible that you installed it in the wrong way. Like a standard diode a Zener have about 0.6V forward voltage...

Set your DMM at DC position and what voltage you have directly across Zener (R4)?


Quote:
Originally Posted by Hayduke View Post
I wondered if I have the heater voltages wrong, so I tried the other position. Upon turning on the amp, I started to smell something burning and saw a tiny amount of smoke, but I had already cut power about the time I saw the wisp of smoke.
I wish you didn't smoke the tube heater filament.


Quote:
Originally Posted by Hayduke View Post
I only have sound out of the right channel

Update: It is a bad tube :x
Moved tubes and the problem switched channels. I got others, so I'll do more testing later. So while I'm glad I figured out the dead channel, I'm bummed about the bad tube. Now to figure out why my voltages are weird.

btw, the one channel does sound nice

Update2:
It's not a bad tube. I ordered 6 different pairs of tubes. I've tried 5 now, and in all of them, only one channel. I have figured out that there is some sound in the bad channel, but it's faint and distorted. In fact, the good channel is distorted as well.

What's really weird is that swapping the tubes makes the problem switch channels. When it follows the tube, makes me think the tube is the problem, but this behavior has me stumped.

I'm gonna walk away for awhile and hope someone has an idea what's going on.
It's difficult to follow. If you install a new set of tube, the left channel doesn't work. Right? And if you swap channels (using the same tube set) then the problem swap to the right and the left channel now works fine. Right?
post #596 of 1686
Hayduke,

Are you not getting ~19V between TB+ and TP on both channels? Or just the channel that is now out?
post #597 of 1686
Quote:
Originally Posted by wolf18t View Post
If you have less than 1V difference and are using a Zener at R4, is it possible that you installed it in the wrong way. Like a standard diode a Zener have about 0.6V forward voltage...

Set your DMM at DC position and what voltage you have directly across Zener (R4)?
good idea, I'll check that now and let you know

Quote:
Originally Posted by wolf18t View Post
I wish you didn't smoke the tube heater filament.
Smoke seemed to come from the back right.


Quote:
Originally Posted by wolf18t View Post
It's difficult to follow. If you install a new set of tube, the left channel doesn't work. Right? And if you swap channels (using the same tube set) then the problem swap to the right and the left channel now works fine. Right?
hmmmm I can't say 100% it was always the left to start, but maybe. The zener in R4 backwards certainly would explain that huh?

Hope I didn't just ruin a bunch of tubes.

Quote:
Originally Posted by wiatrob View Post
Hayduke,

Are you not getting ~19V between TB+ and TP on both channels? Or just the channel that is now out?
Both channels are within 1V of each other.
post #598 of 1686
Eureka! The zeners were backwards!

At first I was worried because I wasn't getting any voltage at all across them. I think I might have popped the fuse in the DMM during testing. I can't even get a reading across a battery now

I'll open it up and look later.
post #599 of 1686
Quote:
Originally Posted by Hayduke View Post
Eureka! The zeners were backwards!
Yay!

Alex, perhaps we should add the orientation of the Zener's to the tweaks section of the website?
post #600 of 1686
Thread Starter 
Good idea. I'll try to get to that later.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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