[FallenAngel]

Hey guys,

I'm thinking of building a SOHA. There is a current PCB by Digi01 at HeadWize, but I'm thinking of a bit different version. I want this one. That more or less leaves me to breadboard, which believe it or not, I actually enjoy somewhat more than PCB.

This being my first tube project and the fact that I'm not sure whether this is consider "high voltage" (read: able to fry my ass), I thought I'd ask for help before starting.

The parts list is really not that hard and I think I can grab all of these between (www.DigiKey.ca or canada.Newark.com) and www.PartsExpress.com, so here comes the big and scary part... I seriously don't understand some parts of this schematic.

I took a copy of the schmetic and circled the things that are totally confusing for me.



RED: I'm guessing this is the "J113 Cascode", uhm... do I need to match these in any way?

GREEN: How do I connect this tube

, sorry guys, first tube project, totally clueless.

BLUE: Aaahhh! Where are the inputs and outputs? I'm hoping I labelled them right, but also, where is the ground? Does this have a virtual ground?


Well, I think that's about it for now, thanks in advance. Oh and if you want to scream "STOP BEFORE YOU KILL YOURSELF!!!", as long as it's called for, it would definitly be appreciated.

 

[a1rocketpilot]

I am also about to build this as soon as the PCB's are made. Lets go in order:

The J113 cascode does not need to be matched if I recall. Also, why not use the LND150? It's performance is only very slightly less on paper and its has a smaller part count and it has a higher voltage limit, giving more headroom in case of accident.

The tube has 9 pins, numbered 1-9, so connect the tube socket accordingly so on the PCB. If I recall, digi01's board has the tube mounted on the opposite side from the components, so keep that in mind.

The purple things you circled are the inputs and outputs. The two RCA inputs are on the left side of the page and the headphone jack is in the approx. center.

As far as being high voltage goes, It is higher than many solid state designs, running at a B+ of about 60v. However this is considerably less than the normal voltages in tube amps, which is over 200v. In other words, this may sting you if you do something wrong, but it probably won't cause any real damage like real tube amps can. Hope this helps!

Aditya

 

[tomb]

FallenAngel,

I would highly recommend that you build Digi's version, first. If this is your first tube amp, you will have problem enough dealing with that. I am still confused, and I've been studying and participating in the design threads since almost the beginning of the SOHA Thread #1 (no longer in existence on Headwize).

A couple of things -
1. a1rocketpilot is correct: the LND150 MOSFET CCS is the superior choice. Being at least on the North American continent, you should have no trouble sourcing the MOSFET. The other CCS designs were developed because the LND150 is almost impossible to get overseas.
2. Digi is developing a plug-in DB for the next Group Buy (#2), but most importantly, Steinchen is working on a MOSFET buffer for Digi's SOHA board. If you are familiar with the Millett Hybrid and Steinchen's DB's, then 'nuff said. The Group Buy #2 is going on as we speak - ending on Sept 30th. I have four of the boards from the 1st Round, and their quality is about as good as anything from Amb or Tangent. Highly recommended.

Digi also has an Assembly Instructions website for the board:
SOHA V3

Also, a1rocketpilot was incorrect on one small thing - Digi's board does not have the tube reversed. Some people build the tube boards with the caps on the bottom. That way, the tube sticks out further, making it easy to replace (and emphasizing the tube). Boutique versions of the high voltage power caps can be as tall as the tube. If you put them all on the same side of the board, you can't even see it. The typical Pana FM's or Nich UPW's don't really have that problem.

 

[Blooze]

Quote:

Originally Posted by tomb I am still confused, and I've been studying and participating in the design threads since almost the beginning of the SOHA Thread #1 (no longer in existence on Headwize).

FallenAngel - here it is if you're interested.
http://headwize.com/ubb/showpage.php?fnum=3&tid=5896


Quote:

Originally Posted by tomb 2. Digi is developing a plug-in DB for the next Group Buy (#2), but most importantly, Steinchen is working on a MOSFET buffer for Digi's SOHA board. If you are familiar with the Millett Hybrid and Steinchen's DB's, then 'nuff said.

I would build the amp, either breadboard (not that hard if you look through the original thread and have done a bit of soldering) or on Digi's board if you're so inclined, using the LND150 if you can get it and just the spec'd parts. I wouldn't worry about trying to tweak the thing out before you've even heard the original version. Everyone always wants to take things to the limit it seems before they've even built or heard the original. I personally don't have a problem with that, but I like to hear the difference to see if the extra expenditure was really worth it. IMO you would be better off spending money on tube rolling than diamond buffers in the beginning.

Just be careful and follow safe practices. You would be wise to remember that you will be dealing with mains voltage coming into the PS which can surely drop you dead quick. It's always been my understanding that anything over 40V should be approached with care.

Good luck with your build!! You'll like this little amp.

 

[FallenAngel]

First of all, thanks for all the feedback.

Although I was pretty much hoping to do this on breadboard (more fun and I can use the diomond buffers), I was convinced (girlfriend really didn't want me to fry myself) to take the PCB route.

About the CSS choice, I'm trying to get everything I need from only one source and Newark has the J113, but neither DigiKey.ca nor Newark Canada has the LND150.

So now I have a few questions about this amp.

1) Transformer choice, I found a cheap transformer in a surplus store near by, its a 30VCT 2Amps, I know the the suggested was like 400mA, hope the extra power woun't do anything bad.

2) Caps... which positions would be most worth it to use boutique parts, I seriously fell in love with BlackGates since comparing them to the Nichicon UPW I had in my PINT, and they're only a few bucks more for the Standard series.

3) Enclosure... I've got a really nice wine case I've been dying to use for an amp. It's wooden and easily big enough to fit this thing with lots of room to spare. Would I be looking at any issues with things like grounding or the transformer causing a hum?

4) Tube socket, I'm thinking of having the tube socket on top of the case so I could switch the tubes without taking the amp apart and for simple appearance.

5) Where do I buy the PCB? There is a link to paypal straight from http://taihu.bjtzh.gov.cn/~digi01/soha-v3.0.htm is that where I can get it? Also, how long does it take to receive it?

Thanks again, I'm already looking forward to this amp.

 

[tomb]

SOHA Group Buy #2 on Headwize

 

[FallenAngel]

Thanks tomb, asked for 2

.

 

[dsavitsk]

Quote:

Originally Posted by FallenAngel Although I was pretty much hoping to do this on breadboard (more fun and I can use the diomond buffers), I was convinced (girlfriend really didn't want me to fry myself) to take the PCB route.

Just don't stick your fingers in the circuit while it is on, and if you need to make adjustments while powered on, do it with one hand behind your back. 60V is enough to give you a nasty shock, but it is not in the same realm of danger as 450V. Just be careful and go slow.



Quote:

1) Transformer choice, I found a cheap transformer in a surplus store near by, its a 30VCT 2Amps, I know the the suggested was like 400mA, hope the extra power woun't do anything bad.

Too big is not a problem. It is simply a measure of what the trafo can supply, not what it will supply. The circuit determins what is drawn.

Quote:

2) Caps... which positions would be most worth it to use boutique parts

The most important are the cathode bypass cap (1000/16) and the coupling cap (330n.) W/r/t/ the 1000uF cap, that is way bigger than necessary. That cap sees basically the plate resistance of the tube || to the cathode bias resistor in this amp to determine frequency rolloff, so I would bet you can lower that by a factor of 10 or more. 100uF will be fine. The bigger cap does no good in this spot.


However, especially if you are going to bread board this, buy cheap caps to get it working, and swap them out later.


Quote:

4) Tube socket, I'm thinking of having the tube socket on top of the case so I could switch the tubes without taking the amp apart and for simple appearance.

It is fine, but the rest of the circuit should be close with the connected resistors right on the pins.

 

[tomb]

Quote:

Originally Posted by FallenAngel Thanks tomb, asked for 2 .

Congrats! Be sure to read that link for Digi's "Walkthrough" and read the "SOHA - Thread 2" on the DIY section of Headwize - lots of good info there. Note that the BOM on Digi's Walkthrough is the correct one for his board. Not all the parts on the Headwize Library article are compatible. The Library article was for the P2P version.

To me, the power supply is the tricky part - much different than anything we use in Solid State or the Millett. I've finally settled on a DIY-walwart approach using two duplicate leads with 15VAC + Gnd (Center Tap) on each that will terminate into two identical walwart plugs for the case. That way, the separation distance will take care of any hum, so you can use your cheap transformer without needing a toroid.

By the way, where did you get the cheap transformer? 30VCT is not that common outside of a toroid.

 

[FallenAngel]

Tomb: I got the transformer from Sayal - www.sayal.com - the part number is "AM-TR-30T-2"

Thanks dsavitsk, I was definitly thinking of doing this on breadboard, but since I ordered 2 PCBs, I'll build up that version, and if I like it, I'll breadboard my own little clone.

I will be looking at this PCB schematic and will be ordering everything from http://canada.newark.com and www.partsconnexion.com

Parts Choices:

C2, C14 (dsavitsk: in my image, it's 330nF, in the PCB version it's 100nF): I've got a few of the 0.1uF and 0.47uF 715P (Orange Drops), so I'm thinking I'll throw those in there. - Newark

C1, C13 (the ones under the Tube connection on the left of the schematic): I'm thinking "1000uF BlackGate STD" at $4.75 or if dsavitsk is right, then I can easily go with 220uF STD at $2.95, but since I don't use these caps often, I usually just get the 470uF/16V STD for $3.75. - PartsConnexion

C3, C4, C5, C6 100uF/100V - If these aren't very important, I'm just going to find any 100uF/100V electrolytics. From manufacturers I recognize, I see Vishay BC Components "22 ASM" Series axial at $1.54.

C7, C8, C11 47uF/16V - Again, if these aren't very important, I'll just stick some Vishay BC Components "AML 138" Series axial at $1.14 - It would actually be cheaper to go with BlackGate PK (miniature) series here, the 47uF/25V is only $0.95 and the STD series is $1.85, probably worth it for only a few cents more.

C9, C10, C12 470uF/35V - The BOM is very strange on this one, it suggests 560uF for C12 and 1500uF for C9 & C10. I can either go with 470uF BC Components, but since these caps are basically the power supply rail caps, perhaps something better is in order?

AC_IN - Can somebody recommend a power jack. I'm guessing I'll need it fused, so what about 92N4150 from newark? Or maybe I'll just get the CONNEX-62491 from PartsConnexion

R4, R14 (TrimPots) - Hope this one fits 01F7709 from Newark

Pot - PartsConnexion sells the Alps "Blue Velvet" 10K and 100K, but no 50K, the 100K should work fine, right? Or should I get the 10K?


---

VERY IMPORTANT: Which Tubes do I buy? I saw quite a few in partsconnexion and I really don't know which ones to try.


Wow, that took a really long time! Thanks for looking, any recommendations would be recommended.

 

[dsavitsk]

Quote:

Originally Posted by FallenAngel C2, C14 (dsavitsk: in my image, it's 330nF, in the PCB version it's 100nF): I've got a few of the 0.1uF and 0.47uF 715P (Orange Drops), so I'm thinking I'll throw those in there. - Newark

Since you are ordering from parts connexion anyway, and since the PCB should eliminate errors, I'd pick up something for this part like a Jensen or a Mundorf or somethig snazzy. In this size they are pretty cheap and it will sound better than the 715. Even better, pick up some Vitamin Q's from ebay.

Quote:

C1, C13 (the ones under the Tube connection on the left of the schematic): I'm thinking "1000uF BlackGate STD" at $4.75 or if dsavitsk is right, then I can easily go with 220uF STD at $2.95, but since I don't use these caps often, I usually just get the 470uF/16V STD for $3.75. - PartsConnexion

I have found the standard series to sound a little flat in this position. You might prefer a Silmic from Digikey, or a BG N series. Also, after a lot of experimentation, I now officially disagree with Rick. Bypass it with a decent 0.1uF film cap (maybe not if you spring for the N series). I have been using either Vit Q's or some russian teflon caps for this and both work great.


Quote:

Pot - PartsConnexion sells the Alps "Blue Velvet" 10K and 100K, but no 50K, the 100K should work fine, right? Or should I get the 10K?

Tangent, Amb, and Percy sell the 50K. Percy has 50K Nobles. You can also get 50K PECs at Digikey that sound better than the Alps or the Noble. However, this amp has a ton of gain, so you need to find a pot that tracks really well at low volumes as you are not likely to turn it up much.


Quote:

VERY IMPORTANT: Which Tubes do I buy? I saw quite a few in partsconnexion and I really don't know which ones to try.

Someone else was asking about 12au7's, but you might not like the answer: http://www.diyaudio.com/forums/showt...threadid=87219

 

[FallenAngel]

C2, C14
Quote:

Originally Posted by dsavitsk Since you are ordering from parts connexion anyway, and since the PCB should eliminate errors, I'd pick up something for this part like a Jensen or a Mundorf or somethig snazzy. In this size they are pretty cheap and it will sound better than the 715. Even better, pick up some Vitamin Q's from ebay.

Mundorf MCap ZN 0.1uF go for $4.40, are they worth it here?
There are also Mundoff Supreme for $10 and Jensens for $11 - $16

Are these really worth it here, or would the MCap ZN's be fine? As for Vitamin Q's, I've found some 0.1uF and 0.22uF for around $5 each.

C1, C13
Quote:

Originally Posted by dsavitsk I have found the standard series to sound a little flat in this position. You might prefer a Silmic from Digikey, or a BG N series. Also, after a lot of experimentation, I now officially disagree with Rick. Bypass it with a decent 0.1uF film cap (maybe not if you spring for the N series). I have been using either Vit Q's or some russian teflon caps for this and both work great.

Never heard somebody saying BG Standards are flag, but ok. BG N Series 470uF/16V and 1000uF/25V are same price at $12.50, would it make that much of a difference between them and the STD, I'll be getting 4 of these and when it's $50 vs $15, I gotta ask.

Also, I can't find any Elna Silmic on DigiKey. - Ok, I just didn't know that Silmic II are RFS series, in which case, damn they're really cheap, maybe I'll go with some of these instead

Quote:

Originally Posted by dsavitsk Tangent, Amb, and Percy sell the 50K. Percy has 50K Nobles. You can also get 50K PECs at Digikey that sound better than the Alps or the Noble. However, this amp has a ton of gain, so you need to find a pot that tracks really well at low volumes as you are not likely to turn it up much.

Well, I was trying to only order from 2 places, but if I gotta get something from DigiKey, sure. If I just get the 100K from PartsConnexion, would it be worth it, or should I go with the 50K "PEC"

What's the part number for that?

Quote:

Originally Posted by dsavitsk Someone else was asking about 12au7's, but you might not like the answer: http://www.diyaudio.com/forums/showt...threadid=87219

[/QUOTE]
Damn, And PartsConnexion is having their 15% off everything sale for only 3 more days, gotta place the order quick. Guess I'll be reading quite a bit.

Ok, well that more or less settles it, I'll be ordering from DigiKey, Newark and PartsConnexion. Guess I'll be redoing the BOM for this one again.

 

[tomb]

C9 and C10 are the power caps that reside between the 12V rails feeding the heaters and the opamp. As such, you can pretty much view them as power caps similar to any SS amp - except even more juice is needed for the heaters. The OPA2134 used in the original design is limited to about 45ma per channel, so 470uF's used in the original design were OK. When the discussion turned to adding BUF634 or plopping a couple of OPA551's on a 2X1 adapter (200ma each!), then it was suggested that you could go as high as 3300uF each for C9 and C10. There is still a happy medium between response and capacitance, and Digi's BOM may reflect that.

C12 is the bootstrap cap that bumps up the 30V to 50-60V for B+. It does not follow the same strategy as C9 and C10.

For your trimmers - don't use the ones you've shown. A standard Bourns top adjust 3299W - like those used in the Millett or on the TREAD - works best.

As for the tubes, search through the thread on Headwize. There is an extensive review based on the original SOHA. Some great tubes (by all reports) are as follows:

NEW Production:
Electro-Harmonix 12AU7
JJ ECC82
EI 12AU7

NOS:
GE 5963
GE & RCA long grey plate 12AU7's

Some have also mentioned the Amperex Bugle Boy 12AU7, but that thing is really expensive. You can get all the ones I mentioned above for $8-$9 ea.

 

[steinchen]

Quote:

Originally Posted by FallenAngel C2, C14 Mundorf MCap ZN 0.1uF go for $4.40, are they worth it here? There are also Mundoff Supreme for $10 and Jensens for $11 - $16 Are these really worth it here, or would the MCap ZN's be fine? As for Vitamin Q's, I've found some 0.1uF and 0.22uF for around $5 each.

Mundorf MCapZN are excellent and a significant improvement over Wima MKP e.g. They are worth the investment here, it is the only cap that is directly in the signal path. The MCapZN is my personal favourite since it has a very good performance and is still affordable.

Quote:

Originally Posted by tomb 2. Digi is developing a plug-in DB for the next Group Buy (#2), but most importantly, Steinchen is working on a MOSFET buffer for Digi's SOHA board. If you are familiar with the Millett Hybrid and Steinchen's DB's, then 'nuff said.

not only a mosfet buffer, a diamond and a hybrid buffer too. None of these will be a direct drop in replacement, you'll have to air-wire the buffers to the SOHA board. Additionally you'll have to heatsink the regulators on the SOHA board due to the significantly higher current draw.

As for Digi's plug-in DBs I doubt that they work with the SOHA because of the unbalanced psu and the lack of an output coupling cap, but we'll see.

 

[tomb]

Quote:

Originally Posted by steinchen ... snip ...not only a mosfet buffer, a diamond and a hybrid buffer too. None of these will be a direct drop in replacement, you'll have to air-wire the buffers to the SOHA board. Additionally you'll have to heatsink the regulators on the SOHA board due to the significantly higher current draw.

I was kind of wondering - that MOSFET buffer photo you posted looked huge.

Think maybe the typical 25¢ stamped metal TO-220 heat sink will work on those regulators? There's no room otherwise.