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Discrete Op Amp PCB Build

Posted 25th May 2017 at 10:18 PM by googlyone

Well I am home again, and the PCBs for the discrete op amp beat me home.

I used PCBcart, who have a very competitive prototype PCB service -it is certainly not worth your time to make fiddly double sided boards like these at home!

They came out like this...

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And no, I don't have a finger the size of a baguette...

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and thru the microscope which I had to use to solder the bits on:

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There are parts on both sides of these PCBs.

A this point I am yet to mount the miller cap, as I want to play with this a bit. On the schematics there is a "guaranteed to work" value. I want to measure the phase margin and stability into some odd loads - as overcooking this will directly affect...
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Superlux HD668B SE classA amp

Posted 23rd May 2017 at 09:58 AM by abraxalito
Updated 27th May 2017 at 08:10 AM by abraxalito

My Superlux HD668Bs are the best value purchase I've ever made in headphone kit, far and away the best bang for the buck at their very modest price-point. If any pair of cans is worth designing a specific amp for, they are most certainly on the short-list. The amp I designed specifically for my AT MSR-7s doesn't have enough ooomph for these as the impedance is appreciably higher and the efficiency lower.

So here is my attempt at a dedicated HD668B amp with no corners cut in SQ while still being cheaper to buy the parts for than the cans themselves. I've designed it to run from a single lithium ion cell - a Panasonic 18650 would work fine and give around 60hrs of listening time. Its a single-ended classA topology driven by a single current-source loaded MOSFET. And naturally enough it sounds like no amplifier at all - a straight wire without gain, seeing as its just a buffer.

In actual fact it certainly does have gain, but only current gain. And no shortage of...
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JFET Amplifer and Filter Project

Posted 17th May 2017 at 10:37 PM by SyncTronX
Updated 17th May 2017 at 11:36 PM by SyncTronX (add images)

JFET Amplifier and Filter Project.


A lab was developed with something a little bit more hands on for participants than just blindly doing the NIDA labs. We would benefit from making an N-Channel JFET amplifier and asked if we would be interested. Of course, we werent told the JFET type or gain and didnt know wed also include a bandpass filter. These parameters wed have a drawing from which to choose. Finally, wed have to document and report the entire process in APA format.

Each project is unique and provided on two stapled pages placed upside down in a mixed pile at the front of the room. Each participant was asked to come up and draw their project. I drew project labeled E on the upper right corner of the second page. Included were the following: JFET 2N5485 or 2N5458, Gain set to 22, the Bandpass to be 15kHz 30kHz, and Total Circuit DC Power as Calculated/Measured.
Other resources were...
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Discrete OpAmp - Replaces Dual Channel DIP8 - design and PCB

Posted 11th May 2017 at 06:28 PM by googlyone

Well here I am travelling again, and this time stuck for an extra week away from home. The design bug has bit, and this time I tried keeping things small rather than crazy.

I have built a number of discrete operational amplifiers in the past. Usually integrating everything onto the board so that they wind up being preamplifiers rather than operational amplifiers.

If that confuses you - the difference is that an operational amplifier needs to be plugged into a circuit to be useful, a preamp is a standalone PCB.

The inspiration was seeing a burson amp on ebay. I looked and thought to myself "I keep starting that, and keep ending up with a preamp". Then I thought "Do it right for once".

The resultant goals:
- Dual channel op amp
- DIP8 pinout
- Good perfromance
- Class A (vs class AB which many are)
- Minimum size, whilst
- Sticking to 0805 SMD resistors caps, and...
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File Type: pdf Final Artwork CH2.pdf (22.9 KB, 26 views)
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Tweeter passive XO based on a transformer

Posted 1st May 2017 at 01:04 PM by abraxalito

As avid readers of my blog over the years may well know, I've modded quite a few active speakers. Mainly Swan/Hivi D1010s and D1080s of various generations along with some 3Nods. The entry level Swans are super-cheap on Taobao nowadays with pairs of older generation D1010s going for about 375rmb.

As a result of all these mods and my learning by doing I haven't got a single pair of actives in working condition, primarily because the mods I currently want to do (including fitting transformers inside and swapping out the classAB chipamps on bass/mid to replace with classD) won't fit in the available space. All the amp and active XO parts have been hacked about almost beyond recognition and are of very dubious reliability. So I have come up with a plan - I've thrown all the electronics boards away and will convert the carcasses into passive speakers. Since I also prefer to feed speakers via step-down transformers for better dynamics, I wanted to explore the possibilities of using...
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Sapphire 4 headphone amplifier [Rev. 4.1m]

Posted 24th April 2017 at 01:02 AM by rjm (RJM Audio Blog)
Updated 29th April 2017 at 11:54 PM by rjm

Development thread starts here.

While I consider this my best work to date, the documentation trail has become hopelessly scattered so let me try to regroup in this blog post and make some sort of gateway/portal to the project. [Update: web page write up now finished.]

The followup to the Sapphire 3 started out as a generic musing on stacked diamond buffers and current mirror amplification, coalesced for a bit as the ill-fated Project Unity before becoming part of the Sapphire line initially as a temporary measure as something I could drop in my existing chassis for testing. The first iteration kept the open loop buffer of previous Sapphire iterations, but now I'm running the 4.1m boards closed loop I realize I've ended up at just the classic, dictionary definition current feedback amplifier. Sigh. So much for originality. It sounds great though, so I'm not as upset about it as I might have been.


I'm being loose with the...
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Now For Something Completely Different - In a Neat Little Box!

Posted 22nd April 2017 at 04:30 AM by googlyone
Updated 22nd April 2017 at 04:40 AM by googlyone

After a few months of playing with loudspeaker drivers and software fixes to code that I wished I had never written in the first place, the lure of the BJT loomed large again...

There I was, sitting at my desk, and pondering where these dark feelings were coming from and how to scratch that evil itch, and my eyes came to rest upon that most ludicrous of things - the amplifier of 100 transistors.

About a year ago, or so it seems to me sitting here, I built this homage to the BC549, or as the engineer in me says, "Laughably stupid thing, which probably shouldn't work. Who would be dumb enough to try and find out?".

It needed a case. It needed a power supply. It deserved each of the above that would allow it to work, and ideally take an equally "left field" approach. it should never have been built, but it had. It should have been put in the back of a very dark, locked cupboard, but it had not.

The idea then...
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CrystalFET 2 MM passive phono preamplifier with JFET amplification

Posted 21st April 2017 at 01:05 AM by rjm (RJM Audio Blog)
Updated 21st April 2017 at 04:44 AM by rjm (revised to 2.0g (capacitors C9 and C10 misslabelled))

In hindsight the original CrystalFET suffered from overreach. To obtain the 55 dB midband gain required for an MC phono stage lots of other things I might have wanted to build into the design like degenerative feedback were discarded.

Degenerative feedback as it applies to JFET amplifiers is the practice of leaving the source resistor partially or completely unbypassed. It reduces the voltage gain, but linearizes the dynamic operation of the amplifier - coercing the gain to the ratio of the source and drain resistors rather than leaving it defined by the JFET transconductance. The circuit gain can then be fixed by the designer without worrying so much about matching devices.

The CrystalFET circuit is a traditional two stage passive photo preamplifier design, the only novelty here is the addition of a second JFET configured as a source follower to each amplifier stage. To obtain above 50 dB gain no degenerative feedback could be used, and the JFETs had to be not...
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Group Buy for Salas Boards

Posted 17th April 2017 at 06:05 PM by Tea-Bag

DCG-3 preamplifier and DCSTB DC Power stabilizer purpose built for it

The boards are going to be in Matte black, like the PS on the right

Click the image to open in full size.

Threads for boards:

Building Thread

GB Thread

DCSTB Board $20
DCSTB Board $25
Transistor Kits for the DCG-3 and the DCSTB are available. Full kits are also available.
DCG-3 Transistor Kit $ 40 - includes matched transistors where needed.
DCG-3 Full Kit (includes transistor kit) $75


Note that DCSTB kits only cover half the board. Additionally, if you wish to run at a higher bias and...
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Hafler 9505 power supply capacitors for sale. IMPOSSIBLE to find!!!

Posted 11th April 2017 at 10:38 PM by wmeyer
Updated 14th April 2017 at 10:59 AM by wmeyer (forgot the caps value though members would look up supplied part #)

Would anyone be interested in purchasing Hafler 9505 PS capacitors? I am looking into acquiring brand new caps not NOS. I'm not sure what I will be charging but if I was to make a guess I would say about $100.00 each. The 9505 uses 4 of these. You can also use these in other projects as well. The small size makes it perfect for a low profile amplifier. These are very hard to find. Here is the part number, DCMX203U100CJ2B Please let me know if this works for anyone out in the diy world, Please chime in. The caps are 20,000uF 100vdc 3.635"tall x 2.5" dia with lead spacing of 1.13".
These are a direct drop in for the Hafler 9505 TransNova amplifier or this number also. i am waiting for a reply. 3186FD203T100BPA1
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