I have an opportunity to upgrade my current computer speakers, which are these anonymous Roland micro-monitors with dog knows what for drivers and amp to tickle them (plastic cases, too).
Plastic cases will stay in the form of huge PVC 90 degree elbows loaded with Tang Band W4-654S drivers (ported enclosure). I'm thinking of a baby Class A amp to drive them, an attenuated version of my L'Fake phony SIT amp. The preliminary schematic is shown below.
Since I spend so much time at my computer, it's time I had more than the current Roland audio-fart generator to accompany my internet journeys....
Plastic cases will stay in the form of huge PVC 90 degree elbows loaded with Tang Band W4-654S drivers (ported enclosure). I'm thinking of a baby Class A amp to drive them, an attenuated version of my L'Fake phony SIT amp. The preliminary schematic is shown below.
Since I spend so much time at my computer, it's time I had more than the current Roland audio-fart generator to accompany my internet journeys....
Attachments
Oh, and in the fine "Amp Camp" tradition, I'll be latching on to an appropriate 24V switching adapter to power the "Amp Cramp". I would advise folks to stay away from all the Apple power book clone adapters that swamp an Ebay search for an appropriate power supply - given the price, what's actually in that pristine plastic shell must hark back to a biblical quote about "whited sepulchres" - pure white exterior, total rottenness within.
Last edited:
That top half drive scheme has been around since 2007's "Half Nelson", that got some play at the very first Burning Amp. What made it perk up was revamping the input circuitry, which for some reason, started life as a follower. I'm afraid it was one of those creations that began life as a result of dragging a quad pad and pencil to the bog with me.
If it seems to you that I'm feeding you straight lines, that's probably so....
If it seems to you that I'm feeding you straight lines, that's probably so....
Always an education.
I think I know what is happening on the bottom half, but is that a current mirror on top made up of two differnt fet types?
I think I know what is happening on the bottom half, but is that a current mirror on top made up of two differnt fet types?
Some background reading showing the evolution of this amp (it evolved over a number of years):
http://www.diyaudio.com/forums/pass-labs/114167-half-nelson-amp.html
http://www.diyaudio.com/forums/pass-labs/220110-lfake-cheap-expedient-replace-elusive-sit.html
A variant that's pretty much all mirrors:
http://www.diyaudio.com/forums/pass-labs/191986-le-mutant-class.html
The Mutant currently resides in my living room, making pleasant noises. I'm laying out a version with higher supply voltages and doubled output devices.
http://www.diyaudio.com/forums/pass-labs/114167-half-nelson-amp.html
http://www.diyaudio.com/forums/pass-labs/220110-lfake-cheap-expedient-replace-elusive-sit.html
A variant that's pretty much all mirrors:
http://www.diyaudio.com/forums/pass-labs/191986-le-mutant-class.html
The Mutant currently resides in my living room, making pleasant noises. I'm laying out a version with higher supply voltages and doubled output devices.
In reply to previous questions:
The open loop gain of the "Amp Cramp" is about 150 (~43 dB). I got this by increasing the gain resistor from 475 ohms to 4.75 Meg and running the simulation with a small input voltage.
The current in the output stage is set by the input jfet, and is multiplied by the asymmetric current mirror at the top to get the final output current. I customarily put both halves of the mirror on the heat sink together for bias stability, though in my first shot at this topology, the "Half Nelson", the small half of the current mirror (the ZVP3306A) was located on the PCB instead of on the heat sink, and the bias current was still stable. As Papa will tell you, oversized heat sinks are a good thing...The bottom output driver is a buffered "fake SIT" (it looks like a triode, with low output impedance) and swallows whatever bias current is delivered by the topside p-channel fet.
The open loop gain of the "Amp Cramp" is about 150 (~43 dB). I got this by increasing the gain resistor from 475 ohms to 4.75 Meg and running the simulation with a small input voltage.
The current in the output stage is set by the input jfet, and is multiplied by the asymmetric current mirror at the top to get the final output current. I customarily put both halves of the mirror on the heat sink together for bias stability, though in my first shot at this topology, the "Half Nelson", the small half of the current mirror (the ZVP3306A) was located on the PCB instead of on the heat sink, and the bias current was still stable. As Papa will tell you, oversized heat sinks are a good thing...The bottom output driver is a buffered "fake SIT" (it looks like a triode, with low output impedance) and swallows whatever bias current is delivered by the topside p-channel fet.
This amp is finally getting built using perf board. The look will be rather utilitarian, as it will probably be hiding somewhere as it drives my computer speakers. One impetus for getting this one up and running is to galvanize a colleague at work to get off his butt and build something. I designed a headphone amp for him (OPA627 + mosfet diamond buffer) that he built and appreciates, so is appetite has been stimulated somewhat.
I've been using PCBWay to get cheap, but nice-looking boards for my various projects. One of the last orders I placed with them was for some little "Amp Cramp" boards. I spent this weekend building them and tuning them up. As per usual, reality was somewhat different from the simulation, so some resistor values needed to change. I may put a couple of pots in the final board to make the tuning-up process easier. One adjust would be for bias current, and the other for output voltage centering (before the output coupling cap). Some time next week they'll get an audition using a bench supply, as the bias levels look pretty stable. I look forward to hearing the little bugger...
Wrenchone, I have to say I always enjoy your threads. I may not post, but I read, and reread so many of your posts, and have learnt quite a bit as an end result. Much apprecated sir!
Actually, I was playing Bill Evans and Gonzalo Rubalcaba, but I like Pauline (R.I.P.) and Eno, too.
This weekend, the cramper will get fitted with a bias trim network so I can better home in on the proper bias level when the heat sinks have warmed up. I could do some thermal coupling of small signal mosfet to large signal output mosfet, but would prefer to avoid that level of obsessive fiddly attention.