Ok, I've been very slow on the diy stuff and not had a lot of time to post as much as I used to
Anyway, I decided to evict a Naim pre-amp from its case and start transfering the SC classA in there, the transformer is external
The internal cable is crap so I'll be replacing that as soon as I can, its mainly used as temp I also intend on using this mainly as a active pre-amp as well as a HA
Another thing I want to try later is the offset corrector , once this is fitted I can remove the 220uf on the LTP output. I currently use no input coupling caps, even with the cap less Buffalo/IVY offset is less than 5mV both channels
LTP transistors are matched BC560C with AK balancing mod on the collectors
The vol pot is a 50k Alps blue wired up as a shunt using 47k resistors, I'll probably swap the pot to a an Attenuator when I can
Supplies are +/15v, you'll notice a JLH in there too, Bias for each channel is 100mA Current limiter transistor are now Toshiba's
Updates will be added when I can, I'm sure Alex will add various bits and pieces too
Last Edit: Jun 2, 2013 9:53:49 GMT by Deleted: Silicon Chip did not publish a Class A HA/ preamp design.
I'll try not to spooge all over this post, but I am massively interested in this amp. I printed this (below) yesterday, and hope to have an etching session tomorrow.
As you can see, I've filled in the fuse holder tracks, but as yet have not implemented the thermal balancing, but I'm sure that will come.
The changes Alex has made make this very intriguing.
Also interested in how you find the dc offset servos, as I think that going capless is a pretty good idea.
If you have the camera out, I'd really appreciate a good, clear close up of the board. It'll help for sanity checks come stuffing time.
If any 'etchers' out there want a copy of the artwork, and it's OK with you Alex, I'll pass them on, so PM me.
Will Nice work. I know a member in Austria who would LOVE to get his hands on a couple of those PCBs. Perhaps he could be persuaded to try and get some more of those lovely dual transistors, which would greatly simplify matters, in return for a couple of PCBs? Unfortunately, he seems to be the only one who has had any luck obtaining them so far.
Quote: are the dual transistors the ones you have on your board
Will I can't remember for sure, but Leo may have the 2N3811A in his 15W Class A amp, that was the original device that the new recommended PNP dual transistor is an equivalent of. The dual transistors that we have been using recently are devices that have been selected for close matching of HFE and VBE, then thermally bonded. This makes this particular build unsuitable for novices. Friedrich has been able to obtain samples of the new dual devices that are suitable, but so far ,Leo, Dave and myself have been unable to obtain them from the device distributors. The dual can devices that we have been trying to obtain,which would simplify construction, are the Linear Integrated Systems LS 313 (NPN) and the LS352 (PNP) Alex
are the dual transistors the ones you have on your board
Will I can't remember for sure, but Leo may have the 2N3811A in his 15W Class A amp, that was the original device that the new recommended PNP dual transistor is an equivalent of. The dual transistors that we have been using recently are devices that have been selected for close matching of HFE and VBE, then thermally bonded. This makes this particular build unsuitable for novices. Friedrich has been able to obtain samples of the new dual devices that are suitable, but so far ,Leo, Dave and myself have been unable to obtain them from the device distributors. Alex
In lieu of the LIS LS313/LS352, I've bought a shed-load (technical term) of BC560C to match (hfe/vbe), put I'll also need to get an equivalent load of MPSA18, to increase chances of matching.
I know you guys have looked for the LIS devices, but I'll have ago, as well. You never know!
Will The selected and matched devices perform very well indeed,and the MPSA18 are only a few cents each if you buy a couple of hundred from Jameco. Friedrich obtained his LS devices from the Austrian distributor, but they won't export them. (That is assuming they still have stocks.) Alex
P.S. Anybody wanting a copy of the schematic should PM Leo or myself. I think that for the moment, it would best be restricted to RG members only.
Dave It is basically a simplified version of mine, with Leo's individual touches,but the same SQ. Leo can't afford a DACT2 attenuator though. Come to think of it , neither can I any more. I expect that Leo will be swapping the Alps for an attenuator shortly, which is likely to lift the quality a little higher again, compared to the SC HA with the standard LM4562. Alex
Those DACT2's are seriously pricey aren't they ... the Rolex of attenuators. Probably be running smooth long after our el-cheapo metal film one's have give up the ghost.
How close does the LM'd SCHA come to this class A in SQ?
Dave I prefer to leave that answer to Leo. Mainly for impartiality, and secondly, that it's a hassle to move things back and forwards between the PC and the lounge room setup. As it is, I have to move the X-DAC V3 and it's outboard dual regulated PSU from room to room when I want to change from PC to lounge room and speakers. Best results are obtained via the HDD and X-DAC V3 at the P.C. using Creative's Media Source Player. The hard part is to disconnect all the inputs to the Class A preamp, and locating it and the combined PSU in it's 2U rack case,( for both my 15W/Ch Class A amp and the Class A preamp) at the PC.in my bedroom. However, a while back I tried the "Jaycar " as a preamp, and although it sounded quite similar tonally, the Class A preamp was much more 3 dimensional on speakers. Alex