In very simplistic terms, is this how the LTP thermal modification is implemented?
Viewed from top of board. Spacing, etc, is a different matter. Once implemented, I'll update the artwork for the board.
Will Yes, but with a 1N5819. (just to show I did look!) Don't forget that the MPSA18 have different lead out to the original devices too. Start off with the trimpot set at around 100 ohms initially. Alex
Connected up to a PSU, and switched on (from the far end of the room, using an extension cable, like a true coward)
No pop. No magic smoke letting. Looking good.
Set Bias to 100ma. Check.
Measure DC offset - 4.96VDC, on each board
Cue an hour of frotting about, only to find I reversed a BC560C. Removed and replaced on both boards.
Offset down to 20mV. Much better.
I should explain that at this stage, I've used BC560 and MPSA18, but not matched them, as once I have these boards working, I'll be putting in the LS313/352 I have. I also have not yet implemented the LTP mod or the DC offset corrector.
Connect up my alien dac and stunt headphones.
oooh. Nice. For three hours.
I'll leave further impressions until I've put in the LTP mod/DC offset corrector and JLH, but it is very, very nice. Even wrung a soundstage out of my Philips stunt-phones.
If you are undecided about this amp, I really would suggest going for it.
Reposting this here, from the LS group buy thread, just in case people miss it.
Hopefully it'll help you decide to go for building this amp, if you are feeling that it is out of your reach.
It really is a case that If I can do it (with the help of the circuit diagram, pretty pictures and a couple of E-mails) then anyone who made the SCHA can. I'm more of a board stuffer than one of these ninjas that now which way around a cap goes, due to the closeness of the nearest transistor etc, but I am learning.
I've put up a few diagrams in the thread, that I created during my build, that have been now proven to be correct (ie my amp works!) that really should help.
The only extra 'technical bits' over the SCHA are probably the transistor matching (which is straight forward with a DMM or the circuit you (cjarchez) posted) and the board etching.
I'll be trying a method of board etching this week that Jon Clancy has tried. It is not very technical at all. A bit of etch fluid and two plastic bags! No real investment in pieces of equipment at all.
I have a post half written detailing how I etch boards. I'll post this soon, so perhaps it may encourage some to have a go, and hopefully help people make the SC Class A. If all else fails, I sure I can be persuaded to help with etching boards.. (oh jeez, did I just post that?!)
If anyone else feels that this may be a bit too technical, please post to your questions. I'm sure that they can be answered to help people feel that they can build this amp.
Will To save people getting confused, this isn't the 15W Class A amplifier, it is the preamp/headphone amp version, which was based around the original SiliconChip 15W Class A amplifier, and uses the same PCBs, but lower power output devices and a few other changes. The LS devices as mentioned in the GB thread, may be used in both the preamp and 15W Class A amplifiers. Alex
P.S. Just a reminder what Leo's rather nice build looked like. The toroidal transformer was in it's own small case.
I've also installed the LS313/352 that I had, and they make the whole business of close matching the BC560C/MPSA18 so much easier - i.e. you dont have to!! Plenty of space left in the Group Buy, by the way ;D
The trimpot is an in-line version, same as the bias trimpot. I used the Bourns 3296W for both posistions (Rapid 68-0310)
I'll post pictures of the modification sometime soon.
I've implemented the LTP thermal mod, and can say that it is a definite improvement, so to that end, I've modified the original artwork for the PCB, and you can download it from here:
Will Are you sure ? Some people still think that I imagine things that I claim to hear ! O.K. I should not have said that ! Nice work Will, did you have any real problem fitting the dual devices due to their different pin out ? Alex
Minor update to the layout. It now includes testpoints for the LTP thermal modification. Use them direct or with flying leads, they'll make setting the 0mv deviation easier to perform when the board is cased up.
I've had a really good listen, and the amp is a wonder. I've jumped at some of the moments in the music, as the transients are stunning in speed. Music is presented in an authorative, liquid style which is very easy to listen too. I need to etch some JLH psrr's for this, but I'm a little scared about how good this amp will be after! (knowing how it transformed the SCHA)
Another thing. If you have AKG K701, you owe it to yourself to make this amp. It drives them beautifully, there seems to be a nice synergy going on.
Next month, Modushop.biz will be hearing from me....
Hmmm, I can see this escalating if we're not careful. Especially as No2 Son has a big yellow lego digger, which blows that crane away....
Anyway, a number of people have contacted me about etching boards for this headphone amp. Individually, I'd have to account for etch and delivery, but if I can get all the interested people out their to have boards in one go, It should bring costs down, for everyone.
So, here it is. Please PM me if you would like a set of the main amp boards and the offset correctors. I would suggest that the PSU board could be bought from Jaycar, and modified as per the SCHA. This makes a lot of sense especially if you live in Australia. Once I've proved Will's Dual Reg Board, we could throw that into the equation. As for the JLH PSRR, well, I'm sure Jon Clancy would love to hear from you!
The big proviso is that I will supply the etched boards, and you will do the drilling. That's the bit that really takes up my time, and If I don't have to do that, then I'm happy etching boards for you guys (and maybe gals, perhaps).
I'll leave this offer open for a couple of weeks, and see how things lie. I'll then contact everyone involved, and we'll from there.