edit: 17th April 2011. Just to set the record straight, although I refer to this chipamp as a "gainclone" strictly speaking it is not. A gainclone is I believe actually a clone of the 47 labs gaincard, however many people refer to chipamps as gainclones incorrectly (myself included).

edit: apologies to anyone viewing this at less than 1280 screen width... It formats properly with that (two images side by side) but I'm not going to go and redo all the images now!!! I already did twice....

Well I said about 4 years ago that I would do a build writeup for my P2P gainclone. I took all of the pictures but never got around to it. I'm off sick and lacking energy so decided it was as good a time as any to have a crack at it

I started out with the BrianGT schematic. I wanted to build something that people were using and was likely to just work. Here is a copy of the schematic
...

Well for me anyway I think I must be borderline obsessive compulsive! It happens every time I start a project (or even before I start it). I spend hours and hours and hours deliberating on whether this would be better or that would be better. Honestly in some ways I wish I'd never become "aware" of alternatives to the kits that were available from the local electronics shops Dick Smith and Jaycar

I got the jfets for my B1 a few weeks ago. I finally started matching them earlier this week, I hadn't started because I was obsessing over the fact I needed a 100 ohm precision resistor and a transistor socket. I hadn't got those because I wanted to order all the other bits as well and only pay for one lot of shipping. But do you think I could make a decision on what parts to get?

First off I obsessed over the resistors (yeah I know).. wanted vishay RN60D's but local sources are 5 - 10 times the price of mouser, problem with mouser is unless...

Its been really cold the last two days.. I was thinking whilst walking to the train this morning that I should have worn my winter coat! (Cold by Sydney standards anyway, The Northern Hemisphere people would laugh at my notion of cold)

The weather has unfortunately had a damping effect on the progress of my speakers too. First problem is that my workspace is the balcony, and it has been very windy and stormy, so I haven't risked doing any more veneering.... Second thing that worried me was that with the wet weather I might have troubles with splitting.

So Last weekend I placed an order for 100 2SK170BL's from Spencer. He sent them on Monday so hopefully will have them in about a week. The plan is to build a B1 buffer (ultimately I suspect there will be more than one), to finally do away with the abomination that passes for a preamp in my Integrated Series 200 amp.

The B1 will I think be the easy bit... I will probably build it on perfboard...

I woke up this morning, and immediately had a slightly uneasy feeling. There was a strange orange glow seeping through the gaps in the blinds! I tentatively pulled aside one of the vertical pannels and saw what appeared to be red fog. Thoughts of armorgedden, and the cover of Midnight Oil's "red sales in the sunset" came to mind. I was about to grab the camera when Mya decided it was time to wake up and have some milk. Best not to delay the milk supply for a 21 month old with a nasty cold, or armageddon might seem like a palletable alternative, so I made the milk and appeased the small one.

By the time I got the camera, the sky was more orange than red, took a snap and started to get ready for work.

It would have been sensible to take the good camera with me but I was not thinking along those lines as I hurried off to work. Driving to the train station was weird in itself, the headlights of oncoming cars had a strange blue tinge, and so did the...

I'm in a way glad that it is taking so long to finish these speakers after my four year break from them! If it had have taken a couple of weekends I would have kicked myself for not having done it sooner!

One thing that having had a four year break does is make it easier to slow down and do things properly, afterall, a month or two is relatively short in comparison

I got over two of the biggest fears with this project on the weekend. One was whether I was going to be able to do the veneering properly (this seems so far not to be an issue) and the other was whether my plans to put a 22mm chamfer on the front baffles was going to work (a test on some scrap MDF worked well). The main thread for the project is here with all the latest pictures

How to finish the veneer is as yet undecided, but I'll worry about that in a few weeks

The next big hurdle after finishing the boxes is going to be getting decent freq response measurements,...

I've long known that the boxes for my current three ways are leaky, more than likely incredibly leaky, but I've never bothered doing anything about it because I intended to build some new not leaky cabinets. Well I haven't and it is probably going to be a while before I do, so today I decided to check the impedance curve for the woofer in box. Thanks to Mick (prickears) for his quest to get to the bottom of his 80 Hz suckout I am finally doing something about my leaks!

The design goal (when I changed to vifa 10" drivers from the original 12" plessey's) was a box tuning freq of about 34Hz. When I measured the impedance today for one of the boxes, the tuning freq was in fact about 30Hz much lower than intended and when modeled with that tuning freq, has a very droopy response starting to roll of from about 220Hz, though the f3 is only 2HZ higher than the inteneded model at 36Hz..

So It looks like one of the diy tasks in the comming weeks will be attempting...

I'm finding DIY'ing a very different ball game now that I have started again. The difference being I have a 21 month old daughter now. Gone are the days when I could sit down for 10 hours straight working on something. I have to take on tasks that I can complete in 2 hours (or work on into the night and suffer sleep deprevation!)

Yesterday I did some more sanding on my speaker boxes, I think I could just keep sanding until the 25mm MDF was only 20mm I better stop soon and get on with the veneering.

Tonight I attacked the problem of the switching thump in the amp. I found some 47uF Nichicon FG caps in the cupboard and soldered them into the negative feedback circuit in the preamp. That brought the DC coming out of the gain stage in the preamp down from -1.1V to about -210 mv. I'm not sure if I will try and tackle getting rid of the rest of the dc, but it is a substantial improvement. The cap basically put between the second resistor in the feedback...

Well I drilled some holes and wired up the new RCA's on the back of my amp, so now I can switch between direct input (bypassing the preamp) and the normal integrated path. Works nicely and I suspect that the slightly soft top end was actually due to the preamp as the sound seems a bit crisper now on the direct input.

However.... when I switch from direct to via the preamp, or vice versa I get a big thump from the speakers I can avoid if I switch the speakers off but it is not particularly family friendly, especially since I have a 21 month old who will love to switch the switch on and off

So I'm not sure if it is switch contact bounce or perhaps just switch on thump (the fact that there is no thump at all when switching to the off position leads me to belive it is more like a switch on thump), but it is rather anoying. Puts a bit of a damper on an otherwise successful mod. The switch is a center off type, I might have to get a three pole...

Sanded down the second box so it is now in a state that is close to ready for veneering. Filled the screw holes with filler and also the bottom of the front baffle which was for some reason a mm or so short. can't remember if that was a measurement stuff up or a problem with the saw accuracy, probably the former I'd guess! Actually now that I think about it, it was an assumption that the mdf was really 25mm thick and it was slightly more... Only problem is the filler takes 48 hours to dry, so next weekend before I can start trying to slap on any veneer.

Also attacked the amp again. Plan was to put in the bypass of the preamp. Got most of the way there but was getting too late so left the drilling of the back pannel (for the rca sockets) for tomorrow. All in all it was a worthwhile excercise. The way the input caps were installed was nothing short of shocking, I think I did it to check it was ok, but never got around to doing it properly, I decided to use cat5e twisted...

Glued the rear baffle onto the second of my MTM's today. Had an OH <insert your choice of expletive here> moment just after I placed the baffle onto the speaker. I'd put it on inside out! Now I realise that might sound a bit weird, but because my binding posts are not quite long enough for 25MM MDF I had to route out a few mm to allow me to put the first of two screws on. Anyway luckily I realised pretty quickly and was able to pull it off wipe off the glue, put some more glue down and get it screwed down and clamped without any problems. Would have been a bit of a disaster If I hadn't noticed straight away.

Was planning on putting in the new RCA's to the amp today as well, as a way of bypassing the preamp, but just didn't have the time or the energy. Maybe next weekend

Tony.