Mrboo's Diy Preamp

[markiemrboo]

Just thought I would report progress, went silent for a while as expected

 

While constructing I ran in to quite an annoying problem. Before constructing, I ran a simulator to simulate the power supply. I was hoping to fit the old ( or not so old, I rebuilt it with new PCB's while I was at it ) power amp stuff and the preamp stuff in the same case, and come up with an 'integrated amp', and the only way I would be able to do this is if I ran all three regulated supplies off of the same transformer (the big 500VA thing).

 

The simulation seemed to pick up on some error in the circuit layout. Upon trying to explain my problem to diyaudio, I was basically told..... no, there's not a problem, what you want to do will work. It ended in a bit of an arguement and all that, but eventually some kind soul came along, read my thread and saved me by seeing where I was coming from and explained what was wrong.

 

So, now I have a possible solution (I say possible, because I simulated this solution before it was suggested and it didn't work either, but he sounds sure it will work etc) I have rebuilt it all yet again, but lack a single bridge rectifier to get it finished. I ordered one this morning, so with any luck it will arrive tomorrow and I can finish the wiring, turn it on, and hope the problem is fixed. If so, all that's left is to redo the RCA input wiring, test for hum / buzz (and if there is any, fix it), and... then it should be finished!

 

Then I think I will redo the case. I can redo that while I am using the completed amp though, then just move the internals over afterwards

 

 

Here's a picture of the current mess. Looks a lot more... uhh... full... than before...!

 

The missing square near the centre of the picture is where the remaining bridge rectifier belongs.

 

[markiemrboo]

Another small update is in order I guess

 

Starting off, I have totally changed my mind about the whole 2U case thing :| I'm gonna send the new one back to Maplin. If I manage to get an RMA number tomorrow and get it sent off it should still be under 14 days, so full refund minus postage.

 

The separate bridge idea still didn't work, so the preamp is going to be using that diddly little transformer. I'm going to put the preamp in a little ABS box from Maplin http://www.maplin.co.uk/Module.aspx?Module...ES&doy=25m1 one of those, MB6. It's pretty small, which I like, and working with plastic is IMO so much easier than metal.

 

I think I am going to have the power amp in two separate boxes with two separate PSU's aswell, this means I can properly have one regulated supply per channel! Again, plastic, so it should be nice and simple to cut. Slightly bigger boxes http://www.maplin.co.uk/Module.aspx?Module...ES&doy=25m1, but still pretty small really. This does probably mean I will have to try and sell my 500VA toroid and get two smaller ones (thinking 160-250VA), but oh well. I'm a fool who likes to waste money obviously The two power amps could then actually sit on the left and right speaker individually, which would mean I only have the little preamp box on my desk for volume control. I like this idea!

 

At the moment I have one regulated supply for both channels, and the preamp on it's own little transformer. It all fit in to the 2U case fairly easy, but it is a bit of a wiring mess and I am liking the idea of separate cases now *cries*. Still, gives me something to do.

 

Anyway, I have tested it and the preamp works fine! Played music through a "dual cone" test speaker and I was getting no low frequency humming, but I was getting a buzzy clicky sort of noise through the tweeter (I think, hard to tell with this dual cone P.O.S). This is just a grounding issue and easily sorted. The input section of the power amps is joined with the grounds of the power ground. I expect it's probably coming from here, so I just have to cut a trace and run a wire basically Not bad at all though. The previous PCB's had some low frequency hum with all the grounds connected on the PCB so it's definately an improvement!

 

The preamp box should be here Saturday(it's rare I have got things on a Saturday, but it has happened???) or Monday. I'll probably get that all sorted the same day it arrives, so I will be taking pictures of that soon I should think. The power amp... well, given that I now want it in separate boxes that might take a bit longer. I might even be able to fit the 500VA in there, so I might be able to get one done sooner, but otherwise possibly the end of next week I think...

[markiemrboo]

Well, the preamp case arrived already! So here's some progress. I'm really quite liking it so far. It's lovely and small, and despite being ABS plasticy VERY sturdy

 

I am not sure what I am to do with the mains safety earth really though, as it's not a metal case. I'll figure it out I guess. I think all I have left is to drill RCA holes and a bit of soldering / crimping (aka bashing things with a hammer)...

 

First thing I did was fit the mains power plug.

 

 

Next up, feet! Salvaged from an old broken 'public address' amp. Will probably try and make some outta wood or something if I can be bothered

 

 

Then comes the power switch.

 

 

After sorting out the insides and mounting everything, I drilled a hole for the volume control. To the right of this will be another one, same knob, for source selection. I don't have enough RCA sockets for anything but a single input and preamp output at the minute, so I have left it for now.

 

 

A shot of insides. It's... pretty much a perfect fit....

 

Edited January 26, 2007 by markiemrboo

[hardnrg]

in true "you've missed a bit" style, i thought i'd point out that the power switch appears to be upside down

 

i dunno what you could do about the earth either, but all my technics separates have the figure eight "cassette/radio" mains connector, so somehow they do without... i guess if you really wanted to use it, you could make some sort of faraday cage over the A/C section... i do wonder how having unshielded A/C lines right next to the audio circuitry would affect the signal, but it looks like you've sorted all that out earlier :thumbs-up:

[markiemrboo]

in true "you've missed a bit" style, i thought i'd point out that the power switch appears to be upside down

 

Just looked at my light switch, and you're right... push down for on, and up for off. For some reason when I was fitting the switch I decided I liked it push up for on and push down for off. Somehow it feels more natural It's not fixed, it's kinda a slide in thing with catches on the back, so if I decide I want it the other way that's easy enough really

 

i dunno what you could do about the earth either, but all my technics separates have the figure eight "cassette/radio" mains connector, so somehow they do without...

Alot of hifi stuff does [used to?] do this to avoid ground loops... I think, so you don't get any humming noise through the speakers. If it's a metal case it's not really the best of ideas though

 

i guess if you really wanted to use it, you could make some sort of faraday cage over the A/C section... i do wonder how having unshielded A/C lines right next to the audio circuitry would affect the signal, but it looks like you've sorted all that out earlier :thumbs-up:

 

I'm not sure at all really! I'm also wondering if it's gonna pick up RF interference or anything with a plastic case. A metal case connected to safety earth would act as a nice shield surely! I'm thinking I could line the inside with foil and have that connected to safety earth I guess though...

 

I've drilled the hole for the source selection switch anyhow. A third knob could be added to the left at about the same distance apart for balance if I ever need it. I don't really use balance so I think I am leaving it out. It's not exactly perfect, I think I should have maybe had the volume closer to the source selection, or vice versa... but... I am ok with it

 

 

Too scared to do the RCA holes now incase I ruin all that hard work I'll have a go at that tomorrow.

 

Speakers have finally been put on the wall so other than that, I have just cleared my desk up a bit. I have space!

Edited January 26, 2007 by markiemrboo

[Makaveli]

Wow, I am very impressed with the outcome, I think I might have to make one as well

[markiemrboo]

Wow, I am very impressed with the outcome, I think I might have to make one as well

 

Thanks It does look better than I thought it would, though still not exactly professional I like it much more than the 2U case though. You also probably can't see it, but the knobs aren't exactly close to the case... and they're really hard to get straight. dang screw on things. It's quite annoying!

 

 

I will try and remember to post PCB layouts soon. I'm still unsure of grounding arrangements to eliminate any funny noises through the speakers (humming, buzzing, clicking, hissing etc). Once I have all that sorted out....!

 

 

The power amps should fit nicely (another perfect fit situation) in to the plasticy cases I posted about earlier, each channel in a separate case. The description of it sounds like it might need some hacking up inside though. It's apparently got PCB mounting slots and things which I don't really want. The large version is also apparently a light grey on the top and bottom panels, which... well, I would prefer all black . Maybe I can paint it

[markiemrboo]

Well, it's so much closer to a finished [or usable even] state today!

 

I drilled the RCA holes.

 

 

I then did proceed to solder wires together, which was insanely tricky as they were fitted directly to the case, which meant I had to solder them... while they were fitted to the case. So I went and though, and figured I could use some scrap PCB, and mount them to the PCB instead, then just mount the PCB to the case. It worked quite well, I am happy with it and it did make life a heck of a lot easier! The screw isn't quite center, even though I did try and mark it out, but never mind

 

(there are vertical lines which show up in the pictures it seems, this is where I removed the masking tape with a razor I was gentle with the blade and I honestly can't see them in real life.)

 

 

Just another shot so you can see what it looks like with the RCA plugs plugged in (and maybe to show off the RCA plugs?)

 

 

I might also add that I cheated yesterday with the source selector. I was using another pot just for demo, so I made the hole slightly bigger and fitted the source selector. It's big. It definately needs to be trimmed down a tad

 

 

 

 

I have plugged it in, turned it on, and played a test tone and it actually seems to work OK on the scope, thankfully! It was just a really quick test though. I need to test it some more before I am happy Tomorrow I think I shall plug my computer in to that, then plug the preamp's output in to my current integrated amp to see what happens (explosions anyone?!). Should be able to test for any funny noises this way. Hopefully I won't get any funny noises though, then I will be happy enough to start rebuilding the power amps

 

Also thinking of drilling some small holes or something in the top, a grid like, just so any heat can at least escape?

Edited January 27, 2007 by markiemrboo

[hardnrg]

maybe a mesh fan filter might be a bit better (easier? neater?) than a hand-drilled grid?

 

 

http://www.thecoolingshop.com/product_info...roducts_id/1378

 

unless you're particularly handy with a centre-punch

 

i dunno, just a thought eh :thumbs-up:

[markiemrboo]

maybe a mesh fan filter might be a bit better (easier? neater?) than a hand-drilled grid?

 

 

http://www.thecoolingshop.com/product_info...roducts_id/1378

 

unless you're particularly handy with a centre-punch

 

i dunno, just a thought eh :thumbs-up:

 

Possibly something like that, yeah. I'm not sure yet. I don't even know if it's gonna get hot enough to need anything like that? The only thing that'll really possibly be putting out any considerable heat is the voltage regulators... and I doubt they are going to be working particularly hard powering a couple of opamps. We shall see I guess... I might be brave enough to try leaving it on all day tomorrow.....

[markiemrboo]

Done a little testing this morning.

 

I don't seem to be hearing any additional noise, which is good. I do get mobile phone interference if I sit my mobile phone RIGHT on top of the case, pretty much right above the preamp board. I guess this is because it is a plastic case. Thinking about buying some "copper foil" adhesive stuff just to stick around the lid and tie that to safety earth in the hope it will make that issue disappear.

 

Twisting the source selection doesn't seem to make any nasty noises, though the other two "sources" are connected to nothing

 

The volume pot is crap. Brand new and dodgy. When turned right down (so... should be silence), one channel is OK, but the other acts like about 1/4 volume, until you turn it up a bit, then it goes quieter and acts like normal. Shall order a better one. Seen two at Maplin which look better to mount the knobs on. The picture illustration shows a solid knob, just like the source selector. These seem to work much better. The source selector knob is nice and flat.

 

Turn on noise seems non existant. Turn off noise, as the power supply collapses, is quite nasty I still don't have that back ordered relay from Farnell either. They're taking their time! I think a bleeder resistor on the caps might help with the issue a bit though. If I am right in my thinking, it should make the power supply caps drain much faster, so the nasty noise will shorten in it's duration.... I think

[jammin]

I expect the Pre will be just fine without any ventilation.

 

If you remember the pics I posted of my Preamp in your other thread, the case has no vents at all.

That thing happily sits on for weeks (if not months) at a time, I only ever turn the power amp on and off.

 

It would be different if there were valves involved, but obviously there aren't.