*** The DIY Audio Thread ***

[AbsenceJam]

Sounds good, I'll look into that.

Why 75 ohm? And surely it varies with length too...

75 ohm is a coax cable standard, made for that output impedance.

 

[MikeHunt79]

As for the enclosures, i really like the look of this:

http://www.head-fi.org/forums/f6/post-pics-your-builds-85561/index328.html

(the build by Ferrari is quoted by someone else on this page, although i amnot sure where the actual pic is in this huge thread)

This is the look i would like to go for - the galaxy enclosure from modushop with a fairly simple front panel, although i would like to add an LCD display. How much is the postage Mike? Has anyone had them machined by modushop + rough cost?

Well I haven't ordered from them yet as I havent decided yet weather to put have my PSU in the same enclosure as my amp9... Postage is around £14 unless you order more than 5kg (which is a lot!). Not bad when you think it's coming all the way from Italy. I'd be up for splitting postage tho if you are ordering also?

Those galaxy enclosures look very nice, I'm really tempted to get a 10mm front panel also, as it would make a very nice heatsink. Also, the t-amp enclosure is worth a look as it already has a knob + hole machined for it. It's a little big for me tho.

If you want custom work done, you'd be best off e-mailing them for a quote.

Mmm... i have just measured the DC offset and have got some odd readings. To clarify, i am measuring by shorting the inputs, disconnecting the speakers and turning the amp on + leaving it running. I am then measuring between the +ve and -ve points of each terminal block with the multimeter set to DC voltage. I get the following readings:

Channel 1: 103 mV (mmm... not good)
Channel 2: -39 mV (??)

The second reading would suggest a short to me, although i cannot see a short anywhere. The first reading just doesnt look too good (although it actually says in the sec section on the manual Max offset voltage 150 mV, so i presume that 103 is OK, but not great). Can someone confirm that i am measuring correctly? Is there anything i can do to correct the problem? I notice adam you pointed out before that the output caps should block the DC offset - i have cleaned the solder joints etc and cannot see any problems with them. Do you have any other suggestions?

Thanks

It looks like you are measuring correctly. Don't worry about the 2nd channel having a minus figure, DC offset can be + or -.

Maybe the 103mV is a little high, but it will probably be less with a speaker connected...

Do you have a 10W 10Ohm resisor or cruddy speaker handy for testing?

 

[ChrissyT88]

Im using some old (my Dad's! - with his permission of course ) mordant short speakers for testing, since they arent used and it doesnt really matter if they get cooked. Measuring when the speaker is connected and music is playing (it works fine - no problems there), i measure the same level of DC offset (although it varies from about 100 - 110 mV). Measuring from ground to each of the speaker terminals also reveals a 0.1 V difference on that particular channel... I do have a 10mOhm resistor - but i presume that it would be to simulate the use of a speaker.

EDIT: I understand that this amp works by using a DC offset (i think...) - does that mean that you should expect some on the output? If the manual indicates a max offset voltage of 150mv (although it doesnt specify 'at the output'), i guess that it would be alright for day to day use?

Thanks for the offer on splitting postage, but im afraid im not 100% sure on what i am after! I think im going to buy a preamp kit, so a lot of the front panel work etc will depend on what kind of features this has (im really hoping to have an LCD display for example). Once again, thanks for the offer, but i think i need a little more time to decide...

 

[MikeHunt79]

No problem, I still can't decide either, my DAC arrived today, so I'm trying to get all the parts I need in a single order so I won't be ordering for a while yet.

As far as DC offset goes, if it's less than 150mV then it's fine for day to day use IMO. If I had some mega high end fragile vintage speakers then I may try and get it lower, but you won't damage commercial speakers like mordant shorts. Also, the amp is bridged so measuring from ground to speaker terminal will give a strange reading... you only need to measure the voltage between the terminals. I think my amp 6 is around 60mv on one chan and I've used it on loads of speakers with no issue.

 

[ChrissyT88]

Thanks mike - its not the Mordant Shorts i am worried about but my brand new B&Ws! Hehe. Im sorry for all the questions, but im pretty new to this, and want to do it right! Anyway, im sure it will be fine - as i said, it works fine, and sounds great, and above all hasn't cooked the MS speakers (yet). I ordered my preamp kit this afternoon, and am looking at some speaker protection stuff to be on the safe side. Im not too sure whether to put the PSU in a different case or not either - i hope to be able to use its 5V rail to power the preamp section, so it would be a lot more convenient to keep everything close together cable wise.

I did 'make' some speaker cable today (although didnt order enough braid, so couldnt do both sets), turning some horrible, pretty cheap blue shark cable into something that looks alright, but i do need some more heatshrink to finnish them off. To be honest, i dont think it sounds any different to the free stuff that came with my denon micro system (essentially bell wire!), but they look a lot prettier... (excuse the poor pic)

 

[BF-Bert]

Is that solid core coax cable your using for speaker cables? Imho its probably not the best cable to look for improvements with.

 

[gurusan]

For those interested over at head-fi there's a guy with spare parts after building a few Millet Starving Student amps...he has enough to assemble 4 kits. I've spoken for one of them. Check the thread out in the DIY section, his name is RDS. Total with international shipping is 62usd...comes with everything except a case.

 

[ChrissyT88]

Its not coax cable, its 'Shark high quality audio cable'... not 100% sure about the quality part, but its stranded copper wire encases in a coax type shielding. In all the area of the copper wire is similar to the free cable i had been using, but i thought i would be interesting to see if it made any difference (the answer is no...).

 

[Dr.EM]

I think I know the cable your using there. It came as a stereo pair (figure 8) but with a small third cable in thier middle? It's designed for in car use with that extra core carrying the 12v trigger signals. It is a signal level cable however so not really designed for speaker use, theres enough copper in the shield so that it'll work alright though. Van Damme petrol blue cable or similar does for just about any speaker hookup (various gauges available depending on power and length of run).

 

[adamau5]

the shield will only work if you ground it.

 

[ChrissyT88]

@Adam - i know, but there is no convenient way of doing so (although i was under the impression that it would shield against very high frequency interference without grounding)! The cable was cheap and i thought i would give it a go. I may look at getting some proper QED cable, or perhaps when i have a little more money making a decent DIY cable.

@Dr.EM - bang on! I must admit i have not been using the shield to carry a signal, only the copper core - i'll give it a go.

 

[Founder_film]

Just finished my first DIY project. A CMoy headphone amp:

I know its not very neat but I am a first timer and the layout was tricky. Cost me about £20 to get all the bits. Sounds amazing and I am not even using the OPA2132PA chip which is a highly recommended chip. I ended up using a MC1458N dual op-amp chip from a well known highstreet electronics store. I am seriously impressed. Its really loud and sounds ace with my sennheiser PX 100's. Anyway anyone else using the CMoy design I would recommend the MC1458 as a cheaper alternative to the OPA chips which are about £3 each.

Right now got to make it neater, smaller and put it in an enclosure

 

[gurusan]

looks good! My cmoy stuff was supposed to arrive today but rs-online wrote my address down wrong and DHL got confused. derned people!!!!

 

[Founder_film]

looks good! My cmoy stuff was supposed to arrive today but rs-online wrote my address down wrong and DHL got confused. derned people!!!!

Rubbish, I signed on to RS but I have just moved so don't think they will deliver to my new address as its not my billing address. Is there an option on RS to have a different delivery address to billing address? I want to get one of the more expensive chips just to see how much better they are.

 

[gurusan]

I don't know, you could give them a ring....

The bay is also a good place for sourcing opamps

 

[p4radox]

Hmm, major problems with the AMP10. I've soldered on the majority of the bits now, apart from the Tripath IC mainly.

I'm trying to test the 5V rail before proceeding any further but I keep blowing fuses. Something is seriously wrong, but I can't find a short anywhere, and my soldering looks perfect.

edit: I think I might have fried a rectifier too...

 

[adamau5]

ouch - fried rectifier probablys means you should look at tranny wiring - or a dead short to ground, check the impedance between ground and whatever voltage you are running at.

 

[ChrissyT88]

@Founder Film - thats a neat CMOY, much neater than mine! I remember the volume being waaayyy too high for my liking and changed some resistors to make it less loud. I did use the OPA2132 and thought it sounded great, very balanced, but its great you had good results with a much cheaper off the shelf chip.

@p4radox - I had problems (including some smoke!) when my amp wasnt properly grounded - a mistake i have learned from.

 

[MikeHunt79]

Hmm, major problems with the AMP10. I've soldered on the majority of the bits now, apart from the Tripath IC mainly.

I'm trying to test the 5V rail before proceeding any further but I keep blowing fuses. Something is seriously wrong, but I can't find a short anywhere, and my soldering looks perfect.

edit: I think I might have fried a rectifier too...

Hmm, not much I can suggest really apart check the voltage after your rectifiers, you should be getting +-24v. Do you have a scope? This would help as you may have grounding problems...

I know it's not much consolation, but I finished my Amp9B today, only got as far as testing it and it works!

I've got some DC offset on some of the channels, I need to test with load really, then attach heatsink, solder in the zener diodes on the input, etc. but I'll have plenty of time to do that over the weekend.

 

[p4radox]

When you say "grounding" problems, what exactly do you mean?