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killie99
05-11-2019, 11:28
I was looking to try my hand out at building a preamp. The reason for this was my amp went faulty (was repaired under warranty) and that got me thinking about how much it would cost for repairs once the warranty ran out - I suspect A LOT!

Amps these days seem to be jammed full of computer control bells and whistles and if these controllers fail then it can leave you with not a lot more than an expensive door stop.

I looked around the DIY Audio forum and there are several great looking pre-amp projects on the go. After a bit of reading I settled on building a Salas DCG3. Its a two stage single ended Class A circuit. It's named DCG3 because it has DC servo and three MOSFETS. Go here (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone.html) to read more .
Salas has also designed a power supply for the DCG3 called the DCSTB - dual mono construction and designed to the same dimensions as the DCG3 so all goes together very neatly.

Pre-Amp and power supply chosen, I had a think about what else I wanted before I plunged head first in to the build. Salas also has an input selector board available called the iSelect. I believe this was originally developed to partner the DCB1 and the neat thing about the iSelect is that you can use a cheap as chips selector switch as all the circuitry is on the iSelect board, the selector switch is only being used to activate a relay to select the desired input. So, super short signal paths and cheap selector switch, win win!

First thing to do was buy all the bits and I found a complete set of boards already built for sale ..... but untested. Boards duely purchased and arrived a few days later. First thing I did was go over the boards with a fine tooth comb to make sure they were built correctly. Glad I did. Found a bunch of parts in the wrong locations or inserted the wrong way round, phew, glad I hadn't applied any power!
All fixed and ready to apply power using a light bulb current limiter. DCSTB power supplies working fine so time to connect up the DCG3. Thankfully no magic smoke! Carried out the set up procedure to minimise DC offset and left it running for a few hours. Connected up the iSelect board and oh nooooooo - magic smoke appears! In my rush to get it working I somehow managed to wire the iSelect up incorrectly and blew the regulator. Note to self - check, check and double check wiring before applying power! Regulator replaced and thankfully no other damage done. iSelect appears to be working - well I can hear the relays click so that's a good sign. Looks a bit Heath Robinson at the moment though!

https://i.postimg.cc/9F7Kp0mt/testing.jpg


Time to be put in the my system to see how it fairs, but before I connect it to my main system I fired it up with an old pair of Mission speakers and a Yamaha amp just in case of any disasters - don't want to fry my main amp or speakers, that would not be good. All good connected to the Yamaha/Missions setup so onwards to the main system. I had to configure my Perreaux integrated amp as a pre/power so that I could use just the power amp section, easily done using the menu option. All connected up and power applied, volume knob slowly turned up all the way to max and there's a wee bit of noise from the speakers, but given it's only loosely wired that's a good sign so time to try some music.

Ouch! Too much gain, I can barely get the volume knob past 9 o'clock but other than that it actually sounds very good for something with no run time. The DCG3 has a gain of 3 as standard, which for my system just now is just too much. I swapped out a couple of resistors and now have the gain down to 2, the minimum it can be set at according to Salas. At least I have a bit more travel on the volume knob before the neighbours complain.
I continued to use the DCG3 for a week or so and the first time I swapped the Perreaux pre back in I noticed how much better the Perreaux was. Better soundstage, better imaging, and much better at the top end. I'd almost go as far as describing the DCG3 as harsh but that's maybe being unkind to it. When you think about how much it cost to build and how it was performing then it's actually a bit of a bargain. In it's current state it was under 300 for all the bits but the tin board it was sitting on and the loose wiring probably weren't helping matters so I ordered a case from AliExpress.
It turned up about 10 days later but unfortunately I had to pay an 18 ranson fee to the courier to get it delivered, grrr!
I then set about getting all the bits in the box. I was surprised at how much space there was in the box when I laid everything out which got me thinking if there was anything else I could stuff in there. A streamer seemed like the perfect answer. I had just bought a Salas L-Adapter power supply board and low and behold it fitted nicely in the case along with the Rpi/Isolator/Boss. Could I possibly fit 2 L-Adapters, one for the BOSS and one for the RPi?Nope, there is no way I can get 2 L-Adapters in there but what I can do is have an external power supply for the RPi, just need to put a jack point on the back panel. Sorted.

https://i.postimg.cc/g23r56mt/amp_1.jpg

Time to break out the tools, drill holes, file bits of metal and generally bodge stuff. A good few weeks later (I kid you not, I had to order various bits and pieces to get everything fitted properly and ordered some of the bits from China) and everything was in it's place ready to be put back in for a listen.

It's sounding better than it was before, definitely less background noise but the top end is still a bit on the harsh side and the imaging and soundstage are still shy of what the Perreaux can do.
I have an inkling of what the problem is, the cheap Alps Blue pot. The other thing that was cheesing me off was no remote volume control. I've got so used to being a lazy sod that having to get up and adjust the volume control almost everytime I changed track was getting to be a pain in the backside. Decision made, the replacement volume control needs to be remote control.

I've tried stepped attenuators before and didn't get on with them, there were too many times I couldn't get the sweet spot volume. I've read great things about the Khozmo's but I don't know that I'm prepared to drop that amount of cash for something that I might not get on with - $200 for the remote volume option just seems nuts. Oh and it's so big it won't fit in the space I have.
All things considered I plumped for a TKD 2CP-2511, now to find a motor controller to drive the bloomin thing. As if by magic a thread appeared on AOS where the topic of remote volume controls was being discussed and Firebottle suggested there was a cheap as chips Chinese controller that would do the job. 9 parted with and a cheap controller was winging it's way to me. The controller was actually very well made, much better than I was expecting for 9 but to be on the safe side I replaced the 2 electrolytic caps with slightly better rated Panasonics. Even the cheap IR handset which came with the controller is reasonably well made. TKD wired up to the controller on the bench to make sure it works. All looks good, so time to get it fitted in to the case.

This is now what the inside of the case looks like - not a lot of room to put anything else in there!

https://i.postimg.cc/Ls0phvv1/dcg3_inside_markup.jpg

Front and rear, the front panel looks pretty decent for an amateur :):

https://i.postimg.cc/3wwhKzjh/dcg3_front_day.jpg

https://i.postimg.cc/x15YV9zg/dcg3-rear.jpg

Back to listening with the TKD in place. Ah the joys of sitting on your backside and adjusting the volume! First thing I noticed with the TKD is how much more travel there is on the pot at lower volume, I guess this is just down to the curve TKD have used. For this alone it's a big plus. Sound wise it's night and day. Harshness gone, poor soundstage gone, poor imaging gone. This has taken the DCG3 to the level of the Perreaux preamp section. They have a different sound signature but they are both excellent. Both are super quiet. The Perreaux is neutral, the DCG3 a bit warmer but it's not by much. Some might even say the DCG3 is the one that's neutral and the Perreaux is a bit clinical.
Let's get this in to perspective though. All in I reckon I've spent around 500 on the DCG3, 700 if you include the Rpi/Isolator/Boss and L-Adapter power supply. The Perreaux is 8k worth of integrated amp so a few k pre-amp section.

Before I make any rash decisions on what to keep or dispose of, I'm going to give the DCG3 an extended stay in the system, probably for a couple of months, and then swap back to the Perreaux. Too many times I've made too quick a decision and a few months down the line wishing I'd taken a different route. For this time though I'm determined to give the DCG3 more than it's fair share of system time.
Time will tell.

ReggieB
05-11-2019, 13:55
Rather splendid!

Marra
05-11-2019, 14:15
A lovely build.

struth
05-11-2019, 14:52
Really well Done Stuart... top marks

Bigman80
05-11-2019, 15:33
Cracking work. Well done!!!!

smangus
05-11-2019, 21:37
Cor there's a lot going on in that case! Great looking build, great stuff.

Sent from my SM-G975F using Tapatalk

Ianmac
07-11-2019, 19:01
Very nice.
Didnt know I had a fellow DIY enthusiast nearby. Im in Stewarton
Have a good Mill and Lathe if you need assistance with metal bashing and an analogue TEK scope for setting up/fault finding.
Ian

Rosewind
07-11-2019, 21:29
What a nice build! Looks very tasty indeed! Hmm.

killie99
07-11-2019, 23:14
Very nice.
Didnt know I had a fellow DIY enthusiast nearby. Im in Stewarton
Have a good Mill and Lathe if you need assistance with metal bashing and an analogue TEK scope for setting up/fault finding.
Ian
Oh a mill and lathe, that’s handy. I’ve got access to all sorts of test and repair equipment , I work at the old Digital Equipment factory in Ayr (I’m the engineering manager) so if you need anything just ask

Idm
08-11-2019, 12:45
Hi Killie,

I also have a DCG3 and it is excellent. I built mine with 4 x Salas UltraBiB regulators. Salas has suggested that in can be worthwhile to swap out the BC560c's for matched BC327-40's, indeed that is how I built mine. There is further information here:

https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-270.html

I would certainly recommend trying the alternative part as long as you can match the transistors. It is also worthwhile putting a cable tie round each pair of BC560C's or BC327-40's as this keeps the transistors at nearly the same temperature and reduces the output offset so the DC servo doesn't have to work so hard.

My DCG3 is feeding a Pass F5 so no coupling caps, though I do have caps on the output of my sources (DAC and phonostage).

Very nice looking build by the way.

Cheers
Ian

killie99
08-11-2019, 13:18
Already swapped out the BC560's for BC327's. Bought a pile of 327's and got 4 very closely matched, all over 400 hfe.

Puffin
14-11-2019, 07:05
Just a straight wire with gain really......:lol:

Lovely build. I have nearly finished a Twisted Pear Diamante discrete opamp pre-amp.