[doobooloo]

Yup. My experimental balanced amp, with two mints.

After about four hours of having a pair of RS-1s in my hand, I've taken it completely apart ready to mod.

One of the mods is, of course, making them balanced.

Each channel will have two BUF634 buffers, and I will be using the AD8620 opamp. It will have a single 4-pin XLR jack with pinouts compatible with the K1000, and two standard 3-pin XLR inputs. The amp will have no volume control, and most likely going to be run off 4x 9V batteries. My source, the Edirol UA-25, already has a volume-controlled balanced output, so no point of having two volume controls in the signal path. Also, maybe a power jack or two (for L/R), if I decide to build a decent power supply with TREAD or something later on.

If the case allows I'll add some larger boutique rail caps and bypass them with some film caps. I haven't decided what case to use, but I want it to be as small as possible. Wouldn't be too hard with two mints and no pots, but the four batteries (and having them accessible) may be a problem. I might get some sort of a wooden jewelry case (to match the RS-1s) or something where I can use one of the compartments for storing batteries... I'll have to see.

I just want to get a flavor of the balanced sound with my RS-1s, so if this turns out to be positive I'll go ahead and build a more sophisticated solution. But this is kind of fun and exciting, a small battery-powered balanced headphone amp!

 

[n_maher]

doo,

Great idea for a relatively low-cost balanced amp. No need to have that third channel empty on something like the PIMETA. A couple of quick comments: one, the 8620 wouldn't be my opamp of choice for anything in the Grado line. You might consider using something a little more laid back like the 2227 or [2]637s on a brown-dog - I'm pretty sure they make a dual SMD adapter, as crazy as that sounds. Two, I think you might be better off try to include volume control on the amp side of things. With any source that I've owned (granted not the Edirol) the variable output has always been inferior to the line out. This does includes a couple of higher end CDPs that I've owned/used in the past. I do understand your reasoning if the only balanced outputs are variable ones though.

And knowing the creations you've produced in the past I can't wait to see this one. I still keep pictures of your PPA and HP-100 around to look at from time to time. They still rank as some of the most impressive DIY work I've seen.

Best of luck,

Nate

 

[DaKi][er]

Quote:

Originally Posted by n_maher I think you might be better off try to include volume control on the amp side of things. With any source that I've owned (granted not the Edirol) the variable output has always been inferior to the line out. This does includes a couple of higher end CDPs that I've owned/used in the past. I do understand your reasoning if the only balanced outputs are variable ones though.

Volume control on a balanced signal to do it well will cost around 3-4X that of the 2 mint amps themselves completely ruining the simplicity of the project

Overall is seems like a great idea for a cheap and simple ballanced amp, just one board for left channel and one for right, you could run seperate batteries for each one (they should discharge at the same rate) to have them completly seperate

 

[n_maher]

Quote:

Originally Posted by DaKi][er Volume control on a balanced signal to do it well will cost around 3-4X that of the 2 mint amps themselves completely ruining the simplicity of the project

I was thinking of two of these:

which would only run you about $75 shipped, with all the resistors. Granted, not small enough to tuck into anything portable, but about the cheapest, decent stepped attenuator that I've run accross and actually used.

Nate

 

[bg4533]

A while back someone let me know SFT was selling 4 channel pots for only $19. They seem like they would be a perfect fit for this amp.

 

[doobooloo]

Quote:

Originally Posted by n_maher variable output has always been inferior to the line out.

Thanks for your kind words!

Regarding the line out, the Edirol UA-25 only has a variable line out. The volume pot is shared for the headphone out, unbalanced line out, and balanced line out. I think it is a shame there is no "real" line out but sometimes equipment limitations like these are good for keeping my budget under control.

If the need ever arises I'll add that feature later on or make a universal passive preamp/switchbox or something (been thinking about this for a while). But it's not something I need at this point of time.

Regarding battery operation, I was thinking about having 2x 9V per channel (4 total), but I thought, why not try to max this thing out... so, with 1.5mA class-A biasing for opamps and using a 100ohm R11 (I'm stacking two buffers), I think the power requirements would go a bit out of hand for 9V batteries. Actually, any estimates of battery usage under those circumstances would be greatly appreciated!

I also ordered 2 TREADs when I ordered my 2 MINT boards since just ordering those seemed a bit, well, wasteful of the flat shipping fee and I knew they were going to come in handy someday.

After ordering that, however, I remembered that I have a small 2x 15V (1A each, IIRC) Avel-Lindberg toroidal transformer from a while back. Perfect!

So, I'll make a (relatively small) dual power supply for this amp as well.

Related question here: what power plugs/jacks are recommended for dual power supplies? I don't want two separate plugs/jacks per channel, I really just want one power cable going to the amp.

Quote:

Originally Posted by n_maher one, the 8620 wouldn't be my opamp of choice for anything in the Grado line. You might consider using something a little more laid back like the 2227 or [2]637s on a brown-dog - I'm pretty sure they make a dual SMD adapter, as crazy as that sounds

Regarding opamps... Regardless of headphones, I have a strong preference for the AD8610/8620 over the OPA637 or OPA627. Since the OPA637s are a good deal more expensive and are reputed by many to be nicer, I really tried to like them but regardless of headphones I find myself liking the AD8610/8620 sound better.

And since the MINT is SMD, I want to go with a path that I've convinced myself over that I clearly prefer over the alternative.

And yes I've seen those crazy single SOIC to dual SOIC adapters...

Not sure if they'll fit on a MINT, though.

 

[doobooloo]

Quote:

Originally Posted by bg4533 A while back someone let me know SFT was selling 4 channel pots for only $19. They seem like they would be a perfect fit for this amp.

Superior gang characteristic... Hmm.

Maybe I should ask him for more details. I might just grab one in case. Thanks for the info!

 

[doobooloo]

OK - I've made up my mind regarding inputs and outputs...

Inputs will be two locking combo Neutrik jacks, so one can choose to use regular XLR balanced inputs or 1/4" balanced inputs (my source has 1/4" balanced outputs).

There will be a single output - a 5-pin Neutrik jack which will accomodate both balanced and non-balanced headphones.

I'm still stuck on the power connector though...

Don't want to use a 4-pin XLR for power... Should I just have the toroid and regulator integrated in the amp and save the trouble of the power connector altogether?

 

[Jazper]

I'm currently building another M3, but here's some ideas:

Power:
Use Ruggedised RJ45 sockets and just use plain old CAT5 or CAT6 for the power, you won't be drawing large amounts of current and if you ground the extra wires you'll be fine

Opamps
You might wana try the AD843 (though you'd need two, and they're not soic). Analog snap and verve with a bit more warmth than the 8610. Alternatively the AD8066 is a good option.

 

[Jazper]

repeat post

 

[The Monkey]

doobooloo,

Do you plan on doing a build log? Because that would be awesome.

 

[doobooloo]

Quote:

Originally Posted by The Monkey doobooloo, Do you plan on doing a build log? Because that would be awesome.

Yeah, I'll keep this thread updated with progress.

But not anytime soon, as it is finals week and parts aren't here yet. I expect to get this done sometime in January.

 

[doobooloo]

I just built two MINTs yesterday night, reasonably maxed out, exact same configuration, with single BUF634s on each channel, one MINT with AD8620 and the other with AD8066.

So far, I'm not all that satisfied with the results... Something's missing, things are sounding rather dull. All the details are there, but theres life missing. For some reason I prefer the RA-1 greatly over both of those amps.

I'm going to leave them running for another day or two and then post some results again.

I am actually now a bit discouraged about using MINTs to do my next amp. Perhaps I should do something like a dual PPA or dual M3 instead and wire them so that I can use them as two regular headamps if the need arises (two separate boxes, perhaps.)

Anyway, the AD8066 is definately interesting... First impressions are that it's not as musical and somewhat grainy, don't know if this is actually additional detail that's actually in the recording that's now being passed through, or if this is just the opamp's sound signature. At any rate, I'm not blown away by the AD8066, but I think it does have interesting potential.

This leads me to another point - I would definately like to be able to switch back and forth over time among a small pool of preferred opamps.

Aah... what started as a low-cost project is now looking like it's going to end up being pretty costly.

 

[flecom]

Quote:

Originally Posted by doobooloo Superior gang characteristic... Hmm. Maybe I should ask him for more details. I might just grab one in case. Thanks for the info!

if you get more info on that pot please share, ive been looking for something affordable for my dynamite

 

[kin0kin]

wow Doo! you can take 8620 with grados?

I'd have slaped in some 627s there