n00b question: A47 with high impedance cans?

[Dr.Hee]

Hi, new memeber here already sorry for his wallet

I've been lurking around for quite a long while, and got bitten by the diy-bug. I bought Beyerdynamic DT880s a while ago and decided to build an amp. I'm not a total newbie with electronics, I have some soldering skills and understand some of the basics... I recently built a cmoy for portable use and it works quite ok.

Now to the question.... I'd like to build an amp for the dt880s for home use, and came across the a47 as a step up from the cmoy. The question is, is the a47 that much of an improvement over the cmoy with hi-z cans? Also, portability wouldn't be an issue so I could use a higher voltage power supply for the a47 to get better voltage swing, as well as bigger caps etc... I guess hi-z cans would also benefit from better current capability of the a47?

I've done some searching, and I came across some comments about the a47 being "faster" sounding than the cmoy, but that the cmoy has a fuller bass and that the a47 can sound thin in comparison. Is this true? Although I love the dt880s, I wouldn't mind slightly beefier sound.

So, should I go for the a47 or something else for the dt880s? I know there's better stuff out there (pimeta etc.) but I'm not really willing to put in the €€ (I'm from Finland).

Thanks.

 

[Kenny12]

are you talking about pimeta or meta47 because the buffers used in the meta47 is discontinued and its not that much more xpsive just to build the pimeta.

and yes i would work fine with high impedance cans and is a big improvement over the cmoy

 

[Dr.Hee]

Quote:

Originally Posted by Kenny12 are you talking about pimeta or meta47 because the buffers used in the meta47 is discontinued and its not that much more xpsive just to build the pimeta. and yes i would work fine with high impedance cans and is a big improvement over the cmoy

No, by A47 I mean the Apheared 47... And isn't it Meta42 and not Meta47?

 

[Kenny12]

my memory has failed me

in that reguard i have no advice to offer sorry

oh and even a mint is a drastic improvement to the cmoy

 

[Dr.Hee]

Quote:

Originally Posted by Kenny12 my memory has failed me in that reguard i have no advice to offer sorry oh and even a mint is a drastic improvement to the cmoy

Thanks for the reply anyway.

I'm aware that the MINT is bettern than the cmoy, but then again isn't it also superior to the A47, which is really just a beefed up cmoy?

 

[Xakepa]

From my expirience, A47 was definitely better than my basic CMoy, but the major improvement came when I soldered the buffer. My cans are just 24ohms and can easily sqweeze 35-45mA out of the A47.

If your cans are not current hungry, I don't think A47 is the best for you - if you don't want to spent too much, try CMoy first. If you're not happy with the sound, you could solder a buffer on it w/o too much hassle...and if you're unhappy again (which I doubt) than you could go for Pimeta

Just my .02

 

[ericj]

The A47 "sounds faster" than the cmoy because the gain stage does indeed slew faster when the current load is handled by the buffer stage, in this case another opamp in unity gain.

As configured by Apheared, the A47 has very little voltage gain, because grados don't need much voltage gain. They just need a lot of current.

You have 250ohm DT-880's? Well, congratulations, they're nice cans. Wearing DT-770 Pro/80 right now. 600ohm DT-990 at home.

Go ahead and build an A47 but work out the feedback resistor values for the gain stage to set it at a gain of 5 to 7, depending on how quiet your source is.

You don't need the high current capability, but separating the gain and current loads will improve the sound a bit anyway.

I recently built an a47-alike amp using an opa404 - a low-current high-speed precision quad opamp. I used a gain of 6 iirc and it sounded pretty good with everything i threw at it - but i have no grados. Which is good, because even in the a47 configuration the opa404 can only sink about 20ma per channel (10ma typical per each of four opamps)

 

[Pars]

The A47s I have built definitely sound better than a CMoy (at least with stock OPA2132s and 2134s), and can somewhat rival a MINT (not as detailed and quite as refined, but kinda fun). These can quite easily be built using some slight mods to Tangent's CMoy layout by moving the existing opamp socket up one row, putting the 2nd dip8 socket right below it, replacing the voltage divider rail splitter with a TLE2426 TO92 package, and a couple extra 47ohm resistors.

 

[Dr.Hee]

Thanks for all the input guys.

ericj, i'm aware that the a47 might need a touch more gain, but the cmoy i built has a gain of about 4-5 and it's still *way* too loud with my cd-player (Denon DCD-635). I built the cmoy as a bit of an experiment just to hear what it sounds like and didn't have a pot readily available, so i didn't install one... i haven't listened to the cmoy much.

Now, i was planning on building an amp to use with the denon, which i guess is a pretty strong source. But i just discovered yesterday that it sounds thin and lacks bass compared to the line out from my soundcard (Terratec EWX24/96)

Anyway, the main thing that keeps me away from the pimeta is that i'd have to order the pcb from "across the pond"... has anyone built it succesfully on protoboard?

 

[Xakepa]

Mine A47 is not the basic Apheared design - it has an additional BUF634T for the ground channel. That buffer improved the sound more than the additional voltage "repeater" opamp (2227PA).

So if you can get it easy - try to buffer the ground channel in CMoy first. It might work better than building whole new amp. And you can lower the gain, my A47 is 3.5 gain with no pot, and it's just right for AV-710 rear out and 24V wallwart with HD201.

 

[rickcr42]

The Apheared 47 Amp is straight from the Burr-brown App notes "doubling opamp power" for the OPA604 but called a "47" because all the parts ended up being numbered 47 (470uf,47 0hms,etc).This was designed because the opamps at the time had limted current output or the ones with high current available sounded like crap so this method of using two identical opamps of good fidelity was come up with by BB (a variant by Mackie before that).

Fast forward to the present day and with the availability of opamps that put out currents formerly the domain of the buffer stage in the area of 100-320 mA with good fidelity and it is my opinion it is time to re-think the simple single-stage single op-amp Chu Moy Pocket Amp type hadphone amps for portable use as the best and most elegant method.
Complicated is only the best choice when there is NO choice.

Just my humble opinion.YMMV

 

[Xakepa]

Rick, do you mind mentioning some of the new opamps you're taliking about, which might work as (hopefully) drop-in replacement in a basic CMoy. I'm up to pull a big order from Digikey (for a Pimeta), and I'll be happy to try some of those.

Thanks.

 

[ericj]

opa551 and opa552 - 551 is unity gain stable, 552 is stable at gains of 5 and greater (iirc) and much faster. These are single channel opamps.

There are others - ad8397? but that's a bipolar chip.

 

[Centinul]

rickcr42 -- Are you basically saying that a CMoy built with the high current output op-amps (that ericj mentioned, OPA551,OPA552) of today would sound better with high impedance cans then an A47? If so I would rather take that route then the A47 I was thinking of building for my Grado SR-80's.

 

[rickcr42]

Quote:

Rick, do you mind mentioning some of the new opamps you're taliking about, which might work as (hopefully) drop-in replacement in a basic CMoy.

The first one,the AD8397 is capable of 320 mA peaks !

It would take more than "few" OP2134 pamps to even come cloe and that battery drain would kill you so this alone beats the concept of the A47 "doubel barrel" amp and like i said was a solution for an earlier time.

the second one does not have as much pure kill power in the current delivery but will one run on very low voltages while offering very high drive current output which may make it ideal for low voltage portable .The 270mA is no joke either


http://www.analog.com/UploadedFiles/...22AD8397_0.pdf

http://www.analog.com/en/prod/0%2C28...8656%2C00.html

here is the head to head :

AD8397 +/- 1.5 to +/- 12 volts,8-15ma or so per channel draw,310mA peak-current output

AD8655/Ad8656 +5 VDC @ 4mA per channel,+/- 220mA output current

compare that to most combination opamp/buffer amps and this single chip pair will whip their ass all else being equal and while a 47 amp is possible it woiuls make zero sense other than just to do it.With Grado cans the Ad8397 does sound better but the AD8656 is no slouch either.As "for drop in" replacements you will need to have an SMD-DIP adapter because no new opamps are being made in the larger packages,only older desings and that most likely not for too much longer.

Quote:

rickcr42 -- Are you basically saying that a CMoy built with the high current output op-amps (that ericj mentioned, OPA551,OPA552) of today would sound better with high impedance cans then an A47? If so I would rather take that route then the A47 I was thinking of building for my Grado SR-80's.

absolutely.It is my personal opinion that the more parts you eliminate from the signal path the better the end result and this new generation of chips eliminates the need for both a separate buffer AND any need for doubling the current on current shy opamps as in the 47 amp and that goes for both senns and grados.
Just be aware the gain setting is not the same for both cans used with the amp.Grados like low voltage gain or you lose any shot at having a volume control with any range while the higher impedance/less sensitive Senns need voltage gain just to get going.