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Gold A-1 Build with Pics

post #1 of 24
Thread Starter 
Does anyone have any experience with or knowledge of this piece? It's offered as a kit, a partial kit, and/or a complete build.

http://www.ebay.com/itm/110669245109?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

I don't know the topology, or if there are counterfeit parts, etc. However, looking at the photos, the thing looks robust, with a decent power supply and lots of finals - many output transistors. I'm not familiar with the MC33078N, but I do know they're a dual opamp. I've never heard them, though.

Thanks in advance!
post #2 of 24

It's impossible to say without the thing in your hands, and even a schematic would help, but it looks pretty damn good to me. Looks like the opamps are socketed so you could easy change them for LM4562s if you wanted a known audio opamp, although the MC33078 is probably OK, if you google it you'll get the datasheet, I did.

 

I read 15V on the transformer, that's 21+ peak, it should mean plenty of output swing (looks like 2 regulators, probably +/-18V) to drive even the most difficult of phones.

 

$61, shipped, for the assembled board. Damn. It's hard to compete with prices like that, you'd be hard put to just buy the parts for that.

 

Looks like the front panel on the assembled one is a solid chunk of aluminium and the whole case is made from ali plates and right-angle extrusions.

 

4 O/P devices PC, complementary push-pull, 'course it's all down to the implementation, but there's the makings of something really good there.

 

w

post #3 of 24
Thread Starter 
Quote:
Originally Posted by wakibaki View Post

It's impossible to say without the thing in your hands, and even a schematic would help, but it looks pretty damn good to me. Looks like the opamps are socketed so you could easy change them for LM4562s if you wanted a known audio opamp, although the MC33078 is probably OK, if you google it you'll get the datasheet, I did.

 

I read 15V on the transformer, that's 21+ peak, it should mean plenty of output swing (looks like 2 regulators, probably +/-18V) to drive even the most difficult of phones.

 

$61, shipped, for the assembled board. Damn. It's hard to compete with prices like that, you'd be hard put to just buy the parts for that.

 

Looks like the front panel on the assembled one is a solid chunk of aluminium and the whole case is made from ali plates and right-angle extrusions.

 

4 O/P devices PC, complementary push-pull, 'course it's all down to the implementation, but there's the makings of something really good there.

 

w




Thank you for that!  I'm exploring possibly building a good to high quality headphone amp (for fun and excitement - no, really!), and this one looked decent.  The Project Sunrise is another one I'm thinking about, as is the "Lehmann Circuit" DIY also on eBay.  You've validated my thinking, and I appreciate it.

post #4 of 24
Thread Starter 

I placed a bid on a Gold A-1 today.  However, I also went ahead and bought a Project Sunrise amp.  I've built many a kit (I currently have Tim Ryan's Seventh Circle Audio T-15 mic preamps - three of them - in my recording rig), starting with a SWTPC Universal Tiger pair way back when I was in college (did I give away my age?).  I enjoy building, much more than sewing!  I'm looking forward to the PS amp.  I'm also looking forward to the Gold A-1, and when I get going on it, I'll start a thread here with my progress and impressions.

post #5 of 24
Thread Starter 

I've done some digging and discovered that the Gold A-1 is made by a company in Guangdong, China, called "Zero-Zone".  It is reputedly an "updated version" of the Beyerdynamic A-1.  According to the write-up I found (and Google translated), the differences between it and the Beyer piece are higher filter capacitance and better regulation in the power supply, as well as double the output transistors with better filtering than the A-1. It has an inverting input, which is said to isolate the circuit from the source impedance, and it's circuit current bias is claimed to be "constant, safe and reliable".  It is supposed to sound very good.  Reading the translation has been quite interesting, and in places a good deal of interpolation is needed along with interpretation.  I gathered that a version of the A-1 was released to some of the Chinese headphone enthusiast community, and that there were some issues (as in fried headphones!).  The "PRADO" version, which this is, was then released, incorporating suggestions from the community as well as needed fixes.  I'm looking forward to hearing it.

 

The Chinese site is here:  www.hifidiy.net


Edited by sarals - 10/4/11 at 3:28pm
post #6 of 24

Is 700mv protection good enough?

post #7 of 24
Thread Starter 
Quote:
Originally Posted by johnwmclean View Post

Is 700mv protection good enough?


That's a good question.  What would you consider adequate?  Yes, I would be leery about using my good cans with it until I know it's stable.

 

post #8 of 24

AMB’s ε12 triggers at 70mv. I’d be concerned at 700mV, a possible fault condition with an idling offset of around 675mv over an extended period would not be good for any cans.


Edited by johnwmclean - 10/4/11 at 10:25pm
post #9 of 24
Thread Starter 
Quote:
Originally Posted by johnwmclean View Post

AMB’s ε12 triggers at 70mv. I’d be concerned at 700mV, a possible fault condition with an idling offset of around 675mv over an extended period would not be good for any cans.
 

 

Indeed!  Checking for DC offset on startup and the adding a lower threshold trigger sometime down the road would be in order, yes?  Also, even though I am not a DIYer (just a kit builder with a solid but basic understanding of circuits), how likely do you suppose an amplifier would go to DC or have any sort of idling offset?  In my experience over the years with pro equipment (some of which was pretty shaky), not often.  Still - I see your point.  Can I hear your suggestions?

 

 

post #10 of 24
Thread Starter 

I need to figure out what kind of ground the A-1 uses, because it may be easier just to add an AMB c12 muting board to it.  I won't jump the gun, though - I'll wait until I have the amp and can check it over for the first time.  It will be learning experience!


Edited by sarals - 10/5/11 at 12:04pm
post #11 of 24

You can wait until you get the amplifier and check the DC offset before connecting your phones. This will let you try the amp at least in the short term. Then you can look at the existing circuit and see if you can modify it to trigger at a lower voltage. It might be as simple as grafting in a voltage divider.

 

0.7V is marginal, it's still only 30mW in 16 ohms and less in a higher impedance. I would expect the very large majority of phones to withstand it, at least briefly, and I would expect phones of 64 Ohms or greater and less sensitive phones of a lower impedance to survive it indefinitely.

 

For example, the Shure e2c's with an impedance of 16 Ohms have an efficiency of 105dB/mW (on the high side). 30mW is 15dB over 1mW and corresponds to 120dB output, which is loud, but shouldn't destroy phones.

 

The AMB circuit is not without problems anyway; because of the summer at the input you could have equal and opposite offsets on L & R channels of any magnitude (or any offsets summing to <0.07V referenced to ground) and the circuit would still not trigger. This is not a likely occurrence, although less unlikely in a fully differential system, but if you're going to be paranoid, then why not go the whole hog? It's not as if it's a problem to design a circuit without this behaviour.

 

w

post #12 of 24

If you're paranoid about the (unlikely) cancellation at the summing amp, just use a separate ε12 per channel.  Problem solved.

post #13 of 24
Thread Starter 
Quote:
Originally Posted by amb View Post

If you're paranoid about the (unlikely) cancellation at the summing amp, just use a separate ε12 per channel.  Problem solved.

That's what I was thinking.  The "paranoid" part!  Actually, I'm going to do as wakibaki suggested - try it first, and measure the offset.  I'll probably add the e12 sometime down the road - thanks, guys!
 

 

post #14 of 24
Thread Starter 

I went ahead and bought the amplifier.  It's a "preassembled" populated circuit board.  Personally, I would have preferred to do the assembly myself, but there weren't any full kits available when I decided to buy.  I'm not crazy about the quality, it's a little on the sloppy side, but it will have to do.  I did dress up some of the more egregious parts, plus I removed the headphone jack from the board.  The board looks nice.  It's robust, the traces are clean, and it looks like a quality piece.  The heat sink for the output transistors is just a piece of cut off aluminum extrusion.  I could have done that!

 

I bought a case, as well.  I'd like to use it for a "nicer" build one day (like an AMB M3 or something similar), so I don't want to drill a bunch of holes in it and mess up it's looks as well as functionality.  So, I bought some adhesive stand offs to put the PCB on, and I'll secure the heatsink to the bottom of the chassis with just a couple of screws.  The toroid will end up living where it will no matter the amplifier in the case.  I'm leaning towards placing it in the front of the chassis, behind the power switch.  In the case of the Gold Amp, I'd like to run the mains behind the heatsink, between it and the side of the chassis, to the toroid.  I think that would keep any stray hum to a minimum.  The other thing is the wires from the input connectors would be quite short - once again, hum injection would be kept to a minimum.

 

I've posted a few pictures here.  The shots of the PCB in the case are taken from the rear of the case looking forward. 

 

I welcome comments, as long as you're nice :~)

 

IMG_6278.jpg

 

IMG_6283.jpg

 

IMG_6282.jpg

 

IMG_6280.jpg

post #15 of 24
Thread Starter 

I don't have a schematic for this amplifier.  However, comparing a photo of the Beyer headphone amp PCB to this one, there is quite a bit of similarity in the topology.  This one has double the output transistors, but everything else looks VERY similar.  I have no idea the quality of the components on this board, nor do I know how well the power supply is designed.  I'll find out!

I haven't applied power to the transformer yet, so the PCB hasn't been powered, either.  I'm going to lay the parts out on a board before I do that.  Yes, I'll be checking for DC offset, first thing!

 

Once again, comments are welcome!  I've built a lot of kits, but this is the first complete DIY I've ever done.  If anyone sees something that could be done better in the layout, please do let me know!

 

IMG_6284.jpg

 

IMG_6285.jpg

 

IMG_6286.jpg

 

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