Linestage design for an Opus (newb alert!!) - diyAudio
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Old 10th June 2010, 02:57 PM   #1
MarcusC is offline MarcusC  United Kingdom
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Default Linestage design for an Opus (newb alert!!)

I hope someone here can help me, I'm looking for a project. I have a TPA opus (wm8741) and want to expand my knowledge of circuits with a new line-stage. Up to now all i have built is the opus modules (a bit of soldering) an amb CKKIII (a bit more soldering) and a cmoy (a bit of fun). I can read most schematics i'm presented with so I think building one from a schematic rather than a board might be quite fun. Does this sound out of my depth?

My problem is the vast number of designs/variations out there. So what i'm looking for is a design for a balanced in/single-ended out linestage for voltage output dac's. I'm inclined to say passive but only because the one i'm using (ballsie) is active so i'm not too fussed on that score.

I have tried searching but haven't had much luck due mainly to the sheer number of results. Also pass labs designs seem to be either balanced or SE not both.

Oh yeah, i'll probably need a matching power supply too...

Thanks for reading,
Martin
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Old 10th June 2010, 05:32 PM   #2
qusp is offline qusp  Australia
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so when you say build from schematic rather than a board, are you going to get the PCB made yourself? you are still going to need a PCB or do it on perfboard some point to point can be done, but can just turn into a mess if you havent any experience. myself I would just whack a pair of transformers on it you only really need a buffer with this dac though, if not trannies, I would go active personally, a discrete buffer could be a bit of fun, you will find several mosfet and jfet type buffers that sound very well. as far as power supplies I would go with the salas shunt regs
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Old 11th June 2010, 07:59 AM   #3
MarcusC is offline MarcusC  United Kingdom
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Thanks for the reply, yeah I wasn't planning on making a pcb, more a case of perfboard/point to point or re-arrange onto stripboard. It's not too much of a problem if it goes wrong a few times.

Quote:
Originally Posted by qusp View Post
myself I would just whack a pair of transformers on it
?? What do you mean by this?

Quote:
Originally Posted by qusp View Post
you only really need a buffer with this dac though, if not trannies, I would go active personally, a discrete buffer could be a bit of fun, you will find several mosfet and jfet type buffers that sound very well. as far as power supplies I would go with the salas shunt regs
There are quite a few designs out there. The problem is they are mostly balanced, do I just take half the balanced signal out, is that what's normally done? It seems a bit wasteful really. Thanks for the name drop on the salas ps, i've bookmarked the thread.
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Old 11th June 2010, 09:13 AM   #4
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I think qusp meant that as "use transformers as your linestage".

You may find certain threads here helpful, such as "DAC Output Using Transformer" and to a lesser extent "Buffalo II & Transformers".

Edit: I have personally used LL1527s with my Buffalo DAC. Easy to implement and pretty good sounding for the price. Might be worth trying on the Opus as well.

Last edited by chipzahoy; 11th June 2010 at 09:17 AM.
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Old 11th June 2010, 11:40 AM   #5
qusp is offline qusp  Australia
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yes, thats what I meant. voltage out dacs do very well with transformer coupling. transformers as a line stage is about as elegant a solution as you will find IMO. they will also give you both balanced and single ended outs with no additional components. the SE output from them uses all of the balanced signal too, rather than half like your example of using half a balanced stage plus ground. I would not do this for exactly that reason, there needs to be a summing stage that folds the balanced output down into a single ended output without wasting much DNR. this process also generally rolls off any DC offset that might be present as well, a nice biproduct of trannies, which does this all in the one process, as well as the bonus of galvanic-ally isolating the ground
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Old 11th June 2010, 02:09 PM   #6
MarcusC is offline MarcusC  United Kingdom
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Thanks for the explanation, i'd never heard of it before. Sounds like it could be worth looking into.
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Old 11th June 2010, 03:55 PM   #7
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If you're unfamiliar with transformer output like I am, try contacting Kevin from K&K audio for specific advice. I find him professional and quite pleasant to deal with.
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Old 12th June 2010, 06:46 AM   #8
qusp is offline qusp  Australia
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no problem Marcus, you must use a line level output transformer and preferably one suited to the output impedance of the dac as well as the input impedance of the next stage. most line out transformers are 600:600 meaning they are 1 to 1 with no gain, with the primaries being of 600R impedance, as with the secondaries. for your purposes you should use a pair of transformers with dual primaries and dual secondaries.

I recommend contacting Bud Purvine (BudP here on the forum) he is very helpful and he makes some quite nice and good value line level trannies
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Old 18th June 2010, 02:10 PM   #9
tassosk is offline tassosk  Greece
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I'm listening a dual mono OPUS with a pair of 1527XL Lundhals terminated with 10K resistors accros the secondaries as an output stage and I'm very pleased by the results.
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