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Old 27th August 2010, 07:16 PM   #3041
sendler is offline sendler  United States
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Originally Posted by Mush View Post
Hi All,
My cheap Chinese Ebay DAC has died and I am now considering a Behringer SRC2496. They are about $200 from Amazon. I have my UTC A-20 transformers and am wondering if anyone has any experience with this kind of combination. Do you have any advice?

Lastly, what is a good way to connecting the transformers to the DAC output? I have seen several approaches. I am no engineer but know how to solder and experiment.

Thanks and I hope all is well with you.

Ciao - Mush
The DEQ2496 is much easier to mod with access to the dac outputs via the ribbon cable. It is also indispensible to have digital EQ in your system even if you use a stock DEQ digital in/ out into something like the big dac board.
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Old 27th August 2010, 08:10 PM   #3042
Mush is offline Mush  United States
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Thanks Scott. I'll look into the DEQ2496.

Is there best way to connect the transformers with regard to load resistors and filtering? Do you know of a circuit to look at? I've gone through a lot of this thread and am not really sure how the DAC outputs need to be loaded.

Ciao
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Old 27th August 2010, 08:15 PM   #3043
McGyver is offline McGyver  Poland
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Originally Posted by Bill Fuss View Post
No, the 1798 is completely different. It requires current to voltage conversion.
However, it's possible to use Monacors LTR110 (or almost any 1:1 trafos with center-tapped primaries) as a desimmetrization. One can put two conversion resistors on primary, or one conversion resistor on secondary. The trafos must be followed by gain stage, for example tube stage shown by me.

Quote:
Originally Posted by Mush View Post
Hi All,
My cheap Chinese Ebay DAC has died and I am now considering a Behringer SRC2496. They are about $200 from Amazon. I have my UTC A-20 transformers and am wondering if anyone has any experience with this kind of combination. Do you have any advice?

Lastly, what is a good way to connecting the transformers to the DAC output? I have seen several approaches. I am no engineer but know how to solder and experiment.

Thanks and I hope all is well with you.

Ciao - Mush
I have one of these Behringers. No matter what have I done, it cannot even equal the performance of carefully modded CS4397 DAC. And PCM1798 DAC is even far better.
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Old 27th August 2010, 08:23 PM   #3044
danrap is offline danrap  Belgium
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But the pcm1798 board in this dac
Upsampling 24bit/192 DAC mainboard ,USB, PCM1798 su eBay.it Processors, Home Audio, Electronics
have an I/V convertor, maybe is it possible to mount Monacors or other 1:1 trafo without modifications..
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Old 27th August 2010, 08:29 PM   #3045
sendler is offline sendler  United States
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Quote:
Originally Posted by Mush View Post
Thanks Scott. I'll look into the DEQ2496.

Is there best way to connect the transformers with regard to load resistors and filtering? Do you know of a circuit to look at? I've gone through a lot of this thread and am not really sure how the DAC outputs need to be loaded.

Ciao
I still prefer to simply cap couple a single output to my amps. Review of the Jensen transformers to follow tomorrow when I get a chance to do some more critical listening.
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Old 27th August 2010, 08:50 PM   #3046
McGyver is offline McGyver  Poland
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Quote:
Originally Posted by danrap View Post
But the pcm1798 board in this dac
Upsampling 24bit/192 DAC mainboard ,USB, PCM1798 su eBay.it Processors, Home Audio, Electronics
have an I/V convertor, maybe is it possible to mount Monacors or other 1:1 trafo without modifications..
Yes, it's possible, but is it optimal? I/V convertor on this board is op-amp based. Me myself dislike solutions like this.
In my present sound system there's no single semiconductor in the analogue signal path. Tubes only. Triodes only. All single ended. And the sound quality is to die for.
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Old 27th August 2010, 09:55 PM   #3047
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Quote:
Originally Posted by Mush View Post
Thanks Scott. I'll look into the DEQ2496.

Is there best way to connect the transformers with regard to load resistors and filtering? Do you know of a circuit to look at? I've gone through a lot of this thread and am not really sure how the DAC outputs need to be loaded.

Ciao
As with many things, it is a bit more complicated.

First, there are two "kinds" of DACs: Voltage out and Current out. The Behringer that was suggested uses a AKM DAC which is a "voltage DAC".

Using that AKM as an example, the + and - legs (it is a balanced out) are riding on a DC. If you are using this as single-ended (a pre-amp with an RCA connector) then you must get rid of the DC. This can be done with capacitors to block the DC or can be done by a transformer (the voltages out are equal and only differ in sign).

The next issue is filtering (anti-aliasing). Part of this may be done by the DAC chip already (this is true of the AKM, but the filtering is insuffiecent). A transformer may provide additional low-pass filtering (or it may not).

The remaining issue is whether the DAC needs to see some series resistance (the AKM specs a 600 ohm or greater requirement). If this is not done then the pre-amp amy try to "draw power or current" from the DAC. They were not designed for that and the series resistance is required. In addition, the transformer may need to be loaded on the secondary side (it may or may not). If this is not done there may be some "ringing" and Bill has posted extensively on this.

So you need to consider the following: 1) is the DAC a voltage or current design, 2) does the DAC provide suffiecient low pass filtering, 3) does the transformer provide any low pass filtering, 4) is the gain sufficient - else you may need a step up transformer and the impedance reflected will correspondingly change), 5) are you using the transformer to cancel DC and convert from balanced to unbalanced. 6) is there a pre-amp? (or are you using this to "drive the cable" to the amp). Ther are others, but these are potentially major headaches.

Until these issues are specified, I would take any advice as being incomplete.
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Old 27th August 2010, 10:47 PM   #3048
Mush is offline Mush  United States
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Thanks for your reply. My old DAC used the CS4398. I had it connected to the UTC A-20s via a resistor per each leg. I used r/c in series and a load resistor across the secondary. It sounded great. After one of the channels blew up, I began to wonder if the values and general network of loads and filters that I used werr at fault.

I am looking for suggestions for a circuit of resistors and capacitors for a voltage DAC connected via transformers could provide good performance and reliability. (I drive a 32 yr old Volvo, reliability is a big deal for me).

Thanks very much for your help.
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Old 27th August 2010, 11:21 PM   #3049
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I haven't seen complete specs on the A20s but generally, output trafos have an unloaded impedance of around 100k or higher.

Voltage out dac chips don't want to see any load,they are voltage sources.

The A20s don't need a load to operate correctly, but filtering with RC circuits is pretty much mandatory for some dac chips. The A20s have limited bandwidth so they supply some filtering themselves, but a filter around 150khz wouldn't hurt anything and could possibly eliminate any ringing. The only sure way to tell is with a sig gen and a scope.

Primary series resistors should be around 250 to 500, depends on the chip. 4398 is OK with 250, others might want 500 depending on source resistance of the dac chip. They act as current limiters at the cutoff frequencies to protect the chip from over current. They also affect the low end cutoff F3 so it is a balancing act.

If you have the upsampling model, possibly the upsampling chip roasted due to the improper voltage on the daughter board. It's been reported by a few people.

Last edited by Bill Fuss; 27th August 2010 at 11:31 PM.
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Old 27th August 2010, 11:23 PM   #3050
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Mush - this diagram helped me when connecting my A-20's to the DAC. Resistor values for the primaries can be played around with to suit prefrences:

Click the image to open in full size.

This is the terminal scheme for the A-20's. Note terminals 3+4 / 9+10 are linked for 1:1 600:600:

Click the image to open in full size.

Last edited by RichLund; 27th August 2010 at 11:32 PM.
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