Transfo & Sigma/Delta DAC integration

[GaryKrant]

Hi all: Tired of searching entry level info about conditioning PWM DAC signal using a tranfo instead a low pass filter :-(... Now I need some help pls.

Taking as base a Marantz CD6000 (DAC stage similar to long tweaked Marantzes CD6x), I'm trying to replace the entire output stage (based on HDAMs or HDAM+Active LPF depending on CD6000 model).

My aim is using a line signal trasnformer to both low-pass filtering PWM DAC output and converting from differential to single ended output, but I have some doubts (because saddly I'm not electronics engineer), so any light over them will be really appreciated:

• Taking the output from a NPC SM5872BS Sigma/Delta DAC (16 times oversampled 44,1kHz digital signal), should this signal be dropped directly to the transformer or, as I think, might it be previously low pass filtered, say, with a RC filter?
• In both previous named low pass filter and transfo, the target bandwidth should be as closest as possible to 20Hz-20kHz? I assume that it is true.

I sent an email to Lundahl who fast and gently answered me suggesting the usage of their LL1527 trafo, however its bandwidth is 10Hz-150kHz. Searching in their catalog I find 5402 being more suitable to my needs: in fact its bandwidth is 20Hz-40kHz. Am I radically wrong when thinking that my target bandwidth is 20Hz-20kHz? Why they offered me a model with a higher bandwidth?

As you can see I'm really confused, so any help, any usage example, any addressing to info will be really really appreciated.

[Bill Fuss]

You want as wide a bandwidth as possible and a secondary RC filter that operates way above the audio band. The LL1527X might be fine but I dont know anything about your chip except the datasheet info. It would seem that the chip will only supply 1ma of output current @ about 4.7V so 5kohms would be the minimum value of the load impedance. A test CD and scope would be the best test.

In general, trafos work great with V out chips as long as you keep the cables short.

[GaryKrant]

Thanks for your repply Bill.

Based on what you point both trafos should work, however it's true that 20Hz base frequency on LL5402 might produce an attenuation on lower frequencies and that LL1527X should work better if secondary RC filter deals with 20kHz-100kHz bandwidth filtering.

In any case I think I understand your concept: transfo deals with a first filtering stage and differential to single ended conversion, then RC filter deals with a second filtering stage, it's it?

[Spartacus]

I'm pretty sure the NPC DACs don't use analog filtering (i.e. switched capacitor) so without a bunch of filtering you'll get a big chunk of noise coming through. Marantz IIRC use 4th order filters (I may be wrong) so a transformer on it's own, even one with a low bandwidth, may not be enough. I'd hunt down a circuit that provides 4th order filtering at around 40-60KHz. Look through transformer manufacturer's datasheets for something that might do.

[GaryKrant]

Originally Posted by Spartacus

I'm pretty sure the NPC DACs don't use analog filtering (i.e. switched capacitor)

True, this DAC outputs signal as PWM, so filtering must be implemented outside the DAC. That's why I love this kind of DACs, because of the flexibility of doing that filtering as you want

Originally Posted by Spartacus

Marantz IIRC use 4th order filters (I may be wrong) so a transformer on it's own, even one with a low bandwidth, may not be enough.

In fact CD6000 uses a 2nd order RC low pass filter followed by an active one. DAC datasheed shows two practical applications: one based on a passive+active and other based on an active+active low pass filter.

Originally Posted by Spartacus

I'd hunt down a circuit that provides 4th order filtering at around 40-60KHz. Look through transformer manufacturer's datasheets for something that might do.

So your advice seems to be in line with Bill's one: transfo + additional filter. In fact Heart's audio tube based CD6000 mod uses the onboard passive filter and then a transfo prior to amplify the signal. My doubt now is knowing experiences in this line, ie, used transfos, schematics, etc, for similar applications.

Thanks for your help, Spartacus.

[Spartacus]

Checked the datasheet and yep, NPC use a second order filter. Doesn't say what the -3db is though. I guess you could try it with and without extra RC filtering and make a decision by ear. For a transformer I'd use one with a bandwidth as low as possible without significantly impacting the audio range.

[GaryKrant]

Oh God, this is really a headache for a electronics absolute fool like me

Let's state that -as Bill pointed based on DAC specs- DAC output drops 1mA @ 4,7V, what forces a matching impedance > 5kOhm in the transfo primary.

After reviewing Lundahl catalog I find two models with freq responses fit to my needs:Lundahl LL1540 (input line transfo) and LL5402 (output line transfo)...

... and here I'm lost with specs: no-load impedances, static resistances...

I'm not able to understand if one of these models fits to my needs. I just assume that matching impedance must be achieved adding a resistor between primary and - (or so I think).

Again, any help about DAC-tranfo integration will be really appreciated.

[Bill Fuss]

Yeah, it gets confusing. To simplify it somewhat, don't use a trafo wound for input duty. The construction is vastly different and complicates the implimentation greatly. Some will argue with this but it is true non the less.
Output trafos are designed to operate without a specified load so the impedance of your preamp is reflected into the primary. It will be on the order of 50k-100kohms, plenty high for the chip. An output trafo's rated impedance is just that, the load it CAN operate into and meet specs, not a required load.
Whether you use a limited bandwidth product or not is up to you, but I would not. I would merely use a simple 1st order filter before and after the trafo

[Bill Fuss]

I do believe the 5402 would be a bad choice. The inductance is too low. If you are dead set on using Lundahls, I would closely adhere to their recommendations. Personally, I would look for a bifilar wound output trafo of another brand, such as the Jensen JT-11-DM, bifilar wound, nickel core.

[GaryKrant]

Hi Bill: I've been looking for that trafo at Jensen but it seems not being in their catalog. Anyway I've found an application note from Jensen very similiar to what I wanna do.

It seems using a low pass filter, as you recommend, prior to goint for the primary. Just one problem: JT-11-SSP costs 140$ (glub)

I'll try, again, the Lundahl approach and I'll follow his and your advice: using the ll1527 and not filtering in the transfo.

Bill, your help is really appreciated. Thanks :-)